Failure Mechanism and Lifetime Prediction of Monolithic Restorations

摘要

In this work, first, a 3D failure prognosis methodology was developed for interface initiated failures of monolithic ceramic crowns, combining experimentally determined fast fracture parameters and finite element multi-axial stress analysis on the basis of fracture mechanics based failure probability model. The complete 3D restoration model was developed using commercially available hardware and software. The proposed method was verified by prior 2D axisymmetric restoration model and experimental data of failure probability of flat onlay specimen with borosilicate glass. A detailed analysis of the stress state (flexural stress, interfacial shear stress and interfacial normal tensile stress) at the ceramic/cement interface was conducted and the impact of reduced cement modulus on these stress states was also analyzed to simulate bond degradation.;Second, by introducing ceramic fatigue in this method, we develop interface-initiated fatigue failure model of monolithic ceramic crowns under simulated masticatory loading. For this purpose, four representative ceramic materials, fluromica (FM), leucite (LR), yttrium-stabilized zirconia (YZ) and lithium disilicate (LD) where material parameters (fast fracture parameters and fatigue parameters) were available in the existing literature were chosen. Fast fracture parameters were converted to multi-axial stress state parameters and fatigue parameters were converted to power-law-based parameters based on existing conversion methods. Crown survival probabilities as a function of loading cycles were obtained from simulations performed on the four ceramic materials utilizing identical crown geometries and loading conditions. Additionally, for two of the model crown systems (FM and LD), region dependent failure probabilities were determined and compared against fractographic analyses of failed crowns available in dental literature.;Third, an approximate but simple relative fatigue life estimation method was established. Careful examination of experimentally measured/converted fatigue parameters of materials (FM, LR, LD and YZ) in the existing literature lead to the finding that, ceramic fatigue relating the maximum cyclic stress and stress corresponding to initial crack size prior to N number of cycled fatigue were somewhat similar. This finding was valid for clinically relevant loading range and mastication frequency. Based on this, an approximate fatigue equation universally applicable to all dental ceramic materials was developed. Utilizing the developed universal fatigue equation, an approximate relative fatigue life estimation method was established considering failures from only high stress region in ceramic/cement interface.