Quantitative anatomy of the lateral masses of the atlas and axis vertebrae.

The study was carried out to determine the safe site of entry and the appropriate trajectory of the screw implantation in the lateral masses of atlas (Cl) and axis (C2) during their fixation using the plate and screw technique. Fifty dried specimens of atlas and axis vertebrae were studied. Various dimensions of the lateral masses were quantitatively measured, laying stress on their relationship with the vertebral artery foramen. As the vertebral artery foramen was present entirely in the transverse process in all specimens, screw implantation in the facet of atlas was relatively safe. Best direction of screw implantation in the facet of atlas was observed to be 15 degrees medial to sagittal plane and 15 degrees superior to axial plane. It should be implanted from the middle of the posterior surface of facet. Vertebral artery foramen formed a deep groove in the undersurface of a majority of superior facets of axis. In 15% facets, vertebral artery foramen occupied the entire undersurface of the superior facet. Safe angle for screw implantation in the facet of axis through its pedicle was seen to be 40 degrees medial to sagittal plane and 20 degrees superior to axial plane. Safe site of screw entry in the axis was superior and medial third of the posterior surface of the pedicle. Quality of cancellous bone in the lateral masses in the proposed trajectory of screw in Cl and C2 was good, providing an excellent purchase of the screw.