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Introduction

The development of (poly)crystalline dental ceramics, particularly yttrium-oxide tetragonal partially stabilized zirconia (Y-TZP), and the introduction of CAD/CAM (computer-aided design/computer-aided machining) technology have increased the use of metal free restorations in dentistry. CAD/CAM systems rely primarily on three steps: scanning the tooth preparation, data processing and computer designing of the structure, and milling the pre-fabricated blocks. The use of pre-fabricated ceramic blocks and CAD/CAM technique can minimize internal defects and imperfections resulting from other fabrication methods, therefore improving the reliability and providing quality control .

Currently, most all-ceramic restorations are fabricated using a high crystalline content ceramic framework to provide strength and a veneering porcelain offering better esthetics . Studies have reported that all-ceramic restorations suffer from chipping, cracking, or delamination of the porcelain veneer during service . Chipping is the greatest cause of failure for zirconia-based all-ceramic restorations . Yet, all ceramic restorations fabricated by heat-pressing have showed less veneering chipping than restorations obtained by traditional layering .

The failure behavior of multilayered structures is influenced by the fracture toughness and elastic modulus mismatch between the ceramic layers, which result in a significant concentration of tensile stresses across the interface within the higher modulus material . In addition, the residual stresses are related to the materials thermal history, such as the coefficient of thermal expansion (CTE) mismatch between veneer and framework materials, fast heating and cooling rates of the restoration, and differences in the thermal diffusivity of the materials .

Lithium disilicate glass–ceramic was introduced to improve the mechanical behavior of feldsphatic porcelains and it has been suggested as an alternative veneering material for Y-TZP . Recently, a new technique named “CAD-on” was developed to produce all-ceramic crowns and bridges using a Y-TZP framework and a lithium disilicate ceramic veneer, which are both fabricated by the CAD–CAM technology. A fusion ceramic is used to bond the framework and the veneer structures. As a fairly new method, the literature is deficient on the multilayer structure obtained by the CAD-on technique.

Most all-ceramic restorations are multilayer structures and should be evaluated as such. Thermal residual stresses generated during the firing process, may induce crack propagation resulting in porcelain chipping or delamination. Such phenomenona can only be observed in multilayer structures . The material under tension and the interaction between the ceramic layers may affect the strength and the failure mode of multilayer structures .

Therefore, the aim of this study was to evaluate the flexural strength, the reliability, the fracture toughness, and the failure behavior of the ceramic structures obtained by the CAD-on technique, testing the null hypothesis that multilayer and monolithic structures show similar values for the evaluated properties.