The pisiform bone, named from the Latin word for “pea,” is the smallest of the eight carpal bones that form the wrist. It is uniquely positioned at the base of the hand on the side closest to the little finger. The pisiform plays a part in the complex mechanics and function of the wrist and hand.
Pinpointing the Location of the Pisiform Bone
The pisiform is found in the carpus (wrist area), specifically within the proximal row of carpal bones closest to the forearm. This small, pea-shaped bone is situated on the palmar (front) side of the wrist on the medial aspect, often called the ulnar side. It is the most prominent of the carpal bones on this side, lying anterior to the others.
A person can easily locate the pisiform bone by gentle palpation at the base of the palm, near the crease of the wrist. It feels like a small, hard bump located directly in line with the little finger. This superficial position makes it a useful anatomical landmark for clinicians.
Its placement is anterior to the other carpal bones, not deep within the wrist. This small bone is situated where the forearm’s ulna bone joins the wrist.
The Pisiform’s Role as a Sesamoid Bone
The pisiform bone is classified as a sesamoid bone, meaning it is embedded within a tendon. It develops directly within the tendon of the flexor carpi ulnaris (FCU) muscle. The FCU is one of the primary muscles responsible for flexing the wrist and deviating it toward the ulnar side.
This sesamoid arrangement means the pisiform acts as an anatomical pulley for the FCU tendon, which enhances the muscle’s mechanical efficiency. By positioning the tendon further away from the center of the wrist joint, the pisiform increases the lever arm of the FCU. This longer lever arm allows the muscle to generate greater force, increasing grip strength and stability during wrist movements.
The pisiform provides a stable base of attachment for several surrounding ligaments and muscles. Its palmar surface serves as an anchorage point for the transverse carpal ligament and the origin of the abductor digiti minimi muscle, which moves the little finger away from the others.
Immediate Neighbors and Connections in the Wrist
The pisiform’s immediate anatomical environment involves both a direct articulation and a relationship with surrounding neurovascular structures. Unlike most carpal bones, the pisiform articulates with only one other bone: the triquetrum. This articulation, known as the pisotriquetral joint, is a smooth surface on the dorsal side of the pisiform that rests against the triquetrum.
The pisiform is a prominent boundary for a narrow passageway on the ulnar side of the wrist called Guyon’s canal. This fibro-osseous tunnel allows the ulnar nerve and ulnar artery to pass from the forearm into the hand. The pisiform bone, along with the tendon of the flexor carpi ulnaris and the abductor digiti minimi muscle, forms the medial wall of this canal.
The bone’s superficial position helps define the tunnel’s structure and offers protection for the nerve and artery as they enter the hand. The canal spans from the proximal edge of the pisiform to the hook of the hamate bone.