Investigation of the Mechanical Response of the Anterior and Posterior Cervical and. Lumbar Disc Bulge. David G. Drucker

Transcription

1 Investigation of the Mechanical Response of the Anterior and Posterior Cervical and Lumbar Disc Bulge By David G. Drucker B.S. University of California, Davis, 2009 THESIS Submitted as partial fulfillment of the requirements for the degree of Master of Science in Bioengineering in the Graduate College of the University of Illinois at Chicago, 2012 Chicago, Illinois Defense Committee: Farid Amirouche, Mechanical and Industrial Engineering, Chair and Advisor David Eddington Jun Cheng

2 ACKNOWLEDGEMENTS I would like to thank research advisor Dr. Farid Amirouche for all of the support and advise he has given me for this project. Thank you for furthering my educational curiosity in biomechanics. I would also like to acknowledge Dr. Giovanni F. Solitro for all of his hard work and support. Special thanks Dr. David Eddington and Dr. Jun Cheng for taking time out of their schedules and lending their expertise in this project. Special thanks to Dr. Ashish Upadhyay for his clinical expertise and disc analysis. Lastly thank you to the students in the lab for offering encouragement. ii

3 TABLE OF CONTENTS SUMMARY...ix 1. ANATOMY OF THE HUMAN LUMBAR AND CERVICAL SPINE Anatomy of the Vertebra Anatomy of the Cervical Spine Anatomy of the Lumbar Spine Soft Tissue Anatomy Intervertebral Disc Annulus Fibrosis Nucleus Pulposus Articular Facet Joints LIGAMENTS MUSCLES MECHANICS OF THE SPINE BIOMECHANICS OF THE SPINE SEGMENT REVIEW OF EXISTING IVD EXPERIMENTAL MECHANICAL TESTING STUDIES Current Literature of lumbar specimen testing Current literature of cervical specimen testing SPECIFIC AIMS TO MEASURE AND ANALYZE THE IVD BULGE METHODS OF EXPERIMENTAL TESTING Specimen Preparation Mechanical Testing Protocol Positioning of Linear Variable Differential Transformer on the Specimens Experimental Data Processing and Analysis Phase 2. Specimen Preparation and Testing: Laminectomy Phase 2. Specimen Preparation and Testing: Facetectomy Specimen Dissection and Inspection RESULTS OF EXPERIMENTAL TESTING Phase 1 Results Intact Cervical Spine Phase 2 Results Laminectomy Cervical Spine Phase 2 Results Facetectomy Cervical Spine iii

4 6.4 Phase 1 Results Intact Lumbar Specimens Phase 2 Results Laminectomy Lumbar Specimens Phase 2 Results Facetectomy Lumbar Specimens Anterior and Posterior Bulge analysis at each Spine Level: Cervical Spine Anterior and Posterior Bulge analysis at each Spine Level: Lumbar Spine Analysis of the Average Deformation Response Analysis of the Removal of the Articular Facets in the Lumbar Region Analysis of the Removal of the Articular Facets in the Cervical Region Dissection of Cervical Specimens Dissection of the Lumbar Specimens Comparison of each Configuration: Lumbar Spine Comparison of each Configuration: Cervical Spine DISCUSSION WORKS CITED iv

5 LIST OF TABLES Table I. Comparison of the results of the studies and deformation Values Table II. Table of rigidity values Table III. Comparison on the amount of bulge with and without facets in the lumbar spine Table IV. Comparison on the amount of bulge with and without facets in the Cervical Spine v

6 LIST OF FIGURES Figure 1 Cervical Spine... 4 Figure 2. Lumbar vertebra... 5 Figure 3 Specimen preparation. As shown here from Lin The vertebrae are constrained in a polyester resin (Lin et al, 1978) Figure 4. Specimen Image after testing as shown here from the Wenger 1997 study. The IVD is clearly bulging Figure 5. Specimen preparation and testing set up for the 2010 Cuchanski study (Cuchanski 2010) Figure 6. Diagram of Anterior Deformation and Load Directions Figure 7. Experimental Setup Lumbar Specimen Figure 8. Sensor Placement Intact Cervical specimen Figure 9. Cervical Laminectomy Cut Figure 10. Lumbar Laminectomy Figure 11. Cervical laminectomy mechanical test Figure 12. Lumbar laminectomy Mechanical setup Figure 13. Cervical facetectomy posterior view Figure 14. Cervical Spine C5-C6 facetectomy mechanical testing Upper posterior view Figure 15. Lumbar facetectomy Figure 16. Lumbar facetectomy mechanical testing Figure 17. Example of Sagittal Cut Specimen L2-L Figure 18. Average Anterior Bulge Intact Specimens Figure 19. Average Vertical Deformation for Cervical Intact Specimens Figure 20. Comparison of the Gap height change in Cervical Spine studies Figure 21. Vertical deformation perpendicular to the axial plane Figure 22. Anterior Bulge in the cervical Spine Figure 23. Posterior Bulge Cervical spine under laminectomy Figure 24a,b. Anterior and posterior IVD bulge slope comparison Figure 25. Vertical Cervical deformation facetectomy Figure 26. Anterior bulge cervical spine facetectomy Figure 27. Posterior cervical bulge: facetectomy Figure 28. Vertical deformation Lumbar intact Figure 29. Anterior lumbar bulge intact Figure 30. Lateral Bulge Lumbar, Intact Figure 31.Comparison of the Existing literature to the current study with respect to the gap deformation Figure 32 Comparison of the Existing literature to the current study with respect to the anterior bulge vi

7 Figure 33. Comparison of the Existing literature to the current study with respect to the lateral bulge Figure 34. Vertical deformation Lumbar laminectomy Figure 35. Anterior Bulge lumbar laminectomy Figure 36. Lateral bulge Lumbar Laminectomy Figure 37. Posterior Bulge Lumbar Laminectomy Figure 38. Vertical Deformation Lumbar Facetectomy Figure 39. Anterior Bulge Lumbar Facetectomy Figure 40. Lateral Bulge Lumbar Facetectomy Figure 41. Posterior Bulge Lumbar Facetectomy Figure 42. Comparison of C3-C4 Anterior and Posterior Bulge Laminectomy Figure 43. C5-C6 Comparison of Anterior and Poster Bulge Laminectomy Figure 44. Comparison of C7-T1 Anterior and Posterior Bulge Laminectomy Figure 45. C3-C4 Comparison of Anterior and Posterior bulge Facetectomy Figure 46. Comparison of C5-C6 Anterior and Posterior Bulge Facetectomy Figure 47. Comparison of C7-T1 Anterior and Posterior Bulge Facetectomy Figure 48. Comparison of T12-L1 Anterior and Posterior Bulge Laminectomy Figure 49. Comparison of L2-L3 Anterior and Posterior Bulge Laminectomy Figure 50. Comparison of L4-L5 Anterior and Posterior Bulge Laminectomy Figure 51. Comparison of T12-L1 Anterior and Posterior Bulge Facetectomy Figure 52. Comparison of L2-L3 Anterior and Posterior Bulge Facetectomy Figure 53. Comparison of L4-L5 Anterior and Posterior Bulge Facetectomy Figure 54. Average Deformations Laminectomy Lumbar Figure 55. Average Deformations Lumbar Facetectomy Figure 56. Average Deformation Cervical laminectomy Figure 57. Average Deformations Cervical Facetectomy Figure 58. Axial cut C3-C Figure 59. Axial cut specimen 1 C7-T Figure 60. Sagittal cut. Specimen C7-T Figure 61. Sagittal Cut. Specimen C5-C Figure 62. Sagittal Cut. Specimen C3-C Figure 63. Axial Cut Specimen T12-L Figure 64. Sagittal Cut and Axial Cut, Specimen T12-L Figure 65. Sagittal Cut. Specimen L2-L Figure 66. Sagittal and Axial Cut. Specimen L4-L Figure 67. Sagittal Cut. Specimen L4-L Figure 68. Average Vertical Deformation after each modification Figure 69. Average Vertical Deformation after each modification Figure 70. Average Posterior Bulge of the lumbar Specimens after each modification. 89 Figure 71. Average Posterior Bulge of the Lumbar Specimens after each modification 90 vii

8 Figure 72. Average Vertical Deformation of the Cervical Specimens after each modification Figure 73. Average Anterior Bulge of the Cervical Specimens after each modification. 91 Figure 74. Average Posterior Bulge of the Cervical Specimens after each modification92 viii

