Effect of curing mode on the polymerization characteristics of dual-cured resin cement systems

journal of dentistry 36 (2008) 418–426

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Effect of curing mode on the polymerization characteristics of dual-cured resin cement systems Cesar A.G. Arrais a, Frederick A. Rueggeberg b, Jennifer L. Waller c, Mario F. de Goes d, Marcelo Giannini e,* a

Department of Operative Dentistry, School of Dentistry, University of Guarulhos, Guarulhos, SP, Brazil Department of Dental Materials, School of Dentistry, Medical College of Georgia, Augusta, GA, USA c Department of Biostatistics, Medical College of Georgia, Augusta, GA, USA d Department of Restorative Dentistry, Dental Materials Section, Piracicaba School of Dentistry, University of Campinas, SP, Brazil e Department of Restorative Dentistry, Operative Dentistry Seciton, Piracicaba School of Dentistry, University of Campinas, SP, Brazil b

article info

abstract

Article history:

Objectives: To evaluate the effects of different curing conditions on the degree of conversion

Received 26 July 2007

(DC) of dual-cured cementing systems [combination of bonding agent (BA) and resin cement

Received in revised form

(RC)] using infrared spectroscopy.

20 February 2008

Methods: Four fourth generation products [Scotchbond Multipurpose Plus/RelyX (3M ESPE),

Accepted 21 February 2008

Optibond/Nexus 2 (Kerr), All Bond2/Duolink (Bisco), and Bond-It!/Lute-It! (Pentron)], and three fifth generation materials [Bond1/Lute-It! (Pentron), Prime&Bond NT Dual-Cure/Calibra (Dentsply), and Optibond Solo Dual Cure/Nexus 2 (Kerr)] were applied to the surface of a

Keywords:

horizontal attenuated-total-reflectance unit, and were polymerized using one of four con-

Degree of conversion

ditions: self-cure (SC), direct light exposure through glass slide (DLE, XL3000/3M ESPE) or

Dual-cured cementing systems

through pre-cured resin discs (shades A2;A4/2 mm thick/Z250/3M ESPE). Infrared spectra of

Indirect restorations

the uncured cementing systems were recorded immediately after application to the ATR,

Self-cure1

after the system was light-cured or left to self-cure, and spectra were obtained 5 and 10 min later. DC was calculated using standard techniques of observing changes in aliphatic-toaromatic peak ratios pre- and post-curing. Data (n = 5) were analyzed by two-way repeated measures ANOVA and Tukey’s test ( p = 0.05). Results: Changes in aliphatic-to-aromatic peak ratios before and after placing RC onto the BA demonstrated that a combined layer was created. All groups exhibited higher DC after 10 min than after 5 min, except the DLE group of Bond-it!/Lute-it!. No significant differences in DC were observed among light-activated groups regardless of the resin disc shade in three of the four fourth generation cementing systems. The SC groups exhibited lower DC than the DLE groups for both fourth and fifth generation products either after 5 or 10 min. Conclusion: The chemistry of the bonding interface changed when RCs were applied to uncured BAs. The presence of an indirect restoration can decrease the DC of some cementing systems and the self-curing mode leads to lower DC than the light-activating one. # 2008 Elsevier Ltd. All rights reserved.

* Corresponding author at: Department of Restorative Dentistry/Operative Dentistry, Piracicaba School of Dentistry, UNICAMP, Av. Limeira, #901, Piracicaba, SP 13414-900, Brazil. Tel.: +55 1934125340; fax: +55 1934125218. E-mail address: [email protected] (M. Giannini). 0300-5712/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2008.02.014

journal of dentistry 36 (2008) 418–426

1.

