Anatomical study of the teres major muscle: description of an additional distal muscle slip.

Background: Understanding muscle and tendon anatomy is of tremendous importance to achieve optimal surgical execution and results in tendon transfers around the shoulder. The aim of this study was to introduce and describe an additional distal muscle slip of the teres major (TM).
Methods: Sixteen fresh-frozen cadaver shoulders were dissected with the deltopectoral approach. The ventral latissimus dorsi (LD) tendon was harvested, and the shoulders were analyzed for the presence/absence of a distal teres major slip (dTMs) and its dimensions and relationship with the TM and LD tendons.
Results: The dTMs was identified in 12 shoulders (75%). It was always distal to the TM tendon and visible during the deltopectoral approach. There was a clear separation between the TM proximally and dTMs tendon distally. At the humeral insertion, both tendons had a common epimyseal sheet around the teres major and inserted continuously at the humerus. The mean width of the dTMs tendon at the insertion was 13 ± 4 mm (range, 7-22 mm). The total lengths of the dTMs tendon and LD tendon were 40 ± 7 mm (range, 25-57 mm) and 69 ± 7 mm (range, 57-79 mm), respectively (p < 0.001). The dTMs muscle showed direct adhesions in ten shoulders (83%) with the LD muscle.
Conclusions: This is the first macroscopic description of an additional distal slip of the teres major muscle. The dTMs has a separate (distal) but continuous (mediolateral) insertion at the humerus within a common epimyseal sheet around the TM. The dTMs tendon is visible during the deltopectoral approach and can therefore provide a lead structure, particularly in ventral LD transfers with the deltopectoral approach.