Muscles extending the wrist

Extensor carpi radialis longus

Extensor carpi radialis brevis

Extensor carpi ulnaris

Extensor digitorum

Extensor indicis

Extensor digiti minimi

Extensor pollicis longus

Extensor pollicis brevis

Extensor carpi radialis longus

Extensor carpi radialis longus lies on the lateral side of the posterior compartment of the forearm, being partly covered by brachioradialis. It arises from the anterior part of the lower third of the lateral supracondylar ridge of the humerus and adjacent intermuscular septum. Occasionally, there may be an attachment to the lateral epicondyle by the common extensor tendon. Approximately in the middle of the forearm, the muscle forms a flattened tendon which runs distally over the lateral surface of the radius. In the lower third of the forearm, the tendon, together with that of extensor carpi radialis brevis, is crossed by the tendons abductor pollicis longus and extensor pollicis brevis. The tendon of extensor carpi radialis longus and brevis pass deep to the extensor retinaculum in a common synovial sheath. Together they groove the posterior surface of the styloid process of the radius. The tendon of extensor carpi radialis longus attaches to the posterior surface of the base of the second metacarpal.

Nerve supply

Extensor carpi radialis longus is supplied by the radial nerve, root value C6, 7, which enters the muscle above the elbow. The skin over the muscle is supplied by roots C5, 6.

Action and palpation

These will be considered with extensor carpi radialis brevis.

Extensor carpi radialis brevis

Extensor carpi radialis brevis lies adjacent to and is partly covered by longus, to which it may be partly fused. It arises from the lateral epicondyle of the humerus via the common extensor tendon, the lateral ligament of the elbow and the adjacent fascia. The tendon of brevis forms halfway down the forearm and runs with that of longus deep to abductor pollicis longus and extensor pollicis brevis. It passes below the extensor retinaculum, in a common synovial sheath, to attach to the posterior surface of the base of the third metacarpal.

Nerve supply

Extensor carpi radialis brevis is supplied by the posterior interosseus branch of the radial nerve, root value C6, 7. The skin over the muscle is supplied by roots C5, 6, 7.

Action

Working with extensor carpi ulnaris, extensor carpi radialis longus and brevis produce extension of the wrist. Working with the flexor carpi radialis, however, they will produce abduction(radial deviation) of the wrist. In addition, extensor carpi radialis longus may help to flex the elbow joint.

Functional activity

Functionally, the wrist extensors work strongly in the action of gripping, where they have a synergic role. The synergy of the two radial extensors and extensor carpi ulnaris is a vital factor in the gripping action. By maintaining the wrist in an extended position, flexion of the wrist under the action of flexor digitorum superficialis and profundus is prevented, with the result that these muscles act on the fingers. If the wrist is then allowed to flex the flexor tendons cannot shorten sufficiently to produce effective movement at the interphalangeal joints. This therefore becomes a state of active insufficiency.

If the radial nerve is damaged the patient is unable to produce an effective grip because of paralysis of the wrist extensors. However, with the wrist splinted in extension, the tendons of superficialis and profundus act on the fingers and a functional grip can be obtained.

Palpation

When the wrist is extended and abducted against resistance, both extensor carpi radialis longus and brevis can be palpated in the upper lateral aspect of the posterior part of the forearm. The tendons, particularly longus, can be palpated in the floor of the “anatomical snuffbox” if the same movement of extension and abduction is carried out.

Extensor carpi ulnaris

Extensor carpi ulnaris arises from the lateral epicondyle of the humerus via the common extensor tendon and the adjacent fascia. There is also a strong attachment, via a common aponeurosis shared with flexor digitorum profundus and flexor carpi ulnaris, from the posterior border of the ulna. The muscle forms a tendon near the wrist which passes below the extensor retinaculum in its own synovial sheath and compartment. This is carried in a groove next to the ulnar styloid process. The tendon attaches to a tubercle on the medial side of the base of the fifth metacarpal. 

Nerve supply

Extensor carpi ulnaris is supplied by the posterior interosseus branch of the radial nerve, root value C7, 8. The skin over the muscles is supplied by roots C6, 7, 8.

Action

Working with extensor carpi radialis longus and brevis produces extension of the wrist. The functional significance of this has been described in the actions of the two former muscles. Working with flexor carpi ulnaris, extensor carpi ulnaris produces adduction(ulnar deviation) at the wrist.

Palpation

The tendon of extensor carpi ulnaris can be identified on the dorsum of the wrist, when the wrist is extended and adducted against resistance. It lies on the lateral side of the ulnar styloid process. 

Extensor digitorum

Extensor digitorum is centrally placed within the posterior compartment of the forearm. It arises from the lateral epicondyle of the humerus via the common extensor tendon, the covering fascia and the intermuscular septa at its sides. In the lower part of the forearm the muscle forms four tendons which pass deep to the extensor indicis. On the dorsum of the hand the tendons diverge towards the medial four digits, being interconnected by obliquely placed fibrous bands. The arrangement of these bands is variable, however, the tendons for the ring and little fingers are attached usually until they pass just proximal to the metacarpophalangeal joints. The tendon to the index finger may not be attached to that of the middle finger.

