In the present anatomical study, we introduced and described a muscle–tendon slip distal to the TM, which seems to be an additional head of the TM muscle. The dTMs has a separate (distal) but continuous (mediolateral) insertion at the humerus within a common epimyseal sheet around the TM. Other than the previously described TM accessorius [16], which lies proximally, the described additional head is located distal to the TM. This distal teres major slip was identified in 75% of our shoulder specimens, and its presence was defined by a distinct and separate bony insertion distal to the tendon insertion of the TM.
The LD tendon attaches more laterally and proximally compared to the TM tendon with the most inferior portion of the insertion area of the TM usually located inferior to that of the LD tendon [16, 25], leaving parts of the TM bony insertion visible during a deltopectoral approach. Yet, separation of the LD and TM tendon insertions during this approach can be challenging. If present, intraoperative exposure of the dTMs tendon may provide an additional distal lead structure for separating the tendon insertions between the LD and TM when using the deltopectoral approach for tendon transfer surgery. In particular, for isolated LD transfers [5], where the TM is left at its anatomical position, an exact differentiation of the insertion sites between LD and TM is fundamentally important to perform a correct tendon transfer.
Beger et al. classified the interconnections between LD and TM into five different types [17]: type 1 referred to no muscular or tendinous connections, type 2/3 to muscular connections, and type 4/5 to tendinous connections. Wang et al. described additional muscle fibers, which cover the TM tendon posteriorly and attach directly to the periosteum of the humerus [10]. Moreover, Dancker et al. described an additional strand of muscle fibers proximal to TM [16]. The authors assumed an additional head of the TM with considerable thickness and separate bony insertion by a short tendon and named it TM accessorius. Also, several studies reported on additional/anomalic slips of the TM which pass between TM and the long head of the triceps [19,20,21], or between TM and the fascia of the shoulder capsule and the brachialis fascia [19, 22], or from the biceps brachii to the TM [23], or from the tendon of coracobrachialis to the TM [19], or even from the infraglenoid tubercle to the TM [24]. However, to the best of our knowledge, this is the first study describing an additional distal muscle slip of the TM. In contrast to the TM accessorious, which inserts proximally and medially to the main insertion of the TM [16], the dTMs inserts as a separate and distal tendon with a continuous insertion at the humerus within a common epimyseal sheet around the TM.
Several surgical techniques have been described to transfer LD in cases of posteroinferior rotator cuff insufficiency, with or without TM [2, 5, 26, 27]. More recently, an isolated LD transfer was favored in concomitant reverse total shoulder arthroplasty, particularly in cases of combined pseudoparalysis [28, 29] of active elevation and external rotation [30,31,32]. This might be even more beneficial in patients with subscapularis muscle insufficiency, to prevent further weakening of the internal rotation force of the shoulder with a transformation of two internal rotators into external rotators. The LD tendon transfer can be performed using a single [5, 26, 30,31,32,33,34] or double incision technique [5, 26, 30,31,32,33,34]. In particular, for the single incision technique, the findings of the present study can contribute to improving the surgical safety by providing a deeper understanding of the surgical anatomy. We identified a separate muscle belly of the TM with a separated and more distal tendon insertion in three-quarters of the shoulder specimen examined. A specific band-like connection between the LD tendon and dTMs was found to be located approximately 5 cm proximal to the tendon insertion. A fascial-like connection between LD and dTMs has been described previously [14]. However, we could describe the distinct location of this important connection that needs to be released (together with the interconnections between LD and TM) in isolated LD tendon transfers in order to achieve appropriate tendon excursion and to minimize tension. Moreover, it remains controversial whether an isolated single-incision LD transfer should be performed underneath TM [31] or through a split within the muscle, as introduced by Popescu et al. [30]. The LD transfer passage within the TM muscle seems to be feasible just between the TM and dTMs. Comparing the figures of that study to our findings, it remains possible that the transfer was actually performed in the study by Popescu et al. [30]. Yet, it remains necessary to study how well and to what extent the TM and dTMs can be separated from each other without jeopardizing them.
In patients with irreparable subscapularis tears, pectoralis major transfer [35,36,37] has been the most commonly performed procedure for glenohumeral joint preservation with good to excellent long-term results [38]. More recently, ventral LD transfer has been described as an alternative technique for irreparable subscapularis tears [7, 9]. As in the single-incision LD transfer for posterosuperior tears, locating and harvesting the tendon in a ventral LD transfer are performed similarly. The findings of this anatomic study could help to improve the safety and efficiency of ventral LD transfers in patients with irreparable subscapularis insufficiency.
A limitation is that we could not study the function as well as neurovascular supply pattern of the dTMs, as it was not feasible in our cadaveric shoulder specimen. The lack of tone and neuromuscular control and the alteration of elasticity due to the freeze-thaw process should also be noticed when interpreting our results. The macroscopic appearance and excursion are suggestive of an additional distal teres major muscle slip. However, in order to confirm the dTMs, further anatomical studies (of function, innervation patterns, histology, and pennation angles) are necessary. Further, we did not evaluate whether the dTMs was present unilaterally or bilaterally, as only one shoulder specimen of each donor was available. We also did not study the presence of the dTMs in different ethnic groups than white American. Therefore, the presence of the dTMs may be the result of ethnical specificity and needs to be further investigated. The authors believe that this new anatomic knowledge is of importance in the field of tendon transfers around the shoulder.