Introduction

The clinical success of composite and ceramic indirect restorations is attributed to the reliable bond between adhesive cementing systems (resin cements/bonding agents) and mineralized dental tissues.1,2 Light intensity reaching the resin cement is strongly attenuated by either the distance from the light source or by the absorbing characteristics of the indirect restorative material. This attenuation results in low degree of conversion (DC) and compromised mechanical properties of the dentin/adhesive interface when only lightcured resin materials are used to bond the restorations.3,4 In an attempt to overcome this problem, manufacturers developed dual-cured resin materials, which contain self-curing components to initiate the polymerization reaction in the absence of light.5,6 Dual-cured cementing systems contain a mixture of monomers and initiators, and are formulated to not depend only on light activation to polymerize. Therefore, light activation of adhesive resins prior to delivering an indirect restoration might not be necessary. Among commercial resinbased indirect cementing systems available, manufacturer instructions differ widely in advocating the pre-curing of the dentin bonding agent. Some products advocate light-curing of the dentin bonding agent prior to cementation, others indicate the clinician can choose to light cure or not, while others state that light curing should not be performed prior to resin cement application. Clinically, however, it would be advantageous not to light-cure the dentin bonding agent separately. If the thickness of polymerized dentin bonding layer is large, its added dimension would result in incomplete seating of the restoration, generating large marginal discrepancies and the necessity to adjust occlusion.7 Acidic resin monomers from two-step total etch and selfetching adhesives may impair the polymerization of dualcured cements and composites that are initiated via a peroxide-amine binary redox system.8,9 As a consequence, low bond strength values are reported when light activation of the dentin bonding agent is not performed.10,11 In order to overcome this chemical incompatibility, chemical co-initiators have been introduced in the dentin bonding agent, such as aryl sulfinic acid salts, organoboron compounds, and barbituric acid/cupric chloride.12 These components react with the acidic resin monomers to produce either phenyl or benzenesulfonyl free radicals that initiate the reaction of dual-cured resin cements when light from the curing unit is not available.12 Several studies have demonstrated that ceramic or resinbased composite inlays/onlays reduce the amount of light reaching the bottom of the restoration, and therefore compromise photo-activation of light-activated luting materials.13,14 Moreover, when evaluating in vitro occlusal wear, quantity of remaining double bonds, and cement system hardness, some authors indicate that the chemical curing mechanism alone is less effective than the light-activated reaction when dual-cured resin cements were used.13,15 However, there is no information regarding the DC of such dual-cured resin cements when they were combined with dual-cured adhesive systems in simulated clinical conditions when light intensity is strongly attenuated or totally absent.

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Thus, the purpose of this study was to measure the DC of representative commercial fourth and fifth generation dualcured cementing systems, when they were applied and lightactivated with little attenuation (through a microscope slide) or when light was attenuated by passing through pre-cured resin discs (shades A2/A4), or when light from the curing unit was totally absent (self-cured (SC)). Most manufacturers suggest the interval between 5 and 10 min after seating the indirect restoration as the most appropriate moment for occlusal adjusts, finishing, and polishing procedures, all of which are capable of generating stress. Therefore, the 5- and 10-min DC analysis was performed to provide information about a possible indicator of the physical properties of the adhesive interface during such procedures. The research hypotheses tested for each dual-cured cementing system were: within a given dual-cured cementing system: (1) conversion using direct light exposure (low light attenuation) will be higher than when the systems are allowed to self-cure only; (2) attenuation of curing light delivered to the dual-cured cementing system by passing through pre-cured resin discs will result in lower DC compared to when light passes through only a glass slide (low attenuation); (3) for similar thickness of pre-cured composite, the conversion of a dual-cured cementing system, when light-cured, will be less for the darkershaded composite; and (4) DC after 10 min will be higher than that measured after 5 min from polymerization initiation for SC and all light-curing modes.

2.

Materials and methods

2.1. Specimen preparation and Fourier transformed infrared analysis Four fourth and three fifth generation adhesive systems (Tables 1 and 2) and their recommended dual-cured resin cements (Table 3) were used (adhesive system/resin cement): All Bond2/Duolink (AB2/DUO; Bisco), Bond-It!/Lute-It! (BIT/ LUTE; Pentron), Optibond/Nexus 2 (OPT/Nexus; Kerr), Scotchbond Multipurpose Plus/Rely X (SBMP/RelyX; 3M ESPE); Bond1/ Lute-It (B1/LUTE; Pentron); Prime & Bond NT Dual-Cure/Calibra (NTD/Cal; Dentsply) and Optibond Solo Dual Cure/Nexus 2 (SOLOD/Nexus, Kerr), respectfully. Light-cured composite resin discs (2 mm thick, 10 mm in diameter – A2/A4 shade – Z250, lot# 5LB; 3M ESPE) were prepared to simulate overlying laboratoryprocessed composite resin restorations. The adhesive systems and resin cements were applied as described in Tables 1 and 2 to a horizontal diamond ATR element (Golden Gate, Specac, Woodstock, GA, USA) in the optical bench of a Fourier transform infrared spectrometer (FTS-40, Digilab/BioRad, Cambridge, MA, USA). Adhesive tape (3M) was placed around the diamond surface to act as a spacer, ensuring standard thickness for all specimens (100–120 mm). All adhesive systems were placed according to manufacturer instructions, but none were lightcured prior to placement of the resin cement. The deposited material was covered with a Mylar strip and polymerized using one of 4 different curing modes: light activation (sn#202149, XL3000, 3M/ESPE power density: 600 mW/cm2;) through a glass slide (2 mm thick) (direct light exposure (DLE)); light activation through A2 or A4-shade pre-cured resin discs (A2/A4) (Fig. 1); or

