The pisiform bone is one of the eight carpal bones forming the wrist. Its position on the pinky-finger side of the wrist makes it easily palpable, located just above the crease where the hand meets the forearm. This bone plays a unique anatomical role in the complex framework of the wrist. The pisiform is a component of the proximal row of carpal bones, and its placement contributes to the overall mechanics of wrist function and strength.
Anatomy and Identification
The pisiform is unique among carpal bones because it is classified as a sesamoid bone. Sesamoid bones are embedded within a muscle’s tendon, rather than articulating directly with other bones at a joint surface. The pisiform develops directly within the tendon of the flexor carpi ulnaris (FCU) muscle, a major muscle responsible for wrist movement.
Its name is derived from the Latin word “pisum,” meaning “pea,” accurately describing its small, pea-shaped form. The bone is situated on the anterior side of the wrist, closer to the palm than the other carpal bones. It is the last carpal bone to ossify, typically completing its development by the age of 12.
The pisiform has a single articulation point, connecting exclusively with the triquetrum, another bone in the proximal row of the carpus. This joint, called the pisotriquetral joint, allows the pisiform a large range of motion against the triquetrum. The smooth, dorsal surface forms this joint, while its palmar surface is rough and serves as an attachment site for various ligaments and the abductor digiti minimi muscle.
Biomechanical Role in the Wrist
The primary function of the pisiform centers on its role as a mechanical spacer and a lever arm for the flexor carpi ulnaris muscle. By housing the pisiform, the FCU tendon gains a mechanical advantage, similar to how the kneecap (patella) enhances the force of the quadriceps muscle. This sesamoid placement increases the distance between the wrist’s center of rotation and the line of action of the muscle’s pull.
This increased distance translates into greater leverage, allowing the FCU muscle to generate improved force during wrist flexion (bending the wrist toward the palm). The FCU muscle is also a primary contributor to ulnar deviation (bending the wrist toward the pinky finger side). The pisiform’s presence amplifies the power of this motion, which is important for activities requiring a strong grip or forceful wrist movements.
The bone also serves as an attachment point for the pisohamate and pisometacarpal ligaments, which extend to the hamate bone and the fifth metacarpal bone. These connections help stabilize the bone and transmit the force of the FCU further into the hand, contributing to the stability of the wrist’s ulnar column.
Injuries and Associated Pain Syndromes
Because of its superficial location on the ulnar side of the wrist, the pisiform is vulnerable to specific types of trauma and chronic stress.
Fractures and Dislocation
Pisiform fractures are rare among carpal bone injuries, but they often result from direct impact. A common mechanism is a fall onto an outstretched hand or a direct blow to the heel of the hand, such as from using a hammer or during certain sports.
Pisiform fractures are frequently difficult to identify on standard X-rays. When a fracture is suspected, specialized views like a carpal tunnel view, or advanced imaging such as a CT scan, may be necessary to delineate the fracture pattern. Treatment for non-displaced fractures typically involves immobilization of the wrist for several weeks.
Another potential injury is pisiform dislocation, where the bone is forced out of its articulation with the triquetrum. This is a rare injury, often occurring with significant hyperextension of the wrist, which causes the FCU tendon to pull the pisiform out of place. Dislocation can occur in isolation or alongside other severe wrist injuries.
Pisiformitis and Treatment
A chronic pain condition associated with the pisiform is pisiformitis, which involves inflammation or tendinopathy of the surrounding structures. This syndrome often results from repetitive microtrauma or prolonged pressure on the bone, such as from activities like cycling, where the handlebar presses into the hypothenar area of the palm.
Diagnosis for both acute injuries and chronic pain is initiated by palpation, which reveals tenderness and swelling directly over the bone. In cases of chronic pain or severely displaced fractures, surgical removal of the pisiform, known as a pisiformectomy, can be an effective treatment option to alleviate symptoms.