ANATOMIC CONSIDERATIONS OF ANTERIOR TRANSARTICULAR SCREW FIXATION FORATLANTOAXIAL INSTABILITY

Study Design. Anatomic parameters of C1 and C2 were measured in 30 dri ed human cervical spines. Anterior transarticular C1-C2 screws were pl aced in 15 cadaveric spines. Objective. To provide anatomic data for a nterior transarticular atlantoaxial screw or C1-C2 screw and plate fix ation. Summary of Background Data. A posterior approach to fixation in the atlantoaxial joint has been well described. Damage to the vertebr al artery is documented as a rare complication of posterior atlantoaxi al transarticular screw fixation. An anterior surgical approach to exp osing the upper cervical spine for internal fixation and bone graft re cently has been developed. No anatomic information regarding the anter ior transarticular atlantoaxial screw or screw and plate fixation betw een C1 and C2 is available in the literature. Methods. Direct measurem ents using digital calipers and a goniometer were taken from 30 pairs of dried hu man C1 and C2 vertebrae. The anterior transarticular C1-C2 screw insertion point is at the junction of the lateral edge of the C 2 vertebral body to 4 mm above the inferior edge of the C2 anterior ar ch. The parameters related to anterior transarticular atlantoaxial scr ew fixation or screw and plate fixation between the C1 lateral mass an d the C2 vertebral body were measured. Fifteen embalmed cadavers were used for anterior C1-C2 transarticular screw placement. Longer screws (30-40 mm) were used to detect whether the screw tips violated the upp er cervical canal or vertebral arteries. Results. In the anterior tran sarticular atlantoaxial screw placement, lateral angulation of the scr ew placement relative to sagittal plane ranged from 4.8 +/- 1.8 degree s to 25.3 +/- 2.6 degrees. The posterior angulation of the screw place ment relative to the coronal plane ranged from 12.8 +/- 3.1 degrees to 22.6 +/- 3.2 degrees. The length of the medial screw path ranged from 14.7 +/- 1.5 mm to 25.4 +/- 2.8 mm. In the anterior screw and prate f ixation, the anteroposterior diameter of the inferior facet articular surface ranged from 16.2 +/- 1.6 mm to 17.1 +/- 1.8 mm. The anteropost erior diameter of the C2 vertebral body ranged from 9.3 +/- 1 mm to 16 .2 +/- 1.8 mm. The anterior prevascular retropharyngeal approach appro priately exposed the atlantoaxial joint for anterior transarticular C1 -C2 screw placement. No screws violated the vertebral artery and cervi cal canal. Conclusions; An anterior transarticular atlantoaxial screw 15-25 mm long can be inserted with a lateral angulation of 5-25 degree s relative to the sagittal plane and a posterior angulation of 10-25 d egrees relative to the coronal plane. Additionally, in C1-C2 anterior plate fixation screws 15 mm long could be anchored in the inferior fac et of the C1, and screws 9-15 mm long could be anchored in the C2 vert ebral body.