9 SUMMARY The spine is an important structure that provides support, flexibility, range of motion and protection. Neck and lower back pain are significant problems in the aging population. In this dissertation the cervical and lumbar intervertebral disc bulge response to compressive forces will be analyzed. Eleven cervical specimens and seven lumbar specimens are compressed in a series of three cycles with increasing loads to 550 N. The vertical deformation and anterior bulge is measured in multiple configurations including intact, laminectomy, and facetectomy the posterior bulge is also measured in the latter two configurations. The changes in the bulging patterns are compared and analyzed. It is shown that as the cervical specimens were compressed stiffening affect in the anterior bulge occurs that did not appear in the vertical deformation response. Upon further investigation, using the laminectomy configuration to gain access to the posterior Intervertebral Disc (IVD) region, it was found that the posterior bulge did not experience a stiffening effect. Additionally, when the articular facets were removed to assess the changes that the articular facets impose on the anterior and posterior disc bulge, it was shown that a translation of the vertebral bodies occurs in the cervical and lumbar regions but in opposite directions suggesting that the orientation of the articular facets may play a role in the deformation response of the IVD. The results from this study can be used as a basis for further studies to develop diagnostic techniques for spinal injuries and ailments like herniation, spondylolisthesis, among others. ix

10 1. ANATOMY OF THE HUMAN LUMBAR AND CERVICAL SPINE The spine is an anatomical structure that begins just below the skull and extends down just past the pelvis; it provides support, stability and flexibility for the upper body (Middleditch et al, 2005). It also protects the spinal cord which is vital to the central nervous system (Middleditch et al, 2005). The spine supports the body to maintain an upright direction, improves flexibility, provides a large range of motion, serves as attachment points for muscles and protects the spinal cord. The spine is composed of a total of 29 vertebrae separated into four regions: cervical, thoracic, lumbar and sacral, each with functional, anatomical, physiological differences (Middleditch et al, 2005). The spinal column is composed of vertebrae, cartilage and ligaments. The vertebral column alternates between vertebral bodies and intervertebral discs in the anterior region and the posterior elements and two articular facet joints in the posterior region (Middleditch et al, 2005). The vertebral bodies are composed of a hard cortical shell about one millimeter thick and softer trabecular bone on the interior (Silva et al, 1994). The trabecular bone is structured into vertical and horizontal struts providing a light, strong support structure to assist in resisting the buckling of the cortical shell in compression or bending (Thomsen et al. 2002). The trabeculae are orientated along the axis of greatest stress and strain (Middleditch et al, 2005) this is in accordance to wolf s law that states that the orientation of bone aligns itself in the direction of the greatest stress (Chamay et al,1972). The pores between the struts are filled with marrow and other cells to provide nutrition and life support (Keaveny et al, 2001). In the posterior 1

11 2 region, the posterior elements or posterior arch is composed of the spinous process, transverse processes, lamina, articular facets, and pedicles (Middleditch et al, 2005). The spinous process can limit the amount of extension that the bending of the joints is allowed as well as providing attachment points for ligaments, tendons and muscles (Middleditch et al, 2005). Transverse process on each of the lateral sides limits the amount of lateral bending as well as providing attachment points for ligaments, tendons and muscles (Middleditch et al, 2005). Pedicles connect the transverse processes to the vertebral bodies. Laminae connect the spinous process to each of the transverse processes. Articular facet joints act in conjunction with the intervertebral disc to connect vertebrae to the adjacent level (Middleditch et al, 2005). The intervertebral disc and articular facets provide flexibility and due to relatively small young s modulus provides a reliable point for bending, rotation and deformation. 1.1 Anatomy of the Vertebra The purpose of the vertebra is to transmit load through the spine, provide for attachment points for ligaments tendons and muscles, and create space between the pelvis, thorax and head (Drake et al, 2010). The vertebrae must resist compressive and shear forces and bending and rotational moments. The vertebral body contains an outer shell of hard cortical bone less than 1 mm of thickness (Silva et al, 1994), young s modulus of 18.6 GPa, encasing soft cancellous trabecular bone young s modulus of 10.4 GPa as measured using a microtensile test (Rho, Jae Young et al, 1993). The trabecular bone is composed of vertical and horizontal struts to increase the strength and resist the buckling of each vertebral body (Aebi et al, 2005).

12 3 1.2 Anatomy of the Cervical Spine The cervical spine is primarily utilized to support the head and allows for great amount of range of motion and flexibility. It is composed of 7 vertebrae including the atlas and axis which provide for the greatest amount of rotational movement (Middleditch et al, 2005). Lower cervical vertebrae have a standard vertebrae structure with vertebral body, pedicles, spinous and transverse processes, lamina and articular facets. It contains a lorodosis curve. In normal usage it must resist compression between 120 N to 1200 N which is the force resulting from the support of the head and muscles of the neck (Patwardhan et al, 2001). The cervical spine can move by rotation, flexion, extension and lateral bending (Shea et al,1991; Prybyla et al, 2007; Moroney et al,1988; Nixon et al,1986). One unique feature of this region is the shape of the endplates: they curve upward in the lateral region to form convertebral joints and unconvertebral processes otherwise known as the joints of Luschka (Middleditch et al, 2005). Due to the concavity and convexity of the endplates of the vertebral bodies the amount of flexion and extension is greatest but lateral bending is limited due to the contact between the joints of luschka (Middleditch et al, 2005). These processes form a U shaped cup that allows the superior vertebrae to fit into a socket of the inferior vertebra cushioned by the Intervertebral disc (Middleditch et al, 2005). Other regions of the spine lack the unconvertebral joints. The articular facets are are oriented near horizontal to allow for greater rotational movement and great degree of freedom (Pal et al, 2002).

13 4 Figure 1 Cervical Spine (Brockway 1893) 1.3 Anatomy of the Lumbar Spine The lumbar spine supports the majority of the upper body weight and as a result these vertebrate are large and can withstand a large amount of force, exceeding 1200 N (Patwardhan et al, 1999). It is made up of 5 vertebrae L1 to L5 and a lorodosis curve (Middleditch et al, 2005). The intervertebral discs are large compared to the other regions and are used to cushion the load. The orientation of the articular facets is near vertical (Van Schaik et al, 1985). Compared to the cervical spine the posterior elements in the lumbar are more posterior whereas the cervical spine the elements are more lateral. The endplates are flat and parallel to the intervertebral disc. The transverse processes are more defined and pronounced in the lumbar spine they also situated more posteriorly to allow muscle and tendon attachment points (Middleditch et al, 2005).

14 5 Figure 2. Lumbar vertebra (Brockway, 1893) 1.4 Soft Tissue Anatomy In addition to the bony segments the spine is also composed of soft tissue which is made up of all of the connective tissue surrounding the spine including ligaments, intervertebral discs, articular facet joints, tendons and muscles. The primary purpose of these connective tissues is to passively or actively resist forces and motions in all directions: axially, laterally, anterior-posteriorly, and twisting.

15 6 1.5 Intervertebral Disc The intervertebral disc is located between each vertebral body of the spine. The intervertebral disc is a composite structure which is composed of a liquid nucleus pulposus in the center which is surrounded by the annulus fibrosis made up of layers of fibers (Phillips et al, 2010). The superior and inferior ends of the intervertebral disc consist of cartilaginous endplates which act as the interface between the rest of the disc and the vertebral body. The endplates are composed of hyaline cartilage consisting of collagen, proteoglycan, and water which allows for nutrient flow into and out of the disc by diffusion (Phillips et al, 2010). 1.6 Annulus Fibrosis The annulus fibrosis is composed of multiple layers of fibrous cartilage which contain the liquid nucleus pulposus. It provides structural support and vertical height to the IVD. Physiological differences may exist between the cervical and lumbar IVDs, particularly in the annulus. In the lumbar region the annulus fibrosis is made up of alternating layers of concentric layers of fibers. In the lumbar region it is generally accepted that the nucleus pulposus is completely surrounded by concentric alternating or a crisscross pattern layers of fibers (Mercer et al, 1999). The thickness of the annulus fibrosis is about even all around the disc (Phillips et al, 2010). On the other hand, the cervical spine intervertebral disc has received debate on its structure. In current studies that dissected the cervical intervertebral disc the researchers discovered significant differences in the make-up of the cervical intervertebral disc (Mercer et al, 1999). In the cervical spine, it has been shown; that the annulus fibrosis in the anterior region consists of thick crescentric mass of collagen and

16 7 that taper laterally and posteriorly toward the uncinate processes until it reaches the posterior-lateral region where it stops (Mercer et al, 1999). It is nearly none existent in the post lateral and posterior regions of the spine. The fibers are oriented upwards toward the midline of the sagittal plane the fibers lack the concentric layers of crisscross fibers that are found in the lumbar spine (Bogduk et al, 2000; Yoganandan et al, 2001). With different anatomic setup the cervical spine is bound to have different physiological and mechanical behaviors under stress and strain. 1.7 Nucleus Pulposus The nucleus pulposus is an avascular area of liquid. Because it is liquid it does not compress but it expands outward maintaining its volume as the spine is compressed axially. When the spine is compressed, the nucleus pulposus expands outward but is constrained inside the annulus causing the annulus to expand and bulge (Jongeneelen 2006). As a person ages and the disc begins to degenerate the nucleus pulposus becomes more fibrous and turns white, losing its non-compressible properties. (Jongeneelen 2006) The Nucleus Pulposus is composed of chondrocytes, proteoglycans and water (Jongeneelen 2006). 1.8 Articular Facet Joints Articular facet joints are synovial joints which are surrounded by soft tissue that lubricate the joint. Facet joints allow smooth movement between vertebrae. They resist motion through bony interactions and aided by ligaments. The facet joints consist of a pad of cartilage between two bony elements.

