Cross-sectional and biomechanical investigation of concurrent infraspinatus and teres minor muscle variations: A case report

Introduction: The infraspinatus and teres minor are scapulohumeral muscles that laterally rotate the arm. Multiple variations in these muscles have been reported; however, the impact of concurrent accessory infraspinatus (a-I) and teres minor (a-TM) muscles has yet to be described. The present study aims to employ gross cross-sectional and biomechanical analyses to investigate a case of coexistent a-I and a-TM muscles for structural effects, functional impacts, and clinical implications. Methods: Bilateral a-I muscles and a left a-TM muscle were discovered during routine dissection of a human cadaver. The aberrations were carefully cleaned of extraneous tissue and photographed. The left glenohumeral joint was sectioned sagittally and photographed to confirm the relation of a-I and a-TM to contiguous structures. The infraspinatus, a-I, teres minor, and a-TM muscles were examined for gross and microscopic structural features that were used to calculate maximal isometric force (F_max) and bilateral comparability (δ). Results: The 8.66 g (bilateral mean) a-I and 13.42 g (left) a-TM inserted on the greater tubercle just distal to their namesake muscle. At the left glenohumeral joint, a-I remained distinct from infraspinatus while a-TM and teres minor exhibited connected bellies. Bilateral a-I and teres minor muscles were structurally similar (δ < 0.30), while left and right infraspinatus muscles approached non-similarity (δ = 0.69). F_max of the left infraspinatus, a-I, teres minor, and a-TM were 117.12 N, 26.45 N, 59.93 N, and 29.88 N, respectively. Conclusions: Coexistent a-I and a-TM muscles can increase the rotator cuff's lateral rotation force by 31.82%, but they may imbalance glenohumeral joint stability, complicate injury and surgical repairs to the region, and increase risk of quadrangular space syndrome. This study provides gross and functional analyses of concurrent a-I and a-TM muscles that may help guide diagnosis, treatment, research, and education for related cases.