Microtensile bond strength of a filled vs unfilled adhesive to dentin ...

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288 Figure 4 (A) Scanning electron micrograph of the resin– dentin interface bonded with One-Step with self-etch technique after argon ion etching. Thickness of the hybrid layer (3.5 mm) was similar to the One-Step total-etch group (Fig. 3(A) and (B)). The unfilled adhesive layer was only 2 mm thick (compare Fig. 2(A) and (B)). c, composite; a, adhesive; h, hybrid layer; d, dentin. (B) Scanning electron micrograph of the resin–dentin interface bonded with One-Step with self-etch technique subjected to sequential acid–base treatment. Thickness of the hybrid layer (3.5 mm) was similar to the One-Step total-etch group (Fig. 3(A) and (B)). c, composite; a, adhesive; h, hybrid layer; d, dentin, g, gap induced in the adhesive layer and between hybrid layer and adhesive layer. adhesive failures (Fig. 6). Using total-etch technique, the filled adhesive One-Step Plus (Fig. 5) and unfilled adhesive One-Step (Fig. 7) showed mostly mixed failures. Discussion E. Can Say et al. Filled adhesives were expected to act as an intermediate shock-absorbing elastic layer between composite resin and dentin, thus increasing the bond strength to dentin. 1,3,20 Many studies evaluated comparisons between commercially available filled and unfilled adhesives however, the advantages of these adhesives as stress buffers remain unpredictable 7 and have not been substantiated with in vitro bond tests 25–29 and a clinical trial. 30 Filler type, size, shape, its surface characteristics, their interaction with the resin matrix and various solvents in the adhesives may affect the bond strength. 20,21,31–33 The unfilled adhesive One- Step and filled adhesive One-Step Plus, used in this study, utilizes acetone as a solvent. Besides, One- Step Plus consists of 8.5% Glass fillers with an average particle size of 1 mm. SEM observations of the resin-dentin interfaces revealed that the thickness of the hybrid layer created with One- Step Plus was almost similar to that achieved with One-Step both with total-etch (approximately 3 mm; Figs. 1(B) and 3(B), respectively) and selfetch (approximately 3.5 mm; Figs. 2(B) and 4(B)) techniques. Conversely, the adhesive layer of the filled adhesive was thicker (Figs. 1(A) and (B), 2(A) and (B)) than the unfilled adhesive (Fig. 3(A) and (B), 4(A)). Filler particles were found around the tubular orifices and in some dentin tubules (Fig. 1(A)) with the total-etch technique, however they were not capable of penetrating into the spaces between collagen fibers because the width of interfibrillar spaces is about 20 nm. 9 The results of the microtensile bond test showed that bond strengths to dentin between filled adhesive One- Step Plus and the unfilled adhesive One-Step were not significantly different when they were used with the total etch (38.8G12.2 and 33.9G8.5 MPa, respectively) and with the self-etch technique (22.4G4.0 and 26.4G7.5 MPa, respectively). These results might indicate that the penetration ability of the resin monomer and the evaporation rate of water/solvent from dentin subsurface are not so different between filled and unfilled Table 4 Failure modes of the specimens after microtensile bond test. Groups Adhesive Mixed Cohesive in dentin Cohesive in composite One-Step Plus total-etch 3 15 2 – One-Step Plus self-etch 17 3 – – One-Step total-etch 5 15 – – One-Step self-etch 14 6 – – Adhesive, between resin and dentin; Mixed, partially adhesive, partially cohesive failure in bonding resin or hybrid layer; Cohesive in dentin; Cohesive in resin.
Microtensile bond strength of adhesive to dentin using self-etch and total-etch technique 289 Figure 5 Scanning electron micrograph of the fractured surface of One-Step Plus total-etch group showing mixed failure. d: cohesive in dentin, b: cohesive in bonding resin, a: adhesive failure. adhesives, although they have different viscosities because of the 8.5% filler load. Moreover, similar bond strengths to dentin may be due to the application of resin composites to a flat dentin surface because the polymerization shrinkage of the composite resin would minimize stress at the bonded interface. 26 Stress distribution during tensile testing between filled and unfilled adhesives has been expected to be different because adhesives containing fillers showed higher elastic modulus than unfilled resins. 25 However, the failure modes of the filled and unfilled adhesives with total-etch technique (Figs. 5 and 7, respectively) were similar Figure 6 Scanning electron micrograph of the fractured surface of One-Step Plus self-etch group showing adhesive failure. Figure 7 Scanning electron micrograph of the fractured surface of One-Step total-etch group showing mixed failure. a: adhesive failure, b: cohesive in bonding resin. showing predominantly mixed failure (Figs. 5 and 7, respectively) and also similar with the self-etch technique showing adhesive failure (Figs. 6 and 8, respectively). This may be due to the fact that the application of a thin layer of a more flexible adhesive (lower elastic modulus) (Figs. 3(A) and 4(A)) led to the same stress relief as thick layers of a less flexible adhesive (higher elastic modulus) 34 (Figs. 1(A) and 2(A)). Self-etch adhesives have been classified on their ability to penetrate smear layers and their depth of demineralization into the subsurface dentin as mild, intermediary strong and strong. 16,35 The self-etching primer Tyrian SPE used in this study Figure 8 Scanning electron micrograph of the fractured surface of One-Step self-etch group showing adhesive failure.
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