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Table 1 – Brand, composition, batch number and manufacturers’ instructions of the fourth generation dual-cured adhesive systems used Product (code) (manufacturer) All Bond 2 (AB2) (Bisco Inc., Schaumburg, IL, USA) Bond-It! (BIT) (Pentron Corp., Wallingford, CT, USA)

Optibond (OPT) (Kerr Corp., Orange, CA, USA)

Scotchbond Multipurpose (SBMP) (3M ESPE, St. Paul, MN, USA)

Composition (manufacturer supplied) (batch number) Primer A: acetone; ethanol; Na-N-tolylglycine; glycidylmethacrylate (0500003574); Primer B: acetone; ethanol; biphenyl dimethacrylate (0500003579); Pre-Bond Resin: bis-GMA, TEGDMA; benzoyl peroxide; BHT (0500004345) Primer A: NTG-GMA magnesium salt in acetone, acetone (123280). Primer B: mixture of PMGDM, a condensation product of PMDA and glycerol (126514). Unfilled dual resin activator: mixture of UDMA and HDDMA resins with self-curing initiator, stabilizer, benzoyl peroxide (110743). VLC adhesive: dental methacrylate resin mixture with photoinitiator, amine, and stabilizer (128389) Primer: ethyl alcohol, alkyl dimethacrylate resins, water (423435). Dual Cure Paste (3B): uncured methacrylate ester monomers; triethylene glycol dimethacrylate (424560); inert glass filler, pigment and stabilizers. Dual Cure Activator Resin (3A): uncured methacrylate; ester monomers; benzoyl peroxide (423073) Primer: water; 2-hydroxyethyl methacrylate; copolymer of acrylic and itaconic acids (5AT). Activator: ethyl alcohol; sodium benzenesulfinate (5KT). Catalyst: bis-GMA; 2-hydroxyethyl methacrylate; benzoyl peroxide (3AP)

Manufacturer’s instructions and exceptions Mix primers A and B. Apply five consecutive coats to dentin; dry all surfaces for 5–6 s with an air syringe; light-cure 20 s*; apply thin layer of Pre-Bond Resin immediately prior to cementation. Air thin. Do not light-cure Mix equal parts of primer A and B. Apply five coats to the etched surface to achieve a shiny appearance; dry only after final coat. Mix equal parts of Bond-It Light cure resin and Dual-Cure Activator and apply. Surface may be light cured or allowed to self-cure

Apply Optibond Prime (bottle 1) to dentin and enamel surfaces with microbrush, scrubbing the surface for (30) s; air dry for 5 s; dispense Optibond Dual Cure paste (3B) and 1 drop of Dual Cure Activator (3A) and thoughtfully mix them for 15 s; apply a thin coat of the mixed dual cure to the dentin surface. Do not air thin and do not light cure Apply activator (1.5) to enamel and dentin. Dry gently for 5 s; apply primer (2.0) to enamel and dentin. Dry gently for 5 s; apply catalyst (3.5) to enamel and dentin; mix and apply a self-cure or dual-cure luting material to the bonding surface of the restoration; seat the restoration. If a dual-cure cement was used, light-cure the margins

TEGDMA, Triethylene glycol dimethacrylate; bis-GMA, bisphenol A diglycidyl ether methacrylate; UDMA, urethane dimethacrylate; PMDA, pyromellitic dianhydride; HADDMA, 1,6-hexanediol dimethacrylate; PMGDM, pyromellitic glycerol dimethacrylate; NTG-GMA, N-tolyl-glycineglycidylmethacrylate; BHP, butylated hydroxytoluene. *The adhesive systems were not light-activated before the cementation of indirect resin composite disc.

they were allowed to self-cure under a Mylar strip and glass slide (SC), with no curing light exposure. In the light-activated groups, light exposure was provided according to manufacturers’ instructions (Table 3) just after placing all components of the cementing systems and the pre-cured resin discs or the glass slide on the ATR.

