Pisiform

The pisiform (os pisiforme) is a small carpal bone on the medial side of the proximal carpal bones row. It is considered a sesamoid bone within the tendon flexor carpi ulnaris.

Gross anatomy

The pisiform sits in an anterior plane to the rest of the carpal bones and articulates with the triquetrum. It has a spherical shape with a slight disto-lateral long axis. The articular dorsal surface is flat, forming the pisotriquetral joint, whilst the palmar surface is rough and round and provides for muscular attachment. It is the only moving structure of the carpal canal .

Attachments

Musculotendinous

• flexor carpi ulnaris
• abductor digiti minimi

Ligamentous

• pisometacarpal
• pisohamate
• flexor retinaculum

Relations

The ulnar artery sits adjacent to a lateral surface groove of the pisiform.

Blood supply

The ulnar artery provides vascularity with branches entering both proximal and distal surfaces. Proximal vessels enter inferior to the triquetral facet near the tendon of the flexor carpi ulnaris. Superior and inferior branches run beneath the articular surface and along the palmar cortex respectively. Distal entering vessels run parallel to the palmar cortex and anastomose with the proximal vessels. Superior proximal and distal vessels form an arterial ring deep to the facet.

Variant anatomy

• os pisiforme secundarium: can be mistaken for a fracture, located at proximal pisiform pole

See Accessory ossicles of the wrist.

Radiographic features

Plain radiograph

Distal avulsion and vertical fracture may occur from a direct blow, especially when the pisiform is tensed from the flexor carpi ulnaris. Mechanism often includes a fall onto an outstretched hand.

The pisotriquetral joint is a common site of wrist osteoarthritis.

Development

Often the last carpal bone to ossify, the pisiform has one ossification center that ossifies in the ninth to twelfth year, later in males.

History and etymology

Derived from the latin word pisum, pisiform denotes to ‘pea shaped’.