Growth pattern and physiological characteristics of the temporomandibular joint studied by histological analysis and static mechanical pressure loading testing

Objective: The aim of this study was to investigate the age-related changes in the articular disc, condyle, condylar fibrocartilage, and subchondral bone of the temporomandibular joint (TMJ) in rabbits. Methods: Female New Zealand white rabbit aged 1, 4, 12, and 32 weeks were obtained. Each age group comprised 5 rabbits and was subjected to chondrocyte culture, histological assessment of the articular disc length, assessment of the anteroposterior diameter of the condyle, measurement of the fibrocartilage thickness, and assessment of the subchondral bone architecture. The production of Collagen, type II, alphal (COL2A1); SOX9; Collagen, type X, alphal (COL10A1); and Runt-related transcription factor 2 (Runx2) was detected via western blot. Six rabbits at the ages of 12 and 32 weeks were sacrificed for the mechanical pressure loading test (75 kPa, 3 days). Changes in the condyles after pressurization were observed via scanning electron microscopy and evaluated by micro-CT. Results: Significant enlargement of the condyle occurred from 4 to 12 weeks of age (p = 0.003); however, the length of the articular disc increased significantly from 1 to 12 weeks of age (p < 0.005). A rapid decrease in the cartilage thickness but an increase in the subchondral bone density occurred from 1 to 4 weeks of age and from 4 to 32 weeks of age in rabbit, respectively (p < 0.05). The expressions of COL2A1 and SOX9 gradually decreased from 1 to 32 weeks of age. The protein expressions of COL10A1 and Runx2 were maintained at a high level at 4-12 weeks of age. After static mechanical pressure loading, the damage and reduction in the bone mineral density (BMD) in the 12-week-old group was greater than those in the 32-week-old group. Conclusions: An inconsistency between the condyle and articular disc during growth and development may be a causal factor for temporomandibular disorders TMDs in adolescents. Our findings suggest that the response of the adult condyle to the mechanical pressure is significantly greater than that of the adolescent condyle; however, with regard to reconstruction, the situation is reversed, which may influence the pathological progress of TMDs.