C1 anatomy and dimensions relative to lateral mass screw placement

STUDY DESIGN. An anatomic surface osteometric analysis of cadaveric cervical spines. OBJECTIVE. To assess the feasibility of placing a 3.5-mm cortical screw in the lateral mass of C1 in a large number of specimens for the purpose of gaining internal fixation for various conditions. SUMMARY OF BACKGROUND DATA. Previous studies have addressed surface dimensions of the atlas vertebra and computerized tomography-measured dimensions of the lateral masses of the atlas. These studies used a limited number of specimens with potentially homogeneous origins. METHODS. A total of 120 atlas (C1) vertebrae for a total of 240 lateral masses were examined and external measurements obtained to assess the feasibility of placing a 3.5-mm cortical screw in the lateral mass. RESULTS. The minimum lateral mass dimensions found from 240 C1 lateral masses were 13.15 mm anterior-posterior, 4.22 mm medial-lateral, and 4.73 mm cephalocaudal. The height of the posterior arch at the groove for the vertebral artery (pedicle analog) was less than 4 mm in 46 of 240 (19.2%) arches. CONCLUSIONS. It is feasible to safely and reproducibly place a 3.5-mm cortical screw in the lateral mass of C1 when the appropriate starting point and trajectory of the screw are chosen.