Effects of solvent evaporation time on immediate adhesive properties of universal adhesives to dentin.

DENTAL-2410; No. of Pages 10

ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx–xxx

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Effects of solvent evaporation time on immediate adhesive properties of universal adhesives to dentin ˜ c, Issis V. Luque-Martinez a , Jorge Perdigão b,∗ , Miguel A. Munoz Ana Sezinando d , Alessandra Reis a , Alessandro D. Loguercio a a

Department of Restorative Dentistry, State University of Ponta Grossa, Paraná, Brazil Department of Restorative Sciences, University of Minnesota, Minneapolis, MN, USA c School of Dentistry, Universidad de Valparaíso, Valparaíso, Chile d Department of Stomatology, University Rey Juan Carlos, Madrid, Spain b

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. To evaluate the microtensile bond strengths (␮TBS) and nanoleakage (NL) of three

Received 5 February 2014

universal or multi-mode adhesives, applied with increasing solvent evaporation times.

Received in revised form

Methods. One-hundred and forty caries-free extracted third molars were divided into 20

19 April 2014

groups for bond strength testing, according to three factors: (1) Adhesive – All-Bond Univer-

Accepted 7 July 2014

sal (ABU, Bisco, Inc.), Prime&Bond Elect (PBE, Dentsply), and Scotchbond Universal Adhesive

Available online xxx

(SBU, 3 M ESPE); (2) Bonding strategy – self-etch (SE) or etch-and-rinse (ER); and (3) Adhe-

Keywords:

because the respective manufacturer recommends a solvent evaporation time of 10 s. After

Microtensile bond strength

restorations were constructed, specimens were stored in water (37 ◦ C/24 h). Resin–dentin

sive solvent evaporation time – 5 s, 15 s, and 25 s. Two extra groups were prepared with ABU

Nanoleakage

beams (0.8 mm2 ) were tested at 0.5 mm/min (␮TBS). For NL, forty extracted molars were

Etch-and-rinse

randomly assigned to each of the 20 groups. Dentin disks were restored, immersed in ammo-

Self-etch

niacal silver nitrate, sectioned and processed for evaluation under a FESEM in backscattered

Universal simplified adhesive

mode. Data from ␮TBS were analyzed using two-way ANOVA (adhesive vs. drying time) for

systems

each strategy, and Tukey’s test (˛ = 0.05). NL data were computed with non-parametric tests

Solvent evaporation

(Kruskal–Wallis and Mann–Whitney tests, ˛ = 0.05).

Dentin

Results. Increasing solvent evaporation time from 5 s to 25 s resulted in statistically higher mean ␮TBS for all adhesives when used in ER mode. Regarding NL, ER resulted in greater NL than SE for each of the evaporation times regardless of the adhesive used. A solvent evaporation time of 25 s resulted in the lowest NL for SBU-ER. Significance. Residual water and/or solvent may compromise the performance of universal adhesives, which may be improved with extended evaporation times. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗

Corresponding author at: 515 SE Delaware St, 8-450 Moos Tower, Minneapolis, MN 55455, USA. E-mail address: [email protected] (J. Perdigão). http://dx.doi.org/10.1016/j.dental.2014.07.002 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Luque-Martinez IV, et al. Effects of solvent evaporation time on immediate adhesive properties of universal adhesives to dentin. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.07.002


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1.

Introduction

New multi-mode or universal one-bottle adhesives have been recently introduced for use as either self-etch or as etch-andrinse adhesives [1]. The respective manufacturers suggest that multi-mode adhesives may also be used with separate etching of enamel margins, or selective enamel etching. Due to the intrinsic wetness of the dentin substrate, hydrophilic monomers have been used in the composition of dentin bonding systems for years [2,3]. Hydrophilic resins result in high dentin bond strengths. However, several studies have demonstrated that degradation of the resin–dentin interface occurs over time [4,5]. It has been questioned whether current monomers have become too hydrophilic [6]. In fact, all self-etch adhesives, including the newest universal adhesives, contain water, which is required for ionization of the hydrophilic acidic monomers [7]. Their hydrophilicity makes one-step self-etch adhesives behave as semi-permeable membranes, allowing fluid transudation across the resin–dentin interface [8]. Some etch-and-rinse adhesives also contain water and hydrophilic monomers, which makes them behave as permeable membranes as well, allowing exudation of dentin fluid [9]. The presence of residual water may accelerate the degradation of the bonding interface [3,10]. Commercial dental adhesives include organic solvents, such as ethanol or acetone, to facilitate monomer infiltration into the humid dentin substrate. Although water and organic solvents are essential components of one-step adhesives, solvents should be completely removed during clinical application of the adhesive. If solvents are not evaporated, residual water and organic solvents may inhibit the polymerization of monomers in current dentin adhesives [11]. Solvent evaporation is usually accomplished by agitating the adhesive on dentin/enamel surfaces followed by solvent evaporation with compressed air [12]. An extended solvent evaporation time has been used to successfully improve the degree of conversion and mechanical properties of 1-step selfetch and 2-step etch-and-rinse adhesives [13,14]. There is no consensus, however, regarding the proper solvent evaporation time for 1-step self-etch [13,15,16], or for 2-step etch-and-rinse adhesives [17–19]. Taking into account that new universal adhesives contain both water and, at least, one organic solvent (ethanol or acetone), the aim of this study was to compare the immediate microtensile bond strengths (␮TBS) and nanoleakage (NL) of three universal or multi-mode adhesives, applied with increasing solvent evaporation times. The null hypotheses tested were that extended solvent evaporation time would not improve: (1) the immediate bond strengths of universal adhesives and; (2) the sealing ability of resin–dentin interfaces formed with universal adhesives.

