The wrist, or carpus, is a complex joint connecting the forearm to the hand. It is not a single joint but a collection of small, irregularly shaped bones working together. The wrist contains eight bones, which form the flexible and robust structure of the carpus.
The Specific Count: Eight Carpal Bones
The wrist is composed of eight bones, collectively known as the carpal bones or carpals. These small bones form the root of the hand, providing a mobile base for the palm and fingers. All eight are considered short bones, meaning they are roughly cube-shaped, which allows them to glide against one another. The eight carpal bones are:
- Scaphoid
- Lunate
- Triquetrum
- Pisiform
- Trapezium
- Trapezoid
- Capitate
- Hamate
Anatomical Organization of the Wrist
The eight carpal bones are organized into two distinct rows: the proximal row and the distal row. This arrangement defines the architecture and movement of the wrist. The proximal row is closest to the forearm, while the distal row is positioned closer to the hand’s metacarpals.
The proximal row includes the Scaphoid, Lunate, Triquetrum, and Pisiform. These bones articulate directly with the radius, one of the two forearm bones, to form the primary wrist joint, known as the radiocarpal joint. This row is more mobile and adapts constantly to hand movements.
The distal row includes the Trapezium, Trapezoid, Capitate, and Hamate. This row forms a stable unit that connects to the five metacarpal bones of the hand. The articulation between the two carpal rows is called the midcarpal joint, and motion across both joints allows for the wide range of wrist movements.
Role of the Carpal Bones in Movement and Stability
The eight separate bones allow the wrist to perform complex movements that a single joint cannot. The multiple small articulations facilitate a wide range of motion, including flexion and extension (bending the wrist forward and back) and radial and ulnar deviation (side-to-side movement). The gliding of the two carpal rows ensures the synchronization of these movements.
The carpal bones also play a role in load transfer and shock absorption between the hand and the forearm. Their structure forms a transverse arch, providing a rigid foundation for gripping objects. Stability is maintained by numerous strong ligaments that interconnect the bones.
The Capitate bone, the largest carpal bone, acts as a central pillar linking the two rows and coordinating their motion. This interplay between bones and ligaments enables the dexterity and strength characteristic of the human hand.
Common Injuries Related to Carpal Structure
The intricate structure of the carpus is susceptible to specific injuries, often resulting from a fall onto an outstretched hand. The Scaphoid bone is the most frequently fractured carpal bone, accounting for approximately 60% to 70% of all carpal fractures. Its vulnerability is partly due to its position bridging the two carpal rows.
The Scaphoid has a precarious blood supply that enters distally and flows backward (retrograde blood flow). A fracture can easily disrupt this supply to the proximal portion of the bone, leading to a high risk of non-union or delayed healing.
The arrangement of the carpal bones forms a concavity on the palm side, covered by a ligament, creating the carpal tunnel. This tunnel serves as a passageway for the median nerve and several tendons. If the space inside this tight tunnel decreases, the median nerve can become compressed, causing Carpal Tunnel Syndrome, which results in numbness, tingling, and weakness in the hand.