The Role of Constituent Materials in Spinal Cord Biomechanics

Spinal cord injury is a devastating and catastrophic occurrence, currently with no effective cure. Traumatic injuries result from a mechanical insult to the cord and most injuries are managed with mechanical interventions. Despite this, little is known about the mechanics of spinal cord injury or the mechanical properties of spinal cord tissue. Therefore the focus of this dissertation is to characterize the behavior of spinal cord tissue and quantify the relationship between tissue properties and injury mechanics.

Material properties of spinal cord constituents significantly affected both the magnitude and distribution of tissue level stresses and strains resulting from a compression injury. White matter properties had the greatest effect on whole cord mechanics (R² = 0.91) and pressure (R² > 0.72) outcomes. These results suggest that age and disease related changes in material properties of the spinal cord may affect injury susceptibility and patterns of injury.

Unconfined compression testing of spinal cord white matter demonstrated the viscoelastic nature of the tissue. The compressive behavior was best characterized with a first order Ogden constitutive model. The optimized model parameters were sensitive to preload, post mortem time and peak strain. The relaxation behavior of the tissue showed rapid reduction in stress (> 75%) in less than 60 seconds. Rapid relaxation of the spinal cord indicates that incremental movement of the spinal cord during surgery could significantly decrease the peak stresses in the cord and thereby reduce the risk of ischemic injury.

Characterization of the compression behavior of the spinal cord is confounded by high variability, which limits statistic power in these studies. Flash-freezing spinal cord tissue prior to specimen preparation significantly reduced variability in the mechanical response without affecting the mean material behavior. The reduction in variation in the mechanical response of flash-frozen specimens allowed for a statistically significant effect of strain rate to be observed with 50% fewer samples.

In summary, the work described herein presents a quantification of the influence of material properties on spinal cord injury mechanics and provides the first experimental evaluation of the compression behavior of spinal cord tissue.

Year	Entry
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Year

Entry

1998

Bilston LE. Finite element analysis of some cervical spinal cord injury modes. In: Proceedings of the 1998 International IRCOBI Conference on the Biomechanics of Impact. September 16-18, 1998; Göteberg, Sweden.365-375.

1993

Galbraith JA, Thibault LE, Matteson DR. Mechanical and electrical responses of the squid giant axon to simple elongation. J Biomech Eng. February 1993;115(1):13-22.

1988

Kearney PA, Ridella SA, Viano DC, Anderson TE. Interaction of contact velocity and cord compression in determining the severity of spinal cord injury. J Neurotrauma. 1988;5(3):187-208.

1970

Metz H, McElhaney J, Ommaya AK. A comparison of the elasticity of live, dead, and fixed brain tissue. J Biomech. July 1970;3(4):453-458.

1993

Keaveny TM, Borchers RE, Gibson LJ, Hayes WC. Theoretical analysis of the experimental artifact in trabecular bone compressive modulus [published correction appears in J Biomech. 1993;26(9):1143]. J Biomech. April–May 1993;26(4-5):599-607.