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Experimental and analytical modeling of the in vivo and in vitro biomechanical behavior of the human lumbar spine

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Title:
Experimental and analytical modeling of the in vivo and in vitro biomechanical behavior of the human lumbar spine
Physical Description:
Book
Creator:
Vestgaarden, Tov I
Publisher:
University of South Florida
Place of Publication:
Tampa, Fla
Publication Date:

Subjects

Subjects / Keywords:
Lumbar spine
Biomechanics
Intradiscal pressure
Physiological loads
Disc degeneration
Dissertations, Academic -- Biomedical Engineering -- Doctoral -- USF   ( lcsh )
Genre:
non-fiction   ( marcgt )

Notes

Summary:
ABSTRACT: This dissertation has two major parts; Analytical and Experimental. The analytical section contains a study using Finite Element Analysis of dynamic instrumentation to demonstrate stress reduction in adjacent level discs. The experimental section contains biomechanical testing of facet fusion allograft technique and finally a comparison between In Vivo and In Vitro intradiscal pressures to determine forces acting on Lumbar spine segment L4-L5. A comprehensive study of available data, technology and literature was done. Conventional fusion instrumentation is believed to accelerate the degeneration of adjacent discs due to the increased stresses caused by motion discontinuity. A three dimensional finite element model of the lumbar spine was obtained which simulated flexion and extension. Reduced stiffness and increased axial motion of dynamic posterior lumbar fusion instrumentation designs results in a ~10% cumulative stress reduction for each flexion cycle.The cumulative effect of this reduced amplitude and distribution of peak stresses in the adjacent disc may partially alleviate the problem of adjacent level disc degeneration. Traditionally a pedicle screw system has been used for fixation of the lumbar spine and this involves major surgery and recovery time. Facet fixation is a technique that has been used for stabilization of the lumbar spine. The cadaver segments were tested in axial rotation, combined flexion/extension and lateral bending. Implantation of the allograft dowel resulted in a significant increase in stiffness compared to control. Facet fusion allograft provides an effective minimally invasive method of treating debilitating pain caused by deteriorated facet joints by permanently fusing them. An In Vitro biomechanical study was conducted to determine the intradiscal pressure during spinal loading. The intradiscal pressures in flexion/extension, lateral bending and axial rotation was compared to In Vivo published data.There is no data that explains the actual forces acting on the spine during flexion, extension, lateral bending or axial rotation. The functional spinal units were tested in combined axial compression and flexion/extension, combined axial compression and lateral bending and combined axial compression and axial rotation using a nondestructive testing method. Overall, this study found a good correlation between In Vivo and In Vitro data. This can essentially be used to make physiological relation from experimental and analytical evaluations of the lumbar spine. It is important to know how much load needs to be controlled by an implant.
Thesis:
Dissertation (Ph.D.)--University of South Florida, 2007.
Bibliography:
Includes bibliographical references.
System Details:
Mode of access: World Wide Web.
System Details:
System requirements: World Wide Web browser and PDF reader.
Statement of Responsibility:
by Tov I. Vestgaarden.
General Note:
Title from PDF of title page.
General Note:
Document formatted into pages; contains 117 pages.
General Note:
Includes vita.

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aleph - 001989304
oclc - 308433842
usfldc doi - E14-SFE0002277
usfldc handle - e14.2277
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SFS0026595:00001


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ABSTRACT: This dissertation has two major parts; Analytical and Experimental. The analytical section contains a study using Finite Element Analysis of dynamic instrumentation to demonstrate stress reduction in adjacent level discs. The experimental section contains biomechanical testing of facet fusion allograft technique and finally a comparison between In Vivo and In Vitro intradiscal pressures to determine forces acting on Lumbar spine segment L4-L5. A comprehensive study of available data, technology and literature was done. Conventional fusion instrumentation is believed to accelerate the degeneration of adjacent discs due to the increased stresses caused by motion discontinuity. A three dimensional finite element model of the lumbar spine was obtained which simulated flexion and extension. Reduced stiffness and increased axial motion of dynamic posterior lumbar fusion instrumentation designs results in a ~10% cumulative stress reduction for each flexion cycle.The cumulative effect of this reduced amplitude and distribution of peak stresses in the adjacent disc may partially alleviate the problem of adjacent level disc degeneration. Traditionally a pedicle screw system has been used for fixation of the lumbar spine and this involves major surgery and recovery time. Facet fixation is a technique that has been used for stabilization of the lumbar spine. The cadaver segments were tested in axial rotation, combined flexion/extension and lateral bending. Implantation of the allograft dowel resulted in a significant increase in stiffness compared to control. Facet fusion allograft provides an effective minimally invasive method of treating debilitating pain caused by deteriorated facet joints by permanently fusing them. An In Vitro biomechanical study was conducted to determine the intradiscal pressure during spinal loading. The intradiscal pressures in flexion/extension, lateral bending and axial rotation was compared to In Vivo published data.There is no data that explains the actual forces acting on the spine during flexion, extension, lateral bending or axial rotation. The functional spinal units were tested in combined axial compression and flexion/extension, combined axial compression and lateral bending and combined axial compression and axial rotation using a nondestructive testing method. Overall, this study found a good correlation between In Vivo and In Vitro data. This can essentially be used to make physiological relation from experimental and analytical evaluations of the lumbar spine. It is important to know how much load needs to be controlled by an implant.
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Physiological loads
Disc degeneration
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Experimental and Analytic al Modeling of the In Vivo and In Vitro Biomechanical Behavior of the Hu man Lumbar Spine by Tov I. Vestgaarden A dissertation submitted in partial fulfillment of the requirement s for the degree of Doctor of Philosophy in Biomedical Engineering Department of Chemical Engineering College of Engineering University of South Florida Co-Major Professor: W illiam E. Lee III, Ph.D. Co-Major Professor: Antoni o E. Castellvi, M.D. Daniel Hess, Ph.D. Ashok Kumar, Ph.D. Shuh Jing Benjamin Ying, Ph.D. Date of Approval: November 2, 2007 Keywords: lumbar spine, bi omechanics, intradiscal pressu re, physiological loads, disc degeneration, spine fusion, posteri or instrumentation, disc stresses, adjacent disc, facet fixation. Copyright 2007, T. I. Vestgaarden

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DEDICATION I would like to dedicate this dissertation to my mom, dad, brother and sister; without their support I would not have had the courage to begin this adventure, or the strength to finish it. Thank You.

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ACKNOWLEDGEMENTS I would like to thank my parents, br other and sister for the continuous support during my years at University of South Florida. Wit hout their love and support, this would not be possible. I would like to thank the people and institutions that supported me duri ng my years as a graduate student at University of South Florida. My heartfelt thanks go to Dr. William E. Lee III and Dr. Antonio E. Castellvi for their guidance and sponsorship for thes e research projects. I would also like to thank my examining committee Dr. D aniel Hess, Dr. Ashok Kumar and Dr. Shuh-Jing Ying for taking their valuable ti me to be involved in this project. A special note of recognition goes to Dr. David Pienkowski, Dr. Hao Huang, Dr. Ke Li, Dr. Wes Johnson and Dr. Murray Maitland, for their valuable time, advice, guidance, and patience th roughout this endeavor. I thank Deborah Clabeaux, for her help with numerous re search and administra tive tasks; without her nothing would be possible. I would like to thank all my friends for all their support during my dad’s fight against cancer. I would also like to th ank MBNA for their 0% interest, so that continuing school has been possible afte r ended support. I would also like to thank my Godson, Jacob, and Goddaught er, Thea, for all the smiles. Jeg vil ogs takke min familie for all hj elp og sttte gje nnom min studie tid. N er det heldigvis ikke alt for lenge igj en, og da er det min ti d til hjelpe dere p alle mulige mter.

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i TABLE OF CONTENTS LIST OF TABLES...............................................................................................v LIST OF FI GURES...........................................................................................vii ABSTRACT ..................................................................................................xi CHAPTER 1 – IN TRODUCT ION........................................................................1 1.1 – Backg round.....................................................................................1 1.1.1 – Spine Anatomy..................................................................2 1.1.1.1 – Norma l Curv es....................................................2 1.1.1.2 – Curvatur e Abnormalit ies......................................3 1.1.1.3 – Div isions..............................................................3 1.1.1.4 – Typica l Vertebra..................................................4 1.1.1.5 – Lumbar Vertebr ae...............................................5 1.1.1.6 – Cervic al Spine.....................................................7 1.1.1.7 – Thorac ic Spine....................................................8 1.1.1.8 – Lum bar Spine......................................................8 1.1.1.9 – Interver tebral Disc...............................................8 1.2 – Signifi cance....................................................................................9 1.3 – Obje ctive.......................................................................................12

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ii 1.4 – Outline of t he Dissertat ion.............................................................13 CHAPTER 2 – MATERIAL S AND METH ODS..................................................14 2.1 – Anal ytical......................................................................................14 2.2 – Exper imental .................................................................................16 2.2.1 Biomechani cal Testing.....................................................16 2.2.2 – Intradiscal Pre ssure Measur ements................................20 2.2.3 – Human Cadaver Tissue and Fi xation..............................22 CHAPTER 3 – FINITE ELEM ENT ANALYSIS OF DYNAMIC INSTRUMENTATION DEMONSTRATES STRESS REDUCTION IN ADJACE NT LEVEL DISCS............................25 3.1 – Intr oduction ...................................................................................25 3.2 – Material s and Met hods..................................................................26 3.2.1 – Stud y Desi gn...................................................................26 3.2.2 – Finite El ement Mode ling..................................................27 3.3 – Resu lts..........................................................................................33 3.4 – Discu ssion....................................................................................43 CHAPTER 4 – BIOMECHANICAL TESTING OF FACET FUSION TECHNIQU E............................................................................50 4.1 Introd uction ....................................................................................50 4.2 Material s and Met hods..................................................................51 4.2.1 Spine Preparati ons..........................................................51 4.2.2 Implant and Fi xation Tech niques .....................................53

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iii 4.2.2.1 Specimen In strumentation..................................53 4.2.2.2 Facet Fusion A llograft Inse rtion..........................54 4.2.3 Study Protocol..................................................................56 4.2.4 Statisti cal Anal ysis...........................................................56 4.3 Resu lts...........................................................................................57 4.4 Discu ssion.....................................................................................60 CHAPTER 5 A COMPARISON BETWEEN IN VIVO AND IN VITRO INTRADISCAL PR ESSURE S...................................................65 5.1 Introd uction ....................................................................................65 5.2 Material s and Met hods..................................................................66 5.2.1 Spine Preparati ons..........................................................67 5.2.2 Test Setup and Bi omechanical Testing............................68 5.2.2.1 Test Setup..........................................................68 5.2.3 Study Protocol..................................................................69 5.3 Resu lts...........................................................................................70 5.4 Discu ssion.....................................................................................76 CHAPTER 6 – SUMMARY ...............................................................................81 6.1 – Conc lusion....................................................................................81 6.2 – Contri bution..................................................................................82 6.3 – Future Work..................................................................................83 REFERENC ES.................................................................................................85 APPENDICE S .................................................................................................97

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iv Appendix A Figures Related to Analytical Results..............................98 Appendix B Figures Related to Experimental Results......................101 Appendix C Tables Related to St atistics and Experimental Data......105 Appendix D Publications Related to the Dissertation Research........117 ABOUT THE AUTHOR..End Page

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v LIST OF TABLES Table 3-1: Materi al Proper ties..........................................................................33 Table 3-2: Peak Calculated St ress (MPa) in t he L3-L4 Di sc.............................35 Table 3-3: Peak Calculated Stre ss (MPa) in the L4 – L5 Dis c..........................37 Table 4-1: Range of Motion of t he Intact and Tr eated Segm ent.......................57 Table 4-2: Stiffness of t he Intact and Tr eated Segm ent...................................58 Table 4-3: Percentage Change of R ange of Motion and Stiffness....................58 Table C-1: Range of Motion Test Resu lts for Individual Specimens During Extension Loading .......................................................................105 Table C-2: Range of Motion Test Resu lts for Individual Specimens During Flexion Lo ading........................................................................... 105 Table C-3: Range of Motion Test Resu lts for Individual Specimens During Bending Lo ading.......................................................................... 106 Table C-4: Range of Motion Test Resu lts for Individual Specimens During Torsion Lo ading........................................................................... 106 Table C-5: Stiffness Test Result s for Individual Specimens During Extension Loading .......................................................................107

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vi Table C-6: Stiffness Test Results fo r Individual Specim ens During Flexion Loading........................................................................................ 107 Table C-7: Stiffness Test Results fo r Individual Specimens During Bending Loading........................................................................................ 108 Table C-8: Stiffness Test Results fo r Individual Specimens During Torsion Loading........................................................................................ 108 Table C-9: Summary of the Singl e Factor ANOVA Performed on the Range of Motion Specimens Du ring Extension Loading..............109 Table C-10: Summary of the Single Factor ANOVA Performed on the Range of Motion Specimens Du ring Flexion Loading..................110 Table C-11: Summary of the Single Factor ANOVA Performed on the Range of Motion Specimens During Lateral Bending Loading....111 Table C-12: Summary of the Single Factor ANOVA Performed on the Range of Motion Specimens During Axial Rotation Loading.......112 Table C-13: Summary of the Single Factor ANOVA Performed on the Stiffness Specimens During Extension L oading..........................113 Table C-14: Summary of the Single Factor ANOVA Performed on the Stiffness Specimens Duri ng Flexion Load ing..............................114 Table C-15: Summary of the Single Factor ANOVA Performed on the Stiffness Specimens During Lateral Bendin g Loading.................115 Table C-16: Summary of the Single Factor ANOVA Performed on the Stiffness Specimens During Ax ial Rotation Lo ading....................116

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vii LIST OF FIGURES Figure 1-1: The Comple te Human Spine............................................................2 Figure 1-2: A Typical Lumbar Vertebra (Gray’ s Anatomy)..................................5 Figure 1-3: A Typical Cervical Vertebra (Gray’ s Anatomy).................................6 Figure 1-4: A Typical Thoracic Vertebra (Gray’ s Anatomy)................................7 Figure 1-5: A Typical Intervert ebral Disc (Gray’ s Anatomy)................................9 Figure 2-1: Displacement Function for Finite Elem ent Method.........................15 Figure 2-2: The MTS 858 Bionix II Spi ne Tester at University of South Florid a............................................................................................17 Figure 2-3: MTS Force Transducer Us ed on the Experimental Apparatus.......18 Figure 2-4: Pressure Pr obe Made by Or thoAR.................................................21 Figure 2-5: A FSU Po tted on Both Sides ..........................................................23 Figure 2-6: X-Ray Image of a Potted Specimen, with No Anchors in the Disc Spac e....................................................................................24 Figure 3-1: Isometric View of the Finite Element Mesh of the Lumbar Spine and the Semi -Rigid Rod.......................................................27 Figure 3-2: Isometric View of an Intervert ebral Dis c.........................................29 Figure 3-3: The Damper Model of the Dynamic Inst rumentation......................30

