Effects of aging on the bond strength of self-etching adhesives and resin luting cements

Journal of Dental Sciences (2013) 8, 61e67

Available online at www.sciencedirect.com

journal homepage: www.e-jds.com

ORIGINAL ARTICLE

Effects of aging on the bond strength of self-etching adhesives and resin luting cementsy Ali Riza Cetin a, Nimet Unlu a*, Mehmet Ata Cebe b a b

Department of Restorative Dentistry, Faculty of Dentistry, Selcuk University, Konya, Turkey Department of Restorative Dentistry, Faculty of Dentistry, Gaziantep University, Gaziantep, Turkey

Received 19 December 2011; Final revision received 20 February 2012 Available online 29 November 2012

KEYWORDS aging; bonding agents; luting cements; microtensile bond strength

Abstract Background/purpose: This study was designed to evaluate the microtensile bond strengths of self-etching adhesives and resin luting cements immediately after bonding and after exposure to aging. Materials and methods: Forty noncarious human molars were divided into five groups and randomly assigned to self-etching systems [AdheSE Bond(ASE), Prompt L-Pop(PLP), and TyranSPEþOnestep Plus(OSP)] or resin luting cement systems [Panavia F(PF) or Duolink(DL)]. All adhesives were applied in accordance with the manufacturers’ instructions. A composite resin build-up was created with composite resins from each respective company (Tetric EvoCeram, Filtek Supreme XT, Aelite Aesthetic, Estenia, and Tescera ATL). Half of the specimens were tested immediately after bonding. The remaining specimens were mechanically loaded in artificial saliva for 60,000 cycles with a wear simulator at 37  C and were then stored in artificial saliva at room temperature for 1 year before undergoing a microtensile test. Data were analyzed using one-way analysis of variance and Tukey’s honestly significant difference test. Results: Differences in immediate bonding values were observed among the adhesives, ASE demonstrating the greatest bond strength. After aging (with loading and after 1 year), a reduced interfacial bond strength was observed for the ASE, OSP, and DL adhesives. Conclusion: PF luting cement and PLP dentin adhesive were less affected by aging than the other dentin adhesive systems studied. Additional in vivo data should be acquired to complement these findings and clarify the clinical efficacies of the tested adhesives. Copyright ª 2012, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.

* Corresponding author. Department of Restorative Dentistry, Faculty of Dentistry, Selcuk University, 42100 Campus, Selc ¸uklu/Konya, Turkey. E-mail address: [email protected] (N. Unlu). y This article was presented in the IADR/AADR/CADR 89th General Session, San Diego, CA, 16e19 March 2011. 1991-7902/$36 Copyright ª 2012, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved. http://dx.doi.org/10.1016/j.jds.2012.02.003

62

Introduction To increase the clinical performance of resinrestorations, various composite resins and adhesive systems have been developed over the years. Polymerization shrink age of composite resins appears to be one of the most important problems. Therefore, both the development of adhesives and the reduction in polymerization shrinkage of composite resins were studied. Different adhesive formulations have recently been proposed to reduce the polymerization shrinkage of composite restorations. Nevertheless, for new adhesive systems, the main focus of researchers is to reduce the clinical steps, avoid mistakes, and improve the bonding stability.1,2 Recently, some composite resins that use a dedicated two-step, self-etching adhesive system claimed shrinkage of nearly 1% by volume, which is lower than the 2e5% exhibited by some bisphenyl-glycidyl-methacrylate composites.3 Although much reduced, all current resin composites still shrink. At the beginning of the 1990s, indirect composite restorations were shown to improve clinical conditions with respect to proximal contact, occlusal anatomy, and marginal adaptation.4 The esthetic results and longevity of these indirect composite resin restorations depend on each step of the clinical and laboratory procedures. Cementation is the most critical step, and involves application of an adhesive system and a resin luting agent.5 The clinical protocol for placing laboratory-processed composite resins includes the use of dual-polymerizing resin cements.6,7 Recently, self-etching adhesives were also introduced to further simplify the bonding procedure. These adhesives combine the etching and priming processes into a single step.8 The bonding mechanism of self-etching adhesives is based upon the simultaneous etching and priming of smearcovered dentin, which uses an acidic primer followed by the application of an adhesive resin.9 Self-etching primers eliminate the separate acid-etching and rinsing steps, which simplifies the bonding technique and reduces its technical sensitivity.10 However, only a few studies exist in the literature regarding the bond strengths of selfconditioning systems and resin cements.11,12 In addition to the materials (adhesives and composites), aging is an important factor that may influence the longterm performance of resin restorations. Several in vitro studies without aging reported high bond strengths for these self-etching and dual-cure resin materials.13,14 Because factors such as normal daily functioning, thermal stresses, malocclusion, and habitual bruxism stresses throughout the tooth and restorative system may affect and destroy adhesive bonds, optimal dentin bonding might not always be obtained in clinical practice.15 The clinical longevity of a hybrid layer seems to involve both physical and chemical factors. Physical factors, such as occlusal chewing forces and repetitive expansion and contraction stresses caused by temperature changes within the oral cavity have been proposed as affecting the interfacestability.16,17 Acidic chemical agents in dentinal fluid, saliva, food, beverages, and bacterial products further influence the toothebiomaterial interface, resulting in various patterns of degradation of unprotected

