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Microtensile bond strength of new self-adhesive luting agents and conventional multistep systems
Microtensile bond strength of new selfadhesive luting agents and conventional multistep systems Ronaldo G. Viotti, DDS,a Alline Kasaz, DDS,b Carlos E. Pena, DDS,c Rodrigo S. Alexandre, DDS, MS,d Cesar A. Arrais, DDS, MS, PhD,e and Andre F. Reis, DDS, MS, PhDf School of Dentistry, University of Guarulhos, Guarulhos, São Paulo, Brazil; Blumenau Regional University, Blumenau, Santa Catarina, Brazil Statement of problem. Several self-adhesive luting agents have recently been introduced on the market. It is crucial to know the effectiveness of such luting agents prior to their clinical application. Purpose. The purpose of this study was to evaluate the microtensile bond strengths (μTBS) produced by different selfadhesive cements and compare them with conventional luting agents. Material and methods. Six self-adhesive cements (RelyX Unicem (UN), RelyX U100 (UC), SmartCem 2 (SC), G-Cem (GC), Maxcem (MC), and SeT (SET), and 2 conventional luting agents, one that uses a 2-step etch-and-rinse adhesive (RelyX ARC (RX)), and one that uses a 1-step self-etching adhesive (Panavia F (PF)), were used in this study. An additional group included the use of a 2-step self-etching primer adhesive system (Clearfil SE Bond) prior to the application of Panavia F (PS). Fifty-four human molars were abraded to expose occlusal surfaces and were assigned to 9 groups according to the luting material (n=6). Five composite resin (Filtek Z250) discs (12 mm in diameter, 5 mm thick) were cemented on the teeth according to manufacturers’ instructions. After 24 hours of water storage, restored teeth were serially sectioned into beams with a cross-sectional area of approximately 1 mm2 at the bonded interface and were tested in tension with a crosshead speed of 1 mm/min. Failure mode was determined using scanning electron microscopy. Data were statistically analyzed by 1-way ANOVA and Tukey’s studentized range HSD test (α=.05). Results. Mean bond strengths (SD) in MPa were: RX, 69.6 (16.6)A; PS, 49.2 (9.7)A; PF, 33.7 (13.9)AB; GC, 16.9 (10.3)BC; UC, 15.3 (3.4)BC; UN, 12.5 (2.4)C; MC 11.5 (6.8)CD; SC, 8.5 (4.9)CD; SET, 4.6 (0.5)D. Groups with different uppercase letters were significantly different from each other (P<.05). The predominant failure mode of the self-adhesive cements was adhesive between the resin cement and dentin. Conclusions. The bond strengths produced by the multistep luting agents were significantly higher than those observed for most self-adhesive cements. (J Prosthet Dent 2009;102:306-312)
Clinical Implications
The early bond strengths of self-adhesive cements varied among materials evaluated. Most self-adhesive cements produced lower bond strengths than conventional multistep luting agents and should be avoided when bonding indirect restorations to nonretentive preparations. Supported by FAPESP (The State of São Paulo Research Foundation), grants #2007/06083-4 and #2007/06447-6. Master student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos; Adjunct Faculty, Blumenau Regional University. b Master student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. c Master student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. d PhD student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. e Assistant Professor, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. f Assistant Professor, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. a
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November 2009 Bonding of resin-based composite materials to tooth hard tissues has been simplified recently. Until a few years ago, most adhesives were available in 3 application steps, which were later combined into 2 steps (etch and rinse or self etching) and, more recently, into a single self-etching application step. Indirect adhesive procedures constitute a substantial portion of esthetic restorative procedures. Until recently, all luting agents required the application of one of these adhesive systems, either self etching or etch and rinse, to prepare the tooth prior to cementation.1-4 The multistep application technique has been reported to be complex and sensitive because it also depends on the performance and technique sensitivity of etch-and-rinse and self-etching adhesive systems, which can influence bonding effectiveness.5-11 However, a new type of luting material has been developed that does not require any pretreatment of the tooth surface, the so-called self-adhesive cement.12-14 This material aims to combine the favorable properties of conventional (zinc phosphate, glass ionomer, and polycarboxylate cements) and resin luting agents.15 After the first self-adhesive cement was introduced commercially (RelyX Unicem; 3M ESPE, St. Paul, Minn), it rapidly gained popularity among clinicians due to its simplified “mistakefree” application technique. Thus, several manufacturers developed selfadhesive cements. A number of studies have evaluated the bond strength and characteristics of the first material of this new class of luting agent and compared it to currently available multistep luting agents.1,13,14,16-25 Dentin bond strengths comparable to those provided by the multistep luting agents have been reported, while lower bond strengths were found on enamel surfaces. However, a wide variety of self-adhesive cements is currently commercially available, and little information is available with regard to the bond strengths produced by the
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self-adhesive systems that were introduced subsequent to RelyX Unicem (3M ESPE), following the same intent.16,19,24 Despite the favorable dentin bond strength behavior reported for RelyX Unicem,26-28 bond strength can vary among materials due to differences in composition.24,29,30 The purpose of this study was to evaluate the microtensile bond strengths to dentin produced by 6 self-adhesive cements, and to compare them with conventional etchand-rinse and self-etching luting agents. The tested null hypothesis was that there would be no difference between the bond strengths to dentin produced by self-adhesive and conventional luting agents.
MATERIAL AND METHODS Fifty-four recently extracted caries-free third molars stored in 0.1% thymol (Symrise GmbH, Holzminden, Germany) solution at 4°C were used in this study. Teeth were obtained by protocols that were approved by the review board of the Guarulhos University (Guarulhos, São Paulo, Brazil). After disinfection and removal of soft tissues, flat coronal dentin surfaces were exposed with 600-grit SiC paper (3M of Brazil Ltd, Sumare, Brazil) under running water to create a standardized smear layer. Teeth were assigned to 9 experimental groups, which were treated with 1 of the 9 luting techniques (n=6). Six self-adhesive cements: RelyX Unicem (UN), RelyX U100 (UC), SmartCem 2 (SC), G-Cem (GC), Maxcem (MC), and SeT (SET), and 2 conventional luting agents, one that uses a 2-step etch-and-rinse adhesive (RelyX ARC, RX), and one that uses a 1-step self-etching adhesive (Panavia F, PF), were used in this study. An additional group included the use of a 2-step self-etching primer adhesive system (Clearfil SE Bond) prior to the application of Panavia F (PS). Luting agents were mixed and placed according to manufacturers’ instructions (Table I).