2.2.

Monomer conversion

Infrared spectra were collected between 1680 and 1500 cm 1 at a rate of 1 s 1 at 2 cm 1 resolution, from the moment when the first layer of adhesive resin was applied to the ATR surface through the next 10 min. Five replications were made for each test condition. When the resin cement was applied to the previously placed, unpolymerized adhesive, a slow change in aliphatic to-aromatic C C absorption ratio indicated the presence of the resin cement at the adhesive-covered diamond surface (Fig. 2). The time at which the first stabilized aliphatic to-aromatic C C absorption ratio was seen served to supply the infrared spectra of the uncured mixed resin (adhesive resin/resin cement) interface. Monomer conversion was calculated by standard methods using changes in the ratios of aliphatic-to-aromatic C C absorption peaks in the

uncured and cured states obtained from the infrared spectra.16,17 The DC of all curing modes was compared within each product at 5 and 10 min from the time the resin cement was applied to the adhesive system, as well as between the two time periods. All polymerized specimens were carefully removed from the ATR plate and measured for thickness to the nearest 0.01 mm using a digital micrometer (Series 406; Mitutoyo America Corp., Aurora, IL, USA) to ensure that pressure applied to either the microscope slide of prepolymerized resin disc provided similar thickness for all specimens.

2.3.

Curing light irradiance

The irradiance (mW/cm2) of the curing unit was determined using a laboratory-grade spectral radiometer (DAS 2100, Labsphere, N. Sutton, NH, USA) with a 7.62 cm diameter integrating sphere. Five measurements were obtained when the glass slide or the A2/A4 pre-cured resin discs were placed between the integrating sphere aperture and the light guide tip. Irradiance values were obtained between 350 and 600 nm by dividing the total emitted power (mW) by the optical area of the light guide end.

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Table 2 – Brand, composition, batch number and manufacturers’ instructions of the fifth generation dual-cured adhesive systems used Product (code) (manufacturer)

Composition manufacturer supplied) (batch number)

Manufacturer’s instructions and exceptions

Bond 1 (B1) (Pentron Corp.)

Activator: methacrylate monomers in ethanol and/or acetone, benzoyl peroxide, acetone (128878). Resin: mixture of PMGDM, a condensation product of PMDA and glycerol, dimethacrylate HEMA and TMPTMA in ethanol and/or acetone with photoinitiator, amine accelerator and stabilizer, pyromellitic dianhydride (129121)

Prime & Bond NT (NTD) (Dentsply Caulk, Milford, DE, USA)

Resin: acetone, urethane dimethacrylate resin, dipentaerythritol pentaacrylate phosphate, polymerizable dimethacrylate resins, polymerizable trimethacrylate resins (050413). Activator: aromatic sodium sulfinate (self-cure initiator), acetone, ethanol (041110)

Optibond Solo (SOLOD) (Kerr Corp.)

Adhesive resin: ethyl alcohol, alkyl dimethacrylate resins, barium aluminoborosilicate glass, fumed silica (silicon dioxide), sodium hexafluorosilicate (428904). Activator: ethyl alcohol, alkyl dimethacrylate resins, sodium salt of benzene sulfinic acid (428260)

Mix one drop of Bond1 Dual Cure Activator with 2 drops of Bond1 Primer/Adhesive. Using a fully saturated brush tip each time, apply two coats of Bond1 Primer/Adhesive to tooth within 10 s; apply a gentle stream of air for a minimum of ten (10) s. Hold air syringe 1 in. from site, positioned so as not to disturb resin surface. Avoid excess of Bond1 Primer/Adhesive in internal line angles or point angles Place 1–2 drops of the adhesive and equal number of drops of Self-Cure Activator into a mixing well; mix contents for 1–2 s with a clean, unused brush tip; using the disposable brush supplied, immediately apply mixed adhesive/activator to thoroughly wet all the tooth surfaces. These surfaces should remain fully wet for 20 s and may necessitate additional applications of mixed adhesive/activator; remove excess solvent by gently drying with a dental syringe for at least 5 s. Surface should have a uniform glossy appearance. Cure mixed adhesive/activator for 10 s using a curing light unita Dispense one drop of Optibond Solo Plus and Optibond Solo Activator into a disposable mixing well. Mix for 3 s; apply mixture to dentin with a light brushing for 15 s to cover dentin surface; lightly air thin for 3s. Light-cure for 20 sa

