Transcript Biomechanical Properties of Formalin Fixed Lumbar

Biomechanical Properties of Formalin
Fixed Lumbar Intervertebral Discs
Emily Brown
Advisor: Dr. Gary Bledsoe
BE@SLU REU
Summer 2009
Saint Louis University
Background
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Clinical relevance
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Over 1 million Americans hospitalized for back injuries*
Over 4 million cases of back pain related to IVD injuries or
degeneration*
Lumbar spine
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Highest loads
Most prone to disc degeneration
* American Academy of Orthopedic Surgeons, 2003
The Intervertebral Disc
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Annulus Fibrosus
Nucleus Pulposus
Vertebral Endplate
Purpose
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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
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Elastic and shear moduli
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Determine capabilities of formalin fixed discs in research
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Application to finite element model of spine
Materials
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Cadaver specimens
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2 male, 1 female, ages unknown
Formalin fixed
Discs
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L1-L2 through L4-L5
Removed with endplates and some adjacent vertebra
Testing Set Up
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Materials Testing System
Grip system
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Serrated metal plates
Maximized contact for torsion
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Universal joint above top platen
Testing
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Tests
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Cyclic loading
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Compression
Torsion
Compression/Torsion Combination
Within physiological range of disc
150 cycles, .5 Hz
3 trials with rest period
Data Collection
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Recorded 20 times/sec by MTS
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Axial and Torque Count
Axial Displacement and Force
Torque Angle and Torque Torque
Calculated stress and strain
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Stress=F/A
Strain=∆h/h
Analysis
Elastic Modulus
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Calculated from stress and strain
10, 75, 149 cycles
ANOVA Test
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Cycles
Trials
Levels
Specimens
Axial Stress Strain
0.35
0.3
0.25
Stress (MPa)
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10 cycles
0.2
75 cycles
149 cycles
0.15
Linear (10 cycles)
0.1
Linear (75 cycles)
Linear (149 cycles)
0.05
0
0.11
0.13
0.15
Strain (mm/mm)
0.17
Results
Average Elastic Moduli (MPa)
LSS1
10.555
L1-L2
± 1.75
9.6545
L2-L3
±.83
12.322
L3-L4
±.50
7.5898
L4-L5
±.41
10.030
Specimen
Averages ±1.97
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LSS2
8.8888
±2.03
12.692
±1.42
11.016
±.70
11.590
±1.29
11.046
±1.60
LSS3
15.377
±.91
8.4138
±.83
17.747
±1.59
8.8335
±.82
12.592
±4.69
Level Ave
11.607
±3.37
10.253
±2.20
13.695
±3.57
9.3378
±2.05
No significant difference between specimens or levels
(p>.14)
Analysis
Shear Moduli
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Disc modeled as ellipse:
Unloading and loading
10, 75, 149 cycles
Torsion Angle and Torque
ANOVA Test
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Cycles
Trials
Levels
Specimens
2.5
2
1.5
1
Torque (N)
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-1.5
-1
-0.5
0.5
10 cycles
0
75 cycles
-0.5
0
0.5
-1
-1.5
-2
-2.5
Angle of Deformation
(degrees)
-3
1
1.5
149 cycles
Results
Average Shear Moduli (KPa)
L1-L2
L2-L3
L3-L4
L4-L5
Specimen
Averages
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LSS1
LSS2
LSS3
Level Ave
105.78
±17.93
64.246
±8.79
73.044
±25.60
47.017
±7.37
72.524
±24.67
81.241
±9.60
63.922
±7.78
66.647
±8.57
80.355
±9.29
73.041
±9.03
153.73
±23.22
121.81
±26.15
119.45
±16.80
84.302
±14.64
119.82
±28.38
113.58
±36.87
83.327
±33.33
86.383
±28.82
70.558
±20.48
LSS3 significantly different than LSS1 and LSS2
(p<.05)
Comparisons to Combination
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Elastic moduli
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LSS1: no significant difference
LSS2 and LSS3: lower in combination
Shear moduli
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No clear trend
No significant difference between specimens or levels in
combination
Discussion
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Compression
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Torsion
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Little variation expected in fixed discs
LSS3 female patient
Sources of error
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Cross-sectional area measurement for stress
Shear moduli ellipse approximation
Actual disc height vs. specimen height
Finite Element Analysis
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Motion segments created in Mimics
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Modeled from female patient
Experimental moduli added to model
Compression loads applied in ALGOR
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Average axial strain throughout disc calculated
Finite Element Analysis Results
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Strain Comparisons
L1-L2
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L2-L3
L3-L4
L4-L5
Actual Disc* .097-.121
.112-.147
.059-.075
.073-.090
Model Disc
.072
.029
.070
.063
Differences between model and actual discs
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Different patients
Bone properties in model
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Cortical and cancellous bone
Actual disc height vs. specimen height
* Range is from 1 to 150 cycles
Acknowledgments
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National Science Foundation
Saint Louis University
Dr. Rebecca Willits
Neva Gillan
The Bledsoe Lab
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Dr. Gary Bledsoe
Becky Cardin
Ted Kremer
References
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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
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Hirsch C, The Reaction of Intervertebral Discs to Compression Forces. J Bone Joint Surg Am,
37: 1188-1196, 1955
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Panjabi M, White A: Basic Biomechanics of the Spine. J of Neurosurgery, 7(1): 76-93, 1980
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Perey O. Fracture of the vertebral end plates in the lumbar spine: an experimental
biomechanical investigation. Acta Orthop Scand (Suppl), 25:65-68, 1957
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Urban J, Roberts S: Review: Degeneration of the intervertebral disc. Arthritis Res Ther, 5:120130, 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
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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.
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Cycles
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Trend toward no significant differences
Some differences from 10 to 75 or 149 cycles
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Increasing and decreasing moduli
Trials
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Much significant difference but no clear trend
Not related to length of rest period
Compression Results cont.
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Levels
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All but LSS1 L1-L2 to L2-L3 and LSS2 L3-L4 to L4-L5
significantly different
Specimens
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Trend toward significant differences