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Os odontoideum is a well-corticated, ovoid ossicle separated from the body of the axis (C2) at the site of the odontoid process, representing either a congenital anomaly or an ununited odontoid fracture.

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It results in atlantoaxial instability and potential spinal cord compression.

![Diagrams depicting an os odontoideum in neutral (A), flexed (B), and extended (C) positions.

The ossicle and the ring of C-1 move as a unit. The spinal cord may be injured anteriorly by the ossicle if there is excessive movement on extension or posteriorly by the posterior ring of C-1 if there is too much movement on flexion. The amount of movement (C-1 translation) was calculated as the distance between the posterior cortical margin of the anterior tubercle of C-1 and the anterior margin of the C-2 vertebral body as indicated by the dotted lines and “a” or “b” in panels B and C. With either anterior or posterior instability, this distance increases.

Klimo P J, Kan P, Rao G, Apfelbaum R, Brockmeyer D. Os odontoideum: presentation, diagnosis, and treatment in a series of 78 patients: Clinical article. Journal of Neurosurgery: Spine SPI. 2008;9(4):332-342. doi:10.3171/SPI.2008.9.10.332](https://prod-files-secure.s3.us-west-2.amazonaws.com/2aa05644-4658-4c26-84d3-64c36b55fb6c/a01d24a2-88fe-47a3-a550-cd139bd52b62/image.png)

Diagrams depicting an os odontoideum in neutral (A), flexed (B), and extended (C) positions.

The ossicle and the ring of C-1 move as a unit. The spinal cord may be injured anteriorly by the ossicle if there is excessive movement on extension or posteriorly by the posterior ring of C-1 if there is too much movement on flexion. The amount of movement (C-1 translation) was calculated as the distance between the posterior cortical margin of the anterior tubercle of C-1 and the anterior margin of the C-2 vertebral body as indicated by the dotted lines and “a” or “b” in panels B and C. With either anterior or posterior instability, this distance increases.

Klimo P J, Kan P, Rao G, Apfelbaum R, Brockmeyer D. Os odontoideum: presentation, diagnosis, and treatment in a series of 78 patients: Clinical article. Journal of Neurosurgery: Spine SPI. 2008;9(4):332-342. doi:10.3171/SPI.2008.9.10.332

Etiopathogenesis

Two main theories:

1. Congenital theory – Failure of fusion between the odontoid process and the C2 body due to developmental defect of the dens–body synchondrosis.
2. Acquired theory – Unrecognized fracture through the odontoid synchondrosis in early childhood that heals as a separate ossicle.

Predisposing factors:

• Developmental syndromes: Down syndrome, Morquio syndrome, Klippel–Feil syndrome.
• Trauma in early childhood.
• Vascular insufficiency affecting dens ossification centers.

Classification

Two recognized morphologic types:

• Orthotopic: Os is in normal position above the axis body, articulating with the anterior arch of C1.
• Dystopic: Os is fused to the basion or anterior rim of foramen magnum, not articulating normally with C1.

Clinical Features

• May be asymptomatic, discovered incidentally.