The wrist is the anatomical region that serves as the bridge between the forearm and the hand. It is a highly complex structure, far from a simple hinge joint, responsible for a wide range of movements. It is formed by a cluster of eight small, irregularly shaped bones that work together, allowing the hand to achieve the dexterity necessary for daily tasks.
The Carpus: Defining the Wrist Joint Region
The bones of the wrist are collectively known as the carpus, and the eight small bones that make it up are called the carpal bones. These bones are grouped into a compact mass that creates an arch, which is concave on the palmar side. The carpal bones are organized into two distinct horizontal rows, stacked one upon the other: the proximal row and the distal row. This two-row organization is foundational to the wrist’s entire function and structure.
The Proximal Row: Connecting to the Forearm
The proximal row is the group of carpal bones situated nearest to the forearm, forming the main articulation point of the wrist joint. Starting from the thumb side and moving across, the four bones in this row are the scaphoid, lunate, triquetrum, and pisiform. The scaphoid, lunate, and triquetrum articulate directly with the radius, the larger of the two forearm bones, creating the radiocarpal joint. Notably, the ulna, the other forearm bone, does not directly contact the carpal bones but is separated by a disc of cartilage. The pisiform is unique because it is a small, pea-shaped bone that rests on the front surface of the triquetrum. It is considered a sesamoid bone, meaning it is embedded within the tendon of the flexor carpi ulnaris muscle.
The Distal Row: Connecting to the Hand
The distal row of carpal bones is positioned closer to the fingers, acting as the foundation for the palm of the hand. This row contains four bones, moving from the thumb side toward the little finger: the trapezium, trapezoid, capitate, and hamate. These four bones articulate with the five metacarpals, which are the long bones that form the palm. The trapezium forms a specialized joint with the first metacarpal, allowing the thumb its wide range of motion. The capitate is the largest of all eight carpal bones and occupies a central position. The hamate is recognizable by a distinctive hook-like projection extending toward the palm.
Structural Importance of the Carpus
The architecture of the carpus, featuring eight separate bones arranged in two rows, provides a biomechanical advantage over a single, solid bone. This segmented structure allows for a combination of mobility and stability. The numerous small joints between the carpal bones permit subtle gliding and sliding motions, which contribute significantly to the overall flexibility of the wrist. This movement enables the hand to perform a wide range of actions, including bending, extending, and rotating. The interlocking nature of the carpal bones creates a strong, stable arch capable of distributing and absorbing forces. This arrangement helps manage the shock and load experienced during activities like gripping, lifting, or supporting weight.