The carpal bones are a collection of eight small, irregularly shaped bones that form the skeletal foundation of the wrist. They are located between the forearm bones—the radius and ulna—and the hand bones, known as the metacarpals. This cluster of bones acts as a flexible, mobile bridge, allowing the hand to move and articulate in a complex range of motions.
The Eight Carpal Bones
The eight bones are arranged into two distinct rows, referred to anatomically as the proximal and distal rows. The proximal row is the set of bones nearest to the forearm, while the distal row connects to the metacarpals of the hand. This two-tiered structure is fundamental to the wrist’s complex movement capabilities.
The proximal row contains four bones: the scaphoid, lunate, triquetrum, and pisiform. The scaphoid is named for its boat-like shape, and the lunate is crescent-shaped, while the pisiform is pea-shaped and sits within a tendon. This row primarily articulates with the radius of the forearm to form the main wrist joint.
The distal row also consists of four bones: the trapezium, trapezoid, capitate, and hamate. The capitate is the largest carpal bone and occupies a central position, often described as the “head” of the carpus. The hamate is distinguished by a small, hook-like projection on its palmar surface.
A common mnemonic helps organize the eight bones from the thumb side (radial) of the proximal row to the pinky side (ulnar) of the distal row. A useful phrase is “Sally Left The Party, To Take Cathy Home.” This sequence corresponds to the Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, and Hamate.
Primary Functions of the Carpal Bones
The collective arrangement of the carpal bones provides the wrist with exceptional mobility and a high degree of stability. The individual articulations between the bones create a complex mechanism that facilitates movements like flexion, extension, and side-to-side deviations.
The proximal row of bones is relatively loosely connected and moves as a single unit against the forearm bones, making it the primary site for wrist flexion and extension. This flexibility allows the hand to be positioned effectively in space. The distal row, conversely, is tightly bound together, moving more rigidly with the metacarpals of the hand.
The alternating tightness and looseness between the two rows allow motion to be distributed across multiple joints, providing a balance of motion and structural integrity. This complex kinematic chain allows the force generated in the forearm muscles to be effectively transmitted to the hand. The carpal bones essentially function as a kinetic bridge, translating muscular action into controlled hand movement.
Common Issues and Injuries
Due to their small size and location, carpal bones are highly susceptible to injury, particularly from falls onto an outstretched hand. Fractures of the carpal bones are relatively common, with the scaphoid being the most frequently broken. The scaphoid bone is vulnerable because it links the proximal and distal rows, making it subject to forces from both sides.
A fracture of the scaphoid bone presents a unique challenge to healing because of its unusual blood supply pattern. The blood vessels that nourish the bone enter mainly at the distal end, meaning a fracture can cut off the blood flow to the broken piece. This tenuous vascularity significantly slows the healing process and increases the risk of avascular necrosis, the death of bone tissue due to lack of blood supply.
The carpal bones are also integral to the anatomy of the carpal tunnel, a narrow passageway on the palm side of the wrist. The bones form a concave arch that serves as the floor and sides of this tunnel. A strong band of tissue, the transverse carpal ligament, stretches across this arch to form the roof.
This tunnel houses nine flexor tendons and the median nerve, which provides sensation to much of the hand. Carpal tunnel syndrome occurs when swelling of the tendons or surrounding tissues causes compression of the median nerve within this fixed, unyielding space.