First Case Report: Supernumerary Humeral Head of Pronator Teres with Median Nerve Embedment

During dissection of the upper limbs in a gross anatomy course for first year medical students, we identified unusual, bilateral, supernumerary humeral heads of the pronator teres muscles in a 70 year‐old, Caucasian female cadaver. The supernumerary humeral head on each side consisted of a substantial muscle belly that originated on the inferomedial aspect of the supracondylar ridge superior to the epicondylar origin of the otherwise typical humeral head, and distally inserted into the rest of the humeral head mass. With these supernumerary humeral heads, the median nerves were effectively surrounded by and embedded in pronator teres muscle mass on both right and left sides. Given that the cause of pronator teres syndrome is typically considered to result from median nerve compression: (1) between the humeral and ulnar heads, (2) by the bicipital aponeurosis, or (3) by the flexor digitorum superficialis arch, such proximal embedment of the nerve within a supernumerary humeral head of the pronator teres may pose an additional risk and source for compression neuropathy. Further studies of cadaver specimens are ongoing to determine the frequency of supernumerary humeral heads of the pronator teres with attendant embedment of the median nerve.