UNILATERAL POSTERIOR ATLANTOAXIAL TRANSARTICULAR SCREW FIXATION

Bilateral posterior C1-2 transfacet screw placement with associated po sterior bone graft wiring is the accepted treatment for patients with atlantoaxial instability. This technique was modified to treat 19 pati ents with atlantoaxial instability and unilateral anomalies that preve nted placement of a screw across the C1-2 facet. In these cases, a sin gle contralateral transarticular screw was placed in conjunction with interspinous bone graft wiring to avoid neural or vertebral artery inj ury and to provide C1-2 stability. Postoperatively, all 19 patients we re placed in Philadelphia collars (mean immobilization 8 weeks, range 6-12 weeks). Unilateral Cl-2 facet screw fixation was needed for the f ollowing reasons: a high-riding transverse foramen of the C-2 vertebra present in 13 patients (left side in eight, right side in five), poor screw purchase in two (left side in both), screw malposition in one ( left side), severe degenerative arthritis in one (right side), neurofi broma in one :right side), and fracture of the C-1 lateral mass in one (left side). Six weeks postsurgery, one patient presented with a brok en screw and required occipitocervical fusion with a Steinmann pin and wire cable from the occiput to C-3 to achieve solid fusion. Solid fus ions were achieved in the other 18 patients (mean follow-up period 31 months, range 14-54 months); there was no delayed screw breakage, wire breakage, or spinal instability. There were no operative or postopera tive neurological or vascular complications. The authors' experience d emonstrates that unilateral C1-2 facet screw fixation with interspinou s bone graft wiring is an excellent alternative in the treatment of at lantoaxial instability when bilateral screw fixation is contraindicate d.