Biomechanics of cervical spine facetectomy and fixation techniques.

Facetectomy, either unilateral or bilateral, significantly altered the capacity of cervical spine functional units to withstand increasing compression-flexion loads applied in a constant mode to different specimen configurations. Unilateral facetectomy resulted in an average 31.6 +/- 9.7 percent decrease in strength whereas bilateral disruption caused an average 53.1 +/- 11 percent decrease in strength. Motion analysis in a two-dimensional plane after facetectomy indicated an anterior displacement of the instantaneous axis of rotation (IAR) with a resultant increased load on the vertebral bodies and disc. This anterior shift of the IAR in the horizontal plane was significantly but not completely resolved by wire fixation of the facet joints. These fixation techniques, consisting of either facet to facet or facet to spinous process wiring, demonstrated a similar capability to restore strength to the functional units as well as reducing excessive motion in the vertical and anterior axes induced by the facetectomies.