Morphometric three-dimensional computed tomography anatomy of the hypoglossal canal.

Optimal surgical exposure during the transcondylar approach may result in injury to the hypoglossal nerve. This study evaluated the utility of three-dimensional computed tomography (3-D CT) as a means of obtaining detailed anatomic information in an individual patient prior to surgery involving the hypoglossal canal. We studied 20 hypoglossal canals in ten patients using 3-D CT reconstructed from 1-mm CT slices. Detailed anatomic measurements were performed to define the relationship of the hypoglossal canal to the occipital condyles and clivus. The relationship of the hypoglossal canal to the occipital condyles and clivus were extremely variable. From the outer table of the clivus in the midline, the extracranial opening of the hypoglossal canal was 20.3 + 2.7 mm (range 15.7-24.7 mm). The intracranial opening was 7.4 + 2.3 mm (range 6.2-11.5 mm) from the inner table of the clivus. From the superior aspect of the condyle, the inner opening was 11.0 + 1.4 mm (range 8.7-12.7 mm) and the outer opening was 19.1 + 2.4 mm (range 14.3-22.8 mm). From the lowest point of the condyle, the outer opening was 12.4 + 2.1 mm (range 9.1-15.6 mm). The posterior condylar emissary vein was 12.2 + 3.0 mm from the intracranial opening of the hypoglossal canal. Three-dimensional CT is a useful tool for assessing critical anatomic relationships and tailoring surgical approaches for individual patients. The amount of bone that can be safely removed without violating the hypoglossal canal can be determined preoperatively for each patient.