Embryology

The odontoid process is formed from the centrum of the first spinal sclerotome, with the centrum of the second spinal sclerotome forming the body of the axis. The tip of the dens is a derivative of the proatlas. The odontoid process at birth is separated from the body of the axis by a cartilaginous band that represents a vestigial disk. This is referred to as the neurocentral synchondrosis. The band lies below the level of the superior articular facets of the axis. In patients with os odontoideum, the gap between the os and the remaining body of the axis lies above the level of the superior articular facets of the axis. The tip of the dens that forms from the proatlas is not ossified at birth. Instead, it ossifies from a separate ossification center and fuses with the odontoid process by the age of 12 years. When it fails to fuse with the odontoid process, it is termed ossiculum terminale persistens.

Pathogenesis

The cause of os odontoideum is unclear. The pathogenesis has been explained by both congenital and traumatic theories.

The posttraumatic theory is the more prevailing. Fielding et al.2 proposed that an unrecognized fracture in the region of the base of the dens or an acute ligamentous injury occurs in childhood with slight separation of the fracture fragments. This is followed by a contraction of the alar ligaments, causing distraction of the fracture fragments, thus pulling the fractured odontoid fragment closer to the occiput. This also disrupts the blood supply of the odontoid process and leads to poor healing and formation of an ossicle, the os odontoideum. The blood supply to the ossicle is taken over by the proximal arterial arcade, which also supplies the anterior arch of the atlas. This could account for the anterior atlantal arch hypertrophy that is seen in these patients. Several authors2–5 have shown that an os odontoideum may occur after a bony or ligamentous injury in early childhood, at which time a complete odontoid process has been demonstrated.

McRae6 believed that os odontoideum had an embryological origin but required the ongoing stresses of life to trigger instability and produce symptoms after childhood.

According to proponents of the congenital theory, os odontoideum results from a failure of fusion of the odontoid process with the body of the axis. This theory is supported by the increased incidence of associated congenital deformities in these patients (e.g., KlippelFeil syndrome, basilar invagination, and assimilation of the atlas) and also by the increased incidence of os in patients with Down syndrome, multiple epiphyseal dysplasia, and other skeletal dysplasia.7,8 However, for this theory to be true, the gap between the os odontoideum and the axis should be located below the level of the superior axis facet, which has never been demonstrated. In addition, in all patients in whom an os odontoideum is present, a neurocentral synchondrosis is nearly always visible. Kirlew et al.9 proposed that os odontoideum results from an incomplete migration rather than an incomplete fusion, with the defect occurring at the site of migratory arrest. Hence, they said that the location of the defect that occurs at the anatomical base of the dens could be consistent with a congenital etiology.

Classification

There are two types of os odontoideum, depending on its position at the craniovertebral junction (CVJ). It is said to be orthotopic if it is present in the place of the normal dens and moves with the axis and atlas. A dystopic os is one that is attached to the clivus and moves with it and the anterior arch of the atlas.

Indicators of Instability

In the absence of a united odontoid process, the atlantoaxial movements appear to be supported only by ligaments. Several authors believe that os odontoideum represents an unstable CVJ, and surgery is indicated irrespective of the clinical presentation. It appears that in cases where the odontoid process is not in continuity with the body of C2, the atlantoaxial joint is inherently unstable.

Radiological evidence of movement of the ossicle and C1 on dynamic films relative to the body of C2 indicates significant instability. The presence of cord signals on magnetic resonance imaging (MRI) in relationship to the superior edge of the body of C2 also indicates instability.

In most cases, the instability of the atlantoaxial joint is observed on flexion of the head (anterior subluxation). In rarer cases, instability may be observed on extension of the head (posterior subluxation). Thus, the motion dynamics may vary in each patient with os odontoideum. Lateral displacement has also been observed in patients with os odontoideum. This multidirectional instability at the articulation between the atlas and the axis is suggestive of defective cruciate and capsular ligaments.

Clinical Presentation

Os odontoideum may be identified incidentally, and the symptoms in these cases are apparently unrelated to the odontoid abnormality. Worsening neurological symptoms may be of acute or gradual onset and progressive in nature. Neurological deficits can range from mild to severe motor and sensory abnormalities. Symptoms related to vertebral artery compromise have also been reported. Trauma may precipitate neurological symptoms in these patients. Neck pain and torticollis are prominent. Ataxia, limb weakness, limited neck movement, swallowing difficulties, and hoarseness of voice are a few of the presenting symptoms.