The human wrist’s core consists of eight small bones, known as the carpal bones. These intricately arranged bones form the hand’s foundation, connecting it to the forearm. They facilitate a wide range of hand functions, from delicate movements to powerful gripping actions.
The Eight Bones
In the proximal row, closer to the forearm, are the scaphoid, lunate, triquetrum, and pisiform bones. The scaphoid, boat-shaped, is positioned on the thumb side of the wrist. Adjacent to it is the lunate, which is crescent-shaped. The triquetrum, a pyramidal bone, sits next to the lunate, and the small, pea-shaped pisiform rests on the triquetrum’s palmar surface.
Moving to the distal row, closer to the fingers, are the trapezium, trapezoid, capitate, and hamate. The multi-faceted trapezium articulates with the thumb’s metacarpal bone. Next to the trapezium lies the trapezoid, the smallest bone in the distal row, wedge-shaped. The capitate is the largest carpal bone, centrally located and head-shaped. Finally, the hamate, with a prominent hook-like projection on its palmar side, is situated on the little finger side of the wrist. A common mnemonic to remember these bones from radial to ulnar, then proximal to distal, is “Some Lovers Try Positions That They Can’t Handle.”
How They Form the Wrist
The proximal row, which includes the scaphoid, lunate, triquetrum, and pisiform, articulates directly with the radius bone of the forearm. This articulation forms the primary wrist joint, allowing movement between the hand and forearm. The ulna, the other forearm bone, also contributes to wrist stability, though it does not directly articulate with the carpal bones.
The distal row, composed of the trapezium, trapezoid, capitate, and hamate, connects to the metacarpal bones of the hand. This arrangement allows for movement between the carpal bones themselves and between the carpals and metacarpals. The carpal bones are tightly bound together by numerous ligaments. This forms a natural arch, known as the carpal arch, on the palmar side of the wrist. This arch provides stability and creates the carpal tunnel, a passageway for nerves and tendons that extend into the hand.
Their Role in Hand Movement
The intricate arrangement of the carpal bones and their multiple small joints enable the wrist to perform a wide array of complex movements. These bones facilitate flexion (bending the hand forward), extension (bending the hand backward), radial deviation (tilting the hand towards the thumb side), and ulnar deviation (tilting the hand towards the little finger side). The collective motion of these small bones allows for smooth, coordinated wrist movement rather than relying on a single, large joint.
The carpal bones also transmit forces from the hand to the forearm. When gripping an object or absorbing an impact, these bones distribute the stress across the wrist, protecting the forearm and hand. Their ability to absorb shock supports daily activities, from typing to lifting. This dynamic interplay allows for the precision and dexterity required for fine motor tasks.
Common Conditions Affecting Carpal Bones
Carpal bones are susceptible to certain conditions due to their complex structure and frequent involvement in hand activities. Fractures are a common injury, particularly to the scaphoid bone, which is frequently broken during a fall onto an outstretched hand. Scaphoid fractures can be challenging to heal due to the scaphoid’s limited blood supply.
Carpal instability occurs when the ligaments connecting the carpal bones are damaged or stretched. This causes abnormal movement between the bones, leading to pain and reduced function. The carpal arch also plays a role in carpal tunnel syndrome, a condition where the median nerve, which passes through the carpal tunnel, becomes compressed. While carpal tunnel syndrome primarily involves the nerve, the bony arch forms the boundaries of the space.