BACKGROUND: A new technique involving screw fixation of the atlas via the posterior arch and lateral mass has recently been reported for atlantoaxial instability. Because the posterior arch is thin, lateral mass screws risk penetrating the upper part of the posterior arch and damaging the vertebral artery running along the upper part of the posterior arch. METHODS: A total of 50 dry bone samples of the atlas from Japanese cadavers were used. We manually measured the shortest distance from the vertebral canal to the transverse foramen and the thickness at the thinnest part of the groove using calipers and investigated the frequency of dorsal ponticuli at the posterior arch. RESULTS: The area from the vertebral canal to the transverse foramen was thick enough to allow screw insertion, but the thickness of the posterior arch at the thinnest part of the groove was less than the screw diameter (3.5 mm) in 22% of vertebrae and <4 mm in 39%. A dorsal ponticuli was present in 10% of these samples. CONCLUSIONS: The size and shape of the posterior arch must be evaluated using radiography and computed tomography before inserting a lateral mass screw of the atlas via the posterior arch.
BACKGROUND: A new technique involving screw fixation of the atlas via the posterior arch and lateral mass has recently been reported for atlantoaxial instability. Because the posterior arch is thin, lateral mass screws risk penetrating the upper part of the posterior arch and damaging the vertebral artery running along the upper part of the posterior arch. METHODS: A total of 50 dry bone samples of the atlas from Japanese cadavers were used. We manually measured the shortest distance from the vertebral canal to the transverse foramen and the thickness at the thinnest part of the groove using calipers and investigated the frequency of dorsal ponticuli at the posterior arch. RESULTS: The area from the vertebral canal to the transverse foramen was thick enough to allow screw insertion, but the thickness of the posterior arch at the thinnest part of the groove was less than the screw diameter (3.5 mm) in 22% of vertebrae and <4 mm in 39%. A dorsal ponticuli was present in 10% of these samples. CONCLUSIONS: The size and shape of the posterior arch must be evaluated using radiography and computed tomography before inserting a lateral mass screw of the atlas via the posterior arch.