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Biomechanical Properties of Formalin Fixed Lumbar Intervertebral Discs

Investigating mechanical characteristics of fixed intervertebral discs, implications for research and spine modeling. Analysis of elastic and shear moduli. Study conducted with cadaver specimens, exploring compression, torsion, and combination loading.

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Biomechanical Properties of Formalin Fixed Lumbar Intervertebral Discs

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  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 • 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)

  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 * 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

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