Anatomical variability and morphometry of the proximal common tendon of the semitendinosus and the long head of the biceps femoris: a morphological study with a proposed classification.

BACKGROUND: The proximal common tendon (PCT), formed by the semitendinosus (ST) and long head of the biceps femoris (lhBF), represents the primary origin of the posterior thigh muscle complex and a frequent site of injury. Previous studies have reported inconsistent descriptions of its morphology and limited quantitative data. The present study aimed to characterize the anatomical variability and morphometry of the PCT and to propose a classification of its structural patterns. MATERIALS AND METHODS: The PCT region was dissected in 61 formalin-fixed adult lower limbs (32 left, 29 right) obtained from 39 body donors (27 limbs from female donors and 34 from male donors). Using the ischial tuberosity as a reference point, we measured the length of the PCT to its bifurcation, distances to distal tendon extensions, and the width and thickness of both the PCT and its components. Measurements were performed three times using electronic calipers, and mean values were analyzed. Morphological variants were identified based on macroscopic appearance and classified accordingly. RESULTS: The mean length of the PCT to the bifurcation was 77.53 ± 20.62 mm, with a proximal thickness of 11.61 ± 3.97 mm and a width of 19.84 ± 4.68 mm. Five morphological variants of the lhBF tendon were identified on the lateral side. On the medial side, a single feather-shaped morphology was observed in all cases. The ST tendon demonstrated marked variability, being present laterally in 90.2% of limbs and medially in 67.2%. Absence of the ST tendon was noted in 9.8% (lateral) and 32.8% (medial) of specimens. CONCLUSIONS: The PCT exhibits substantial morphological variability, with pronounced asymmetry between lateral and medial tendon components. The proposed classification system and morphometric reference values may improve anatomical understanding and support surgical and imaging-based assessment of PCT injuries.