Five-mm-thick composite resin discs, 12 mm in diameter, were prepared by layering 2-mm-thick increments of a microhybrid composite resin (Filtek Z250, shade A2; 3M ESPE) into a silicone mold. Each increment was light activated (700 mW/mm2) for 40 seconds with a halogen light (Optilux 501; Kerr Corp, Orange, Calif ). One side of the composite resin discs was abraded with 600-grit SiC paper (3M of Brazil Ltd) under water cooling to create a flat surface with standardized roughness. The composite surface was airborne-particle abraded with 50-µm aluminum oxide particles (Asfer Indústria Química Ltda, São Caetano do Sul, Brazil) for 10 seconds. Before luting procedures were performed, the composite resin discs were ultrasonically cleaned in distilled water for 10 minutes, rinsed with running water, air dried, and silanated (RelyX Ceramic Primer; 3M ESPE). After application of the luting agent according to the manufacturer’s instructions, the composite resin disc was pressed on the cement using light pressure, after which excess cement was removed. Specimens were light activated for 40 seconds with the same halogen light from the buccal, lingual, and occlusal directions. Bonded specimens were stored in distilled water for 24 hours. Afterwards, teeth were serially sectioned perpendicular to the adhesive-tooth interface into slabs, and the slabs into beams with a cross-sectional bonded area of approximately 1 mm2 using a diamond saw (IsoMet 1000; Buehler Ltd, Lake Bluff, Ill).31 Beams were fixed to the grips of a universal testing machine (EZ Test; Shimadzu Corp, Kyoto, Japan) using a cyanoacrylate adhesive (Loctite Super Bonder Gel; Henkel, Düsseldorf, Germany) and tested in tension at a crosshead speed of 1 mm/min until fracture. Maximum tensile load was divided by specimen cross-sectional area to express results in units of stress (MPa). Five beams were selected from each restored tooth, and the average value for each tooth was used
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Table I. Cements, lot number, manufacturers, delivery system, composition, and application technique Manufacturer (Lot Numbers)
Delivery System (Cement)
Composition
Dual-polymerizing resin cement + 2-step etch-and-rinse adhesive system
RelyX ARC (GEHG) + Adper Single Bond 2 (8RW) 3M ESPE, St. Paul, Minn
Automatic dispenser, 2 pastes, hand mixed for 10 s
Etchant: 35% H3PO4 Adhesive: bis-GMA, HEMA, UDMA, dimethacrylates, ethanol, water, camphorquinone , photoinitiators, polyalkenoic acid copolymer, 5-nm silica particles Cement: bis-GMA, TEGDMA polymer, zirconia/silica filler
a (15 s); b (15 s); c; d; e; i (10 s); mix cement; apply mixture
Dual-polymerizing resin cement + 1-step self-etching adhesive
Panavia F (paste A, 00248C; paste B, 0026B) + ED Primer (primer A, 00255A; primer B, 00131A) Kuraray Medical, Inc, Tokyo, Japan
One-step self-etching adhesive + resin cement, dual polymerizing
Primer A: HEMA, 10-MDP, 5-NMSA, water, accelerator Primer B: 5-NMSA, accelerator, water, sodium benzene sulphinate Paste A: 10-MDP, silanated silica, hydrophobic aromatic and aliphatic dimethacrylate, hydrophilic dimethacrylate photoinitiator, dibenzoyl peroxide Paste B: silanated barium glass, sodium fluoride, sodium aromatic sulfinate, dimethacrylate monomer, BPO
h (A+B) (leave undisturbed for 60 s); mix cement; apply mixture; i (40s)
Panavia F (paste A, 00248C; paste B, 0026B) + Clearfil SE Bond (00788A) Kuraray Medical, Inc
Two-step self-etching adhesive + ED Primer + resin cement, dual polymerizing
f (20 s); e; g; i (10 s); h (ED Primer); e; mix cement; apply mixture; i (40 s)
2 pastes, hand mixed
Primer: MDP, HEMA, hydrophilic dimethacrylate, dl-camphorquinone, N,N-diethanol p-toluidine, water Bond: MDP, bis-GMA, HEMA, hydrophobic dimethacrylate, dl-camphorquinone, N, N-diethanol p-toluidine, silanated colloidal silica Paste A and Paste B: as described above
Dual-polymerizing self-adhesive resin cement
G-Cem (0702191) GC America, Inc, Alsip, Ill
Capsules, mechanically mixed 10 s
Powder: fluoroaluminosilicate glass, initiator, pigment Liquid: 4-META , phosphoric acid ester monomer, water, UDMA, dimethacrylate, silica powder, initiator, stabilizer
Automix cement; apply mixture; i (40 s) or j (5 min)
Dual-polymerizing self-adhesive resin cement
RelyX U100 (287269) 3M ESPE
Clicker dispenser 2 pastes, hand mixed
Base: glass fiber, methacrylated phosphoric acid esters, dimethacrylates, silanated sílica, sodium persulfate Catalyst: glass fiber, dimethacrylates, silanated sílica, p-toluene sodium sulfate, calcium hydroxide
Mix cement; apply mixture; i (40 s) or j (5 min)
Dual-polymerizing self-adhesive resin cement
Maxcem (2954635) Kerr Corp, Orange, Calif
Paste/paste dual syringe, direct dispensing through mixing tip
Resin: multifunctional DMAs, GPDM, proprietary Redox initiators and photoinitiators Filler: barium, fluoroaluminosilicate, fumed silica (66 wt%)
Automix cement; apply mixture; i (20 s) or j (3 min)
Dual-polymerizing self-adhesive resin cement
SmartCem 2 (0807311) Dentsply Caulk, Milford, Del
Paste/paste dual syringe, direct dispensing through mixing tip
Dual-polymerizing self-adhesive resin cement
SeT (50711292) SDI Ltd, Bayswater, Australia
Capsules, mechanically mixed for 10 s