The distal attachment of extensor digitorum is complex in that each tendon helps to form an aponeurosis over the dorsum of the fingers. This is called the dorsal digital expansion or extensor hood. In its simplest form it is best thought of as a movable triangular hood, the base of which lies proximally over the metacarpophalangeal joint. From here the sides of the hood wrap around the phalanx with the apex pointing distally. The extensor hood forms the dorsal part of the capsule of the metacarpophalangeal joint and extends forwards either side of the metacarpal head to fuse with the deep transverse metacarpal ligament. As the extensor hood approaches the proximal interphalangeal joint, it narrows and is reinforced on either side by the interosseus and is reinforced on either side by the interosseus and lumbrical muscles for that finger. At the distal end of the proximal phalanx, the extensor hood divides into three parts(figure b). The central part, which is directly continuous with the extensor tendon, inserts into the base of the middle phalanx on its dorsal aspect. The two collateral parts, which are continuous with the tendons of the interossei and lumbricals, reunite to insert into the base of the distal phalanx on its dorsal aspect. There may be an attachment to the base of the proximal phalanx but this is unusual.

The way in which the extensor hood wraps around the phalanges towards the palm facilitates the attachment of the lumbrical and C7, 8. The skin over the muscle is supplied by roots C6, 7.

Action

Extensor indicis assists extensor digitorum in its actions with respect to the joints of the index finger. It does, however, enable the index finger to be used independently, and also aids extension of the wrist.

Palpation

The tendon of extensor indicis, lying medial to that of extensor digitorum, can be palpated on the dorsum of the hand when the index finger is extended. The muscle belly can be palpated by deep pressure over the lower part of the ulna during the same movement.

Extensor pollicis longus

Extensor pollicis longus lies deep to extensor digitorum in the posterior compartment of the forearm. Here it arises from the lateral part of the middle third of the posterior surface of the ulna and the adjacent interosseus membrane above extensor indicis. Above the wrist it forms a single tendon which passes in its own synovial sheath deep to the extensor retinaculum, lying in a groove on the back of the radius medial to the dorsal tubercle. Winding around the dorsal tubercle, the tendon changes direction and crosses the tendons of extensor carpi radialis longus and brevis and the radial artery. As the tendon crosses the metacarpophalangeal joint it forms the dorsal part of the joint capsule. It is joined in this region by slips from abductor pollicis brevis laterally and adductor pollicis medially. The whole arrangement forms a triangular expansion, not unlike that of extensor digitorum. The expansion may also be joined by extensor pollicis brevis. The tendon of extensor pollicis longus attaches to the dorsal surface of the base of the distal phalanx of the thumb. In its course across the back of the hand, the tendon forms the medial boundary of a region known as the anatomical “snuff-box”.

Nerve supply

Extensor pollicis longus is supplied by the posterior interosseus branch of the radial nerve, root value C7, 8. The skin over the muscle is supplied by roots C6 and 7.

Action

Extensor pollicis longus extends all of the joints of the thumb, and assists in extension and abduction of the wrist. Once this has been achieved, the obliquity of the tendon will pull the first metacarpal into a laterally rotated and abducted position. In full abduction or extension of the thumb, extensor pollicis longus can act as an adductor. In addition, it may contribute slightly to supination, due to its oblique course across the lower part of the forearm.

Palpation

The tendon of extensor pollicis longus is easily identified with the thumb extended as the medial border of the “anatomical snuff – box”. The tendon frequently ruptures in conditions, such as rheumatoid arthritis or post Colles’ fracture. As a result, the patient is incapable of extending the interphalangeal joint of the thumb.

Extensor pollicis brevis

Extensor pollicis brevis lies on the lateral side and is adjacent to extensor pollicis longus, and distal to abductor pollicis longus to which it closely adheres. It arises from the middle part of the posterior surface of the radius and adjacent interosseus membrane. The tendon is formed above the wrist, and runs with that of abductor pollicis longus deep to the extensor retinaculum in a common synovial sheath, in a groove on the lateral surface of the radial styloid process. Below this point, these two tendons form the lateral boundary of the “anatomical snuff – box”. The tendon of extensor pollicis brevis partially replaces the dorsal part of the capsule of the metacarpophalangeal joint of the thumb and inserts into the dorsal surface of the base of the proximal phalanx. Occasionally the tendon may be prolonged, so that it runs with and attaches to the tendon of extensor pollicis longus.

Nerve supply

Extensor pollicis brevis is supplied by the posterior interosseus branch of the radial nerve, root value C7 and 8. The skin over the muscle is supplied by roots C6 and 7.

Action

Extensor pollicis brevis extends both the carpometacarpal and metacarpophalangeal joints of the thumb. It may also help in extending and abducting the wrist, particularly against resistance.

Palpation

The tendons of extensor pollicis brevis and abductor pollicis longus run together from the lower lateral aspect of the radius where they form the lateral boundary of the “anatomical snuff – box”. Both tendons can be palpated when the thumb is extended, that of abductor pollicis longus being the most anteriorly placed.

Application

The synovial sheath of extensor pollicis brevis and abductor pollicis longus frequently becomes inflamed in the region of the radial styloid process(De Quervain’s syndrome). Techniques, such as transverse frictions, ultrasound or injection can be usefully applied to this point.