Acid Etching on Glass-Ceramics’ Tridimensional Microstructure and Flexural Strength

Objectives: The aim of this study was to evaluate the glass-dissolution of different etching protocols on tridimensional ceramic’s surfaces, and the correlation with its flexural strength.
Methods: Two glass-ceramic materials, leucite (IPS Empress), and lithium disilicate (IPS e.max) bar-shaped samples (4×12×0.3mm) were prepared. Specimens were divided into 5 groups according to surface protocol: (1). Control ; (2). 5% hydrofluoric acid for 20s (HF5% 20s); (3). (HF5%60s); (4). (HF10% 20s); (5). (HF10%60 s). The flexural strength was evaluated through three-point bending test. Then, the fractured specimens were submitted to a morphological glass-dissolution analysis. Their four sides were evaluated (applied-surface, internal, lateral and contralateral) using scanning electron microscopy (SEM) and micro-CT analysis. A new classification of glass-dissolution by HF was created, to measure lateral and internal corrosion. Data were correlated with flexural strength values (MPa) and submitted to ANOVA, followed by Bonferroni test (α= 0.05). Weibull analysis and Pearson correlation coefficient were also calculated.

Results: Weibull analysis showed lower reliability and characteristic strength of leucite when applying HF10%60s. Lithium disilicate values were similar among the groups. For both materials, HF10%60s had the highest internal glass-dissolution (F=6.172)(p≤ 0.004). However, only on leucite ceramic had an inverse correlation with its flexural strength. On the lateral surfaces, HF acid seems to flow over and corrode them with different patterns. Contralateral surface’s edges of both ceramics were also affected by all etched protocols.

Conclusions: HF acid flow over restoration’s lateral surfaces and corrode them in different degrees, with the possibility of also affecting all margins. Protocols of greater time and concentration are more related to internal glass-dissolution and the lessening of ceramic flexural strength.