2.

Material and methods

2.1.

Tooth selection and preparation

One hundred and forty extracted, caries-free human third molars were used. The teeth were collected after obtaining

the patient’s informed consent under a protocol approved by the local Ethics Committee Review Board. The teeth were disinfected in 0.5% chloramine, stored in distilled water and used within six months after extraction. A flat occlusal dentin surface was exposed in all teeth after wet grinding the occlusal enamel with # 180 grit SiC paper. The exposed dentin surfaces were further polished with wet # 600-grit silicon-carbide paper for 60 s to standardize the smear layer.

2.2. Experimental design, restorative procedure and specimen preparation Teeth were randomly assigned into 20 groups (n = 7) according to the adhesive strategy and different solvent evaporation times of three universal adhesive systems: All-Bond Universal (ABU – Bisco Inc., Schaumburg, IL, USA); Prime&Bond Elect (PBE – Dentsply Caulk, Milford, DE, USA); and Scotchbond Universal Adhesive (SBU – 3 M ESPE, St. Paul, MN, USA). Each adhesive was applied (1) as etch-and-rinse (ER) adhesive or as self-etch (SE) adhesive; and (2) with three adhesive solvent evaporation times (5 s, 15 s, and 25 s). Two extra groups were tested to include the recommended manufacturer’s solvent evaporation time of 10 s for ABU in both adhesive strategies. All details regarding the adhesive composition are displayed in Table 1. Solvent evaporation was accomplished with an oil-free airwater syringe. The air pressure was adjusted to 1 bar using a pressure regulator, and the air nozzle was held at 45◦ to the dentin surface at a distance of 1.5 cm. The adhesive systems were applied as per the respective manufacturer’s instructions, except for the different experimental solvent evaporation times. Please refer to Table 1 for more details. After the bonding procedures, a nanofilled composite restoration (Filtek Z350, 3M ESPE, St. Paul, MN, USA) was built in two increments of 2 mm. Each increment was light polymerized for 40 s using a LED light-curing unit set at 1200 mW/cm2 (Radii-cal, SDI Limited, Bayswater, Victoria, Australia).

2.3.

Microtensile bond strength (TBS)

After storage in distilled water for 24 h at 37 ◦ C, one-hundred restored teeth (n = 5 for each experimental group) were sectioned longitudinally in a mesio-distal and buccal-lingual directions across the bonded interface with a low-speed diamond saw (Isomet, Buehler Ltd, Lake Bluff, IL, USA) with water irrigation to obtain resin–dentin beams with a cross sectional area of approximately 0.8 mm2 measured with a digital caliper (Digimatic Caliper, Mitutoyo, Tokyo, Japan). Resin–dentin bonded beams were attached to a Geraldeli jig [20] (Odeme Biotechnology, Joaçaba, SC, Brazil) with cyanoacrylate adhesive and tested under tension (Model 5565, Instron, Norwood, MA, USA) at 0.5 mm/min until failure. The ␮TBS values (MPa) were calculated by dividing the load at failure by the cross-sectional bonding area. The failure mode was classified as cohesive ([C] failure exclusively within dentin or resin composite), adhesive ([A] failure at the resin/dentin interface), or mixed ([M] failure at the resin/dentin interface that included cohesive failure of the neighboring substrates). The failure mode analysis was



Materials

Composition

Application mode Self-etch Manufacturer recommendations

1. Etchant: 35% Phosphoric acid, benzalkonium chloride (SELECT HV-Etch) 2. Adhesive: MDP, Bis-GMA, HEMA, ethanol, water, initiators

Prime & Bond Elect – PBE (1102221)

1. Etchant: 34% Tooth Conditioner Gel (34% phosphoric acid) 2. Adhesive: Mono-, di- and trimethacrylate resins; PENTA Diketone; Organic phosphine oxide; Stabilizers; Cetylamine hydrofluoride; Acetone; Water

Scotchbond Universal Adhesive – SBU (448716)