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viii Figure 3-4: Comparison of Stress in L3-L4 with Di fferent Variables for R and G.............................................................................................39 Figure 3-5: Stress Distr ibution of L3-L4 at 45 Flexi on.....................................40 Figure 3-6: Stress Distri bution of L4-L5 Disk at 45 Flexion.............................41 Figure 3-7: Stress Distri bution of L3-L4 Disk at 15 Extens ion.........................42 Figure 3-8: Stress Distri bution of L4-L5 Disk at 15 Extens ion.........................42 Figure 3-9: Two Approaches to Generate 2 of Rota tion..................................45 Figure 4-1: Posterior View of Placem ent of Facet Fusion Allograft in Facet Jo ints...................................................................................53 Figure 4-2: Superior View of Placem ent of Facet Fusion Allograft in Facet Join ts...................................................................................54 Figure 4-3: Percentage Reduction of Facet Joint Due to the Implant (Panjabi )........................................................................................55 Figure 4-4: Typical Flexion-Extens ion Results, Showing Comparison Between Intact and Tr eated Spec imen..........................................59 Figure 4-5: Stiffness Results for t he Intact and Treat ed Specim ens.................61 Figure 4-6: Stiffness Results for t he Intact and Treat ed Specim ens.................62 Figure 4-7: Comparison of Percent Change of Stiffness to Published Data...............................................................................................63 Figure 5-1: Torque vs. Angle Data for the Extension and Flexion Experimental Test..........................................................................70

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ix Figure 5-2: Pressure vs. Angle Da ta for the Extension and Flexion Experimental Test..........................................................................71 Figure 5-3: Torque vs. A ngle Data for the Lateral Bending Experimental Test................................................................................................72 Figure 5-4: Pressure vs. Angle Data for the Lateral Bending Experimental Test..........................................................................72 Figure 5-5: Torque vs. A ngle Data for the Axial Rotation Experimental Test................................................................................................73 Figure 5-6: Pressure vs. Angle Data for the Axial Rotation Experimental Test................................................................................................74 Figure 5-7: Extension Flexion Intradiscal Pressure In Vitro of Selected L4-L5 Segments with Respect to the Total Motion in a Single Level..............................................................................................75 Figure 5-8: Lateral Bending Intradiscal Pressure In Vitro of Selected L4L5 Segments with Respect to t he Total Motion in a Single Level..............................................................................................75 Figure 5-9: Axial Rotation Intradiscal Pressure In Vitro of Selected L4-L5 Segments with Respect to the Total Motion in a Single Level.......76 Figure A-1: Stress Distri bution of L3-L4 Disk at 15 Flexion.............................98 Figure A-2: Stress Distri bution of L3-L4 Disk at 30 Flexion.............................98 Figure A-3: Stress Distri bution of L3-L4 Disk at 45 Flexion.............................99 Figure A-4: Stress Distri bution of L4-L5 Disk at 15 Flexion.............................99

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x Figure A-5: Stress Distri bution of L4-L5 Disk at 30 Flexion...........................100 Figure B-1: Typical Lateral Bend ing Results, Demonstrating a Comparison Between Intact and Treated S pecimen....................101 Figure B-2: Typical Axial Rotation Results, Demonstrating Comparison Between Intact and Tr eated Specim en........................................102 Figure B-3: Range of Motion Compar ison Between the Different Intact Specimens ...................................................................................102 Figure B-4: Range of Motion Compar ison Between the Different Treated Specimens ...................................................................................103 Figure B-5: Stiffness Comparis on Between the Different Intact Specimens ...................................................................................103 Figure B-6: Stiffness Comparis on Between the Different Treated Specimens ...................................................................................104

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xi EXPERIMENTAL AND ANALYTI CAL MODELING OF THE IN VIVO and IN VITRO BIOMECHANICAL BEHAVIOR OF THE HUMAN LUMBAR SPINE Tov I. Vestgaarden ABSTRACT This dissertation has two major parts; Analytical and Experimental. The analytical section contains a study usin g Finite Element Analysis of dynamic instrumentation to demons trate stress reduction in adjacent level discs. The experimental section cont ains biomechanical testing of facet fusion allograft technique and finally a comparison between In Vivo and In Vitro intradiscal pressures to determine forces acti ng on Lumbar spine segment L4-L5. A comprehensive study of available data, technology and literature was done. Conventional fusion instrumentation is believed to accelerate the degeneration of adjacent discs due to th e increased stresses caused by motion discontinuity. A three dim ensional finite element model of the lumbar spine was obtained which simulated flexion and ex tension. Reduced stiffness and

PAGE 15

xii increased axial motion of dynamic poste rior lumbar fusion instrumentation designs results in a ~10% cumulative st ress reduction for each flexion cycle. The cumulative effect of this reduc ed amplitude and distribution of peak stresses in the adjacent disc may partia lly alleviate the problem of adjacent level disc degeneration. Traditionally a pedicle screw system has been used for fixation of the lumbar spine and this involves major su rgery and recovery time. Facet fixation is a technique that has been used for stabi lization of the lumbar spine. The cadaver segments were test ed in axial rotation, comb ined flexion/extension and lateral bending. Implantation of the al lograft dowel result ed in a significant increase in stiffness compar ed to control. Facet fusion allograft provides an effective minimally invasive method of treating debilitating pain caused by deteriorated facet joints by permanently fusing them. An In Vitro biomechanical study was conducted to determine the intradiscal pressure during spinal lo ading. The intradiscal pressures in flexion/extension, lateral bending and axial rotation was compared to In Vivo published data. There is no data that explains the actual forces acting on the spine during flexion, extens ion, lateral bending or axial rotation. The functional spinal units were tested in combined ax ial compression and flexion/extension, combined axial compression and lateral bending and combined axial compression and axia l rotation using a nondestructi ve testing method. Overall, this study found a good correlation between In Vivo and In Vitro data. This can

PAGE 16

xiii essentially be used to make physiologic al relation from experimental and analytical evaluations of t he lumbar spine. It is im portant to know how much load needs to be controlled by an implant.

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1 CHAPTER 1 – INTRODUCTION 1.1 – Background First, I want to introduce some commonly used terms in medicine to describe directions, planes and motions. A person that is orientated in the “anatomical” position is facing forward, wit h arms and legs on a slight angle. The “palms of hands” are facing forward. Directional terms commonly used are Anterior, Posterior, Superior, Inferior, Medial and Lateral. Anterior, also referred to as Ventral, means toward the front. Posterior (Dorsal) is toward s the back, and as an example we can look at the vertebra. When you look at the vertebra, you have the vertebral body and the posterior elements. These poste rior elements are towards the back. Superior (cranial) is towards the top and inferior (caudal) is towards the bottom. Medial describes the mid line of the body a nd Lateral means away from the midline of the body. In general there are three planes; frontal, midsagittal and transverse plane. The frontal plane is the plane that goes from inferior-superior and rightleft. As an example, right side bending wi ll occur within the frontal plane. The

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2 other planes are midsagittal (anterio r-posterior and inferior-superior) and transverse (anterior-posterior and right to left) planes. 1.1.1 – Spine Anatomy 1.1.1.1 – Normal Curves The spine consists of four curvat ures, and they alter between convex and concave. The cervical region (neck) has a concave curvature and the same does the lumbar region (lower back). The thoracic region (mid region) and Sacral region are both convex curved. Figure 1-1: The Complete Human Spine

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3 1.1.1.2 – Curvature Abnormalities There are some curvature abnormalities that might be present at birth, while others might be the caused from a disease, uneven muscle force or bad posture. The most frequently seen cu rvature abnormalities are scoliosis, kyphosis and lordosis. Scoliosis is a spine curvature that is abnormal in the lateral curvature and the spine should normally be straight in this position. While the spine will always have a slight scoliosis (lateral cu rvature in the frontal plane), it will not cause problems with most people. Scoliosis is more common for females and is most common to occur in late childhood. Kyphosis is a change in the thor acic curvature towards the back (posterior). The spine is rounded, and the vertebral bodies are usually compressed into a wedge shape. This is most commonly caused by compression fractures due to osteoporosis. Lordosis is an exaggerated lumbar cu rvature and is often referred to swayback. 1.1.1.3 – Divisions Three of these four regions are build up from vertebral and intervertebral disc. The vertebrae consist of a vert ebral body, lamina, pedicle, spinous process, transverse process, superior fa cet and inferior facet. The disc that

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4 connects the vertebral bodies is made from an incompressible center named nucleus pulposus and the nucleus pu lposus is surrounded by the annulus fibrosus. The annulus fibrosus is build up by annulus grounds and layers of annulus fibers. These fibers have an alternating mesh that is aligned at an approximate 30 degrees. 1.1.1.4 – Typical Vertebra Different regions have different charac teristics to the vertebrae, but they have all some common features. A typica l vertebra consists of the vertebral body, vertebral arch and seven processes. The body is the solid construction of the vertebrae and is exposed to high compression loads. The majority of the loads are distributed through the vertebral body and the inte rvertebral disc act as the “shock absorber”. While the superior and inferior parts of the vertebr al body are roughened for attachment of the intervertebral disc. The intervertebral disc is a thick, disc shaped construct. The anterior and posterior surfaces have ligaments running from superior to inferior on the spine. The anterior and la teral surfaces have nutrient foramina for blood vessels.

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5 1.1.1.5 – Lumbar Vertebrae The Lumbar vertebrae are the larges ve rtebrae in the spi ne. These are in the lower spine and carry the highest loads. A lumbar vertebra consists of the body, pedicle, transverse process, spin ous process, lamina, inferior and superior facets. The vertebral body is t he largest part of the vertebrae and the vertebral body is connected to the interver tebral disc. The disc is carrying about 70 percent of the load, while the two face t joints carry the remaining 30 percent. The pedicle connects the posterior elem ents to the vertebra body and this is a very strong and rigid part of the vertebrae. Typically the L4 vertebra is the lar gest vertebrae and the L4 vertebrae is typically located at the same level as the superior part of the ileum crest. Figure 1-2: A Typical Lumbar Vertebra (Gray’s Anatomy)

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6 As seen in figure 1-2, the transvers e process is attached to the pedicle and the transverse process is directed in the lateral direction. The facets (labeled as Inferior Articulated Process) are also connected to the pedicle and the facets are directed in the superior and inferior directions. The facet joints consist of the superior facets of one ve rtebra and the inferior facets of the adjacent vertebrae. These facet joins add st ability to the segment and it is also load bearing. The posterior elements create the spinal canal, which protects the spinal cord. Figure 1-3: A Typical Cervical Vertebra (Gray’s Anatomy)

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7 Figure 1-4: A Typical Thoracic Vertebra (Gray’s Anatomy) 1.1.1.6 – Cervical Spine The cervical spine consists of 7 ve rtebrae (C1-C7), wher e C1 and C2 are very unique. A typical cervical vertebrae c onsist of the C1 is also referred to as the atlas and the C2 is referred to as the axis. The atlas has a primary function to support the head and it does not hav e the body, pedicle, lamina, spinous processes like the vertebral usually do. It consists of two large lateral masses.

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8 The axis is a rigid vertical axis, fo r rotation of the atlas. The C7 is referred to as the “vertebr al prominens” and is the most prominent. It has many characteristics of t he thoracic vertebrae. Cervical spine is the most flexible region (the greatest Range of Motion) of the spine and is also the region wit h the lowest load bear ing capabilities. 1.1.1.7 – Thoracic Spine The thoracic region has twelve vertebr ae. This is also the region where the ribs are connected to the verbal co lumn. The typical thoracic vertebrae are T2-T10 and the an-typical are T1 and T11-T12. 1.1.1.8 – Lumbar Spine The lumbar region consists of 5 vertebrae and they have wide massive bodies. The Lumbar region is the section of t he spine that has the highest load bearing capabilities, and lim ited Range of Motion (R OM). The Lumbar region has good ROM in Flexion. 1.1.1.9 – Intervertebral Disc The intervertebral disc is the flex ible portion between the vertebral bodies. This intervertebral disc consis ts of two major components: nucleus

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9 pulposus and annulus fibrosus. The nucleus pulposus is the center portion of the intervertebral disc and it is an incompressible material. This nucleus pulposus is a gelatinous cushioning part of the intervertebral disc and as the pressure increases, the nuc leus bulges and this leads to the disc bulging. The annulus fibrosus are several la yers of cartilaginous laminae. Figure 1-5: A Typical Interver tebral Disc (Gray’s Anatomy) 1.2 – Significance The most common disease, next to the common cold, is Low Back Pain (LBP)1.

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10 Fusion of adjacent vertebrae is wi dely used for treating degenerated disc disease, but this procedure does not always alleviate pain2 and has a degree of comorbidity.3 Use of conventional (rigid) po sterior instrumentation commonly accompanies fusion to prevent motion and aid fusion healing; however, such rigid fixation is believed to accele rate the radiographically observed degeneration of the discs adjacent to the fused segments due to the increased stresses caused by the abrupt sti ffness and motion discontinuity.4-8 As an alternative to rigid fixation different methods of “soft”9 or “dynamic”10-11 stabilization have emerged.12 Regardless of the name used, these stabilization methods feature some type of less-than -rigid instrumentation design connected to modified pedicle screws for the purpos e of gaining more favorable movement and load transmission across non-fus ed segments. Less than rigid instrumentation seeks to distribute moti on rather than eliminate it, and thereby reduce the likelihood of adj acent level disc disease while improving the long term outcome of lumbar fusion procedures.13 Treatment of lower back pain can be performed by several different procedures. These procedures typically invo lve an internal fixation of the lower spine, which is a well established met hod of reducing lower back pain. To allow fusion, several methods of fixation are used14-21. Metal is traditionally used to achieve fixation, which is done by pedicle screw system, translaminar facet screws or facet interference screws22-27. Lately, the surgical methods and fixation devices have been rapidly evolvi ng. When internal fixation first began in

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11 the 1940’s, Don King develo ped and implemented a somewhat simple idea for fixation28. This method is very similar to what is now referred to as translaminar facet fixation29-34. This idea restricts the motion in the facet joint, leading to a fusion of the joint28,29,35-46. The idea introduced in t he late 1940’s was modified by Boucher in 1959 and it is referred to as t he “True transfacet” method36. This method changes the angle which the screws are inserted, and provides for similar stability and a safer approach. Facet fixation was brought back in 1984 by Magerl, referred to as translaminar transfacet fixation29. This is a modification of the original method developed by King28. This method is considered easier to perform, more stable and safer than the initial tr anslaminar facet method developed by King28. In the 1980’s the pedicle screw system became the golden standard, while it might not be the most ergonomical ly method of fixing the lumbar spine for fusion24-26. The pedicle screw system has several disadvantages, but in some cases it is the only option for a successful healing47-58. With an increase of medical device development to treat low back pain (LBP), there is also an increasing n eed for testing of medical devices In Vitro1,59. Currently, there are no published data that supports the actual forces in the spine during flexion, extensi on or lateral bending. There are published articles that give In Vivo intradiscal pressure measurements for these motions, but there are no correlation performed against In Vitro testing results60-65.

