The aim of this study was to investigate dentin bonding durability of different etch-and-rinse (ER) adhesive systems under fatigue stress and to compare morphological features of resin/dentin interfaces using SEM.
Two three-step ER adhesives, a two-step ER adhesive, and a universal adhesive in ER mode were evaluated. Before application of either primer or adhesive, phosphoric acid etching of human dentin was completed. Fifteen bonded specimens for each adhesive system were stored in distilled water at 37 °C for 24 h, then subjected to a shear bond strength (SBS) test. Bonding durability was assessed from the perspective of biomechanical stress. 25 bonded specimens for each adhesive system were subjected to shear fatigue strength (SFS) testing with a repeated subcritical load at a frequency of 20 Hz for 50,000 cycles or until failure.
Mean SBS and SFS values ranged from 33.3 to 41.2 MPa, and from 18.3 to 20.3 MPa, respectively. Three-step adhesives showed higher SBS and SFS values than the other adhesive systems. Under SEM, resin tags in different adhesive systems showed similar features, but morphology below the hybrid layer was material dependent. The universal adhesive in ER mode showed an obvious thin, high-density reaction layer below the hybrid layer.
Three-step adhesives showed higher dentin bond durability than the other ER adhesives; no significant differences in SFS were found between the universal adhesive in ER mode and the three-step ER adhesives. The results of this in vitro study indicate that some ER adhesives might establish chemical bonding with intact dentin below the hybrid layer in addition to micromechanical retention.
Etch-and-rinse adhesive systems (ER) use phosphoric acid etching to treat enamel and dentin before application of adhesive [ ]. ER adhesive systems are grouped into three-step and two-step systems based on the use of a priming step [ ]. ER adhesive systems are thought to enhance enamel bond effectiveness [ , ], and remove bonding inhibitors such as saliva, blood, gingival crevicular fluid, and dental plaque owing to the use of strong acid and a water rinse after etching [ ]. Based on a systematic review and meta-analysis of dentin bonding performance with caries-affected dentin, ER adhesive systems show better dentin bonding than self-etch (SE) adhesive systems [ ]. Furthermore, a systematic review of post-operative sensitivity for posterior resin composite restorations found that adhesive strategy (ER or SE) for posterior resin composite restorations does not influence the risk and intensity of post-operative sensitivity [ ]. Despite these adhesive systems being used worldwide in dentistry, bonding strategies using phosphoric acid pre-etching before application of adhesive are not the most recent technology and increase the number of clinical steps.
Fundamentally, the bonding mechanism of ER adhesive systems is thought to be micromechanical interlocking between cured adhesive and hard tissue [ , ]. The strong acid etching not only increases surface energy and bonding area but also enhances porosity in mineralized tissue [ , ]. Different enamel etching patterns exist [ ], and very fine undulations created on enamel surfaces are effective for establishing strong and durable enamel bonds [ , ]. The formation of a hybrid layer (HL) and of resin tags in dentinal tubules plays some role in maintaining mechanical interlocking for dentin bonding. However, in contrast to enamel bonding, concerns remain for dentin bonding due to the possibility of naked collagen fibrils that are not protected by resin monomers [ , ]. From biomechanical and biological degradation perspectives, insufficiently resin-impregnated areas may be vulnerable and thus more susceptible to structural deterioration.
In contrast, bonding of self-etch (SE) adhesive systems involves a chemical interaction between hydroxyapatite (HAp) and functional resin monomers, followed by micromechanical interlocking with etched mineralized tissue [ , ]. In particular, the Ca salt formed by reaction between HAp and functional resin monomer is both water-insoluble, and acid-resistant [ , ]. Further, the Ca salt can be created within a clinically realistic timeframe and shows excellent stability even after ultrasonication [ ].
Universal adhesive systems are widely accepted because of their simplified bonding procedures, similar to single-step SE adhesive systems [ , ]. Furthermore, they have broad applicability because they can be used in either ER or SE mode [ , ]. Although the dentin bonding mechanisms of universal adhesives in different etching modes are dissimilar, previous studies have shown that dentin bond strength in ER mode is almost unchanged in SE mode [ ]. A scanning electron microscopy (SEM) investigation observed universal adhesive/dentin interfaces in different etching modes, and universal adhesives in SE mode exhibited similar morphological features to conventional single-step self-etch adhesives. However, universal adhesives in ER mode exhibit different morphological features from three- and two-step ER adhesive systems. Most universal adhesives show a thin high-density layer (reaction layer) underneath HL that is not visible in three and two-step ER adhesive systems [ ]. Most universal adhesives use an effective functional monomer that creates a stable self-assembled nano-layering at the bonding interface, 10-methacryloyloxydecyl dihydrogen phosphate (MDP) [ ]. Some conventional ER adhesive systems do not form this layering. Therefore, functional monomers of universal adhesives in ER mode interact in some way with HAp underneath the demineralized dentin. In contrast, conventional three- and two-step ER adhesive systems contain different functional monomers or a polyalkenoic acid copolymer that might interact with HAp after acid etching. However, little information is available to date concerning dentin bonding mechanisms of universal adhesives in ER mode and conventional ER adhesive systems.
The purpose of this study was to investigate the differences in dentin bonding mechanisms among different ER adhesive systems based on shear fatigue strength tests and SEM observation of the resin/dentin interface. The null hypothesis to be tested was: the dentin bond durability and morphological features of resin/dentin interfaces of different ER adhesives, including universal adhesives in ER mode, would not differ.
