To determine the effects of intrinsic wetness on the formation of dentin bonding interfaces of four resin cement systems bonded to dentin under different pulpal pressures.
Thirty-six freshly extracted third molars were selected and processed for dentin μTBS. The teeth were randomly assigned into 12 experimental groups, according to the adhesive luting system [Adper Single Bond Plus (3 M ESPE) combined with two luting agents RelyX ARC (3 M ESPE) and heated Filtek Z250 Universal Restorative (3 M ESPE), Clearfil CD Bond (Kuraray) combined with Clearfil Esthetic Cement (Kuraray), and RelyX Unicem 2 Automix (3 M ESPE)] and pulpal pressure (0, 5, and 20 cm of simulated pulpal pressure). Leucite-reinforced glass-ceramic slabs (IPS Empress CAD, Ivoclar Vivadent) of 3 mm thickness were bonded to dentin. The samples were stored in distilled water for 24 h and then sectioned in X/Y directions across the adhesive interface to obtain specimens with a cross section of 0.8 ± 0.2 mm 2 . All sticks were fractured by tension at a crosshead speed of 1.0 mm/min and the data were submitted to Kruskal-Wallis and Mann-Whitney Tests (α=0.05). Ultrastructural analysis of the interfaces was performed using Confocal Laser Scanning Microscopy (CLSM) and Scanning Electron Microscopy (SEM).
The statistical analyses showed that pulpal pressure decreased μTBS for all groups. Significantly higher μTBS values were obtained in heated Z250 group restored without any pulpal pressure. CLSM showed that the uptake of water through the dentin tubuli and their anastomosis of lateral branches during the adhesive luting procedures prevented adequate formation of the dentin bonding interfaces. SEM showed that the luting film created is material- dependent and all adhesive failure occurred at the resin-dentin interface.
The constant intrinsic wetness replenishment prevents adequate formation of the hybrid layer.
Intrinsic moisture during adhesive luting procedures significantly affects the interaction between luting materials and dentin subtract and decreases the quality and bonding strength of the resin-dentin bond.
Water is both essential and detrimental to the etch-and-rinse adhesive technique and significantly influences long-term bonding effectiveness to dentin. Water is necessary to keep the dentin moist after acid etching (etch-and-rinse technique) to avoid the collapse of the exposed collagen matrix and ensures adequate penetration of the monomer into the demineralized dentin. On the other hand, in vital teeth, the increased dentin permeability caused by acid etching leads to increased intrinsic wetness due to the tubular transudation of dentinal fluid driven by pulpal pressure . Intrinsic moisture seeping from the pulp chamber during adhesive application prevents the formation of sealed interfaces . This excess of moisture prevents the adequate penetration and polymerization of the monomers into the demineralized dentin complex, leaving collagen fibrils unprotected and surrounded by water within the hybrid layer
Self-etching and self-adhesive luting systems seem a good alternative for bonding indirect restorations to vital dentin. However, studies have shown that water can diffuse across the polymerized adhesive layer of self-etching luting systems and form water droplets along the adhesive–resin cement, which results in lower dentin bond strengths . Self-adhesive resin cements rely on the chemical reaction between phosphoric acid monomers and hydroxyapatite of dental hard tissues to bond on the tooth structure . During this reaction water is crucial for the luting agents to release hydrogen ions required for smear layer demineralization, however, excess of intrinsic water may affect sealing capacity and mechanical properties of self-adhesive cements .
Dentin bond strength and ultramorphologic characteristics of the resin-dentin interface created by those luting agents in the presence and absence of simulated pulpal pressure have been evaluated . However, it is still not clear how intrinsic wetness uptake from the pulpal chamber during the luting procedures affects the resin-dentin interface formation quality and bonding strength, as well as how the different adhesive luting systems interact with dentin subtract under physiologic pulpal pressure .
The reduction of pulpal pressure by the application of the local anesthesia with vasoconstrictor might improve the quality of the resin-dentin bond formed . Therefore, the aim of this study was to evaluate the influence of intrinsic water on resin-dentin interface formation produced by four different luting agents. The research null hypotheses were: 1) intrinsic wetness does not affect bond strength or the sealing ability and bonding integrity resin-dentin interfaces; 2) different adhesive luting systems are equally affected by intrinsic water permeation.
Materials and methods
Thirty-six intact freshly extracted, non-carious, non-restored human third molars were selected after the donors’ informed form had been obtained under a protocol reviewed and approved by the Institutional Review Board. The teeth were scaled, cleaned, stored in 0.5% chloramine solution at 4 ∘ C to prevent bacteria growth and used within three months after extraction.
The teeth were sectioned perpendicular to the longitudinal axis of the tooth, 3 mm below the cement-enamel junction (CEJ) using a diamond saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA), under distilled water-cooling, to remove the roots. A second parallel cut removed the occlusal enamel and superficial dentin to create a flat dentin surface of middle dentin (2.5 mm above the CEJ). The pulpal tissue was gently removed from the exposed pulp chamber, without damaging the pre-dentin. The occlusal surfaces of the crown segments were polished using a waterproof 600-grit silicon carbide paper under running water for 60s to create a standard smear .
To simulate pulpal pressure on dentin surface, the crown segments were attached to perforated composite resin disk using cyanocrylate glue (Zapit, Dental Ventures of North America, Corona, CA, USA) and connected to a polyethylene tubing of 18-gauge. Positive water pressures of 0, 5, or 20 centimeters (cm) were delivered to the prepared dentin surface during the adhesive procedures and the luting of the ceramic blocks as schematically illustrated in Fig. 1 .
Ceramic block preparation
Nine CAD/CAM blocks (17 × 8 × 9 mm) of leucite-reinforced glass-ceramic (IPS Empress CAD, Ivoclar Vivadent AG, Schaan, Principality of Liechtenstein) were used in this study. Each block was attached to an acrylic resin cylinder, mounted into a cutting machine (Isomet 1000), and cut using a low-speed diamond saw, under distilled water-cooling, into 4 slabs with 3 mm of thickness.
All slabs were conditioned with 5% hydrofluoridric acid (IPS Ceramic Etching gel, Ivoclar Vivadent) for 60 s, rinsed with running distilled water for 60 s, and cleaned in an ultrasonic bath of distilled water for 60 s. The slabs were dried with oil- and dust-free air (Dust-Off, Falcon Safety Products, Inc. Branchburg, NJ) for 20 s and a coupling agent (Clearfil Ceramic Primer, Kuraray Medical, Inc. Okayama, Japan) was applied on the ceramic surface. After 60 s, the remaining silane agent was evaporated with oil- and dust-free air for 5 s.
Luting of ceramic blocks
Four luting systems were selected for this investigation: an etch-and-rinse adhesive system (Adper Single Bond Plus, 3 M ESPE, St. Paul, MN, USA) combined with two luting agents (RelyX ARC, 3 M ESPE and heated Filtek Z250 Universal Restorative, 3 M ESPE), a self-etch adhesive system (Clearfil CD Bond, Kuraray Medical Inc. Tokyo, Japan) combined with Clearfil Esthetic Cement (Kuraray Medical Inc.), and a self-adhesive resin cement (RelyX Unicem 2 Automix, 3 M ESPE). The luting systems used in the study and their chemical compositions are listed in Table 1 .