A.R. Cetin et al collagen fibrils, elution of resin monomers (probably due to suboptimal polymerization), and degradation of resincomponents.18e20 Areas within the hybrid or adhesive layers that are not completely polymerized may allow water to diffuse, which may impair the bond strength and compromise the longevity of adhesive restorations.21,22 A high-quality hybrid layer requires optimal infiltration of the adhesive monomer into the enamel and dentin substrates, and optimal polymerization within the substrate.23 Application of cyclic mechanical stress can be used to simulate physiological conditions and allows researchers to examine the effects on a restoration and storage in saliva. This simulation can provide valuable information on the durability of dentin bonding, especially for newly introduced agents. The purpose of this in vitro study was to evaluate the effects of mechanical load cycling and1 year of storage in artificial saliva on the microtensile bond strength (mTBS) values of two dual-polymerizing luting cements and three self-etching dentin adhesives. As a null hypothesis, it was proposed that the bond strengths of the adhesive systems after aging would be lower than the bond strengths of the adhesive systems before aging.

Materials and methods This study used 40 freshly extracted noncarious human third molars. Following a positive review by the Dental Faculty Ethics Committee of Selcuk University (no:2006/0204;Konya, Turkey), the extracted teeth were thoroughly cleaned to remove hard and soft deposits and then stored at 4 C in a 0.5% chloramine solution prior to use. The occlusal dentin surfaces of the teeth were exposed by removing the occlusal enamel and superficial dentin with a slow-speed diamond saw (Isomet; Buehler, Lake Bluff, IL, USA) under water-cooling. The teeth were randomly assigned to five experimental groups: three self-etching systems [AdheSE Bond (ASE), Prompt L-Pop (PLP), and Tyran SPEþOnestep Plus (OSP)] and two resin luting cement systems [Panavia F (PF) and Duolink (DL)]. The compositions of the adhesive systems and their uses are summarized in Table 1. A smear layer was applied to the dentin surface by polishing the occlusal surface with 600-grit silicon carbide sandpaper. The bonded interface was prepared according to each adhesive group and applied in accordance with the manufacturers’ instructions (Table 1). After adhesive application, the dentin surfaces were restored with composite resins obtained from the respective producer of each adhesive (Tetric Evo Ceram, Filtek Supreme XT, Aelite Aesthetic, Estenia, and Tescera ATL) (Table 2). However, the direct composites have different volumetric shrinkage values of 3.18%for Aelite Aesthetic, 2.09% for Filtek Supreme XT, and 1.49% for Tetric Evo Ceram,24 so to compensate for this difference, we used an incremental placement technique. Direct composite resins were built up incrementally on the bonded surface to a height of 4 mm. Each 2-mm increment of resin composite was then cured for 40 seconds with a light source (Hilux Expert; Benlioglu Dental, Ankara, Turkey) at a 0.5-mm standard curing distance and a light intensity of 660 mW/cm2, as constantly

Effects of aging on bonding of adhesives and cements Table 1

63

Chemical compositions, dentin pretreatment, and bonding procedures of the dentin adhesives tested.

