Biomechanical evaluation of posterior cervical stabilization after a wide laminectomy

Study Design: In vitro biomechanical investigation with nondestructive and destructive testing in a human cadaverio model simulating wide postlaminectomy condition. Objectives. To determine the relative stability conferred by a posterior cervical spinal rod system and posterior cervical plating. Summary of Background Data. Posterior cervical plate fixation has been shown to be biomechanically superior to wiling techniques but lateral mass screws may injure neurovascular structures or facet joints if they are inserted improperly. A cervical rod system has been developed to enhance the safety of lateral mass instrumentation. Methods. The cervical spines of 12 cadavers under went biomechanical testing. After completion of the nondestructive intact testing; a wide laminectomy with subtotal facetectomies from C4 to C6 was performed. The specimens in two subgroups (group A, cervical spine rods with unicortical fixation, and group B reconstruction plates with bicortical fixation) were tested in flexion, lateral bending and torsion. Finally destructive testing inflexion was performed. Stiffness, neutral zone failure moment, energy to failure, and mechanism of failure were determined for each specimen. The data were analyzed using paired tests and ANOVA. Results. Group B had a greater mean screw, torque value. The instrumented constructs had a greater stiffness ratio (instrumented/intact) then the intact specimens in flexion, lateral bending, and torsional testing. Group A had a significantly greater flexural stiffness than Group B. Neutral zone ratio values were significantly lower during flexural testing for the cervical rod construct. Destructive testing resulted in significantly grater failure moment and energy to failure values for group A. In the cervical rod construct failure occurred primarily by superior screw loosening with pull- out from the lateral mass. Reconstruction plates consistently failed with fracture of the lateral mass and superior screw loosening. Conclusion. Significantly greater stability was noted in the cervical rod construct during nondestructive and destructive flexural testing.