The wrist, known anatomically as the carpus, is a complex joint that forms the bridge connecting the forearm to the hand. It is not a single bone but rather a collection of small, irregularly shaped bones that work together to allow a wide range of motion. This intricate bony structure provides both the flexibility needed for delicate hand movements and the stability required for gripping and bearing weight.
The Eight Carpal Bones
The bones of the wrist are collectively called the carpal bones, totaling eight. These small, irregularly shaped bones are arranged in two distinct rows, each containing four bones. They are typically identified starting from the thumb side (radial) and moving inward toward the little finger (ulnar) side.
The eight carpal bones are the scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, and hamate. The scaphoid is boat-shaped, the lunate is crescent-shaped, the triquetrum is pyramid-shaped, and the pisiform is a small, pea-shaped bone.
The trapezium is quadrangular and positioned at the base of the thumb. The trapezoid is a smaller, wedge-shaped bone next to it. The capitate is the largest carpal bone, situated near the center of the wrist, and the hamate is distinguished by a small, hook-like projection on its palmar surface.
Organizing the Wrist Bones into Rows
The eight carpal bones are functionally grouped into two transverse rows: the proximal row and the distal row. The proximal row is closest to the forearm and includes the scaphoid, lunate, triquetrum, and pisiform. This row is considered more mobile because it lacks direct tendon attachments and moves primarily in response to surrounding articulations. The scaphoid is particularly important as it acts as a structural link between the two rows.
The distal row is positioned closer to the hand, consisting of the trapezium, trapezoid, capitate, and hamate. This row is more rigid and stable compared to the proximal row. Strong ligamentous connections within the distal row mean that it tends to move as a single unit. The primary role of this row is to articulate with the bones of the hand, providing a stable base for movement.
Connecting the Forearm and Hand
The carpal bones function as an intermediary structure, connecting the long bones of the forearm to the smaller bones of the hand. The main wrist joint, known as the radiocarpal joint, is formed by the articulation of the proximal carpal row with the forearm bone called the radius. The lunate, scaphoid, and triquetrum of the proximal row work together to form this joint. This articulation allows for flexion, extension, and side-to-side movements.
It is the radius, positioned on the thumb side of the forearm, that bears the majority of the force and articulates directly with the wrist bones. The other forearm bone, the ulna, does not directly participate in the radiocarpal joint. Instead, a disc of cartilage separates the ulna from the carpal bones.
Distally, the four carpal bones of the distal row articulate with the bases of the five metacarpal bones of the hand, forming the carpometacarpal joints. These five metacarpals form the framework of the palm, and from them extend the bones of the fingers.