Adhesive system AdheSE Bond Lot:17659

Primer Bond

Prompt L-Pop Lot:7187444

BlisterA

BlisterB TyranSPE Lot:0600004679

Primer

Onestep Plus Panavia F Lot:00368A A and B pastes Lot:00366A

Bond ED primer

Duo-Link Lot:0600009836 A and B Paste Lot:0459

One step Bond

Principle ingredients

Steps of application

Dimethacrylate, phosphoric acid acrylate, water, stabilizers Dimethacrylate, HEMA, silica, initiators and stabilizers Liquid 1 (red blister): methacrylated phosphoric esters; Bis-GMA; Initiator: camphorquinone; stabilizers Liquid 2 (yellow blister): water; HEMA; Polyalkenoic acid; stabilizers A: Ethanol B: 2-Acrylamido2-methyl propanesuflonic acid, bis 2methacryloyloxy-ethylphosphate, ethano Bis-GMA, BPDM, HEMA acetone, glass frit HEMA,5-NMSA, MDP, sodiumbenzenesulfonate, water Paste A: MDP, dimethacrylate, filler, photoinitiator, chemical initiator Paste B: dimethacrylate, photoinitiator BPDM, HEMA, acetone

Apply primer, leave for 30s, air-dry

Paste A: Bis-GMA, TEGDMA, UDMA, glass filler Paste B: Bis-GMA, TEGDMA, glass filler

Apply bond, air-thin, light-cure for 10s Mix blisters A and B, scrub continuously for 15s, gently air-dry, re-apply, light-cure for 10s

Mix primers A and B, apply primer(2 coats), air-dry

Apply bond 15s, air-thin,light-cure for 10s Mix primers A and B,apply primer for 30s, gently air-dry Mix A and B pastes for 20s minimum, apply the mixture of paste, light-cure the margins for 20s Etch 15s, wash acid gel, air-dry for 2e3 s, apply bond, air-thin,light-cure for 10s Mix A and B pastes for 10e15 s, apply the mixture of paste, light-cure each surface of the restoration for 40 s

Abbreviations: 5-NMSA Z N-methacrilol-5-aminosalicylic acid; Bis-GMA Z bisphenyl-glycidyl-methacrylate; BPDM Z biphenyl dimethacrylate; HEMA Z 2-hydroxyethyl methacrylate; MDP Z 10-methacryloxidesil dihydrogen phosphate; MMA Z methyl methacrylate; TEGDMA Z triethylene glycol dimethacrylate; UDMA Z urethane dimethacrylate.

monitored with a radiometer. The inlays were incrementally built up on the dentin surface to a height of 4 mm, and each increment was polymerized for 120e180 seconds in the occlusal direction with a curing unit. The inlays were then postcured in a light oven for 180 seconds, placed in a heat oven for 10 minutes at 114 C, and luted with resin cement. After bonding, each group was further divided into two subgroups (n Z 4) to be tested either immediately or after artificial aging. Samples from one of the subgroups, which was tested immediately, were stored for 24 hours before their mTBSs were determined, while the remaining specimens were mechanically loaded.

Mechanical load-cycling test The root surfaces of the teeth were covered with a 1-mmthick light polyether impression material (Impregum Penta DuoSoft; 3M ESPE, Seefeld, Germany) with a dispenser gun (3M ESPE), and the excess silicone material was removed

Table 2

with a scalpel blade to provide a flat surface 2 mm below the facial cementoenamel junction of each tooth. The teeth were then embedded in acrylic resin (GC Pattern Resin LS; GC, Alsip, IL,USA). The thin layer of silicone material simulated the periodontal ligament on the root surfaces of the teeth. Occlusal contact loading was simulated in an artificial oral environment sliding wear tester (Chewing Simulator; University of Selcuk, Research Laboratory Center, Konya, Turkey).25 Specimens were stored in artificial saliva in a simulator chamber at 37 C in occlusal contact similar to physiological conditions (Fig. 1). Four specimens of each material were tested in a pin-on-block design with a 0.3e0.8 mm radius with eccentric sliding of a spherical natural enamel antagonist (palatinal cusps of human maxillar molars, which were extracted and embedded in acrylic resin) under permanent contact with specimens at a vertical loading of 50 N for 60,000 cycles at a frequency of 1.2 Hz.26

Color, manufacturer, and lot numbers of the composite resins used in this study.

