Effect of decreased loading on the metabolic activity of the mandibular condylar cartilage in the rat.

The aim of this study was to measure the effect of decreased temporomandibular loading on the proliferative activity and the level of matrix production of the condylar cartilage. The effect of reduced joint loading on the activity of stromelysin-1 (MMP-3), which has been associated with conditions of articular cartilage matrix breakdown, was also examined. Eighty 14-day-old female rats were assigned to two groups. Following weaning at 20 days, the experimental group was fed a soft diet and the incisors were shortened regularly to keep them out of occlusion. The controls were fed a hard diet. The activity of tritiated thymidine incorporation and the incorporation of radiolabelled sulphur were measured 2, 6, 12, 24 and 48 hours after initiation of the experiment. The radiolabelled sulphur intake was significantly lower in the condylar cartilage of the experimental group 6-24 hours after initiation of the experiment, and tritiated thymidine activity was lower after 12-24 hours, indicating lower proliferation and matrix production. The cartilage in the experimental group showed marked immunostaining against MMP-3 in all cartilage layers 9 days after initiation of the experiment. In the control group, the staining was clearly seen only in the superficial fibrous layer and in the erosion front. A marked reduction in proliferative activity and proteoglycan synthesis in mandibular condylar cartilage was found after a continuous soft diet and suppressed incisal mastication in the rat. The results show that sufficient loading is important for condylar cartilage growth, to maintain both ideal proliferation and matrix chondrocyte production.