Additional radial wrist extensor muscles

Last revised by Dr Joachim Feger on 10 Sep 2021

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

Additional wrist extensor muscles occur in 10-24% of the population and are more common in women 1,2.

  • 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
  • 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

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.

The extensor radialis intermedius and accessory extensor carpi radialis muscles have been first reported by John Wood in 1867 and later classified by Alexander Macalister 2,4,5. The extensor carpi radialis tertius has been first described by Soubhagya R Nayak and colleagues in 2007 6.

Additional wrist extensors can be mistaken for soft tissue masses or split tears of the extensor carpi radialis longus and brevis tendons 2.

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.

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