Finite element analysis is a standard method to simulate the stress distribution in all-ceramic dental restorations in order to estimate the loading capacity of the brittle components. The hypothesis of this study was that stresses in the connector area of a veneered FDP are strongly influenced by the framework dimensions and the veneering material.
Finite element analyzes of bilayered fixed dental prostheses with three different framework-designs and three different veneering materials were conducted, applying the loads onto the veneering as well as directly onto the framework. The outer shape of the veneering ceramic remained constant for all cases.
The maximum first principal stresses in the framework of the fixed dental prostheses (FDP) decreased with smaller framework dimensions when the load was applied on the veneering. By applying the load directly onto the framework of the FDP without veneering a converse tendency was found. The variation of the veneering material lead to the conclusion that stresses in the framework became higher with decreasing Young’s modulus of the veneer, while the stresses in the veneer increased at the same time.
The veneering material plays a significant role for the failure of a FDP and cannot be neglected neither in testing nor in simulation. Thus the loading capacity of dental restorations can only be reasonably evaluated when the whole restoration is taken into account, including framework and veneering.
All-ceramic fixed dental prostheses (FDP) are an esthetic and long lasting solution for the replacement of missing teeth. All-ceramic FDPs are usually fabricated of two different materials, one for the framework and one for the veneering. Nevertheless, failures can occur during lifetime of the restorations such as chipping of the veneering ceramic or fracture of the whole structure .
Due to the possibility of failure, the mechanical reliability should be evaluated very carefully. For ceramic materials, tensile stresses affect strength much more than compressive stresses . Their resistance to fracture is dependent on various factors. Some factors are already identified, such as the material type for the framework and veneering as well as the connector dimensions . Also material thickness ratio, design, processing conditions, as well as elastic and mechanical properties influence the strength of a multi-layered restoration .
To avoid a fracture of the FDP a minimum dimension for the cross section of the framework material in the connector area is recommended . Moreover, the use of a framework material with higher strength as shown by zirconia is recommended to achieve higher fracture loads . However, some findings indicate that not only the framework material, but also the composition of the used materials has an influence on the fracture strength . Some authors observed that the veneering is the main restricting factor to the FDP’s strength and recommend a specific veneering thickness to minimize tensile stresses . Different layer thicknesses of the materials in simplified test specimens seem to influence the failure mode and failure origin . Furthermore, the tensile stress distribution and the strength of the single layers affect the location of crack initiation .
Hence, in order to estimate the clinical reliability and fracture strength of dental restorations, it is important to have knowledge of the stress distribution in both components as well as the fracture strength parameters. A suitable engineering tool to predict the stress distribution in dental restorations is the finite element method . Only a few finite element analyzes on connector design and the influence of material properties were found. Herein were two-dimensional and three-dimensional evaluations.
In the present study, anatomical FDPs with different framework designs and veneering Young’s moduli were three-dimensionally modeled to estimate the maximum tensile stresses in the framework and the veneering component. The hypothesis of this study was that stresses in the connector area of a veneered FDP are strongly influenced by the framework dimensions and the veneering material.