Pisiform
Os Pisiforme
location_on Palmar surface of the wrist, on the ulnar side, anterior to the triquetrum
The pisiform is the smallest carpal bone and the only carpal bone that is a sesamoid bone, developing within the tendon of the flexor carpi ulnaris. It sits on the palmar surface of the triquetrum and is easily palpable on the ulnar side of the wrist. Despite being classified as a carpal bone, it functions more as a sesamoid, increasing the mechanical advantage of the flexor carpi ulnaris.
Key Anatomical Features
- Only sesamoid bone among the carpal bones
- Develops within the tendon of the flexor carpi ulnaris
- Easily palpable on the palmar ulnar aspect of the wrist
- Single articular facet on its dorsal surface for the triquetrum
- Serves as a pulley to increase mechanical advantage of flexor carpi ulnaris
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Flexor carpi ulnaris | Inserts onto the pisiform (contains the pisiform as a sesamoid) | Flexes and adducts the wrist |
| Abductor digiti minimi | Originates from the pisiform | Abducts the little finger |
| Flexor retinaculum | Medial attachment to pisiform and hook of hamate | Forms the roof of the carpal tunnel |
| Pisohamate ligament | Pisiform to hook of hamate | Transmits flexor carpi ulnaris force to the distal carpus |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Pisotriquetral joint | Synovial plane | Triquetrum |
Common Pathologies
Pisiform fracture
Uncommon fracture usually from a direct fall onto the palm. May compress the ulnar nerve in Guyon's canal. Best seen on a carpal tunnel view radiograph.
Pisotriquetral arthritis
Degenerative joint disease of the pisotriquetral joint causing ulnar-sided wrist pain. Pisiform excision provides definitive relief without significant functional loss.
Guyon's canal syndrome
Compression of the ulnar nerve as it passes adjacent to the pisiform in Guyon's canal, causing hand weakness and numbness in the ring and little fingers.
Clinical Relevance
The pisiform is a key landmark for Guyon's canal (the ulnar tunnel), where the ulnar nerve and artery enter the hand between the pisiform and the hook of the hamate. The pisiform can be moved side to side when the wrist is flexed, distinguishing it from other carpal bones. Pisiform excision for arthritis or fracture nonunion has minimal functional impact because the pisohamate ligament maintains force transmission.
Development and Ossification
The pisiform is the last carpal bone to ossify, with its center appearing between ages 9 and 12. As a sesamoid bone, it develops within the tendon of the flexor carpi ulnaris. It is the only carpal bone with a single articular surface.
Did You Know?
- Pisiform means pea-shaped in Latin (pisum = pea)
- The pisiform is the only carpal bone classified as a sesamoid bone
- The pisiform is the last carpal bone to ossify and has only one articular surface
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