An Unusual Accessory Muscular Belly of the Flexor Carpi Radialis Muscle.

Variations of the upper limb musculature are of clinical interest, potentially impacting diagnostic interpretation and surgical outcomes. This report describes a rare anatomical variation identified in a 72-year-old male cadaver, i.e., an accessory flexor carpi radialis (FCR) originating from the medial border of the brachioradialis (BR). While current classifications categorize FCR variants based on origins from the bicipital aponeurosis, biceps brachii, or pronator teres, this case represents a unique morphology. Embryologically, such bundles likely arise from the incomplete separation of the common flexor-pronator mass during morphogenesis. Clinically, these accessory bellies may be misidentified as soft tissue tumors on imaging or complicate surgical procedures like tendon transfers and volar approaches to the radius. Furthermore, their proximity to the superficial branch of the radial nerve creates potential sites for nerve compression and entrapment syndromes.