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 tissue disruption required to produce a subaxial cervical unilateral facet dislocation.

OBJECTIVES

To develop an understanding of the relative contributions to stability of the subaxial cervical soft tissues and to define an anatomic threshold of injury necessary to produce a unilateral cervical facet dislocation.

SUMMARY OF BACKGROUND DATA

The literature at this time is unclear regarding the precise pathomechanics of a cervical unilateral facet dislocation and the required threshold of soft tissue injury necessary for its genesis. Published clinical reports do not make any specific reference to these factors or are unclear in their objectivity.

METHODS

Two adjacent vertebra at a time in 10 fresh-frozen subaxial cervical spine specimens (C2-C3 to C6-C7) were transfixed in the coronal plane with 3.5-mm Schanz screws. A steady unilateral vertical distraction force resulting in lateral cervical flexion was applied to these screws as the surrounding cervical soft tissue structures were sequentially ablated. Four experimental models were developed, varying the order of soft tissue disruption.

RESULTS

The physiologic coupling of subaxial cervical unilateral distraction and rotation, because of the spatial orientation or inclination of the cervical facet joints, allowed the creation of a unilateral facet dislocation without an additional flexion moment. Disruption of the ipsilateral articular 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 appeared to facilitate or lessen the force required to dislocate a unilateral facet. Disruption of the anterior and posterior longitudinal ligaments and intertransverse ligaments was not necessary to create a unilateral facet dislocation.

CONCLUSION

This anatomic study further supports the theory that discontinuity of the anterior and posterior longitudinal ligaments is not necessary for a unilateral facet dislocation to occur. The ipsilateral facet capsule, anulus fibrosus, and ligamentum flavum appear to be the physical soft tissue restraints that need to be disrupted to produce a unilateral facet dislocation.