Micro-tensile bond strength of adhesives bonded to class-I cavity ...

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1000 Introduction Thermo-cycling is a widely used artificial aging methodology. The ISO TR 11450 standard [1] indicates that a thermo-cycling regimen comprising 500 cycles in water between 5 and 55 8C is an appropriate artificial aging test. A recent literature review [2] concluded that 10,000 cycles corresponds approximately to 1 year of in vivo functioning, rendering 500 cycles, as proposed by the ISO standard, very minimal to mimic long-term bonding effectiveness. The artificial aging effect induced by thermocycling can be two-fold: (1) hot water may accelerate hydrolysis of non-protected collagen and extract poorly polymerized resin oligomers [3–5]; (2) due to the higher thermal contraction/expansion coefficient of the restorative material (as compared to that of tooth tissue) repetitive contraction/expansion stresses are generated at the tooth–biomaterial interface. This may result in cracks that propagate along bonded interfaces, and once a gap is created, changing gap dimensions can cause in- and outflow of pathogenic fluids, a process known as ‘percolation’ [2,6]. In the light of the first aging effect (diffusion-dependent hydrolysis and elution), thermo-cycling should be applied to micro-specimens, of which the interface is directly exposed to the changing temperature environment. Then, degradation of the adhesive–tooth interface is most severe, clinically corresponding to the most vulnerable restoration margins. Aging of mTBSspecimens might also be more appropriate than aging of the larger ‘shear-bond’-strength specimens, because the surrounding composite and tooth tissue may thermally protect the adhesive– tooth interface. If the second aging effect (repetitive contraction/expansion stress) is focused upon, thermo-cycling should be applied to specimens in which stress similar to that occurring clinically can be generated. In vivo stress will be generated if the ratio of bonded to unbonded surfaces or the ‘C-factor’ is high [7]. Clinically, about the highest C-factor is generated in narrow occlusal class-I cavities. In this case, thermo-cycling of the whole tooth including the high C-factor restoration will result in the highest possible stress imposed at the interface. It is, however, not clear whether or not thermocycling affects the bond strength to dentin. A recent meta-analysis [8], concerning data published between 1992 and 1996, concluded that thermo-cycling has no significant effect on bond strength. Most studies included in the meta-analysis were carried out following the ISO standard of 500 cycles (mean number of cycles in the studies analyzed was 630). This number of cycles was probably too low to obtain an aging effect [2,8,9]. Also specimen geometry has often not been taken into account. In most studies of this review, relatively large composite cylinders bonded to flat surfaces were thermo-cycled, prior to being pulled apart following a shear or tensile bond strength test protocol [8]. As a result, a large part of the interface must have been thermally protected by surrounding dentin [10] and composite [11] (which are known to be good thermal insulators). Because of the low C-factor of a flat restored surface (about 1/6), little repetitive expansion/ contraction stress must have been generated at the interface. Both reasons might explain why thermocycling did not affect bonding effectiveness in those studies. Therefore, the purpose of this study was to evaluate the relative contribution of diffusiondependent chemical degradation and repetitive contraction/expansion stress on the thermocycling-induced degradation of dentin–adhesive interfaces at the bottom of occlusal class-I cavities. The hypothesis tested was that thermo-cycling of restored class-I cavities (repetitive contraction/expansion stress), as well as of mTBS-sticks (diffusiondependent hydrolysis and elution) does not decrease bonding effectiveness. Materials and methods Specimen preparation J. De Munck et al. For this study, non-carious human third molars (gathered following informed consent approved by the Commission for Medical Ethics of the Catholic University of Leuven), stored in 0.5% chloramine solution at 4 8C were used within 1 month after extraction. First, all teeth were mounted in gypsum blocks in order to ease manipulation. A standard box-type class-I cavity (4.5!4.5 mm) was then prepared at the occlusal crown center with the pulpal floor ending at mid-coronal dentin, using a high-speed hand piece with a cylindrical mediumgrit (100 mm) diamond bur (842; Komet, Lemgo, Germany) mounted in a MicroSpecimen Former (University of Iowa, Iowa City, IA, USA). Next, the cavities were subjected to a bonding treatment (Table 1) using either a three-step etch and rinse adhesive (OptiBond FL), a two-step self-etch adhesive (Clearfil Protect Bond) or a one-step self-etch adhesive (iBOND). Subsequently, the cavities were filled in three horizontal layers with a resin-composite (Z100, 3M ESPE, St Paul, MN,
Micro-tensile bond strength of adhesives bonded to class-I cavity-bottom dentin after thermo-cycling 1001 Table 1 Adhesives used. Adhesive Composition Application OptiBond FL Etchant: 37.5% phosphoric acid, Apply the etchant for 15 s; rinse for 15 s; gently (Kerr, Orange, silica thickener [301194] air dry for 5 s; scrub the surface for 15 s with CA, USA) Primer: HEMA, GPDM, PAMM, ethanol, water, primer; apply a thin coat of bonding agent and photoinitiator [212652] Bond: TEGDMA, UDMA, GPDM, HEMA, bis-GMA, filler, photoinitiator [301335] light cure for 30 s Protect Bond Primer: MDP, MDPB, HEMA, initiator, water Apply the primer for 20 s using a rubbing (Kuraray, [ABB-002] motion; gently air dry; apply the bonding Osaka, Japan) Bond: MDP, HEMA, dimethacrylates, colloidal SiO2, surface treated NaF, initiator [ABP-001] agent; light cure for 10 s iBOND (Heraeus Adhesive: UDMA, 4-MET, gluteraldehyde, Apply in three consecutive times and rub for Kulzer, Hanau, acetone, water, stabilizer, photoinitiator 30 s; gentle air dry until adhesive moves no Germany) [010028] more; thoroughly air dry for 5 s; light cure for 20 s Bis-GMA, bisphenol-glycidyl methacrylate; GPDM, glycerol phosphate dimethacrylate; HEMA, 2-hydroxyethylmethacrylate; MDP, 10-methacryloyloxydecyl dihydrogen phosphate; MDPB, 12-methacryloyloxydodecylpyridinium bromide; PAMM, phthalic acid monoethyl methacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate; 4-MET, 4-methacryloyloxyethyl trimellitic acid. USA). Light-curing was performed using a highpower LED curing device (L.E.Demetron 1, Demetron/Kerr, Danbury, CT, USA). Then, the restored cavities were sectioned perpendicular to the adhesive–tooth interface using an Isomet diamond saw (Isomet 1000, Buehler Ltd, Lake Bluff, IL, USA) to obtain rectangular sticks (1.8!1.8 mm wide; 8–9 mm long). Out of each tooth, four sticks were sectioned from the central cavity floor (Fig. 1). They were mounted in the pin-chuck of the MicroSpecimen Former and trimmed at the biomaterial–tooth interface to a cylindrical hour-glass Figure 1 Schematic study design. shape with a bonding surface of about 1 mm 2 using a fine cylindrical diamond bur (835KREF, Komet, Lemgo, Germany) in a high-speed handpiece under air/water spray coolant. Specimens were then fixed to Ciucchi’s jig with cyanoacrylate glue (Model Repair II Blue, Sankin Kogyo, Tochigi, Japan) and stressed at a crosshead speed of 1 mm/min until failure in a LRX testing device (LRX, Lloyd, Hampshire, UK) using a load cell of 100 N. The mTBS was expressed in MPa, as derived from dividing the imposed force (N) at the time of fracture by the bond area (mm 2 ). When specimens failed before actual testing, the mTBS was determined from
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