Anatomical relationship of the internal carotid artery to C-1: clinical implications for screw fixation of the atlas

OBJECT: Various C1-2 instrumentation techniques have been developed to treat atlantoaxial instability. Screw fixation of C1-2 poses a risk of injury to the vertebral artery and internal carotid artery (ICA). Injury to the ICA caused by C-1 screws is extremely rare, but has been described. To characterize this risk, the authors studied the anatomical relationship of the ICA to the lateral mass of C-1. METHODS: The authors studied 100 patients who had undergone computed tomography scanning and magnetic resonance imaging of the neck to assess the position of the ICA in association with the C-1 lateral mass. Each ICA was classified into 1 of the following 4 zones: Zone 1 (medial to lateral mass), Zone 2 (medial half of lateral mass), Zone 3 (lateral half of lateral mass), and Zone 4 (lateral to lateral mass). For patients with an ICA ventral to the lateral mass, the shortest distance between the ICA and lateral mass was measured to determine the margin of error with an overpenetrated bicortical screw. RESULTS: Of the 100 patients, 58% had a left ICA in Zones 2 and 3 with a mean distance from the anterior cortex of 3.5+/-1.5 mm (+/- standard deviation), and 74% had a right ICA in Zones 2 and 3 with a mean distance from the anterior cortex of 3.9+/-1.6 mm. Both ICAs anterior to the lateral mass were noted in 47% of patients, and 84% had >or= 1 ICA anterior to the lateral mass. When the ICA was anterior to the lateral mass, it was more commonly in the lateral half (left ICA in 91% and right ICA in 92%). The left ICA was in Zone 1 in 1% and Zone 4 in 41%. The right ICA was in Zone 1 in 1% and Zone 4 in 25%. CONCLUSIONS: A high percentage of patients demonstrate an ICA directly ventral to the C-1 lateral mass, which poses a risk of ICA injury caused by an overpenetrated bicortical screw.