BIOMECHANICAL EVALUATION OF POSTERIOR CERVICAL STABILIZATION AFTER A WIDE LAMINECTOMY

Study Design. In vitro biomechanical investigation with nondestructive and destructive testing in a human cadaveric model simulating a wide postlaminectomy condition. Objectives. To determine the relative stabi lity conferred by a posterior cervical spinal rod system and posterior cervical plating. Summary of Background Data. Posterior cervical plat e fixation has been shown to be biomechanically superior to wiring tec hniques, but lateral mass screws may injure neurovascular structures o r facet joints if they are inserted improperly. A cervical rod system has been developed to enhance the safety of lateral mass instrumentati on. Methods. The cervical spines of 12 cadavers underwent biomechanica l testing. After completion of the nondestructive intact testing, a wi de laminectomy with subtotal facetectomies from C4 to C6 was performed . The specimens in two subgroups (group A, cervical spine rods with un icortical fixation, and group B, reconstruction plates with bicortical fixation) were tested in flexion, lateral bending, and torsion. Final ly, destructive testing in flexion was performed. Stiffness, neutral z one, failure moment, energy to failure, and mechanism of failure were determined for each specimen. The data were analyzed using paired t te sts and ANOVA. Results. Group a had a greater mean screw torque value. The instrumented constructs had a greater stiffness ratio (instrument ed/intact) than the intact specimens in flexion, lateral bending, and torsional testing. Group A had a significantly greater flexural stiffn ess than Group B. Neutral zone ratio values were significantly lower d uring flexural testing for the cervical rod construct. Destructive tes ting resulted in significantly greater failure moment and energy-to-fa ilure values for group A. In the cervical rod construct, failure occur red primarily by superior screw loosening with pull-out from the later al mass. Reconstruction plates consistently failed with fracture of th e lateral mass and superior screw loosening. Conclusion. Significantly greater stability was noted in the cervical rod construct during nond estructive and destructive flexural testing.