There always has been some disagreement about whether the os terminale and the os odontoideum are the same or different bones. However, I have always considered them to be the same, believing that the os terminale becomes the os odontoideum when it enlarges in association with hypoplasia of the dens [1]. This also has been suggested by others [2], and the concept is depicted in Fig. 4.1.

Embryologically, the os terminale is derived from the fourth occipital sclerotome and eventually fuses with the dens, as shown earlier in Fig. 4.1. If, on the other hand, the dens undergoes hypoplasia, the os terminale overgrows and becomes the os odontoideum (Fig. 4.1). What is important about all this is that the os odontoideum, as it enlarges, can assume an almost endless, and often very bizarre, variety of configurations. At the same time, there is underdevelopment of the ligaments stabilizing C1 and C2, and because of this the whole problem is associated with varying degrees of hypermobility and instability of C1 and C2, frequently requiring surgical stabilization.

The normal os terminale sits atop the dens in a V-shaped notch (Fig. 4.2), and in some cases its location is more posterior than anterior (Fig. 4.3). In some of these cases the os terminale is a little larger than usual. When the os terminale is seated deep within the notch of the dens, it is not seen on lateral view, but in other cases it sits well above the tip of the dens and appears as a single, smooth ossicle (Fig. 4.4a). In some of these cases the dens is a little dysplastic/deformed in appearance but not overtly hypoplastic (Fig. 4.4b). In other cases, especially on CT evaluation, confusing, but still normal, bony fragmentation of the dens may mimic a comminuted dens fracture (Fig. 4.5). In this regard, however, one might recall that extensively comminuted fractures of the tip of the dens are basically nonexistent.

As noted earlier, when the dens becomes hypoplastic, the os terminale enlarges and overgrows to become the os odontoideum. The os odontoideum can be relatively small or very large, round or oval, or very bizarre and irregular in appearance. At any rate, when this complex of findings is seen the upper cervical spine becomes unstable (Figs. 4.6 and 4.7). Generally, if there is forward flexion motion of C1 on C2 greater than 2 mm, pathologic instability is present. Surgical stabilization is required, and this is important because many, of these cases are incidentally discovered in the Emergency Room when trauma to the head and/or cervical spine has been sustained. Therefore, while in some cases the findings may be minor and of no clinical consequence, in other instances significant instability of the upper cervical spine is present and the problem becomes just as great as if an unstable fracture were present.