FIGURE 3.

Method for the measurement of the cranial translation ratio (CTR). The distance of cranial translation (A) is measured parallel to the long axis of the dens axis (white dashed line), from the tip of the dens (white line) to the cranial border of the atlas ventral arch (gray line). The length of the dens axis (B) is defined as the distance between the tip (white line) and the ventral base of the dens (gray line). The CTR is then calculated by dividing the distance of cranial translation (A) by the length of the dens axis (B). A step formation occurs when the ventral base of the dens is located rostrally to the caudal border of the atlas ventral arch. In this situation, the gray line at the ventral base of the dens is not superimposed on the white line anymore