Modified Magerl technique of lateral mass screw placement in the lower cervical spine: an anatomic study

Twelve adult embalmed cadaveric cervical spines were used to modify the traditional Magerl technique for screw placement in the lower cervical spine. The starting point for the drill bit was 2 mm inferior to the inferiormost edge of the superior facet and 1-2 mm medial to the posterior midline of the lateral mass. The orientation of the drill bit was parallel to the superior facet in the sagittal plane, and 25-30 degrees lateral in the transverse plane. After drilling, we made direct measurements of the screw path length between the dorsal and ventral cortexes of the lateral mass and screw-path angles in both the sagittal and transverse planes. The results showed the mean screw-path length for all levels ranged from 11 to 15 mm with the smallest value at C7. The mean sagittal and transverse angles of the screw path ranged from 41 to 43 degrees and from 26 to 28 degrees. Penetration of the superior facet was found in three (2.5%) cases. Possible spinal nerve violation was seen in 13 (10.8%) cases if the drill bit was excessively overpenetrated. This study suggested that our modified Magerl technique for lateral mass screw placement be as high as possible without impinging on the facet joint, and drilling be as cranial and lateral as possible to leave the superior articular process as high and lateral as possible. This may further decrease the incidence of spinal nerve injury. Careful drilling and tapping technique is recommended to avoid overpenetration.