An accessory muscle belly or an accessory muscle head? An unusual arrangement of muscles in the anterior compartment of the forearm

Purpose Knowledge of the unusual arrangement of the flexor pollicis longus (FPL) muscle is important as the variable tendon may be a rare cause of carpal tunnel syndrome. Methods During a routine dissection at the Department of Anatomy, an unusual formation of the FPL muscle was observed in a formalin embalmed Central European cadaver. Results This report presents a variation of the FPL muscle, where the muscle split and formed a separate accessory head inserting into the first lumbrical muscle. Moreover, a tendinous interconnection was present between the FPL muscle tendon and the tendon of the aberrant muscle head. Conclusion The cases described by previous literature, concerning the Linburg–Comstock variation or the accessory head of the first lumbrical muscle originating from the FPL muscle, are closest to the present case. Such variation has a clinical significance ranging from the functional limitation of the thumb and index finger movement to the potential median nerve compression.