Experimental characterization and viscoelastic finite element modeling of a porcine thoracic spinal cord submitted to axial compression

Abstract

Axial compression is one of the main loading sustains daily by the spinal cord, while transverse compression loads, much less frequent, often results in spinal cord injury. To increase our understanding of the spinal cord biomechanics under normal and traumatic conditions, measuring the behavior of the spinal cord components material properties is of utmost importance. Recent studies expressed the importance of identifying the behavior and material properties of the spinal cord white and grey matter separately. In addition, recent studies stated the importance to determine the relaxation response of the spinal cord tissues and to improve the behavior of spinal cord finite element models by using derived viscoelastic properties. This work has not been completely achieved yet. The main objective of this study is to characterize and simulate numerically the mechanical behavior of the spinal cord white and grey matter tissue under unconfined axial compression including a stress relaxation phase. Two spinal cord were harvested from the thoracic region of two pigs immediately following sacrifice. Each spinal cord was cut in four samples of 2.25 mm thick, for a total of eight samples. Samples were placed in a petri-dish, wrapped in a sprayed gauze with Phosphate- Buffer saline to avoid dehydration and tested under unconfined axial compression using a mechanical testing device. The throughout process was completed in less than six hours to reduce the influence of time post-mortem on the mechanical properties of the spinal cord tissue. All specimens were compressed up to 40% strain at a strain rate of 0.01/sec, and was then allowed to relax for 1000 seconds. A finite element model (FEM) of a typical sample was created and used to derive, using an inverse method, the hyper-viscoelastic material properties of the grey matter, with white matter material properties derived from literature. A first order Ogden hyperelastic model coupled with a 3-term prony series best captured the compressive response of the spinal cord and its viscoelastic behavior during stress-relaxation. It has been found that grey matter is about 3.2 times stiffer than the white matter. For relaxation time, it has been determined that the spinal cord tissue completely relax after approximately 931 seconds following axial compression. These results highlight the importance of distinguishing the type of tissue when using computational models to deepen our understanding of the biomechanics of the spinal cord.

Date	22 Apr 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Éric Wagnac (Supervisor) & Yvan Petit (Co-supervisor)