The Calculation Of Reliability Of The Strength Of The All Ceramic Crown Of Mandibular First Molar

Optimization design and lifetime prediction has always been the main topic in the field of prosthodontics.This is mainly because that the failure of oral prosthesis is determined by many uncertain factors (such as oral environment, cyclic biting force, shape and characteristic of certain material). Furthermore, it is a random and probabilistic incident. Therefore, With respect to single factor, some deterministic methods (such as laboratory approach ,finite element analysis ,et al )which are presented in the former research are not so valuable. The theory of reliability analysis is a systematic method, which is based on the probability statistics and numerical calculation. It has already been applied in the field of architecture, electronic fields, aeronautics and astronautics, and so on. With respect to the uncertain nature of the factors in oral environment, to apply the theory of reliability analysis in the optimization design and life-time prediction of oral prosthesis will make the analysis more close to clinical practice. All-ceramic crowns exhibit outstanding esthetics and excellent biocompatibility,but it is an essentially brittle material, and thus fractures are possibility when all-ceramic crowns are used. Therefore, to construct reliability analysis model of all-ceramic crowns with respect to the randomness of physical parameters and loading state. To make a probe study in the fatigue reliability analysis by using stress-strength inferencetheory; To explore a way for the further study of mechanical strength in the studyof reliability analysis, and to lay a foundation for the application of reliability analysis in other oral prosthesis.To construct a reliability analysis model of mandible first molar all-ceramic crown by using reverse techniques combined with software such as Geomagic Studio CaitaV5 and AlgorV18. To acquire some random parameters in the reliability analysis (such as load spectrum of routine biting force, load spectrum of routine temperature load, occlusal contact locations and areas during different occlusions ) by reviewing literatures and statistic analyzing . In this study, to calculate the reliability by using stress-strength inference theory based on normal-normal distribution probabilistic model. To summary the law of the changes of fatigue limits with respect to time, and make an analysis of its mechanism. The results are as follows:1,The mold of the mandibular first molar was constructed according to the geometry data of the mandibular first molar acquired by Wang Huiyun.The solid model was constructed by CATIA V5 according to the mold ,which is with good characteristics, distinct structures and good geometric similarity to the clinical realities and was saved as .STP . A three-dimensional finite element model was finished after inputting the STP format model to the Algor and thus .igs format model was acquired. The model fnished include the surppoting teeth, cement, corwn and alveolar bone.2,Statistic analysis of the routine biting forces show that the forces are obeyed to the normal distribution.The maximum and minimum occlusal contact areas of the normal occlusion and the occlusal contact area of the extreme conditons were acquired by reviewing the literatures. The loads was divded into 3 load conditions: First, load was applied to the surface of the tooth at the locations of the fossa,inclined surface and dental cusp respectively about 0.6 mm diameter area to represent the cases when use the crown to bite too hard food such as a piece of sand or the premature points; second, load was applied to the surface of the tooth of the fossa,inclined surface and dental cusp respectively about 1.2 mm diameter area and at the location of the occlusal surface about 3.5 mm diameter area to represent two extreme occlusal contact areas during daily chewing.3,Statistic analysis of the routine temperature loads show that the loads are obeyed to the normal distribution.4,when the bite forces were applied vertically only, the all- ceramic crown experienced a concentration of tensile stresses in areas around the loading points on the surface of the crowns at which loads were applied 0.6 mm diameter and 1.2 mm diameter areas and in areas around mesial margin of the buccal surface on the surface of the crowns at which the load was applied 3.5 mm diameter area. When the temperature loads were applied only, the tensile stresses became lower and lower from the occlusal surface to the margin. When the bite forces and temperature loads were applied together ,the model experienced similar tensile stress distribution patterns as the model under bite forces only.5,while the load was applied at the position of the occlusal surface about 3.5 mm diameter area,at the positions of the fossa,inclined surface about 1.2 mm and at the at the position of the fossa about , the reliability datas are 100﹪; while the load was applied at the position of dental cusp 1.2 mm diameter area and at the position of inclined surface 0.6 mm diameter area, the failure possibility are 0.1; while the load was applied at the position of dental cusp, the failure possibility is 0.3.To conclude based on results:1,The occlusal contact areas and locations have much effect on the reliability of the strength of the all-ceramic crowns.The reliability of the all-ceramic crowns decreases form the locations of the fossa to the dental cusp.The all-ceramic crowns exhibit high failure possibility when the load was applied at the position of the fossa,inclined surface and dental cusp about 0.6mm diameter area which represent the cases when use the crown to bite too hard food such as a piece of sand or the premature points.2,The reliabilities of the four kinds of all-ceramic crowns including EmpressⅠ,EmpressⅡ,In-Ceram Alumina,ZrO2 are all 100﹪subjected to the single temperature load,from which we conclude that only the temperature load has little effect on the strength of the all-ceramic crowns.3,By comparing the results acquired by the reliability analysis to the ones acquired by the clinical research,we found that the reliability analysis we studied in the research are suitable tu be used in the evaluation of the safe factor and the lifetime of the all-ceramic crowns.However,more factors which have effect on the safe factor and lifetime of the all-ceramic crowns are expected to considered in the future.