Atypical course of the rare accessory extensor carpi radialis muscle.

The description of a bilaterally well developed, bicipital and bipennate accessory extensor carpi radialis muscle is reported. The muscle arose between the origins of the long and short radial carpal extensor muscles and inserted by 2 tendons into the 1st 2 metacarpal bones. The ulnar tendinous insertion was simultaneously the origin of abductor pollicis brevis. Both heads of the accessory extensor carpi radialis muscle were innervated by a branch of the deep branch of the radial nerve. The present observation supplements our knowledge on variations in the antebrachial and carpal regions, which are important in hand surgery.