The wrist joint (also known as the radiocarpal joint) is a synovial joint in the upper limb, marking the area of transition between the forearm and the hand.
In this article, we shall look at the structures of the wrist joint, the movements of the joint, and the relevant clinical syndromes.
Structures of the Wrist Joint
The wrist joint is formed by:
- Distally – The proximal row of the carpal bones (except the pisiform).
- Proximally – The distal end of the radius, and the articular disk (see below).
The ulna is not part of the wrist joint – it articulates with the radius, just proximal to the wrist joint, at the distal radioulnar joint. It is prevented from articulating with the carpal bones by a fibrocartilaginous ligament, called the articular disk, which lies over the superior surface of the ulna.
Together, the carpal bones form a convex surface, which articulates with the concave surface of the radius and articular disk.
Like any synovial joint, the capsule is dual layered. The fibrous outer layer attaches to the radius, ulna and the proximal row of the carpal bones. The internal layer is comprised of a synovial membrane, secreting synovial fluid which lubricates the joint.
There are four ligaments of note in the wrist joint, one for each side of the joint
- Palmar radiocarpal – Found on the palmar (anterior) side of the hand. It passes from the radius to both rows of carpal bones. Its function, apart from increasing stability, is to ensure that the hand follows the forearm during supination.
- Dorsal radiocarpal – Found on the dorsum (posterior) side of the hand. It passes from the radius to both rows of carpal bones. It contributes to the stability of the wrist, but also ensures that the hand follows the forearm during pronation.
- Ulnar collateral – Runs from the ulnar styloid process to the triquetrum and pisiform. It acts to prevent excessive radial (lateral) deviation of the hand.
- Radial collateral – Runs from the radial styloid process to the scaphoid and trapezium. It acts to prevent excessive ulnar (medial) deviation of the hand.
The wrist joint receives blood from branches of the dorsal and palmar carpal arches, which are derived from the ulnar and radial arteries (for more information, see Blood Supply to the Upper Limb)
Innervation to the wrist is delivered by branches of three nerves:
- Median nerve – Anterior interosseous branch.
- Radial nerve – Posterior interosseous branch.
- Ulnar nerve – deep and dorsal branches.
Movements of the Wrist Joint
The wrist is an ellipsoidal (condyloid) type synovial joint, allowing for movement along two axes. This means that flexion, extension, adduction and abduction can all occur at the wrist joint.
All the movements of the wrist are performed by the muscles of the forearm.
Flexion – Produced mainly by the flexor carpi ulnaris, flexor carpi radialis, with assistance from the flexor digitorum superficialis.
Extension – Produced mainly by the extensor carpi radialis longus and brevis, and extensor carpi ulnaris, with assistance from the extensor digitorum.
Adduction – Produced by the extensor carpi ulnaris and flexor carpi ulnaris
Abduction – Produced by the abductor pollicis longus, flexor carpi radialis, extensor carpi radialis longus and brevis.
Clinical Relevance: Injuries to the Wrist Joint
The scaphoid bone of the hand is the most commonly fractured carpal bone – typically by falling on an oustretched hand (FOOSH).
In a fracture of the scaphoid, the characteristic clinical feature is pain and tenderness in the anatomical snuffbox.
The scaphoid is at particular risk of avascular necrosis after fracture because of its so-called ‘retrograde blood supply’ which enters at its distal end. This means that a fracture to the middle (or ‘waist’) of the scaphoid may interrupt the blood supply to the proximal part of the scaphoid bone rendering it avascular.
Patients with a missed scaphoid fracture are likely to develop osteoarthritis of the wrist in later life.
Anterior Dislocation of the Lunate
This can occur by falling on a dorsiflexed wrist. The lunate is forced anteriorly, and compresses the carpal tunnel, causing the symptoms of carpal tunnel syndrome.
This manifests clinically as paraesthesia in the sensory distribution of the median nerve and weakness of thenar muscles. The lunate can also undergo avascular necrosis, so immediate clinical attention to the fracture is needed.
The Colles’ fracture is the most common fracture involving the wrist, caused by falling onto an outstretched hand.
The radius fractures, with the distal fragment being displaced posteriorly. The ulnar styloid process can also be damaged, and is avulsed in the majority of cases.
This clinical condition produces what is known as the ‘dinner fork deformity’.