Variations of the pronator teres muscle: predispositional role to median nerve entrapment.

The variability of the human pronator teres muscle is studied in 60 upper limbs. The humeral head was present in all cases and was double in 3 cases (5.0%). The ulnar head was present in 47 cases (78.3%). The ulnar head was muscular in 11 cases, tendinous in 6 cases, and mixed in 30 cases. The collateral branches of the median nerve destined to the pronator teres muscle were found to be arranged in three main patterns: arising directly from the median nerve, arising from the superficial flexor antebrachial muscles nerve, and mixed type. Special reference is made to the influence of variations in the pronator teres muscle on the compression or the entrapment of the median nerve (pronator syndrome). The proposed determinant variations are: short and tendinous ulnar head, ulnar head joined to the arch of the flexor digitorum superficialis muscle, ulnar head with triple origin slips, and humeral head perforated by the median nerve.