Although the anatomy and movements of the atlantoaxial joint are now well documented, much remains unknown regarding pathological conditions of this area. Infections of varying etiologies, as well as trauma, are well-known factors in the production of a forward atlantoaxial subluxation (26, 29, 37, 38). Recently, forward subluxations of the atlas on the axis have been demonstrated in rheumatoid arthritis and ankylosing spondylitis (20, 21, 25, 27, 35), as well as in mongolism (30, 31). Also well documented are the forward subluxations associated with congenital anomalies such as an absent or separate odontoid process or an atlanto-occipital fusion (3, 12, 22, 23, 33). In all these various conditions, there is either a defect of congenital nature, a fracture, or an attenuation or rupture of a ligament to permit the subluxation forward of the atlas on the axis. A posterior subluxation of the atlas on the axis also becomes possible due to the congenital absence of both the anterior arch of the atlas and the ...