Composite materials for adhesive systems

Manufacturer

Lot number

Color

Filtek Supreme XT for Prompt L-Pop Tetric Evo Ceram for AdheSE Bond Aelite Aesthetic for Tyran SPEþOnestep Plus Tescera ATL for Duolink Estenia for Panavia F

3M ESPE (USA) Ivoclar Vivadent (Liechtenstein) Bisco (USA) Bisco (USA) Kuraray (Japan)

6ER J23353 05697 0004691 00239A

A2 A2 A2 A2 A2

Enamel Enamel Enamel Body Enamel

64

A.R. Cetin et al

Figure 1 Photograph of the chewing simulator employed in the study. The insert at the right schematically illustrates the alignment of one specimen in one chamber of the chewing simulator. (A) Antagonist tooth; (B) composite specimen; (C) tooth specimen; (D) medium.

Loaded specimens were then stored in artificial saliva27 (0.4 gNaCl, 0.4 g KCl, 0.795 g CaCl2$2H2O, 0.78 g NaH2PO4$2H2O, 0.005 g Na2S$9H2O, 1 g urea, and 1000 mL distilled water) at room temperature for 1 year before undergoing the microtensile test. The artificial saliva solution was changed monthly, and its pH was monitored weekly. To avoid bacterial growth, we added 0.05% by weight thymol to the artificial saliva.

Microtensile test procedures After the aging period (load cycling and 1 year of storage), specimens were retrieved from the storage medium for mTBS determination. Microtensile testing was undertaken using the nontrimming technique, which was first described by Sano et al.28 Four teeth were used for each bonding system, and each tooth was sectioned with a slow-speed saw (Isomet; Buehler) under water-cooling, into multiple 0.9-mm  0.9-mm beams. Five standard beams for both the aging period and the control were obtained from each tooth. Specimens were then attached to a Bencor Multi-T testing apparatus (modified by Bernard Ciucchi, Danville Engineering, Danville, CA, USA) with cyanoacrylate adhesive (Zapit; DVA, Anaheim, CA, USA) and subjected to tensile forces in a microtensile testing machine (Microtensile Tester; Bisco, Schaumburg, IL, USA) at a crosshead speed of 1 mm/min. The cross-sectional area at the site of failure was measured to the nearest 0.01 mm with digital calipers (model CD-6BS; Mitutoyo, Tokyo, Japan), and this area was used to calculate the mTBS value, which was expressed in MPa: MPa Z F(N)/bond area (mm2).

Failure mode analysis After microtensile testing, the fractured surfaces of all specimens were examined using a stereomicroscope (LGP52; Olympus, Tokyo, Japan) to determine the mode of failure at 50 magnification. Failure was classified as

adhesive (interfacial failure), cohesive in dentin, cohesive in resin (including failures either within the resin composite or adhesive layer), or mixed failure.

Statistical analysis Data were analyzed using one-way analysis of variance (ANOVA) to evaluate the effects of the two experimental factors: the type of adhesive and the aging regimen. Tukey’s honestly significant difference (HSD) test was used to compare between adhesives at a significance level of 0.05. Statistical analyses were performed using SPSS software (version 13.0; SPSS Inc., Chicago, IL, USA).

Results The means and standard deviations of the mTBSs of each group are summarized in Table 3. Statistically significant differences were found among mTBS values of the materials (P < 0.05). In the control group, ASE showed the greatest bond strength, while DL showed the weakest bond strength. In the aging group, PF showed a higher bond strength than OSP. ANOVA revealed a significant influence of both the adhesive system and aging on mTBS values (P < 0.05). The results of Tukey’s HSD test showed that mTBS values of the load-cycled groups of ASE, OSP, and DL were significantly lower than those of the respective control groups of ASE, OSP, and DL (P < 0.05). However, no significant differences were observed between mean mTBS values of the loadcycled and control groups for the PLP and PF adhesives (P > 0.05). The distribution of failure modes among the materials is shown in Table 4.The light microscopic analysis demonstrated that load-cycled groups showed exclusively adhesive-type failures (100%) for both the DL and OSP materials. After aging, adhesive-type failure was mainly observed in ASE, PLP, and PF materials (at 60%, 65%, and 70%, respectively). In the control groups without aging, adhesive-type failure was mainly observed in the DL, OSP, and PLP groups (at 55e85%). However, for the ASE and PF

Effects of aging on bonding of adhesives and cements Table 3

65

Mean microtensile bond strength and standard deviation of adhesive systems (n Z 20).

