Anatomy of subaxial cervical foramens: the safety zone for lateral mass screwing.

Introduction: The purpose of this study was to measure the structures of the ventral of lateral masses using cadaver specimens and to quantitatively compare the safety zone for the two major techniques used on each vertebral level from C3 to C6.

Methods: This study is based on 52 cervical vertebrae of 13 cadavers. The anatomical measurements focused on the anterior surface of the lateral mass. We investigated the safety width, heights, and the height of nerve roots.

Results: The mean values of the safety width of the Magerl technique from C3 to C6 were 6.1, 7.3, 6.4 and 4.3 mm, respectively. The mean values of the safety width of the Roy-Camille technique were 6.7, 6.6, 5.8 and 5.4 mm, respectively. The mean values of the safety height of the Magerl technique were 5.0, 5.4, 5.8 and 5.2 mm, respectively. The mean values of the safety height of the Roy-Camille technique were 4.9, 4.0, 1.0 and -1.2 mm, respectively. The mean values of the nerve root height were 3.9, 4.9, 5.9 and 6.9 mm, respectively.

Conclusion: The safety width of the Magerl technique was shorter at C6 because the vertebral artery runs more laterally at C6. The height for the Magerl technique was not significantly different from C3 to C6, however, the safety height for the Roy-Camille technique was significantly shorter at C5 and C6. Our findings suggest that it is important to ensure that the screw(s) penetrate through the cranial side of the ventral aspect of a lateral mass when performing the Magerl technique at all vertebral levels, and to carefully select the screw length when using the Roy-Camille technique, especially at C5 and C6, in order to avoid nerve root injury.