OBJECTIVE: In this study, forty dried specimens of atlas and axis vertebrae were examined using direct anatomical measurements.

METHODS: Various dimensions of the atlas and the axis vertebrae were quantitatively measured, with special emphasis on their relationship with the vertebral artery foramen.

RESULTS: As the vertebral artery foramen was present entirely in the transverse process of all specimens, screw implantation in the facet of atlas proved relatively safe.

CONCLUSIONS: The anatomy of the atlas and axis vertebrae exhibits complex, three-dimensional structures, showing extensive variability in morphology. Characteristics of the atlas and axis vertebrae must be noted before any spinal operation such as transpedicular screw fixation, transarticular screw fixation, screw fixation of dens axis, interspinous wiring, and interlaminar clamp.