Fractures during clinical function have been reported as one of the major causes for failure associated with all-ceramic dental restorations. Comparison of fracture strength and fracture modes of different all-ceramic crown systems is not straightforward. Established methods for reliable testing of all-ceramic crowns are not currently available.

Dental materials are subject repeatedly to masticatory forces, both directly on an occlusal surface, and indirectly through interactions with an abutment or via a connecting unit.

Ageing of zirconia (zirconium dioxide) based materials is associated with surface structural changes caused by humidity at elevated temperatures. Tetragonal phase is the stable structure of zirconia at the sintering temperature. Yttria, magnesia or other oxides are added to zirconia materials to maintain this favourable crystalline form also at room temperature. With variable temperatures in the mouth, catalyzed by humidity, the structure at the surface will slowly transform to a monocline phase. A tetragonal to monocline phase transformation is assumed to reduce the bond strength between zirconia cores and veneering ceramics. This transformation is also regarded as detrimental to monolithic zirconia prostheses or implants due to the volumetric expansion of the crystals at the surface.