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The pisiform bone is a small, pea-shaped bone found in the wrist or hand. It is one of the carpal bones, specifically one of four proximal row carpal bones. The pisiform bone is the last carpal bone to ossify, usually between the ages of 7 and 12. It acts as an attachment site for several muscles, tendons, and ligaments, including the flexor carpi ulnaris and abductor digiti minimi muscles. The pisiform bone contributes to the formation of the ulnar canal, which provides a passageway for the ulnar nerve and artery.
| Characteristics | Values |
|---|---|
| Bone Shape | Pea-shaped or knobbly |
| Bone Type | Sesamoid |
| Bone Location | Proximal row of carpal bones in the wrist |
| Bone Function | Provides smooth surface for tendons to glide over; acts as an attachment site for muscles and ligaments |
| Ossification | One of the last bones to ossify, typically between 7-12 years of age |
| Blood Supply | Ulnar artery |
| Injuries | Fractures, dislocation, avulsion injuries, and osteoarthritis are common issues |
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What You'll Learn
Pisiform bone structure
The pisiform bone is a small, pea-shaped bone found in the wrist, or more specifically, in the proximal row of the carpal bones. It is also known as the os pisiforme, derived from the Latin word "pisum", meaning pea. The pisiform is a sesamoid bone, which means it is embedded within a tendon, in this case, the flexor carpi ulnaris tendon. It is the last carpal bone to ossify, usually between the ages of 7 and 12, and it has a single ossification centre.
The pisiform bone has a unique structure among the carpal bones. It has a smooth and oval dorsal surface that articulates with the triquetral bone, forming the pisotriquetral joint. The palmar surface, on the other hand, is rough and rounded, providing attachment sites for various tendons and ligaments, including the flexor retinaculum, flexor carpi ulnaris, and abductor digiti minimi. The lateral surface is rough and concave, while the medial surface is convex and rough.
The pisiform is the only carpal bone that provides attachment sites for the abductor digiti minimi and flexor carpi ulnaris tendons. It also acts as one of the four attachment points of the flexor retinaculum, contributing to the stability of the wrist. The pisiform's articulation with the triquetral bone allows for a surprising range of movement, about 1 cm, which is necessary for the stability of the abductor digiti minimi tendon.
The pisiform bone plays a crucial role in the ulnar canal or Guyon canal, which is a fibro-osseous tunnel. It forms the ulnar border of this canal, through which the ulnar nerve and ulnar artery pass. This canal is important for providing vascularity to the wrist and hand.
In terms of evolutionary significance, the pisiform bone in humans is notably shorter compared to other non-human primates. This difference is believed to be due to a lost growth plate in hominins, possibly between Australopithecus afarensis and Homo neanderthalensis. Some scholars suggest that the reduction in the size of the pisiform bone allowed for greater extension in the wrist, increasing the capacity for throwing and clubbing in our hominin ancestors.
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Flexor carpi ulnaris muscle
The flexor carpi ulnaris (FCU) is a muscle of the forearm that flexes and adducts at the wrist joint. It is the most powerful wrist flexor and the only anterior forearm compartment muscle completely innervated by the ulnar nerve. The FCU tendon is a landmark for locating the ulnar nerve and artery during surgery, as these structures lie immediately lateral to the muscle.
The flexor carpi ulnaris has two heads: a humeral head and an ulnar head. The humeral head originates from the medial epicondyle of the humerus via the common flexor tendon. The ulnar head originates from the medial margin of the olecranon of the ulna and the upper two-thirds of the dorsal border of the ulna by an aponeurosis. The FCU inserts onto the pisiform, hook of the hamate (via the pisohamate ligament), and the anterior surface of the base of the fifth metacarpal (via the pisometacarpal ligament). The pisiform bone is a sesamoid bone that lies embedded within the flexor carpi ulnaris tendon, providing a smooth surface for it to glide over. It acts as an important attachment site for both the flexor carpi ulnaris and abductor digiti minimi muscles.
The flexor carpi ulnaris is innervated by the ulnar nerve's muscular branch, which arises from the C7, C8, and T1 nerve roots. The ulnar nerve enters the forearm between the two heads of the FCU. Proximally, a branch of the posterior ulnar recurrent artery supplies the muscle as it passes between the humeral and ulnar heads. Branches of the ulnar artery supply the middle and distal parts of the muscle, with an additional supply present distally via the inferior ulnar collateral artery.
The flexor carpi ulnaris is the most medial of the superficial flexors of the forearm, which include the pronator teres, palmaris longus, flexor digitorum superficialis, and flexor carpi radialis. Together with other muscles of the anterior forearm, flexor carpi ulnaris flexes the hand at the wrist. It can also move the hand sideways due to its position and direction in the forearm. When the flexor carpi ulnaris contracts with the extensor carpi ulnaris muscle in the posterior compartment, their counteracting forces produce adduction of the hand at the wrist, also known as ulnar deviation or ulnar flexion.
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Abductor digiti minimi muscle
The abductor digiti minimi muscle is a short intrinsic muscle of the hand. It is also referred to as Musculus abductor digiti minimi. It is an abductor of the little finger at the metacarpophalangeal joint. It also contributes to the extension of the middle phalanx of the little finger through its connection to the finger's extensor mechanism. The name is derived from the Latin "-ab", meaning "away from"; "ducere", meaning "to draw"; "digitus", meaning "digit"; and minimum or quintus, meaning "smallest" or "fifth", which together mean "abductor of the smallest or fifth finger".
