Finite element analysis of cervical facetectomy

Study Design. Moment-rotation responses and disc anulus stresses of intact and facetectomized C4-C6 cervical spinal units were analyzed using detailed, three-dimensional, finite element models. Objectives. To evaluate biomechanical effects of progressive unilateral and bilateral facet resections on cervical spine segmental mobility (external response) and disc anulus stress (internal response). Summary of Background Data, Experimental studies have demonstrated that facetectomy significantly increases segmental mobility of the cervical spine. The biomechanical effects of facetectomy on the internal response, however, have not been investigated. Methods. Moment-rotation responses of C4 with respect to C6 and von Mises stress in the disc anulus were examined using finite element models of a 0% (intact), 25%, 50%, 75%, and 100% unilaterally and bilaterally facetectomized cervical spinal unit. The model simulations were conducted under the pure-moment loading of 1.8 Nm in flexion, extension, lateral bending, and axial torsion. The intact model also was Validated experimentally under the same conditions. Results. The moment-relation responses of the intact unit were within the ranges of experimental data. Cervical rotations increased with the increased degree of facet resection. The greatest change occurred between 50% and 75% facet resections in bilateral facetectomy. Similar patterns were found for disc anulus stresses, but to a greater extent. The maximum increase in rotation (11%) and in anulus stress (30%) occurred in lateral bending. Torsion was the least affected loading mode. The effects of unilateral facetectomy were considerably less than those of 75% bilateral facetectomy. Conclusions. Facetectomy has a greater effect on anulus stress than on intervertebral joint stiffness. Significant increase in anulus stresses and segmental mobility may occur when bilateral facet resection exceeds 50%.