The wrist, along with the shoulder girdle and the elbow, is one of the three, chained mechanisms that comprise the upper limb. Among the most complex joints in the body, the wrist is the flexible, narrow connection between the forearm and the hand. It has eight bones called carpal bones, which are arranged in two rows of four – the proximal row sits near the crease of the wrist and the distal row sits closer to the fingers. Starting at the thumb side of the wrist, the proximal row of carpal bones consists of the scaphoid, lunate and triquetrum and connects the two bones of the forearm – the radius and the ulna – to the hand bones. The distal row of carpal bones consists of the trapezium, trapezoid, capitate, hamate and pisiform. Contributing to the complexity of the wrist joint is the fact that each small carpal bone forms a joint with the bone next to it. Therefore, the wrist is actually a joint comprised of many smaller joints.
Articular cartilage, which covers the ends of the bones at each joint, is thinner in the wrist joint than in other large weight-bearing joints of the body – yet provides a smooth buoyant surface that allows the joint surfaces to glide across one another without causing damage. It’s function is to serve as a type of shock absorber and facilitate movement. Articular cartilage in the wrist covers the sides of all of the carpal bones, as well as the ends of the bones connecting from the forearm to the fingers.
The joint is surrounded by a capsule, which is strengthened by the volar radiocarpal, dorsal radiocarpal, the ulnar, and the radial collateral ligaments. The synovial membrane lines the surfaces of these ligaments. The collateral ligaments are two important ligaments supporting each side of the wrist and connecting it to the forearm. The ulnar collateral ligament (UCL) is located on the ulnar side of the wrist – originating at the ulnar styloid, the bony bump at the edge of the wrist on the side furthest from the thumb. The UCL provides support to a small body of cartilage where the ulna meets the wrist, which is called the triangular fibrocartilage complex (TFCC). The TFCC is located between the ulna and two carpal bones, the lunate and the triquetrum. It helps cushion the area of the wrist joint in which it is located and works to improve the range of motion and ease of movement within the wrist joint.
The radial collateral ligament (RCL) is located at the thumb side of the wrist and originates at the outer edge of the radius on a small bump called the radial styloid. It connects at the side of the scaphoid and prevents the wrist from bending too far to the side opposing the thumb.
In the wrist, the median nerve and the tendons that flex the fingers to form a fist travel through the carpal tunnel compartment. The median nerve supplies sensation to the thumb, the first two fingers, part of the third finger, and some of the hand muscles. It also sends a nerve branch to control the thenar muscles of the thumb. The carpal tunnel is comprised of two walls – one of which is thick and consists of the carpal bones in the wrist and the other is shallow and consists of a thick ligament called the transverse carpal ligament located just under the skin at the palm side of the wrist. The ulnar nerve passes through the Guyon’s canal, which is comprised of the two carpal bones the pisiform and the hamate.
The nerves traveling through the wrist are subjected to constant bending and straightening of the wrist, which can lead to pressure on the nerves within their tunnels (Carpal tunnel and Guyon’s canal) and result in pain, weakness in the hand and numbness.
Because the wrist is comprised of many small bones and ligaments connecting to and supporting these bones and the large role it plays in both daily activities and sports, it is often subject to injury. It is vulnerable to both repetitive stress conditions as well as a variety of wrist fractures – as it is often instinctively used to shield the upper body from an impact or soften a fall. And ligaments in the wrist that are stretched or torn can eventually lead to arthritis.
In wrist lacerations, the tendons to the middle and ring fingers are most likely to be injured, because they run closer to the surface of the skin than those of the index and little fingers. Such loss of tendon function often indicates damage to the median nerve, which is also superficially located.