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-rotation 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%.
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-rotation 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%.