The wrist, or carpus, is a complex, flexible network of eight distinct bones known as the carpal bones. This intricate arrangement provides the hand with a remarkable range of motion and the necessary mobility for daily activities.
Identifying the Eight Carpal Bones
The eight carpal bones are irregularly shaped and organized into two transverse rows of four bones each. The proximal row is positioned closer to the forearm, while the distal row sits closer to the hand.
The proximal row consists of the Scaphoid, Lunate, Triquetrum, and Pisiform bones, moving from the thumb side toward the little finger side of the wrist. The Scaphoid is the largest bone in this row and serves as an important structural link between the two carpal rows. The Pisiform is unique as a small, pea-shaped sesamoid bone embedded within a tendon, making it the least involved in the main wrist joint.
The distal row includes the Trapezium, Trapezoid, Capitate, and Hamate bones. The Capitate is typically the largest of all eight carpal bones and occupies a central position within the wrist structure. The Hamate is distinctive for its hook-like projection on the palm side, which acts as an attachment point for certain wrist ligaments.
The Wrist Joint: Connecting Forearm to Hand
The main wrist joint, called the radiocarpal joint, is formed by the articulation between the proximal row of carpal bones and the radius bone of the forearm. The concave surface of the radius fits over the convex surfaces of the Scaphoid, Lunate, and Triquetrum bones.
The Ulna, the other bone in the forearm, does not directly articulate with the carpal bones because a fibrocartilage disc separates it from the wrist joint. Its involvement is primarily at the distal radioulnar joint, which allows for the rotation of the forearm. Distally, the carpal bones connect to the five Metacarpal bones, which form the skeletal structure of the palm.
This complex system of joints, including the radiocarpal and midcarpal joints, allows for significant movement, such as flexion, extension, and side-to-side deviation. The numerous small articulations and supporting ligaments work together to ensure both stability and mobility. The carpal bones essentially act as a flexible bridge, transferring forces from the hand to the forearm.
Vulnerabilities of the Carpal Structure
The dense clustering of the carpal bones makes them susceptible to specific injuries and conditions. The Scaphoid bone is the most frequently fractured carpal bone, often from a fall onto an outstretched hand. Its vulnerability is compounded by a unique blood supply that enters mainly from one end and flows backward toward the other.
A fracture, particularly in the middle of the bone, can severely disrupt this limited blood flow, leading to complications like non-union or bone tissue death known as avascular necrosis. Another common issue is Carpal Tunnel Syndrome, which results from the tight anatomical space created by the carpal bones. This arch of bones, along with the transverse carpal ligament spanning across them, forms a narrow tunnel.
The median nerve and nine flexor tendons pass through this restricted carpal tunnel. Swelling or inflammation of the surrounding tissues can compress the median nerve within this confined space. This compression is what causes the symptoms of Carpal Tunnel Syndrome, such as numbness and tingling in the fingers.