1. Etchant: 32% phosphoric acid, water, synthetic amorphous silica, polyethylene glycol, aluminum oxide (Scotchbond Universal Etchant) 2. Adhesive: MDP Phosphate monomer, dimethacrylate resins, HEMA, methacrylate-modified polyalkenoic acid copolymer, filler, ethanol, water, initiators, and silane

1. Apply two separate coats of adhesive, scrubbing the preparation with a microbrush for 10–15 s per coat. Do not light cure between coats. 2. Evaporate excess solvent by thoroughly air-drying with an air syringe for at least 10 s, there should be no visible movement of the material. The surface should have a uniform glossy appearance 3. Light cure for 10 s 1. Apply generous amount of adhesive to thoroughly wet all tooth surfaces 2. Agitate for 20 s 3. Gently dry with clean air for at least 5 s. Surface should have a uniform, glossy appearance 4. Light-cure for 10 s 1. Apply the adhesive to the entire preparation with a microbrush and rub it in for 20 s 2. Direct a gentle stream of air over the liquid for about 5 s until it no longer moves and the solvent is evaporated completely 3. Light-cure for 10 s

Experimental groups

Manufacturer recommendations

Experimental groups

1. Apply adhesive as recommended by the manufacturer 2. The only difference is the solvent evaporation time: 5, 15 and 25 sa

1. Apply etchant for 15 s 2. Rinse thoroughly for 10 s 3. Remove excess water with air syringe for 5 s. 4. Apply adhesive as in the self-etch strategy

1. Apply etchant for 15 s 2. Apply adhesive as recommended by the manufacturer The only difference is the solvent evaporation time: 5, 15 and 25 sa

1. Apply adhesive as recommended by the manufacturer 2. The only difference is the solvent evaporation time: 15 and 25 s

1. Apply etchant for 15 s. 2. Rinse for 15 s. 3. Dry with air syringe for 5 s. 4. Apply adhesive as in the self-etch strategy

1. Apply etchant for 15 s 2. Apply adhesive as recommended by the manufacturer 3. The only difference is the solvent evaporation time: 15 and 25 s

1. Apply adhesive as recommended by the manufacturer 2. The only difference is the solvent evaporation time: 15 and 25 s

1. Apply etchant for 15 s 2. Rinse for 10 s 3. Air dry 5 s 4. Apply adhesive as in the self-etch strategy

1. Apply etchant for 15 s 2. Apply adhesive as recommended by the manufacturer 3. The only difference is the solvent evaporation time: 15 and 25 s

d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx–xxx

All-Bond Universal – ABU (1200002722)

Etch-and-rinse

HEMA: 2-hydroxyethyl methacrylate; MDP: methacryloyloxydecyl dihydrogen phosphate; bis-GMA: bisphenol glycidyl methacrylate; TEGDMA: Triethylene glycol dimethacrylate; PENTA: dipentaerythritol penta acrylate monophosphate. a Bisco recommends 10 s of solvent evaporation time. However, to standardize different groups, we also tested this adhesive for 5, 15, and 25 s of solvent evaporation time.

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Table 1 – Adhesive materials (batch number), composition and application mode of the adhesive systems used.

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Table 2 – Number and percentage of specimens (%) according to fracture pattern mode and the premature debonded specimens from experimental group. Adhesive system

Application mode

Air-dry time

Fracture pattern A

Etch-and- rinse All-Bond Universal (ABU) Self-etch

Etch-and-rinse Prime&Bond Elect (PBE) Self-etch

Etch-and- rinse Scotchbond Universal Adhesive (SBU) Self-etch

C

A/M

PF

5 10 15 25 5 10 15 25

50 (83.3) 42 (72.4) 46 (74.1) 38 (70.4) 61 (86.1) 54 (88.5) 52 (81.3) 62 (94.0)

6 (10.1) 4 (6.9) 6 (9.7) 8 (14.8) 0 (0.0) 0 (0.0) 2 (3.1) 0 (0.0)

2 (3.3) 4 (6.9) 4 (6.5) 6 (11.1) 6 (8.3) 2 (3.3) 8 (12.5) 2 (3.0)

2 (3.3) 8 (13.8) 6 (9.7) 2 (3.7) 2 (5.6) 5 (8.2) 2 (3.1) 2 (3.0)

5 15 25 5 15 25

48 (77.7) 62 (89.0) 58 (82.9) 44 (75.9) 52 (74.3) 34 (68.0)

2 (3.0) 0 (0.0) 2 (2.9) 4 (6.9) 4 (5.7) 0 (0.0)

4 (6.1) 4 (5.5) 4 (5.7) 4 (6.9) 8 (11.4) 6 (12.0)

12 (12.3) 4 (5.5) 6 (8.6) 6 (10.3) 6 (8.6) 10 (20.0)

