Mechanistic Links Underlying the Comorbidity of Osteoporosis and Osteoarthritis: Cell Fate Plasticity Driven by the Subchondral Bone Microenvironment

Osteoporosis (OP) and osteoarthritis (OA) are two common degenerative musculoskeletal disorders associated with aging and are traditionally classified and managed as distinct disease entities. Emerging evidence suggests that OP and OA may share bidirectional associations and common biological mechanisms, and that under specific pathological conditions they may develop into a mutually reinforcing comorbid state. The comorbidity of osteoporosis and osteoarthritis (OP–OA) is not a simple superimposition of bone loss and cartilage degeneration; rather, it represents a disorder of the osteochondral unit centered on disruption of the subchondral bone microenvironment. Alterations in the structural strength, remodeling dynamics, vascular and neural status, and bone marrow lesions of subchondral bone collectively reshape the local microenvironment, thereby directly affecting mechanical signal transmission and cellular behavior within the joint. Focusing on the subchondral bone microenvironment as the central pathological nexus, this review systematically summarizes how mechanical imbalance, aberrant bone remodeling, inflammatory activation, metabolic dysregulation, and cellular senescence jointly remodel the local niche in OP–OA comorbidity. These microenvironmental changes further induce phenotypic remodeling and fate deviation of bone marrow mesenchymal stem cells, bone remodeling-related cells, osteoimmune cells, and chondrocytes. On this basis, we integrate the regulatory roles of developmental signaling, mechanotransduction pathways, and inflammatory–immune signaling networks, and propose that microenvironment-driven cell fate plasticity may serve as a key mechanistic hub promoting the initiation and progression of OP–OA comorbidity as well as the persistent destabilization of the osteochondral unit. This perspective may help overcome the limitations of current studies that address OP and OA separately, and may provide a theoretical framework for early identification and stratification, biomarker discovery, and combined precision-targeted interventions for this comorbid condition.