Articular circumference of head of ulna

The articular circumference of the head of the ulna (circumferentia articularis capitis ulnae) is the smooth, convex, cartilage-covered band that wraps around the lateral and part of the anterior/posterior aspect of the ulnar head, and which articulates with the ulnar notch of the distal radius to form the distal radioulnar joint (DRUJ).

Location and orientation — It occupies the peripheral (lateral) portion of the rounded ulnar head, situated just distal to the collum ulnae (ulnar neck) and proximal to the distal (inferior) surface of the head. It is distinct from the flat distal/inferior articular surface (the "pole" and fovea) of the head, which faces the triangular fibrocartilage complex (TFCC) of the wrist.

Articulation and function — By articulating with the concave ulnar notch (incisura ulnaris) of the radius, it permits the radius to rotate around the fixed ulna during pronation and supination of the forearm. In some older/clinical descriptions the convex articular band is referred to as the "seat" of the ulnar head.