Etching of ceramic blocks with 10% hydrofluoric acid

Prof. Guilherme Carpena Lopes and Deise Rebelo Consoni

New ceramic systems have been developed and commercialized in the international market. The adequate adhesion to the ceramics is reached by means of two principles; application of hydrofluoric acid (HF) in a low concentration for a few seconds followed by the application of the bonding agent, silane. Respectively, those phases result in a ceramic surface with higher surface free energy and chemical bonding to the silica in the ceramics, be it feldspathic or ceramic glass.

With the popularity of ceramic systems for use with CAD/CAM, it became crucial to know the etching standard of each commercial brand of fluoric acid in the selected ceramic block for clinical use.

Objective: through magnetic field scanning electron microscopy, analyze the surface-etching standard of 3 ceramic blocks using a hydrofluoric gel in the 10% concentration.

Materials and methods: the ceramic blocks (n=2) selected for this laboratorial study were: feldspathic ceramics (VITABLOCS Mark II, VITA Zahnfabrik, Germany); reinforced glass-ceramic with lithium desilicate (IPS e.max CAD, Ivoclar Vivadent, Liechtenstein); and reinforced glass-ceramic with a high density of lithium disilicate obtained by micronizing (HDM, GC Initial LiSi CAD, GC Co., Japan). The blocks were crystalized in appropriate ceramic kiln, cut in two section directions using a diamond disk in low rotation for the obtainment of rectangular samples (4x4x6mm3). The blocks were polished with silicon carbide sandpaper up to #2000 grains, cleaned in ultrasound with purified water and separated in two groups: Control = polished ceramic surface; or, surface treated with hydrofluoric acid at 10% (Condac Porcelana, FGM).

The times for the application of the gel, in accordance with the instructions of each block manufacturer, were respectively 60 seconds, 20 seconds and 20 seconds. Following that, the hydrofluoric acid was thoroughly washed with air/water spray for 30 seconds. The ceramic samples were then cleaned again in ultrasound for 10 minutes, dried with air jet without moisture or oil for approximately 1 minute, assembled in a sample holder, covered with gold- palladium and observed in scanning electron microscopy SEM (FESEM, JSM-6701F, JEOL) with augmentation from 2,000X to 20,000X.

Results: the control groups did not show a standard in the formation of micro retentions comparable to the samples treated with hydrofluoric acid at 10%. The samples treated with HF resulted in the creation of irregularities in all ceramic blocks. The feldspathic ceramics showed an etching standard that looked like honeycombs in the vitreous matrix3 with the maintenance of the integrity of the ceramic crystals.

The glass-ceramic blocks reinforced with lithium disilicate and micronized lithium disilicate exhibited ceramic etching standard with similar depth. Additionally, that standard was homogenous in the whole surface, with the dense concentration of lithium disilicate crystals exposed and apparently in areas of over-etching of the ceramics. For both glass-ceramic blocks, the lithium disilicate crystals were confined to the vitreous phase.

Conclusion: for all the ceramics analyzed (feldspathic, glass ceramic reinforced with lithium disilicate and glass-ceramic reinforced with micronized lithium disilicate), hydrofluoric acid at 10% (Condac Porcelana, FGM) resulted in an appropriate standard of ceramic etching from the adhesive and integrity perspectives.

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