Grooved Surface of the Obturator Internus Muscle With Two Distinct Adjacent Parts

The specific anatomical features of the obturator internus, particularly those of its medial surface, remain insufficiently characterized. This study investigated the morphology of the obturator internus muscle by focusing on the shape of its medial surface and potential age-related changes. Through dissection of anatomical specimens and in vivo pelvic magnetic resonance imaging (MRI) analysis, we identified a consistent groove-like structure on the medial surface of the muscle and divided it into distinct regions: a superior portion with radiating muscle fibers and an inferior portion with parallel-aligned fibers. This division was characterized by differing fascicle orientations, with interspersed adipose tissue occasionally observed at the boundary. MRI scans of 27 women aged 25-83 years demonstrated that this groove was consistently visible; its location and depth were quantified. The groove position showed a weak negative trend with age, but this relationship was not statistically significant and may have been underpowered. No significant correlation was found for groove depth. By delineating the consistent bipartite architecture of the obturator internus and identifying its age-related variation, this study provides a detailed anatomical characterization of the medial surface of the obturator internus that may serve as a reference for future anatomical and clinical investigations involving the hip-pelvic interface.