The carpus, commonly known as the wrist, is a complex anatomical structure connecting the forearm to the hand. This region is composed of a collection of small, irregularly shaped bones that facilitate a wide range of motion. Understanding the carpus involves knowing the precise number of bones present and how they are arranged to form the wrist joint.
The Eight Bones of the Carpus
One hand contains eight carpal bones, which form the carpus and provide the primary skeletal support for the wrist. These bones are classified as short bones due to their irregular, cube-like shapes. They are distinct from the long bones of the forearm (radius and ulna) and the long bones of the hand (metacarpals).
The eight individual bones each have a unique name based on their shape or position. The capitate is the largest of the carpal bones, while the pisiform is the smallest and is considered a sesamoid bone, meaning it is embedded within a tendon.
The eight carpal bones are:
- Scaphoid
- Lunate
- Triquetrum
- Pisiform
- Trapezium
- Trapezoid
- Capitate
- Hamate
How the Carpal Bones Are Organized
The eight carpal bones are structured into two distinct transverse rows, providing a stable yet flexible joint for the hand. The proximal row is closer to the forearm, while the distal row is closer to the fingers.
The proximal row includes the scaphoid, lunate, triquetrum, and pisiform bones. This row articulates with the radius of the forearm to form the primary wrist joint, allowing for the majority of wrist movement. The scaphoid bone is important because it bridges the two rows, contributing to the stability of the entire wrist structure.
The distal row consists of the trapezium, trapezoid, capitate, and hamate bones. This row connects directly to the bases of the five metacarpal bones, which lead out to the fingers. Unlike the proximal row, the distal row is relatively stable and acts as a fixed base for the hand bones, ensuring structural integrity during gripping and manipulation tasks.
The Role of the Carpal Bones
The collective function of the carpal bones is to connect the distal ends of the radius and ulna in the forearm to the metacarpals of the hand. This connection enables the complex, multi-directional movements that characterize the wrist joint. The two rows of bones glide and rotate against each other, allowing for synchronized motion of the hand.
The bones also form a natural arch on the palm side of the wrist, which creates a narrow passageway known as the carpal tunnel. This tunnel is formed by the concave arrangement of the bones and is spanned by a thick ligament. The tunnel protects the median nerve and nine flexor tendons that pass through to the hand, making the carpal bones fundamental to both mobility and sensory function.