1. Biomechanical Properties of Formalin Fixed Lumbar Intervertebral Discs Emily Brown Advisor: Dr. Gary Bledsoe BE@SLU REU Summer 2009 Saint Louis University

2. Background  Clinical relevance  Over 1 million Americans hospitalized for back injuries*  Over 4 million cases of back pain related to IVD injuries or degeneration*  Lumbar spine  Highest loads  Most prone to disc degeneration * American Academy of Orthopedic Surgeons, 2003

3. The Intervertebral Disc  Annulus Fibrosus  Nucleus Pulposus  Vertebral Endplate

4. Purpose  Hypothesis: If a level of the spine is removed, the force will be transferred to the other levels of the spine.  Mechanical characterization of fixed IVDs  Elastic and shear moduli  Determine capabilities of formalin fixed discs in research  Application to finite element model of spine

5. Materials  Cadaver specimens  2 male, 1 female, ages unknown  Formalin fixed  Discs  L1-L2 through L4-L5  Removed with endplates and some adjacent vertebra

6. Testing Set Up  Materials Testing System  Grip system  Serrated metal plates  Maximized contact for torsion  Universal joint above top platen

7. Testing  Tests  Compression  Torsion  Compression/Torsion Combination  Cyclic loading  Within physiological range of disc  150 cycles,.5 Hz  3 trials with rest period

8. Data Collection  Recorded 20 times/sec by MTS  Axial and Torque Count  Axial Displacement and Force  Torque Angle and Torque Torque  Calculated stress and strain  Stress=F/A  Strain=∆h/h

9. Analysis  Elastic Modulus  Calculated from stress and strain  10, 75, 149 cycles  ANOVA Test  Cycles  Trials  Levels  Specimens

10. Results Average Elastic Moduli (MPa) LSS1LSS2LSS3Level Ave L1-L2 10.555 ± 1.75 8.8888 ±2.03 15.377 ±.91 11.607 ±3.37 L2-L3 9.6545 ±.83 12.692 ±1.42 8.4138 ±.83 10.253 ±2.20 L3-L4 12.322 ±.50 11.016 ±.70 17.747 ±1.59 13.695 ±3.57 L4-L5 7.5898 ±.41 11.590 ±1.29 8.8335 ±.82 9.3378 ±2.05 Specimen Averages 10.030 ±1.97 11.046 ±1.60 12.592 ±4.69  No significant difference between specimens or levels (p>.14)

11. Analysis  Shear Moduli  Disc modeled as ellipse:  Unloading and loading  10, 75, 149 cycles  ANOVA Test  Cycles  Trials  Levels  Specimens

12. Results  LSS3 significantly different than LSS1 and LSS2 (p<.05) Average Shear Moduli (KPa) LSS1LSS2LSS3Level Ave L1-L2 105.78 ±17.93 81.241 ±9.60 153.73 ±23.22 113.58 ±36.87 L2-L3 64.246 ±8.79 63.922 ±7.78 121.81 ±26.15 83.327 ±33.33 L3-L4 73.044 ±25.60 66.647 ±8.57 119.45 ±16.80 86.383 ±28.82 L4-L5 47.017 ±7.37 80.355 ±9.29 84.302 ±14.64 70.558 ±20.48 Specimen Averages 72.524 ±24.67 73.041 ±9.03 119.82 ±28.38

13. Comparisons to Combination  Elastic moduli  LSS1: no significant difference  LSS2 and LSS3: lower in combination  Shear moduli  No clear trend  No significant difference between specimens or levels in combination

14. Discussion  Compression  Little variation expected in fixed discs  Torsion  LSS3 female patient  Sources of error  Cross-sectional area measurement for stress  Shear moduli ellipse approximation  Actual disc height vs. specimen height

15. Finite Element Analysis  Motion segments created in Mimics  Modeled from female patient  Experimental moduli added to model  Compression loads applied in ALGOR  Average axial strain throughout disc calculated

16. Finite Element Analysis Results  Strain Comparisons  Differences between model and actual discs  Different patients  Bone properties in model  Cortical and cancellous bone  Actual disc height vs. specimen height L1-L2L2-L3L3-L4L4-L5 Actual Disc*.097-.121.112-.147.059-.075.073-.090 Model Disc.063.072.029.070 * Range is from 1 to 150 cycles

17. Acknowledgments  National Science Foundation  Saint Louis University  Dr. Rebecca Willits  Neva Gillan  The Bledsoe Lab  Dr. Gary Bledsoe  Becky Cardin  Ted Kremer

18. References  Brown T, Hansen RJ, Yorra AJ: Some mechanical tests on the lumbosacral spine with particular reference to the intervertebral discs. J Bone Joint Surg [Am], 39A: 1135-1164, 1957  Farafan HF, Cossette JW, Robertson GH, Wells RV, Kraus H: The Effects of Torsion on the Lumbar Intervertebral Joints: The Pole of Torsion in the Production of Disc Degeneration. J Bone and Joint Surg Am. 52: 468-497, 1970  Hirsch C, The Reaction of Intervertebral Discs to Compression Forces. J Bone Joint Surg Am, 37: 1188-1196, 1955  Panjabi M, White A: Basic Biomechanics of the Spine. J of Neurosurgery, 7(1): 76-93, 1980  Perey O. Fracture of the vertebral end plates in the lumbar spine: an experimental biomechanical investigation. Acta Orthop Scand (Suppl), 25:65-68, 1957  Urban J, Roberts S: Review: Degeneration of the intervertebral disc. Arthritis Res Ther, 5:120- 130, March 2003  Virgin,WJ: Experimental Investigations into the Physical Properties of the intervertebral Disc. J. Bone and Joint Surg., 33-B: 607-611, Nov. 1951  Wilke H, Krischak S, Claes L: Formalin Fixation Strongly Influences Biomechanical Properties of the Spine. J. of Biomechanics, 29(12): 1629-1631, Dec. 1996

19. Compression Results cont.  Cycles  Trend toward no significant differences  Some differences from 10 to 75 or 149 cycles  Increasing and decreasing moduli  Trials  Much significant difference but no clear trend  Not related to length of rest period

20. Compression Results cont.  Levels  All but LSS1 L1-L2 to L2-L3 and LSS2 L3-L4 to L4-L5 significantly different  Specimens  Trend toward significant differences