Adhesive systems AdheSE Bond Prompt L-Pop TyranSPEþOnestep plus Panavia F Duo-Link

Immediately after bonding 26.11 14.16 16.38 19.48 8.07

    

Mechanically loaded and aged in artificial saliva

a

10.78 5.34b,c 7.69c,e 5.10e 3.23f

13.48 12.10 4.94 18.33 5.25

    

4.04b 4.11b 1.18d 4.86e 1.50d

Different letters indicate statistical significance (P < 0.05).

materials, mixed-type failure was more dominant in the control groups (50%) (Table 4).

Discussion In this study, load cycling and water storage negatively affected the bond strengths of some dentin adhesives (ASE, DL, and OSP), while not negatively affecting the bond strengths of others (PLP and PF). The PF luting cement and PLP dentin adhesive were more resistant to the effects of load cycling and water storage. Compared to other adhesive bond-strength tests, the mTBS test has several advantages, including an improved stress distribution during testing, prevention of cohesive failure in the dentin, the ability to measure regional differences in resinedentin bond strength, and the ability to measure higher bond strengths of newly developed materials.28 Therefore, the mTBS test method was used in this study. Reliable and durable bonding between resin materials and dentin is important in the field of adhesive dentistry, and long-term clinical trials have been the gold standard for evaluating the quality of new bonding systems.29 The execution of a long-term clinical trial, however, is difficult due to operator variability, substrate differences, recall failure, and the time and resources involved.30 Thus, the present study, which subjected samples to loading and artificial salivary aging, was designed to simulate clinical situations while trying to avoid some of the usual hurdles of clinical studies. As a hybrid layer is created by a mixture of the dentinorganic matrix, residual hydroxyapatite crystallites,

Table 4

resinmonomers, and solvents, aging may affect each of the individual components differently or may result in synergistic combinations of degradation phenomena within the hybridlayer.29 Except for the PLP bonding resin and PF luting cement, the results of our study showed that loaded and aged specimens had lower mTBS values than unloaded specimens. Lodovici et al31 reported that, in self-etching adhesive dentin bonding systems, the presence of acidic monomers resulted in the formation of a thin hybrid layer on the dentin surface. This layer provides micromechanical retention for restorations. It is thought that this hybrid layer is the weakest link in achieving long-term durable bonding. Thus, according to our results, the weakest hybrid layer of self-etching bonding systems tested may be considered to be one of the reasons for the reduced bond strengths of dentin adhesives. After the aging of resin-bonded specimens in artificial saliva, Tay et al32 described the transition from initial nanoleakage from isolated silver grains to water trees in the adhesiveresin matrices as a series of events starting with water sorption. Water movement was shown to begin as a diffusion-type mechanism and became more rapid as transport pathways formed relatively larger water-filled channels.32 Similar water movements within the adhesive layer can be driven by osmotic pressure gradients due to high concentrations of dissolved inorganic ions and hydrophilic resin monomers, which were shown to result in the formation of water blisters over the adhesive layer.33 According to results of the present study, when mechanical load cycling was applied, a concentration of main stresses in the hybrid layer interface may have led to plastic deformation of the adhesive interface. In addition,

Failure modes of specimens after the microtensile bond strength test.

Groups

n

Adhesive

Mixed

Cohesive in composite

Cohesive in dentin

AdheSE Bonddcontrol AdheSE Bonddwith aging TyranSPEþOnestep Plusdcontrol TyranSPEþOnestep Plusdwith aging Duo-Linkdcontrol Duo-Linkdwith aging Adper Prompt L-Popdcontrol Adper Prompt L-Popdwith aging Panavia Fdcontrol Panavia Fdwith aging

20 20 20 20 20 20 20 20 20 20

8 12 13 20 17 20 11 13 9 14

9 6 6 0 3 0 7 5 10 5

1 1 1 0 1 0 1 2 0 0

2 1 0 0 0 0 1 0 1 0

Adhesive Z failure occurred entirely within the adhesive; Cohesive in composite Z failure occurred exclusively within the composite; Cohesive in dentin Z failure occurred exclusively within the dentin; Mixed Z failure continued from the adhesive into either in composite or dentin.

