Influence of post-etching cleaning and connecting porcelain on the microtensile bond strength of composite resin to feldspathic porcelain

Abstract

There are different methods to fabricate layered feldspathic porcelain restorations, including the refractory die technique and the hot-press technique. Standard adhesive protocol for such restorations requires etching and silanating the fitting surface of the porcelain. Variations in bond strength between porcelain and composite resin might result from the different fabrication methods for porcelain restorations. The purpose of this study was to determine differences in microtensile bond strength between composite resin (used as a luting agent) and feldspathic porcelain generated from the refractory die technique using 2 different connecting porcelains and the hot-press technique. The effect of post-etching cleaning was also investigated. Sixteen pairs of ceramic blocks (7 x 8 x 4 mm) were fabricated. Eight pairs were fabricated using feldspathic porcelain (D-B4) on refractory dies. For 4 pairs, the refractory dies were coated with a paste (Ducera Lay Connector Paste; group CON) as a connecting porcelain, and the other 4 pairs were coated with a clear translucent porcelain (CL-O; group CLO). Another 8 block pairs were fabricated using the hot-press technique (Authentic porcelain SL B00+; groups AUTH and AUTH-N). Surface conditioning of the ceramic blocks included airborne-particle abrasion followed by hydrofluoric acid etching (all groups), and post-etching cleaning with a brush/H3PO4 and ultrasonic bath immersion (cleaning not applied to group AUTH-N). All specimens were then silanated/heat dried, and blocks of the same porcelain were bonded to each other using an adhesive resin (Optibond FL) and a light-polymerizing composite resin (Z100). Specimens were stored in water for 24 hours. A nontrimming microtensile bond strength test was applied. Ten beams (0.9 x 0.9 x 8 mm) from each pair of blocks were selected for testing. Bond strength data (MPa) were analyzed with a Kruskal-Wallis test, and post hoc comparison was done using the Mann-Whitney U test (alpha = .05). Additional specimens (1 block per group) were also evaluated for the effect of conditioning steps and mode of fracture using optical microscopy and scanning electron microscopy (SEM) analysis. The mean microtensile bond strengths of CLO and AUTH groups were not significantly different from one another at 46.3 and 49.7 MPa, respectively. For both CON and AUTH-N groups, the mean bond strengths at 37.9 MPa and 24.1 MPa, respectively, were significantly different (P < .05) from the other 3 groups. Optical microscopy revealed a significant amount of white residue for all groups as a result of hydrofluoric etching. Cleaning with a microbrush and 37.5% phosphoric acid for 1 minute resulted in the removal of the crystalline debris. The SEM analysis of specimens cleaned by phosphoric acid brushing alone revealed microscopic deposits still contaminating the etched surface; those were efficiently removed after ultrasonic cleaning. The SEM analysis of fractured beams demonstrated a trend for more mixed-type failure in CON and AUTH-N specimens involving both the composite resin and the surface of the porcelain, whereas CLO and AUTH fractured surfaces were primarily confined to the composite resin. Increased resin bond strength to refractory-generated porcelain is obtained with CLO as the connecting porcelain compared to the CON paste. The AUTH porcelain exhibited the highest mean bond strength, but omission of post-etching cleaning resulted in the lowest bond strength.