Os Odontoideum

Excerpt

Os odontoideum is an anatomic anomaly of the upper cervical spine and was first described by Giacomini in 1886 (see Image. Os Odontoideum). Radiologically, this condition is defined as an oval or round-shaped ossicle with smooth circumferential cortical margins, representing a hypoplastic odontoid process (dens) without continuity with the C2 vertebral body. Typically, the ossicle migrates cranially to the expected position of the odontoid tip and can manifest in 2 anatomical types—an ossicle located in the normal odontoid (orthotopic) position and another near the occiput in the area of the foramen magnum (dystopic).

Researchers have described radiological dimensions of os odontoideum, with most being approximately half the size of a normal odontoid process. Some are exceptionally small and cephalic, posing challenges for diagnosis via plain x-rays or computed tomography (CT). The ossicle is located slightly posterior and superior to the anterior arch of C1. The os odontoideum is often attached to the anterior arch of C1 through a transverse ligament.

One of the primary risks associated with this anatomical entity is anterior atlantoaxial subluxation, while posterior atlantoaxial subluxation is extremely rare. This instability in the atlantoaxial joint can result in cervical spinal stenosis, leading to cervical myelopathy due to vascular compromise, bony compression, or stretching of the spinal cord.