Accessory Muscle from the Trapezius Muscle into the Thoracolumbar Fascia

Background: As part of the superficial back musculature, the trapezius and latissimus dorsi muscles play an integral role in upper body mobility. The trapezius functions in neck extension and movement of the scapula, allowing motions such as elevation, depression, upward rotation, and retraction. The latissimus dorsi functions to internally rotate, adduct, and extend the arm. Anatomical variations of these muscles have been previously reported in literature and are associated with aplasia, hypoplasia, agenesis, and variations in vasculature and innervation.

Case Information: We present our discovery of a unique trapezius muscle variant that was discovered upon standard cadaveric dissection. The variant is approximately 6.0 cm in length. The accessory muscle is observed to originate from the inferolateral fibers of the trapezius and inserts into the superomedial fibers of the latissimus dorsi muscle. The inferior aspect of the muscle-tendon runs continuously with the latissimus dorsi muscle tendon, ultimately inserting into the thoracolumbar fascia. The superior aspect of the muscle belly narrows to fuse with the deep surface of the trapezius muscle. The middle portion of the accessory muscle was surrounded by its own fascial sheath, separating it from both the superficial cutaneous tissue and the deep musculature.

Conclusion: Anatomical variants arise commonly in individuals, and their clinical significance - especially in the routinely used upper body muscles - can either impair quality of life or be asymptomatic. More specifically, accessory muscle variants, like the one we found, are important to note due to their use in surgical procedures, relevance during diagnostic imaging, and their potential for clinical manifestations (such as pain syndromes and scoliosis). We suggest that accessory muscle variants could offer alternative options to surgeons when considering tendon transfer procedures. Next, we propose that the presence of a unilateral accessory muscle could play a role in scoliosis. The trapezius and paraspinal muscles, and their connections with the spinal vertebrae, contribute to the balance of forces that, when imbalanced, may present as idiopathic scoliosis. Previous studies have shown that abnormalities in the paraspinal muscles, like the trapezius, have a strong correlation with idiopathic scoliosis. Lastly, we suggest the presence of unilateral accessory muscles may give rise to pain symptoms.