Effect of neutralization and hydrofluoric acid precipitate remotion on the compressive strength of monolithic lithium disilicate crowns

Abstract

Background The cleaning protocol for the ceramic surface after acid etching resulted in a decrease in bond strength and flexural strength of a glass ceramic. This study aims to evaluate the effect of different ceramic surface treatments after hydrofluoric acid etching (HF) on the compressive strength of monolithic lithium disilicate crowns. Methods Forty (40) human third molars received conventional full coverage preparation. After performing digital impressions of teeth preparations, ceramic blocks were machined using a CAD/CAM system in order to obtain the crowns. The crowns were distributed in 4 groups as ceramic surface treatment (n=10): (HF) - 4.9% HF for 20s + air-water spray for 30s; (HFN) - HF + neutralizing agent for 5 min (N); (HFU) - HF + ultrasonic bath for 5 min (U); e (HFNU) - HF + N + U. SEM and EDS analysis was performed in each group in order to characterize the ceramic surface and to verify the chemical element distribution after HF cleaning protocols. A silane layer was applied (for 60s), and crowns were then cemented with dual resin cement. A compressive load was applied on the middle of the occlusal crown surface with a crosshead speed of 1 mm/min until fracture. Data were analysed using ANOVA and Tukey test (α=0.05). Results Fluoride ions were found in samples of all post-etching cleaning protocols. The mean value (Kgf) was: HF = 169.92 ± 21.37; HFN = 187.34 ± 34.79; HFU = 166.63 ± 40.22 and HFNU = 175.26 ± 40.22. The ceramic surface treatment after HF etching did not significantly influence (p>0.05) the compressive strength of the tested ceramic crowns. Conclusions Surface treatments with neutralizing agent associated with the ultrasonic bath as the pre-cementation protocol was the most efficient protocol in eliminating the precipitate deposited on the porosities created by acid etching.