Abstract
Introduction
Atlanto-occipital dislocation is rare and usually fatal. Stabilisation is typically from Occiput to C2, sacrificing atlanto-axial movement. To preserve movement, screw fixation from the articular mass of C1 to the occipital condyle has been described. Amongst other structures, the hypoglossal nerve is at risk. No previous study has addressed the anatomy of the hypoglossal canal in relation to screw trajectory. We aim to identify landmarks to aid safe screw passage into the occipital condyle.
Methods
20 dry skulls provided 40 hypoglossal canals (HCs) and 40 occipital condyles (OCs). No distinction was made between sex, race or age. 9 parameters were measured for each HC, and relation to skull base was noted.
Results
The mean length of the HC was 10mm (range 8-14). The extra-cranial foramen of the HC is located lateral to the intra-cranial foramen (30 degrees, range 19-45). 19 out of 20 skulls had HCs with intra-cranial foramina more caudal than their extra-cranial foramina, ie the HC angled cranially (22 degrees, range 7-51). 36 out of 40 OCs were found to be wholly inferior to the rim of the foramen magnum, with 4 (in 2 skulls) whose bodies lay below, but extended above, this landmark. Every single HC was situated, in its entirety, superior to the rim of the foramen magnum.
Conclusions
The trajectory of the hypoglossal canal from its intra-cranial foramen is antero-supero-lateral. It is situated, in its entirety, superior to the rim of the foramen magnum. The thickest portion of the occipital condyle is antero-medial. The screw passage from posterior through the C1 articular mass ought to aim for the anterior, superior, medial quadrant of the occipital condyle, and should not pass cranial to the rim of the foramen magnum in order to minimise the risk to the hypoglossal nerve.
