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|Towards Understanding the Injury Mechanics and Clinical Outcomes of Traumatic Subaxial Cervical Facet Dislocation and Fracture-Dislocation
|Quarrington, Ryan David
|School of Mechanical Engineering
|Despite potentially devastating outcomes, the injury mechanisms of traumatic subaxial cervical facet dislocation (CFD) and fracture-dislocation (CFD+Fx) are not well understood and have not been reliably produced in biomechanical testing. In particular, bilateral CFD (BFD) with concomitant facet fracture (BFD+Fx) has not been produced experimentally, possibly due to a lack of neck muscle replication. Muscle activation may impose intervertebral compression and anterior shear during injury, increasing loading of the facets and preventing isolated dislocation via intervertebral separation – such separation has been observed during inertially-produced CFD. The mechanical behaviour of the facets during these scenarios, and the effect of axial distraction on the risk of facet fracture or dislocation, have not been investigated. The aim of this thesis was to improve understanding of the epidemiology, clinical outcomes, and injury mechanisms of CFD and CFD+Fx, and to investigate the biomechanics underlying the injury. In Study 1, a large-cohort medical record and radiographic review of subaxial cervical subluxations, dislocations, and fracture-dislocations presenting at an Australian tertiary hospital over the decade to 2014 was performed. Two primary injury causations were identified: motor vehicle accidents in younger adults, and falls in the elderly. BFD frequently caused spinal cord injury (SCI) and concomitant facet fracture was common. The C6/C7 vertebral level was most commonly involved, and injury to this level most often caused SCI. In Study 2, the bilateral inferior facets of 31 isolated human cadaver subaxial cervical vertebrae (6×C3, C4, C5, and C7, 7×C6) were loaded quasi-statically in simulated supraphysiologic anterior shear and compressive-flexion directions using a materials testing machine – these motions are thought to be associated with BFD. Facet stiffness and failure load were significantly greater in the simulated compressive-flexion loading direction, and sub-failure deflection and surface strains were higher in anterior shear. Facet tip fractures occurred during anterior shear loading, while failure through the pedicles was most common in compressive-flexion. In Study 3, the effect of intervertebral axial separation on human cadaver C6 inferior facet biomechanics during non-destructive anterior shear, axial rotation, flexion, and lateral bending motions of twelve C6/C7 functional spinal units (FSUs) was investigated. Axial compression generally increased facet deflection and strains, when compared to intervertebral distraction. In Study 4, a method was developed to reliably apply 20 mm of constrained anterior shear motion with superimposed intervertebral axial compression or distraction to twelve human cadaver cervical FSUs in a materials testing machine. The effect of superimposed axial compression vs distraction on the type of fractures observed was assessed for the subset of specimens that successfully achieved 20 mm of anterior shear. BFD+Fx was produced in five of 12 specimens, of which three had axial compression superimposed. The mechanical behaviour of the C6 inferior facets at the point of initial anatomical failure did not appear to be affected by intervertebral axial separation. This thesis presents the first large-cohort clinical investigation of CFD and provides quantitative information about the biomechanical response of the subaxial cervical facets to simulated traumatic loading. Axial compression generally increased facet surface strains and deflections when superimposed on intervertebral motions, and constrained intervertebral anterior shear can produce BFD+Fx. It is anticipated that this thesis will inform the development of improved preventative measures and provide data for the validation of models of cervical trauma.
|Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2018
|Cervical facet dislocation
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