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12 With this increased demand for devel opment and validation of medical devices, the relation to physiological relev ance is critical. Currently, there is no physiological rationale for the forces and moments applied during cadaver testing of medical devices. Another increasing problem is the supply of cadaver tissue and mathematical models are in creasing in popularity. By collecting scientific data, this data can be used to validate mathematical models. 1.3 – Objective There are three main objectives to th is dissertation. As earlier stated, these are both analytical and experim ental. The analytical section is accomplished by the use of a finite elem ent model to calcul ate and compare the stresses in the adjacent level disc t hat are induced by conventional and “dynamic” posterior lumbar fusion inst rumentation. The hypothesis of this particular study was validation of the inci dence of adjacent le vel disc disease in the lumbosacral spine will be decreased with the use of semi-rigid rods. The second section of this dissert ation contains the experimental evaluation. Here, a comparison of the biom echanical properties of a facet fusion allograft In Vitro was performed. The hypothesis is to investigate that the stiffness and stability of spine will increas e by implanting facet fusion allograft. The last objective of this dissertat ion was to find relationship between In Vivo and In Vitro spinal mechanical loads. Th is was done by comparing the

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13 published In Vivo intradiscal pressures to In Vitro intradiscal pressures and evaluate the effects of moments applied In Vitro 1.4 – Outline of the Dissertation The remaining of the dissertation is organized as follows. In Chapter 2, the general materials and methods of the analytical and experimental work is described. Application specifics are ex plained in the respective chapters. Chapter 3 describes the analytical section of the dissertation, which contains a three dimensional finite elements study of the lumbar spine. The experimental work is shown in c hapter 4 and 5. In chapter 4, a facet fusion allograft is investigated. In Vitro Intradiscal pressure measurements are conducted in chapter 5 and compared to published In Vivo data. This comparison shows how much mechani cal load is acting on the spine. Chapter 6 summarizes the dissert ation research, outlines the contributions and provides some re commendations fo r future work.

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14 CHAPTER 2 – MATERIALS AND METHODS 2.1 – Analytical Engineering is in general problem solving by using mathematical models of physical situations. In traditional engineer ing, finite element method has been used extensively and is increasing in popularity in the medical field. The mathematical models are differentia l equations developed to solve the boundary and initial conditions. By applyi ng fundamental laws and principles, these differential equations are der ived based upon mass, force or energy. There are two methods; Force method, where the forces are unknown and displacement method, where displacem ents are unknown. Ther e are limitations to the force method, so the current use in finite element method is the displacement method. The governing equation for finite el ement method is a relation between the force, displacement and the stiffness. Seen below, is a sample of a two dimensional finite el ement method equation.

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15 (Equation 2-1) This equation shows the force (F), the displacement (d) and the stiffness (k). There are generally eight steps to solving a problem wi th finite element method. The first step is to select an element. Depending on the problem, a one, two or three dimensi onal element can be used. A first or second order element, as well as the shape of t he element must be used. Second order elements have more nodes, and gives be tter accuracy. The next step is to choose the displacement functions as shown below. Figure 2-1: Displacement Functi on for Finite Element Method

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16 Next, a definition of stre ss/strain and strain/displacement relationship is needed. This is done by applying boundary conditions. From this, the element stiffness matrix can be defined and the global equations can be assembled. With the global equations, a solution fo r displacement can be found. The displacements will be used to find the stre ss and the strain and the results are interpreted. The specifics for this particular st udy, is explained in detail later on in the dissertation. 2.2 – Experimental 2.2.1 Biomechanical Testing A nondestructive spine biomechanics test setup was used to find the biomechanical properties. This particula r setup is based on an axial servohydraulic materials testing system (MT S Systems Inc., 858 Bionix II, Eden Prairie, MN) and is modified to allow be nding as well as ax ial rotation. Axial compression is integrated in the MTS 858 Bionix II and the l oad is measured by the use of a load cell. The MTS 858 Bioni x II with the modifications can be seen below in figure 2-2.

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17 Figure 2-2: The MTS 858 Bionix II Spine Te ster at University of South Florida The load cell is an electroni c device (transducer) that is used to find the axial force applied. The l oad cell measures strain, by the use of a Wheatstone bridge strain gage. Since the load cell measures dy namic load, there is a constant feed back and error correction process. This will generally cause the signal to oscillate, but by the use of a controller system, this oscillating effect is minimized. The control systems consist of an actuator that actively dampens the effect of the oscillation. This me thod offers great performance, but the process is complex and costly.

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18 These load cells are calibrated on si te by the manufacturer. The general method of calibration is simply to apply a known force by the help of gravity. Since the applied static force is known, the load cell can be calibrated accordingly. This procedure is done with a series of different loads, and a calibration equation is developed. The l oad cell has an accuracy of 0.13% error for force measurements and 0.10% for displacement measurements. Figure 2-3: MTS Force Transducer Used on the Experimental Apparatus The displacement is measured by a linear variable differential transformer. The linear vari able transformer measures the absolute position by using the magnetostrictive measuring pr inciple developed by J. Tellermann. This method uses magnetic fields and waveguides to determine the distance

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19 the ultrasonic wave travels. These linea r variable differential transformers are calibrated by the manufacturer and have an accuracy of 0.01% The torsion and bending motions are measured by linear variable differential transformers. The linear vari able transformers record the angular displacement and an approximate error of 1%. The angular displacement is calibrated by positioning the device in series of different known angles and finding the proper gain settings for the particular device. The torque is measured by an elec tronic device (transducer) that is called a torque cell. In a very similar manner to the load cell, the torque cell measures strain, by the use of a W heatstone bridge strain gage. Since the torque cell measures dynamic load, ther e is a constant feed back and error correction process. This will generally c ause the signal to oscillate, but by the use of a controller system, this oscillat ing effect is minimized. The control systems consist of an actuator that ac tively dampens the effect of the oscillation. This method offers great performance, but the process is complex and costly. The torque cells are calibrated by inputting a linear series of known torque to find the proper gain. The accuracy of these torque cells are approximately 1%. The axial force and axial displacem ent are continuously recorded and can be used to interpret the axial stiff ness of the specimen. Axial torsion is measured by fixing one end of the specimen and applyi ng an axial torque on the other end of the specim en. By measuring the torque and the axial rotational

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20 angle, the rotational resistance can be ca lculated. The bending consists of a superior and inferior moment and an equal, but opposite bending moment is applies at both ends. This allows fo r pure bending moment and no shear is present. The bending mom ent and the angle are reco rded throughout the cycle for an accurate measurement of the bending stiffness. This bending moment is used to measure flexion/extension and by turning the specimen 90 degrees, it will measure lateral bending. 2.2.2 – Intradiscal Pressure Measurements The intradiscal pressure measurement s were performed by inserting a cannulated needle into the center of the nucleus propulsus66,67. The nucleus propulsus is uniformly hydro static and gi ves a comparable reading through out the majority of the nucleus. An approxim ation of the center of the nucleus was done by measuring the radiogr aphic images. Once the center of the nucleus was found, a calibrated pressure probe (OrthoAR Model No: 0571521-57, Medical Measurements Inc., Hackens ack, NJ) was inserted through the cannulated needle and the pressure sens or was exposed to the hydrostatic pressure of the nucleus propulsus.

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21 Figure 2-4: Pressure Probe Made by OrthoAR The pressure probe is a Piezoresist ance of semiconductor device, based on a microelectromechanical system (ME MS) Wheatstone bridge strain gage. The strain gage changes the resistance accordingly to the strains in the pressure probe. The output voltage is ch anging as a result of the change in resistance, and the voltages are recorded and interpreted by the MTS software. The pressure probe is calibrated by using nitrogen pressure. A known pressure of nitrogen is released into a sealed container, where the pressure probe is inserted. This procedure is done with small increm ents and a graph of the known pressure can be plotted agai nst the change of resistance in the strain gage in the tip of the pressure probe. The gain on t he pressure probe can be adjusted accordingly and verificati on is done. The pressure probe has a certified sensitivity of 0.496 V/V-kPa at a pressure of 2 MPa with an error of 0.3% at 1 MPa according to Nationa l Bureau of Standards. The pressure sensor was horizontal oriented, as t here is no significant difference in orientation68.

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22 2.2.3 – Human Cadaver Tissue and Fixation The human cadaver tissue is suppli ed by National Disease Research Interchange to be used for research only. This tissue is harvested at the hospital within 12 hours and stored at -80 degrees Celsius. The tissue has passed all the serologic testing before sh ipping, while care must still be taken. The tissue is inspected upon arrival and stored at -80 degrees until use. Tissue is handled professionally, with respect, ca re and disposed in a proper manner. The lumbar spine segments are disa rticulated and potted into 4” x 4” aluminum fixtures by the use of polyester resin and anchors. Figure 2-5 below is a sample image of a FSU potted on bot h sides and securely fastened in the fixture.

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23 Figure 2-5: A FSU Potted on Both Sides An important aspect of potting is not to disturb the disc space. A digital Faxatron (Model No: MX-20, Wheeling, Illi nois) is used to capture an X-Ray to verify that the disc space is not viol ated. Figure 2-6 below show a sample XRay of the potted FSU, and there are no objects in the disc space to alter the biomechanical behavior.

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24 Figure 2-6: X-Ray Image of a Potted Spec imen, with No Anchors in the Disc Space Once the potting is performed, t he FSU are covered with gauss and sprayed with saline solution. When the specimens are not in use, they are stored at +4 degrees Celsius to minimize tissue degradation.

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25 CHAPTER 3 – FINITE ELEM ENT ANALYSIS OF DYNAMIC INSTRUMENTATION DEMONSTRATES STRESS REDUCTION IN ADJACENT LEVEL DISCS 3.1 – Introduction Conventional fusion instrumentation is believed to accelerate the degeneration of adjacent discs due to th e increased stresses caused by motion discontinuity. Fusion instrumentation that employs reduced rod stiffness and increased axial motion, i.e. “dynamic” in strumentation, may partially alleviate this problem, but the effects of this instrumentation on the stresses in the adjacent disc are unknown. The objective of this study was to use a finite element model to calculate and compare the stresses in the adjacent level disc that are induced by conv entional and “dynamic” posterior lumbar fusion instrumentation. The efficacy of dynamic stabilizatio n remains controversial, and is therefore a suitable topic fo r continuing investigation2,70-73. Although several clinical outcome studies de scribe preliminary results obtained from the use of dynamic stabilization3,4,12,23,26, these studies lack a randomized controlled design, a statistically adequate sample size, or long-term follow-up data that

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26 would enable the clinical efficacy of these methods to be properly evaluated10. Early data suggests that the results are at least no worse than those observed from conventional ri gid instrumentation2. Information is also lacking from a scientific perspective because dynamic stabilization methods have largely been developed based on clinical suggestions inste ad of quantitative engineering design efforts, and thus the biomechanics of these methods remain relatively unstudied. Therefore, the purpose of the present study was to: 1) quantify the biomechanics of rigid and one other specific type of dynamic instrumentation when biomechanically tested in a simulate d laboratory model, 2) use these data in a finite element model of a fused and fixed lumbar spine to calculate the flexion-induced peak stresses in the adj acent level discs, and 3) compare these results to determine if a biomechanical basis exists for believing that the reduced stiffness and increased axial motion conferred by dynamic instrumentation can alter the st resses in adjacent level discs. 3.2 – Materials and Methods 3.2.1 – Study Design This laboratory study, performed at Un iversity of Kentucky, used both standardized compressive testing of dynam ic instrumentati on on an established lumbar spinal segment model, as well as a finite element modeling technique

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27 which enabled quantification of the stresse s induced in an esta blished model of lumbar spinal discs74 as a function of instrumentation design (rigid or conventional vs. dynamic). This experi mental design, i.e., stiffness testing followed by finite element analyses, is consistent with prior studies75-76. 3.2.2 – Finite Element Modeling A three dimensional finite element model of the lum bar spine (L1-L5 including discs) was developed by firs t obtaining a validated finite element mesh74 for the L3-L5 spine sectio n. (Figure 3-1) Figure 3-1: Isometric View of the Finite Element Mesh of t he Lumbar Spine and the Semi-Rigid Rod

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28 Isometric view of the finite element mesh adapted from a model created and validated by Smit et al from which a m odel of the lumbar sp ine was used and to which the semi-rigid inst rumentation was applied. The geometry had been developed base d on a series of computed tomography scans of the L4 vertebra of a 44 year male with no pathologies74. The L4 mesh was then replicated to m odel the other lumbar spine vertebrae. Note that this validated model of L1 – L5, previously developed by Smit et al., consists of a series of fi ve dimensionally equivalent L4 vertebrae. This resulting mesh of L1-L5 vertebrae was positione d such that the angle between the inferior surface of L2 and the superior surface of L5 was 40 degrees. This model consisted of a fused (totally ri gid) L5-S1 segment and a L4-L5 segment that was modeled to imitate fixation with ei ther rigid or dynamic instrumentation. The dimensions for the instrumentation us ed in this model were obtained from direct measurement of exem plar instrumentation (Isobar TTL, Scient’X USA Inc, Maitland, FL, USA). The fused segment s between L5-S1 were modeled by specifying the material properties of the L5-S1 disc to be the same as those of cortical bone. Adjacent pairs of vert ebrae were connected by intervertebral discs that were modeled by a nucleus in the center surrounded by 3-4 rings of annulus fibrosus. The nucleus typically occupies about 30-50% of the area of the disc; therefore the fraction used for the nucleus in the model obtained was 43%77.(Figure 3-2)

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29 Figure 3-2: Isometric View of an Intervertebral Disc Isometric view of an intervertebral di sc used in the model. Model shows the annulus fibrosis (outer three layers of mesh elements) and the nucleus pulposis (darker inner mesh elements) The entire finite element model contained 18,128 thr ee-dimensional 8node linear brick elements. Loading of the model was accomp lished by combined flexion or extension plus axial load ing. The axial load of 400 N was applied as a “follower” load thereby allowi ng the axial load to follow the motion of the spine. The model simulated forward flexion at discrete angular increments of 15, 30 and 45 and a backwards extension of 15 by applying relative angular

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30 displacements between L1-L2, L2-L3, and L3-L4 segments, respectively, based upon values equal to those obtained from a normal spine during forward flexion and backward extensions78. The damper of the dynamic instru mentation, located between the instrumented L5 and L4 vertebrae, permitted the upper segment of the fixation rod to have a reduced stiffness and a limi ted amount of axia l micromotion. These two features of this damper me chanism were modeled by employing a softer segment (having variable stiffness values, all of which were less than those of titanium alloy) placed in seri es with an axial motion connector (which allowed axial motion only). (Figure 3-3) Figure 3-3: The Damper Model of the Dynamic Instrumentation

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31 Expanded schematic illustrat ion of the mechanical co mponents of the damper element of the dynamic in strumentation component (shown in Figure 3-1). Two parameters, R and G, were used in this model to quantify the reduced stiffness and the axial micr omotion of the damper mechanism, respectively. Note that the damper is an integral component of the TTL device which is responsible for these two features. The parameter, R=Krigid/Kdynamic (Equation 3-1) was used to quantify the reduced stiffne ss of the damper. (This dimensionless stiffness ratio quantified how much stiffer the rigid instrumentation was relative to the dynamic instrumentation. The Krigid term of equation [3 -1] represents the elastic stiffness of the rigid instrumentation, while the Kdynamic term represents the elastic stiffness of the dynamic instrumentation. Values for Krigid and Kdynamic were obtained from the material properties of titanium alloy and the variable reduced stiffness material comprising the softer segment. The G parameter was defined as the maximum axial mo tion allowed by the damper mechanism. To study the effects of axial motion on the resulting pressures inside the disc, five discrete maximum allowable axial di splacements (0 to 0.8 mm in 0.2 mm increments) were used in the model. Changes to both R and G permit the changes in pressure within the disc to be quantified as a result of varying

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32 instrumentation elastic sti ffness and axial micromotion. Before reaching the maximum axial motion, the damper also functioned as an axial spring with a stiffness of 175 KN/m (calculated from the product manual accompanying the Isobar TTL instrumentation). The inferior portion of the sacrum was modeled as a block and the lower surface of the block was considered fixed. A static compressive (“follower”) load of 400 N was axially applied to the super ior surface of the L1 vertebra and this load was maintained perpendicular to the superior surface of the L1 segment throughout axial load induced deformati on. All components in the assembly shown (Figure 3-1) were m odeled by using linear elastic materials. The material properties assigned to these components74,78 in the finite element model are shown (Table 3-1).

