| Background:Tooth defection is a kind of common oral disease and it is often found on mandibular first molar.According to position and size,defections can be divided into multiple types.Meanwhile,root canal treatment(RCT)has impact on the structure and decreases mechanical properties of teeth,which makes the situation even more complicated in clinical practice.With the continuous development of stomatology,more and more therapies for tooth defections have been invited,and the restorations such as inlay,onlay and ceramic crown have been widely used.Under different circumstances,appropriate selection of restoration,which influences biomechanical properties,has become the key to successful treatment.In recent years,three-dimensional finite element method has become an effective and widely used research method to evaluate biomechanical properties of restorations in stomatology science.Purposes:To build up a model of mandibular first molar and analyze its mechanical properties under load by three dimensional finite element method(FEM).So as to further clarify the indication of different restorations and to study mechanism of failure by analyzing the stress distribution situation.The results can provides references for the clinical useMethods:1.A model of first mandible molar was established according to CBCT data.2.The Mesial-Occlusal(MO)and Mesial-Occlusal(MOD)defections were built in the first molar models,and the defections were restored with CAD/CAM inlay,onlay and full crown.The mandibular first molars model with RCT was constructed,in which the same defection forms and restorations were added.A total of 12 groups were obtained.3.Vertical load of 225N and a lateral load of 200N were applied at each group respectively and the von Mises stress distribution of each group was evaluated.Results:1.The von Mises stress distribution of the intact tooth is relatively uniform,and the peak stress is located on the enamel of the loading site.The stress of the mesial part is larger when the vertical load is applied and stress concentrated at the bifurcation of the two root canals.The peak stress in dentin is located at the lingual side of pulp cavity under the lateral loading,and the lingual neck surface of the teeth.2.Compared with the intact tooth model,the von Mises stress of the inlay,onlay and the full crown were higher,and the rank of maximum stress of enamel was full crown,onlay and inlay.The maximum stress of crown was 188.497 MPa,and the peak stress of the enamel exposed to the load was higher than that of the intact tooth and the rank of maximum von Mises stress was inlay,intact tooth,crown and onlayˇ3.In the root canal treatment(RCT)groups,the maximum von Mises stress of the restorations were higher than that of the group without RCT,and the rank of stress was full crown,inlay and onlay.The peak stress of enamel exposed in the embedded body group was 30%higher than that of the teeth without RCT,and the area of stress concentration was reduced.The rank of maximum von Mises stress was full inlay,intact tooth,crown and onlay.4.Under the same vertical load,the maximum von Mises stress of the inlay and full crown increased with the defection of the tooth,while the peak stress of the onlay decreased.Different defections had little effect on the peak stress under lateral loading occurs.Conclusion:1.The restorations made of IPS e.max CAD lithium disilicate ceramics can meet the current requirements for the strength in clinical work.2.Tooth defection and tooth preparation will produce significantly higher von Mises stress values and stress concentration.3.For teeth with dental pulp,the stress distribution of inlay and onlay is better than that of ceramic crowns to restore MO defection.Therefore,in the case of small defection,the restoration therapy to retaining healthy dental tissue should be adopted as far as possible.4.When the MOD defection of the tooth without RCT 1s founded,the protection ability of the inlay to the remaining tooth will be reduced,but the stress on the prosthesis is still low,so that the treatment therapy should be selected basing on the clinical situation.5.It is not recommended to treat MO or MOD defection teeth with inlay after RCT.Meanwhile onlay and ceramic crown should be used to protect the remaining dental tissues.What's more the stress distribution of onlay is better than that of full crown and inlay,which lead to better outcome in such situations. |
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