66 fatigue could be a facilitating factor for failure in the hybrid layer. Moreover, Hashimoto et al19 described two degradation patterns within a hybrid layer after 1 year of storage in water. These degradation patterns included disorganization of collagen fibrils and hydrolysis of resin from interfibrillar spaces within the hybrid layer, which subsequently weakened the strength of the resinedentin bond. Hydrolysis is a chemical process that breaks covalent bonds between polymers through the addition of water to ester bonds, which was shown to result in loss of the resin mass.17 This is considered to be one of the main reasons for resin degradation within the hybrid layer and contributes to reductions in bond strengths of dentin adhesives over time.34,35 In our study, failure in specimens restored with these adhesive systems predominantly occurred as adhesive failure in the aged groups (Table 4), which in turn may be considered to be the weakest portion of the bonded interface. Although many other factors may affect the mechanical properties of resinedentin interfaces, it is possible that such differences in failure patterns may be indirect evidence of water absorption and hydrolysis of the hybrid layer. However the PLP resin and PF luting cement showed no significant decreases in bond strengths, probably due to their individual chemical components. In the present study, a functional monomer in the composition of PF luting cement, 10-methacryloxidesil dihydrogen phosphate (MDP), contains phosphate groups in its molecular structure that effectively chemically interact with hydroxyapatite; this might also have contributed to the superior and stable bonding effectiveness.36,37 The chemical bonding potential of MDP with hydroxyapatite was significantly high and hydrolytically stable.37 Additionally, Hashimoto et al found that crystal growth formed within the resinedentin bonds during long-term water storage of fluoride-containing resins.38 They speculated that fluoride released from restorative materials might be a factor responsible for crystal formation, and the ability to grow crystals between fluoridated restorative compounds and hard dental tissues may protect tooth surfaces in interfacial gaps and thus contribute to long-term stability.38 The PLP resin is composed oaf zinc fluoride complex, and our results are in agreement with those data. Thus, water absorption and hydrolysis of the bonded interface may have affected these adhesive systems less. Our results confirm previous studies suggesting that fatigue and water storage can decrease resinedentin bond strengths.39e41 It was reported that demineralized dentin becomes weaker after cyclic loading.42 Nikaido et al11 evaluated the effects of thermocycling and mechanical loading on the bond strengths of a self-etching primer system and concluded that surface preparation, the C-factor, cavity depth, and dentin substrate all influence bond strength values after thermal and fatigue loadings. In our study, bond strengths of the PLP adhesive and PF luting cement also decreased after loading and aging, but the results were not statistically significant. Within the limitations of our study, it could be hypothesized that using the PLP resin and PF luting cementfor restoration of the posterior teeth could be beneficial because of their moredurable bonding.

A.R. Cetin et al A study on a total-etching adhesive system showed that mechanical cycling alone does not affect bond strength. When thermal and mechanical load cycling was performed, however, the bond strength significantly decreased.43 When both compressive loading and water aging were applied in our study, significant decreases were observed in the bond strengths of all of the groups except PF and PLP (i.e., the bond strengths of the unloaded and loaded samples in the PF and PLP groups did not significantly differ). In contrast to our study, some studies indicated that no significant differences existed between the shear bond strengths of unloaded and loaded samples when cyclic compressive loading was applied.44,45 However, it should be emphasized that randomized controlled clinical trials are the most reliable way to assess the long-term behavior of adhesive systems. Therefore, further studies regarding the bond durability of the resin systems used in this study are needed. Under the limitations of the present study, the mTBS values of ASE dentin adhesive and PF luting cement were found to be significantly greater than those of OSP and PLP dentin adhesives and DL luting cement in the control groups. Moreover, load cycling and water storage negatively affected the bond strengths of these dentin adhesives. However, the PF luting cement and PLP dentin adhesive were more resistant to the effects of load cycling and water storage.

Acknowledgments This research project was supported by the Scientific Research Foundation of Selcuk University, Konya, Turkey.

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