Investigating the effect of axial compression and distraction on cervical facet mechanics during supraphysiologic anterior shear

Bilateral cervical facet dislocation (BFD) with facet fracture (Fx) often causes tetraplegia but is rarely recreated experimentally, possibly due to a lack of muscle replication. Intervertebral axial compression (due to muscle activation) or distraction (due to inertial loading), when combined with excessive anterior translation, may influence interfacet contact or separation and the subsequent production of BFD with or without Fx. This paper presents a methodology to produce C6/C7 BFD+Fx using anterior shear motion superimposed with 300 N compression or 2.5mm distraction. The effect of these superimposed axial conditions on six-axis loads, and C6 inferior facet deflections and surface strains, was assessed. Twelve motion segments (70±13 yr) achieved 2.19mm of supraphysiologic anterior shear without embedding failure (supraphysiologic shear analysis point; SSP), and BFD+Fx was produced in all five specimens that reached 20mm of shear. Linear mixed-effects models (a=0.05) assessed the effect of axial condition. At the SSP, the compressed specimens experienced higher axial forces, facet shear strains, and sagittal facet deflections, compared to the distracted group. Facet fractures had similar radiographic appearance to those that are observed clinically, suggesting that intervertebral anterior shear motion contributes to BFD+Fx.