Variation of Posterior Deltoid Muscle

Background: Shoulder anatomy develops into a complex structure with some of the widest ranges of motion displayed on the human body. Held together entirely by ligaments and muscles, the shoulder a relatively weak joint with many clinical presentations. An overlooked attribute is the anatomic variability within the shoulder region. During a routine dissection, bilateral posterior variants of the deltoid muscle, with an interesting orientation of muscle fibers, were discovered on a 78-year-old female cadaver. These variants show an almost perpendicular orientation to the expected direction of deltoid muscle fibers and are not consistent with any other expected muscle bellies in the region. Case Information: While performing a routine dissection in the gross anatomy laboratory, aberrant deltoid muscle straps were observed bilaterally on a 78-year-old female cadaver. These fibers, seen in the figures below, extend superficially from the lateral aspect of the overlying deltoid and lateral triceps brachii fascias. These fibers then lay on top of the infraspinatus muscle belly. Directionally they run at a perpendicular angle to the posterior deltoid fibers and are contained within a separate fascial sheath lying superficially over the existing muscle bellies. An additional, albeit rather small, set of fibers run from the same deep deltoid and triceps brachii fascias to then lay over the scapular spine origin of the posterior deltoid. Although these fibers are contained within a separate fascial sheath, there appears to be no change in the neurovascular anatomy associated with the posterior shoulder. A separate neurovascular bundle was not noted to accompany either of the variant muscle straps. Conclusions: Aberrant straps of muscle in the deltoid region have been described on several occasions. The shoulder joint is an unstable joint with one of the widest ranges of motion of any joint in the body. Its mobility is derived from the construction of the capsule, which is composed of a network of tendon and connective tissue. Consequently, this joint is the source of a great number of acute and chronic pain cases. Perhaps a portion of these cases may be attributed to the presence of variant muscles influencing the shoulder joint. In addition to pain, the shoulder is a frequent location for a variety of surgical procedures. Effective clinical management of patients with concerning shoulder issues should include consideration of atypical structures, whether intra-operatively or not.