17 8 In different levels of the spine the facet joints change orientation from near horizontal in the cervical region (Pal et al, 2002) to near vertical in the lumbar region(van Schaik et al, 1985) as explained above and the orientation of which determines the level and ability of that level. This change in direction may dictate the range of motion in that region and at various levels (Van Schaik et al, 1985). 1.9 LIGAMENTS Ligaments are a form of soft tissue that connects bone segment to bone segment. They are composed of collagen fibers aligned together (Drake et al, 2010). The primary purpose of Ligaments is to passively prevent tensile movements in the joint and to resisting tensile loads and are composed of fibers which act uniaxially (Drake et al, 2010). There are usually multiple ligaments on different sides of the joint. Each ligament usually has multiple layers of fibers each having slightly different attachment points to the bone (Yoganandan et al, 2000). The ligaments are made of fibers oriented in distinct layers and patterns (Mercer et al, 1999) and each layer can be limited to crossing one vertebrae or multiple vertebral segments. There are many ligaments that are associated with the spinal motion segment. The anterior longitudinal ligament is positioned on the anterior of the vertebra and is attached along the entire spine. The posterior longitudinal ligament is attached on the posterior region of the vertebral body and spans the entire spine. The anterior longitudinal ligament and the posterior longitudinal ligament may play a role in preventing the annulus fibrosus of the IVD to bulge. Capsular ligaments enclose the articular facets and provide lubrication to the synovial joints.

18 MUSCLES Muscles actively keep the spine erect and stable under compression and movement. Each muscle is connected to a tendon which in turn is fixed to the bone. The muscle can connect single levels or connect multiple regions (Drake et al, 2010). In Patwardans et al, 2001 study a hypothesis was introduced suggesting that the muscles exert forces to causes the resulting force to go directly through the center of rotation, the follower load. The muscles balance the load through the spine and maintain the erect structure of the spine. (Patwardhan et al, 2001).

19 2. MECHANICS OF THE SPINE The spine is designed to maintain structure and provide controlled movement by resisting forces and moments in three dimensions. Forces include compressive, tensile, and shear forces in addition to bending and torsion moments The spine is a complex structure mechanically; each level of the spine has limited independent movement although some movements are coupled between the different levels. An example of this coupled motion is in the cervical region where lateral bending is coupled with rotation (Panjabi et al, 2001). Because of the variability in the anatomy of each level there are significant differences in the mechanical responses of each level. When one applies a force on a specimen, the specimen will have a deformation response. The vertebra is assumed to be a rigid body and therefore most of the deformation in the spine occurs in the intervertebral disc. The linear ratio between the load and deformation is termed the stiffness. Stress is a force that is distributed over an area; a strain is a ratio between the deformation and length of an object. The stress and strain are normalized measurements of force and deformation used to minimize the geometrical factors associated with the specimen. The linear ratio between stress and strain is called the elastic modulus or Young s modulus. Elastic modulus is fundamentally defined: Elastic Modulus = (Rand et al, 2005) EQ1 10

20 11 Mechanical properties of an object depend on both the geometric properties and material properties of the object. Geometric properties depend on the shape of the object and are constant in all objects of the same shape whereas the material properties depend on how the object is composed and varies depending on the material used and forces applied (Rand et al, 2005). Depending on the material an object can deform uniformly, an isotropic material, no matter the orientation of the object of force applied to the object or non-uniformly, an anisotropic material (Rand et al, 2005). An isotropic material is one that has the same mechanical material properties in each plane when considering a point of the object (Rand et al, 2005). On the other hand an anisotropic material is one that has different material properties in each direction. General material properties depend on a number of factors including the orientation of individual fibers or atoms and whether they are orientated along a single axis or orientated randomly in a volume (Rand et al, 2005). As an object is stressed it is strained and similarly if the object is strained then it becomes stressed. One way to visual this relationship is in a stress-strain curve. The initial constant slope is called the elastic modulus and is a measurement of the stiffness of the material. The steeper the slope, the more stiff the material. 2.1 BIOMECHANICS OF THE SPINE SEGMENT Due to the alignment of fibers in the collagenous matrix the annulus fibrosis material can be considered to be anisotropic in all levels of the spine. Although each fiber individually is monotropic, as a whole the annulus is anisotropic due to the layered design with alternating fiber directions. Therefore strain in the anterior direction is

21 12 different from strain in the axial and lateral direction whereas the lumbar spine with its concentric layers has a somewhat consistent response to force applied to the sample. In addition to the stiffness difference another key aspect of the disc which allows for healthy repeatability of movement the fluid filled nucleus pulpopus. Water has an important property in that it is an incompressible material; therefore when it is trapped in a container and the container is compressed the fluid expands outward in all directions causing the container to bulge (Jongeneelen 2006). As the disc is compressed the fluid expands stretching the fibers in the annulus which are strongest in the axial forces tension and compression. Therefore instead of bending and outward bulking of fibers the liquid is forced against the inner wall making the fiber taut and act in tension. This prevents the unregulated bending or compression of the fibers reducing the risk for fiber fracture (Jongeneelen 2006). If a disc with a degenerated nucleus is compressed the fibers in the annulus experiences a compressive force which buckles the fibers because the fibers are weak in compression increasing the risk of fracture in the fibers and therefore permanent damage as the fibers are bent and torn (Jongeneelen 2006). All of these factors affect the mechanics of the disc. As a functional spinal unit (FSU) is compressed the deformation is assumed to occur in the disc because of the great differences in the elastic moduli between the vertebral body and the IVD. The gap distance between the vertebra decreases. Because the volume of the IVD is considered to be constant when there is deformation in the disc height then there is deformation in the anterior, posterior, and lateral directions.

22 13 In natural physiological in vivo situations these deformations occur in daily life. Bulge is defined as the relative changes in deformation of the IVD in each direction, anterior, posterior, or lateral, compared to a preloaded condition. Extreme bulge in any direction that results in the failure of the annulus fibrosis is considered to be a herniation.

23 3. REVIEW OF EXISTING IVD EXPERIMENTAL MECHANICAL TESTING STUDIES There are limited studies pertaining to the response of the cervical and lumbar spine. Many studies investigate the range of motion of the motion segments. There are few studies that investigate the deformation response of the IVD under compression of the lumbar spine and even fewer investigating the cervical IVD response to compression. 3.1 Current Literature of lumbar specimen testing There are limited amount of studies examining the deformation response of the Lumbar disc in various situations. Various studies have examined the axial deformation or the bulging response of the IVD individually but there are few papers that analyze the axial deformation and the bulging of the disc at the same time. Methods of data collection and analysis vary from study to study. Some studies utilize transducers, lasers or imaging to determine the amount of bulge under various loads. Additionally these studies contained a wide variance in terms of set-up of the in-vitro experiments specimens in some studies were constrained in the inferior or superior vertebra or both. Posterior elements were retained in some studies and removed in other studies. In 1974, Keith Markolf completed a study in which 24 Functional Spinal Units (FSUs) were placed unconstrained on a leveling fixture. The posterior elements were removed from the specimens. Each specimen was compressed to 1000 N. A transducer was used to measure the axial deformation of the specimen which resulted in a vertical deformation of 0.7 mm (Markolf et al, 1974). In order to analyze the effects of degeneration the specimens were visually examined. This study by Markolf took only 14

24 15 into account axial deformation of the disc therefore the relationship between the deformations in different directions was not analyzed. A number of studies have measured the amount of bulge under axial compression but do not report the amount of vertical deformation. In 1978, a study done by J.S. Shah in which six specimens with two discs and three vertebral body segments were tested. The specimens were stored frozen at -25 degrees Celsius in polythene bags. The specimens were constrained between two parallel plates and compressed. Transducers were used to measure the bulge of the IVD when compressed to 490 N (50 lbs.) and 981 N (100 lbs.) the amount of bulge was 0.4 mm at 490 N and 0.6 mm at 981 N (Shah et al, 1978). Another study by Lin in 1978, 19 fresh FSU s were tested at humidity levels % with the posterior elements retained. The specimens were loaded to 450 N. Strain gages were used to measure the bulge and vertical deformation. The specimens were constrained in a polyester resin during testing; the amount of bulge in the anterior direction was found to be mm, in the left lateral region: 0.43 mm and in the right lateral region: mm. The specimens were graded for disc degeneration using the Galante grading scheme where 1 is a normal disc 4 is a severely degenerated disc (Lin et al, 1978). The goal of this study was to investigate the response of complex loading on the lumbar specimens.

