The Effect of Head-Forward Posture on Lower Neck Loads and Dislocation Risk in Head-First Impact: A Computational and Experimental Investigation.

Background: Subaxial cervical facet dislocation (CFD) is a severe neck injury associated with head-first impact (HFI). However, CFD has not been reliably reproduced in experimental or numerical studies, limiting understanding of its underlying mechanics. This study quantified the influence of preimpact head-forward posture on lower-neck sagittal-plane loading associated with CFD during HFI. Methods: A detailed finite element head-neck model was enhanced and evaluated against literature data, then used to investigate HFI response at a 2 m/s vertical drop impact velocity. A reposturing procedure achieved graded head-forward eccentricity from 0 to 50 mm in 5 mm increments, reflecting postures observed during anticipatory neck bracing. A matched cadaveric HFI experiment was subsequently performed with 30 mm eccentricity. Results: Simulations demonstrated that eccentricities of 15 mm or more produced an S-shaped neck deformation during impact, as observed in prior experiments that produced CFD, accompanied by marked increases in anterior intervertebral shear and flexion bending at C7/T1. The matched cadaveric experiment produced bilateral C7/T1 dislocation, consistent with the predicted loading and deformation trends. However, complete CFD was not reproduced computationally because current intervertebral soft-tissue failure formulations did not capture the observed failure progression. Conclusions: Preimpact head-forward eccentricity can substantially increase lower-neck loading and susceptibility to CFD under the inverted-drop conditions studied.