The effect of glazing and thermocycling on microhardness of different milled ceramic laminate veneers with SEM observations

Abstract

CAD/CAM technology is one of the fastest growing areas of digital restorative dentistry. The aim of the study was to evaluate in vitro the effect of glazing and thermocycling on the microhardness of ceramic veneers under the assumption that there is a significant difference between the microhardness values of different glazed and unglazed CAD/CAM materials.
Materials and methods.
The microhardness of 80 veneer specimens from four different ceramic CAD/CAM materials were measured: 1) IPS E.max CAD lithium disilicate ceramic (Ivoclar, Germany); 2) IPS Empress CAD leucite glass ceramic (Ivoclar); 3) Cerec feldspathic ceramic (Dentsply Sirona, Germany); and 4) Cerasmart hybrid ceramic (GC, Japan). All specimens were divided into 4 equal groups, and in each group, half (10) of the veneers were glazed. All veneer specimens were subjected to 10,000 thermocycles to simulate one year of service. Vickers microhardness values were measured at three stages: after milling, after glazing, and after thermocycling. SEM images of the surface of the samples after milling and after thermocycling were also obtained.
Results.
Statistically significant differences between microhardness values of different materials after glazing and thermocycling were revealed. Microhardness values after glazing decreased by 5% for E.max, by 7—9% for Empress and Cerec, and by 34% for Cerasmart. E.max specimens showed higher microhardness values after glazing (601±13 MPa) compared to Empress (575±12 MPa), Cerec (453±16 MPa), and Cerasmart (76 MPa; p<0.05). Glazed surface analysis also revealed statistically significant differences after thermocycling (p<0.05). Tamhein's T2 post hoc test confirmed that the microhardness values of the E.max group were significantly higher after thermocycling compared to the Cerasmart, Empress and Cerec groups (p<0.05). The effect of thermocycling was also evident for unglazed samples where the E.max group showed higher microhardness values (641±7 MPa) compared to Cerasmart, Empress and Cerec groups (p<0.05).
Conclusion.
 Glazing and thermocycling reduced the microhardness of veneers made of the investigated materials. E.max and Empress samples showed the highest microhardness of about 600 kgf/mm2. Comparable values of about 500 kgf/mm2 were shown by Cerec feldspathic ceramics. The microhardness of Cerasmart samples was much lower, in the order of 100 kgf/mm2 (p<0.05). The study highlights the importance of glazing and thermocycling in evaluating the microhardness values of ceramic CAD/CAM materials, revealing the peculiarities of their behavior in clinical use. Thermocycling did not significantly decrease the microhardness.