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33 Material properties obtained from sources lis ted and used in the finite element model. Units of Young’s Modulus are gigaPascals ; Poisson’s ratio is dimensionless. Table 3-1: Material Properties Material Young’s Modulus, GPa Poisson's Ratio Cortical Bone 12 0.3 Cancellous Bone 3 0.2 Fibrous 0.03 0.45 Nucleus 0.001 0.49 Steel 190 0.3 Titanium 116 0.33 Peak stress values in the disc, as well as the areas of the 2D projections of the 3D volumes of disc tissue expos ed to > 80% of peak stress volumes, were calculated for varying values of R and G by using commercially available finite element analysis software (ABAQ US/Standard, ABAQUS Inc., Pawtucket, RI). 3.3 – Results The experimental testing performed at University of Kentucky, showed mean value of the elastic sti ffness (axial load divided by actuator displacement)

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34 of the rigid instrumentation was 21, 960 8,034 N/mm, while the mean elastic stiffness of the dynamic in strumentation was less than one-third this value (p = 0.01), i.e., 6,169 1,298 N/mm. Using these data, the resulting R and G values for the rigid instrumentation were 1 (“cont rol” stiffness value) and 0 (meaning no axial micromotion – obtained from the m anufacturer), respectively, whereas the R and G values for the dynamic inst rumentation were 3.6 and 0.4 mm, respectively. Other values for R and G were also used in the model calculations to compute the effect of al ternative values for elastic stiffness and axial micromotion. (Tables 3-2 & 3-3). Table entries (italicized values) are t he peak stresses (units of gigaPascals) induced in the L3 – L4 disc superior to the dynamic instrumentation component as calculated from the finite element model as a function of: 1) flexion (+ value)/extension (value) angle (extre me left column), 2) dimensionless stiffness ratio R (second column from le ft), and 3) axial motion parameter G (column headings, units of mm).

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35 Table 3-2: Peak Calculated Stress (MPa) in the L3-L4 Disc Angle, degrees G(mm) R (ratio) 0.0 0.2 0.4 0.6 0.8 1 7.70967.53647.37157.20677.0422 3.6 7.63767.45787.28667.11576.9453 10 7.56447.38677.21747.04856.8800 45 44 7.3416 1 5.04834.87674.71334.55034.3882 3.6 4.99994.82114.65114.48144.3123 10 4.95244.77544.60694.43884.2712 30 44 4.8044 1 2.47762.30782.14722.08592.0859 3.6 2.45322.27592.10771.94041.9101 10 2.43032.25422.08701.92091.8515 15 44 2.3569 1 4.24204.04283.85083.85083.8508 3.6 4.23484.02513.80663.74313.7431 10 4.22154.01263.79473.70933.7093 -15 44 4.2085

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36 Table entries (italicized values) are t he peak stresses (units of gigaPascals) induced in the L4 – L5 disc spanned by the dynamic instrumentation component as calculated from the finite element model as a function of: 1) flexion (+ value)/extension (value) angle (extre me left column), 2) dimensionless stiffness ratio R (second column from le ft), and 3) axial motion parameter G (column headings, units of mm).

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37 Table 3-3: Peak Calculated Stress (MPa) in the L4 – L5 Disc Angle, degrees G(mm) R (ratio) 0.0 0.2 0.4 0.6 0.8 1 2.59722.73772.87133.00433.1369 3.6 2.71412.87653.03173.18723.3429 10 2.76012.92423.08123.23863.3964 45 44 2.9633 1 1.72211.85791.98732.11652.2448 3.6 1.80101.95912.11052.26242.4147 10 1.83091.99122.14482.29902.4537 30 44 1.9626 1 0.85220.98441.11061.15881.1588 3.6 0.89211.04701.19551.34431.3713 10 0.90671.06421.21531.36671.4300 15 44 0.9717 1 0.43190.82081.20291.20291.2029 3.6 0.48030.82141.18281.28821.2882 10 0.53680.86161.20551.34101.3410 -15 44 0.8796

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38 Calculated values are shown for the peak von Mises stresses induced in the L3-L4 disc for the 400 N axial load applied with each of the two instrumentation designs at each of the four flexi on/extension positions (15 30 and 45 flexion and 15 extension) and for varying values of R and G (Table 32). The data showed that the use of dynamic instrumentation was associated with a 5.5% reduction in peak stress for the L3-L4 disc and a 16.7% increase in peak stress for the L4-L5 disc compared to the rigid instrumentation at 45 of flexion. It was also observed that, by maintaining the G value at 0.0 (allowing no axial micromotion) but a llowing the stiffness of t he proximal segment of the dynamic instrumentation to decrease, caused a reduction in the peak stress in the L3-L4 disc by approximat ely 1-2%. Alternatively, maintaining the same stiffness of this proximal segment as is found in the rigid case, i.e., maintaining the R-value at 1, but increasing the ax ial micromotion, i.e., increasing the Gvalue, results in reducing the peak stre ss in the L3-L4 disc by approximately 89%. Thus, increasing the G-parameter (specifically, increasing axial micromotion) was shown to be more effect ive in reducing the peak stress in the L3-L4 disc than was decreasing the R-param eter (specifically, decreasing the rod stiffness). The effects not ed above were also observed at 15 and 30 of flexion as well as at 15 of extension, but less pr ominently (Figure 3-4).

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39 0.0 2.0 4.0 6.0 8.0 -150153045 Angle [Degrees]Stress [MPa] R=1 G=0.0 R=10 G=0.0 R=1 G=0.8 R=10 G=0.8 R=3.6 G=0.4 Figure 3-4: Comparison of Stress in L3-L4 with Differ ent Variables for R and G Representative values for the calculat ed stresses induced in the L3-L4 disc as function of one of four different flex ion/extension angles (abscissa) and for varying indicated (color-coded values of re lative stiffness (R-parameter values) and axial motion (G-par ameter values). Note that the minimal value for peak stress in the L3-L4 disc in the 45 flexion case was achieved for R and G va lues of 10 and 0.8 mm, respectively. To graphically visualize the stress r eduction caused by reduced stiffness and increased axial micromotion associat ed with dynamic instrumentation, the

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40 stress levels in the L3-L4 disc located above the rigid instrumentation were contrasted with those of the same disc located above dynamic instrumentation which have the “optimal” dynamic paramet ers (R=10, G=0.8 mm) noted. (Figure 3-5). Figure 3-5: Stress Distr ibution of L3-L4 at 45 Flexion. Anterior and posterior views of calculat ed stress distribution in the L3-L4 disc at a 45 flexion angle for discs associated with rigid instrumentation (right side) and “dynamic” (left side) inst rumentation (1/10 stiffness, i.e., R = 0.1) for 0.8 mm axial motion.

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41 A representation of the peak stresses for the extreme motions are shown below in figures 3-5, 3-6 and 3-7. These cases are all achieved with R and G values of 10 and 0.8 mm, respectively The remaining representations are shown in Appendix A. Figure 3-6: Stress Distri bution of L4-L5 Disk at 45 Flexion

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42 Figure 3-7: Stress Distri bution of L3-L4 Disk at 15 Extension Figure 3-8: Stress Distri bution of L4-L5 Disk at 15 Extension

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43 Note from these stress contours that the volume of L3-L4 disc tissue located above the dynamic instrumentation that was exposed to stresses of 6.17 MPa or greater was 47% less than the volume of L3-L4 disc tissue located above the rigid instrumentat ion that was exposed to stresses of 6.17 MPa or greater. The stress value 6.17 MPa wa s 80% of the peak stress in the L3-L4 disc located above the rigid instru mentation when calculated at 45 of flexion. 3.4 – Discussion Reduced stiffness and increased ax ial motion of dynamic posterior lumbar spinal fixation in strumentation resulted in both lower peak stresses and smaller volumes of tissue exposed to high amplitude stresses in simulated adjacent level discs. While the stre ss reduction effect was small (~10% cumulatively for a single forward flexion) this is important because this benefit will be repeated over many loading cycle s (1 – 10 million/year). Classic material fatigue studies show that sm all reductions in peak load amplitude produce substantial increases in mate rial longevity, and this finding is substantiated by analogous studies conducted in ca daveric lumbar vertebrae70. Although the reduced stiffness and increas ed axial motion also increased the peak stress in the L4-L5 disc by up to 28%, this load increase needs to be considered in light of the peak stress am plitude in the L4-L5 disc which was 2 to 3 times less than that in the adjacent L3-L4 disc. The reduced stiffness and increased axial motion of dynamic instrum entation also allows some rotation of

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44 the L4 vertebra with respect to L5. This rotation is not permitted by rigid instrumentation designs. To achieve t he same overall level of flexion when both types of devices are used, the L3-L4 disc experiences smaller rotation demands when this type of dynamic inst rumentation is used. This reduced rotation then leads to a corresponding stress reduction in this disc. There are only a few publ ished studies that ar e reasonably comparable to the present study. Three of these used cadaveric spinal segments that were mechanically tested In Vitro in conjunction with another type (Dynesis) of dynamic instrumentation. A ll showed that this type of dynamic instrumentation can favorably alter load transmission an d movement yet can also provide adequate stability. None of these studies quantified the changes in pressure within the disc that remain at the basis of adjacent segment degeneration79-81. Another study used computational models to compare materials selection, but not device design. This study also focu sed on overall mechanical stability and load transmission rather than pressures within the disc76. A fifth study used a finite element method to compute pre ssures within adjacent discs, but did not study the effects of dy namic instrumentation82. The one study most closely similar to that done presently83 also used a finite elem ent model of the lumbar discs, but concluded that dynamic instrumentation does not alter pressures within the discs. The reason for this disparit y in findings may be reflective of the mechanical performance differences betwe en the Isobar system (present study) and the Dynesis system (Z ander et al. study).

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45 It is important to note that dynamic instrumentation also permits axial distraction, which in turn changes the c enter of rotation. Consider the two instrumented segments, A and B. (Figure 3-9a). Figure 3-9: Two Approaches to Generate 2 of Rotation Saggital view of a schematic illustrati on of the damper me chanism that shows two approaches regarding how rotation can be obtained for instrumentation that allows “dynamic” motion (a): (b) pure bending only with no axial motion – notice the location of the Center of Rota tion (COR), or (c) bending with axial compression/extension – note the altered (more physiological) location of the Center of Rotation.

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46 If axial distraction (i.e., increase of the inter pedicular distance) is permitted, then the center of rotation sh ifts and falls within the L4-L5 disc and not on the posterior side of the posterio r lateral ligament. (Figure 3-9a) When no axial motion is allowed, the center of rotation is located at the level of the damper (which is acting as a type of “hinge”, Figure 3-9b). This shift in the center of rotation reduces the effectiv e moment arm for L4 rotation, which in turn causes a reduced moment and lowe r stresses in the L3-L4 disc since L1 will have the same displacement in both cases. This allows a more physiological motion than can otherwise be obtained with instru mentation that does not allow distraction. As noted in the results, decreas ing the R-parameter alone has the effect of reducing the stif fness of the material resisting the rotation, while decreasing the G-parameter alone has the effect of adjusting the axis of rotation for L4. The numerical results obtained in the present study demonstrate that within t he range of values for stiffness and axial motion (parameters R and G) used herein, moving the center of rotation anteriorly is more effective in reducing stress amplitud es in the adjacent level disc than is reducing the elastic stiffness of the inst rumentation. Alt hough the particular type of dynamic instrumentation studi ed has both features, i.e., anterior translation of the center of rotation and reduced elasti c stiffness, the former feature is considered to be c linically more important. Increased load demands at the adjacent level disc accompanying fusion has been associated with a ccelerated degeneration of that disc in animal

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47 models14 and is also associated with adj acent level disc problems in humans5. Rigid fixation has been a ssociated with increased pre ssures within the disc which are as much as 73% greater in adjacent cervical discs84. Others suggest that not just the amplitude, but the altered pattern of loading may also have a role in this process of adjacent level disc disease12. Given the current findings, some6 argue that there remains less than adequate proof of the difference between rigid and dynamic stabilization, while others10 claim that the lack of differences provides support for the c oncept. This assumption will be best evaluated from long-term follow-up data obtained from adequately powered randomized controlled clinical trials which study dynamic versus conventional instrumentation. It is important to reme mber that “dynamic” is an appellation for a generic class of load-sharing fixation instrumentation; due to differences in designs and materials of such devices, varying levels of stiffness and motion will result. Outcomes of computational or in vivo studies employing dynamic devices are likely to be different due to their biomechanical heterogeneity. Only the resulting clinical studies will enabl e those with superior performance to be identified. Limitations of the present study in clude the less than ideal anatomical model used. The lumbar vertebrae employ ed in this finite element model were not size-adjusted for the various vertebr al levels, but were all identical and based upon the dimensions of an L4 vertebr al body. However, this model was developed and validated previously74 and thus is not considered a major

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48 limitation because the focus of the study was the comparative, not absolute, differences in pressures within the disc. Also, as loading de forms the in vivo spine, the load likely does not remain perpendicular: however for the model used in this study, it was assumed to remain perpendicular. This assumption introduces a limitation to the absolute ac curacy of the inter nal stress results reported, but the m agnitude of this error is considered small and the comparative (between rigid and dynamic stabilization instrume ntation) effects are believed negligible. The model used also did not include the effects of degenerative disc material properties, strain dependent disc swelling pressures/material permeabilit y, or nonlinear elastic ma terial behavior. While these may be important from an absol ute perspective to understand the behavior of individual discs85, their relative contribution in the present study involving comparison of two different fixa tion types is considered insignificant. Assuming that adjacent level disc det erioration is partially caused by repetitive high amplitude l oading and non-physiologic axes of rotation, reduced elastic bending stiffness and increased axial motion attri butable to an anteriorly shifted axis of rotation in posterior instrumentation will more favorably distribute the motion demands of the lumbar spine. This finding supports emerging clinical evidence that such mechanical al terations to posterior spinal fixation devices have a beneficial effect on disc tissue and thereby delays the onset, reduces the severity of, or prevents entirely, the phenomenon of accelerated adjacent level disc deterioration.

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49 In conclusion, reducing the stif fness, increasing the axial motion, and anteriorly translating the axis of ro tation of posterior spinal fixation instrumentation may be part of the solution to the problem of adjacent level disc degeneration.

