A Thesis entitled A Parametric Study of Physiological Changes to Develop a Finite Element Model of Disc Degeneration by Leonora A. Felon Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Bioengineering Dr. Vijay Goel, Committee Chair Dr. Scott Molitor, Committee Member Dr. Mohamed Samir Hefzy, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo December 2010 ii An Abstract of A Parametric Study of Physiological Changes to Develop a Finite Element Model of Disc Degeneration by Leonora A. Felon Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Bioengineering The University of Toledo December 2010 Finite Element spine models have been created to simulate disc degeneration in the past; however they have not taken into account physiology and how other levels are affected by a change at another level. Here in lies the problem with those models. Several models were created to explore how physiological changes change an experimental validated L3/S1 spine model. Physiological conditions taken into account included Poisson’s ratio of nucleus, tearing of the annulus and physiologically relevant compressive loads on the spine. Much was learned through this process and a final set of three models were created and biomechanically evaluated. The model with the greatest degeneration was also “graded” based on clinical grading schemes and studies. The model was capable of simulating motion similar to cadaver studies done by others. It showed increased pressure in the disc and loading on the facets as the model was farther degenerated. Finally, based on the clinical grading schemes and studies, the large degenerated disc model was determined to be a grade IV degeneration of the L4/5 disc. iii For my ever supporting parents, Darrell & Karen, and all my family and friends who encouraged me along. For all the moms in my life who kept on me to finish, thank you. iv Acknowledgements This thesis would not have been possible without the help and support of my advisor, Dr. Goel, and all my lab mates. I thank them for their help and entertainment over the years. I’d like to thank my family for all the love and support for my education over the years, especially my parents, who have showed me what love and devotion look like. I’d like to thank my brothers of Theta Tau and sisters of the UT softball team. You definitely made my college experience better and more educational than just book learning and I thank you for that. I love that I can count some of my best friends as my brothers and sisters. Also thank you to all my friend moms who kept nagging at me to finish; Kate, Christine, Mother Ana, Mrs. Tkacz and all my church ladies. Finally, thank you to God who has given me more talents than I deserve and for guiding me on how to use them to his glory. v Contents Abstract iii Acknowledgments v Contents vi List of Tables x List of Figures xvi 1 Introduction 1 1.1 Chapter Overview 1 1.2 Back Pain 1 1.3 Treatment Options 2 1.4 Degenerated Disc Basics 4 1.5 Scope of Study 5 1.6 Overview of Chapters 6 2 Literature Review 7 2.1 Chapter Overview 7 2.2 Aging Spine 7 2.3 Spinal Disorders 8 2.3.1 Vertebral body degeneration 8 vi 2.3.2 Disc Degeneration 9 2.3.3 Osteophytes 12 2.3.4 Facet Joint Degeneration & Osteoarthritis 12 2.3.5 Spinal Stenosis 13 2.3.6 Other degenerative conditions 14 2.4 Diagnosis 14 2.5 Treatment Options 15 2.5.1 Conservative 15 2.5.2 Decompression 16 2.5.3 Fusion Surgery 19 2.5.4 Non-Fusion Options: Spinal Arthosis & Dynamic 22 Stabilization 2.6 Spinal Arthosis 23 2.6.1 Total disc replacement 23 2.6.2 Facet replacement technologies 26 2.7 Dynamic stabilization 26 3 Methods & Materials 29 3.1 Chapter Overview 29 3.2 Initial model: nuclear changes alone 29 3.3 Examination of Physiological elements 31 vii 3.3.1 Constant Load with Nuclear Changes, Variable Tear Types 31 & Amount 3.3.2 Constant Tear Amount with Nuclear, Tear Type and 32 Load Changes 3.3.3 More Tears and their locations & patterns 33 3.3.4 Worst case scenarios 37 3.4 Degenerated Models: small, medium & large 39 4 Results 42 4.1 Chapter Overview 42 4.2 Initial model: nuclear changes alone 42 4.3 Examination of Physiological elements 46 4.3.1 Constant Load with Nuclear Changes, Variable Tear Types 46 & Amount 4.3.2 Constant Tear Amount with Nuclear, Tear Type and 55 Load Changes 4.3.3 