PMDA, Pyromellitic dianhydride; PMGDM, pyromellitic glycerol dimethacrylate; BHP, butylated hydroxytoluene; TMPTMA, trimethylolpropane trimethacrylate; HEMA, 2-hydroxyethyl methacrylate. a The adhesive systems were not light-activated before the cementation of indirect resin composite disc.

2.4.

Statistical analyses

A two-way repeated measures analysis of variance (ANOVA) (effect of curing mode and time) was performed for each

product when the DC was the variable selected, and one-way ANOVA was performed to compare the differences in light intensity. All statistical tests were performed at a pre-set alpha of 0.05 and followed by Tukey’s post hoc test.

Table 3 – Brand, composition and batch number of the dual-cured resin cements used Product (manufacturer) Duolink (DUO) (Bisco Inc.) Nexus 2 (Nexus) (Kerr Corp.) Lute-It! (LUTE) (Pentron Corp.) Calibra (CAL) (Dentsply Caulk)

Rely X (Rx) (3M ESPE)

Composition (batch number) Base: bis-GMA; TEGDMA; glass filler; urethane dimethacrylate. Catalyst: bis-GMA; TEGDMA; glass filler (0500003751) Activator: ethyl alcohol; alkyl dimethacrylate resins; benzene sulfinic acid sodium salt monomers of methacrylic acid esters, Ba–Al–borosilicate glass, chemical and photoinitiators (base: 423638; catalyst: 423975) In both base and catalyst: UDMA, HDDMA, amine and inorganic pigments (in base only), benzoyl peroxide (in catalyst only), UV stabilizers (in both base and catalyst), barium glass, inorganic fluoride*, borosilicate glass, silane silica zirconia (base: 130666; catalyst: 126388) Base paste: barium boron fluoroalumino silicate glass, bis-GMA resin, polymerizable dimethacrylate resin, polymerizable dimethacrylate resin, hydrophobic amorphous fumed silica, titanium dioxide, other colorants are inorganic iron oxides. Catalyst paste: barium boron, fluoroalumino silicate glass, bis-GMA resin, polymerizable dimethacrylate resin, hydrophobic amorphous fumed silica, titanium dioxide, benzoyl peroxide (base: 0504111; catalyst: 0505121) Paste A: silane treated ceramic, triethylene glycol dimethacrylate (tegdma), bis-GMA, silane treated silica, functionalized dimethacrylate polymer, paste B: silane treated ceramic, TEGDMA, bis-GMA, silane treated silica, functionalized dimethacrylate polymer (EYFH)

Time of light exposure (s) 40 40

40

30

40

TEGDMA, Triethylene glycol dimethacrylate; bis-GMA, bisphenol A diglycidyl ether methacrylate; UDMA, urethane dimethacrylate; HDDMA, 1,6-hexanediol dimethacrylate

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Fig. 1 – Illustrative diagram demonstrating the interaction between the infrared beam and the specimen, as well as the position of the pre-cured resin discs or the glass slide and the light-curing unit tip.

3.

Results

3.1. Monomer conversion—within dual-cured cementing systems For most dual-cured cementing systems, a decrease in aliphatic-to-aromatic C C absorption ratio was noted when the dual-cured resin cements were applied to the ATR surface containing uncured bonding agents (Fig. 1). The only exception was observed for SOLOD/Nexus, which showed an increase in this ratio when the resin cement was applied.

Table 4 displays DC results of all fourth and fifth generation dual-cured cementing systems, respectively. No significant differences in DC were observed when A2 values were compared to DLE for the fourth generation cementing systems, except for OPT/Nexus at 5 min, where DC of A2 was significantly lower. However, with the fifth generation systems, the A2 groups exhibited lower DC than the DLE groups for all materials (Table 4). The DC of A4 specimens was significantly lower than that of the DLE group in most of the dual-cured cementing systems. The only exception was when AB2/DUO, BIT/LUTE (at 10 min) and SBMP/RelyX were used, which showed no significant differences in DC between DLE and A4 groups.

