Additional flexor muscles of the forearm: case report and clinical significance

Anomalous muscles usually do not cause symptoms but are of academic interest. They become a surgical problem when they produce symptoms or are difficult to differentiate from soft tissue tumours. During routine cadaveric dissection for the undergraduate students at the Kasturba Medical College, Mangalore, we came across two additional muscles in the deep flexor compartment of the right forearm of a 69-year-old woman. The anomalous muscles were located on the ventral aspect of the proximal forearm, in a plane deep to the flexor digitorum superficialis (FDS). Both the muscles originated from the deep surface of the FDS. The muscle on the radial side was a Gantzer's muscle as it was inserted into the tendon of the flexor pollicis longus. The muscle on the ulnar side formed an independent tendon for the middle finger, 14 cm above the proximal edge of the flexor retinaculum and passed through the carpal tunnel; surprisingly the tendon of the additional muscle and flexor digitorum profundus tendon for the middle finger gave origin to the second lumbrical in the carpal tunnel. The passive traction on the tendon of the additional muscle resulted in flexion of the distal and medial phalanges. The presence of such an additional tendon and origin of lumbrical muscle in the carpal tunnel should be considered in the aetiology of carpal tunnel syndrome.