||Substructural improvement in 3-unit porcelain-fused-to-zirconia fixed partial denture
||Department of BioMedical Engineering
fixed partial denture
finite element method
Substructural improvement in 3-unit porcelain-fused-to-zirconia fixed partial denture
Department of Biomedical Engineering, National Cheng Kung University
Fixed partial denture is one of the most common methods for restoring missing teeth. With the development of medical technology, the average life span of human being was prolonged. Therefore, the structural strength of fixed partial denture needs to be improved for longer use. The purpose of this study was to seek an optimal geometrical design of framework of zirconia-based fixed partial dentures which would improve its strength. Through the combination of finite element method and the bi-directional evolutionary optimization algorithm (BESO), the framework of fixed partial denture framework was optimized to enhance its structural strength.
The numerical model of the present study was based on the images from micro computerized tomography, and the contours of two abutment teeth were constructed via reverse engineering. The contour of fixed partial denture model was sketched using the software exocad.
The process of structural optimization was to search for the optimal material distribution of porcelain and zirconia in the structure of the fixed partial denture by altering a different zirconia framework design. The results showed that the structural optimization improved the design of the fixed partial denture. The maximum principal stress at the both sides of connector decreased by 9.7% and 19.2% respectively, From the biomechanical point of view, the structural optimization improved the framework design of fixed partial denture.
Compared with the traditional design, the optimized fixed partial denture design was expected to extend its life span by minimizing the maximum principal stress within the porcelain layer. It also provides a new design reference for dental professionals to develop fixed partial dentures with higher strength for more complex clinical conditions.
Key words: fixed partial denture, structural optimization, finite element method
Fixed partial dentures and single implant are common methods of treating missing tooth clinically. Compared to dental implant, fixed partial dentures have been widely used because of its cost efficiency .The structure of the fixed partial denture usually consists of a framework and veneering porcelain. The framework played an important role for the strength of the fixed denture structure. The zirconia-based fixed partial dentures became mainstream because of the high mechanical strength, biocompatibility and aesthetic characteristics.
Clinical literatures recorded failures of fixed partial dentures that includes framework fracture, chipping, loss of retention, abutment tooth fracture, secondary caries. This present study discussed the failure situations from the mechanical aspect including framework fracture, chipping, and loss of retention.
This study aimed to reduce the occurrence of porcelain chipping through the framework design rather than the fracture resistance of the framework. The objective of this study was to reduce the maximum principal stress of porcelain by altering framework geometry in fixed partial denture.
Materials and Methods
Finite element modeling
The second premolar and the second molar were scanned by micro computerized tomography and the image was used to construct a digital model. The tooth model was imported into the software exoCAD to sketch a fixed partial denture, and the digital model was imported into the finite element software. After setting material properties, boundary conditions, and loading, the model of traditional design was completed. This model was also used to compare the mechanical performance with optimized model.
Description of topology improvement method
BESO was based on python and can be ran parallelly with the finite element software. In this study, the objective of optimization was to prevent the veneering porcelain from chipping by altering framework configuration of the fixed partial denture.
The BESO is an approach which changes the geometry of structure by progressively replacing material to where it is most needed. The BESO calculated the weighting factor of changing each element according the filtering radius. The maximum principal stress of each element within design domain was extracted and multiplied with weighting factor. According to the stress after weighting, the BESO ranked the elements and assigned the highly stress concentrated element with hard material property.
The BESO found the peak maximum principal stress of the design domain. After the assigned volume fraction was reached, the program checked the condition of convergence, when it was reached, the altering process was completed. Because remeshing was not incorporated in the optimization process, the edge of optimized model presented jagged, after smoothing the edge of the model, the optimized design was finished.
Result and Discussion
Stress distribution and improvement of fixed partial denture
The location of the peak value of maximum principal stress was the same as the clinical situation that occurred in the porcelain within the connector. According to the results of finite element analysis, the maximum principal stress at both sides of the connector decreased by 9.7% and 19.2% respectively.
The shear stress and tensile stress at the fused-interface and the maximum principal stress of the porcelain were improved by 9% or more, it was expected that porcelain chipping can be reduced.
Framework variation after optimization
The objective function of optimization was to reduce the maximum principal stress of the porcelain, and that framework was not considered during optimization. Framework fracture may occur due to concentration of high maximum principal stress. However, the maximum principal stress of the framework was increased from 57.500 MPa to 57.619 MPa, implying that structural strength of the framework remained at a similar level after optimization.
It can be seen from the cross sectional view of the fixed denture, the connector of framework has a greater cross sectional area and smooth curvature after optimization. This results in the maximum principal stress of the porcelain, which had better mechanical performance with less normal stress and shear stress of the fused-interface
1.The values of maximum principal stress, normal stress, and shear stress were found reduced after optimization.
2.From biomechanic a standpoint point, this suggests that the improvement of traditional framework design is neccessary.
3.The approach of optimized design reduced maximum principal stress and interfacial stress and provided a better reference of framework.
Extend Abstract II
第一章 緒論 1
第二章 材料與方法 15
2.4.2 程序與結構最佳化雙向演進法 27
第三章 結果 31
第四章 討論 45
第五章 結論 54
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