In Vivo Cervical Spine Posture Changes During Non-impact Inverted Freefalls

Axial headfirst impacts can cause catastrophic cervical spine injuries when the head is rapidly decelerated and the cervical spine is compressed by the torso's inertia. The goal of this study was to quantify the cervical vertebral translations, rotations, eccentricity and curvature, and the head rotation in human subjects exposed to non-impact inverted freefalls that represented the pre-impact dynamics of a headfirst impact.

Eleven human subjects were exposed to 4 headfirst freefalls (2 relaxed, 2 with pre-bracing) while secured in a race car seat fixed to a carriage that was inverted and released to freefall over 312.5 ms (0.479 m) before being decelerated to rest. Sagittal fluoroscopy of the cervical spine was acquired and analyzed at freefall onset and end to extract vertebral and head posture variables. Linear mixed models were used to assess the effect of time (freefall onset/end) and condition (relaxed/braced) on the posture variables.

Subjects consistently moved their cervical spine anteriorly and inferiorly, rotated their vertebrae and head in flexion, and increased their spinal eccentricity. Small changes in spinal curvature and intervertebral angles suggested that the subjects responded using an "en bloc" rotation of the cervical spine and head about a point inferior to C6. Compared to the relaxed condition, pre-freefall bracing produced a different initial posture but a similar end posture.

Despite considerable inter-subject variability, a consistent neck and head reorientation was observed, albeit with variable underlying segmental cervical spine posture, providing valuable input for cadaveric tests and computational models simulating headfirst impacts to improve injury prediction.