Posterior Screw Placement on Lateral Mass of Atlas: An Anatomical Perspective

Abstract

Introduction: Fusion of the occipitocervical or atlantoaxial vertebra is an accepted treatment option in upper cervical spine instability caused by cervical trauma or various other disorders. The C1-C2 fusion techniques have limitations which may be overcome by the lateral mass screws. Posterior screw placement in the atlas lateral mass avoids the loss of occipitocervical motion and enables posterior C1-C2 fusion in patients. This is followed in patients who are not suitable for transarticular screw fixation. Aim: To define ideal entry point, exit point, trajectory and dimensions of the lateral mass screw during the posterior screw fixation of atlas vertebra, to avoid damage to vertebral artery and contents of vertebral canal. Material and Methods: For this study 50 intact dried human atlas vertebrae from the Indian population were measured using a digital Vernier calliper that provides accurate resolution up to 0.01 mm and a goniometer. The measurements were entered in an excel sheet. Mean and standard deviation were calculated. Results: In the present study, the ideal entry point for the lateral mass screw to prevent damage to 1st part of vertebral artery was found to be the midpoint of pedicle analog of atlas. The ideal exit point for the screw to avoid injury vertebral canal contents and 2nd part of vertebral artery was on the anterior surface of lateral mass at a distance of 16.3 mm (SD=1.8) from the anterior tubercle. The horizontal angulation of screw trajectory was found to be 4 degrees medial and vertical angulation 5.4 degrees superior. The distance between entry point and ideal exit point was taken as the optimum length of screw (17.3 mm). The optimum width of screw was calculated by adding the distance of lateral mass inferior to pedicle analog and the thickness of vertebral artery groove (7.2 mm). Conclusion: The above findings and landmarks will be of great help to spine surgeons while performing posterior screw fixation of lateral mass of atlas vertebra.

Keywords

• 1st cervical vertebra
• entry point
• exit point
• trajectory