In vitro genesis of subaxial cervical unilateral facet dislocations through sequential soft tissue ablation

Study Design. In vitro anatomic study investigating the degree of soft tiss ue disruption required to produce a subaxial cervical unilateral facet disl ocation. Objectives. To develop an understanding of the relative contributions to st ability of the subaxial cervical soft tissues and to define an anatomic thr eshold of injury necessary to produce a unilateral cervical facet dislocati on. Summary of Background Data. The literature at this time is unclear regardin g the precise pathomechanics of a cervical unilateral facet dislocation and the required threshold of soft tissue injury necessary for its genesis. Pu blished clinical reports do not make any specific reference to these factor s or are unclear in their objectivity. Methods. Two adjacent vertebra at a time in 10 fresh-frozen subaxial cervic al spine specimens (C2-C3 to C6-C7) were transfixed in the coronal plane wi th 3.5-mm Schanz screws. A steady unilateral vertical distraction force res ulting in lateral cervical flexion was applied to these screws as the surro unding cervical soft tissue structures were sequentially ablated, Four expe rimental models were developed, varying the order of soft tissue disruption . Results. The physiologic coupling of subaxial cervical unilateral distracti on and rotation, because of the spatial orientation or inclination of the c ervical facet joints, allowed the creation of a unilateral facet dislocatio n without an additional flexion moment. Disruption of the ipsilateral artic ular capsule, ligamentum flavum, and more than half of the anulus fibrosus was necessary for the genesis of a unilateral facet dislocation, Disruption of the supraspinous and interspinous ligaments was not necessary but appea red to facilitate or lessen the force required to dislocate a unilateral fa cet. Disruption of the anterior and posterior longitudinal ligaments and in tertransverse ligaments was not necessary to create a unilateral facet disl ocation. Conclusion. This anatomic study further supports the theory that discontinu ity of the anterior and posterior longitudinal ligaments is not necessary f or a unilateral facet dislocation to occur. The ipsilateral facet capsule, anulus fibrosus, and ligamentum flavum appear to be the physical soft tissu e restraints that need to be disrupted to produce a unilateral facet disloc ation.