Freehand determination of the trajectory angle for cervical lateral mass screws: how accurate is it?

Different methods of lateral mass screw placement in the cervical spine have been described with separate trajectories for each technique in the sagittal and parasagittal planes. In the latter, plane 30° has been recommended in the modified Magerl's technique as the optimum angle to avoid injury to the vertebral artery and nerve root. The estimation of this angle remains arbitrary and very much operator dependant. The aim of this study was to assess how accurately the lateral trajectory angle of 30° is achieved by visual estimation amongst experienced surgeons in a tertiary spinal unit and to determine the likelihood of neurovascular injury during the procedure. We chose an anatomical 'sawbone' model of the cervical spine with simulated lordosis. The senior author marked the entry points. Five spinal consultants and five senior spinal fellows were asked to insert 1.6-mm K wires into the lateral masses of C3 to C6 bilaterally at 30° to the midsagittal plane using the marked entry points. The lateral angulation in the transverse plane was measured using a custom protractor and documented for each surgeon at each level and side. The mean and standard deviation (SD) of the data were obtained to determine the inter observer variability. Utilising this data, measurements were then made on a normal axial computerised tomography (CT) scan of the cervical spine of an anonymous patient to determine if there would have been any neurovascular compromise. Among the 10 surgeons, a total of 80 insertion angles were measured from C3 to C6 on either side. The overall mean angle of insertion was 25.15 (range 20.4-34.8). The overall SD was 4.78. Amongst the 80 measurements between the ten surgeons, two episodes of theoretical vertebral artery violation were observed when the angles were simulated on the CT scan. A moderate but notable variability in trajectory placement exists between surgeons during insertion of cervical lateral mass screws. Freehand estimation of 30° is not consistently achieved between surgeons and levels. In patients with gross degenerative or deformed cervical spine anatomy, this may increase the risk of neurovascular injury. The use of the ipsilateral lamina as an anatomical reference plane is supported.