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Biomechanical Properties of Formalin Fixed Lumbar Intervertebral Discs Emily Brown Advisor: Dr. Gary Bledsoe BE@SLU REU Summer 2009 Saint Louis University
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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
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The Intervertebral Disc Annulus Fibrosus Nucleus Pulposus Vertebral Endplate
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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
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Materials Cadaver specimens 2 male, 1 female, ages unknown Formalin fixed Discs L1-L2 through L4-L5 Removed with endplates and some adjacent vertebra
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Testing Set Up Materials Testing System Grip system Serrated metal plates Maximized contact for torsion Universal joint above top platen
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Testing Tests Compression Torsion Compression/Torsion Combination Cyclic loading Within physiological range of disc 150 cycles,.5 Hz 3 trials with rest period
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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
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Analysis Elastic Modulus Calculated from stress and strain 10, 75, 149 cycles ANOVA Test Cycles Trials Levels Specimens
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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)
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Analysis Shear Moduli Disc modeled as ellipse: Unloading and loading 10, 75, 149 cycles ANOVA Test Cycles Trials Levels Specimens
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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
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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
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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
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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
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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
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Acknowledgments National Science Foundation Saint Louis University Dr. Rebecca Willits Neva Gillan The Bledsoe Lab Dr. Gary Bledsoe Becky Cardin Ted Kremer
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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
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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
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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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