Study design: Prospective study.
Objective: To evaluate the safety of the lateral mass screw (LMS) fixation.
Summary of background data: LMS fixation has been known to have risk of injury to vertebral artery, nerve root, or facet joints.
Methods: A consecutive series of 178 patients undergoing planned LMS fixation were studied. Screw fixation was performed using the freehand technique. Entry point of screws was 2-mm medial from the center of the lateral mass. Planned divergent angle of the screw was 30º. Bicortical fixation was tried in all cases. After the surgery, all patients underwent 3-dimensional computed tomographic scan of the cervical spine. We checked the number of screws in each level, divergent angle of the screws, and violation of foramen transversarium (FT), intervertebral foramen, or facet joint by screws. A reliability test was performed.
Results: Total 1256 screws were fixed with 269 at C3, 318 at C4, 331 at C5, and 338 screws at C6. Mean divergent angle of the screws were 34.7º/33.1º at C3, 33.9 º/32.1º at C4, 34.7º/32.7º at C5, and 33.6º/30.7º at C6. Incidence of FT violation was 0.876%. FT violation was most common at C6 (6/11 violations). Mean divergent angle in cases of FT violation was 15.0º and was significantly smaller than that of safe cases. There was no injury to vertebral artery. There was no violation of intervertebral foramen. Incidence of facet violation was 1.433%. Seventeen facet violations were within fusion segment. Only 1 screw violated healthy facet. Facet violation was most common at C3 (12/18 violations).
Conclusion: LMS fixation is a safe stabilizing technique with very low incidence of violation of FT, intervertebral foramen, and facet. There is a possibility of FT violation if the divergent angle was small. FT violation was most common at C6. Facet violation was most common at C3.