Materials and methods
Materials used in this study are shown in Table 1 . Two three-step ER adhesives, OptiBond FL (OL, Kerr, Orange, CA, USA) and Adper Scotchbond Multi-Purpose Plus (SM, 3M Oral Care, St. Paul, MN, USA), and a two-step ER adhesive, Single Bond Plus (SB, 3M Oral Care), were used. A universal adhesive, Scotchbond Universal (SU, 3M Oral Care), was used as a comparison adhesive. Phosphoric acid pre-etching was performed using Ultra-Etch (Ultradent Products, South Jordan, UT, USA). The microhybrid resin composite Clearfil AP-X (Kuraray Noritake Dental, Tokyo, Japan) was used for bonding to dentin. A tungsten halogen visible-light curing unit, Spectrum 800 Curing Unit (Dentsply Sirona, York, PA, USA), was used, and light irradiance (average 600 mW/cm 2 ) was checked during the experiment.
|Code||Three-step ER adhesives||Main components||Manufacturer|
|OL||OptiBond FL||Primer: HEMA, GPDM, BHT, ethanol, water, CQ||Kerr|
|6902900 (Primer)||Adhesive: bis-GMA, UDMA, TEGDMA, GDMA, HEMA, filler, CQ, ODMAB, filler (fumed SiO 2 , barium aluminoborosilicate, Na 2 SiF 6 ), ytterbium trifluoride, coupling factor A174||Orange, CA, USA|
|SM||Scotchbond Multi-purpose plus||Primer: HEMA, polyalkenoic acid, water||3 M Oral Care, St. Paul, MN, USA|
|N852287 (Primer)||Adhesive: bis-GMA, HEMA, triphenylantimony, amines|
|Code||Two-step ER adhesive|
|SB||Adper Single Bond Plus||bis-GMA, HEMA, UDMA, Vitrebond copolymer, GDMA, ethanol, water, silane treated silica, diphenyliodonium hexafluorophosphate, EDMAB, CQ||3 M Oral Care|
|Code||Universal adhesive in ER mode|
|SU||Scotchbond Universal||bis-GMA, MDP, HEMA, Vitrebond copolymer, silane treated silica, ethanol, water, CQ, silane||3 M Oral Care|
|Ultra-Etch G017||35% phosphoric acid||Ultradent Products, South Jordan, UT, USA|
Extracted and de-identified human molar teeth were used. After extraction, soft tissue attached to the root was immediately removed with hand instruments, and teeth were then stored frozen (−20 °C) until use. To assure homogeneity, teeth with almost the same size and shape were carefully selected from stored extracted teeth, and teeth with any sign of caries or cracking of the enamel were discarded. Approximately two-thirds of apical root structures were then removed using a low-speed saw (IsoMet 1000 Precision Sectioning Saw, Buehler, Lake Bluff, IL, USA). Selected teeth were prepared by sectioning the teeth mesiodistally. Each tooth segment was then mounted in an aluminum ring with a diameter of 25 mm using Triad DuaLine (Dentsply Sirona, Charlotte, NC, USA). Dentin bonding surfaces were ground flat using a water coolant and a sequence of silicone carbide (SiC) papers (Struers, Cleveland, OH, USA) ending at 4000 grit. The study protocol was reviewed and approved by the Ethics Committee for Human Studies at our institution (#2015-06).
Bonding procedures and shear bond strength (SBS)
Experimental protocols for dentin bonding procedures are provided in Table 2 . Forty specimens were used for each test group: 15 to determine dentin SBS and 25 to determine the SFS in ER mode (phosphoric acid application for 15 s before adhesive application). The sample size for SBS was based on ISO specification 29022 (Dentistry-Adhesion-Notched-edge shear bond strength test) [ ]. It indicates that the sample size should be fifteen, and we adopted this number as our SBS testing protocol is a modified version of ISO 29022. All bonding procedures were performed following the manufacturer’s instructions ( Table 2 ), and were conducted by a single operator. After bonding, a stainless steel metal ring was set over the bonding site and held in place with a custom fixture in a modified Ultradent Bonding Jig (Ultradent Products). Metal rings were used to place resin composite on dentin surfaces for shear bond strength (SBS) and shear fatigue strength (SFS) tests. The bonding procedure resulted in a resin composite cylinder inside the ring 2.36 mm in diameter and approximately 2.5 mm in height. The ring was left in place for both tests.
|Adhesive application protocol|
|Three-step ER adhesive|
|OL||Dentin surface was phosphoric acid etched for 15 s. Etched surface was rinsed with water for 15 s. Dried gently for 3 s (do not desiccate). Primer was applied to dentin surface with light scrubbing motion for 15 s. Air dried for 5 s. Using same applicator, adhesive was applied with light scrubbing motion for 5 s. Air thinned for 3 s. Light irradiated for 20 s.|
|SM||Dentin surface was phosphoric acid etched for 15 s. Etched surface was rinsed with water for 15 s. Air dried gently for 2 s. Left moist. Primer was applied to dentin. Air dried gently for 5 s. Adhesive was applied to dentin. Light cured for 10 s.|
|Two-step ER adhesive|
|SB||Dentin surface was phosphoric acid etched for 15 s. Etched surface was rinsed with water for 10 s and blotted dry. Apply 2−3 consecutive coats of adhesive for 15 s with gentle agitation. Air dried gently for 5 s. Light cured for 10 s.|
|SU in ER mode||Dentin surface was phosphoric acid etched for 15 s. Etched surface was rinsed with water for 10 s. Adhesive was applied to air-dried dentin surface with rubbing motion for 20 s and then medium air pressure applied to surface for 5 s. Light irradiated for 10 s.|