The Role of Facet Angle Asymmetry in Fractures of the First Cervical Vertebra

Compression in the head-neck complex often causes fractures of the first cervical vertebra (C1). The type of fracture often determines whether the injury is stable or unstable, which significantly affects the ultimate injury severity. The often unstable bursting fracture of C1 is thought to be caused by a transformation of axial compressive forces into lateral bursting forces by the wedge-shaped lateral masses. The purpose of this study was to measure the orientation of the joint surfaces of C1 to determine whether this symmetry exists. An additional goal was to determine whether the orientation of the joint surfaces varied significantly with location on the surface. Direct measurements of surface coordinates were taken from 40 dried vertebrae. The angles of two areas on each of the four joint surfaces of the lateral masses of C1 were then calculated. The resulting angles agreed with previous investigations of upper cervical vertebral anatomy. It was found that, for the anterior zones, the superior facet angles were significantly different than the inferior facet angles; this was not true, however, for the posterior zones. Comparisons of anterior vs. posterior zones showed a significant difference for the superior facets. This suggests that sagittal-plane angulation between the occiput and C1 may affect the load path through C1. These findings lend support to the idea that compression of the head-neck complex may result in different types of injury depending both on the shape of the first cervical vertebrae and its orientation relative to adjacent levels.