Odontoid process

a toothlike projection on the axis that articulates above with the atlas. This process is also called the dens. Also, see dens of axis.

2. Pathology

At times an abnormal separation occurs between the anterior arch of the atlas and the odontoid process, termed anterior atlantoaxial subluxation. This abnormality is seen in patients with rheumatoid arthritis, sometimes representing a very early stage in the course of the disease. It results from the presence of transverse ligament laxity due to synovial inflammation and hyperaemia of the adjacent joints. In addition, vertical atlantoaxial subluxation (atlantoaxial impaction or cranial settling) can occur in rheumatoid arthritis and may be fatal. This event, also termed vertical translocation of the odontoid process, results from disruption and collapse of bone and joint structures between the occiput and the atlas and between the atlas and the axis.

Odontoid process erosions are observed in rheumatoid arthritis as the natural consequence of synovial inflammation in adjacent joints. Considerable osteolysis and pathologic fracture of the weakened bone can ensue even after minimal trauma. Erosions also occur in ankylosing spondylitis, juvenile chronic arthritis and psoriatic arthritis.

Fractures of the odontoid process probably result from a combination of extreme flexion, extension or rotation along with a shearing force. In most patients no neurologic deficit occurs, although a neural deficit may develop weeks or months after the injury owing to gradual or delayed atlantoaxial subluxation. Three types of fracture have been described. Type 1 fractures are unilateral oblique fractures occurring through the tip of the odontoid process; these injuries are probably secondary to an avulsion of the alar ligament. This injury does not produce instability even if nonunion ensues. Type 2 fractures, the most common, occur at the junction of the odontoid process with the body of the axis and disrupt the blood supply to the odontoid process. Nonunion occurs with high frequency owing to fracture displacement or angulation, leading to formation of an os odontoideum. Type 3 are horizontal or oblique fractures adjacent to the base of the odontoid process that extend into the cancellous bone of the axis vertebral body. Nonunion of these fractures is unusual. Fractures of the odontoid process are best demonstrated on the anteroposterior open-mouth view, although Mach bands produced by the inferior margins of the incisors, the occiput, the posterior arch of the atlas and the tongue may simulate fractures of the odontoid process on this view.

Agenesis or failure of segmentation of the odontoid process may occur during development. Aplasia and hypoplasia may be seen as an isolated congenital defect or after trauma.

The odontoid process may also be involved in calcium pyrophosphate dihydrate crystal deposition disease, Downs syndrome, gout, metaphyseal chondrodysplasia, metatropic dysplasia, Morquios syndrome and spondyloepiphyseal dysplasia congenita.

DR