Pisiform bone

The pisiform is the smallest of the eight carpal bones—a pea-shaped bone located on the palmar (volar) and ulnar side of the proximal carpal row. It is frequently described as a sesamoid bone embedded within the distal tendon of the flexor carpi ulnaris (FCU), although in humans its small, mobile character has led to some debate over whether it is a true carpal or a sesamoid.

Articulation: It forms a single synovial joint, the pisotriquetral joint, articulating dorsally with the triquetrum. It is the last carpal bone to ossify, typically by around age 12.

Muscular and ligamentous attachments: The FCU tendon attaches to its proximal anterior surface, and it serves as the proximal origin of the pisohamate and pisometacarpal ligaments; the abductor digiti minimi and the transverse carpal ligament also attach to it.

Relationships: It forms the medial (ulnar) wall of Guyon's canal (the ulnar tunnel), through which the ulnar nerve and artery pass, and together with the hook of the hamate contributes to the medial boundary of the carpal tunnel and the pisohamate arcade.

Function: It acts as a lever/fulcrum (hypomochlion) that increases the flexion force of the FCU and contributes to stability of the ulnar column of the wrist, helping hold the triquetrum in position.

Gray's description:

The pisiform bone may be known by its small size, and by its presenting a single articular facet. It is situated on a plane anterior to the other carpal bones and is spheroidal in form.

• Its dorsal surface presents a smooth, oval facet, for articulation with the triangular: this facet approaches the superior, but not the inferior border of the bone.

• The volar surface is rounded and rough, and gives attachment to the transverse carpal ligament, and to the Flexor carpi ulnaris and Abductor digiti quinti.

• The lateral and medial surfaces are also rough, the former being concave, the latter usually convex.

The pisiform articulates with one bone, the triquetrum.