5 15 25 5 15 25

54 (87.0) 64 (80.0) 60 (88.2) 50 (89.3) 46 (67.6) 64 (82.0)

0 (0.0) 4 (5.0) 0 (0.0) 2 (3.6) 10 (14.7) 2 (2.6)

4 (6.5) 8 (10.0) 8 (11.8) 4 (7.1) 8 (11.8) 2 (2.6)

4 (6.5) 2 (5.0) 0 (0.0) 0 (0.0) 4 (5.9) 10 (12.8)

A: adhesive fracture mode; C: cohesive fracture mode; A/M: adhesive/mixed fracture mode; PF: premature failures.

performed under a stereomicroscope at 100× magnification (Olympus SZ40, Olympus Corporation, Tokyo, Japan). Specimens with premature failures (PF) were included in the tooth mean. We have attributed them the average value between zero and the lowest bond strength value obtained in all experiment. In this specific study, the value of 4.7 MPa was attributed when PF were recorded.

2.4.

Nanoleakage (NL) evaluation

Forty restored teeth (n = 2 for each experimental group) were used for nanoleakage evaluation assigned to each experimental group described in Table 1. Two 1 mm-thick dentin disks were sectioned from each tooth parallel to the occlusal surface, using a slow-speed diamond saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) under water-cooling. After ensuring that no enamel remains were present, a standard smear layer was created in the bonding surfaces of each disk (pulpal surface of the occlusal side disk and the occlusal surface of the pulpal side disk) with 600-grit SiC paper under water for 60 s to standardize the smear layer. The adhesives were then applied to each of the four bonding surfaces as described above. A 0.3 mm-thick layer of flowable composite (Filtek Bulk Fill, 3M ESPE, St. Paul, MN, USA) was then applied and cured for 40 s. The restored dentin disks were sectioned buccolingually in two halves using a slow-speed diamond saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) under watercooling to exposed the resin–dentin interface. All surfaces were coated with two layers of nail polish except for the bonding interface. After the nail polish dried, the specimens were immersed in an aqueous solution of 50 wt% ammoniacal silver nitrate (pH 9.5) for 24 h at 37 ◦ C, followed by 8 h in a photo-developing

solution in order to permit reduction of the diammine silver ions to metallic silver grains [21]. The specimens were washed in water for 1 min and the nail vanish removed with a periodontal scaler. Fixation was carried out with 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer at pH 7.4 for 12 h at 4 ◦ C. After fixation, the specimens were rinsed with 20 mL of 0.2 M sodium cacodylate buffer at pH 7.4 for 1 h with three changes, followed by distilled water for 1 min. The specimens were dehydrated in ascending grades of ethanol: 25% for 20 min, 50% for 20 min, 75% for 20 min, 95% for 30 min, and 100% for 30 min [22]. Specimens were polished with waterproof silicon carbide papers of decreasing abrasiveness (600-grit, 800-grit and, 1200grit), followed by soft tissue disks with increasingly fine suspensions of 1 ␮m and 0.3 ␮m for 1 min each. The specimens were ultra-sonicated in 95% ethanol for 10 min, thoroughly dried, and demineralized in 0.5% silica-free phosphoric acid for 1 min to remove polishing debris. Sections were mounted on Al stubs with carbon adhesive tape and graphite paint (Ted Pella, Inc., Redding, CA, USA). Then, specimens were evaporated with carbon under a DV502A Vacuum Evaporator (Denton Vacuum, Moorestown, NJ, USA) for 1 min and observed in backscattered mode under a Hitachi S-4700 FESEM (Hitachi High Technologies America, Inc., Dallas, TX, USA) with an Autrata-modified YAG detector at an accelerating voltage of 8.0 kV and working distance of 13.0–13.2 mm. A series of 9–10 micrographs were obtained from each of the four interfaces to include the entire length of the interface in secondary and in backscattered mode simultaneously. Micrographs were assembled through Adobe Photoshop CS3 Extended Version 10.0.1 (Adobe Systems Incorporated, San Jose, CA, USA) to reproduce the entire interface. The interface total length (in mm) was measured using ImageJ 1.44o



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22.2 (3.7) d 32.6 (3.4) b 38.9 (2.0) a 22.0 (5.1) d – –

c

25 s 15 s 10 s 5s

18.8 (1.5) d 18.9 (2.6) d 32.3 (4.8) b 41.0 (4.1) A 35.6 (5.2) B 40.9 (2.3) A 40.5 (2.8) A 29.9 (4.4) C 38.7 (2.6) A,B 40.8 (5.0) A – – 34.4 (4.3) B,C 16.8 (2.4) D 36.2 (3.3) B

Similar capital (etch-and-rinse) and lower (self-etch) is not statistically significant (p < 0.05). The manufacturer recommends 10 s of solvent evaporation time. The manufacturer recommends 5 s of solvent evaporation time.

