Accessory joints between basiocciput and atlas/axis in the median plane.

To explore the many osseous irregularities that are found in the area between the basiocciput, the anterior arch of the atlas and the tip of the dens axis we studied 99 cadaver specimens using magnetic resonance tomography (MRT), computed tomography (CT), median saw-cut sections, and histological sections. Additionally, "dry" specimens of the skull (n = 110), atlas (n = 56), and axis (n = 33) were investigated. In the median plane, the dry and cadaver specimens exhibited osteoarthritis-related osseous outgrowths and osteophytes of the articular surfaces of the median atlanto-axial joint (n = 63), and the presence of congenitally developed free ossicles (n = 22) and of third occipital condyles (n = 3). The largest osteophytes (giant osteophytes) (n = 4) of the anterior arch of the atlas formed osseous contact zones with the basiocciput that were visible histologically as real joints and were designated accessory median atlanto-occipital joints. The third occipital condyles also formed osseous contact zones, visible histologically as real joints, with the anterior arch of the atlas or with the tip of the dens, and were designated accessory atlanto-occipital or occipito-odontoid joints. Frequent free ossicles, incorporated into the accessory joint, were found by histological examination to be covered with hyaline cartilage.