Unilateral Conjoint Tendon Formation of the Piriformis, Superior Gemellus, and Tendonized Obturator Internus: A Rare Cadaveric Variation of the Deep Gluteal Region.

Anatomical variations of the deep gluteal musculature are uncommon but may have important implications for surgical approaches, radiologic interpretation, and the evaluation of posterior hip pain. During routine cadaveric dissection of an 89-year-old male donor, we identified a rare unilateral variation involving the deep external rotators of the right hip. The piriformis (Pirif.), superior gemellus (SG), and a markedly tendonized obturator internus (OI) converged to form a prominent common tendon inserting onto the greater trochanter. Although the inferior gemellus contributed to the common insertion, it did not substantially participate in the enlarged tendon, while the quadratus femoris (QF) maintained its typical independent insertion. Examination of the contralateral side demonstrated normal anatomy, confirming the unilateral nature of the variation. The observed configuration expands the spectrum of known variations involving the deep external rotators of the hip. Embryologically, the anomaly may be explained by incomplete separation of the Pirif. and OI tendon complexes during development. The extensive tendonization of the OI and fusion with adjacent muscles may alter force transmission, hip stabilization, and external rotation mechanics. Furthermore, the close anatomical relationship between the fused tendon complex and the sciatic nerve suggests potential clinical relevance in deep gluteal syndrome, posterior hip pain, and sciatic nerve irritation. This cadaveric case highlights the anatomical complexity and variability of the deep gluteal region and underscores the importance of recognizing such variants during anatomical dissection, surgical procedures, and radiologic evaluation. Awareness of these uncommon configurations may improve understanding of posterior hip pathology and aid clinicians in diagnosing and managing deep gluteal disorders.