Rare Forearm Muscle Variations Seen in Two Cadaveric Dissections

Abstract

IntroductionAnatomical variations of the forearm muscles have been widely reported within the scientific literature, including variations or absence of the extensor pollicis brevis (EPB) and extensor pollicis longus (EPL) muscles. Rare instances have been found of an additional extensor forearm muscle, the extensor carpi radialis accessorius (ECRA). The authors describe a unique extra muscle originating from the muscle belly of the extensor carpi radialis longus (ECRL) and the distal humerus in two cadavers. One of these cadavers also had a complete absence of the EPB and an EPL with two sites of insertion.Case PresentationsDuring a cadaveric dissection course, two cadavers were found to have an extra extensor muscle of the left forearm. In both cases this muscle originated from the lateral surface of the ECRL muscle belly and the distal humerus; both muscles had a single tendon that traveled through the first extensor tunnel with the tendons of the abductor pollicis longus (APL) to insert onto the base of the first metacarpal. In addition, in one cadaver the EPB muscle was absent and the tendon of the EPL divided distally to insert into both the proximal and distal phalanx of the thumb.DiscussionThe ECRA is a rare additional extensor forearm muscle described to originate from the humerus below the ECRL and inserts into the first metacarpal bone of the thumb. The ECRA found in each of our two cadavers had an unusual origin from the muscle belly of the ECRL and from the distal humerus superior to the ECRL. This finding adds to the handful of known variations in the ECRA. The EPL and EPB are deep posterior forearm muscles. The EPL usually originates from the middle third of the ulna and inserts onto the base of the distal phalanx of the thumb. The EPB usually originates from the dorsal surface of the interosseous membrane and inserts onto the proximal phalanx of the thumb. In addition to the ECRA found in the left forearm, one of the cadavers also had anatomical variations in the extensor pollicis muscles on the same side. The EPB was absent and the EPL tendon divided distally to insert into the proximal and distal phalanx of the thumb. Variations in the EPB are common, and instances of an absent EPB have been cited in the literature. The EPL variation is most consistent with Turker Classification 1g. It’s possible this represents an anatomical adaptation to compensate for the absent EPB.ConclusionWhile an ECRA muscle is already rare, even more unusual is an ECRA originating from the ECRL and from the distal humerus superior to the ECRL and has not been reported. In addition, the concurrent existence of the three anatomic abnormalities within the same forearm has not been described previously. Knowledge of possible variants in forearm anatomy is crucial for surgeons to have a full understanding of possible causes of pathology. Alternatively, lack of awareness may cause diagnostic confusion or treatment failures.