Grooved surface of the obturator internus muscle: Overlap of two distinct parts and its age-related changes

The obturator internus is traditionally described as a disc-shaped muscle involved in hip joint movement, and it plays a role in pelvic floor support because of its anatomical proximity to the levator ani. However, the specific anatomical features of the obturator internus that contribute to pelvic floor support remain unclear. Therefore, 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 cadaveric dissection 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 two distinct regions: a superior portion with fan-shaped muscle fibers and an inferior portion with parallel-aligned fibers. This division is primarily characterized by differing fascicle orientations, with interspersed adipose tissue occasionally observed at the boundary. Surface curvature analysis of photogrammetry-based three-dimensional models confirmed the presence of a shallow groove at the interface. MRI scans of 27 women aged 25–83 years demonstrated that this groove was consistently visible, and 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. Given that the origin of the levator ani lies along the fascia on the medial surface of the obturator internus, the characteristic morphology of this surface, particularly the identified groove, may be relevant to the structural stability of the pelvic floor. By delineating the consistent bipartite architecture of the obturator internus and identifying its age-related variation, this study establishes an anatomical foundation for future investigations into the interplay between hip joint morphology and pelvic floor function.