Type
Dual-polymerizing resin cement + 2-step self-etching adhesive system
2 pastes, hand mixed
Application Technique
UDMA, di- and tri-methacrylate resins, phosphoric acid Automix cement; modified acrylate resin, barium boron apply mixture; fluoroaluminosilicate glass, organic peroxide i (40 s) or j (6 min) initiator, camphorquinone photoinitiator, phosphene oxide photoinitiator, accelerators, butylated hydroxytoluene, UV stabilizer, titanium dioxide, iron oxide, hydrophobic amorphous silicon dioxide Methacrylated phosphoric esters, UDMA, photoinitiator 67 wt% (45 vol%), fluoroaluminosilicate glass, pyrogenic silica
Automix cement; apply mixture i (20 s) or j (5 min)
Application technique = a: acid etch; b: rinse surface; c: dry with cotton pellet; d: apply 1-bottle adhesive; e: gently air dry; f: apply primer; g: apply adhesive; h: apply mixture; i: light polymerize; j: autopolymerize. Bis-GMA: bisphenol A diglycidyl ether methacrylate; 4-META: 4-methacryloyloxyethyl trimellitate anhydride; UDMA: urethane dimethacrylate; HEMA: 2-hydroxyethyl methacrylate; 10-MDP: 10-methacryloxydecyl dihydrogen phosphate; 5-NMSA: N-methacryloxyl-5-aminosalicylic acid; TEGDMA: triethylene glycol dimethacrylate; GPDM: glycerophosphate dimethacrylate
in the calculations. Bond strength values were statistically evaluated using a 1-way ANOVA and the Tukey’s studentized range HSD test (α=.05). Pretest failures were not included in the statistical analysis.31,32 Statistical analyses were performed using a
statistical software program (SAS for Windows V8; SAS Institute, Inc, Cary, NC). Failure modes were determined by examination of fractured specimens with a scanning electron microscope (SEM) (LEO 435 VP; LEO Electron
The Journal of Prosthetic Dentistry
Microscopy Ltd, Cambridge, UK). Specimens were mounted on aluminum stubs and gold-sputter coated (MED 010; BAL-TEC AG, Balzers, Liechtenstein) prior to viewing at different magnifications. Failure mode at the fractured interface was classi-
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November 2009 fied into 1 of 4 types: CD (cohesive failure in dentin), AD (adhesive failure between cement and dentin), CC (cohesive failure in the cement), or ADR (adhesive failure between the luting agent and composite resin). Instead of classifying failures as mixed, the area percentage of each type of failure
in each specimen was recorded.
RESULTS Mean (SD) µTBS values are presented in Table II. The ANOVA revealed a significant difference among groups (df=50; F=25; P<.01). For MC
and SC, only 5 and 4 teeth could be tested, respectively. The Tukey test revealed significant differences among the different luting techniques (P<.001). The multistep etch-andrinse system RX and the 2-step selfetching technique PS produced the highest bond strength values and
Table II. Dentin bond strength values for different dual-polymerizing resin cements Material (Number of Teeth)
Mean (SD)
Tukey
Pretest Failure/ Number of Beams
RelyX ARC + Single Bond (n=6)
69.6 (16.6)
A
0/30
Two-step self-etching adhesive/resin cement
Panavia F + SE Bond (n=6)
49.2 (9.7)
A
8/30
One-step self-etching adhesive/resin cement
Panavia F + ED Primer (n=6)
33.7 (13.9)
AB
0/30
Self-adhesive cement
G-Cem (n=6)
16.9 (10.3)
BC
0/30
Self-adhesive cement
RelyX U100 (n=6)
15.3 (3.4)
BC
0/30
Self-adhesive cement
RelyX Unicem (n=6)
12.5 (2.4)
C
3/30
Self-adhesive cement
Maxcem (n=5)
11.5 (6.8)
CD
20/30
Self-adhesive cement
SmartCem 2 (n=4)
8.5 (4.9)
CD
13/30
Self-adhesive cement
SeT (n=6)
4.6 (0.5)
D
3/30
Product Type Two-step etch-and-rinse adhesive/resin cement
Values are means (SD) in MPa. Values identified by different uppercase letters are significantly different (P<.05). RelyX ARC Panavia + SE Panavia F
CD
SeT
AD
SmartCem 2
CC
Maxcem
ADR
G-Cem RelyX U100 RelyX Unicem 0
20
40
60
80
100
Percent 1 Distribution of failure modes within groups. CD, cohesive failure in dentin; AD, adhesive failure between dentin and luting agent; CC, cohesive failure in resin cement; ADR, adhesive failure between luting agent and composite resin.
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A
B
2 Representative SEMs of most predominant failure mode of self-adhesive cements. A, Adhesive failure is observed for G-Cem (x60 magnification). B, At higher magnification (x1500 magnification), smear layer and smear plugs can be observed over dentin surface. were significantly different from the self-adhesive cements. The 1-step self-etching luting agent PF did not differ significantly from RX and PS. The self-adhesive systems GC and UC were not significantly different from PF, but were lower than RX and PS. Except for SET, no significant difference in bond strength was noted among all self-adhesive materials. The lowest bond strengths were recorded for SET, which were not significantly different from those of MC and SC. A high number of pretesting failures was recorded for MC (20) and SC (13). The distribution of failure modes among luting materials is shown in Figure 1. A representative image of an adhesive failure between the luting agent and dentin is shown in Figure 2. This was the predominant failure mode for the self-adhesive cements and for the self-etching system PF.
