Distinct biological and biomechanical features in jaw and knee cartilage contribute to their degenerative susceptibility

The temporomandibular joint (TMJ) and knee joint are both prone to degenerative diseases such as osteoarthritis (OA); however, TMJ-OA generally manifests earlier than knee OA, with underlying mechanisms largely unknown. In this study, we explored potential explanations through comprehensive biological and biomechanical analyses in mice. Structurally, the mandibular condylar cartilage (MCC) exhibits prominent differences from tibial articular cartilage (AC) at occlusion establishment, evidenced by distinct extracellular matrix organization and enriched transcription factor expression toward the posterior TMJ. Both jaw and knee cartilage house progenitor-like, slow-cycling cells at the cartilage surface, but these cells are depleted more rapidly in the MCC, associated with faster tissue turnover and cellular replenishment. Biomechanically, the MCC exhibits a lower elastic modulus (EIT) than the tibial AC. This biomechanical feature is correlated with its unique extracellular matrix composition to support the force-bearing jaw movement at the anterior TMJ, while providing cushioning and compliance to protect a putative stem/progenitor cell niche at the posterior TMJ. Together, these findings highlight fundamental biological and biomechanical distinctions between jaw and knee cartilage that may underlie their different susceptibility to degeneration.