| Decreased OVD (Occlsual Vertical Dimension) is one of the common problems in dental clinic, which can be caused by severe tooth wear, the denture for complicated dentition defect, periodontal disease at late stage, and bad restoration. In traditional opinion, the normal occlusion was changed with decreased OVD, and the posterior neighboring tissues of the TMJ will be loaded by condyles, resulting in overloading TMJ and the remodeling of condylar cartilage..But, recent researches have showed that there were no significant differences in the position and the constituent ratio of condyles, and the retroposition of condyles was not so obvious. There is no direct evidence for condylar cartilage remodeling induced by the decreased OVD.The present study first established an stable rat model of decreased OVD by reducing the crown height on the molar and the length of the anterior teeth. HE staining was carried out on TMJ samples for observing the morphology of the condylar cartilage and its subchondral bone. Then, the thickness of condylar cartilage was measured and analyzed. The ultrastructure of subchondral bone was observed by the Micro-CT.The results are as follows:(1) An stable rat model of decreased OVD can be established successfully by reducing the crown height on bilateral upper molars and the length of the anterior teeth.(2) Obvious morphologic changes of condylar cartilage were induced by decreased OVD. The pyknotic nuclei were observed in proliferative layer of condylar cartilage in 3d experimental group. The homogenous area without chondrocyte appeared in 10d experimental group and the arrangement of proliferative layer of condylar cartilage was abnormal. The morphology of condylar cartilage in 20d and 30d experimental groups almost recovered to normal.(3) In both expermental group and sham-control group, the posterior part of the cartilage was thicker than the middle part, and the middle part was thicker than the anterior part. The middle part was significantly thicker in 3d and 10d experimental groups than their age-matched sham-control groups(p<0.05), but no such differences in 20d and 30d experimental groups (p>0.05). The cartilage thickness of the posterior part was significantly higher in experimental group at all the examined time-points than their age-matched sham groups.(4) The ultrastructural changes of the condylar subchondral bone were induced by decreased OVD. There was no significant difference in sham-control group and experiment group under light microscope. But the results of Micro-CT showed that the surface of the subchondral bone was smooth and regular before experiment, obvious pits, which was induced by bone-absorption, appeared in condylar subchondral bone in 10d experimental group. The surface of the subchondral bone became smooth again in 30d experimental group.(5) BV/TV and Tb.N were reduced in the experimental group, while Tb.Sp increased in the experimental group. BS/BV and Tb.Th turned up dynamic balance. The condylar subchondral bone remodeled in the experimental group.Conclusions:(1) A stable rat model of decreased OVD was established successfully in the present study .(2) The pyknotic nuclei in proliferative layer of condylar cartilage, and the homogenous area without chondrocytes, as well as the ultrastructual changes of subchondral bone, can be induced by decreased OVD.(3) The above-mentioned morphologic changes appeared at the early stage of experiment. These changes went stable with time. |
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