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50 CHAPTER 4 – BIOMECHANICAL TESTING OF FACET FUSION TECHNIQUE 4.1 Introduction Traditionally a pedicle screw system has been used for fixation of the lumbar spine and this involves major su rgery and recovery time. Facet fixation is a technique that has been used for st abilization of the lu mbar spine and the proposed facet fixation technique c an be performed as a percutaneous procedure. The proposed technique stabiliz es the facet joints in a similar manner as the translaminar facet stabilization. Minimal invasive surgery has had an increase in popul arity the last couple of years, instead of a traditi onally open back surgery. For minimal invasive surgery, a facet fixation wi ll be more feasible than a pedicle screw system86. The minimal invasive pedicle screw method is very time consuming and technically demanding. The procedure discussed in this paper is a percutaneous facet fixation where an allograft is used for fixation. This method will us e human bone for the fixation and this will allow the facets to fuse together and prov ide fixation of the facet joints. The stability of the functional spinal unit (FSU) wi ll be restricted by no motion of the facet joint, which will lead to fusion of the facet joint. While all

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51 other available procedures for FSU stabi lization use pedicle or transfacet screw fixation, this procedure uses an allogra ft bone dowel that is pre-formed to a specific shape22,87,88. 4.2 Materials and Methods Three human cadaveric lumbar spine segments were tested, using a nondestructive testing method. The lumbar spines were disarticulated at L1-L2 and L3-L4. The segments were tested in axial rotation, combined flexion/extension and latera l bending. The specimens were first tested intact as control. Next, the same spine segments were implanted with the facet fusion allograft by a board certified or thopedic surgeon according to the manufacturer’s specification. Axial ro tation, flexion/ext ension and lateral bending were performed with a constant l oad of 100 N and a moment of 6 Nm was applied in 6 cycles. The first 5 cycles were used to precondition the specimen and the data for the 6th cycle was interpreted. 4.2.1 Spine Preparations A total of three adult human cadaver lumbar spine segments were harvested. The donor’s av erage age was 65.5 1.8 (range 61-73) years and the donor group consisted of 2 males and 1 female. The medical history of all the donors was reviewed, where donors wit h any disease that will affect the

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52 spine biomechanics or trauma were excluded. These thr ee lumbar spine segments were investigated visually, as well as the specimens went through a radio graphically screening to excl ude any major abnormalities such as osteolycis, fractures or damage to the vert ebral bodies or the intervertebral disc. The disarticulation was chosen based on the quality of the particular articulations found in the radio graphicall y screening. The lumbar spines were disarticulated to create a variation of Functional Spinal Units (FSU) from different levels to be used in this study This method allows for the most FSU’s to be extracted from each lumbar spi ne, but certain spines produced more FSU’s than others. En Block specimens were stored at -80 degree Celsius and thawed to +4 degrees Celsius in a refrigerator. The specimens where covered by gauss, sprayed with saline solution and left at room temperature bef ore testing. To securely attach the specimens to the test fixture, the specimens were reinforced by inserting metal screws in the ve rtebral endplate and potted in a two part polyepoxide based resin. All extraneous musculature was removed from each spine, keeping all the ligament s and posterior elements intact.

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53 4.2.2 Implant and Fixation Techniques 4.2.2.1 Specimen Instrumentation Each FSU was instrumented with face t fusion allograft as shown in Figures 4-1 and 4-2. Figure 4-1: Posterior View of Placement of Facet Fusion Allograft in Facet Joints

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54 Figure 4-2: Superior View of Placement of Facet Fu sion Allograft in Facet Joints. 4.2.2.2 Facet Fusi on Allograft Insertion To implant the facet fusion allogra ft the facet joint needs to be accessed either by direct visualization during open surgery or indirectly by fluoroscopy during percutaneous surgery. Once the facet joint is identified, the posterior facet joint capsule is removed, as well as any significant osteophytes. The facet joints will then be cleared of any remainin g cartilage or debris, as well as this, clinically, will help the joint to fuse. T he drill guide is then centered between the inferior and superior facets, where the drill guide stabilizing teeth are placed

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55 superior and inferior in the facet joint ope ning. This will prevent the drill guide to move around on the facets, but still allow for changing the angle medially and laterally to drill in the plane of the facet joint. Once the drill guide is in position, the tapered compaction drill bit can be used to drill facet implantation site. This will lead to a removal of less than 50% of the superior facet and less than 50% of the inferior facet as shown in Figure 4-3. 0 10 20 30 40 50 60 T1T2T3T4T5T6T7T8T9T10T11T12L1L2L3L4L5 Vertebrae% Reduction Left Superior Right Superior Left Inferior Right Inferior Figure 4-3: Percentage Reduction of Face t Joint Due to the Implant (Panjabi) The drill bit has a drill stop set at 10 mm and it will allow the drill bit to drill slightly deeper (2 mm) than the height of the implant (8 mm), but not so deep it might cause any potential damage. The drill bit and drill guide is now

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56 removed, and this void will now be filled with the taper ed facet fusion allograft. The facet fusion allograft implant is inserted with the plac ement and impaction tool, in the same direction as the site was drilled. This implant is now impacted into place and will be counter sunk 1-2 mm into the compaction-drilled tunnel. This procedure will be repeat ed for the other facet joint at the particular level that is being treated. 4.2.3 Study Protocol The segments were tested in axial ro tation, combined flexion/extension and combined left/right lateral bending. The specimens are tested intact (control) before they wher e treated with the facet fusion allograft implant. Axial rotation, flexion/extensi on and lateral bending were performed with a constant axial load of 100 N and a mo ment of 6 Nm was applied in 6 cycles. The loading rate used for all the different case s is 0.125 Hz for one part of the cycle89. 4.2.4 Statistical Analysis The collected data was evaluated by us ing one-way analysis of variance (ANOVA) followed by a Tuke y-Kramer comparison for ev aluating the significant difference of the stiffness between th e intact and treated specimen. All statistical tests were performed on SAS (re lease 9.1, SAS Institute Inc., Cary,

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57 NC), with a significance defined at a 95% confidence interval. The values are given as the mean standard deviation. 4.3 Results The stiffness and range of motion (ROM) of intact and treated specimens, during flexion/extension, lateral bending and axial rotation are shown in Figures 4-1 and 42. Tables 4-1, 4-2 and 4-3 summarize the results of stiffness, ROM and percentage change due to treatment. Table 4-1: Range of Motion of t he Intact and Treated Segment Intact [Degree]Treated [Degree] Flexion 4.281.101.590.52 Extension 2.180.581.030.04 Bending 6.050.563.121.39 Torsion 2.511.411.820.64

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58 Table 4-2: Stiffness of t he Intact and Treated Segment Intact [Nm/Degree]Treated [Nm/Degree] Flexion 0.99 0.252.450.78 Extension 2.00 0.744.110.22 Bending 1.51 0.163.561.80 Torsion 3.64 1.764.211.29 Table 4-3: Percentage Change of Range of Motion and Stiffness Change of ROMChang e of Stiffness Flexion 49.62% 10.73%126.76%35.71% Extension 40.85% 21.02%119.88%4.16% Bending 54.44% 13.84%148.58%48.78% Torsion 26.32% 2.09%26.80%11.43% In comparison to the intact specimen, the facet fusion allograft shows a significantly higher (P < 0.05) stiffness in flexion and extension (Table 4-1). There is a noticeable change of stiffness in lateral be nding and axial rotation, but this change is not statistical signifi cant. The stiffness increased 127% in

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59 flexion (1.1 Nm/Degree to 2.5 Nm/D egree) and 120% in extension (1.8 Nm/Degree to 4.0 Nm/Degree) following bilateral implant ation of the allograft. For lateral bending, the stiffness increased by 149% (1.6 Nm/Degree to 4.0 Nm/Degree) and for axial torsion there was a 27% change of stiffness (3.0 Nm/Degree to 3.8 Nm/Degree). These values are interpreted from full range of motion grafts. The sample graph in figure 4-4 below, show the typical flexion-extension results. -4 -2 0 2 4 -8-6-4-202468 Torque [Nm]Angle [Degree] Intact Facet Fusion Allograft Figure 4-4: Typical Flexion-Extension Results, Showing Comparison Between Intact and Treated Specimen. The sample graphs for lateral bending and axial rotation are shown in Appendix B.

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60 4.4 Discussion Fixation of the facet joint has been performed by inserting metal screws perpendicular through the facet joint. This has shown to give a good fixation, but it is at high risk of causing pe rmanent damage. It is also a technically demanding procedure28-29,35-46. The proposed technique is similar to the Lumbar Facet Interference screw, but this impl ant is made from allograft and has a press fit27,86,. A potential problem with allograft im plant is the biological process of absorption of the bone. When the bone is absorbed the implant reduces in size and there is a chance of the implant to become loose90-93. This method is also very similar to the procedure propos ed by Stein et al., while the proposed procedure has a pre shaped im plant and the proper inst ruments for insertion86. The purpose of this study was to find t he biomechanical stab ility of the facet fusion allograft and compare to publishe d data of various facet fixation techniques27. There are some limitations in this study to consider. As any in vitro experimental testing, t he study will be limited by th e lack of muscular lumbar spine stability. This will be the case for a ll the groups included in this study, and the change as a percentage will be comp ared. Since each FSU is used for control and treatment, each FSU are te sted twice. This might change the stiffness of the last treatm ent from fatigue, but accordi ng to Panjabi there is little or no effect for the short duration the specimen is tested94.

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61 Lumbar facet fixation devices have been discussed in several biomechanical in vitro studies27. These fixation methods provide good fixation, but they are technical dem anding and the biom echanical properties are usually not ideal in axial rotation. This proposed method inserts the im plant in the plane of the facet, perpendicular to the traditiona l method. For this reason, the implant is compressed between the inferior and s uperior facet and better axial rotation results are seen. The comparative inta ct and treated results for stiffness and range of motion are shown in figure 4-5 and 4-6 below. 0 1 2 3 4 5 6 FlexionExtensionLateral bendingTorsionStiffness [Nm/Deg] Intact Facet Fusion Allograft Figure 4-5: Stiffness Results for the Intact and Treated Specimens

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62 0 1 2 3 4 5 6 7 FlexionExtensionLateral bendingTorsionRange of Motion [Deg] Intact Facet Fusion Allograft Figure 4-6: Stiffness Results for the Intact and Treated Specimens The comparisons in difference bet ween specimens are shown in Appendix B. In the comparison shown in Figure 4-7, the facet fusion allograft is shown as a standalone procedure, wh ile the other methods ar e presented with a cage. This might cause the facet fusion allograft to show a higher gain of stiffness in axial rotation.

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63 -100% 0% 100% 200% 300% 400% 500% FlexionExtensionLateral bendingTorsionChange of Stiffness [%] Cage + Translaminar Screw (Kandziora) Cage + Lumbar Facet Interference Screw (Kandziora) Cage + Pedicle Screw (Kandziora) Facet Fusion Allograft (Vestgaarden) Figure 4-7: Comparison of Percent C hange of Stiffness to Published Data The facet fusion allograft pres ented in this study demonstrates comparable demobilization of flexion and extension to traditional methods. The percentage change of stiff ness in lateral bending demonstrate a great percentage change, but it is not statistical signi ficant. One out of three specimens only had a minor change in stiff ness and therefore, the statistical significance is not present. The stiffness of this fixation me thod is lower than the pedicle screw systems. This can be explained by the absence of metal through the pedicles,

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64 which allows for deflection of the pedicle. Deflection of the pedicle also allows for some deflection of the vertebral body and higher stress level in the disc are occurring. By increased stress, the disc will remain in better condition and reduce the chance of adjac ent degenerative disc desease90-93. The pre-shaped allograft dowel is effe ctive in restricting facet joint movement. This method provides stabilizat ion and fixation for minor instabilities, which can allow the joint to fuse th rough integration with the allograft. The allograft also gives a smooth change of st iffness in the spine and reduces the chance of adjacent degenerative disc disease. This study demonstrates that the biomechanical properties of the facet fusi on allograft are simila r to existing facet fixation methods. Results of this pilot st udy shows a potential for this technique and additional biomechanical st udies with a greater sample size is desiered.

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65 CHAPTER 5 A COMPARISON BETWEEN IN VIVO AND IN VITRO INTRADISCAL PRESSURES 5.1 Introduction There is no data that explains the actual forces acting on the spine during flexion, extension, lateral bending or axial ro tation. There are published articles that give intradiscal pressu re measurements for these motions, but there are no correlation performed against In Vitro testing results. All these issues will be addressed in this dissertat ion and it will be presented in sections. Many models have been made to estima te loads during lifting activity. Some are simplified, while others hav e used EMG measurements to find the muscle forces with or without the co mbination of intradiscal pressure measurements95-109. Wilke et al made continuous dynamic In Vivo measurements for flexion, extensi on, lateral bending and axial rotation110. This is the only published study with this data. The motions and pressure curves described in this study are very simila r to experimental cadaver testing. Finite Element Method has been used to evaluate spinal implants, but these models do not necessarily give a direct correlation to physiological loads acting on the spine111-114. By using known forces and moments, their respective

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66 displacements and the use of intradiscal pressure, these models can be very accurate. There are models that take t hese aspects into considerations, but they are not validated by t he use of physiological data115-118. The prediction of muscle forces and spinal loadings are dependent on the trunk models and the posture119-120. The effect of the abdominal pressure is controversially, but the In Vivo intradiscal pressure is measured with all the physiological loads present121. The abdominal pressures are usually not simulated during In Vitro testing or in analytical models. There have been several pap ers published in the 60s and 70s discussing intradiscal pressures122-127. These pressures are absolute values, rather than complete data sets published more rec ently. Pressure transducers are also an important aspect of meas uring intradiscal pressure s and there has been made some major advantages with the technology used in more recent publications66. The purpose of this paper is to provide a database with correlation to previously published In Vivo intradiscal pressure curves to the current In Vitro pressure curves. This data will enable a proper adjustment and validation of a computer model and to give physiol ogical meaning to loading data used on cadavers for In Vitro testing of medical devices. 5.2 Materials and Methods A study of the intradiscal pressure during motion of an intact specimen will be performed to compare to In Vivo results as descri bed in literature.

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67 Human cadaver lumbar spines were disart iculated to get functional spinal units (FSU). The FSU’s were tested in combined axial compression and flexion/extension, combined axial compression and lateral bending and combined axial compression and axial ro tation using a nondestructive testing method. 5.2.1 Spine Preparations A total of 6 adult human cadaver lumbar spine segments were harvested. The donor’s av erage age was 50.5 1.8 (range 45-65) years and the donor group consisted of 5 males and 1 female. The medical history of all the donors was reviewed, where donors wit h any disease that will affect the spine biomechanics or trauma were exclud ed. These six lumbar spines were investigated visually, as well as the s pecimens went through a radio graphically screening to exclude any major abnormalities such as osteolycis, fractures or damage to the vertebral bodies or the intervertebral disc. All the FSU’s were disarticulated to give L4-L5 specimens containing the L4 and L5 vertebral bodies, posterior elements, ligament s and the intervertebral disc. En Block specimens were stored at -20 degree Celsius and thawed to +4 degrees Celsius in a refrigerator128. The specimens where covered by gauss, sprayed with saline solution and left at ro om temperature prior to testing. To securely attach the specimens to the test fixture, the specimens were reinforced

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68 by inserting metal screws in the ve rtebral endplate and potted in a two part polyepoxide based resin. All extraneous musculature was removed from each spine, keeping all the ligaments and pos terior elements intact. Plain film radiographs (Faxatron Model Ni: MX-20, Wheeling, IL) was used to verify that none of the reinforcing metal screws interf ered with the intervertebral disc. 5.2.2 Test Setup and Biomechanical Testing 5.2.2.1 Test Setup A nondestructive spine biomechanics test setup was used to find the biomechanical properties. This particu lar setup is based on an axial servohydraulic materials testing system (MT S Systems Inc., 858 Bionix II, Eden Prairie, MN) and is modified to allow be nding as well as ax ial rotation. Axial compression is integrated in the MTS 858 Bionix II and the l oad is measured by the use of a load cell. T he load cell has an accuracy of 0.13% error for force measurements and 0.10% for displacement measurements. The linear variable differential transformers used to measur e torsion have an approximate error of 1%. The axial force and axial displace ment are continuously recorded and can be used to interpret the axial stiff ness of the specimen. Axial torsion is measured by fixing one end of the specimen and applyi ng an axial torque on the other end of the specim en. By measuring the torque and the axial rotational

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69 angle, the rotational resistance can be ca lculated. The bending consists of a superior and inferior moment and an equ al, but opposite bending moment is applies at both ends. This allows fo r pure bending moment and no shear is present. The bending mom ent and the angle are reco rded throughout the cycle for an accurate measurement of the bending stiffness. This bending moment is used to measure flexion/extension and by turning the specimen 90 degrees, it will measure lateral bending. 5.2.3 Study Protocol The segments were tested in axial ro tation, combined flexion/extension and combined left/right lateral bending und er constant axial compression. Axial rotation, flexion/extension and lateral bending were performed with a constant load that repr esents the load of a person standing relaxed. From published data, the init ial intradiscal pressure was set to 0.5 MPa and resulted in a constant axial compressive load of 500-700 N depending on the specimen63-65,110,129-130. A moment of 6 Nm was applied in 6 cycles. The loading rate used for all the different cases is 0.125 Hz for one part of the cycle. With the pressure probe secured in the center of nucleus, the FSU was tested in all the motions and measuremen ts were made. No losses of spinal fluids were noted during the pressure te sting, while some of the specimens appeared to have severely dehydrated nucleus.