DISCUSSION In the present investigation, the bond strengths of 6 self-adhesive cements were compared with conventional multistep systems. An etchand-rinse and a self-etching cement were used as control groups, and an additional group was used for comparison, in which a 2-step self-etching primer adhesive system was used before the application of Panavia F. The null hypothesis was rejected, because significant differences in bond
strength values were observed among the different luting materials. The greatest advantage of self-adhesive cements is the easy and fast application technique, which is among the most desirable features in any dental material. The multistep application technique has been reported to be complex and sensitive, and can compromise bonding effectiveness.5 The self-adhesive systems presented bond strength values significantly lower than those presented by 2 of the control groups, RX and PS. When the 2-step self-etching primer adhesive Clearfil SE Bond was used prior to the application of the self-etching primer (ED Primer), bond strength increased. Panavia F bond strength values were not significantly different from RX or PS. However, no significant differences were observed between PF and the self-adhesive cements GC and UC. The lower bond strengths observed by PF, in comparison with PS, probably occurred because PF uses a 1-step self-etching dual-polymerizing primer (ED Primer).4 It has been reported that inhibition of the polymerization of the luting agent (Panavia F) could occur due to the presence of acidic monomers within the ED Primer composition.4 However, as light activation was performed immediately upon luting, this effect is probably negligible.1 When Clearfil SE Bond was used prior to the application of the Panavia F system, increased bond strengths
The Journal of Prosthetic Dentistry
were observed. This observation can be attributed to the hydrophobic, filled adhesive layer that is applied over the self-etching primer and polymerized before application of ED Primer and the luting agent itself. This hydrophobic layer can reduce permeability of dentinal fluids between dentin and the luting agent.6 In addition, direct light activation of the adhesive system probably resulted in a better monomer conversion within the hybrid and adhesive layers, resulting in higher bond strengths. In the PS group, ED Primer was applied over Clearfil SE Bond to assist in the chemical polymerization of the luting agent. It might be expected that the acidity of ED Primer would not be buffered, as it did not contact the mineralized dentin surface; however, the presence of aromatic sodium sulphinate salts probably reduced the concentration of acidic monomers.4 All of the other self-adhesive cements presented significantly lower values than the multistep systems RX, PS, and PF. The self-adhesive cements RelyX U100 (UC) and RelyX Unicem (UN) were developed by the same manufacturer and are marketed under the same name in some countries. According to the manufacturer, the only difference between these products is the delivery system. While UN requires an activator, triturator, and applicator, UC can be hand mixed. Bond strength values were not signifi-
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November 2009 cantly different between UC and UN; however, UN µTBS values were significantly lower than those of the multistep systems RX, PS, and PF. The quality of the dentin-adhesive-cement interface is closely related to the extension of monomer infiltration into the demineralized collagen network.7 Despite the low initial pH of UN (pH<2 in the first minute, according to the manufacturer), almost no demineralization and hybrid layer formation was observed on the dentin surface.1,16,29 This finding might be attributed to the high cement viscosity, which hinders the wetting and infiltrating of the dentin surface by the luting agent.1 These factors, along with the cement thixotropic properties, may also explain the better fitting of the indirect restoration to the tooth when such a luting agent is subjected to seating pressure.16 This fact is clinically relevant, considering that most indirect restorations must be cemented under slight digital pressure. At that moment, the multifunctional monomers having acidic phosphate groups are supposedly capable of demineralizing and infiltrating the substrate simultaneously. The primary polymerization can be initiated by either light exposure or an autopolymerizing reaction. The polymerization mechanism results in a highly crosslinked polymer with high molecular weight.15 Furthermore, an increase in pH from 1 to 7 is observed as a consequence of the reaction between phosphate groups and both alkaline filler particles and hydroxyapatite from enamel and dentin, to neutralize resin acidity.2,30 The pH neutralization results in water formation and a more hydrophilic cement, which enhances the cement’s wetting ability on the dentin surface and the cement tolerance to water. Water is crucial for self-adhesive luting agents to release hydrogen ions required for smear layer demineralization,8 and is also reused in the reaction between multifunctional acidic phosphate monomers and alkaline filler particles. Such a phenomenon is speculated to be
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responsible for a change in the nature of the cement from hydrophilic to hydrophobic, which is thought to improve adhesive stability. The favorable bond strength observed for UN has been attributed to the micromechanical retention and chemical interaction between monomer acidic phosphate groups and dentin/enamel hydroxyapatite.15 A large number of specimens failed prematurely during preparation for the microtensile bond strength test in the groups that presented the lowest bond strength values, mainly when SmartCem 2 and Maxcem were used. This finding corroborates the observations by Goracci et al,16 who also showed a great number of fractures occurring prematurely before testing. In the present investigation, pretest failures were not included in the statistical analysis. It is known that when calculations are based on specimens that survived preparation procedures, there is an overestimation of the bonding potential. However, if zero values are attributed, bond strengths are underestimated, as some amount of stress was probably necessary to produce the pretest failure.14,31,32 The low bond strengths recorded for the self-adhesive cements are probably related to the cements’ limited ability to demineralize and infiltrate dentin substrate.1,14,29 Despite their initial low pH, the higher viscosity of the self-adhesive cements, if compared to self-etching primers, may explain why no true hybrid layer is formed when applied to dentin.1,29 To promote a micromechanical interlocking with dentin collagen fibrils, these cements should be able to etch the substrate in a relatively short time, requiring optimal wetting properties to ensure a fast interaction with dentin.9 Luting of porcelain veneers with self-adhesive cements is not recommended by manufacturers. Despite the shorter working time, luting of a nonretentive preparation by use of self-adhesive cements should be avoided with these systems. The bonding ability of self-adhesive ce-
ments can be attributed, in part or primarily, to their ability to chemically interact with dentin hydroxyapatite.10,25 This observation can explain the high number of adhesive failures for the self-adhesive materials. Failure mode analysis revealed that for the self-adhesive cements, and for the self-etching system PF, the predominant type of failure was adhesive between the luting agent and dentin. For the multistep systems RX and PS, which presented the highest bond strengths, cohesive failures in dentin and cohesive failures in the luting agent were commonly observed. The self-adhesive cement GC presented favorable bond strengths, which were not significantly different from PF. The bonding mechanism of GC has been reported by the manufacturer to be based on the glass ionomer technology modified by exchanging polyacrylic acid with the acidic functional monomers 4-MET and phosphoric acid esters.19 Water in the cement composition of GC is expected to aid the conditioning reaction, reducing the time needed for interacting with the substrate. However, the relatively weak chemical bonding potential of 4-MET and the high molecular weight of the functional monomer are expected to contribute inadequately, within a clinically relevant time, to the supposed chemical reaction.10 The lowest bond strengths were observed when the self-adhesive cements MC, SC, and SET were used. According to the manufacturers, the self-etching capacity is attributed to the presence of different monomers in the luting agent formulation, such as GPDM in Maxcem, the hydrophilic monomer 4-MET in SmartCem 2, and methacrylated phosphoric esters in SET. Han et al30 reported low pH values for GC, MC, SC, and UN a few seconds after manipulation. However, after 48 hours, only UN presented a neutral pH (pH 7.0). According to the authors, the pH reported 48 hours after polymerization was 2.4 for MC, 3.6 for GC, and 4.0 for SC.
