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

The Intervertebral Disc Annulus Fibrosus Nucleus Pulposus Vertebral Endplate

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

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