The wrist, or carpus, is a complex joint connecting the forearm to the hand, allowing for a wide range of motion necessary for daily activities. This anatomical region requires a balance of mobility and structural support to manage forces transmitted from the hand to the arm. The wrist’s structure, made up of multiple small bones, facilitates movement while maintaining stability during gripping and other functional tasks.
The Precise Count and Arrangement
The human wrist contains eight small bones, collectively known as the carpal bones, which form the carpus. These bones are classified as short bones, meaning they are irregularly shaped. The arrangement of these small bones, rather than one or two larger structures, provides the flexibility needed for movements like flexion, extension, and side-to-side deviation.
The eight carpal bones are organized into two distinct, horizontal rows of four bones each. The proximal row is situated closer to the forearm bones (the radius and ulna), while the distal row lies closer to the five metacarpal bones of the hand. The two rows articulate at the midcarpal joint, allowing them to glide and shift in a coordinated manner during hand movement. This bi-level organization distributes stress and enables synchronized motion.
Naming the Eight Carpal Bones
The proximal row contains four bones, starting on the thumb side and moving toward the pinky side:
- The scaphoid, which is the largest bone in this row and is sometimes described as boat-shaped.
- The lunate, named for its crescent or moon-like shape.
- The triquetrum, which has a pyramidal form.
- The pisiform, a small, pea-shaped sesamoid bone that sits on the front surface of the triquetrum.
The distal row also consists of four bones, maintaining the radial-to-ulnar (thumb-to-pinky) order:
- The trapezium, a quadrangular shape that articulates directly with the first metacarpal bone of the thumb, enabling its wide range of motion.
- The trapezoid, a smaller, wedge-shaped bone, adjacent to the trapezium.
- The capitate, the largest of all the carpal bones, centrally located in the wrist.
- The hamate, which is recognizable by a distinct bony projection on its palmar side called the hook of the hamate.
Common Issues Related to Carpal Structure
The complex structure of the carpal bones makes them susceptible to specific injuries, particularly fractures. The scaphoid bone is the most frequently fractured carpal bone, typically resulting from a fall onto an outstretched hand. Fractures to the middle or proximal part of the scaphoid are concerning because the bone has a tenuous blood supply that enters mostly from one end.
If the fracture disrupts this limited blood flow, the bone tissue can die, a condition known as avascular necrosis. This complication often requires surgical intervention to prevent long-term wrist degeneration. Damage to the ligaments connecting these small bones can also lead to carpal instability, causing the bones to shift abnormally and resulting in chronic pain and eventual arthritis.