The abductor digiti minimi muscle is part of a group of muscles collectively called hypothenar muscles due to their action on the fifth finger. Other hypothenar muscles include flexor digiti minimi brevis and opponens digiti minimi. The abductor digiti minimi is the most medial of the hypothenar muscles, situated along the ulnar border of the flexor digiti minimi brevis and superficial to opponens digiti minimi muscle. It is also the most variable hypothenar muscle.
The main function of the abductor digiti minimi muscle involves the abduction and flexion of the fifth finger at the metacarpophalangeal joint, moving it away from the fourth finger. At the interphalangeal joint, this muscle also aids the flexion of the fifth finger due to its attachment to its extensor expansion. This muscle plays an important role when the hand is grasping large objects with outspread fingers, such as a basketball.
The abductor digiti minimi muscle originates from the pisiform bone, a small, pea-shaped carpal bone on the medial side of the proximal carpal bones row. Other sites of origin include the pisohamate and pisometacarpal ligaments, which connect the pisiform bone to the hamate and metacarpal bones, respectively. The muscle fibres converge into a tendon and insert into the ulnar side of the proximal phalanx of the little finger and its extensor expansion.
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Pisiform's role in the wrist
The pisiform is a small, pea-shaped bone found in the wrist. It is one of the carpal bones, specifically a sesamoid bone, and is embedded within the flexor carpi ulnaris tendon. The pisiform bone is the last carpal bone to ossify, usually between the ages of 7 and 12, and is situated where the ulna joins the wrist. It sits in an anterior plane to the rest of the carpal bones and articulates with the triquetrum or triquetral bone. The pisiform has a smooth, oval dorsal surface and a rough, rounded palmar surface, which provides an attachment site for muscles and ligaments.
The pisiform plays a role in wrist stability and allows for wrist flexion and adduction. It acts as an attachment site for the flexor retinaculum, a fibrous band that spans the wrist, and for tendons of the abductor digiti minimi and the flexor carpi ulnaris. The pisiform is the only carpal bone with insertions for these tendons. The contraction of the flexor carpi ulnaris is necessary for the pisiform to remain stable enough for the abductor digiti minimi to function effectively.
In addition to providing attachment sites, the pisiform, along with the hamulus of the hamate, defines the medial boundary of the carpal tunnel. It also contributes to the formation of the ulnar canal (Guyon canal), which provides a passageway for the ulnar nerve and artery. The ulnar artery sits adjacent to a lateral surface groove of the pisiform and provides vascularity to the area.
Clinical studies have examined the impact of the pisiform on wrist function by studying its removal as a treatment for osteoarthritis in the pisotriquetral joint. Some studies suggest that the pisiform contributes to the stability of the ulnar column of the wrist, while others found that excision only slightly impaired the range of motion of the wrist, particularly in wrist extension. However, individuals who underwent excision reported impaired function when performing heavy lifting and weight-bearing activities.
The reduction in size of the pisiform bone during hominin evolution may have allowed for greater extension in the human wrist, increasing our capacity for throwing and clubbing. This reduction has been linked to the Hoxa11 and Hoxd11 genes, which are also associated with forearm development.
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Pisiform's evolutionary history
The pisiform bone is a small, knobbly bone found in the wrist, forming the ulnar border of the carpal tunnel. It is the last carpal bone to ossify, usually between the ages of 7 and 12. The pisiform bone has undergone significant evolutionary changes, particularly in hominoids (apes and humans). Humans have a short pisiform bone compared to other non-human primates, which is thought to be the result of a lost growth plate in hominins. This growth plate loss has led to the reduction of the pisiform bone in humans, which is hypothesized to have occurred around 1.5 million years ago with the emergence of Homo erectus.
The human pisiform is truncated and has lost one of the two ossification centers and its associated growth plate, a feature that is extremely rare among mammals. This raises questions about the homology of the human pisiform, specifically whether its sole ossification center is homologous to the primary or secondary center of other species. Studies have been conducted to understand the evolutionary changes in the pisiform bone by examining ossification patterns and timing in macaques, apes, and humans.
The drastic evolutionary changes in the human pisiform are associated with the evolution of novel locomotor patterns. Hominoid wrists are characterized by the proximal retreat of the distal ulna, resulting in the loss of articulation between the ulnar styloid process and the pisiform and triquetral bones. This allows for greater ulnar deviation, or bending of the wrist toward the ulnar side. The evolutionary changes in the pisiform bone may also be related to the development of the forearm, as the modification of Hoxa11 or Hoxd11 genes, which influence forearm length, could have contributed to the reduction of the pisiform bone during hominin evolution.
The pisiform bone plays a functionally significant role in the wrist, contributing to the stability of the ulnar column. Its reduction in size may have implications for wrist function, particularly in weight-bearing activities. However, further studies are needed to understand the effects of pisiform morphology on wrist functionality.
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Frequently asked questions
The pisiform is a small, pea-shaped bone found in the wrist. It is one of the carpal bones of the hand, sitting in the proximal row.
The pisiform acts as an attachment site for the flexor carpi ulnaris and abductor digiti minimi muscles.
The pisiform provides a smooth surface for the flexor carpi ulnaris tendon to glide over. It also contributes to the formation of the ulnar canal (Guyon canal), which allows the ulnar nerve and artery to pass through.