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Volume 102 Issue 5 Even though an initial low pH is important for etching of enamel and dentin, if a low pH is maintained for a long time, it can adversely influence the adhesion of the mixed cement to dentin.11,30 Most self-adhesive luting agents evaluated in the current study provided low bond strength values on dentin surfaces. Therefore, further studies evaluating the ultramorphological features of bonding interfaces created by such products over short and long terms are required to explain these unfavorable results.
CONCLUSIONS Despite the favorable technique associated with their use, self-adhesive cements should be avoided when bonding indirect restorations, such as veneers, to nonretentive preparations.
REFERENCES 1. De Munck J, Vargas M, Van Landuyt K, Hikita K, Lambrechts P, Van Meerbeek B. Bonding of an auto-adhesive luting material to enamel and dentin. Dent Mater 2004;20:963-71. 2. Reich SM, Wichmann M, Frankenberger R, Zajc D. Effect of surface treatment on the shear bond strength of three resin cements to a machinable feldspatic ceramic. J Biomed Mater Res B Appl Biomater 2005;74:740-6. 3. Hikita K, Van Meerbeek B, De Munck J, Ikeda T, Van Landuyt K, Maida T, et al. Bonding effectiveness of adhesive luting agents to enamel and dentin. Dent Mater 2007;23:71-80. 4. Mak YF, Lai SC, Cheung GS, Chan AW, Tay FR, Pashley DH. Micro-tensile bond testing of resin cements to dentin and an indirect resin composite. Dent Mater 2002;18:60921. 5. Frankenberger R, Krämer N, Petschelt A. Technique sensitivity of dentin bonding: effect of application mistakes on bond strength and marginal adaptation. Oper Dent 2000;25:324-30. 6. Carvalho RM, Pegoraro TA, Tay FR, Pegoraro LF, Silva NR, Pashley DH. Adhesive permeability affects coupling of resin cements that utilise self-etching primers to dentine. J Dent 2004;32:55-65.
7. Hashimoto M, De Munck J, Ito S, Sano H, Kaga M, Oguchi H, et al. In vitro effect of nanoleakage expression on resin-dentin bond strengths analyzed by microtensile bond test, SEM/EDX and TEM. Biomaterials 2004;25:5565-74. 8. Tay FR, Pashley DH. Aggressiveness of contemporary self-etching systems. I: Depth of penetration beyond dentin smear layers. Dent Mater 2001;17:296-308. 9. Moszner N, Salz U, Zimmermann J. Chemical aspects of self-etching enamel-dentin adhesives: a systematic review. Dent Mater 2005;21:895-910. 10. Yoshida Y, Nagakane K, Fukuda R, Nakayama Y, Okazaki M, Shintani H, et al. Comparative study on adhesive performance of functional monomers. J Dent Res 2004;83:454-8. 11. Wang Y, Spencer P. Continuing etching of an all-in-one adhesive in wet dentin tubules. J Dent Res 2005;84:350-4. 12. Behr M, Rosentritt M, Regnet T, Lang R, Handel G. Marginal adaptation in dentin of a self-adhesive universal resin cement compared with well-tried systems. Dent Mater 2004;20:191-7. 13. Abo-Hamar SE, Hiller KA, Jung H, Federlin M, Friedl KH, Schmalz G. Bond strength of a new universal self-adhesive resin luting cement to dentin and enamel. Clin Oral Investig 2005;9:161-7. 14. Goracci C, Sadek F T, Fabianelli A, Tay FR, Ferrari M. Evaluation of the adhesion of fiber posts to intraradicular dentin. Oper Dent 2005;30:627-35. 15. Radovic I, Monticelli F, Goracci C, Vulicevic ZR, Ferrari M. Self-adhesive resin cements: a literature review. J Adhes Dent 2008;10:251-8. 16. Goracci C, Cury AH, Cantoro A, Papacchini F, Tay FR, Ferrari M. Microtensile bond strength and interfacial properties of self-etching and self-adhesive resin cements used to lute composite onlays under different seating forces. J Adhes Dent 2006;8:327-35. 17. Yang B, Ludwig K, Adelung R, Kern M. Micro-tensile bond strength of three luting resins to human regional dentin. Dent Mater 2006;22:45-56. 18. Piwowarczyk A, Bender R, Ottl P, Lauer HC. Long-term bond between dual-polymerizing cementing agents and human hard dental tissue. Dent Mater 2007;23:211-7. 19. Zicari F, Couthino E, De Munck J, Poitevin A, Scotti R, Naert I, et al. Bonding effectiveness and sealing ability of fiber-post bonding. Dent Mater 2008;24:967-77. 20. Kececi AD, Ureyen Kaya B, Adanir N. Micro push-out bond strengths of four fiber-reinforced composite post systems and 2 luting materials. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;105:121-8. 21. Wang VJ, Chen YM, Yip KH, Smales RJ, Meng QF, Chen L. Effect of two fiber post types and two luting cement systems on regional post retention using the push-out test. Dent Mater 2008;24:372-7. 22. Cantoro A, Goracci C, Papacchini F, Mazzitelli C, Fadda GM, Ferrari M. Effect of precure temperature on the bonding potential of self-etch and self-adhesive resin cements. Dent Mater 2008;24:577-83.
