Biomechanical Properties of Formalin Fixed Lumbar Intervertebral Discs
Emily BrownAdvisor: Dr. Gary Bledsoe
BE@SLU REU
Summer 2009Saint Louis University
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
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
Materials Cadaver specimens
2 male, 1 female, ages unknown Formalin fixed
Discs L1-L2 through L4-L5 Removed with endplates and some adjacent
vertebra
Testing Set Up Materials Testing System Grip system
Serrated metal plates Maximized contact for torsion
Universal joint above top platen
Testing Tests
Compression Torsion Compression/Torsion Combination
Cyclic loading Within physiological range of disc 150 cycles, .5 Hz
3 trials with rest period
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
Analysis Elastic Modulus
Calculated from stress and strain 10, 75, 149 cycles
ANOVA Test Cycles Trials Levels Specimens
Results
Average Elastic Moduli (MPa)
LSS1 LSS2 LSS3 Level Ave
L1-L210.555± 1.75
8.8888±2.03
15.377±.91
11.607±3.37
L2-L39.6545±.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-L57.5898±.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)
Analysis Shear Moduli
Disc modeled as ellipse: Unloading and loading 10, 75, 149 cycles
ANOVA Test Cycles Trials Levels Specimens
Results
LSS3 significantly different than LSS1 and LSS2 (p<.05)
Average Shear Moduli (KPa)LSS1 LSS2 LSS3 Level Ave
L1-L2105.78±17.93
81.241±9.60
153.73±23.22
113.58±36.87
L2-L364.246±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-L547.017±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
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
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
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
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-L2 L2-L3 L3-L4 L4-L5
Actual Disc*
.097-.121 .112-.147 .059-.075 .073-.090
Model Disc
.063 .072 .029 .070
* Range is from 1 to 150 cycles
Acknowledgments National Science Foundation Saint Louis University Dr. Rebecca Willits Neva Gillan The Bledsoe Lab
Dr. Gary Bledsoe Becky Cardin Ted Kremer
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
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