A biomechanical assessment of soft-tissue damage in the cervical spine following a unilateral facet injury.

Background: Unilateral cervical spine facet injuries encompass a wide spectrum, including subluxations, dislocations, and fractures, and the instability produced varies greatly. The extent of anatomical disruption secondary to a unilateral facet injury is poorly understood, and few biomechanical studies have quantified the associated kinematics. The purpose of this study was to develop an experimental method that reliably produces an impending unilateral facet dislocation (perched facet) in cadaveric cervical spines and to identify the soft-tissue damage and resulting changes in cervical spine range of motion and neutral zone associated with this injury.

Methods: Nine fresh-frozen cadaveric human spinal motion segments (C4-C5 or C6-C7) were mounted in a spinal loading simulator to induce a perched unilateral facet injury based on a previously described mechanism of flexion and bending with increasing rotation. Loads were applied to simulate and measure flexion-extension, lateral bending, and axial rotation motions before and after achieving a perched facet. Preinjury and postinjury range of motion and neutral zone were analyzed with use of paired t tests for each movement. Systematic qualitative inspection and gross dissection were then performed to define the soft-tissue injury pattern.

Results: Range of motion and neutral zone increased following the reduction of this injury; the largest increase (294%) occurred in contralateral axial rotation (i.e., right axial rotation after a perched left facet). Postinjury dissections revealed bilateral capsular tears, 50% disc disruption, and 50% tearing of the ligamentum flavum in most specimens. The interspinous and supraspinous ligaments were stretched in less than half of the specimens and were never completely disrupted. The longitudinal ligaments were occasionally torn as extensions of anulus fibrosus disruptions.

Conclusions: This study indicates that the anulus fibrosus, nucleus pulposus, and ligamentum flavum are important cervical spine stabilizers. Facet capsules were often torn bilaterally, implying a more advanced injury than a unilateral facet injury. These discoligamentous injuries result in increases in range of motion and neutral zone.

Clinical Relevance: The results from this work provide further insight into the expected injury and associated instability present in a traumatic unilateral facet injury in the cervical spine.