Overall, the ER strategy resulted in greater NL than the SE strategy for each solvent evaporation time (Table 4). Examples of each nanoleakage pattern are displayed seen in Fig. 1 (ER) and in Fig. 2 (SE).

a

Nanoleakage (NL)

b

3.2.

ABU PBEc SBUc

For the ER strategy, increasing solvent evaporation time from 5 s to 25 s resulted in statistically higher mean ␮TBS for all adhesive systems tested (Table 3; p < 0.001). For SBU, only the solvent evaporation time of 25 s resulted in statistically significant higher mean ␮TBS compared to 5 s of solvent evaporation time (p < 0.01). For ABU, the use of 10 s (recommended by manufacturer), 15 s, or 25 s of solvent evaporation times resulted in statistically similar mean ␮TBS, which were statistically higher than those of 5 s (p < 0.01). For PBE, mean ␮TBS progressively increased with extended solvent drying time (5 s < 15 s < 25 s, p < 0.001). PBE was the only adhesive for which increasing the solvent evaporation time from 15 s to 25 s resulted in significant improvement in the mean ␮TBS. However, the mean ␮TBS of PBE at 25 s was still statistically lower than those of either SBU or ABU (p < 0.01) at 25 s. For the self-etch strategy, a 25 s evaporation time for ABU and PBE resulted in statistically higher mean ␮TBS than those obtained with the recommended solvent evaporation time of 10 s for ABU, and 5 s for PBE (p < 0.01 and p < 0.001, respectively). For SBU an evaporation time of 15 s resulted in the highest mean ␮TBS, which was statistically higher than that obtained with the respective manufacturer’s recommended 5 s evaporation time (p < 0.001).

25 s

Microtensile bond strength (TBS)

15 s

3.1.

10 s

The fracture pattern is shown in Table 2. Most specimens resulted in adhesive failures (Table 2).

5s

Results

Etch-and-rinse

3.

Adhesive

Each tooth was considered as a statistical unit. As the p-value obtained with the Levene’s test was >0.05, data from ␮TBS were analyzed separately using two-way ANOVA (adhesive vs. solvent evaporation time) for the etch-and-rinse and selfetch strategies, followed by Tukey’s post hoc test (˛ = 0.05). Nanoleakage data were analyzed with non-parametric tests (Kruskal–Wallis and Mann–Whitney pair-wise comparisons, ˛ = 0.05) for the etch-and-rinse and self-etch strategies considering each tooth-interface as an independent statistical unit.

Self-etch

Statistical analysis

Table 3 – Microtensile bond strength (␮TBS) values (means ± standard deviations) of different experimental groupsa .

2.5.

b

(NIH, Bethesda, MD, USA). Nanoleakage areas were identified as the areas of the interface that displayed silver ions. All the measurements were calibrated to the same pixel/mm ratio. For each interface, the observed length of the interface with signs of silver infiltration was added up and the percentage calculated as: (length of interface with silver ions/total length of the interface) × 100. Representative areas of each interface were imaged at high magnification (×5000) to analyze the nanoleakage patterns.

29.2 (4.5) b,c 35.6 (4.5) a,b 34.5 (3.6) a,b




6


Fig. 1 – Micrographs of resin–dentin interfaces for the ER strategy, observed under a FESEM with a backscattered detector. Original magnification – ×5000. A = Adhesive layer; C = Composite resin; H = Hybrid layer; T = Resin tag. For ABU-ER, solvent evaporation time of 5 s (a), 10 s (b), 15 s (c) and 25 s (d). The circle marks a reticular pattern of silver ions infiltration. Filled arrows denote silver spots dispersed in the adhesive layer. For PBU-ER, solvent evaporation time of 5 s (e), 15 s (f) and 25 s (g). The circle marks a reticular pattern of silver ions infiltration. The pointer marks filled resin tags. The adhesive layer was not readily visible. For SBU-ER, solvent evaporation time of 5 s (h) and 15 s (i). The unfilled arrow denotes silver accumulation at the base of the hybrid layer. The small line arrows point to a fine dendritic pattern of silver ions within the hybrid layer and glomerular-like precipitates at the transition between the hybrid layer and the adhesive layer. The interface between the adhesive layer and the hybrid layer also displayed a shag-carpet structure. For SBU-ER, solvent evaporation time of 25 s (j). This particular interface is silver free. The shag-carpet pattern was also observed in some areas of the interface for this group (not observed in this micrograph).

