thumb|Pisiform of the right hand as seen on X-ray (labeled Pi)
The pisiform bone ( or ), also spelled pisiforme (from the Latin pisiformis, pea-shaped), is a small knobbly, sesamoid bone that is found in the wrist. It forms the ulnar border of the carpal tunnel.
Structure
The pisiform is a sesamoid bone, with no covering membrane of periosteum. It is the last carpal bone to ossify. The pisiform bone is a small bone found in the proximal row of the wrist (carpus). It is situated where the ulna joins the wrist, within the tendon of the flexor carpi ulnaris muscle.
It only has one side that acts as a joint, articulating with the triquetrum. It is on a plane anterior to the other carpal bones and is spheroidal in form.
The pisiform bone has four surfaces:
- The posterior surface is a smooth, oval facet, for articulation with the triquetrum. This facet approaches the superior, but not the inferior, border of the bone.
- The anterior or palmar surface is rounded and rough, and gives attachment to the transverse carpal ligament, the flexor carpi ulnaris and the abductor digiti quinti.
- The lateral surface is rough, and concave.
- The medial surface is rough, and convex.
Etymology
The etymology derives from the Latin pisum which means "pea" ultimately derived from the Greek "pison" (pea).
Function
The pisiform bone is most recognizable as an unassuming palmar projection forming the heel of human hand.
The pisiform bone, along with the hamulus of the hamate, defines the medial boundary of the carpal tunnel It also acts as an attachment site for tendons of the abductor digiti minimi and for the flexor carpi ulnaris - the tendon in which it develops.
In clinical studies, the pisiform has been removed as treatment for osteoarthritis in the pisotriquetral joint. While some studies came to the conclusion that the pisiform "contributes to the stability of the ulnar column of the wrist", others suggested that while excision slightly impairs the range of motion of the wrist (especially wrist extension), the forces generated within the wrist are not significantly impacted. Subjects in the latter study did report impaired function after excision when performing heavy lifting and weightbearing activities, but this is suggested to be subjective considering that they did not have to change occupation or their level of activity as a result of the excision. Scholars with this point of view would believe that these anatomical changes would improve the action of clubbing in our hominin ancestors.
Others suggest that the pisiform's link with Hoxa11 and Hoxd11 could tie its developmental history to that of the forearm, whose length is determined by Hox gene expression. In mammals and non-human primates, the pisiform is an enlarged and elongated bone that articulates with the distal ulna.