25 16 Figure 3 Specimen preparation. As shown here from Lin The vertebrae are constrained in a polyester resin (Lin et al, 1978) A third lumbar study by Karl Wenger in 1997 took 16 Lumbar FSU segments which were stored frozen. The posterior elements were redacted and the specimens were compressed from 100 N to 2500 N to ensure plastic deformation. The specimens inferior vertebra was embedded in an alloy and the superior vertebra was prevented from sliding. The specimens were then measured for amount of bulge by imaging the specimen before loading and at maximum loading: the mean anterior bulge was found to be 0.37 mm with a standard deviation (SD) of 0.27 mm and range mm. It was found to have a mean lateral bulge of 0.65 mm SD 0.42 and range mm

26 17 (Wenger et al, 1997). The aim of this study was to measure the disc bulge by using a non-contact method. Figure 4. Specimen Image after testing as shown here from the Wenger 1997 study. The IVD is clearly bulging. (Wenger et al, 1997) In 2008, Frank Heuer did a series of papers measuring the bulge of six Lumbar FSUs. The specimens were preconditioned to 500 N to reduce water content. The FSUs were coated with an elastic spray in order to increase the roughness pattern on the disc so that image processing algorithms can be used to analyze the output from lasers which were used to image the disc bulge. In the experiments, the FSUs were compressed to 500 N with the posterior elements intact and the bulge was measured to

27 18 have a mean anterior bulge of 0.7 mm with a range of mm and a mean lateral bulge of 0.23 mm and range of mm (Heuer et al, 2008). The aim of this study was to determine the relationship between disc bulge and surface strain. Another study was completed in 2010 by Grace O Connell. In this study, the posterior elements were removed and 20 FSUs were precondition to 20 N. The specimens were then rapidly compressed to 1000 N and held at 1000N for 20 minutes. At which time the specimens were imaged with high resolution magnetic resonance imaging. The mean anterior bulge under 1000N was found to be 0.9 mm, range of mm, the mean lateral bulge was found to be 0.5 with a range of mm. The degeneration was based on the T1 MRI images and the relaxation time (O Connell et al, 2010). The aim of this study is to measure the disc mechanics under axial loads and using MRIs to determine strains and correlate with disc degeneration. There were only a few studies examining the relationship between vertical deformation and lateral, anterior, or posterior bulge in the lumbar IVD. One such experiment was in 2010: M. Cuchanski did a study in which 15 specimens were stored frozen at -20 degrees Celsius. The posterior elements were removed and the specimens were rigidly fixed into mounting devices. The disc bulge was measured by placing three LEDS on the disc and using an Optotrack motion tracker system to measure the change. It was shown that when the specimen was compressed to 250 N the mean lateral disc bulge is 0.4 mm and range of mm. It was also shown that the lumbar disc vertically deformed to 0.61 mm with a range of mm. The discs degeneration was graded on a scale: 0-healthy 1-mild degeneration, 2-moderate

28 19 degeneration, 3-severely degenerated. (Cuchanski, et al, 2010). The aim of this experiment was to measure the disc bulge under dynamic loading conditions. Figure 5. Specimen preparation and testing set up for the 2010 Cuchanski study (Cuchanski 2010)

29 20 Table I. Comparison of the results of the studies and deformation Values Mean Anterior Bulge Mean Lateral Bulge Author Year Load (N) Mean Axial Deformation Markolf Lin Shah Wenger Heuer Cuchanski O Connell Mean Posterolateral Bulge 3.2 Current literature of cervical specimen testing In contrast there have been limited studies done on the cervical spine. Most cervical spine studies investigate range of motion, roles of anatomical components, axial deformation among others. There is limited information on the bulging of the intervertebral discs of the cervical spine. Moroney completed a study in which 35 cervical motion segments were tested in compression, shear, flexion, extension, lateral bending and axial torsion tests. The segments were constrained in a testing apparatus. The posterior elements and ligaments aside from the longitudinal ligaments were excised from the specimens. A preload of 50 N was applied. The stiffness was measured in various loading conditions and motions. (Moroney et al, 1988) The goal of this study is to measure the load displacement behavior of the cervical spine in different loading configurations including

30 21 compression, shear, lateral bending, torsion flexion and extension. Moments were measured at failure. M. Shea completed a study in 1991 in which 27 cervical spinal units specimens with two discs and three vertebral body segments with the posterior elements retained were taken constrained in Poly methyl methacrylate resin and compressed in different loading methods including compression, flexion, and extension. The pure compression test cycled from 200N in tension to 800 N in compression at a rate of 5 mm/min for a vertical displacement of 0.7 mm (Shea et al, 1991). the goal of this study was to investigate the fracture mechanics and ligamentous injury of multi-axial forces. In another study by Andrzej Prybyla in 2007, 22 cervical Functional Spinal Units (FSU) were stored frozen at -20 degrees Celsius, secured in aluminum cup with plaster and were compressed until failure at 2000 N and deformed 1.5 mm in the vertical direction. (Prybyla, et al, 2007). The goal of this experiment was evaluate the strength of the cervical spine under compression and bending. Another goal of this study was to assess the resistive abilities of each structure in bending.

31 4. SPECIFIC AIMS TO MEASURE AND ANALYZE THE IVD BULGE It has been shown that the when a functional spinal unit is compressed the Intervertebral discs bulges. In the lumbar region the bulge occurs in all directions anterior, lateral, and posterior. But in the cervical region, due to the anatomical and physiological differences when compared with the lumbar region it is unclear how the disc responds to compression. In preliminary testing the anterior bulging of the disc plateaued at a value of 0.1 mm whereas the vertical deformation continued to increase, therefore the problem is why the bulge in the anterior plateaus and how the disc compensates for such a phenomenon. This information is useful in determining the natural response of the IVD under compression so that in the future spine segments with ailments can be tested and compared to baseline data. The objective of this study is to analyze 11 normal cervical Functional Spinal units and 7 lumbar normal FSUs and characterize the IVDs response under compression. Understanding the normal response for IVDs will allow researchers to better characterize abnormal responses and clinical ailments explained previously. Utilizing engineering principles of material properties stress strain relationships and loading properties data will be taken from the functional spinal units. In particular analysis will focus on the anterior bulge which is used as a comparison standard through the testing as the specimens are modified with laminectomies and facetectomies in order to measure the posterior bulge. The posterior region is important to analyze because when the disc bulges in the posterior region 22

32 23 there is risk of herniation and interaction with the spinal cord therefore a series of specific aims were developed as explained below. The first aim of this dissertation is to measure the anterior disc bulge of the cervical spine and to validate the methods of measurement of spine intervertebral disc bulge by using Linear Variable Differential Transformers and compare the results to literature. Due to limited literature on the cervical disc bulge the lumbar specimens were used as a comparison to literature. The second aim is to analyze the posterior bulge in healthy specimens. There are many ailments that affect the bulge like spondyloysis or scoliosis or have a high risk of occurring due to bulge, herniation. Therefore data collected on the posterior bulge in a normal patient can be used to characterize the intervertebral disc. A third aim is to determine the mechanical reasons of the stiffening of the anterior IVD. This includes analyzing the disc for anisotropy as properties in each direction are different. One method to acquire this is to perform laminectomies and facetectomies to analyze the changes in bulging patterns and magnitudes. Lastly the role that the articular facets have on the anterior and posterior disc bulge is assessed. This can have direct influence in analyzing the effects of damage articular facets. Depending on the responses of the bulges it could signify a change in the center of rotation or translation of the vertebral bodies relative to each other. 23

33 5. METHODS OF EXPERIMENTAL TESTING 5.1 Specimen Preparation This study was divided into two phases, phase 1: intact testing consisting of 11 cervical specimen and 7 Lumbar specimens and phase 2: modified specimen testing consisting of 6 cervical and 6 lumbar specimens in two configurations: laminectomy and facetectomy. The specimens were first tested intact in order to maintain conditions found in a physiologically healthy subject. With the intact specimens it was not feasible to measure the posterior bulge therefore in order to gain access to the posterior region the posterior elements were removed in stages: first the spinous process with a laminectomy then the articular facets were removed to assess the role of the articular facets. In order to minimize the complex responses of the spine including coupled motion and moments a Functional Spinal Unit (FSU) is used for testing. A FSU consists of adjacent vertebra and the associated soft tissues including the intervertebral disc, articular facets and ligaments. In the first phase five intact cervical spines were sectioned into 5 C3-C4 Functional Spinal Units (FSUs) 4 C5-C6 FSUs 2 C7-T1 FSUs. Three intact lumbar spines were sectioned into 2 T12-L1 FSUs, 2 L2-L3 FSUs and 3 L4-L5 FSUs which were used as validation and comparison. The FSUs were cleaned of muscles and tendons but the IVDs and associated ligaments were left intact. The specimens were dissected such that a smooth level surface was prepared on the superior and inferior sides of the FSU to allow for symmetric and uniform load distribution as shown in Figure 9. The posterior elements and ligament structures remained on the specimen to mimic physiological conditions. The specimens were stored frozen in a negative 20 degree 24

