Bilateral symmetrical supernumerary heads of biceps brachii with rare pectoralis major insertion.

During a routine dissection of a 51-year-old Caucasian male cadaver, bilateral symmetrical supernumerary heads (SH) of the biceps brachii muscles with insertion into the pectoralis major (PM) muscles were discovered. Multiple case reports have documented supernumerary heads for the biceps brachii; however, none have shown bilateral insertion into the pectoralis major. This study describes a previously undocumented variation of the SH that has potential for clinical impact. Healthcare providers could be confounded by patients presenting with shoulder pain or muscle tears as a result of the anomaly. Furthermore, MRI studies on patients with possible shoulder muscle tears could reveal unexpected results. Such cases would warrant consideration of SH anomaly and treatment should be adjusted accordingly. The significant bulk and angle of the SH insertion on the PM we observed changes force vectors which would have an unknown effect in performance, surgical interventions and pain syndromes. A second biceps brachii anomaly was observed on the left brachium in addition to the bilateral SH. We postulate that these variants provide the potential for clinical complications regarding muscular injury to these aforementioned muscle groups.