The human wrist is composed of eight separate bones that link together -- the carpal bones. Two of the carpal bones create the wrist joint with the ulna and radius bones of the forearm, while the remaining six form the palm. Each of the carpal bones helps enable the many movements of the hand and wrist.
There are multiple ways to classify the eight carpal bones. Medically and biomechanically, it makes the most sense to view the bones as three columns:
Alternatively, most texts and physicians describe the bones as existing on two rows.
"Proximal” describes a body part that is closest to the center of the body. In the case of the carpal bones, the proximal row is the one closest to the wrist. The scaphoid has a boat-like shape and is the largest carpal bone in the proximal row. The lunate is a moon-shaped bone that is the most commonly dislocated carpal bone. Together, the scaphoid and the lunate create half of the wrist joint, with the other half being the ulna and radius bones of the arm. The final two carpal bones on the proximal row are the triquetrum and the pisiform. The triquetrum has a pyramidal shape while the pisiform is small and round.
The carpal bones on the distal row are closer to the center of the hand and make up the center portion of the palm. Beginning at the thumb side of the hand, the trapezium has four distinct sides. The trapezoid has a wedge shape and is similar to the trapezium. It is the smallest bone in the distal row. The central capitate is the largest carpal bone. Finally, the hamate has a wedge shape and a hook-like process that reaches towards the surface of the palm.
Within the wrist are three separate groups of ligaments that connect the carpal bones to the metacarpal bones of the fingers as well as the radius and ulna.
In anatomy, when a bone articulates with another bone, they create a movement, forming a joint, or both. For example, the scaphoid and lunate articulate with the ulna and radius to form the wrist joint and create wrist movements. The pisiform articulates with the triquetrum bone. The trapezium articulates with the first two metacarpals, as well as the scaphoid and trapezoid. The trapezoid also articulates with the second metacarpal, the capitate, and scaphoid bone. The capitate articulates with the scaphoid, hamate, and lunate, in addition to its first two articulations. Finally, the hamate articulates with the fourth and fifth metacarpals, the triquetrum, and the lunate.
Anatomists use specific terms to describe the movements that the body can perform. Radial abduction is a sideways movement -- the wrist turning to move the thumb closer to the forearm, such as when waving. In this movement, the scaphoid tilts towards the palm while the trapezium and trapezoid move closer to the radius bone. Because of the movement of each of the carpal bones, the pisiform has the longest path.
Ulnar adduction is the opposite movement, where the wrist turns to move the pinkie finger closer to the forearm. In this movement, the proximal row of the carpal bones tilts towards the back of the hand. The head of the capitate bone is like an axis point for both movements.
If radial abduction and ulnar adduction are moving the wrist side-to-side, palmar flexion and dorsiflexion are the up and down movements. During palmar flexion, the palm is moving closer to the forearm. As the hand moves, the proximal carpal bones begin to shift towards the back of the hand. In dorsiflexion, the proximal carpal bones shift towards the palm. The lunate bone and the capitate bone are the axis points for these movements.
When a fetus begins to develop, her bones are not the hard, rigid bones that she will eventually have. Instead, the bones harden through the process of ossification. The carpal bones undergo endochondral ossification, which means they begin to harden from the inside first. The centers of ossification for the carpal bones only appear after birth, and they follow a spiral pattern. First the capitate and hamate harden, with the triquetrum following shortly after. Then the ossification order continues with the lunate, trapezium, trapezoid, and scaphoid. The pisiform is the last bone to develop fully.
Together, the eight carpal bones form an arch. On the side closest to the palm, the arch is concave and provides the walls for the carpal tunnel. Specifically, the bones that connect to the tunnel are the scaphoid, trapezium, pisiform, and hamate. A membranous tissue stretches over the tunnel to complete its shape and act as its roof. This tissue is the flexor retinaculum. The carpal tunnel is important for several reasons, most notably because it protects the median nerve and the flexor tendons that allow the fingers to bend. Carpal tunnel syndrome occurs when the tunnel narrows, placing pressure on the median nerve. This typically causes weakness, shock-like sensations, and pain.
There are many ways a person can fracture the carpal bones, but the most common is a fall onto an outstretched hand. Alternatively, an injury may result from direct trauma such as in sports like golf or tennis, where a person holds an instrument that receives force. Most often, the scaphoid is the carpal bone that experiences the fracture, because of its size and location within the hand. In cases where direct trauma hits the hand, the hamate bone is the most common location of the fracture.
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