For ABU applied as an ER or as a SE adhesive, NL did not vary significantly for the four evaporation times (p = 0.62 for ER; p = 0.53 for SE). The same was observed for PBE (p = 0.47 for SE; p = 0.23 for SE). Although the percentage of NL obtained with ABU at 25 s with ER (56.8%) and SE strategy (37.6%) were lower than that measured at 5 s (ER = 85.7%; SE = 77.1%), the difference did not reach the significance level due to the data variance. When SBU was applied as an ER adhesive, 15 s of solvent evaporation time resulted in statistically lower NL than that of 5 s (p = 0.04). The solvent evaporation time of 25 s resulted in statistically lower NL than that of 5 s (p = 0.02) and that of 15 s (p = 0.04). For the SE strategy, solvent evaporation time did not result in any statistically significant difference among solvent evaporation times (p = 0.30), although the percentage of NL obtained at 25 s with the SE strategy (24.1%) was lower than a half that measured at 5 s (51.8%). When the NL pattern was observed under the FESEM at high magnification (×5000) with a backscattered detector, the interfaces formed with ABU-ER and PBE-ER displayed silver accumulation in the hybrid layer with a reticular pattern (Fig. 1a to g), regardless of the solvent evaporation

time. For ABU-ER, silver spots were dispersed over the adhesive layer regardless of the solvent evaporation time (Fig. 1a to d). For PBE-ER, penetration of composite filler particles into the dentinal tubules occurred frequently, as observed in Fig. 1f and g. The adhesive layer was not observed in PBUER specimens observed with the magnification used in this project, even when observed in secondary mode. For SBU-ER, a solvent evaporation time of 5 s (Fig. 1h) resulted in silver accumulation mainly at the base of the hybrid layer. A fine dendritic pattern of silver ions within the hybrid layer and glomerular-like precipitates at the transition between the hybrid layer and the adhesive layer were also observed for SBU-ER regardless of the solvent evaporation time (Fig. 1h and i, not shown in Fig. 1j). When the solvent was evaporated for 5 s and 15 s (Fig. 1h and i), the interface formed by SBU-ER displayed a shag-carpet structure at the top of the hybrid layer, with nano-sized fibrilar extensions intruding into the adhesive layer. This pattern was also observed in some areas of the interface when the solvent was evaporated for 25 s. Extensive areas of the interface formed with SBU-ER were silver-free when the solvent was evaporated for 25 s (Fig. 1j).




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Fig. 2 – Micrographs of resin–dentin interfaces for the SE strategy, observed under a FESEM with a backscattered detector. Original magnification – ×5000. A = Adhesive layer; C = Composite resin; H = Hybrid layer; T = Resin tag. For ABU-SE, solvent evaporation time of 5 s (a), 10 s (b), 15 s (c) and 25 s (d). Silver ions were observed predominantly at the base of the hybrid layer (filled arrow). Only in (b), areas of debonding [G] displayed numerous dense water-trees on the top of the hybrid layer (circles). For PBU-SE, solvent evaporation time of 5 s (e), 15 s (f) and 25 s (g). The hybrid layer and resin tags were infiltrated with silver particles. Only in (f), areas of debonding [G] displayed numerous dense water-trees on the top of the hybrid layer (circles). For SBU-SE, solvent evaporation time of 5 s (h), 10 s (i) and 15 s (i). Silver ions were observed aligned in a shag-carpet pattern (asterisk) at the transition between the hybrid layer and the adhesive layer, similarly to the pattern observed for SBU-ER interfaces.

For ABU-SE silver ions were observed predominantly at the base of the hybrid layer and periphery of resin tags (Fig. 2a to d). For PBE-SE, the hybrid layer was profusely infiltrated with silver particles independent of the solvent evaporation time (Fig. 2e to g), with silver ions lining the resin tags. Areas of debonding, as in Fig. 2f, displayed numerous dense watertrees on the top of the hybrid layer. The resin–dentin interfaces formed with SBU-SE (Fig. 2h to j) depicted silver ions aligned in a shag-carpet pattern at the transition between the hybrid layer and the adhesive layer similarly to the pattern observed for SBU-ER interfaces. Silver was observed lining profusely the periphery of resin tags when the solvent was evaporated for 5 s. The adhesive layer displayed a few dispersed silver spots regardless of the evaporation time.

4.

Discussion

Universal adhesives have a composition similar to that of 1-step self-etch adhesives. Although water is added to trigger the ionization of the respective phosphate monomer in self-etch mode, all adhesives used in this study contain an organic solvent. ABU and SBU contain ethanol, while PBE contains acetone. Organic solvents act not only as carriers of the monomers into the collagen interfibrillar spaces, but also as

diluents to lower the resin viscosity. Additionally, these solvents enhance infiltration of resins into the microporosities created onto by the etchant or by the conditioner [23]. Solvent volatilization can facilitate the polymerization reaction because the distance among monomers is reduced, increasing the degree of conversion [24]. Ideally, solvents should be completely volatilized from the applied mixture prior to polymerization. The choice of solvents impacts the polymerization in several manners. Solvent type affects the diffusion of the polymer chains, the viscosity, the intermolecular termination rate, the primary chain length, the gel point conversion, among others [25]. The evaporation of solvents with compressed air is a technique-sensitive step difficult to accomplish using current clinical techniques. Some reports have suggested that solvents may take up to 20 min to almost completely evaporate [11,26]. Acetone has a higher vapor pressure than ethanol and water, which may reduce the time required for evaporation compared to ethanol [27]. This was not the case in our study with the acetone-based adhesive PBE, which required a longer evaporation time than that recommended by the respective manufacturer. It has been shown that a high acetone content in bonding solutions may be difficult to evaporate, leaving residual solvent in the adhesive resin, which results in pores in the cured adhesive layer [28].


