Different methods of lateral mass(LM) screw placement in the cervical spine have been described. In the axial plane, 30 degrees is the recommended angle to avoid neurovascular injury. The estimation of this angle remains arbitrary and operator dependant.

To assess how accurately the lateral trajectory angle (LTA) for cervical LM screws is achieved by visual estimation amongst experienced spinal surgeons.

A sawbone model of cervical spine with simulated lordosis was used. Five spinal consultants and five senior spinal fellows were asked to insert 1.6 mm K wires into lateral masses of C3 to C6 bilaterally to simulate screws. The LTA in transverse plane was measured using a customised protractor. Basic statistical analyses of all the data were obtained. Using all the angles derived, a virtual screw trajectory was drawn in the lateral plane, on a normal axial Computerised Tomography scan of cervical spine of an anonymous patient using PACS system.

The overall mean LTA for the group was 25.15 degrees, that of the fellows 24.4 and consultants 26.2 degrees. Mean deviation from 30 degrees for fellows was 5.2 and 6.4 degrees for consultants. Overall standard deviation was 4.78, for fellows and consultants it was 3.3 and 5.8 respectively. Two episodes of vertebral artery injury occurred at 15 and 16 degrees with simulated angles on CT.

A moderate variability in visual estimation of the trajectory angle exists even amongst experienced surgeons during insertion of cervical LM screws. An anatomical landmark would be useful to improve the reliability of the procedure.