More Tears and their locations & patterns 57 4.3.4 Worst case scenarios 59 4.4 Degenerated Models: small, medium & large 60 5 Conclusions & Discussion 79 5.1 Chapter Overview 79 5.2 Initial model: nuclear changes alone 79 viii 5.3 Examination of Physiological elements 80 5.3.1 Constant Load with Nuclear Changes, Variable Tear Types 80 & Amount 5.3.2 Constant Tear Amount with Nuclear, Tear Type and 82 Load Changes 5.3.3 More Tears and their locations & patterns 82 5.3.4 Worst case scenarios 83 5.4 Degenerated Models: small, medium & large 84 References 86 A Spine Anatomy 97 A.1 Vertebras 97 A.2 Intervertebral Disc 103 A.3 Ligamental Tissue 104 A.4 Muscular Tissue 110 B Lumbar Biomechanics 118 ix List of Tables 3.1 Material properties of model elements 31 3.2 Various daily activities and the loads seen at L4/5 disc (19) 33 3.3 Combinations of Circumferential & Radial Tears at various amounts 34 of total tearing 3.4 Combinations of Circumferential & Radial Tears at various amounts 34 of total tearing 4.1 Angular motion (deg) in various loading modes for 400 N preload 43 and 10.6 Nm of bending moment across L4-5 segment. 4.2 Maximum Von Mises Stress (N/mm2) in the Nucleus during various 44 loading modes for 400 N preload and 10.6 Nm of bending moment across L4-5 segment 4.3 Maximum Von Mises Stress (N/mm2) in the Annulus during various 44 loading modes for 400 N preload and 10.6 Nm of bending moment across L4-5 segment 4.4 Facet Loads (N) in various loading modes for 400 N preload and 10.6 45 Nm of bending moment across L4-5 segment 4.5 Contact area in various loading modes for 400 N preload and 10.6 46 Nm of bending moment across L4-5 segment x 4.6 Foramen Space (mm) changes across L4-5 during various loading 46 modes for 400 N preload and 10.6 Nm of bending moment; Neutral is before the moment is applied with only the preload. 4.7 Disc height percentage change caused by varying percentage of 47 circumferential tears and various nuclear material property (possion’s ratio) changes 4.8 Disc height percentage change caused by varying percentage of both 48 tear types & various nuclear material property (possion’s ratio) changes. 4.9 Disc height percentage change caused by varying percentage of radial 49 tears and various nuclear material property (possion’s ratio) changes. 4.10 Disc height percentage change caused by each tear scenario and 50 amount of tears for only a possion’s ratio of 0.1. 4.11 Trend line equations relating percentage of disc height loss (x) to 52 amount of tears (y) for each possion’s ratio. Value within parenthesis is the R2 value of the equation. 4.12 Values of tears needed to reach 50% disc height loss for each trend 53 line, if it exists based on trend lines from Table 4.11. 4.13 Trend line equations relating possion’s ratio’s (x) to percentage of 55 disc height loss (y) for various amounts of tears. Value within parenthesis is the R2 value of the equation. 4.14 Values of possion’s ratio needed to reach 50% disc height loss based 55 on trend lines from Table 4.13. xi 4.15 Percentages of disc height loss across various load conditions for 56 each set of tears at a constant amount and two nucleus pulposus possion’s ratio changes (0.1 & 0.25). 4.16 Trend line equations relating load and disc height loss for each set of 57 tear groupings at a constant amount and two nucleus pulposus possion’s ratio with R2 values of trend lines in parenthesis. Calculated load to 50% disc height loss for each trend line case. 4.17 Trend line equations (forced through origin) relating load and disc 57 height loss for each set of tear groupings at a constant amount and two nucleus pulposus possion’s ratio with R2 values of trend lines in parenthesis. Calculated load to 50% disc height loss for each trend line case. Final column shows difference between tables 4.16 & 4.17 4.18 Disc height loss results from the 12.5% torn row of tables 3.3 and 58 3.4 for each cluster/pattern type. 4.19 Disc height loss data in the turquoise set (last column of table 3-3) 59 of tear combinations at increasing percentage of tears within the annulus for each cluster/pattern. 4.20 Disc height loss data for the worst case scenarios. 60 4.21 Numerical values for the relative angular motion across L4/5 for 61 large degenerated disc model in various modalities of motion.