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29.0 (26.6) 37.6 (35.2) 24.1 (23.5) 39.0 (31.1) 32.5 (31.7) 26.5 (22.5) 53.7 (15.4) – –

c

a

b

Only in the SBU in the etch-and-rinse mode was statistically different at p < 0.05 for all comparisons. The manufacturer recommends 10 s of solvent evaporation time. The manufacturer recommends 5 s of solvent evaporation time.

48.6 (38.5) 77.1 (12.5) 51.8 (26.7) 92.3 (9.3) 56.8 (38.4) 33.1 (9.6) 92.2 (4.9) 78.1 (9.2) 56.1 (14.2) 87.1 (11.9) – – 94.6 (6.7) 85.7 (5.6) 75.5 (6.7) ABU PBEc SBUc

15 s 5s

10 s

15 s

25 s

5s

10 s

Self-etch Etch-and-rinse Adhesive

Table 4 – Nanoleakage (%) values (means ± standard deviations) of different experimental groupsa .

b

25 s


As for ethanol solvent, ABU contains 30–60 wt% ethanol [29], whereas SBU contains a lower concentration, 10–15 wt% [30]. The monomer concentration increases dramatically with the evaporation of ethanol, reducing the vapor pressure of the remaining ethanol [31]. This increase in monomer concentration prevents further solvent evaporation, resulting in residual ethanol being trapped inside the adhesive layer [27]. This explains the spotted silver accumulation observed in our study for ABU, which has a relatively high ethanol contents [29]. Changes in solvent concentration and solvent type affect the quality of cross-linking and polymer network [25]. Excess solvent in the cured adhesive may result in a porous structure at the adhesive/dentin interface [28,32]. This situation may be more relevant for the most hydrophilic simplified 1-step adhesives, as the amount of residual solvent/water retention in polymer networks is directly correlated with the hydrophilicity of the adhesive solution [33]. It was shown that ethanol-based adhesives, such as ABU and SBU, present a similar degree of monomer to polymer conversion regardless of the concentration of ethanol, but the mechanical properties of the polymerized adhesive differed significantly [34]. Ultimate tensile strength and modulus of elasticity decrease with an increase in ethanol content [34], which may explain the increased bond strength with extended evaporation times. For the SE strategy, the amount of ethanol in the adhesive bottle was not a crucial factor, quite possibly due to the absence of a fully wet demineralized collagen network. In fact, the mean ␮TBS were significantly higher for SBU-SE for each of the solvent evaporation times, in spite of being the adhesive with the lowest ethanol concentration in this study. All adhesives used in this study contain water. An increase in water concentration results in decreased degree of conversion and lower bond strength for BisGMA/HEMA mixtures [34], therefore preventing optimal polymerization of the adhesive. Increasing the water concentration in SE adhesives from 0 to 60 vol% results in improved acidic monomer ionization, with a gradual increase in the depth of the hybrid layer created by the SE adhesives and a concomitant decrease in bond strengths when the water concentration is above 20% [35], This might be the reason why 5 s evaporation time (in our study) resulted in lower mean ␮TBS compared to 25 s evaporation time for all adhesives in ER mode, and for two out of three adhesives in SE mode. With shorter evaporation times water may have reached a concentration threshold above which the monomers did not polymerize adequately. Additionally, the residual hypertonic layer of uncured acidic monomer may have drawn water from the dentin structure, as reported by Van Landuyt et al. [36]. PBE is an acetone- and water-based HEMA-free universal adhesive system. In our study, the mean ␮TBS of PBE increased with extended solvent evaporation times. Although a variation in solvent evaporation time cannot be directly compared to a variation in the intensity of the air pressure used, a recent study reported that strong air-blowing of a HEMA-free adhesive resulted in higher ␮TBS and less frequent droplets entrapped in the adhesive as compared to a mild air-blowing technique [37]. It is plausible to extrapolate that both strong evaporation pressure and long evaporation times remove more residual water from the interface.




