Microstructure and matrix-filled lacunae impact mechanical response of temporomandibular joint cartilage under physiological loading

To compare cell morphology, extracellular matrix (ECM) composition, microstructure, and mechanical response to dynamic physiological loading between knee and temporomandibular joint (TMJ) condylar cartilage to better understand the unique fibro-hyaline bilayer structure of TMJ condylar cartilage. Cartilage samples were harvested from the femoral and TMJ condyles of the same pigs. Cell morphology was quantified using confocal imaging and 3D image analysis. Total extracellular matrix (ECM) composition was assessed histologically, and the spatial distribution and synthesis rates of nascent glycosaminoglycans and collagen were measured using a new click chemistry technique. Mechanical properties and phase lag were evaluated across a range of loading magnitudes and frequencies to simulate physiological dynamic loading conditions. TMJ condylar cartilage exhibited a distinct hyaline microstructure compared to knee cartilage, including higher cell density (3.6 [3.1 4.2] vs 1.7 [1.3 2.2] · 104 cells/mm3, p=0.03), smaller chondrocytes (642 [611 672] vs 1247 [1187 1306] μm3, p<0.001), enlarged cell lacunae densely filled with glycosaminoglycans (GAGs), and narrowed interterritorial spaces enriched in collagen. These spatial features contributed to a load-responsive mechanical behavior. At low physiological stresses (e.g., during speech), TMJ cartilage remained compliant, while under higher stresses (e.g., chewing), it exhibited stiffness comparable to knee cartilage. TMJ condylar cartilage displays a distinct hyaline microstructure characterized by matrix-filled lacunae, high cell density, and narrow collagen-rich interterritorial spaces. These features underlie its ability to remain compliant under low stress and stiffen under high stress, supporting its load-dependent mechanical function when speaking and chewing.