Cadaveric morphometric anatomy of C-1 vertebra in relation to lateral mass screw placement

Background The increasing use of lateral mass screw placements for atlanto-axial arthrodesis has made it necessary to have a detailed knowledge of the anatomy of the C-1 vertebra for optimal placement of the screws. Methods In the present study, morphometeric analysis was performed on 54 dry C-1 vertebrae. Measurements were taken by a digital caliper to define the working area available for screw fixation. Results The mean height of the entry point was critical for determining the screw diameter and was found to be 3.55 mm. Most specimens were found to be adequate to accept 3 mm diameter screws. The means of the minimum and maximum screw lengths were measured as 15.31 and 23.67 mm. Again, most specimens would accept a screw of 22 mm length. The mean distance from the lateral edge of the screw entry zone to nearest edge of the transverse foramina was 2.11 mm, while the mean of the shortest distance from the entry zone to the vertebral artery groove was 4.93 mm. Conclusions A 3 mm diameter screw with a length of 22 mm would be acceptable in most C-1 vertebrae. The vertebral artery is relatively safe during screw placement. Additional space may be gained by drilling the posterior arch as well as by mobilizing the vertebral artery. The lateral mass of the atlas is anatomically ideally suited for screw fixation to achieve C1-C2 arththrodesis.