There is scarce information in literature about PBE adhesive. A recent in vitro study [38], using manufacturers’ recommended solvent evaporation time, compared the ␮TBS of PBE with those of ABU and SBU, used as ER or as SE adhesives. When applied in ER mode, all three adhesives resulted in statistically similar mean ␮TBS. When applied in the SE mode, PBE resulted in statistically higher mean ␮TBS than those of ABU [38]. The mean ␮TBS associated with SBU-SE were statistically similar to those of the other two adhesives [38]. These results are not in agreement with those of our study, which may be a result of different testing setups. In fact, in our study, PBE was the adhesive that resulted in the lowest bond strengths when adhesives were applied as per the respective manufacturer’s directions. When PBE was used in ER mode, the FESEM observations (Fig. 1e to g) showed composite resin filler particles penetrating into the dentin tubules. This finding is not common unless the adhesive does not form a polymerized resin layer over etched dentin. It has been reported that increasing the acetone contents results in thinner adhesive layers and lower dentin bond strengths [28]. Additionally, for PBE, the manufacturer only recommends one application of “generous amount of adhesive to thoroughly wet all tooth surfaces” (Table 1), which was shown, in this study, to be insufficient to produce an adhesive layer over the etched dentin. Platt et al. [39] found that Prime & Bond NT (Dentsply Caulk, Milford, DE), an ER adhesive with a composition similar to that of PBE (also contains PENTA monomer, cetylamine hydrofluoride, and acetone), needs twice the number of applications recommended by the respective manufacturer to result in acceptable dentin bond strengths. One application of Prime & Bond NT did not form a uniform thickness of adhesive across the interface under the confocal microscope [39]. This characteristic may be a consequence of acetone evaporating very rapidly, therefore not allowing enough time for the monomers to infiltrate etched dentin. It remains to be seen if a double application of PBE would increase its bonding effectiveness. Nanoleakage is an indirect method to evaluate the sealing potential of the dentin-resin hybrid layer [21,40]. By using ammoniacal silver nitrate as a tracer, it is also possible to assess resin hydrolysis and degradation of collagen fibers [4,10]. Water-trees, which represent reticular silver deposits of residual solvent or water entrapped within the adhesive interface, such as those observed in Fig. 1a to g, 2a to c, f, may correspond to areas of deficient polymerization [41]. The silver agglomeration observed at the bottom of the hybrid layer may correspond to the porous non-infiltrated etched substrate and/or hydrophilic phases within the adhesives [21,42]. This may include areas of the hybrid layer where water remained after evaporating the solvents. The morphological findings associated with SBU included a shag-carpet structure at the top of the hybrid layer forming fibril-like extensions into the adhesive layer. These findings are in line with those of Tay et al. [21] who used Transmission Electron Microscopy to investigate the adhesive layer associated with the 1-step self-etching adhesive One-Up Bond F (Tokuyama Co., Tokyo, Japan). It is likely that this similarity is related to the composition of SBU and that of One-Up Bond F. Both contain HEMA, water, a phosphate monomer, a carboxylate monomer, and a functional methacrylic monomer. It

9

is known that HEMA is able to create a hydrogel within the hybrid layer and adhesive resin in some cases [43], that allow water permeation and affects the durability of bonds, especially when poly-HEMAs of low molecular weight are formed [43]. The monomer 10-MDP contains a polymerizable methylmethacrylate group and a phosphate group responsible for ionic interaction with calcium. Both ABU and SBU contain 10-MDP and HEMA. Questions have been raised about the chemical bonding potential of adhesives that include both monomers in their composition [44]. HEMA seems to inhibit the nanolayering chemical bonding mechanism associated with 10-MDP. Clinical studies may shed light on this potential chemical incompatibility between these two monomers. We reject the first null hypothesis as extended solvent evaporation time improved the overall bond strengths of universal adhesives. We partially reject the second null hypothesis, as sealing ability of resin–dentin interfaces formed with universal adhesives improved with increasing evaporation time for all adhesives, albeit the statistically significance was only observed for SBU-ER.

5.

Conclusions

An extended solvent evaporation time may improve the bonding effectiveness for specific universal adhesives depending on the adhesive strategy used.

references

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Immediate adhesive properties to dentin and enamel of a universal adhesive associated with a hydrophobic resin coat.

solubility and nanoleakage of adhesive systems.

Effects of endodontic tri-antibiotic paste on bond strengths of dentin adhesives to coronal dentin.

Surface reactions of adhesives on dentin.

Effects of solvent volatilization time on the bond strength of etch-and-rinse adhesive to dentin using conventional or deproteinization bonding techniques.

Permeability effects of two dentin adhesive systems.

Effects of Er:YAG laser on bond strength of self-etching adhesives to caries-affected dentin.

The effect of saliva decontamination procedures on dentin bond strength after universal adhesive curing.

Performance of universal adhesives on bonding to leucite-reinforced ceramic.

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Effect of dentin biomodifiers on the immediate and long-term bond strengths of a simplified etch and rinse adhesive to dentin.

BONDING OF EXPERIMENTAL POLYMERIC ACP ADHESIVES TO HUMAN DENTIN.

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Dentin Interface Bonding.

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