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The wrist joint is the complex joint formed between the distal ends (furthest from the body) of the Radius and Ulna (two forearm bones) and the carpal bones. It connects the forearm to the hand and allows a good range of motion. Repetitive use does however frequently lead to injuries.
The Ulna is the larger of the two forearm bones, although it tapers at the wrist end, to become narrower than the Radius at this point. The Radius is positioned on the thumb side of the wrist, and the ulna on the little finger side.
They form the wrist joint with the carpal bones. Altogether there are 8 carpal bone which are arranged in two rows, proximal and distal:
- Lunate -proximal
- Triquetrum - proximal
- Pisiform - proximal
- Capitate - distal
- Trapezium - distal
- Trapezoid - distal
- Hamate - distal
The scaphoid bone crosses both rows as it is the largest carpal bone. The scaphoid and the lunate are the two bones which actually articulate with the radius and ulna to form the wrist joint.
Each bone within the wrist is joined to the one next to it by one or more ligaments. As you can imagine, this results in a large number of ligaments! Two of the largest ligaments of the wrist are the medial (ulnar) and lateral (radial) collateral ligaments. The MCL passes from the distal end of the ulnar and crosses the wrist to attach to the triquetrum and the pisiform. The LCL passes from the end of the radius, across the joint to the scaphoid.
Most of the muscles which act on the wrist joint, are situated within the forearm, with only the tendon crossing the joint and inserting on the hand. The muscles on the back of the forearm (dorsal aspect) act to extend the wrist or pull it back as if pulling a ring-pull:
- Extensor carpi radialis brevis
- Extensor carpi radialis longus
- Extensor carpi ulnaris
- Extensor digitorum communis
- Extensor pollicis longus
The muscles on the front of the forearm (palmer aspect) act to flex the wrist, such as when you push a roundabout:
- Flexor carpi radialis
- Flexor carpi ulnaris
- Flexor digitorum superficialis
- Flexor pollicis longus
Some of these muscles also help to perform radial and ulnar deviation. Radial deviation is the act of tilting the wrist in a radial direction (or with the thumb leading). Extensor carpi radialis brevis, longus and flexor carpi radialis all perform this movement. Ulnar deviation is the opposite movement, of tilting the wrist so that the little finger leads. Extensor carpi ulnaris and flexor carpi ulnaris perform this movement.
Three nerves pass from the forearm, across the wrist and into the hand. These are:
The radial nerve is on the radial, or thumb side of the wrist joint. It provides feeling to the back of the hand from the thumb to the middle finger
The median nerve is responsible for the development of carpal tunnel syndrome. It passes through the carpal tunnel and splits into four branches which each travel to the thumb and next three fingers. It provides sensation to all of these fingers, although only the inside half of the ring finger.
The ulnar nerve supplies the small finger and the outer half of the ring finger.