Fig. 2 – Representative spectra exhibiting changes in composition after placing the resin cement on the uncured bonding agent (solid line: spectrum of the bonding agent; dotted line: spectrum after adding the resin cement to the bonding agent). The increase in peaks corresponding to the aromatic C C bonds demonstrates the formation of a combined layer composed of bonding agent and resin cement (dotted line).

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Aa Ba Ab Bab Ab Bb Aa Ba Ac Bbc Ab Bb Ab Bbc 58.4 61.5 53.5 59.3 62.0 64.7 48.1 52.6 52.3 57.4 54.6 57.8 65.8 68.8 Aa Ba Aab Bab Ab Bab Aa Ba Ab Bb Ab Bb Ab Bb (0.4) (0.6) (2.8) (2.0) (0.8) (0.5) (3.4) (3.1) (0.5) (0.2) (1.1) (0.9) (1.2) (1.2) SOLOD/Nexus

NTD/Cal

B1/LUTE

SBMP/RelyX

OPT/Nexus

BIT/LUTE

AB2/DUO

5 10 5 10 5 10 5 10 5 10 5 10 5 10

61.2 63.4 63.4 66.3 65.0 66.8 52.0 55.1 59.3 62.2 59.0 61.5 70.5 72.6

(0.8) (0.9) (1.2) (0.8) (0.3) (0.3) (2.5) (2.4) (0.7) (0.9) (0.7) (0.9) (1.9) (1.8)

Aa Ba Aa Aa Aa Ba Aa Ba Aa Ba Aa Ba Aa Ba

58.6 61.7 58.5 62.8 62.2 65.2 50.1 53.8 54.1 58.3 55.3 58.1 66.6 69.2

A2 DLE Time after mixing or exposure (min) Bonding agent/ cement

Table 4 – Degree of conversion (%) (DC) for adhesive/resin cement systems (mean (S.D.))

Within an adhesive/cement system only, similar letters indicate no significant difference among values (capital letters, columns; lower case letters, rows). DLE, Direct light exposure; A2; A4, system light-cured through 2 mm thick pre-cured composite disc of specific shade; SC, no light exposure, total self-curing. AB2, All Bond 2; DUO, Duolink; BIT, Bond-it!; LUTE, Lute it!; OPT, Optibond; Nexus, Nexus 2; SBMP, Scotchbond Multipurpose Plus; RelyX, Rely X; B1, Bond 1; NTD, Prime & Bond NT Dual Cure; Cal, Calibra; SOLOD, Opitbond Solo Dual Cure. No comparisons were performed among products.

Fifth

Fifth

Fifth

Fourth

Fourth

Fourth

Fourth

(4.3) (1.0) (9.7) (3.2) (2.4) (0.7) (3.0) (2.4) (0.7) (0.9) (2.2) (1.7) (2.0) (1.9) (1.2) (0.9) (3.2) (2.0) (0.6) (0.5) (1.1) (1.3) (0.8) (0.7) (1.5) (0.9) (1.8) (1.7)

37.1 49.5 45.7 57.3 39.5 55.9 36.2 44.3 48.3 56.1 46.6 52.6 61.1 65.8

SC A4

Ab Bb Ac Bb Ac Bc Ab Bb Ad Bc Ac Bc Ac Bc

Generation

journal of dentistry 36 (2008) 418–426

Table 5 – Power density (mW/cm2) measured through glass slide and through 2 mm thick A2/A4-shade precured resin discs (mean (S.D.)) Glass slide 545.4 (6.3)

A2 60.8 (0.2)

A4 45.4 (0.3)

All values were significantly different from one another ( p < 0.05). N = 5 replications per test condition.

For most of the fourth and fifth dual-cured cementing systems, the SC group exhibited lower DC than DLE, A2, and A4 experimental groups at either 5- or 10-min (Table 4). The only exceptions were observed for BIT/LUTE at 10 min, which showed no significant differences among SC, A2, and A4 groups, and for B1/LUTE and SOLOD/Nexus, which showed no significant difference among SC and A4 groups at 10 min. The thickness of all specimens ranged from 100 to 120 mm.

3.2.

Monomer conversion—within curing modes

All fourth and fifth generation cementing systems exhibited lower DC at 5 min than at 10 min in all curing modes. The only exception was seen in DLE of BIT/LUTE where no difference in DC was found between 5 and 10 min (Table 4).

