FIGURE 2.

Method for the measurement of the ventral compression index (VCI). A line (white dashed) is drawn between the midpoints of the atlas ventral and dorsal arches. The ventral atlantodental interval (VADI, white) is measured along this line between the dorsal aspect of the atlas ventral arch and the ventral aspect of the dens axis. The dorsal atlantodental interval (DADI, gray) is measured along the same line between the dorsal aspect of the dens axis and the ventral aspect of the atlas dorsal arch (A). If the dens axis did not cross the line due to hypoplasia or dorsal angulation (B), a second line (white dotted) is drawn from the tip of the dens perpendicularly to the first line, defining the VADI and DADI. The VCI is calculated by dividing the VADI by the DADI. Note the severe occipital dysplasia in both cases and the atlantooccipital overlapping in case b (gray arrow)