Quantitative anatomy of the lateral mass of the atlas

Study Design. The lateral mass of the atlas was quantitatively measured. Objectives. To measure the ranges and external dimensions of the lateral mass of the atlas quantitatively, and to provide the anatomic basis for its posterior screw fixation. Summary of Background Data. Lateral mass screw fixation in cervical vertebra has been reported, but quantitative data regarding the anatomy of the lateral mass of the atlas are unknown. Methods. Atlas specimens from 30 fresh cadavers were used for dissection to measure the width, thickness, and height of the lateral mass of the atlas, diameter of the atlantoaxial joint articular surface, and obliquity of the facet atlantoaxial joint in the coronary plane. Results. According to measurements and statistics, the mean width of the lateral mass of the atlas is 15.47 mm, the mean thickness 17.21 mm, the mean height 14.09 mm, the mean transverse diameter of the atlantoaxial joint articular surface 17.90 mm, the mean longitudinal diameter of the atlantoaxial joint articular surface 15.63 mm, and the obliquity of the facet atlantoaxial joint in coronary plane 34.57degrees. Conclusions. Very little information on the lateral mass of the atlas can be found. This study gives some parameters of the lateral mass of the atlas. On the basis of this study, it may be concluded that there is enough space for lateral mass screws in posterior up-cervical internal fixation.