Lateral radiologic evaluation of lateral mass screw placement in the cervical spine.

STUDY DESIGN: Assessment of the value of lateral radiographs in evaluation of lateral mass screw placement in the cervical spine.

OBJECTIVES: To assess the value of lateral radiographs in determining the safe or hazardous locations of the tips of screws used in lateral mass screw fixation.

SUMMARY OF BACKGROUND DATA: Posterior plating with lateral mass screw fixation is frequently used to stabilize the cervical spine and improve fusion. Injury to the spinal nerves caused by screws that are too long must be identified quickly to minimize neurologic complication. No previous radiologic study in which lateral mass screw placement was evaluated using lateral radiographs has been reported.

METHODS: Six cervical spines were removed from embalmed cadavers. Three screws using the Roy-Camille technique and another three using Magerl technique were placed into the lateral mass at C3-C5 in each specimen. Four screw placements under direct visualization, including placement of the screw tip staying the ventral cortex and 2-mm, 4-mm, and 6-mm overpenetration of the ventral cortex, were performed separately on each specimen for each of the two techniques. After each placement, a lateral radiograph was taken. Each vertebral body was divided vertically into four equal zones with Zone I the most posterior. Another equal zone, posterior to the posterior border of the vertebral body was defined as pre-Zone I. The number of screw tips seen in each zone were quantified for each placement.

RESULTS: In the screws placed using the Roy-Camille technique, 77.8% of screws placed without perforating the ventral cortex were found in Zone I; 72.2% placed with 2-mm overpenetration of the ventral cortex were noted in Zone II; and 61.1% of the screws with 4-mm overpenetration of ventral cortex and 77.8% with 6-mm overpenetration were located in Zone III. For the use of the Magerl technique, 44.4% of the screws placed without perforating the ventral cortex were found in pre-Zone I; 72.2% of the screws placed with 2-mm overpenetration were located in Zone I; and 66.6% with 4-mm overpenetration and 89.7% with 6-mm overpenetration were noted in Zones I and II, respectively.

CONCLUSIONS: Lateral radiographs may be valuable in evaluating lateral mass screw placement. Ideal screw tip positions on lateral radiograph for the Roy-Camille technique may be in Zone I, and for the Magerl technique may be in pre-Zone I.