The veneering process of frameworks induces residual stresses and can initiate cracks when combined with functional stresses. The stress distribution within the veneering ceramic as a function of depth is a key factor influencing failure by chipping. This is a well-known problem with Yttria-tetragonal-zirconia-polycrystal based fixed partial dentures. The objective of this study is to investigate the influence of veneer thickness on the stress profile in zirconia- and metal-based structures.

 The hole-drilling method, often used for engineering measurements, was adapted for use with veneering ceramic. The stress profile was measured in bilayered disc samples of 20mm diameter, with a 1mm thick zirconia or metal framework. Different veneering ceramic thicknesses were performed: 1mm, 1.5mm, 2mm, 2.5mm and 3mm.

 All samples exhibited the same type of stress vs. depth profile, starting with compressive at the ceramic surface, decreasing with depth up to 0.5–1.0mm from the surface, and then becoming compressive again near the framework, except for the 1.5mm-veneered zirconia samples which exhibited interior tensile stresses. Stresses in the surface of metal samples were not influenced by veneer thickness. Variation of interior stresses at 1.2mm from the surface in function of veneer thickness was inverted for metal and zirconia samples.

 Veneer thickness influences in an opposite way the residual stress profile in metal- and in zirconia-based structures. A three-step approach and the hypothesis of the crystalline transformation are discussed to explain the less favorable residual stress development in zirconia samples.

  Source: Dental Materials

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