The authors identified 6 patients with an anomalous flexor carpi radialis brevis muscle in a series of 172 patients who were treated surgically for distal radius fractures. There were 3 male and 3 female patients, 4 patients were Caucasian, and 2 patients were non-Caucasian. The anomalous muscle was found to originate from fibrous tissue adjacent to the radial insertion of the pronator quadratus muscle. The tendon projected distally within the flexor carpi radialis tendon sheath. The authors were unable to determine insertion sites from their limited surgical exposures.
Several previous descriptions of the flexor carpi radialis brevis have been published, but primarily in cadaveric studies. The aberrant muscle is attached to the volar-radial surface of the distal radius, overlying or adjacent to the radial margin of the pronator quadrates muscle. The tendon courses distally and may insert into the carpus and/or base of the index metacarpal. Recognition of this anomalous muscle-tendon unit is understandably important in avoiding confusion with the flexor pollicis longus.