Protection of the C1 posterior arch in atlantal lateral mass fixation: technical case report.

Screw fixation of the C1 lateral mass is a relatively new technique designed to allow for C1/C2 fixation in scenarios where transarticular screw fixation is not safe or possible. In order to place the screw at the base of the C1 arch, it has been recommended to drill at the junction of the C1 posterior arch and the lateral mass of C1, to accommodate the screw head. This may, however, weaken the C1 arch, making it prone to fracture. In this new technique, we describe a modification to the current technique to allow placement of this screw without compromising the C1 arch. A case of atlantoaxial instability secondary to os odontoideum is described. C1 lateral mass fixation is achieved by selecting a screw 10 mm longer than required and placing the screw 10 mm above the bony entry point, for easier placement of the rod and avoidance of drilling at the base of the C1 arch. Adequate and safe C1/C2 fixation was achieved. Sublaminar wiring was performed around the C1 arch with no weakening or fracture of the arch. We believe that in order to place reasonable C1 lateral mass screws, it is inadvisable to drill the base of the junction between the C1 posterior arch and lateral mass as this may lead to arch weakening and failure. Easier instrumentation can be performed and the integrity of the C1 arch maintained using this alternate technique.