The cotyloid fossa fat pad as a hip-specific orthobiologic niche: lessons from the infrapatellar fat pad for mesenchymal stem/stromal cell and extracellular vesicle therapy in osteoarthritis

Hip osteoarthritis (OA) is a leading cause of pain and disability worldwide, yet no validated disease-modifying orthobiologic therapy currently exists for the hip joint. The infrapatellar fat pad (IFP) has emerged as a well-characterized intra-articular adipose-synovial niche in the knee, serving as an accessible source of mesenchymal stem/stromal cells (MSC) with robust immunomodulatory, anti-fibrotic, and chondroprotective properties. IFP-MSC have been extensively characterized through surface immunophenotyping, clonogenic assays, secretome profiling, and multilineage differentiation studies, and their therapeutic capacity has been substantially enhanced through cytokine-based, hypoxic, and hormonal priming paradigms. Their derived extracellular vesicles (EV) further extend this therapeutic framework through paracrine immunomodulation, macrophage polarization, and engineered cargo delivery. The cotyloid fossa fat pad (CFFP), an anatomically analogous adipose-synovial structure within the acetabular fossa of the hip, has received comparatively little biological characterization despite its proximity to the acetabular cartilage, labrum, and ligamentum teres. In this review, we use the well-characterized IFP/synovium complex as a reference framework to systematically evaluate what is established and what remains undefined about the CFFP/pulvinar region in the context of hip OA pathogenesis and cell-based therapy. We synthesize evidence across joint pathology, cellular and molecular composition, synovial interactions, immunometabolic phenotype, and translational readiness. Our review revealed a consistent asymmetry in which the IFP reflects an established and growing evidence base while the CFFP has received limited but emerging direct characterization, with its MSC potency, priming responses, and EV function still undefined. We conclude that the IFP/CFFP comparison is scientifically testable and clinically compelling, but that CFFP-MSC and CFFP-EV cannot yet be presented as validated hip-specific orthobiologic candidates until foundational characterization and head-to-head functional comparisons have been completed.