Biomechanics of the lower cervical spine during shear loading

The biomechanics of the cervical spine during shear loading are not well-established as compared to other loading regimes. This deficit may be problematic as there is evidence that shear loading may contribute to fracture-dislocation injuries, which often lead to spinal cord injury. Because of this deficit, existing safety standards, such as those used in the automotive industry, may not provide sufficient protection against spinal cord injuries in the cervical region. The present work aims to address this deficit in two ways: through the characterization of the load-displacement behaviour of the cervical spine during shear loading, and through an analysis of the effect of test apparatus design on specimen artefact loading during shear testing. In the mechanical testing phase of the project, fresh-frozen human cervical functional spinal units were loaded to 100 N using a materials testing machine and custom-designed test apparatus. Three directions (anterior, posterior, lateral) were tested in each of three specimen conditions (intact, posterior ligamentectomy, disc-only). Significant decreases in stiffness were found in both the anterior (∆81 N/mm) and posterior (∆15 N/mm) directions between the intact and disc-only conditions, respectively. A computational model was then developed to investigate the effects of test apparatus design on artefact loading and coupled rotations, which had proved problematic during previous attempts to apply axial compression preloads during shear testing. Three axial compression force application methods (point load, rotationally constrained, follower load) were modeled during testing up to 10 mm anterior shear, with axial compressive loads up to 800 N for each method. A subset of the simulations were validated experimentally using porcine functional spinal units. It was found that the follower load provided the best reduction of both artefact moments and coupled flexion-extension rotations. This work provides additional scope to existing shear biomechanics data, as well as insight into how test apparatus design may influence results during shear testing of the cervical spine. These results may be used to improve the definition and validation of existing finite element models of the human neck, where such models may reduce the incidence or severity of spinal cord injury through improved automotive safety.Applied Science, Faculty ofBiomedical Engineering, School ofGraduat