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23. Duarte S Jr, Botta AC, Meire M, Sadan A. Microtensile bond strengths and scanning electron microscopic evaluation of self-adhesive and self-etch resin cements to intact and etched enamel. J Prosthet Dent 2008;100:203-10. 24. Mazzitelli C, Monticelli F, Osorio R, Casucci A, Toledano M, Ferrari M. Effect of simulated pulpal pressure on self-adhesive cements bonding to dentin. Dent Mater 2008;24:1156-63. 25. Gerth HU, Dammaschke T, Züchner H, Schäfer E. Chemical analysis and bonding reaction of RelyX Unicem and Bifix composites--a comparative study. Dent Mater 2006;22:934-41. 26. Piwowarczyk A, Lauer HC, Sorensen JA. Microleakage of various cementing agents for full cast crowns. Dent Mater 2005;21:445-53. 27. Schenke F, Hiller KA, Schmalz G, Federlin M. Marginal integrity of partial ceramic crowns within dentin with different luting techniques and materials. Oper Dent 2008;33:516-25. 28. Ibarra G, Johnson GH, Geurtsen W, Vargas MA. Microleakage of porcelain veneer restorations bonded to enamel and dentin with a new self-adhesive resin-based dental cement. Dent Mater 2007;23:218-25. 29. Monticelli F, Osorio R, Mazzitelli C, Ferrari M, Toledano M. Limited decalcification/diffusion of self-adhesive cements into dentin. J Dent Res 2008;87:974-9. 30. Han L, Okamoto A, Fukushima M, Okiji T. Evaluation of physical properties and surface degradation of self-adhesive resin cements. Dent Mater J 2007;26:906-14. 31. Pashley DH, Carvalho RM, Sano H, Nakajima M, Yoshiyama M, Shono Y, et al. The microtensile bond test: a review. J Adhes Dent 1999;1:299-309. 32. Reis A, Loguercio AD, Azevedo CL, de Carvalho RM, da Julio Singer M, Grande RH. Moisture spectrum of demineralized dentin for adhesive systems with different solvent bases. J Adhes Dent 2003;5:183-92. Corresponding author: Dr Andre F. Reis Department of Operative Dentistry, Guarulhos University Rua Dr. Nilo Peçanha 81, Predio U, 6o. Andar Guarulhos, Sao Paulo BRAZIL 07011-040 Fax: +55 11 2464-1758 E-mail: [email protected] / reisandre@yahoo. com Acknowledgements The authors thank Dr Elliot Kitajima (University of São Paulo) for technical support with electron microscopy. Copyright © 2009 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications
The early bond strengths of self-adhesive cements varied among materials evaluated. Most self-adhesive cements produced lower bond strengths than conventional multistep luting agents and should be avoided when bonding indirect restorations to nonretentive preparations. Supported by FAPESP (The State of São Paulo Research Foundation), grants #2007/06083-4 and #2007/06447-6. Master student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos; Adjunct Faculty, Blumenau Regional University. b Master student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. c Master student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. d PhD student, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. e Assistant Professor, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. f Assistant Professor, Department of Operative Dentistry, School of Dentistry, University of Guarulhos. a
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November 2009 Bonding of resin-based composite materials to tooth hard tissues has been simplified recently. Until a few years ago, most adhesives were available in 3 application steps, which were later combined into 2 steps (etch and rinse or self etching) and, more recently, into a single self-etching application step. Indirect adhesive procedures constitute a substantial portion of esthetic restorative procedures. Until recently, all luting agents required the application of one of these adhesive systems, either self etching or etch and rinse, to prepare the tooth prior to cementation.1-4 The multistep application technique has been reported to be complex and sensitive because it also depends on the performance and technique sensitivity of etch-and-rinse and self-etching adhesive systems, which can influence bonding effectiveness.5-11 However, a new type of luting material has been developed that does not require any pretreatment of the tooth surface, the so-called self-adhesive cement.12-14 This material aims to combine the favorable properties of conventional (zinc phosphate, glass ionomer, and polycarboxylate cements) and resin luting agents.15 After the first self-adhesive cement was introduced commercially (RelyX Unicem; 3M ESPE, St. Paul, Minn), it rapidly gained popularity among clinicians due to its simplified “mistakefree” application technique. Thus, several manufacturers developed selfadhesive cements. A number of studies have evaluated the bond strength and characteristics of the first material of this new class of luting agent and compared it to currently available multistep luting agents.1,13,14,16-25 Dentin bond strengths comparable to those provided by the multistep luting agents have been reported, while lower bond strengths were found on enamel surfaces. However, a wide variety of self-adhesive cements is currently commercially available, and little information is available with regard to the bond strengths produced by the
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self-adhesive systems that were introduced subsequent to RelyX Unicem (3M ESPE), following the same intent.16,19,24 Despite the favorable dentin bond strength behavior reported for RelyX Unicem,26-28 bond strength can vary among materials due to differences in composition.24,29,30 The purpose of this study was to evaluate the microtensile bond strengths to dentin produced by 6 self-adhesive cements, and to compare them with conventional etchand-rinse and self-etching luting agents. The tested null hypothesis was that there would be no difference between the bond strengths to dentin produced by self-adhesive and conventional luting agents.
MATERIAL AND METHODS Fifty-four recently extracted caries-free third molars stored in 0.1% thymol (Symrise GmbH, Holzminden, Germany) solution at 4°C were used in this study. Teeth were obtained by protocols that were approved by the review board of the Guarulhos University (Guarulhos, São Paulo, Brazil). After disinfection and removal of soft tissues, flat coronal dentin surfaces were exposed with 600-grit SiC paper (3M of Brazil Ltd, Sumare, Brazil) under running water to create a standardized smear layer. Teeth were assigned to 9 experimental groups, which were treated with 1 of the 9 luting techniques (n=6). Six self-adhesive cements: RelyX Unicem (UN), RelyX U100 (UC), SmartCem 2 (SC), G-Cem (GC), Maxcem (MC), and SeT (SET), and 2 conventional luting agents, one that uses a 2-step etch-and-rinse adhesive (RelyX ARC, RX), and one that uses a 1-step self-etching adhesive (Panavia F, PF), were used in this study. An additional group included the use of a 2-step self-etching primer adhesive system (Clearfil SE Bond) prior to the application of Panavia F (PS). Luting agents were mixed and placed according to manufacturers’ instructions (Table I).
