Impact of polishing, glazing and firing, restoration thickness, point of loading and aging on the edge chipping resistance of lithium silicate ceramics.

Abstract

To investigate the edge chipping resistance (ECR) of four lithium silicate ceramics at different thicknesses and points of loading after various surface treatment, firing and aging protocols. 288 rectangular specimens were cut from CAD/CAM ceramics (lithium-di-silicate: Amber Mill, Amber Mill Direct, IPS e.max CAD; lithium-alumino-silicate: CEREC Tessera) in three thicknesses (1.5 mm, 2 mm, 3 mm) and underwent different surface treatments (polishing, glazing, no surface treatment) and/or firing protocols (high translucency, medium opacity). Specimens were bonded to 4 mm thick dentine analogues and loaded 0.25 mm or 0.30 mm from the edge using a Vickers diamond indenter. ECR was determined initially, after thermocycling (5/55 °C, 10,000 cycles) and after hydrothermal aging (134 °C, 0.2 MPa, 120min). Force when chipping occurred was recorded and ECR calculated. Data were analyzed with Kolmogorov-Smirnov, Kruskal-Wallis, Mann-Whitney U, Friedmann and Wilcoxon tests (p < 0.05). For 7/18 groups, glazed and medium opacity fired Amber Mill showed higher ECR than all other groups. In comparison with polishing or exclusive firing, a surface treatment with glazing led to the highest ECR. The influence of specimen thickness and point of loading was negligible. While aging reduced the ECR in 50 % of the glazed groups, the ECR of those groups remained among the highest. With the majority of groups showing no impact of the specimen thickness, a reduced restoration thickness of 1.5 mm seems to present limited disadvantages and should thus be considered for minimal invasive treatments. With regards to ECR, glazing can be recommended as the preferred surface treatment method for CAD/CAM lithium silicate ceramics.