The third occipital condyle, a constituent part of a median occipito-atlanto-odontoid joint: a case report.

A compound conjunction between basiocciput, atlas (anterior arch), and dens of axis (odontoid process) was found incidentally during routine dissection of the head of an 83-year-old man. According to the patient's history, no neck disability had been recorded. A median saw-cut of the head-and-neck conjunction revealed that the basion was anteriorly reinforced by an osseous pillar 4 mm in length and 3 mm in width. Such a formation is known as a third occipital condyle (condylus tertius, CT). In our case it exhibited one surface oriented in an anterior-inferior direction and articulating with the superior border of the anterior arch of the atlas, and another surface oriented in a posterior-inferior direction and articulating with the superior portion of the dens of axis. The dens of axis itself articulated with the anterior arch of the atlas, forming the (normal) median atlanto-axial joint. Post-mortem computed tomography and post-mortem histological examination completed the investigation. The cartilage of the articular compartment between the CT and the anterior arch of the atlas and the related anterior fibrous disc exhibited severe arthrosis. The findings were discussed in the light of developmental and comparative anatomy.