An uncommon form of the rare extensor carpi radialis accessorius.

Two accessory muscles were found in the lateral compartment of the forearm while dissecting a 92-year-old female cadaver. One of these originated from the extensor carpi radialis brevis, became tendinous and travelled between the two radial extensor tendons. It inserted independently into the second metacarpal bone, and may be regarded as an extensor carpi radialis intermedius. The other accessory muscle originated from the extensor carpi radialis longus, passed superficially over the parent tendon and inserted into the abductor pollicis brevis. This variation appeared to be a rare extensor carpi radialis accessorius, an additional muscle, which usually arises from below the extensor carpi radialis longus and inserts into the first metacarpal bone. Various forms of the accessorius have been described previously, including one that inserts into the abductor pollicis brevis. The tendon of the accessory muscle described in this study passed through its own dorsal tunnel under the extensor retinaculum, making it an uncommon form of the rare accessorius.