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Biomechanical Properties of Formalin Fixed Lumbar Intervertebral Discs

Biomechanical Properties of Formalin Fixed Lumbar Intervertebral Discs. Emily Brown Advisor: Dr. Gary Bledsoe BE@SLU REU Summer 2009 Saint Louis University. Background. Clinical relevance Over 1 million Americans hospitalized for back injuries*

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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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