| Molars bear greater stress during mastication.After the operation of endodontic treatment,especially for those teeth with severe destruction,often the crown or post-and –core crown are given priority to restore the damaged teeth,but they both will remove more healthy tooth structure during preparation.With the development and progress of restorative materials and adhesive materials,it is possible to maximize the preservation of healthy dental tissue and reestablish the lost function.More and more onlays and overlays are applied in the clinical than full crown.The cavity design plays a key role in the long-term stability of the restorations.In this study,three-dimensional finite element method(FEM)was used to simulate the mandibular first molar with full-cusps preparation and form the mesial-occlusal-distal(MOD)cavity.And then on this basis of the simulation,forming two cavity designs,one with a round right shoulder around the buccal and lingual wall(OWS),the other without(ONS).In the experiments,the base thickness and the width of the central cavity were chose as variables to explore the difference of stress distribution between the two types of cavity designs,aim to obtain the best base thickness value respectively and determinewhich design was better,finally provided the theoretical basis for clinical application.Objectives: Making the base thickness and the width of the central cavity as the variable factors,the stress analysis of two MOD cavity designs was carried out,comparing the stress distribution consequence difference with the normal teeth,a comprehensive analysis was needed to determine the best base thickness value and gave the better cavity design.Methods: 1.Experiment 1 An initial analysis of stress distribution of normal tooth Collecting a normal mandible first molar,Micro-CT scanning technology,reverse engineering and finite element analysis method were combined to complete the model simulation.According to human normal mastication,the experiment had simulated two typical loading direction with a static way: axial loading and oblique loading(lingual 45° to the central axis).In the axial situation was eight loading points of 280 N,the other was three points of 105 N.2.Experiment 2 Stress analysis of the ONS Using the mandibular first molar model exported from experiment 1,simulating full-cusps preparation,forming the mesial-occlusal-distal cavity(MOD).The corresponding four models were grouped according to different base thickness in the pulp chamber floor,the models were respectively 0 mm group(no base material),1 mm group,2 mm group and 5 mm group(fill the center cavity).The loading conditions were the same as experiment 1,and the effect of base thickness on the stress distribution was observed.3.Experiment 3 Stress analysis of the OWS Be different from the cavity design of experiment 2,experiment 3 added the round right shoulder around the buccal and lingual wall.The grouping and loading conditions were the same as experiment 2,and the effect of base thickness on stress distribution was observed and analyzed.4.Experiment 4 Analyzing the stress distribution of the two cavites with changed central cavity widthsBased on the result obtained from experiment 2 and experiment 3,choosing a suitable base thickness,and adjusted the two cavities respectively,the ONS design formed 3.5 mm group,4.5 mm group,5.5 mm group and 6.5 mm group and the OWS design formed 3.5 mm group,4.5 mm group and 5.5 mm group,the loading conditions were the same as experiment 1,and the effect of cavity width on stress distribution was observed and analyzed.Results: 1.Experiment 1 In the axial loading condition,the overall strain at the buccal cusps was more obvious and the overall equivalent stress was concentrated at the loading point.The equivalent stress of the root was concentrated in the area of the distal buccal part of the cervical.In the oblique loading condition,the overall strain was more obvious at the lingual cusps,and similar to the axial condition,the loading point had the maximum value of equivalent stress.The equivalent stress of the root was concentrated in the area of the contralateral root of the cervical.2.Experiment 2 Under the two loading conditions,the overall strain and equivalent stress distribution were both similar to experiment 1.The pulp chamber,the adhesive layer and the abutment were all emerging stress concentration.With the increase of base thickness,the maximum value of equivalent stress at the pulp chamber was decreased gradually,and the shear stress of the adhesive layer and the equivalent stress of the abutment were increased gradually.After comprehensive analysis,the base thickness of 1mm showed best stress distribution consequence.3.Experiment 3 Under the two loading conditions,the overall strain and equivalent stress distribution were also similar to experiment 1.The pulp chamber,the adhesive layer and the abutment were also all emerging stress concentration,but the concentration areas were different from experiment 2.With the increase of base thickness,their maximum values' trend of stress were the same as experiment 2,and the corresponding values were smaller than that in experiment 2.In this experiment,the stress distribution of base thickness with 1mm also showed the best result.4.Experiment 4 Under the two loading conditions,the maximum equivalent stress ofthe ONS design both showed a small decreasing trend in the cervical area,the root and the base material both showed a small increasing trend.The three areas of OWS all presented a small increasing trend.But combined with other consequences of the analysis,the stress distribution of the OWS design showed better result.Conclusions: 1.The total strain and equivalent stress distribution of the two overlay designs were similar to the normal teeth,as a whole,their stress effects were like normal teeth.2.The stress distribution of the pulp chamber,the adhesive layer and the abutment were all affected by the base thickness.Combined with clinical practice,the stress distribution consequence of the two designs showed best with the thickness of 1mm.3.When the width of central cavity increased,the influence in the cervical part,the layer part and the root part of OWS and ONS were small,but when the inlay became the major part,the stress in the layer part would have a greater impact,so enough tissue to be retained was suggested.4.The maxmum stress of ONS was mostly higher than OWS,shoulder design was suggested When the tissue was enough. |
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