Axis ring fractures due to simulated head impacts

Background: We investigated mechanisms of axis ring fractures due to simulated head impacts.

Methods: Our model consisted of a human upper cervical spine specimen (occiput through C3) mounted to a surrogate torso mass on a sled and carrying a surrogate head. We divided 13 specimens into 3 groups based upon head impact location: upper forehead in the midline, upper lateral side of the forehead, and upper lateral side of the head. Post-impact fluoroscopy and anatomical dissection documented the injuries. Average occurrence times of the peak loads and accelerations were statistically compared (P<0.05) using ANOVA and Bonferroni pair-wise post-hoc tests.

Findings: Of the 13 upper cervical spines tested, 5 specimens sustained axis ring fractures with the most common mechanism being impact to the upper left lateral side of the forehead. The first local force peaks at the impact barrier and neck and all peak head accelerations occurred between 18.0 and 22.8 ms, significantly earlier than the absolute force peaks. The average peak neck loads reached 1761.2N and the axis ring fractures occurred within 50 ms.

Interpretation: We observed asymmetrical fractures of the axis ring including fractures of the superior and inferior facets, laminae, posterior wall of the vertebral body, pars interarticularis, and pedicles. The fracture patterns were related to the morphology of the axis as a transitional vertebra of the upper cervical spine. Understanding the mechanisms of axis ring fractures may help in choosing the optimal reduction technique and stabilization method based upon the specific fracture pattern.