Effect Of Etching And Resin Bonding On The Flexural Strength Of Glass-ceramic

Numerous HF etching periods have been developed for use in clinical practice ever since it was first suggested as a ceramic surface pretreatment for resin bonding. Most frequently, the ceramic was etched by2.5%～10%HF acid for20s-3min in order to form suitable ceramic surface roughness, whereby the resin cement permeated into and bonded intimately with the undercut structure of the etched ceramic to achieve good mechanical retention. The optimal etching time correlates with the structure and nature of the ceramic. Shorter etching time is not sufficient to produce honeycomb structure on the ceramic surface whereas longer etching time would damage the ceramic by reducing the crystal structure. Therefore, it is of clinical importance to determine proper etching time without weakening the ceramic under the premise to obtain a higher bonding strength. Furthermore, ceramic surface etched by HF would finally be cemented to the underlying tooth structure utilizing a resin luting cement. As a result, the flexural strength of the ceramic etched by HF may not exactly reflect the actual strength of all-ceramic restoration. We evaluate the effect of HF acid etching time and resin bond on the flexural strength of a novel lithium disilicate-based glass ceramic (IPS e.max(?) Press) in this study, trying to figure out whether different HF etching time and resin bond procedure produces different flexureal strength values.Objective:The purpose of the study to analyze the effect of hydrofluoric acid(HFA) etching time and resin bonding on the flexural strength of IPS e.max(?) Press glass ceramic, while at the same time evaluating the efficacy of resin cements to heal the cracks of the etched ceramic.Methods:240bars (25mm×3mmx2mm) of specimens were fabricated from IPS e.max(?) Press ingots using a lost wax, hot press technique. The ceramic surface were ground with silicon carbide paper and polished down to0.5μm.The centre of each specimen was etched using9.5%HF acid gel according to the etching time (0,20,40,60and120s). Forty specimens of each group were randomly selected and divided into two subgroups within each group. Silane coupling agent was then applied to the etched surfaces of subgroup A and lightly air thinned. Then the silanated ceramic surfaces received a layer of unfilled resin luting agent and dual-polymerized composite resin, then polymerized for40s. Samples of subgroup B remained untreated. Half of the every subgroups’ specimen were thermocycled for0or10000times before the3-point bending test.Six remaining specimens of each group were selected, the treated samples surfaces of each group were observed by FE-SEM and the roughness parameters were measured by an AFM. The interfaces between resin cement and ceramic were analyzed under a FE-SEM after grinding and gold-coating.Results:The roughness parameters of IPS e.max(?) Press glass ceramic increased significantly with the increase of etching time. The untreated ceramic surface was smooth. As the etching time increasing, it was possible to quantify image features, such as height and surface roughness. The ceramic/cement interface of the untreated surfaces remained smooth, only a few of resin luting agent penetrated into the flaws and cracks of the ceramic surface. When the etching time prolonged from20s to120s, there were intimate bonding of etched ceramic and resin luting agent at the interface, where resin cement adequately penetrated into pits and microcracks produced by HF acid etching on the glass ceramic surfaces, with the depth of the penetration about5～8μm.The etching time and resin cement significantly affect the flexural strength of the samples (p<0.05). HF etching significantly reduced the mean flexural strength as the etching time increased (p<0.05). The mean flexural strength of each group except group0s increased significantly after the application of resin cement. However, in the present investigation no significant differences in thermocycling on the flexural strengths was observed.Conclusion:Overtime HF etching could have a wakening effect on IPS e.max(?) Press glass ceramic. Appropriately applied resin luting agent could make up the strength loss of the acid etched glass ceramic.