Kinematics, kinetics, and new insights from a contemporary analysis of the first experiments to produce cervical facet dislocations in the laboratory

BackgroundThe first experimental study to produce cervical facet dislocation (CFD) in cadaver specimens captured the vertebral motions and axial forces that are important for understanding the injury mechanics. However, these data were not reported in the original manuscript, nor been presented in the limited subsequent studies of experimental CFD. Therefore, the aim of this study was to re-examine the analog data from the first experimental study to determine the local and global spinal motions, and applied axial force, at and preceding CFD.MethodsIn the original study, quasistatic axial loading was applied to 14 cervical spines by compressing them between two metal plates. Specimens were fixed caudally via a steel spindle positioned within the spinal canal and a bone pin through the inferior-most vertebral body. Global rotation of the occiput was restricted but its anterior translation was unconstrained. The instant of CFD was identified on sagittal cineradiograph films (N = 10), from which global and intervertebral kinematics were also calculated. Corresponding axial force data (N = 6) were extracted, and peak force and force at the instant of injury were determined.ResultsCFD occurred in eight specimens, with an intervertebral flexion angle of 34.8 +/- 5.6 degrees, and a 3.1 +/- 1.9 mm increase in anterior translation, at the injured level. For seven specimens, CFD was produced at the level of transition from upper neck lordosis to lower neck kyphosis. Five specimens with force data underwent CFD at 545 +/- 147 N, preceded by a peak axial force (755 +/- 233 N) that appeared to coincide with either fracture or soft tissue failure.ConclusionsRe-examining this rich dataset has provided quantitative evidence that small axial compression forces, combined with anterior eccentricity and upper neck extension, can cause flexion and shear in the lower neck, leading to soft tissue rupture and CFD. Despite several decades of experimental cadaver work, the mechanisms underlying subaxial cervical facet dislocation (CFD) remain unclear. We re-examined the analog data from the first experimental study (and one of only two to date) to produce CFD in full cervical spines (n = 8), and determined the local and global spinal motions, and applied axial force, that occurred during the injury event. The results of this contemporary analysis demonstrate empirically that small axial compression forces (545 +/- 147 N), combined with anterior eccentricity of the head and upper neck extension, can cause supraphysiologic flexion and shear in the lower neck that produce CFD. image