Concerns of Hydrothermal Degradation in CAD/CAM Zirconia

Abstract

Zirconia-based restorations are widely used in prosthetic dentistry; however, their susceptibility to hydrothermal degradation remains elusive. We hypothesized that CAD/CAM machining and subsequent surface treatments, i.e., grinding and/or grit-blasting, have marked effects on the hydrothermal degradation behavior of Y-TZP. CAD/CAM-machined Y-TZP plates (0.5 mm thick), both with and without subsequent grinding with various grit sizes or grit-blasting with airborne alumina particles, were subjected to accelerated aging tests in a steam autoclave. Results showed that the CAD/CAM-machined surfaces initially exhibited superior hydrothermal degradation resistance, but deteriorated at a faster rate upon prolonged autoclave treatment compared with ground and grit-blasted surfaces. The accelerated hydrothermal degradation of CAD/CAM surfaces is attributed to the CAD/CAM machining damage and the absence of surface compressive stresses in the fully sintered material. Clinical relevance for surface treatments of zirconia frameworks in terms of hydrothermal and structural stabilities is addressed.

Surface roughness and hardness of yttria stabilized zirconia (Y-TZP) after 10 years of simulated brushing

“…Several papers reported the occurrence of t→m phase transformation due to heating of the material in procedures of grinding and/or sandblasting 8,16 , mechanical stress of chewing 10,22 , pH fluctuations 17,18 and humidity 9,10,20 . However, in this study, the mechanical stress caused by the bristles of the toothbrushes and by the abrasiveness of the dentifrice (silica with 3.3µm and RDA equal to 105, calcium carbonate with 2.13μm and RDA equal to 33) 24 were not sufficient to induce a t→m phase transformation or to change the roughness and hardness values of the zirconia.…”

“…Furthermore, mechanical wear such as abrasion can also contribute to changes in the material surface, directly reflecting on the roughness and accelerating the process of degradation 5,16 . Moreover, zirconia is susceptible to chemical degradation due to its dependence on pH and the concentration of fluoride 17 , since alkaline and acid pH values may cause a type of corrosion on the surface of the material 18 ; free fluoride in an acid medium can dissolve Y 3+ ions, forming YF 3 and destabilizing the material 17 .…”

Degradation of dental ZrO<Sub>2</Sub>-based materials after hydrothermal fatigue. Part I: XRD, XRF, and FESEM analyses

“…However, not all the scratches would lead to t→m transformation in their surrounding areas, as only the deepest scratches have the most deleterious effect on transformation. Only the grains along the scratch edges, where the tensile stresses are located, may undergo transformation into m-phase 36) . Zirconia-based materials used for orthopedic surgery have been reported to undergo LTD in humid environments.…”

Surface roughened zirconia: towards hydrothermal stability

“…Since the aging behavior of zirconia of sandblasted plus annealed specimens is similar to sintered plus polished specimens, the lower LTD in the sandblasted state must be related to the superficial compressive layer and the local plastic deformation generated. (Chintapalli, 2012;Kim et al, 2010;Kosmač, 2008). Nevertheless, specimens obtained maximizing the strength by choosing optimum sandblasting conditions, may suffer a drop in strength after artificial degradation, being the drop severe after long and aggressive aging treatments (Chintapalli, 2012;Kosmač, 2008;Papanagiotou et al, 2006).…”