The human wrist plays a central role in nearly every daily activity, from writing and typing to lifting and grasping. This complex anatomical structure allows for a wide range of motion and provides the dexterity that defines human interaction with the world. Understanding its intricate components offers insight into its remarkable capabilities.
The Carpal Count and Location
The wrist is composed of eight small, irregularly shaped bones known as the carpal bones. These bones are situated between the forearm’s long bones—the radius and ulna—and the metacarpal bones of the hand. They collectively form the carpus, which acts as a bridge connecting the hand to the forearm. The carpal bones are arranged into two distinct rows, contributing to the wrist’s flexibility and robust structure.
Naming the Carpal Bones
The eight carpal bones are divided into two rows: the proximal row, closer to the forearm, and the distal row, closer to the hand. The proximal row consists of four bones, moving from the thumb side (radial) to the pinky finger side (ulnar): the scaphoid, lunate, triquetrum, and pisiform. The scaphoid is boat-shaped and the largest bone in this row, while the lunate is crescent-shaped. The pisiform is a small, pea-shaped bone embedded within a tendon.
The distal row also contains four bones, moving from the thumb side to the pinky finger side: the trapezium, trapezoid, capitate, and hamate. The trapezium is a four-sided bone that articulates with the thumb’s metacarpal, allowing for its wide range of motion. The trapezoid is a wedge-shaped bone, and the capitate is the largest of all carpal bones, located centrally in the wrist. The hamate is identifiable by a distinctive hook-like projection. A helpful mnemonic to remember the names of these bones in order is “She Likes To Play, Try To Catch Her,” corresponding to Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate.
Role of Carpal Bones
The carpal bones work in concert to provide the wrist with extensive range of motion, stability, and adaptability. They articulate with the radius and ulna of the forearm to form the radiocarpal joint, which facilitates movements such as flexion, extension, and side-to-side (radial and ulnar) deviation. The two rows of carpal bones also articulate with each other at the midcarpal joint, enabling intricate gliding motions.
Beyond their role in forming joints, these bones also connect with the five metacarpal bones of the hand, which extend into the fingers. This complex network of articulations, supported by numerous strong ligaments, allows the wrist to distribute forces evenly across the hand and absorb impacts. This structure ensures the hand can perform a wide array of tasks, from powerful gripping to delicate manipulations, by providing a stable yet mobile base.
Common Wrist Concerns
The intricate anatomy of the carpal bones makes them susceptible to specific injuries and conditions. Fractures of the carpal bones are common, with the scaphoid bone being the most frequently fractured, often occurring from a fall onto an outstretched hand. The scaphoid’s unique blood supply can make healing slow and sometimes complicated.
Another common condition related to the carpal bones is carpal tunnel syndrome. The carpal bones form the floor and sides of a narrow passageway in the wrist called the carpal tunnel. This tunnel houses the median nerve and several tendons that control finger movement. When tissues within this tunnel swell or the space narrows, it can compress the median nerve, leading to symptoms like numbness, tingling, pain, and weakness in the hand and fingers.