The human wrist features a complex arrangement that allows for a wide range of motion. The cluster of small, irregularly shaped bones forming this structure is known as the carpus, which connects the forearm bones to the hand. The wrist contains exactly eight individual carpal bones. This intricate group functions as a flexible unit, providing both stability and dexterity at the joint.
Naming the Eight Carpal Bones
These eight bones are given distinct names based on their unique shapes and positions. The carpal bones include the scaphoid, which is boat-shaped and is the most commonly fractured carpal bone. Beside it lies the lunate, named for its crescent-like appearance, which is centered near the forearm.
The triquetrum is a pyramidal bone situated on the little-finger side of the wrist. Resting on the triquetrum is the smallest carpal, the pisiform, which is pea-shaped and considered a sesamoid bone embedded within a tendon. The trapezium is a small, four-sided bone that articulates directly with the thumb metacarpal.
The trapezoid is wedge-shaped and the smallest bone in this second group, nestled between the trapezium and the capitate. The capitate is the largest of all eight carpal bones, serving as the central pillar of the wrist structure. Finally, the hamate is identifiable by a distinct hook-like projection on its front surface called the hook of the hamate.
Organization into Proximal and Distal Rows
The eight carpal bones are organized into two distinct, horizontal rows of four bones each. This arrangement allows the structure to move as two separate yet coordinated units. The proximal row is the set of bones closest to the forearm, forming the initial point of contact with the radius.
This proximal row includes the scaphoid, lunate, triquetrum, and pisiform. The bones in this row do not have muscle tendons directly attaching to them; their movement depends entirely on forces transmitted from surrounding joints and ligaments. The distal row is situated closer to the hand and connects to the metacarpal bones of the palm.
The distal row is composed of the trapezium, trapezoid, capitate, and hamate. The articulation between these two rows is known as the midcarpal joint. This joint permits gliding and rotational movements between the proximal and distal carpal groups, contributing significantly to the overall flexibility of the wrist.
The Role of Carpal Bones in Wrist Movement
The presence of eight small, independently articulating carpal bones, rather than a single large joint, provides the immense range of motion in the wrist. This segmented design allows the hand to move in two primary planes: flexion and extension (bending the hand down and up), and radial and ulnar deviation (moving the hand side-to-side).
Movement is distributed across two primary joint spaces: the radiocarpal joint and the midcarpal joint. During extension, approximately 60% of the motion occurs at the midcarpal joint between the two rows of carpal bones. Conversely, during flexion, the majority of the movement is concentrated at the radiocarpal joint where the proximal row meets the forearm bone.
The individual bones must glide and shift against each other synchronously to execute multi-directional movements. This coordinated motion is facilitated by strong intercarpal ligaments that bind the small bones together while permitting subtle shifts. This biomechanical system ensures the hand has the stability to grip and the flexibility to perform delicate tasks.