Distinct Biological and Biomechanical Features in TMJ and Knee Cartilages

The temporomandibular joint (TMJ) and the knee joint are two of the most frequently used joints in the body, with the mandibular condylar cartilage (MCC) and the articular cartilage (AC) covering the joint bone surfaces, respectively. Compromised MCC functions lead to various temporomandibular disorders (TMD), including TMJ osteoarthritis (TMJ OA); however, the mechanisms governing MCC homeostasis and its biomechanical properties are still poorly understood. In this study, we comprehensively compared the biological and biomechanical features of the MCC and AC in mice. Histological analysis on P1, P21, 3-month, and 10-month mice revealed the most drastic structural differences between MCC and AC at occlusion establishment (P21), with MCC found to be more susceptible to age-associated cartilage degeneration. Immunostaining revealed differentially distributed cartilage extracellular matrix components in MCC and AC, including collagen type I, II, and X, and highly enriched expression of several key transcriptional factors at the posterior region of the MCC, including sex determining region Y-box 9 (SOX9), runt-related transcription factor 2 (RUNX2), and scleraxis (SCX). The posterior MCC also houses a group of long-lasting, slow-proliferative cells, as evidenced by the BrdU/EdU incorporation assay, suggesting the presence of a potential stem/progenitor cell niche at the posterior TMJ. Unbiased nanoindentation analysis revealed distinct biomechanical features between these joint cartilages. MCC exhibits a significantly lower elastic modulus (E_IT) than AC, with the highest E_IT observed at the anterior TMJ, which is oppositely associated with the fibrous layer thickness, but positively correlated with the ratio of the collagen type X-positive matrix in the cartilaginous layer. Altogether, this study provides a basic understanding of the biological and biomechanical features of the cartilaginous tissues in two important joints, which may facilitate our understanding of the physiology in the TMJ and knee joint, and support the applications of mouse models to study TMJ dysfunctions.