Supernumerary extensor tendon to the thumb: a report on a rare anatomic variation.

A supernumerary extensor tendon to the left thumb was observed in a black female cadaver. It came from a bipinnate extensor indicis proprius and passed through the fourth synovial compartment, where it fused with the extensor pollicis longus at the midshaft level of the first metacarpal. This appears to be a rare anatomic variation. Since the review of LeDouble in 1897, only two cases have been reported. A reminder of this oddity at this time seems appropriate because of its perceivable clinical significance.