Impact of lateral mass anatomic variation on ideal polyaxial screw head mobility: technical considerations.

OBJECTIVE: To assess optimal angulation characteristics of lateral mass screws for subaxial (C3 to C6) fixation of the cervical spine in the neutral position. BACKGROUND: In the typical Magerl or Anderson placement technique, the screw trajectory is ideally parallel to the facet joint. For the rod and screw to align properly, the screw head must rotate enough to become perpendicular to the rod. If the optimal angle for the screw head is limited by the screw design, abnormal torsional forces will be generated at the rod/screw interface inducing kyphosis. In this paper, we examined the spinal anatomy in patients with normal CTs to determine the necessary range of motion between tulip head and screw to prevent forced persuasion and abnormal cervical spine alignment. METHODS: We examined subaxial radiographs of 292 vertebrae from C3 to C6 in 73 normal subjects. Computed tomography (CT) scans of the cervical spine with multiplanar reconstructions were evaluated in the axial and sagittal planes. A planned screw entry angle of 30° based upon the midpoint of the lateral mass was used in the axial plane, and parallel to the facet joint in the sagittal plane. The screw head angle (SHA) was then calculated from this 3D measured angle. RESULTS: The measured SHA ranged from 27 to 60°. The average SHA was 43.8°. The average SHA was not significantly different between the levels measured with consistent range and standard deviation. Seventy-six percent (223/292) of levels measured required a SHA >40°, and 12% (36/292) required a SHA >50°. CONCLUSION: The authors recommend using cervical instrumentation systems that allow for at least 55° of freedom of the polyaxial head to prevent abnormal segmental forces. In systems with lesser angulation, technique modifications must be applied to prevent translational forces.