02 Settembre 2019

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Effect of hydrofluoric acid etching time on flexural and adhesive properties of lithium disilicate

Original article

Francesco De Angelis, Mirco Vadini, Maurizio D’Amario, Matteo Buonvivere, Camillo D’Arcangelo

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Objectives  Hydrofluoric acid (HF) etching is able to enhance adhesive bonding of glass ceramics. Specifically, it has been shown that HF acid etching is able to produce superficial alterations on lithium-disilicate (LD) glass-ceramics, which result from the preferential removal of the glass matrix and a secondary crystalline phase, lithium orthophosphate; the so achieved rough surface increases surface energy, promoting both the successful application of the silane coupling agent and the resin bonding. Nevertheless, uncertainty regarding the most effective etching protocol for LD still remains. Moreover, little is known about the effects of decreased or increased HF acid concentrations and etching periods on the flexural strength or bonding ability of this glass-ceramic. Aim of this study was therefore to compare the effect of three different etching periods on the 3-point flexural strength and on the microtensile bond strength (μTBS) to dentine of a lithium disilicate (LD) glass ceramic.

Materials and methods  LD bar specimens (25x2x2 mm) were randomly assigned to four groups (n = 10) according to the exposure time to 5% HF acid (no exposure, 20 sec, 60 sec, 120 sec). Bars were observed under scanning electron microscope (SEM) and their 3-point flexural strength was recorded through three point bending test. LD slices (3 mm thick) were also produced, randomly divided into four groups according to the etching time (no-etch, 20 sec, 60 sec, 120 sec) and adhesively luted to flat dentine surfaces. The bonded assemblies were sectioned into 1 mm2 wide beams (n = 12), then subjected to tension to record the ultimate μTBS. Means and standard deviations were calculated for μTBS (MPa) and flexural strength (MPa) in each experimental group and were compared by one-way ANOVA tests (α = 0.05). After the μTBS test, both the dentin and LD sides of each specimen were observed under SEM at 150x or higher magnification to analyze the fracture mode.

Results and conclusions  Morphological changes of the acid etched LD surfaces appeared significantly more distinct after 120 sec of 5% HF acid than after 60 sec or after 20 sec. No-etched LD samples showed smoother surfaces but not significantly reduced flexural strength mean values compared to the 5% HF acid etched groups. Extending the etching time from 20 sec to 120 sec did not significantly improve the bond strength to dentine: the highest mean μTBS value was registered with 120 sec etching time, without any statistically significant difference compared to the 60 sec and 20 sec groups; the no-etched samples provided significantly reduced μTBS compared to all the other groups. In short, extending LD exposure to 5% HF acid, up to 120 sec, may lead to the formation of a visibly more distinct etching pattern without producing detrimental effects on its flexural properties. Concurrently, acid etching beyond 20 sec does not improve the bond strength to dentine.

Clinical significance  A 20-seconds etching protocol with 5% HF appears suitable to promote an adequate bond strength to dentine, allowing the clinician to save time and reduce the risk of exposure to a potentially hazardous product.

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doi: https://doi.org/10.19256/d.cadmos.07.2019.06