Additional radial wrist extensors are normal anatomical variants and accessory muscles of the forearm and the wrist. The following additional wrist extensors have been described 1-6:
- extensor carpi radialis intermedius
- extensor carpi radialis accessorius
- extensor carpi radialis tertius
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Epidemiology
Additional wrist extensor muscles occur in 10-24% of the population and are more common in women 1,2.
Summary
Extensor carpi radialis intermedius (ECRI)
- origin: between the extensor carpi radialis longus or extensor carpi radialis brevis muscles
- course: between the extensor carpi radialis longus and extensor radialis brevis muscles, through the second or variably in a separate extensor compartment
- insertion: base of the second and/or third metacarpal bones
- innervation: posterior interosseous nerve
Extensor carpi radialis accessorius (ECRA)
- origin: fascia of the extensor carpi radialis longus or extensor carpi radialis brevis muscles
- course: superficial to the extensor carpi radialis longus or extensor carpi radialis brevis muscles, through the second or variably in a separate extensor compartment
- insertion: base of the first metacarpal bone or on the abductor pollicis brevis muscle
- innervation: posterior interosseous nerve
Extensor carpi radialis tertius (ECRT)
- origin: common extensor origin at the lateral epicondyle
- course: between the extensor carpi radialis longus and extensor digitorum communis
- insertion: base of the second and/or third metacarpal bones after division below the abductor pollicis longus
- innervation: posterior interosseous nerve
Radiographic features
The tendon of an additional wrist extensor muscle might be found passing through the second extensor compartment on ultrasound or MRI and can be identified by their origin, course and insertion 3.
History and etymology
The extensor radialis intermedius and accessory extensor carpi radialis muscles have been first reported by the British surgeon, anatomist and physiologist John Wood in 1867 and later classified by the Irish anatomist Alexander Macalister 2,4,5. The extensor carpi radialis tertius has been first described by the Bengali anatomist Soubhagya R Nayak and colleagues in 2007 6.
Clinical importance
Additional wrist extensors can be mistaken for soft tissue masses or split tears of the extensor carpi radialis longus and brevis tendons 2.
Related pathology
Additional wrist extensor muscles might be affected by tenosynovitis. It is imaginable but still unknown if they might contribute to intersection syndrome or lateral epicondylitis 1.