The feasibility of inserting atlas lateral mass screws via the posterior arch.

STUDY DESIGN:
A total of 709 skeletally mature atlas specimens were obtained from the Hamann Todd Collection at the Cleveland Museum of Natural History. Using digital caliper, the thickness of the vertebral artery groove was measured to determine the feasibility of potential screw placement.

OBJECTIVE:
To determine the feasibility of screw insertion into the atlas lateral mass via the posterior arch.

SUMMARY OF BACKGROUND DATA:
The originally described starting point for atlas lateral mass screws, at the base of the lateral mass, can be associated with excessive bleeding and irritation of the greater occipital nerve. In order to avoid such problems, we routinely start the screw at a more cranial location on the dorsal, posterior-lateral arch of the atlas, when the anatomy permits. In cases where the arch is too small, we notch the undersurface of the arch so as to recess the screw into the notch. We undertook this cadaveric morphometric study to determine the feasibility of using such techniques in the general population.

METHODS:
A total of 709 atlas specimens were measured bilaterally to determine the thickness of the posterior-lateral arch at the level of the lateral mass. The presence of the ponticulus posticus was recorded as well.

RESULTS:
The average minimal thickness of the posterior-lateral atlas was 3.95 mm. There was a total of 157 complete and 34 incomplete ponticuli posticus. Overall, 85.2% of the specimens had a thickness greater than 3 mm, 654 (46.2%) greater than 4 mm, and 194 (13.7%) greater than 5 mm.

CONCLUSIONS:
Our results suggest that although only a small percentage of patients can accept a screw that is directly inserted via the posterior-lateral arch into the lateral mass, the notching technique is possible in the vast majority of patients. To our knowledge, this is the largest study to examine the possibility of using the posterior-lateral arch as the starting point for these screws. Our results suggest that alternative starting points for these screws are possible in a large percentage of the cases.