Odontoid

Odontoid, or dens, is a tooth-like projection from the upper part of the second cervical vertebra (C2, the axis). It extends upward to form a pivot against the atlas (C1) and enables much of the rotation of the head.

Anatomy aside, the dens lies anterior to the spinal cord and is held against the anterior arch of the atlas by the transverse ligament of the atlas. Together, the dens and atlas form the atlantoaxial joint, which accounts for a large portion of cervical rotation. The alar ligaments connect the dens to the skull, helping to limit excessive movement.

Development and variation: The dens develops as part of the axis during embryogenesis. Anomalies include os odontoideum, in which a separate ossicle exists above a nonfused axis and can cause atlantoaxial instability. Incomplete fusion or hypoplasia of the odontoid process can also occur, sometimes with clinical consequences.

Clinical significance: Fractures of the dens are a common trauma finding. The Anderson and D’Alonzo classification divides them into Type I (tip, relatively stable), Type II (base, most common and unstable with a high risk of nonunion), and Type III (through the body extending into the axis). Type II fractures often require surgical stabilization. Instability may also arise from os odontoideum or inflammatory conditions, with potential risk to the spinal cord.

Imaging and management: Evaluation typically begins with cervical radiographs, including an open‑mouth view, followed by computed tomography for detailed bony anatomy. Magnetic resonance imaging assesses ligamentous injury and spinal cord involvement. Treatment ranges from external immobilization in a collar or halo to surgical stabilization with posterior fusion, chosen based on fracture type, stability, and patient-specific factors.