34 25 Centigrade freezer. The specimens were thawed slowly in a refrigerator for 24 hours before testing. The specimens were thawed and were placed, unconstrained, into an Instron Mechanical Testing machine 5500 series uniaxial load system. 5.2 Mechanical Testing Protocol A preload of 50 N was applied before testing to ensure full contact between the load cell rod and the specimen. This also serves to minimize slippage of the specimen during the cyclic loading cycle. 50 N was chosen as it is comparable to load the head applies to the cervical spine (Moroney 1988). Linear Variable Differential Transformers were positioned on the specimen to measure the displacement of the IVD and vertebra. Each specimen was then compressed in the machine for 3 cycles for each maximum load. Cyclic loading was used, as a simple method to allow for comparison of the LVDT results to the instron results. The maximum load increased from 100 N to 550 N in steps of 25 N. The range 100 N to 550 N was chosen because it includes the physiological normal force. Therefore by applying a load which includes the range up to 550 N it provides a range of data that can be used to map the response of each sample. These tests were completed at a rate of 1 mm/min which provided for static testing conditions. The change in vertical deformation was measured using the values provided by the Instron machine 5500 series dual column tabletop uniaxial loading testing machine, load limit 50 kn. The software controller for the Instron used was Merlin. Schematic of experiment set-up is shown in figure 6.

35 26 Figure 6. Diagram of Anterior Deformation and Load Directions. There was an axial load applied to the specimen the deformation was measured in the vertical and anterior directions. 5.3 Positioning of Linear Variable Differential Transformer on the Specimens During the testing, the bulge was measured using Linear Variable Differential Transformers (LVDTs) Trans-tek series 330 miniature AC gaging LVDT sensors (accuracy of +/- 0.25%) nonlinearity less than 0.2% of full scale. The sensors were powered by 30 Volts DC. The signals were converted into a visual display utilizing labview software and data outputted to text files. Flat tips were attached to each of the

36 27 LVDTs, diameter of 5 mm, in order to reduce the amount of pressure needed to displace the sensors. Two LVDTs were placed to measure the relative displacement of the disc to the bone. Due to the size of the cervical vertebrae, it was more effective in the intact samples to place one LVDT on the anterior side of the specimen measuring the displacement of the disc and the other LVDT was placed on the posterior elements positioned on the spinous process to measure the displacement of the bone. Each sensor was adjusted such that the sensor was partially displaced to ensure recording of any movement of the sensor whether it be extension or contraction which signifies a translation of the vertebra, or slippage of the IVD or vertebra. The sensor on the disc was position such that it measured the maximum disc deformation. In this study the bulge is measured as the relative change of the two sensors to the relative displacement of the bone. Only the anterior bulge in the intact cervical specimens was measured because the posterior elements were included to obtain realistic results which reduce space available for placing sensors and measuring the posterior regions of the disc as shown in figure 7 for the lumbar spine and figure 8 in the cervical spine. The lateral sides of the cervical specimens were similarly blocked due to the transverse processes and transverse foramina as well as the unconvertebral processes preventing reliable measurement of the lateral bulge in the cervical specimens. For the lumbar specimens, a similar protocol was used. In addition to measuring the anterior bulge with sensors, an additional two sensors were added to measure the displacement of the lateral disc and bone as in figure 10 in the lumbar spine. This was possible due to the anatomical differences between the lumbar and cervical regions of

37 28 the spine for instance the increased size and lack of lateral anatomical features in the lumbar region. Figure 7. Experimental Setup Lumbar Specimen

38 29 Figure 8. Sensor Placement Intact Cervical specimen The specimen contains two sensors the anterior sensor measures anterior bulge, the posterior sensor tracks the displacement of the bone movement Experimental Data Processing and Analysis After data collection the data was process and analyzed. Noise was reduced using a moving average. A matlab script was developed to automatically measure the peaks and valleys of each cycle at each load. The data was separated into each cycle and the peak and valleys were taken from each cycle. The differences in the peaks and valleys for each cycle of each load were calculated and denote the distance traveled by each sensor. Incorporating the direction traveled of each sensor and the position of

39 30 each sensor (one for the deformation of the IVD and one for the movement of the bone) the relative motion of the disc to bone was calculated for each cycle this was considered to be the bulge for the cycle. The average bulge of the three cycles was then taken providing for the bulge of each load. The average bulges for each load of each specimen were graphed against each load value results shown below in the results section. Using the graphs and figures the trends were analyzed and compared to the other specimens. In order to measure a characteristic property of the specimens and further validate the methods the rigidity of the IVDs was measured. The rigidity in the axial plane was calculated using the equation: Rigidity MPa = EQ 2 Rigidity is a measurement of the stiffness of the IVD in the vertical direction. The loaddeformation ratio is comparable to the stiffness of the specimen was based on the slope of the initial linear portion of the load deformation curve in the axial direction. The initial disc height was measure using a digital micrometer caliper. The cross sectional area in meters 2 is estimated as shown below. In order to calculate the cross sectional area, the area of the vertebral body was considered to have the same area as the IVD, the measurements were manually taken of the distance between the lateral sides of the disc (coronal plane) and the distance from the anterior edge to the posterior edge of the disc (sagittal plane) of the IVDs. The IVD was assumed to be defined as the same area as the vertebral body measurements

40 31 were taken with a digital caliper, accuracy 0.01 mm. Next, images were captures of each specimen in the axial plane. The images were then uploaded to Rhinoceros modeling software. The cross-sections were reconstructed using NURBS curves scaled to actual specimen measurements in order to calculate the area. By taking into account the real life measurements of the disc dimensions in the sagittal and coronal planes the image was scaled in two dimensions and the area was calibrated appropriately to evaluate the cross sectional area in the axial plane. In order to determine statistical significance for the anterior and posterior bulge for the small sample size (n=6) a paired student t-test was used for each applicable configuration of the lumbar and cervical samples. 5.5 Phase 2. Specimen Preparation and Testing: Laminectomy In order to measure the posterior bulge six cervical and six lumbar FSU s were modified to gain access to the posterior region. A laminectomy was performed to remove the posterior arch. A second round of testing was completed in which the posterior bulge was measured. Laminectomies were performed to remove the spinous process and approximately half of each lamina as shown in figures Cuts were made using a narrow bladed hand saw through each of the four laminae on each specimen. The articular facet joints were retained. The rest of the specimen was left unmodified: the posterior longitudinal ligament was left intact. This method provided the most reliable positioning of the sensors. Additionally because of the removal of the spinous process the LVDT sensor was place perpendicular to the plane of the posterior IVD wall.

41 32 Methodology and analysis was performed the same as stated above. Processing and analysis of the additional posterior sensor information will be completed using the same methodology as stated above. The LVDT sensor contacting the bony portion of the FSU was positioned such that it was directly superior or inferior to the sensors measuring the disc deformation depending on the space available. Figure 9. Cervical Laminectomy Cut The spinous process was removed leaving the articular facets intact.

42 Figure 10. Lumbar Laminectomy The spinous process was removed leaving the articular facets intact. 33

43 Figure 11. Cervical laminectomy mechanical test Sensor placement on the cervical specimen. Posterior and anterior bulges were measured. 34

44 Figure 12. Lumbar laminectomy Mechanical setup Sensor placement on the lumbar specimen. Posterior, anterior, and lateral bulges were measured. 35

45 Phase 2. Specimen Preparation and Testing: Facetectomy Additionally after laminectomy testing the articular facets were removed in a facetectomy to determine their roles in IVD bulge and retested the specimen in the above manner. The superior articular facets were removed by making cuts through the pedicles as shown in Figure This phase of testing contributed information of the effects of the articular facets on the bulging of the IVD. In the case of the cervical specimens the only elements left on the specimens in addition to the vertebral bodies were the inferior facets, transverse foramina. In the lumbar region only the vertebral bodies and inferior facets remained. Testing and placement of the sensors was like that of the laminectomy testing.

