The carpal bones are eight short bones, arranged into two rows of four, situated in the wrist. The proximal row is the row closest to the arm, situated next to the ulna and radius. The distal row is situated next to the metacarpals, which are the first row of bones in the hand.
The proximal row of carpal bones, which lay closest to the ulna and radius, from the lateral to the medial, consist of the scaphoid, lunate, triquetrum, and pisiform. In this context, lateral means the side furthest away from the body, and medial means the side of hand closest to the middle of the body. The distal row of bones, which lay farthest away from the radius and ulna, closest to the bones of the hand, from medial to lateral, consists of the hamate, capitate, trapezoid, and trapezium.
The carpal bones are short bones. Short bones have a compact bone surface surrounding a cancellous, or porous, bone center. The bone center contains small spaces which are usually filled with marrow.
As a group, the eight carpal bones form a concave structure anteriorly, meaning that they have an inward curve, forming an indent or shallow dip when viewed from the front. Viewed from the back, these bones form a convex structure, meaning that they bulge outward. The flexor retinaculum is a large, tough ligament which stretches over the carpal bones on the anterior side. This ligament is attached to a small protrusion on the trapezium at the base of the thumb, and is also attached to a small protruding hook-like structure on the hamate bone. The indented carpal bones, covered by the flexor retinaculum, form the carpal tunnel.
The median nerve, the flexor tendons of the fingers, and blood vessels pass through the carpal tunnel, running from the forearm into the hand. The carpal bones and the flexor retinaculum have a very limited capacity to stretch. This means if any inflammation, accumulation of fluid, irritation, or inappropriate deposits of tissue occurs within the carpal tunnel, the median nerve or one of the other structures in the tunnel becomes trapped against the bones or the flexor retinaculum. This condition is known as carpal tunnel syndrome.
The anatomical position of the carpal bones forms a joint, allowing for flexibility and rotation of the hand and wrist. Four surfaces of the bones are surrounded by cartilage. This allows the joint or the bones to articulate and move against one another smoothly, without causing pain or discomfort. When this cartilage degrades, the bones grate against one another and can cause moderate to severe pain.
The structure of the carpal bones, and the range of motion in the wrist, means that this area is particularly prone to injury. Injury can happen during sporting activities, as well as being caused by falls and repetitive motion. Most injuries occur when the wrist is in a flexed position, as the carpal bones, tendons, and ligaments are under the most stress when flexed.