RARE DEVELOPMENTAL ANOMALIES OF THE ODONTOID PROCESS OF THE AXIS

Abstract

Background Development of the odontoid process (dens) involves a primary ossification center and a secondary ossification center. The primary center gives rise to the body of the dens. The secondary ossification center, called the chondrum terminale , ossifies and becomes the ossiculum terminale, which normally fuses with the body of the dens and is the tip of the dens. Disruption in the fusing process can give rise to 3 distinct pathologies—ossiculum terminale persistens, os odontoideum, and dystopic odontoideum. In general, these conditions are not symptomatic, but their importance lies in the need to differentiate them from other more clinically relevant conditions. A radiographic case for each type is presented. Radiologic findings Three cone beam computed tomography (CBCT) cases, each with a type of variation on the development of the odontoid process, are present. Ossiculum terminale persistens arises as a result of the failure of the ossiculum terminale to fuse with the body of the dens and presents as a small ossicle superior to the dens. The os odontoideum is a large ossicle superior to a hypoplastic dens. The dystopic odontoideum results from fusion of the ossiculum terminale with the basiocciput. Differential interpretation The most important differential is with regard to fractures of the odontoid process. Other considerations are inflammatory diseases or neoplasms. Conclusions CBCT is an integral tool in the armamentarium of oral and maxillofacial radiologists (OMRs). CBCT acquisition frequently includes anatomic structures that are outside the oral and maxillofacial region and must be interpreted. Knowledge of the normal and developmental anatomy of these structures is imperative. Developmental anomalies of the odontoid process can mimic acquired conditions. The proper identification of theses anomalies is crucial because their management and prognosis are different.