46 Figure 13. Cervical facetectomy posterior view 37

47 Figure 14. Cervical Spine C5-C6 facetectomy mechanical testing Upper posterior view. 38

48 39 Figure 15. Lumbar facetectomy A narrow hand saw was used to remove the spinous process leaving the posterior longitudinal ligament exposed.

49 Figure 16. Lumbar facetectomy mechanical testing. In this specimen the inferior facets are removed instead of the superior facets. Mechanically there is no difference. The Inferior Facets were removed because of relative thinness of the inferior pedicles. 40

50 Specimen Dissection and Inspection After the testing phase was completed each specimen was dissected by an orthopedic resident surgeon an example of the cut made is shown in figure 17. The IVD was then examined for damage to the disc and bone, degeneration of the disc and quality of the bone. In five of the six lumbar specimens a sagittal cut was made through the midline of the specimen. This provided information on the quality of bone relative degree of osteoporosity, as well as disc height through the sample. In three of the lumbar specimens additional slices were made just lateral on each side of the initial sagittal cut. Afterwards an axial cut was made through the center of the IVD to examine the quality of the IVD in terms of degeneration and evidence of fracture. Cervical Spine specimens were similarly dissected. It was realized that axial cuts through the specimens destroyed the IVD due to the small disc height and interference from the joints of lucshka, therefore where possible, axial cuts were made otherwise only sagittal cuts were performed in the cervical specimens.

51 Figure 17. Example of Sagittal Cut Specimen L2-L3 42

52 6. RESULTS OF EXPERIMENTAL TESTING 6.1 Phase 1 Results Intact Cervical Spine Five cervical spines with the posterior elements intact: five C3-C4 FSUs, four C5- C6 FSUs and two C7-T1 FSUs were tested. Considering the average values, anterior IVD bulging value increased up to about 0.11 mm. At a load value of 350 N the slope became steeper as shown in figure 18; however, the axial deformation continued to increase as shown in figure 19. Stiffness is defined as the slope of the load deformation curve. The average slope (stiffness) of the initial anterior displacement is 2700 N/mm then increases three fold to 8400 N/mm. The average slope of the axial deformation is 1000 N/mm as shown in figure 19. On examination of the specimens there was no IVD herniation and the endplates remained intact. Calculated axial rigidity values are shown in Table 2. The rigidity was evaluated using the equation: =

53 Load (N) Load (N) Average Intact Cervical Anterior Bulge y = 8416x y = x Deformation (mm) Figure 18. Average Anterior Bulge Intact Specimens Average Intact Cervical Vertical Deformation y = x Deformation (mm) Figure 19. Average Vertical Deformation for Cervical Intact Specimens

54 Load (N) 45 The intact cervical IVD deformation results were then compared to existing literature. Due to the lack of bulging data in literature the gap height (axial deformation) was used for comparison. As shown in figure 20 below, the current study compares well with existing literature in a linear relationship (Shea et al, 1991.Prybyla, et al, Moroney et al, 1988) Cervical Literature cpmparison Gap Height Prybyla, Moroney, 1988 Current Study, 2012 Shea, Deformation (mm) Figure 20. Comparison of the Gap height change in Cervical Spine studies

55 46 The rigidity values were calculated using equation 2 and the methodology stated above and are presented in Table 2 below. The higher the rigidity value the more rigid the specimen which decreases the amount of bulging. Table II. Table of rigidity values Cervical Specimens Rigidity (MPa) Lumbar Specimens Rigidity (MPa) 1 C3-C T12-L C3-C L2-L C3-C L4-L C3-C T12-L C3-C L2-L C5-C L4-L C5-C L4-L C5-C C5-C C7-T C7-T It has been shown in the studies that as the specimens continue to be deformed in the vertical direction the bulging in the cervical anterior bulge stiffens. It appears that the anterior bulge stiffens at higher loads. The lumbar rigidity matches with existing studies (Smeathers, et al, 1988).

56 47 Rigidity is a measurement of normalized stiffness much like the modulus of elasticity. By comparing the rigidity values shown in Table 2, it is shown that the rigidity of the cervical specimens are greater that the lumbar specimens. This is also shown in the relatively low amount of bulge in the cervical region compared to the lumbar region. This supports the data in the limited deformation some of the cervical specimens experienced and the dissection images. Depending on the level, there may also be anatomical considerations for the relationship between bulge and rigidity. For instance, in the T12-L1 specimens there was less posterior bulge than the other levels. One possible influence of this low posterior bulge is that this level is the thoraco-lumbar junction which may pinch a portion of the posterior disc because the spine changes from a kyphoris curve to a lorodorsis curve. 6.2 Phase 2 Results Laminectomy Cervical Spine Six of the cervical specimens and six of the lumbar specimens were tested and the results are shown below. The results are displayed in a number of different methods including analysis of the response of each specimen, comparison of each level of the spine, analysis of the response of the disc in each configuration under max loading, and ratio analysis of the posterior to anterior bulge of each configuration. The results for the vertical deformation, anterior bulge and posterior bulge were analyzed and presented below in figures As shown in figure 24a the anterior bulge shows signs of stiffening in the anterior region though not as well define still occurs in the 300 N to 400 N range. Average anterior bulge seems to be limited to 0.1

57 Load (N) 48 mm. The posterior bulge was analyzed due to interference from the anatomical structures in the posterior arch therefore a full laminectomy was performed. The anterior bulge was measured to maintain continuum in the data and to determine the effects of modification Vertical Cervical Deformation: Laminectomy Deformation (mm) Average Gap Height change Figure 21. Vertical deformation perpendicular to the axial plane.

58 Load (N) Load (N) 49 Anterior Cervical Bulge: Laminectomy Average Anterior Bulge Deformation (mm) Figure 22. Anterior Bulge in the cervical Spine 600 Posterior Cervical Bulge: Laminectomy Average Posterior Bulge Deformation (mm) Figure 23. Posterior Bulge Cervical spine under laminectomy

59 Load (N) Load (N) 50 In the intact configurations as shown in figure 18 the anterior bulge increases in stiffness by a factor of three from about 2700 N/mm to 8400 N/mm. This phenomenon can be explained in a number of ways. If bulge stops or slows in one direction but the specimen continues to lose gap height then the specimen must bulge in a different direction for example the posterior region. Further analysis was provided through the laminectomy testing. The bulging response of the anterior IVD and posterior IVD were compared to assess the properties of each region as shown in figure 24 A and B. Figure 24A. Anterior Cervical Bulge: Laminectomy Figure 24B. Posterior Cervical Bulge: Laminectomy y = x R² = y = 4386x R² = y = x R² = Deformation (mm) Deformation (mm) Figure 24a,b. Anterior and posterior IVD bulge slope comparison

60 51 There appears to be no stiffness change in the posterior region as shown in Figure 24b. On the other hand the slope of the anterior bulge doubles from about 5000 N/mm to 10,000 N/mm. Although the magnitude of the posterior bulge is less than the anterior bulge as the load increases further it can be extrapolated that the posterior bulge amount will exceed that of the anterior bulge. This anisotropic behavior signifies that the posterior direction and anterior direction have different bulging properties where at lower loads the anterior region bulges but at higher loads the posterior direction bulges more. The reason that the testing was limited to 550 N was to reduce the risk of fracture and prevent plastic deformation. One reason for this shift in load that the change in slope occurs is that by removing the posterior arch it increases the instability of the specimen. This instability of the segment may create a shearing effect in the samples which influences the relative deformation of the IVD. Because access to the lateral region of the cervical spine is blocked by several anatomical structures and removing them will disrupt the structure of the disc it is unclear as to the amount of lateral bulge occurring in the specimens. 6.3 Phase 2 Results Facetectomy Cervical Spine In most specimens the posterior bulge was lower than the anterior bulge signifying that there are other factors involved. As a further analysis of the specimens facetectomies were performed to determine the effects of the removal of part of the support system. Results are shown for the axial deformation, anterior bulge and

61 Load (N) 52 posterior bulge in figures Due to the greater deformation response in the facetectomy configuration, a shearing of the specimens maybe occurring which can cause instability in the specimens Vertical Deformation Cervical: Facetectomy Devormation (mm) Average Gap Height Change Figure 25. Vertical Cervical deformation facetectomy

62 Load (N) Load (N) Cervical Anterior Bulge: Facetectomy 300 Average Anterior Bulge Deformation (mm) Figure 26. Anterior bulge cervical spine facetectomy Cervical Posterior Bulge: Facetectomy 300 Average Posterior Bulge Deformation (mm) Figure 27. Posterior cervical bulge: facetectomy

