Atlanto-Axial Articulation

This is a trochoid or pivot joint, of a rather peculiar character. The articular surfaces are: (1) On the lateral masses of the atlas, two somewhat saddle-shaped facets, which are usually confluent ventrally; (2) on the axis, reciprocal saddle-shaped surfaces which extend upon the odontoid process and are confluent on its ventral aspect. It will be observed that the joint surfaces are not at all accurately adapted to each other, so that only limited areas are in contact at any time.

The joint capsule is attached around the margins of the articular surfaces. It is loose and ample enough laterally to allow extensive movement.

The superior atlanto-axial ligament (Ligamentum interarcuale) is membranous and reinforces the capsule dorsally.

The interspinous ligament (Ligamentum inter-spinale) consists of two elastic bands which extend from the dorsal arch of the atlas to the spine of the axis.

The inferior atlanto-axial ligament (Ligamentum dentis externum) arises from the ventral tubercle of the atlas and is attached by two branches on the ventral spine of the axis.

The odontoid ligament (Ligamentum dentis internum) is short, very strong, and somewhat fan-shaped. It extends from the rough concave dorsal surface of the dens or odontoid process, widens in front, and is attached to the transverse rough area on the inner surface of the ventral arch of the atlas.

Movements.—The atlas and the head rotate upon the axis; the axis of rotation passes through the center of the odontoid process and body of the axis.