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70 5.3 Results There have been previous st udies that have reported In Vivo intradiscal pressures for daily activities. One study has reported series of data points during flexion-extension, lateral bending and axial rotation. These motions have been repeated In Vitro For this study, the comparable In Vivo intradiscal pressures are relaxed standing 0.43 0.50 MPa, standing flexed forward 1.08 MPa, standing extended backwards 0.6 MPa, lateral bending 0. 59 MPa (decreasing to 0.38 MPa) and axial rotation 0.6 0.7 MPa110. Figure 5-1: Torque vs. Angle Data for the Extension and Flex ion Experimental Test

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71 Figure 5-2: Pressure vs. Angle Data fo r the Extension and Fl exion Experimental Test In Vitro intradiscal results for the same motions are 0.68 MPa for flexion, 0.50 MPa in extension, 0.57 MPa during lateral bending (decreasing to 0.26 0.36 MPa) and axial rota tion 0.51 .53 MPa.

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72 Figure 5-3: Torque vs. Angl e Data for the Lateral B ending Experimental Test Figure 5-4: Pressure vs. Angle Data for the Lateral Bending Experimental Test

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73 In Vitro results corresponds to the following moments and angular displacements; flexion 6.5 Nm and 5. 9 degrees, extension 6.5 Nm and 3.1 degrees, lateral bending 1.8 Nm and 0.9 degree (decreas ed pressure at 6.3 Nm and 1.6 degrees) and axial rotation 3. 6 5.7 Nm moment and 2.5 degrees angular displacement. Figure 5-5: Torque vs. Angl e Data for the Axial Ro tation Experimental Test

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74 Figure 5-6: Pressure vs. Angle Data fo r the Axial Rotation Experimental Test A comparison between the In Vivo and In Vitro curves are shown in Figures 57, 5-8 and 5-9.

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75 Figure 5-7: Extension Fl exion Intradiscal Pressure In Vitro of Selected L4-L5 Segments with Respect to the To tal Motion in a Single Level Figure 5-8: Lateral Bending Intradiscal Pressure In Vitro of Selected L4-L5 Segments with Respect to the To tal Motion in a Single Level

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76 Figure 5-9: Axial Rotation Intradiscal Pressure In Vitro of Selected L4-L5 Segments with Respect to the To tal Motion in a Single Level. The line with square marker s is an import from the In Vivo publication, while the measured In Vitro results are represented by circular marks. 5.4 Discussion The purpose of this study is to create a database of the correlation between In Vivo and In Vitro data. The In Vivo published pressure measurements have been used, where the In Vitro pressure measurements from the current study show a close relation to the In Vivo pressures110. This

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77 database can be used for physiological re levance to experimental testing and for validation of mathematical models. As far as the authors are aware, there has never been done a study to find these correlations. There are studies that show In Vivo intradiscal pressures, but the referred study is the only paper with dynamic In Vivo intradiscal pressure results published. Duri ng the testing of the cadaver spines, there were several specimens that coul d not reproduce the dynamic intradiscal pressures. These specimens were only used for the absolute values to achieve a reasonable sample size. In present papers, the absolute values of the pressure in the center of L4-L5 are described as well. The values pr esented in this study are roughly the same to the values presented by Wilke et al in exception of flexion and extension110. In flexion and extension, it is clear that the moments applied during the cadaver testing are not suffici ent to achieve the pressures presented by Wilke et. al.110 The flexion and extension re sults demonstrate a correlation, but the applied moments during In Vitro testing are not great enough to simulate the complete cycle of forward flex ion and backwards extension. During In Vitro flexion and extension testing, the applied moment does not have to work against abdominal forces and pressures. This is not the case for In Vivo and it is clear that this will create a higher mo ment to achieve the same pressure. This shows us that during biomechanical eval uation of medical de vices, the applied

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78 moment in flexion and extension needs to be greater than for lateral bending and axial rotation. The results from the In Vitro testing give an accurate representation of the In Vivo intradiscal pressures during latera l bending. A symmetrical curve, roughly, is shown in lateral bending, wi th the same characteristics as seen during In Vivo measurements. It is seen both In Vivo and In Vitro that the pressure raises to a maximum, before the pressure decreases at the highest measured angular deflection. Wilke et al describes the possibility of muscles trying to stabilize the spine actively, before the muscles relaxed and stabilized the spine passively110. Since the same phenomenon is occurring In Vitro this can be dismissed. A likely possibility is that the superior facet impacts the inferior facet on one side and acts like a pi vot point. This will give increased disc height on one side of the disc and the c hance of the nucleus to relieve pressure. During axial rotation, the slope of the pressure is fairly stable for the In Vitro results. During In Vivo testing there is an increas e of pressure at the end of the cycle. This can be explained by t he axis of rotation being fixed during In Vitro testing and no translation allowed. This axis of rotation can have some translation In Vivo were shear forces will be acting on the disc131. This could be the reason for the increased pressure observed by Wilke et al110.

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79 For all the specimens tested, the pressures at the maximum angular displacements were collected. These are similar to the pressures reported by Wilke et al and these values are now verified by a higher sample size110. The published intradiscal pressure curves from the In Vivo measurements have a close correlation to the In Vitro measurements in the current study. This is a good guide for researchers to give a physiological relation to the loads that is applied during cadav er testing. It is very important to know how much load needs to be controll ed by the implant. This can lead to optimization of implant s and reduce the size. During In Vitro flexion, the pure bending mo ment of 6.5 Nm gives an angular displacement of 5.9 degrees and an intradiscal pressure of 0.68 MPa. These measurements indicate that the physi ological motion is equal to a flexion of 20 degrees. During In Vitro testing the physiologica l maximum flexion was not achieved, so higher mo ments should be applied during In Vitro testing. Similarly, during extensi on the angular displacement was 3.1 degrees and this gives an intradiscal pressure of 0. 50 MPa and a physiological backwards extension of 10 degrees. Lateral bending had pressure peak of 0.57 MPa when the angular displacement was 0.9 degree and at a bending mom ent of 1.8 Nm. This corresponds to a physiological lateral bending of 18 23 degr ees. During lateral bending, the highest angular displacement of 1.6 degree was reached with a

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80 moment of 6.3 Nm and a pressure of 0.26 0.36 MPa. This correlates to a person bending 29 degrees laterally. An axial rotation of 2.5 degrees and a pressure of 0.51 0.53 MPa was achieved by applying a moment of 3.6 Nm to one side and 5.7 Nm to the other side. During In Vivo measurements, this same axial rotation gave 17 degree rotation to one side and 24 degr ees to the other side. Overall, this study found a good correlation between In Vivo and In Vitro data. The variation of data is likely to o ccur from lack of translation of motion during In Vitro testing. It is also shown that a higher moment needs to be applied during testing in Fl exion/Extension. This ca n essentially be used to make physiological relation from experimental and analyt ical evaluations of the lumbar spine.

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81 CHAPTER 6 – SUMMARY 6.1 – Conclusion This dissertation contains both anal ytical and experimental hypothesis. Three hypotheses were look ed at and all three hypothes es were answered. The first hypothesis was: The incidence of adjacent level disc disease in the lumbosacral spine will be de creased with the use of se mi-rigid rods. As earlier shown in this dissertation, semi-rigid rods increase axial motion and anteriorly translating the axis of rotation. These factors reduce stress in adjacent disc, while maintains a stress level in the disc at the instrumented level. By reducing the stress in the adjacent disc, the di sc will degrade at a lower rate, and the incident of adjacent level disc disease is decreased. The second hypothesis was to va lidate increased biomechanical stiffness by the use of a facet fusion allograft In Vitro It was found that facet fusion allograft significantly changes the stiffness and could be used for treatment of minor instability. The last hypothesis to be answer ed was that there is a correlation between In Vivo and In Vitro intradiscal pressures. A comparison of the published In Vivo intradiscal pressures to In Vitro intradiscal pressures was

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82 performed. The pressures were evaluat ed and the effects of loads applied In Vitro was considered. A clear correlation was found between the In Vivo and In Vitro intradiscal pressures and physiological relevance c an be used In Vitro and in analytical models. This study also determined how much load to control while testing medical devices. 6.2 – Contribution There are five important discoveries m ade in the research that has led to this dissertation. One of these is t he discovery of reduced stresses in the adjacent disc by the use of semi-rigid rods The incidence of ad jacent level disc disease in the lumbosacra l spine will be decreased wit h the use of semi-rigid rods. From this research, there has already been made impr ovements to this traditional fusion technique, and there are many patients that benefit from this. Semi-rigid rods also increas e axial motion, anteriorly translating the axis of rotation, which leads to reduction of stress in adjacent disc. Facet fusion has been performed sinc e the 1940’s, but it has always been a technically demanding procedure. Because of th is, there has been very limited popularity to these methods. In th is dissertation, a comparison of the biomechanical properties of a facet fusion allograft In Vitro was done. These results showed that there is merit for this procedure. Facet fusion allograft significantly changes the stiffness and co uld be used for treatment of minor instability.

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83 Biomechanical testing of spinal implants has been performed on human cadaver lumbar spines, but there has never been any scientif ic reasoning for the loads that has been applied. There ar e several studies that look at the intradiscal pressures of living humans, and this data was used to find a correlation to the mechanical loads acti ng on the spine. These experimental results from the in vitro testing we re compared to the published In Vivo intradiscal pressures. A clear correlati on was found and physiological relevance can be used In Vitro and in analytical mo dels, as well as a definition for how much load to control was found. 6.3 – Future Work When conducting a intradiscal pressure study, it is important to have intervertebral discs that are in good shape and well hydrated. Also, with all biological tissue ther e will be differences Therefore, a large study needs to be conducted to give the most optimal representation of the correlation between In Vivo and In Vitro intradiscal pressures. This st udy should also contain study parameters to give a good i dea of the effect of different ligaments and facet joint capsule. Eventually, the intervertebral disc will be subjected to all independent loading situations, with al l the ligaments and posterior elements removed. By removing all the ligaments and posterior el ements, the disc ca n be modeled by using continuum mechanics. These specimens should also be tested at a series of different physiological strain rates.

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84 All individual specimens used for this study, should also be scanned by high resolution Computed Tomography (CT) scans. These scans could be used to create a high quality three dimensiona l finite element mesh. This mesh can be created by using commercially availa ble software, by importing the images into medical imaging software. In the selected software package, the tissue is selected a rendered into a th ree dimensional model. The rendering parameters are adjusted to accomplish the desired mo del. This model will be exported as a three dimensional model, bef ore imported into a finite element mesher. Once the model is meshed, the exact experiment al data for that particular finite element mesh can be created into a uniq ue finite element model with verified values. This can be done to all of the individual specimen s and statistical significance can be achieved by using finite element method. With the current limited supply and increasing demand for human cadaver spines, there will be advantages of creating thes e verified and accurate finite element models. These model s will reduce the demand for human cadaver spines. These models could also be used for preliminary testing of implants and have the design optimization performed at an early stage. These models can also be used to predict failu res, instead of meeting the minimum requirements set by the Food and Drug Ad ministration (FDA). The FDA, an American governmental agency, is already showing an interest in finite element modeling of medical devices.

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85 REFERENCES 1. Wilke HJ, Neef P, Caimi M, et al. New in vivo measurements of pressures in the intervertebral disc in daily life. Spine 1999;24:755–62. 2. Schnake KJ, Schaeren S, Jeanneret B. Dynamic stabilization in addition to decompression for lumbar spinal stenosis with degenerative spondylolisthesis. Sp ine 2006;31:442-9. 3. Phillips FM, Voronov LI, Gaitanis IN, et al. Biomech anics of posterior dynamic stabilizing device (DIAM) after facetectomy and discectomy. Spine J 2006;6:714-22. 4. International Society of Biomechanics Homepage., 2004. 5. Gillet P. The fate of the adj acent motion segments after lumbar fusion. J Spinal Disord Tech 2003;16:338-45. 6. Korovessis P, Papazisis Z, Koureas G, et al. Rigid, semirigid versus dynamic instrumentati on for degenerative lumbar spinal stenosis: a correlative radiological and clinical ana lysis of short-term results. Spine 2004;29:735-42. 7. Kumar MN, Jacquot F, Hall H. Long-term follow-up of functional outcomes and radiographic changes at adjacent levels following lumbar spine fusion for degenerative disc disease. Eur Spine J 2001;10:30913. 8. Park P, Garton HJ, Gala VC, et al. Adjacent segment disease after lumbar or lumbosacral fusion: re view of the lit erature. Spine 2004;29:1938-44. 9. Brechbuhler D, Markwa lder TM, Braun M. Surgic al results after soft system stabilization of the lumbar spine in degenerative disc disease-long-term results. Acta Neur ochir (Wien) 1998;140:521-5.

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86 10. Stoll TM, Dubois G, Schwarze nbach O. The dynamic neutralization system for the spine: a mu lti-center study of a novel non-fusion system. Eur Spine J 2002;11 Suppl 2:S170-8. 11. Wild A, Jaeger M, Bu she C, et al. Biomechani cal analysis of Graf's dynamic spine stabilisation system ex vivo. Biomed Tech (Berl) 2001;46:290-4. 12. Mulholland RC, Sengupta DK. Ration ale, principles and experimental evaluation of the concept of soft stabilization. Eur Spine J 2002;11 Suppl 2:S198-205. 13. Sengupta DK. Dynamic stabilization devices in the treatment of low back pain. Orthop Clin No rth Am 2004;35:43-56. 14. Mayer HM. The ALIF concept. EurSpineJ2000;9(suppl 1):S35– 43. 15. Regan JJ, Aronoff RJ, Ohnmeiss DD, et al. Laparoscopic approach to L4 –L5 for interbody fusion using BAK cages: experience in the .rst 58 cases. Spine1999;24:2171– 4. 16. Zdeblick TA, David SM. A prospec tive comparison of surgical approach for anterior L4 –L5 fusion: laparoscopic versus mini anterior lumbar interbody fusi on. Spine2000;25:2682–7. 17. Zelle B, Konig F, Ende rle A, et al. Circumferenti al fusion of the lumbar and lumbosacral spine using a carbon .ber ALIF cage implant versus autogenous bone graft: a comparative study. JSpinalDisordT ech2002;15:369 –76. 18. Kanayama M, Cunningham BW, H aggerty CJ, et al. In vitro biomechanical investigation of the st ability and stress-shielding effect of lumbar interbody fusion devices JNeurosurg2000;93(suppl):259–65. 19. Tsantrizos A, Andreou A, Aebi M, et al. Biomechanical stability of .ve stand-alone anterior lumbar in terbody fusion constructs. EurSpineJ2000; 9:14 –22. 20. Barnes B, Rodts GE, McLaughlin MR, et al. Threaded cortical bone dowels for lumbar interbody fusion: over 1-year mean follow-up in 28 patients. JNeurosur g2001;95(suppl):1– 4.