3.3.

Light attenuation of pre-cured resin discs

Table 5 presents irradiance values measured when light passed through the microscope glass slide, as well as through A2- and A4-shade pre-cured resin discs. When the A2-shade pre-cured resin disc was used, irradiance decreased approximately 89%, while 92% lower irradiance was noted when using the A4 disc.

4.

Discussion

The results demonstrate that the effect of all curing modes on DC depends on both product and the evaluation period. The first research hypothesis stated that conversion using direct light-cure within a given dual-cured cementing system would be higher than when the systems were allowed to self-cure within the same dual-cured cementing system. This hypothesis was accepted for both cementing system generations, at 5 and 10 min. This finding agrees with other reports, which showed that the self-curing mode was less effective when compared to the dual-cured or photo-cured ones.13,15 One possible explanation for this finding may be related to an inability of radicals to migrate, due to the change in viscosity during the polymerization, so the monomer conversion at 10 min is impaired as a consequence.18 Studies comparing the DC developed in the SC mode with that obtained with DLE mode after 24 h were not evaluated in the present work, but would help understand the long-term implications among these products. Compared to the power density delivered through the glass slide, only 11 and 8% of the total irradiance reached the cementing systems when pre-cured A2 and A4 shade resin discs were used, respectively (Table 5). However, even when

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light intensity decreased by approximately 92% (A4 disc), no difference in DC was observed for most of the fourth generation cementing systems when compared to the values of DLE groups after 10 min. Therefore, the research hypothesis stating that the attenuation of curing light delivered to the dual-cured cementing system by passing through pre-cured resin discs will result in lower DC when compared to when light passed through only a glass slide within a given dualcured cementing system was invalidated for the fourth generation products evaluated after 10 min. Considering the fact that the main feature of such cementing systems is the presence of benzoyl peroxide not only in the resin cements, but also in the adhesive systems, the composition of the mixture obtained from resin cements and adhesive systems has higher content of self-curing components than that of these materials when they are not mixed together. Because the higher content of the self-curing components affect the DC of self-cured materials,19 the higher amine/benzoyl peroxide content may have ensured that differences in light intensity did not affect the DC of the cementing systems. The importance of the dual-cure nature of bonding agents with respect to conversion of the combined layer when light is attenuated by the presence of indirect restoration might be confirmed when the results of the current study were compared with the results of El-Mowafy and Rubo20 and Hofmann et al.21 In those studies, the effects of light intensity attenuation on mechanical properties (hardness and flexural strength) of dual-cured resin cements without their respective dual-cured bonding agents were evaluated. It was found that the self-curing components of most resin cements were not capable of compensating for the attenuation of light intensity. El-Mowafy and Rubo also evaluated the light intensity after it passed through a 2 mm thick indirect ceramic restoration, and observed a decrease in light intensity from 700 mW/cm2 to approximately 100 mW/cm,2,20 which was higher than that observed in the current study (Table 5). However, in contrast to the results observed by these authors, in the present work, such attenuation did not affect the DC of the fourth generation dual-cured cementing systems in comparison to the DC of the same products when light-activation was delivered directly through a glass slide. Therefore, the coupling of fourth generation dual-cured bonding agents with dual-cured resin cements was shown to be important for not only avoiding any chemical incompatibility, but also for providing more effective monomer conversion when the curing light was severely attenuated. On the other hand, use of pre-cured resin discs resulted in lower DC than the use of glass slide (DLE group) after 10 min for each fifth generation dual-cured cementing system. Thus, the same research hypothesis discussed above was confirmed for the fifth generation systems. In contrast to the fourth generation cementing systems, the fifth generation dualcured cementing systems use benzoyl peroxide only in the resin cements and contain parabenzene sulfinic acid sodium salts in the adhesive components. These aryl sulfinic acid salts are used to reduce or even eliminate the incompatibility between acidic monomers within the adhesive system and the tertiary aromatic amine from the resin cement.12,22 Therefore, the lower content of self-curing components in the fifth generation systems may have not been capable of compensat-

