The dens (odontoid process) is a tooth-shaped bony projection arising from the superior surface of the body of C2 (axis). It represents the embryonic body of C1 (atlas) that fused to the axis during development. The dens articulates with the anterior arch of the atlas in the atlantoaxial joint and is held against it by the transverse ligament of the atlas. The tip of the dens provides attachment for the apical dental and alar ligaments.
Odontoid fractures are the most common cervical spine fractures in elderly patients from low-energy falls. The Anderson-D'Alonzo classification defines Type I (tip avulsion, stable), Type II (base fracture, most common and most unstable), and Type III (fracture into the axis body, better healing). Type II fractures have a high non-union rate with conservative treatment in elderly patients and are usually managed surgically by anterior odontoid screw or posterior C1-C2 fusion. In rheumatoid arthritis, pannus formation erodes the dens, leading to atlantoaxial instability.
Fracture through the base of the odontoid process in elderly patients from a low-energy fall produces atlantoaxial instability with neck pain and the risk of delayed neurological injury; managed by anterior odontoid screw fixation in good bone or posterior C1-C2 arthrodesis when the fracture geometry or bone quality precludes screw fixation.
Pannus formation from rheumatoid arthritis erodes the dens and destroys the transverse and alar ligaments, producing atlantoaxial subluxation with the ADI greater than 3 mm on flexion radiograph, risking cervical cord compression during anaesthesia and managed by posterior C1-C2 fusion.
