[Effects of bite raising on fine structure of condylar cartilage in rats].

The present study was investigated the response in the early stage and the adaptability in the maturation stage of condylar cartilage, when the bite was raised at the molars so that the mandible was continuously displaced posteroinferiorly during the developmental stage. 3-week-old male SD rats were used in this study. Amount of the bite raising was approximately 1 mm at the bilateral first and second molars. The short-term (1, 2, 3 and 4 weeks) and long-term (8, 12 and 16 weeks) effects were histologically examined by electron and light microscopy. 1. On examining the short-term effects, at 2-3 weeks the thickness of the maturative and hypertrophic zones was markedly reduced, and the ratio of cartilaginous cell layer was increased in the fibrous zone and decreased in the hypertrophic zone. Light microscopy revealed a reduction of the cell count in the maturative and hypertrophic zones at 2 weeks, an irregularity of cell arrangement at 3 weeks and a cell-less layer in some areas at 4 weeks. From these findings and the anatomical structure, it is presumed that the brackets exerted a compressive force on the condylar cartilage. 2. Short-term electron microscopic observation disclosed a vacuole-like structure in the cartilaginous matrix and some vacuolation of cell at 3 weeks. This seems to suggest the inhibited growth of mandibular condylar cartilage in the short-term. 3. Throughout the long-term, the thickness and ratio of the cell layer was reduced in the maturative and hypertrophic zones and were increased in the fibrous zone. Light microscopy revealed a decrease in the cell count and poor trabecular formation in the hypertrophic zone. These seem to suggest that, under the present experimental conditions, the mandibular condylar cartilage is also affected by long-term elevation. 4. Long-term electron microscopic observation disclosed in the hypertrophic zone the cells that rather morphologically resembled cells existing in the maturative zone. Disturbance of normal calcification was also observed. These seem to indicate that the growth of mandibular condylar cartilage tended to be inhibited in long-term. The above results suggested that continuous bite raising in juvenile rats caused a compressive force to the condylar cartilage, inhibited the growth relatively fast and continued to exert its effect for a long time, though the effect is diminished with time.