Acid dissolution of ultrathin glass-ceramic and its correlation with flexural strength

Statement of problem. When glass-ceramics are treated with hydrofluoric acid (HF), not only the area of application is affected but also other surfaces. Information regarding the correlation of the dissolution caused by HF and the flexural strength of the ceramic is lacking. Purpose. The purpose of this in vitro study was to investigate the effect of HF etching protocols on the flexural strength of 2 glass-ceramics, the correlation of their flexural strength with the internal and lateral dissolution, and the differences in Weibull characteristics concerning the etching protocols. Material and methods. Specimens (4x12x0.3mm) of leucite-reinforced glass-ceramic-LEU (IPS Empress) and of lithium disilicate-reinforced glass-ceramic-LD (IPS e.max) were prepared. The specimens were divided into 5 groups (n=12) according to etching protocol: control-untreated; hydrofluoric acid (HF) 5% for 20 seconds (HF5%20s); HF5%60s; HF10%20s; and HF10%60s. Flexural strength was evaluated through 3-point bend testing. The fractured specimens were submitted to a morphological and metrical analysis of each pattern (LEU I-IV, LD I-III) on the lateral surface (LS) and of the dissolution areas (mu m(2)) on the internal surface (IS) using a scanning electron microscopy (SEM) and a software program (FIJI). Flexural strength (MPa) values were analyzed by 1-way ANOVA, Bonferroni test, and Weibull and the correlation between flexural strength and dissolution by Pearson correlation coefficient (alpha=.05). Results. ANOVA revealed that the HF etching protocol was significant (P<.001) for LEU ceramic. When HF10%60s was applied, LEU showed Weibull characteristic strength (sigma theta) values lower than those of the other protocols. LD showed no difference in sigma theta and reliability (m) among the groups. The internal dissolution areas concerning LEU had a significant high negative correlation with the flexural strength (r=-0.6; P<.001). LEU-IV on the LS was the only pattern to show a significant decrease in flexural strength (r=-0.3, P<.05). Conclusions. Special care should be taken when applying hydrofluoric acid on glass-ceramic restorations. Protocols with higher hydrofluoric acid concentrations and longer application times were more related to greater internal areas of dissolution which reduced the flexural strength of leucite ceramics.