こう合を想定したこう合面への荷重負荷による上顎小臼歯歯根歯頚部のひずみ 荷重負荷部位と負荷方向の影響

Abstract

The shapes and structures of teeth are associated with mastication and swallowing. Morphologically, the premolars may be regarded as “intermediate” or “transitional” forms between canines and molars. Lesions of the hard tissues, which might have been induced by occlusal force, are often observed on premolars, especially on first premolars. In the present study, therefore, the effects of the occlusal loading point and direction on the stress generation in the root cervical surface of maxillary premolar were examined using strain gages.In the first experiment, the strains on the four root cervical regions involving buccal, palatal, mesial and distal surfaces were measured for extracted intact human maxillary first and second premolars when one concentrated load was applied perpendicularly to each of the prescribed occlusal surfaces. The compressive component of the load generally yielded axial shortening and barreling, and lateral bending toward the loading side. The strain values increased as the loading point located farther and outer from measuring area. It suggests that the shortening of the crown or outward shifting of occlusal contacts by attrision may have a risk of injuring teeth and periodontal tissues.Second, a load of 7 kg was applied to epoxy resin models duplicating first premolars mesially in sagittal plane at an angle of -10, 0, 10 or 20 degree. In addition, the same experiment carried out for the columnar epoxy resin model. Excessive mesial inclination seemed to make the teeth bend to mesial direction and induce unusual tensile stress on the opposite side of the loading. It was confirmed from the finding that the cervical one-third exhibited a deep depression and the mesial concavity extends from the crown down to the root surface.Third, a load of 7 kg was applied to the epoxy resin model on the 3 points (palatal cusp, mesial and distal marginal ridges) of the occlusal surface at the same time. The compressive strain became smaller than that in 1 point loading.The foregoing results suggest that the imbalanced loading, even though the magnitude of the occlusal force is adequate, may give some effect on the teeth aspects, if the shape of teeth and the directions of loading are varied by attrisions. It is necessary to consider the configuration of teeth and the occlusal direction on this basis in the occlusal reconstruction.