Five-mm-thick composite resin discs, 12 mm in diameter, were prepared by layering 2-mm-thick increments of a microhybrid composite resin (Filtek Z250, shade A2; 3M ESPE) into a silicone mold. Each increment was light activated (700 mW/mm2) for 40 seconds with a halogen light (Optilux 501; Kerr Corp, Orange, Calif ). One side of the composite resin discs was abraded with 600-grit SiC paper (3M of Brazil Ltd) under water cooling to create a flat surface with standardized roughness. The composite surface was airborne-particle abraded with 50-µm aluminum oxide particles (Asfer Indústria Química Ltda, São Caetano do Sul, Brazil) for 10 seconds. Before luting procedures were performed, the composite resin discs were ultrasonically cleaned in distilled water for 10 minutes, rinsed with running water, air dried, and silanated (RelyX Ceramic Primer; 3M ESPE). After application of the luting agent according to the manufacturer’s instructions, the composite resin disc was pressed on the cement using light pressure, after which excess cement was removed. Specimens were light activated for 40 seconds with the same halogen light from the buccal, lingual, and occlusal directions. Bonded specimens were stored in distilled water for 24 hours. Afterwards, teeth were serially sectioned perpendicular to the adhesive-tooth interface into slabs, and the slabs into beams with a cross-sectional bonded area of approximately 1 mm2 using a diamond saw (IsoMet 1000; Buehler Ltd, Lake Bluff, Ill).31 Beams were fixed to the grips of a universal testing machine (EZ Test; Shimadzu Corp, Kyoto, Japan) using a cyanoacrylate adhesive (Loctite Super Bonder Gel; Henkel, Düsseldorf, Germany) and tested in tension at a crosshead speed of 1 mm/min until fracture. Maximum tensile load was divided by specimen cross-sectional area to express results in units of stress (MPa). Five beams were selected from each restored tooth, and the average value for each tooth was used
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Table I. Cements, lot number, manufacturers, delivery system, composition, and application technique Manufacturer (Lot Numbers)
Delivery System (Cement)
Composition
Dual-polymerizing resin cement + 2-step etch-and-rinse adhesive system
RelyX ARC (GEHG) + Adper Single Bond 2 (8RW) 3M ESPE, St. Paul, Minn
Automatic dispenser, 2 pastes, hand mixed for 10 s
Etchant: 35% H3PO4 Adhesive: bis-GMA, HEMA, UDMA, dimethacrylates, ethanol, water, camphorquinone , photoinitiators, polyalkenoic acid copolymer, 5-nm silica particles Cement: bis-GMA, TEGDMA polymer, zirconia/silica filler
a (15 s); b (15 s); c; d; e; i (10 s); mix cement; apply mixture
Dual-polymerizing resin cement + 1-step self-etching adhesive
Panavia F (paste A, 00248C; paste B, 0026B) + ED Primer (primer A, 00255A; primer B, 00131A) Kuraray Medical, Inc, Tokyo, Japan
One-step self-etching adhesive + resin cement, dual polymerizing
Primer A: HEMA, 10-MDP, 5-NMSA, water, accelerator Primer B: 5-NMSA, accelerator, water, sodium benzene sulphinate Paste A: 10-MDP, silanated silica, hydrophobic aromatic and aliphatic dimethacrylate, hydrophilic dimethacrylate photoinitiator, dibenzoyl peroxide Paste B: silanated barium glass, sodium fluoride, sodium aromatic sulfinate, dimethacrylate monomer, BPO
h (A+B) (leave undisturbed for 60 s); mix cement; apply mixture; i (40s)
Panavia F (paste A, 00248C; paste B, 0026B) + Clearfil SE Bond (00788A) Kuraray Medical, Inc
Two-step self-etching adhesive + ED Primer + resin cement, dual polymerizing
f (20 s); e; g; i (10 s); h (ED Primer); e; mix cement; apply mixture; i (40 s)
2 pastes, hand mixed
Primer: MDP, HEMA, hydrophilic dimethacrylate, dl-camphorquinone, N,N-diethanol p-toluidine, water Bond: MDP, bis-GMA, HEMA, hydrophobic dimethacrylate, dl-camphorquinone, N, N-diethanol p-toluidine, silanated colloidal silica Paste A and Paste B: as described above
Dual-polymerizing self-adhesive resin cement
G-Cem (0702191) GC America, Inc, Alsip, Ill
Capsules, mechanically mixed 10 s
Powder: fluoroaluminosilicate glass, initiator, pigment Liquid: 4-META , phosphoric acid ester monomer, water, UDMA, dimethacrylate, silica powder, initiator, stabilizer
Automix cement; apply mixture; i (40 s) or j (5 min)
Dual-polymerizing self-adhesive resin cement
RelyX U100 (287269) 3M ESPE
Clicker dispenser 2 pastes, hand mixed
Base: glass fiber, methacrylated phosphoric acid esters, dimethacrylates, silanated sílica, sodium persulfate Catalyst: glass fiber, dimethacrylates, silanated sílica, p-toluene sodium sulfate, calcium hydroxide
Mix cement; apply mixture; i (40 s) or j (5 min)
Dual-polymerizing self-adhesive resin cement
Maxcem (2954635) Kerr Corp, Orange, Calif
Paste/paste dual syringe, direct dispensing through mixing tip
Resin: multifunctional DMAs, GPDM, proprietary Redox initiators and photoinitiators Filler: barium, fluoroaluminosilicate, fumed silica (66 wt%)
Automix cement; apply mixture; i (20 s) or j (3 min)
Dual-polymerizing self-adhesive resin cement
SmartCem 2 (0807311) Dentsply Caulk, Milford, Del
Paste/paste dual syringe, direct dispensing through mixing tip
Dual-polymerizing self-adhesive resin cement
SeT (50711292) SDI Ltd, Bayswater, Australia
Capsules, mechanically mixed for 10 s
Type
Dual-polymerizing resin cement + 2-step self-etching adhesive system
2 pastes, hand mixed
Application Technique
UDMA, di- and tri-methacrylate resins, phosphoric acid Automix cement; modified acrylate resin, barium boron apply mixture; fluoroaluminosilicate glass, organic peroxide i (40 s) or j (6 min) initiator, camphorquinone photoinitiator, phosphene oxide photoinitiator, accelerators, butylated hydroxytoluene, UV stabilizer, titanium dioxide, iron oxide, hydrophobic amorphous silicon dioxide Methacrylated phosphoric esters, UDMA, photoinitiator 67 wt% (45 vol%), fluoroaluminosilicate glass, pyrogenic silica
Automix cement; apply mixture i (20 s) or j (5 min)
