Evaluation of cervical posterior lateral mass screw placement by oblique radiographs.

STUDY DESIGN: The present study analyzed the two-dimensional representation of cervical lateral mass screws by oblique radiographs compared with cadaveric placement. This was accomplished by posterior and lateral cervical dissection of the lateral masses and intervertebral foramina, keeping the emerging nerve roots intact. The intervertebral foramina were divided into two zones for the study.

OBJECTIVES: To identify and describe the value of oblique radiographs in evaluating posterior lateral mass screw placement in the cervical spine.

SUMMARY OF BACKGROUND DATA: Posterior plate-screw fixation is an effective method of stabilizing the traumatized cervical spine. Because of the surrounding anatomy, precise placement of screws must be attained to avoid iatrogenic injury to the nerve roots, and incorrectly placed screws must be identified quickly to minimize the neurologic complication. No previous radiologic study regarding evaluation of the lateral mass screw placement has been reported.

METHODS: Six cervical spines were removed from embalmed cadavers. Posterior and lateral removal of soft tissue ensued until the lateral masses and spinal nerves were clearly and completely exposed. Two specimens and 20 screws were used for each of the following methods: Roy-Camille, zone 1 placement, and zone 2 placement. Zone 1 was defined as the area between pedicles of adjacent vertebrae. Zone 2 was defined as the area between transverse processes of adjacent vertebrae. Forty-five degrees oblique left and right, anteroposterior, and lateral radiographs were taken.

RESULTS: All screws placed by the Roy-Camille technique and 19 of 20 screws intentionally placed in zone 1 were represented accurately by oblique radiographs. Nineteen of 20 screws placed in zone 1 were well appreciated in the foramen in oblique view. However, 13 of 20 screws placed in zone 2 and approximating the nerve root were inaccurately represented or ambiguous in oblique radiographs.

CONCLUSIONS: Oblique radiographs are valuable to view the relationship between screw placement and foramina. Screws crossing the line connecting the posterior borders of the intervertebral foramina and appearing in the pedicle actually exit the bone and may risk damaging the nerve root.