Factors associated with good outcome using lateral mass plate fixation.

The immediate and long-term outcomes of 70 consecutive patients who underwent subaxial lateral mass fixation between June 1996 and June 2001 were reviewed. Intraoperative fluoroscopy and somatosensory evoked potential (SEP) monitoring were used in all patients. Immediate postoperative computed tomography (CT) was performed to determine screw trajectory and placement. Follow-up ranged from 2 to 7 years. Postoperative CT showed 206 (58%) of 356 screws had unicorticate and 42% bicorticate purchase. Furthermore, 96 (27%) screws had suboptimal trajectory, but only 5 of these screws minimally penetrated the foramen transversarium without resultant vascular or neurological sequelae. A sudden unilateral intraoperative SEP amplitude decrease during screw placement in 2 patients resolved with screw removal and alteration of screw trajectory. The overall fusion success rate was 91.5% and screw pull-out developed in 2 patients. The recommended drilling technique and trajectory (15 degrees - 25 degrees rostral in the sagittal plane, 20 degrees - 30 degrees lateral in the axial plane), supplemented bone grafting, and intraoperative SEP monitoring are all associated with good screw placement, fusion, and neurological outcome and are recommended for all lateral mass fusion procedures.