The wrist is a highly flexible junction connecting the long bones of the forearm to the hand. This area allows for a wide range of motion, including bending, straightening, and side-to-side movements. Anatomically, the wrist is a sophisticated assembly of many smaller, uniquely shaped bones, which enables the hand to perform intricate tasks.
The Carpus: The Wrist Bone Group
The bones that form the wrist are collectively known as the carpal bones, or the carpus. This grouping is made up of eight small, irregularly shaped bones tightly packed and interconnected by numerous ligaments. Their role is to provide a flexible skeletal bridge between the forearm and the hand. These eight bones are organized into two distinct, transverse rows, each containing four bones, which facilitate stability and movement.
The Proximal Row: Connecting to the Forearm
The row closest to the arm is called the proximal row, and it includes four bones that articulate directly with the radius bone of the forearm. These bones are named the Scaphoid, Lunate, Triquetrum, and Pisiform, moving from the thumb side toward the pinky side.
Scaphoid and Lunate
The Scaphoid is the largest bone in this row and is boat-shaped, playing a role in midcarpal stability. Due to its shape and location, the Scaphoid is the most frequently fractured bone in the wrist. The Lunate, named for its crescent shape, sits in the center of the proximal row, articulating with the radius alongside the Scaphoid.
Triquetrum and Pisiform
Moving toward the little finger side is the pyramid-shaped Triquetrum. The Pisiform is a small, pea-shaped bone that sits on the Triquetrum. The Pisiform is classified as a sesamoid bone, meaning it is embedded within the tendon of the flexor carpi ulnaris muscle.
The Distal Row: Connecting to the Hand
The second set of four bones forms the distal row, which is rigidly attached to the five metacarpal bones of the hand. This row includes the Trapezium, Trapezoid, Capitate, and Hamate, moving across the wrist from the thumb side.
Trapezium and Trapezoid
The Trapezium connects specifically to the first metacarpal, forming the highly mobile joint at the base of the thumb. This saddle-shaped articulation is fundamental for the thumb’s ability to oppose the other fingers, allowing for grasping. The Trapezoid is a smaller, wedge-shaped bone that primarily articulates with the second metacarpal of the index finger.
Capitate and Hamate
The Capitate is the largest of all the carpal bones and occupies a central position, articulating mainly with the third metacarpal of the middle finger. The Hamate connects with the metacarpals of both the ring and pinky fingers. A feature of the Hamate is a prominent projection on its palm-facing surface known as the hook of the Hamate.