ing for the decrease in light intensity promoted by the presence of pre-cured resin discs. Among all fifth generation adhesive systems evaluated, B1 was the only adhesive system containing benzoyl peroxide instead of aryl sulfinic acid salts in its composition. However, the DC of this dual-cured cementing system was also affected by the presence of the pre-cured resin discs. One possible explanation for this finding may be related to the lack of the aryl sulfinic acid salts in its composition. Although the total amount of benzoyl peroxide was increased due to its presence in both the resin cement and adhesive system, it may be speculated that the chemical incompatibility between the adhesive resin and dual-cured resin cement may have impaired formation of initiating radicals and consequently the self-curing reaction was compromised.10,23 Thus, the polymerization may have relied mostly on light exposure, which was drastically reduced by the presence of pre-cured resin discs. This speculation might also explain the lower DC of A4 experimental group when compared with that of A2 when B1/LUTE was used at 5 min interval. Despite the lower light intensity observed when the A4-shade pre-cured resin disc was used (Table 5), B1/LUTE was the only cementing system showing significantly lower DC values of A4 compared to A2 groups at the 5 min interval. However, the effects of selfcuring mechanisms in compensating for light attenuation can only be confirmed when comparing the DC of light-activated and dual-cured adhesive systems applied together with dualcured resin cements. This study compared DC at 5 and 10 min after initial mixture of components. Most manufacturers advise the clinician to wait at least five min prior to adjusting a newly cemented indirect restoration. Except for BIT/LUTE, in which no significant difference in DC was observed between 5 and 10 min in DLE group, all other dual-cured cementing systems exhibited lower DC after 5 min than the values observed after 10 min within A2, A4, and SC experimental groups. Thus, the research hypothesis, that the cementing systems would show higher DC at 10 min than at 5 min, was confirmed in almost all experimental groups. When light was not available, the difference in DC between 5and 10 min intervals ranged from 4 to 17% approximately (Table 4). As a consequence, mechanical properties, such as flexural and compressive strengths, elastic moduli, and hardness may be proportionally affected by this change in DC.24,25 Therefore, a longer time period prior to performing occlusal adjustment or removing excess marginal resin cement may be indicated over what is currently advocated (5 min). The results of this study need to be considered with respect to many aspects related to testing conditions. Because penetration of the infrared beam into the materials placed above the crystal ranges from 1 to 7 mm (depending on the refractive index, the wavelength, and the angle of incidence),26 changes in resin content upon introducing the uncured cement paste into the unpolymerized dentin bonding agent were only observed within this thickness. However, the area of infrared observation, and thus characterization of conversion, is considered to be clinically relevant, as it is similar to that of a hybrid layer that would be present on acid etched dentin. Also, some of the dentin bonding agents used in this study are acidic in nature.10 It has been demonstrated that the acidity of selfetching adhesive systems is neutralized after placement by the

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dissolution of hydroxyapatate-containing dentin matrix.27 Such neutralization is thought to optimize conversion, as opposed to when polymerization takes place at low pH values.8 The ATR plate upon which the acidic bonding agents were placed could not provide any buffering capacity to the dentin bonding agents, and thus could not truly simulate the clinical situation. For this reason, no self-etching adhesive systems were evaluated in this study. As both fourth and fifth generation adhesive systems are applied to previously acid-etched, demineralized dentin, which does not contain hydroxyapatite, no change in pH is expected in the clinical situation, considering the evidence that resin monomers do not fully infiltrate the demineralized dentin and therefore do not react with hydroxyapatite.28,29 Thus, the experimental system used seems to not be different from the in vivo condition when applying acidic bonding agents to dentin regarding pH and environmental conditions. However, other limitations of the method, such as differences in the surface energy, temperature, and humidity of the surfaces contacting the cement systems in comparison to those of demineralized dentin, may lead to some differences in monomer conversion. Only methodologies capable of evaluating monomer conversion of bonding agents within demineralized dentin itself would confirm the effects of these conditions on the degree of conversion. This in vitro study was based on well-controlled laboratory conditions. However, other variables, such as the presence of water and quality of resin infiltration into demineralized dentin, can affect DC and the resulting mechanical properties of polymer created when they are applied to teeth.30,31 Monomer conversion values for dentin bonding agents applied and light cured alone were not determined in this study. Because the chemistry of the analyzed interface changed when the resin cement was applied and diffused into the uncured bonding agent, direct comparison of conversion values between the light-cured bonding agent alone and that of the mixture of bonding agent and cement cannot be made. Thus, the significance of these differences is not known. Only additional testing, such as bond strength comparison among specimens of similar curing modes, would reveal the importance of these factors.

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