PAGE 103

87 21. Lubbers T, Bentlage C, Sandvoss G. Anterior lumbar interbody fusion as a treatment for chronic refr actory lower back pain in disc degeneration and spondylolis thesis using carbon cages-stand alone. ZentralblNeurochir2002; 63:12–17. 22. Jang JS, Lee SH, Lim SR. Guide device for percutaneous placement of trans-laminar facet screws after anterior lumbar interbody fusion. Technical note. JNeurosurg2003;98(suppl):100 –3. 23. Thalgott JS, Chin AK, Ameriks JA et al. Minimally invasive 360 degrees instrumented lumbar fusi on. EurSpineJ2000;9(suppl 1):S51– 6. 24. Christensen FB, Hansen ES, Ei skjaer SP, et al. Circumferential lumbar spinal fusion wit h Brantigan cage versus posterolateral fusion with titanium Cotrel-Dubousset inst rumentation: a prospective, randomized clinical study of 146 patients. Spine2002;27:2674–83. 25. Schofferman J, Slosar P, Reynolds J, et al. A prospective randomized com-parison of 270 degrees fusi ons to 360 degrees fusions (circumferential fu-sio ns). Spine2001;26:207–12. 26. Suk KS, Jeon CH, Park MS, et al. Comparison between posterolateral fusion with pedicle scr ew .xation and anterior interbody fusion with pedicle screw .xatio n in adult spondylolytic spondylolisthesis. YonseiMedJ2001;42: 316 –23. 27. Kandziora F, Schleicher P, Scholz M, et al. Biomechanical testing of the lumbar facet interference screw. Spine 2005; 30; E34-E39 28. King D. Internal .xation of lum bosacral fusion. J Bone Joint Surg Am 1948; 30:560–5. 29. Magerl F. Stabilization of the lower thoracic and lumbar spine with external skeletal .xatio n. Clin Orthop 1984;189:125–41. 30. Heggeness MH, Esses SI. Translami nar facet joint screw .xation for lumbar and lumbosacral fusion. A clinical and biomechanical study. Spine1991;16: S266 –9. 31. Kornblatt MD, Casey MP, Jacobs RR. Internal .xation in lumbosacral spine fusion. A biomechani cal and clinical study. ClinOrthop1986;203:141–50.

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88 32. Vanden Berghe L, Mehdian H, Lee AJ et al. Stability of the lumbar spine and method of instrumentati on. ActaOrthopBel g1993;59:175– 80. 33. Ferrara LA, Secor JL, Jin BH, et al. A biomechanical comparison of facet screw .xation and pedicle screw .x ation: effects of short-term and long-term repetitive cycling. Spine2003;28:1226 –34. 34. Deguchi M, Cheng BC, Sato K, et al. Biomechanical evaluation of translami-nar facet joint .xation. A comparative study of poly-L-lactide pins, screws, and pedicle .x ation. Spine1998;23:1307–12. 35. Benini A, Magerl F. Select ive decompression and translaminar articular facet screw .xation for lumbar canal stenosis and disc protrusion. Br J Neurosurg 1993;7:413–8. 36. Boucher HH. Method of spinal fusion. Clin Orthop 1997;335:4–9. 37. Graham CE. Lumbosacral fusion using internal .xation with a spinous pro-cess for the graft: a review of 50 patients with a .v e-year maximum follow-up. Clin Or thop 1979;140:72–7. 38. Grob D, Humke T. Translaminar scr ew .xation in the lumbar spine: tech-nique, indications, resu lts. Eur Spine J 1998;7:178–86. 39. Grob D, Rubeli M, Scheier HJ, et al. Translaminar screw .xation of the lumbar spine. Int Orthop 1992;16:223–6. 40. Heggeness MH, Esses SI. Translami nar facet joint screw .xation for lumbar and lumbosacral fusion: a clinical and biomechanical study. Spine 1991;16(6 Suppl):S266–9. 41. Humke T, Grob D, Dvorak J, et al. Translaminar scr ew.xation of the lumbar and lumbosacral spine: a 5-year follow-up. Spine 1998;23:1180–4. 42. Jacobs RR, Montesano PX, Jackson RP. Enhancement of lumbar spine fu-sion by use of transla minar facet joint screws. Spine 1989;14:12–5. 43. Margulies JY, Seimon LP. Clinical ef.cacy of lumbar and lumbosacral fusion using the Boucher facet screw .xation technique. Bull Hosp Jt Dis 2000;59: 33–9.

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89 44. Plotz GM, Benini A. Anterior lu mbar vertebral translation following trans-laminar screw .xation: a r eport of .ve cases. Int Orthop 1998;22:77–81. 45. Reich SM, Ku.ik P, Neuwirth M. Translaminar facet screw .xation in lum-bar spine fusion. Spine 1993;18:444–9. 46. Stonecipher T, Wright S. Posterior lumbar interbody fusion with facetscrew .xation. Sp ine 1989;14:468–71. 47. Blumenthal S, Gill K. Complica tions of the Wiltse Pedicle Screw Fixation System. Spine 1993;18:1867–71. 48. Esses SI, Sachs BL, Dreyzin V. Complications associated with the technique of pedicle screw .xation: a selected survey of ABS members. Spine 1993;18: 2231–8, discussion 2238–9. 49. Georgis T, Rydevik B, Weinstein JN, et al. Complications of pedicle screw .xation. In: Gar.n SR, ed. Complications of Spine Surgery. Baltimore: Wil-liams & Wilkins; 1989:200–10. 50. Gertzbein SD, Robbins SE. Accura cy of pedicular screw placement in vivo. Spine 1990;15:11–4. 51. Ginsburg HH, Scoles PV. Scoliosi s Research Society Morbidity and Mortal-ity Committee: Complication Report 1990. Park Ridge, IL: Scoliosis Re-search Society; 1990. 52. Hsu J, Zuckermann JF, White AH, et al. Internal.xation with pedicle screws. In: White AH, Rothman RH, Roy CD, eds. Lumbar Spine Surgery. St. Louis: Mosby; 1987:322–38. 53. Lonstein JE, Denis F, Perra JH, et al. Complications associated with pedicle screws. J Bone Joint Surg Am 1999;81:1519–28. 54. Okuyama K, Abe E, Suzuki T, et al. Posterior lumbar interbody fusion: a retrospective study of comp lications after facet joint excision and pedicle screw .xation in 148 cases. Act Orthop Scand 1999;70:329–34. 55. Steffe AD, Biscup RS, Sitkowski DJ. Segmental spine plate with pedicle screw .xation: a new internal .xation device for disorders of the lumbar and thoracolumbar spi ne. Clin Orthop 1986;203:45–54.

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90 56. Weinstein JN, Spratt KF, Spengler D, et al. Spinal pedicle .xation: reliability and validity of r oentgenogram-based assessment and surgical factors on successful screw placement. Spine 1988;13:1012. 57. Whitecloud TS, Butler JC, Cohen JL, et al. Complications with the variable spinal plating system. Spine 1989;14:472–6. 58. Yahiro MA. Comprehensive literat ure review: pedicle screw .xation devices. Spine 1994;19(20 Suppl):2274S–8S. 59. Niosi CA, Oxland TR. Degenerativ e mechanics of the lumbar spine. Spine J 2004;4(s uppl 6):202– 8. 60. van Deursen DL, Snijders CJ, v an Dieen JH, et al. The effect of passive vertebral rotation on pre ssure in the nucleus pulposus. J Biomech 2001;34: 405–8. 61. van Deursen DL, Snijders CJ, Kingma I, et al. In vitro torsion-induced stress distribution changes in porci ne intervertebral discs. Spine 2001;26:2582–6. 62. Janevic J, Ashton-Miller JA, Schul tz AB. Large compressive preloads decrease lumbar motion segment fl exibility. J Orthop Res 1991;9:228– 36. 63. Wang JL, et al. The dynamic response of L(2)/L(3) motion segment in cyclic axial compressive loading. Clin Biomech (Bristol, Avon) 1998;13(suppl 1): 16–25. 64. Wilke HJ, Claes L, Schmitt H, et al A universal spine tester for in vitro experiments with muscle force simula tion. Eur Spine J 1994;3:91–7. 65. Wilke HJ, Wolf S, Claes LE, et al. Influence of varying muscle forces on lumbar intradiscal pressure: an in vitro study. J Biomech 1996;29:549–55. 66. Cakir B, Ulmar B, Koepp H, et al [Posterior dynamic stabilization as an alternative for dorso-ventral fusion in spinal stenosis with degenerative instability]. Z Ort hop Ihre Grenzgeb 2003;141:418-24. 67. McNally DS, Adams MA, Goodshi p AE. Development and validation of a new transducer for intradiscal pressure measurement. J Biomed Eng 1992; 14:495–8.

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91 68. Grob D, Benini A, Junge A, et al. Clinical experience with the Dynesys semirigid fixation system for the lumbar spine: surgical and patient-oriented outcome in 50 case s after an average of 2 years. Spine 2005;30:324-31. 69. McNally DS, Adams MA Internal intervertebr al disc mechanics as revealed by stress profilo metry. Spine 1992;17:66–73. 70. Markwalder TM, Wenger M. Dynamic stabilization of lumbar motion segments by use of Graf's ligaments: results with an average follow-up of 7.4 years in 39 highly selected, consecutive patients. Acta Neurochir (Wien) 2003;145:209-14; discussion 14. 71. Yantzer BK, Freeman TB, Lee WE, et al. Torsion-induced pressure distribution changes in hum an intervertebral discs 72. Saxler G, Wedemeyer C, von Knoc h M, et al. [Follow-up study after dynamic and static stabilisation of the lumbar spine]. Z Orthop Ihre Grenzgeb 2005;143:92-9. 73. Adams MA, Freeman BJ, Morrison HP, et al. Mechanical initiation of intervertebral disc degenerat ion. Spine 2000;25:1625-36. 74. Bose B. Anterior cervical arth rodesis using DOC dynamic stabilization implant for improvement in sagitta l angulation and controlled settling. J Neurosurg Spin e 2003;98:8-13. 75. Kanayama M, Hashimoto T, Shi genobu K, et al. Adjacent-segment morbidity after Graf ligamentoplas ty compared with posterolateral lumbar fusion. J Neur osurg Spine 2001;95:5-10. 76. Putzier M, Schneider SV, Funk J, et al. [Application of a dynamic pedicle screw system (DYNESYS) for lumbar segmental degenerations comparison of clinical an d radiological results for different indications]. Z Orthop Ihre Grenzgeb 2004;142:166-73. 77. Smit TH, Odgaard A, Schneider E. Structure and function of vertebral trabecular bone. Sp ine 1997;22:2823-33. 78. Martinez JB, Oloyede VO, Broom ND. Biomechanics of load-bearing of the intervertebral disc: an exper imental and finite element model. Med Eng Phys 1997;19:145-56.

PAGE 108

92 79. Vena P, Franzoso G, Gastaldi D, et al. A finite element model of the L4-L5 spinal motion segment: biom echanical compatib ility of an interspinous device. Comput Me thods Biomech Biomed Engin 2005;8:7-16. 80. White AA, Panjabi M. Clinical Biomechanics of the Spine. Second ed. Philadephia: Lippincott, 1990. 81. Goel VK, Monroe BT, Gilbertson LG, et al. Interlaminar shear stresses and laminae separation in a disc. Finite element analysis of the L3-L4 motion segment subjected to axial compressive loads. Spine 1995;20:689-98. 82. Niosi CA, Zhu QA, Wilson DC, et al. Biomechanical characterization of the three-dimensional kinematic behaviour of the Dynesys dynamic stabilization system: an in vitro study. Eur Spine J 2006;15:913-22. 83. Schmoelz W, Huber JF Nydegger T, et al. Dynamic stabilization of the lumbar spine and its effects on adjacent segments : an in vitro experiment. J Spinal Di sord Tech 2003;16:418-23. 84. Xu HZ, Wang XY, Chi YL, et al Biomechanical evaluation of a dynamic pedicle screw fixation devic e. Clin Biomech (Bristol, Avon) 2006;21:330-6. 85. Chen CS, Cheng CK, Liu CL, et al. Stress analysis of the disc adjacent to interbody fusion in lumbar spine. Med Eng Phys 2001;23:483-91. 86. Zander T, Rohlmann A, Burra NK, et al. Effect of a posterior dynamic implant adjacent to a rigi d spinal fixator. Clin Biomech (Bristol, Avon) 2006;21:767-74. 87. Eck JC, Humphreys SC, Lim TH, et al. Biomechanical study on the effect of cervical spine fusion on adj acent-level intradiscal pressure and segmental motion. Spine 2002;27:2431-4. 88. Natarajan RN, Williams JR, Andersson GB. Recent advances in analytical modeling of lumbar disc degeneration. Spine 2004;29:273341.

PAGE 109

93 89. Stein M, Elliott D, Glen J, Mo rava-Protzner I: Young Investigator Award: Percutaneous facet joint fu sion—Preliminary experience. J Vasc Interv Radiol 4:69–74, 1993. 90. Foley KT, Gupta SK. Percutaneous pedicle screw .xation of the lumbar spine: preliminar y clinical results. JNeu rosurg2002;97(suppl):7– 12. 91. Verheyden P, Katscher S, Schulz T, et al. O pen MR imaging in spine surgery: experimental investigations and .rst clinical experiences. EurSpineJ 1999; 8:346 –53. 92. Wilke HJ, Wenger K, Cl aes L. Testing criteria for spinal implants: recom-mendations for the standardization of in vitro stability testing of spinal implants. EurSpineJ1998;7:148 –54. 93. Benzel EC.Biomechanics of Spine Stabilization. Rolling Meadows, IL: American Association of Neurological Surgeons; 2001. 94. Cheng BC, Moore DK, Zdeb lick TA, et al. Load-Sharing Characteristics of Two Anterior Cerv ical Plate Systems. The Cervical Spine Research Society Meeting; Rancho Mirage, California; 1997. 95. Treharne RW. Review of Wolff’ s law and its proposed means of operation. Orthop Rev 1981;10:35. 96. Wolff J. Das Gesetz der Tr ansformation der Knochen. Berlin: Hirschwald Verlag; 1892. 97. Panjabi, M. M., Krag, M., Summer s, D. et al. Biomechanical timetolerance of fresh cadaveric human spine specimens. J Orthop Res. 1985;3(3):292-300. 98. Bradford FK, Spurling RG. The inte rvertebral disc. In: Charles C. Thomas, Springfield, 1945. 99. Morris JM, Lucas DB, Bresler B. Role of the trunk in stability of the spine. J Bone Jt Surg [Am] 1961;42-A(3):327-51. 100. Dieen JJv, Creemers M, Draisma I, Toussaint HM. Re petitive lifting and spinal shrinkage, effects of age and lifting technique. Clin Biomech 1994;9:367-74.