63 Load (N) Phase 1 Results Intact Lumbar Specimens Lumbar specimens were tested in the same manner as the cervical specimens. Analysis for the lumbar specimen is similar to the cervical specimens. Due to the increase size and positioning of the posterior elements the lateral bulge was measured for comparison to the other directions. Results for the intact lumbar specimens including vertical deformation, anterior and lateral bulge are presented below in figures 28 through 30. Compared to the cervical specimens there is no distinct slope change in the anterior direction. The average lateral, anterior and posterior bulge are similar in magnitude. The stiffness is much less in the lumbar than in the cervical region which may be contributes to the lumbar IVDs greater size as a result the lumbar specimens deform greater Vertical Deformation Lumbar: Intact 300 Average Gap Height Change Deformation (mm) Figure 28. Vertical deformation Lumbar intact

64 Load (N) Load (N) 55 Anterior Lumbar Bulge: Intact Average Anterior Bulge Deformation (mm) Figure 29. Anterior lumbar bulge intact Lateral Lumbar Bulge: Intact 300 Average Lateral Bulge Deformation (mm) Figure 30. Lateral Bulge Lumbar, Intact

65 Load (N) 56 The Intact Lumbar specimens IVD deformation response was compared to existing literature in the axial deformation and anterior ant lateral bulging. It was found that the results of the current study correspond well with existing literature on the deformation response of the Lumbar IVD (Heuer 2008, Wenger 1997, Cuchanski 2010, O Connell 2010, Markolf, 1974 and Shah 1978). The results from the current study were compared to existing literature and displayed in figures Vertical Deformation Literature Comparison Lumbar Deformation (mm) Existing Literature Current Study, 2012 Figure 31.Comparison of the Existing literature to the current study with respect to the gap deformation. Existing literature includes Markolf et al, 1974 and Cuchanski et al, 2010

66 Load (N) 57 Anterior Bulge Literature Comparison Wenger, O connell, Current Study, 2012 Lin, Lin, 1978 Heuer, Deformation (mm) Figure 32 Comparison of the Existing literature to the current study with respect to the anterior bulge. Existing literature includes Wenger, et al, 1997, Oconnell et al, 2010, Lin et al, 1978, Heuer et al, 2008)

67 Load (N) 58 Lateral Bulge Literature Comparison Wenger, Current Study, 2012 O connell, 2010 Lin, Lin, 1978 Heuer, 2008 Cuchanski, Deformation (N) Figure 33. Comparison of the Existing literature to the current study with respect to the lateral bulge Existing literature includes Wenger, et al, 1997, Oconnell et al, 2010, Lin et al, 1978, Heuer et al, 2008.

68 Load (N) Phase 2 Results Laminectomy Lumbar Specimens Results for lumbar specimens with a laminectomy including vertical deformation, anterior, posterior and lateral bulge are presented below in figures The average anterior, lateral, and posterior bulge have similar magnitudes and trends suggesting a uniform deformation Vertical Deformation Lumbar Laminectomy Average Gap Height Change Deformation (mm) Figure 34. Vertical deformation Lumbar laminectomy

69 Load (N) Load (N) Anterior Lumbar Bulge: Laminectomy 300 Average Anterior Bulge Deformation (mm) Figure 35. Anterior Bulge lumbar laminectomy Lateral Lumbar Bulge: Laminectomy 200 Average Lateral Bulge Deformation (mm) Figure 36. Lateral bulge Lumbar Laminectomy

70 Load (N) Posterior Lumbar Bulge: Laminectomy Average Posterior Bulge Deformation (mm) Figure 37. Posterior Bulge Lumbar Laminectomy

71 Vertical (N) Phase 2 Results Facetectomy Lumbar Specimens Results for lumbar specimens with a facetectomy including vertical deformation, anterior, posterior and lateral bulge are presented below in figures The average trends are shown in black. Further analysis of the roles of the articular facets is below. 600 Vertical Deformation Lumbar: Facetectomy Average Gap Height Change Deformation (mm) Figure 38. Vertical Deformation Lumbar Facetectomy

72 Load (N) Load (n) Anterior Lumbar Bulge: Facetectomy 300 Average Anterior Bulge Deformation (mm) Figure 39. Anterior Bulge Lumbar Facetectomy Lateral Lumbar Bulge: Facetectomy 300 Average Lateral Bulge Deformation (mm) Figure 40. Lateral Bulge Lumbar Facetectomy

73 Load (N) Posterior Lumbar Bulge: Facetectomy 300 Average Posterior Bulge Deformation (mm) Figure 41. Posterior Bulge Lumbar Facetectomy 6.7 Anterior and Posterior Bulge analysis at each Spine Level: Cervical Spine Trends for the anterior and posterior bulge at each level were examined. There was variability in the results at each level. The relative magnitudes of bulging in the anterior and posterior region change in each level and specimen. These affects can be due to relative rigidity of the disc, quality of the disc or level of the specimens. It was shown that for laminectomies the disparity in posterior and anterior bulging is greater in the C3-C4 and C5-C6 level than in the C7-T1 level. This may suggest that the anatomy at the cervico thoraco junction affects the IVD deformation response since the C7-T1 level is a transition level between the cervical and thoracic regions meaning that there are elements of both cervical and thoracic spines at this level. The C7-T1 level lacks the uncinate processes found in the C3-C4 and C5-C6 levels. Furthermore differences

74 Deformation (mm) 65 range in different specimens. Comparison at each level of the cervical spine for laminectomy and facetectomy are shown in figures C3-C4 Level Comparision Laminectomy C3-C4 Anterior 1 C3-C4 Posterior 2 C3-C4 Anterior 2 C3-C4 Posterior Load (N) Figure 42. Comparison of C3-C4 Anterior and Posterior Bulge Laminectomy

75 Deformation (mm) Deformation (mm) C5-C6 Level Comparison Laminectomy 1 C5-C6 Anterior 1 C5-C6 Posterior 2 C5-C6 Anterior 2 C5-C6 Posterior Load (N) Figure 43. C5-C6 Comparison of Anterior and Poster Bulge Laminectomy 0.25 C7-T1 Level Comparison Laminectomy 1 C7-T1 Anterior 1 C7-T1 Posterior 2 C7-T1 Anterior 2 C7-T1 Posterior Load (N) Figure 44. Comparison of C7-T1 Anterior and Posterior Bulge Laminectomy

76 Deformation (mm) 67 Similar analysis was performed for the data on the facetectomy configuration. It is shown below that the disparity of the amount of bulge increases at each level when compared to the same specimens in the laminectomy. It is shown that in each specimen in the facetectomy configuration the anterior bulge is greater than the posterior bulge. This may indicate a shift in the center of rotation due to the removal of the articular facets. It can be assumed that there is much variability in each specimen and cadaver which can contribute to variability of results and responses. This variability can contribute to the variability of literature results and complexity of spine ailments and neck and back pain. C3-C4 Level Comparison Facetectomy 1 C3-C4 Anterior 1 C3-C4 Posterior 2 C3-C4 Anterior 2 C3-C4 Posterior Load (N) Figure 45. C3-C4 Comparison of Anterior and Posterior bulge Facetectomy

77 Deformation (mm) Deformation (mm) 68 C5-C6 Level Comparison Facetectomy 1 C5-C6 Anterior 1 C5-C6 Posterior 2 C5-C6 Anterior 2 C5-C6 Posterior Load (N) Figure 46. Comparison of C5-C6 Anterior and Posterior Bulge Facetectomy C7-T1 Level Comparison Facetectomy 1 C7-T1 Anterior 1 C7-T1 Posterior 2 C7-T1 Anterior 2 C7-T1 Posterior Load (N) Figure 47. Comparison of C7-T1 Anterior and Posterior Bulge Facetectomy

78 Deformation (mm) Anterior and Posterior Bulge analysis at each Spine Level: Lumbar Spine Similar comparison was completed in each lumbar level section: shown in figures For the laminectomy configuration, the L4-L5 level the posterior bulge is greater than the anterior bulge, the L2-L3 level the anterior bulge is greater than the posterior bulge the T12-L1 level contains specimens that vary in which the bulging can occur greatest in the posterior or anterior region. T12-L1 Level Comparison Laminectomy 1 T12-L1 Anterior 1 T12-L1 Posterior 2 T12-L1 Anterior 2 T12-L1 Posterior Load (N) Figure 48. Comparison of T12-L1 Anterior and Posterior Bulge Laminectomy

79 Deformation (mm) Deformation (mm) 70 L2-L3 Level Comparison Laminectomy 1 L2-L3 Anterior 1 L2-L3 Posterior 2 L2-L3 Anterior 2 L2-L3 Posterior Load (N) Figure 49. Comparison of L2-L3 Anterior and Posterior Bulge Laminectomy L4-L5 Level Comparison Laminectomy 1 L4-L5 Anterior 1 L4-L5 Posterior 2 L4-L5 Anterior 2 L4-L5 Posterior Load (N) Figure 50. Comparison of L4-L5 Anterior and Posterior Bulge Laminectomy

