Use of spinous processes to determine the laterally angulated trajectory for placement of lateral mass screws: An image analysis

Background : Lateral mass screw placement techniques have been broadly described in the literature. Differences in these techniques are related to entry points, lateral angulations and the cephalocaudal axis.
Aim : We evaluated 20 patients who underwent lateral mass screw placement between 2007 and 2009. Computed tomography (CT) scans of the cervical vertebrae were analyzed for each patient.
Material and Methods : We measured the maximal transition from the midpoint of the lateral mass to a proposed intersection point by a line connecting the corresponding spinous process and outermost rim of the transverse foramen at each level. This determined an optimal entry point during the tip of screw tilted on the same level of spinous process.
Results : The results revealed that a screw entry point less than 3 mm medial to the midpoint of the lateral mass could safely avoid violation of the vertebral artery.
Conclusions : The current study uses imaging analysis to demonstrate that spinous processes are an intraoperative landmark to aid surgeons in determining safe lateral mass screw trajectories. The limited-scale case results support our prediction from the image analysis. Depending on intraoperative landmarks, lateral mass screws could be safely and comfortably placed with good clinical outcomes.

Keywords
Cervical spine, fusion, instrumentation, lateral mass screw, spinous process