28 - Co-occurrence of Bilateral Upper and Lower Limb Muscle Variations: A Rare Case Study

Poster Board Number: 28
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Abstract Body : INTRODUCTION. Variation in human anatomical structures is a hallmark of gross anatomy, reflecting the complexities of embryological development. Such anatomical variations include the existence of accessory and supernumerary muscles in the limbs, an awareness and understanding of which offer insight into development processes and potential clinical manifestations of these variants. Described supernumerary muscles vary not only in their anatomical features, but also their relative frequencies in the population. Concurrent appearance of multiple variant muscles in a single individual may offer a glimpse into aberrations in embryonic limb development. OBJECTIVE. This case report describes unilateral and bilateral upper limb and rare bilateral lower limb muscle variations discovered during routine dissection of a male donor at the University of California, Davis School of Medicine. MATERIALS AND METHODS. An 80-year-old male donor was selected for full body dissection as part of the first-year medical school course in 2020. Two experienced dissectors followed the dissection protocols laid out in Grant’s Dissector, with appropriate modifications to meet the needs and pacing of the curriculum. Regions were dissected in the following order: thorax, back, shoulder, axilla, upper limb, abdomen, pelvis, and head and neck. Due to COVID-19, the course ended early, and the cadaver was stored until dissection could be resumed. The donor’s lower limbs were subsequently dissected in late 2022, with the dissection plan approximately following Grant’s Dissector. Changes to the dissection plan were made in response to the identified muscle variations to better characterize their anatomical features. RESULTS. Bilateral axillary arch muscles (left: 8 cm long, 0.6 cm wide at midpoint; right: 7 cm long, 0.6 cm wide at midpoint) and a left pectoralis quartus muscle (12 cm long, 1.2 cm wide near distal attachment) were identified during dissection of the anterior thoracic wall and axilla in 2020. When dissection was resumed in October 2022, a variant muscle was identified in the left posterior compartment of the thigh (36 cm long, 1.4 cm at widest point, 3.5 cm circumference) with a tendinous origin on the most proximal part of the linea aspera near the gluteal tuberosity and insertion on the posterior and medial aspect of the femur, near the origin of the medial head of the gastrocnemius. A similar hamstring variation was identified on the right side (34 cm long, 2.2 cm at the widest point, 4.3 cm circumference) with nearly identical origin and insertion. Although this variant muscle shares similar characteristics with the previously described supernumerary semitendinosus muscle, there are notable differences, including the site of insertion. Thus, this may be the first description of this specific posterior thigh muscle variation. CONCLUSION. Co-existing bilateral upper and lower limb muscle variations were identified in a single individual, which is a rare finding and may include a newly identified posterior thigh muscle variation. SIGNIFICANCE/IMPLICATION. Although several muscle variations found in the upper and lower extremities have been described in the literature, there is a paucity of research describing not only bilateral muscular anatomical variations, but also muscular variations across multiple regions in a single individual. This is one of very few case reports to identify, characterize, and describe variant muscles present bilaterally in both the upper and lower limbs.