80 Deformation (mm) 71 For the facetectomy configuration the L2-L3 level the anterior bulge is still greater than the posterior bulge T12-L1 and L4-L5 levels contain specimens that vary in which the bulging can occur greatest in the posterior or anterior region as shown in figures It can be assumed that there is much variability in each specimen and cadaver which can contribute to variability of results and responses. This variability can contribute to the variability of literature results and complexity of spine ailments and neck and back pain. T12-L1 Level Comparison Facetectomy 1 T12-L1 Anterior 1 T12-L1 Posterior 2 T12-L1 Anterior 2 T12- L1 Posterior Load (N) Figure 51. Comparison of T12-L1 Anterior and Posterior Bulge Facetectomy

81 Deformation (mm) Deformation (mm) 72 L2-L3 Level Comparison Facetectomy 1 L2-L3 Anterior 1 L2-L3 Posterior 2 L2-L3 Anterior 2 L2-L3 Posterior Load (N) Figure 52. Comparison of L2-L3 Anterior and Posterior Bulge Facetectomy L4-L5 Level Comparison Facetectomy 1 L4-L5 Anterior 1 L4-L5 Posterior 2 L4-L5 Anterior 2 L4-L5 Posterior Load (N) Figure 53. Comparison of L4-L5 Anterior and Posterior Bulge Facetectomy

82 Deformation Analysis of the Average Deformation Response Averages of the lumbar anterior and posterior bulge of all specimens are presented in Figures error bars denote one standard deviation. Averages of the deformations at certain loads were calculated and plotted for the cervical specimens. By using a paired one tailed student t-test it was shown that there is no statically significant difference between the anterior and posterior bulge due to the variance of the bulging values. More testing may be needed to further test the significance of the anterior and posterior disc bulge. Average Deformations Laminectomy Lumbar Load (N) Average Vertical Deformation Average Anterior Bulge Average Posterior Bulge Figure 54. Average Deformations Laminectomy Lumbar

83 Deformation (mm) 74 Average Deformations Facetectomy Lumbar Load (N) Average Vertical Deformation Average Anterior Bulge Average Posterior Bulge Figure 55. Average Deformations Lumbar Facetectomy

84 Deformation (mm) 75 Similar analysis was used for the cervical specimens. As shown in the figures 56 and 57 below the values for the anterior and posterior bulge in the laminectomy are similar compared to the anterior and posterior bulge in the facetectomy configuration. By using a one tailed paired student T test it was found that for the facetectomy configuration the bulge in the anterior was significantly greater than the posterior bulge with a probability value of less than 5 %. For the laminectomy configuration the anterior and posterior bulge were not found to be significantly different. The error bars in figures 56 and 57 denote one standard deviation. The vertical deformation in each case is significantly different that either of the bulging responses. 0.6 Average Deformation Cervical Laminectomy Average Vertical Deformation Average Anterior Bulge Average Posterior Bulge Load (N) Figure 56. Average Deformation Cervical laminectomy

85 Deformation (N) 76 Average Deformation Cervical Facetectomy Load (N) Average Vertical Deformation Average Anterior Bulge Average Posterior Bulge Figure 57. Average Deformations Cervical Facetectomy 6.10 Analysis of the Removal of the Articular Facets in the Lumbar Region The ratio of Posterior to anterior bulge was calculated in each of the laminectomy and facetectomy configurations to determine the role of articular facets in the IVD disc bulge. A closer investigation of the differences of each configuration follow below in table 3.

86 77 Table III. Comparison on the amount of bulge with and without facets in the lumbar spine 400 N Posterior Bulge Posterior to anterior bulge ratio With Facets mm Without Facets mm N Posterior Bulge Posterior to anterior bulge ratio With Facets mm Without Facets mm In the lumbar region the removal of the articular facets increases the ratio between the anterior bulge and the posterior bulge which means that the values become more equal and thus less significantly significant. As shown in table 3, after the removal of the facets, the posterior bulge increases suggesting the possibility that herniation is proportional to the amount of posterior disc bulge and compressive load. When the facets are removed the posterior to anterior bulging ratio increases suggesting that the center of rotation shifts such that the segments experience more of a sliding motion or translation. With diagnostic imaging this phenomenon can be used as an indicator to estimate the amount of articular facet degradation which can and may affect the quality and health of the intervertebral disc. With MR imaging the orientation of the vertebral bodies can be measured and any change from a standard orientation may suggest that there may be damage to one or more of the support structures.

87 78 Without the articular facets there is a great component of the force going through the disc which will increase the degradation and therefore risk of herniation or other ailments due to increased stress on the disc. With better diagnostic methods doctors will be able to determine the cause of back and neck pain with greater accuracy Analysis of the Removal of the Articular Facets in the Cervical Region The ratio of Posterior to anterior bulge was calculated in each of the laminectomy and facetectomy configurations to determine the role of articular facets in the IVD disc bulge. A closer investigation of the differences of each configuration is shown below in table 4. The ratio of the anterior to posterior bulge decreases when the articular facets are removed. As shown in table 4, it appears that the ratios have the opposite effect in the cervical spine compared to the lumbar spine. In the cervical spine the ratios decrease as the facet are removed which means that the anterior bulge increases as the posterior bulge decreases. This signifies a translation of the center of rotation in the opposite direction than in the lumbar spine. The rotation of one vertebral body relative to another may cause pinching of the posterior IVD. This has significant importance in diagnostics when assessing the quality of the articular facets in a patient. By studying the relative changes in the bulging of the intervertebral disc and orientation of the spinal elements one can better determine the cause for pain or injury.

88 79 Table IV. Comparison on the amount of bulge with and without facets in the Cervical Spine 400 N Posterior Bulge Posterior to anterior bulge ratio With Facets mm Without Facets mm N Posterior Bulge Posterior to anterior bulge ratio With Facets mm Without Facets mm Dissection of Cervical Specimens After testing each specimen was dissected in the manner depicted in the methods section shown below in figures The specimens were inspected by an orthopedic surgeon. The analysis was used to help explain the behavior of specimens with low deformation responses and anotomical differences between the levels. Due to the small disc height sagittal cuts were completed but where applicable axial cuts were made. A distinction must be made between healthy discs and normal discs. A healthy disc is one that contains a nucleus that contain a uniform bulging gel and discrete fibrous lamellas. (Thompson, 1990). Whereas for this study a normal disc is one that does not contribute to physiological problems resulting in pain for the subject.

89 80 The image shown below in Figure 58 is the endplate of the superior vertebra of the vertebral body of a specimen at the C3-C4 level. The axial cut was through the disc. Note the curved endplate in the lateral sides this is where the uncinate processes of the inferior vertebra rests. In figure 59 an axial cut was made through the cleanly through the disc at the C7- T1 level. Note that the C7-T1 specimens lack the lateral uncinate processes and the transverse processes are located more posterior than the other cervical levels. In figure 60 is a sagittal cut of a second C7-T1 note the small height of the disc especially in the posterior region (Toward the left of the image). There is limited nucleus pulposis available to resist the deformation. In Figure 61 is a sagittal cut of a C5-C6 Specimen. In this specimen there is a very limited amount of disc. It is almost a fused disc. This contributes to the extremely low deformation response in the above.

90 81 Figure 58. Axial cut C3-C4 Figure 59. Axial cut specimen 1 C7-T1

91 82 Figure 60. Sagittal cut. Specimen C7-T1 Figure 61. Sagittal Cut. Specimen C5-C6

92 Figure 62. Sagittal Cut. Specimen C3-C4 83

93 Dissection of the Lumbar Specimens The lumbar specimens were also dissected by an orthopedic surgeon. Images are shown below in figures The lumbar IVD specimens appeared healthier overall compared to the cervical specimens. The disc heights were larger compared to the cervical specimens therefore there may be greater deformation response in the lumbar samples. Figure 63. Axial Cut Specimen T12-L1

94 85 Figure 64. Sagittal Cut and Axial Cut, Specimen T12-L1 Figure 65. Sagittal Cut. Specimen L2-L3

95 Figure 66. Sagittal and Axial Cut. Specimen L4-L5 86

96 87 Figure 67. Sagittal Cut. Specimen L4-L Comparison of each Configuration: Lumbar Spine In order to compare each configuration the values were chosen from each lumbar specimen from the max load applied: 550 N then averaged and compared. As shown in figures the values of deformation in the vertical direction, anterior posterior and lateral bulging of each modification are shown to compare the changes to each configuration. Percentages are based off of the ratio between the modification and the intact specimen or in the case of the posterior bulge were compared to the laminectomy modification. In each case the deformation in the laminectomy configuration is the greatest and the facetectomy configuration is the least. This may be due to increased instability in the specimens as the articular facets are removed.