Pronator teres accessorius: A new variant in flexor antebrachial musculature

Muscles of flexor compartment of forearm have a common origin from medial epicondyle of humerus. Additional bellies of flexor muscles are commonly reported but presence of supernumerary muscles is an infrequent phenomenon. The present study describes an unusual muscle mass in flexor compartment of forearm simulating pronator teres. During routine dissection the upper limb of a 50 years old male cadaver, a supernumerary muscle was found on left side of the upper limb in the flexor compartment. The origin of the muscle was 2cm wide and aponeurotic in nature and attached to an oblique line extending from the inferior surface of the medial epicondyle and the medial surface of the trochlea. It was inserted on an oblique line 2.5cm wide on the radius in area between supinator superiorly and flexor digitorum profundus inferiorly. Existence of accessory muscles, which connect flexor muscles, could be explained embryologically by incomplete cleavage of flexor mass during development. The flexor muscles of the forearm develop from the flexor mass which subsequently divides into two layers: superficial and deep. The deep layer gives rise to flexor digitorum superficialis, flexor digitorum profundus and flexor pollicis longus. These supernumerary muscles are extremely rare entities and probably represent deranged embryological development or the process of atavism in which the anomalous part persist for a longer time in the tree of evolution.