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
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.
2006	Ning X, Zhu Q, Lanir Y, Margulies SS. A transversely isotropic viscoelastic constitutive equation for brainstem undergoing finite deformation. J Biomech Eng. December 2006;128(6):925-933.
1998	Arbogast KB, Margulies SS. Material characterization of the brainstem from oscillatory shear tests. J Biomech. September 1998;31(9):801-807.
1990	Spilker RL, Suh J-K, Mow VC. Effects of friction on the unconfined compressive response of articular cartilage: a finite element analysis. J Biomech Eng. May 1990;112(2):138-146.
2005	Miller K. Method of testing very soft biological tissues in compression. J Biomech. January 2005;38(1):153-158.
2006	Jin X, Lee JB, Leung LY, Zhang L, Yang KH, King AI. Biomechanical response of the bovine pia-arachnoid complex to tensile loading at varying strain rates. Stapp Car Crash J. 2006;50:637-649. SAE 2006-22-0025.
1995	Tran NT, Watson NA, Tencer AF, Ching RP, Anderson PA. Mechanism of the burst fracture in the thoracolumbar spine: the effect of loading rate. Spine. September 15, 1995;20(18):1984-1988.
2002	Miller K, Chinzei K. Mechanical properties of brain tissue in tension. J Biomech. April 2002;35(4):483-490.
2004	Sparrey CJ. The Effect of Impact Velocity on Acute Spinal Cord Injury [Master's thesis]. Vancouver, BC: University of British Columbia; July 2004.
2003	Wang CC-B, Chahine NO, Hung CT, Ateshian GA. Optical determination of anisotropic material properties of bovine articular cartilage in compression. J Biomech. March 2003;36(3):339-353.
1982	Anderson TE. A controlled pneumatic technique for experimental spinal cord contusion. J Neurosci Meth. 1982;6(4):327-333.
2008	Greaves CY, Gadala MS, Oxland TR. A three-dimensional finite element model of the cervical spine with spinal cord: an investigation of three injury mechanisms. Ann Biomed Eng. March 2008;36(3):396-405.
1997	Miller K, Chinzei K. Constitutive modelling of brain tissue: experiment and theory. J Biomech. 1997;30(11-12):1115-1121.
2000	Bain AC, Meaney DF. Tissue-level thresholds for axonal damage in an experimental model of central nervous system white matter injury. J Biomech Eng. December 2000;122(6):615-622.
2004	Gefen A, Margulies SS. Are in vivo and in situ brain tissues mechanically similar? J Biomech. September 2004;37(9):1339-1352.
2001	Bain AC, Raghupathi R, Meaney DF. Dynamic stretch correlates to both morphological abnormalities and electrophysiological impairment in a model of traumatic axonal injury. J Neurotrauma. May 2001;18(5):499-511.
1986	Mizrahi J, Maroudas A, Lanir Y, Ziv I, Webber TJ. The "instantaneous" deformation of cartilage: effects of collagen fiber orientation and osmotic stress. Biorheology. 1986;23(4):311-330.
2007	Garo A, Hrapko M, van Dommelen JAW, Peters GWM. Towards a reliable characterisation of the mechanical behaviour of brain tissue: the effects of post-mortem time and sample preparation. Biorheology. 2007;44(1):51-58.
2000	Carter J, Mirza S, Tencer A, Ching R. Canal geometry changes associated with axial compressive cervical spine fracture. Spine. January 2000;25(1):46-54.
2007	Cheng S, Bilston LE. Unconfined compression of white matter. J Biomech. 2007;40(2):117-124.
1996	Bilston LE, Thibault LE. The mechanical properties of the human cervical spinal cord in vitro. Ann Biomed Eng. January–February 1996;24(1):67-74.
2001	Sekhon LH, Fehlings MG. Epidemiology, demographics, and pathophysiology of acute spinal cord injury. Spine. December 15, 2001;26(24)(suppl):S2-S12.
2002	Prange MT, Margulies SS. Regional, directional, and age-dependent properties of the brain undergoing large deformation. J Biomech Eng. 2002;124(2):244-252.
2002	Cusick JF, Yoganandan N. Biomechanics of the cervical spine, IV: major injuries. Clin Biomech (Bristol, Avon). January 2002;17(1):1-20.
1990	White AA III, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia, PA: J.B. Lippincott Company; 1990.
1983	Denis F. The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine. November–December 1983;8(8):817-831.
1999	Arbogast KB, Margulies SS. A fiber-reinforced composite model of the viscoelastic behavior of the brainstem in shear. J Biomech. August 1999;32(8):865-870.
1988	Chang G-L, Hung T-K, Feng WW. An in-vivo measurement and analysis of viscoelastic properties of the spinal cord of cats. J Biomech Eng. May 1988;110(2):115-122.

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.