Quantitative evaluation of facet deflection, strain and failure load during simulated cervical spine trauma

Traumatic cervical facet dislocation (CFD) is often associated with devastating spinal cord injury. The injury mechanisms leading to CFD are complex and have not been replicated in biomechanical testing; however, anterior shear and flexion loading modes are likely associated with dislocation. Concomitant facet fracture is commonly observed in cases of CFD, yet quantitative measures of facet strain, stiffness and failure load have not been reported. The aim of this study was to determine the mechanical response of the facets when loaded in directions thought to be associated with traumatic CFD. Thirty functional spinal units (FSUs; 6×C2/3, C3/4, C4/5, C5/6 and C6/7) were dissected from thirteen fresh-frozen human cadaver cervical spines (mean age = 70 years [range 48-92], seven male). Uniaxial, low-rate loading was applied to the inferior facets of the inferior vertebra in directions to simulate in-vivo 1) flexion and 2) anterior shear loading. Specimens were subjected to sub-failure loading (10 to 100N) in each direction before being failed in a randomly assigned direction. Facet strain, stiffness, deflection and failure load were measured. Paired and independent t-tests were used for comparison of non-destructive and destructive parameters, respectively (α=0.05). Facet stiffness and failure load were significantly greater in flexion, and facet deflection and surface strain were higher in the anterior shear loading direction. Failure occurred through the facet tip when subjected to anterior shear loading, while failure through the pedicles was most common for simulated flexion loading. Subsequent linear mixed effects models will be used to account for vertebral level, donor demographics, and bone quality. It is anticipated that this information will be used to validate and inform computational models of cervical trauma and will assist with the development of preventative measures.