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The radiocarpal joint is the anatomical name for the articulation between the radius bone of the forearm and the carpal bones of the hand, more commonly known as the wrist joint. Classified as a synovial joint, this articulation is held together by ligaments and features a fluid and cartilage-filled cavity between the bones that is referred to as the synovial capsule. Movements that may be performed at the radiocarpal joint include adduction and abduction, or tilting the hand from side to side upon the wrist, as well as flexion and extension, or bending the hand from front to back upon the wrist.
Named for the bones it joins, the radiocarpal joint involves a total of four bones: the radius, scaphoid, lunate, and triquetrum. The radius is the long bone of the forearm whose distal or lower end meets the bones of the carpus in the hand. These bones, the scaphoid, lunate, and triquetrum, form what is known as the proximal row of the carpus, the cluster of eight small bones below the wrist. While the scaphoid and lunate come into contact with the radius bone in the radiocarpal joint, the triquetrum really only does so when the hand is adducted, or tilted in the direction of the pinky finger. This joint between the radius and the carpal bones is referred to as a condyloid or ellipsoid joint, which means that the concave surface of the radius curves around the adjacent convex surface of the carpus.
Components of the radiocarpal joint may be classified as intrinsic or extrinsic to the joint. Intrinsic to the joint is a fluid-filled capsule that is surrounded by the synovial membrane. Continuous with similar cavities among and between the carpal bones, this space between the radius and the carpus contains the membrane, which releases the synovial fluid that fills and lubricates the joint. Also within the membrane is the joint cartilage, which pads the space so that the bones do not rub directly against one another. This space is further penetrated by blood vessels that supply nutrients to the joint.
Extrinsic to the radiocarpal joint are the wrist ligaments. Ligaments are mostly made of collagen, strong fibers of connective tissue that link the bones and surround and protect the joint. On the palm side of the wrist are the palmar radiocarpal ligaments, which run between the radius and the scaphoid, the radius and the lunate, and the radius and the triquetrum. Similarly, the dorsal ligaments on the back of the wrist connect the radius to these bones on their opposite sides. Also extrinsic to the radiocarpal joint is a large articular disk immediately alongside the joint on the medial or pinky-finger side of the wrist, between the distal or lower end of the ulna bone in the forearm and the triquetrum and pisiform bones of the carpus.
By linking the forearm to the hand, the radiocarpal joint permits multiple movements at the wrist. Muscles in the anterior forearm on the palm side can flex or curl the hand, while those on the posterior forearm on the dorsal side can extend the hand, or bend it backward. Additional muscles of the forearm can adduct or abduct the hand on the wrist, tilting it in the direction of the thumb or pinky. More complex motions can be performed through the simultaneous movement of the radiocarpal joint, the radioulnar joint, and the intercarpal joints of the hand.