Triquetrum bone

The triquetrum bone (os triquetrum) is a pyramidal (wedge-shaped) carpal bone located in the proximal row of the carpus on the ulnar (medial) side of the wrist, sitting between the lunate radially and the pisiform volarly. Its name derives from its three-sided pyramidal shape, and it is almost entirely covered with articular cartilage.

The triquetrum has three articular facets:

• A flat/lateral facet articulating with the lunate

• A concave distal facet articulating with the hamate (a helicoidal joint important in midcarpal motion)

• An oval palmar facet articulating with the pisiform (pisotriquetral joint)

Proximally, it does not articulate directly with the ulna but instead contacts the triangular fibrocartilage complex (TFCC). It participates in the motion of both the midcarpal and radiocarpal joints.

Ligamentous attachments.

The triquetrum is a key stabilizing "force nucleus" of the ulnar wrist. Extrinsic ligaments connect it to the radius and ulna (dorsal/volar radiotriquetral and ulnotriquetral ligaments), and intrinsic ligaments include the scaphotriquetral, lunotriquetral, triquetrohamate, and triquetrocapitate ligaments. The dorsomedial surface bears a transverse ridge (posterior tubercle) where the dorsal radiotriquetral and triquetroscaphoid ligaments insert.

Clinical relevance.

It is the second most commonly fractured carpal bone after the scaphoid, accounting for roughly 15–19% of carpal fractures. Its dorsal (non-cartilaginous) surface transmits about 60% of its blood supply.

Gray's description:

The triquetrum (triangular) may be distinguished by its pyramidal shape, and by an oval isolated facet for articulation with the pisiform bone. It is situated at the upper and ulnar side of the carpus.

• The superior surface presents a medial, rough, non-articular portion, and a lateral convex articular portion which articulates with the triangular articular disk of the wrist.

• The inferior surface, directed lateralward, is concave, sinuously curved, and smooth for articulation with the hamate.

• The dorsal surface is rough for the attachment of ligaments.

• The volar surface presents, on its medial part, an oval facet, for articulation with the pisiform; its lateral part is rough for ligamentous attachment.

• The lateral surface, the base of the pyramid, is marked by a flat, quadrilateral facet, for articulation with the lunate.

• The medial surface, the summit of the pyramid, is pointed and roughened, for the attachment of the ulnar collateral ligament of the wrist.

The triquetrum articulates with three bones: the lunate laterally, the pisiform in front, the hamate distally; and with the triangular articular disk which separates it from the lower end of the ulna.