Influence of Grading and Layer Thickness on Stress Distribution in Dental Ceramics: A Finite Element Analysis.

Introduction: Functionally graded designs have been explored in dental ceramics to reduce stress concentrations in layered dental ceramics and enhance their mechanical properties. Materials and Methods: Varying wt.% of 3 mol% yttria-stabilized zirconia (20, 35, 50 wt.% YSZ) in fluormica glass systems were processed and their elastic moduli were determined using nano-indentation. A three-layered functionally graded design (20, 35, and 50 wt.% of YSZ in glass for the bottom, middle, and top layer respectively) was designed. The graded designs were further differentiated as layers with uniform and non-uniform thickness. Three-dimensional geometric models of graded, 10-layered functionally graded, bilayered, and monolithic ceramic designs as controls were constructed (ANSYS Version 15.0) for a 2mm thickness of crown over 8 mm of dentin substrate. Plain strain models were built with solid elements (hex8) of element type SOLID 45 of size 0.2mm. Constraints were placed at the inferior border of the substrate with a load of 600 N in the middle of the top layer. Results: Higher maximum principal stress values were observed in 10-layered FGM designs than in bi-layered, monolithic, and experimental designs. In the experimental design, both uniform thickness of 0.6 mm for each layer and non-uniform thickness design combinations showed the least stress values. There was effect of elastic modulus and no significant effect by the uniformity of layers due to similar stress values between the experimental designs. Conclusions: Graded designs irrespective of the layer thickness, dissipate stress and thus, have the potential to reduce structural failures in dental ceramic systems.