| Clinically,the most frequent method to restore the teeth with large defect was onlay or fiber post-and-core crowns,but how to choose between the two methods has not yet clearly elucidated,basically dental clinicians were used to choose one of them according to their own experiences,therefore subjectivity became inevitable.By using three-dimensional finite element analysis and statistical methods,the present study intended to compare the two restoration methods,and tried to provide a quantitative basis for the restoration choices.Finite element analysis(FEA)was a method of using mathematical approximation to simulate real physical systems.The basic principle was to use simple and interacting elements so that a finite number of unknowns could be used to approximate an infinite number of unknown real systems.Nowadays FEA has been widely applied in the field of biomechanics.The technique of Micro-CT could accurately reflect the characteristics of the fine and complex structure of a object without destroying it,so when FEA was used to combined with Micro-CT and other reverse engineering methods,the fine three-dimensional model could be reconstructed,which even includes the internal structures.Furthermore,finite element analysis software could be used to get accurate finite element model and the related solutions.Because of its superiority,FEA was generally considered to be the most advanced and effective research methods in the field of biomechanics.In this paper,three groups of mandibular first premolar models with different defects were established by using Micro-CT,mimics software(reverse engineering model reconstruction),Geomagic Studio(model optimization),Pro/Engineer(model assembly)and ANSYS(finite element stress analysis).The details of the three-dimensional finite element models were:in Group A the crown had two axial wall defects(mesial and distal wall defects),in Group B the crown had three axial wall defects(mesial,buccal and distal wall defects),in Group C the crown had all axial wall defects.Simulated restorations of post-and-core crown and onlay were applied to each group,respectively.The model of the post-core crown included residual tissue of the tooth,gutta-percha,the fiber post,the adhesive,the resin core and the all-ceramic crown,while the onlay model included residual tissue of the tooth,the gutta-percha,the enamel and the onlay.Alveolar bone was simplified into 30mm~3 cube,and was divided into cortical bone and cancellous bone,the2mm thickness outermost layer of the cube was cortical bone,and the rest was cancellous bone.The alveolar bone was located 2mm below the enamel-dentinal junction.The periodontal ligament was set between the root of tooth and the alveolar bone.Each group of onlay restorations and post-and-core crown restorations were divided to four subgroups according to the amount of residual tooth tissue,totally 24 subgroups of models were generated.The models were imported to ANSYS after modeling completion,and the maximal equivalent stress of the remaining tooth tissue was obtained by analysis in ANSYS software after the procedure of meshing,applied loading force and degree of freedom constraints,solving the model and evaluating the results,then subjecting the results in ANSYS.After that the SPSS software was used for statistical analysis.The maximal equivalent stress value of residual tooth tissue were got after ANSYS software stress analysis and calculation for group A,B and C,in group A it was got in onlay subgroup with 81.475MPa,in group B and group C it was also got in onlay subgroups with 79.808MPa and 78.658MPa,respectively.The maximal equivalent stress of remaining tooth tissue after the two restoration methods in each group were statistically analyzed,and the results were calculated by SPSS software.Value P of group A,B and C were 0.0503,0.623,and 0.082,respectively,showing no significant statistical difference between the three groups(P>0.05).The above results implied that,large defects of mandibular first premolar in group A,B and C could be restored either by onlay or post-and-core crown.However,onlay could preserve tooth tissue as much as possible,and increase the bending resistance of the residual tooth tissue,and protect the root as well.Furthermore,the outline did not stimulate the periodontal tissue and was convenient for cleaning.Therefore,for the crown with two or three axial wall defects,onlay was regarded as a better choice;while for that with all axial wall defects,post-and-core crown was a better choice,because the Von-Mises stress of the bottom surface of hole retention shape exceeded 51MPa when applying onlay restoration,and the residual Von-Mises stress of the post-core crown restoration decreased significantly. |
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