PAGE 110

94 101. Looze MPd, Kingma I, Thunissen W, Wijk van MJ, Toussaint HM. The evaluation of a practical model estimating lumbar moments in occupational activities. Ergonomics 1994;38:1993-2006. 102. Gagnon D, Gagnon M. The influence of dynamic factors on triaxial net muscular moments at the L5/S1 joint during asymmetrical lifting and lowering. J Biomech 1992;25(8):891-901. 103. Marras WS, Sommerich CM. A three-dimensional motion model of laods on the lumbar spine: I. Model structure. Hum Factors 1991;33:123-37. 104. Hughes RE, Bean JC, Chan DB Evaluating the effect of cocontraction in optimization m odels. J Biomech 1995;28(7):875-8. 105. Jager M, Luttmann, A. The load on the lumbar spine during asymmetrical bi-manual materi als handling. Ergonomics 1992;35(78):783-805. 106. Hughes RE, Chan DB. The effect of strict muscle stress limits on abdominal muscle force predictions for combined torsion and extension loadings. J Biomec h 1995;28(5):527-33. 107. Plamondon A, Gagnon M, Gravel D. Moments at the L5/S1 joint during asymmetrical lifting: effects of different load trajectories and initial load positions. Cli n Biomech 1995;10(3):128-36. 108. Cappozzo A. Compressive loads in the lumbar vertebral column during normal level walking. J Orthop Res 1984;1:292-301. 109. Khoo BC, Goh JC, Bose K. A bi omechanical model to determine lumbosacral loads during single stance phase in normal gait. Med Eng Phys 1995;17(1):27-35. 110. Cromwell R, Schultz AB, Beck R, Warwick D. Loads on the lumbar trunk during level walking. J Orthop Res 1989;7(3):371-7. 111. Schultz A, Andersson G, Ortengren R, Haderspeck K, Nachemson A. Loads on the lumbar spi ne. Validation of a biom echanical analysis by measurements of intradisca l pressures and myoelectric signals. J Bone Joint Surg Am 1982;64:713-20. 112. Wilke HJ, Neef P, Barbara H, et al. Intradiscal pressure together with anthropometric data – a data set for t he validation of m odels. Clinical Biomechanics 2001;16:S111-S126.

PAGE 111

95 113. Chaffin D. A computerised biom echanical models development and use in studying gross body ac tions. J Biomech 1969;2:429-41. 114. McGill SM, Norman RW. Dynamically and statically determined low back moments during lifting. J Biomech 1985;18(12):877-85. 115. McGill SM, Norman RW. Partiti oning of the L4-L5 dynamic moment into disc, ligamentous and muscula r components during lifting. Spine 1986;11(7):666-78. 116. McGill SM. Estimation of force and extensor moment contributions of the disc and ligaments at L4/ L5. Spine 1988;13:1395-402. 117. Shirazi-Adl A. Finite-element eval uation of contact loads on facets of an L2-L3 lumbar segment in comple x loads. Spine 1991;16(5):533-41. 118. Shirazi-Adl A. Nonlinear stress a nalysis of the whole lumbar spine in torsionmechanics of facet articu lation. J Biomech 1994;27(3):289-99. 119. Goel VK, Komg W, Han JS, Wein stein JN, Gilbertson L. A combined finite element and optimization invest igation of lumbar spine mechanics with and without muscles. Sp ine 1993;18(11):1531-41. 120. Lavaste F, Skalli W, Robin S, Roy-Camille R, Mazel C. Threedimensional Geometrica l and Mechanical Mode ling of the Lumbar Spine. J Biomech 1992;25(10):1153-64. 121. Parnianpour M, Wang JL, ShiraziAdl A, Sparto P, Wilke H-J. The effect of variations in trunk model s in predicting muscle strength and spinal loading. J muscul oskeletal Res 1997;1:55-69. 122. Shirazi-Adl A, Parnianpour M. Ro le of posture in mechanics of the lumbar spine in compression. J Spinal Disord 1996;9(4):277-86. 123. Pope MH. Biomechanics of the lumbar spine. Ann Med 1989;21(5):347-51. 124. Nachemson A. The influence of spinal movements on the lumbar intradiscal pressure and on the tens ile stresses in the annulus fibrosus. Acta Orthop Scand 1963;33:183-207. 125. Nachemson A, Elfstrom G. Intravital dynamic pressure measurements in lumbar discs. A study of common movements, maneuvers and exercises. Scand J Rehabil Med Suppl 1970;1:1-40.

PAGE 112

96 126. Nachemson A. The effe ct of forward leaning on lumbar intradiscal pressure. Acta Ort hop Scand 1965;35:314-28. 127. Nachemson A. In vivo discome try in lumbar discs with irregular necleograms. Acta Orthop Scand 1965;36:418-34. 128. Nachemson A. The load on lumbar disks in different positions of the body. Clin Orthop 1966;45:107-22. 129. Nachemson A, Morris JM. J Bone Jt Surg. In vivo Measurements of Intradiscal Pressure 1964;46-A(5):1077-92. 130. Dhillon N, Bass EC, Lotz JC. Effe ct of frozen storage on the creep behavior of human intervertebral discs. Spine 2001;26:883–8. 131. Mimura M, Panjabi MM, Oxland TR et al. Disc degeneration affects the multidirectional flexibility of the lumbar spine. Spine 1994;19:1371– 80. 132. McMillan DW, McNally DS, Garbu tt G, et al. Stress distributions inside intervertebral discs: the va lidity of experimental ‘stress profilometry.’ Proc Inst Mech Eng H 1996;210:81–7. 133. White III AA, Panjabi MM. Spinal kinematics. T he Research Status of Spinal Manipulative Therapy. NI NCDS Monograph (No. 15), p. 93. Washington, D.C., U.S. Department of Health, Education and Welfare, 1975.

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97 APPENDICES

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98 Appendix A – Figures Related to Analytical Results Figure A-1: Stress Distri bution of L3-L4 Disk at 15 Flexion Figure A-2: Stress Distri bution of L3-L4 Disk at 30 Flexion

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99 Appendix A. (Continued) Figure A-3: Stress Distri bution of L3-L4 Disk at 45 Flexion Figure A-4: Stress Distri bution of L4-L5 Disk at 15 Flexion

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100 Appendix A. (Continued) Figure A-5: Stress Distri bution of L4-L5 Disk at 30 Flexion

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101 Appendix B – Figures Related to Experimental Results -4 -2 0 2 4 -8-6-4-202468 Torque [Nm]Angle [Degree] Intact Facet Fusion Allograft Figure B-1: Typical Lateral Bending Results, Demo nstrating a Comparison Between Intact and Treated Specimen

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102 Appendix B. (Continued) -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 -6-4-20246 Torque [Nm]Angle [Degree] Intact Facet Fusion Allograft Figure B-2: Typical Axial Rotation Resu lts, Demonstrating Comparison Between Intact and Treated Specimen 0 1 2 3 4 5 6 7 FlexExtBendTorRange of Motion [deg] Specimen 1 Specimen 2 Specimen 3 Figure B-3: Range of Motion Com parison Between the Different Intact Specimens

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103 Appendix B. (Continued) 0 1 2 3 4 5 FlexExtBendTorRange of Motion [deg] Specimen 1 Specimen 2 Specimen 3 Figure B-4: Range of Mo tion Comparison Between the Different Treated Specimens 0 1 2 3 4 5 6 FlexExtBendTorStiffness [N/mm] Specimen 1 Specimen 2 Specimen 3 Figure B-5: Stiffness Comparison Betw een the Different Intact Specimens

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104 Appendix B. (Continued) 0 1 2 3 4 5 6 FlexExtBendTorStiffness [Nm/deg] Specimen 1 Specimen 2 Specimen 3 Figure B-6: Stiffness Comparison Betw een the Different Treated Specimens

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105 Appendix C – Tables Related to St atistics and Experimental Data Table C-1: Range of Motion Test Resu lts for Individual Specimens During Extension Loading Intact [Nm] Treatment [Nm] Specimen 1 2.0816230.983444 Specimen 2 2.8000831.035348 Specimen 3 1.6500001.070861 Mean 2.1772351.029884 Standard Deviation 0.5809720.043964 Table C-2: Range of Motion Test Resu lts for Individual Specimens During Flexion Loading Intact [Nm] Treatment [Nm] Specimen 1 3.2235101.589901 Specimen 2 5.4198681.068129 Specimen 3 4.1987592.117136 Mean 4.2807121.591722 Standard Deviation 1.1004700.524506

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106 Appendix C. (Continued) Table C-3: Range of Motion Test Resu lts for Individual Specimens During Bending Loading Intact [Nm] Treatment [Nm] Specimen 1 5.4799672.955795 Specimen 2 6.0836921.813907 Specimen 3 6.6027324.583941 Mean 6.0554643.117881 Standard Deviation 0.5619151.392112 Table C-4: Range of Motion Test Resu lts for Individual Specimens During Torsion Loading Intact [Nm] Treatment [Nm] Specimen 1 4.1140732.513245 Specimen 2 1.4915571.256623 Specimen 3 1.9286421.688245 Mean 2.5114241.819371 Standard Deviation 1.4050350.638491

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107 Appendix C. (Continued) Table C-5: Stiffness Test Results for Individual Specimens During Extension Loading Intact [Nm] Treatment [Nm] Specimen 1 2.1553694.125380 Specimen 2 1.2071113.882020 Specimen 3 2.6616204.321470 Mean 2.0080334.109623 Standard Deviation 0.7383630.220149 Table C-6: Stiffness Test Results fo r Individual Specim ens During Flexion Loading Intact [Nm] Treatment [Nm] Specimen 1 1.2595332.343231 Specimen 2 0.7557653.266234 Specimen 3 0.9443331.725766 Mean 0.9865442.445077 Standard Deviation 0.2545230.775267

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108 Appendix C. (Continued) Table C-7: Stiffness Test Results fo r Individual Specimens During Bending Loading Intact [Nm] Treatment [Nm] Specimen 1 1.6804253.264725 Specimen 2 1.4985875.496485 Specimen 3 1.3596101.931985 Mean 1.5128743.564398 Standard Deviation 0.1608841.801047 Table C-8: Stiffness Test Results fo r Individual Specimens During Torsion Loading Intact [Nm] Treatment [Nm] Specimen 1 1.8326272.859055 Specimen 2 5.3405535.433570 Specimen 3 3.7359764.329021 Mean 3.6363854.207215 Standard Deviation 1.7560831.291572

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109 Appendix C. (Continued) Table C-9: Summary of the Single Fa ctor ANOVA Performed on the Range of Motion Specimens During Extension Loading Groups Count Sum Average Variance Column 1 36.5317052.1772350.337529 Column 2 33.0896521.0298840.001933 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 1.97462211.97462211.633840.027006 7.708647 Within Groups 0.67892440.169731 Total 2.6535455

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110 Appendix C. (Continued) Table C-10: Summary of the Single Fact or ANOVA Performed on the Range of Motion Specimens During Flexion Loading Groups Count Sum Average Variance Column 1 312.842144.2807121.211034 Column 2 34.7751661.5917220.275106 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 10.846110.84614.59620.01877 7.708647 Within Groups 2.97228140.74307 Total 13.818285

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111 Appendix C. (Continued) Table C-11: Summary of the Single Fact or ANOVA Performed on the Range of Motion Specimens During Lateral Bending Loading Groups Count Sum Average Variance Column 1 318.166396.0554640.315748 Column 2 39.3536433.1178811.937975 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 12.94409112.9440911.486850.02755 7.708647 Within Groups 4.50744641.126861 Total 17.451545

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112 Appendix C. (Continued) Table C-12: Summary of the Single Fact or ANOVA Performed on the Range of Motion Specimens During Axial Rotation Loading Groups Count Sum Average Variance Column 1 37.5342722.5114241.974124 Column 2 35.4581131.8193710.407671 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 0.71840610.7184060.6032480.480711 7.708647 Within Groups 4.76358941.190897 Total 5.4819955

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113 Appendix C. (Continued) Table C-13: Summary of the Single Fact or ANOVA Performed on the Stiffness Specimens During Extension Loading Groups Count Sum Average Variance Column 1 36.02412.0080330.54518 Column 2 312.328874.1096230.048465 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 6.6250216.6250222.319770.009142 7.708647 Within Groups 1.18729140.296823 Total 7.8123115

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114 Appendix C. (Continued) Table C-14: Summary of the Single Fact or ANOVA Performed on the Stiffness Specimens During Flexion Loading Groups Count Sum Average Variance Column 1 32.9596310.9865440.064782 Column 2 37.3352312.4450770.60104 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 3.1909813.190989.5850910.036363 7.708647 Within Groups 1.33164340.332911 Total 4.5226235

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115 Appendix C. (Continued) Table C-15: Summary of the Single Fact or ANOVA Performed on the Stiffness Specimens During Lateral Bending Loading Groups Count Sum Average Variance Column 1 34.5386221.5128740.025884 Column 2 310.693193.5643983.243769 ANOVA Source of Variation SS df MS F P-value F crit Between Groups 6.31312716.3131273.861650.120845 7.708647 Within Groups 6.53930541.634826 Total 12.852435

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116 Appendix C. (Continued) Table C-16: Summary of the Single Fact or ANOVA Performed on the Stiffness Specimens During Axial Rotation Loading Groups Count Sum Average Variance Column 1 310.909163.6363853.083826 Column 2 312.621654.2072151.668159 ANOVA Source of Variation SS dfMS F P-value F crit Between Groups 0.4887710.488770.2057120.673666 7.708647 Within Groups 9.50397242.375993 Total 9.9927425

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117 Appendix D – Publications Related to the Dissertation Research 1) In Press 1.1) Finite Element Analysis of Dynamic Instrumentation Demonstrates Stress Reduction in Adjacent Level Discs. Published in SAS Journal 2) Manuscripts in Preparation 2.1) Biomechanical Testing of Percutaneous Lumbar Facet Fusion Allograft—A Pilot Study. To be submitted to Journal of Biomechanics 2.2) A comparison between in-vivo and in-vitro intradiscal pressures. To be submitted to Spine Journal Authors Include: Antonio E. Castellvi, Debor ah H. Clabeaux, Hao Huang, William E. Lee, Sunil Saigal, David Pienkowski

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ABOUT THE AUTHOR Tov Inge Vestgaarden completed his Bachelors degree in Mechanical Engineering, as well as dual Master degrees in Mechanical and Biomedical Engineering. During his gr aduate studies, the author co ncentrated his research in material sciences and mechanics. In his doctoral research he applied these sciences in the field of spine biomec hanics. During his studies, he was also teaching undergraduate courses. He was nominated and received the Provost Commendation For Outstandi ng Teaching By A Graduate Teaching Assistant for his teaching accomplishments. He has also submitted and been accept ed to present his research at the World Spine Conference in 2007. As we ll as having publication accepted for publication in peer reviewed journal. Prior to his education, Tov I. Vest gaarden served in the Royal Norwegian Air force, where he wa s honorably discharged.