Application technique = a: acid etch; b: rinse surface; c: dry with cotton pellet; d: apply 1-bottle adhesive; e: gently air dry; f: apply primer; g: apply adhesive; h: apply mixture; i: light polymerize; j: autopolymerize. Bis-GMA: bisphenol A diglycidyl ether methacrylate; 4-META: 4-methacryloyloxyethyl trimellitate anhydride; UDMA: urethane dimethacrylate; HEMA: 2-hydroxyethyl methacrylate; 10-MDP: 10-methacryloxydecyl dihydrogen phosphate; 5-NMSA: N-methacryloxyl-5-aminosalicylic acid; TEGDMA: triethylene glycol dimethacrylate; GPDM: glycerophosphate dimethacrylate
in the calculations. Bond strength values were statistically evaluated using a 1-way ANOVA and the Tukey’s studentized range HSD test (α=.05). Pretest failures were not included in the statistical analysis.31,32 Statistical analyses were performed using a
statistical software program (SAS for Windows V8; SAS Institute, Inc, Cary, NC). Failure modes were determined by examination of fractured specimens with a scanning electron microscope (SEM) (LEO 435 VP; LEO Electron
The Journal of Prosthetic Dentistry
Microscopy Ltd, Cambridge, UK). Specimens were mounted on aluminum stubs and gold-sputter coated (MED 010; BAL-TEC AG, Balzers, Liechtenstein) prior to viewing at different magnifications. Failure mode at the fractured interface was classi-
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November 2009 fied into 1 of 4 types: CD (cohesive failure in dentin), AD (adhesive failure between cement and dentin), CC (cohesive failure in the cement), or ADR (adhesive failure between the luting agent and composite resin). Instead of classifying failures as mixed, the area percentage of each type of failure
in each specimen was recorded.
RESULTS Mean (SD) µTBS values are presented in Table II. The ANOVA revealed a significant difference among groups (df=50; F=25; P<.01). For MC
and SC, only 5 and 4 teeth could be tested, respectively. The Tukey test revealed significant differences among the different luting techniques (P<.001). The multistep etch-andrinse system RX and the 2-step selfetching technique PS produced the highest bond strength values and
Table II. Dentin bond strength values for different dual-polymerizing resin cements Material (Number of Teeth)
Mean (SD)
Tukey
Pretest Failure/ Number of Beams
RelyX ARC + Single Bond (n=6)
69.6 (16.6)
A
0/30
Two-step self-etching adhesive/resin cement
Panavia F + SE Bond (n=6)
49.2 (9.7)
A
8/30
One-step self-etching adhesive/resin cement
Panavia F + ED Primer (n=6)
33.7 (13.9)
AB
0/30
Self-adhesive cement
G-Cem (n=6)
16.9 (10.3)
BC
0/30
Self-adhesive cement
RelyX U100 (n=6)
15.3 (3.4)
BC
0/30
Self-adhesive cement
RelyX Unicem (n=6)
12.5 (2.4)
C
3/30
Self-adhesive cement
Maxcem (n=5)
11.5 (6.8)
CD
20/30
Self-adhesive cement
SmartCem 2 (n=4)
8.5 (4.9)
CD
13/30
Self-adhesive cement
SeT (n=6)
4.6 (0.5)
D
3/30
Product Type Two-step etch-and-rinse adhesive/resin cement
Values are means (SD) in MPa. Values identified by different uppercase letters are significantly different (P<.05). RelyX ARC Panavia + SE Panavia F
CD
SeT
AD
SmartCem 2
CC
Maxcem
ADR
G-Cem RelyX U100 RelyX Unicem 0
20
40
60
80
100
Percent 1 Distribution of failure modes within groups. CD, cohesive failure in dentin; AD, adhesive failure between dentin and luting agent; CC, cohesive failure in resin cement; ADR, adhesive failure between luting agent and composite resin.
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A
B
2 Representative SEMs of most predominant failure mode of self-adhesive cements. A, Adhesive failure is observed for G-Cem (x60 magnification). B, At higher magnification (x1500 magnification), smear layer and smear plugs can be observed over dentin surface. were significantly different from the self-adhesive cements. The 1-step self-etching luting agent PF did not differ significantly from RX and PS. The self-adhesive systems GC and UC were not significantly different from PF, but were lower than RX and PS. Except for SET, no significant difference in bond strength was noted among all self-adhesive materials. The lowest bond strengths were recorded for SET, which were not significantly different from those of MC and SC. A high number of pretesting failures was recorded for MC (20) and SC (13). The distribution of failure modes among luting materials is shown in Figure 1. A representative image of an adhesive failure between the luting agent and dentin is shown in Figure 2. This was the predominant failure mode for the self-adhesive cements and for the self-etching system PF.
DISCUSSION In the present investigation, the bond strengths of 6 self-adhesive cements were compared with conventional multistep systems. An etchand-rinse and a self-etching cement were used as control groups, and an additional group was used for comparison, in which a 2-step self-etching primer adhesive system was used before the application of Panavia F. The null hypothesis was rejected, because significant differences in bond
strength values were observed among the different luting materials. The greatest advantage of self-adhesive cements is the easy and fast application technique, which is among the most desirable features in any dental material. The multistep application technique has been reported to be complex and sensitive, and can compromise bonding effectiveness.5 The self-adhesive systems presented bond strength values significantly lower than those presented by 2 of the control groups, RX and PS. When the 2-step self-etching primer adhesive Clearfil SE Bond was used prior to the application of the self-etching primer (ED Primer), bond strength increased. Panavia F bond strength values were not significantly different from RX or PS. However, no significant differences were observed between PF and the self-adhesive cements GC and UC. The lower bond strengths observed by PF, in comparison with PS, probably occurred because PF uses a 1-step self-etching dual-polymerizing primer (ED Primer).4 It has been reported that inhibition of the polymerization of the luting agent (Panavia F) could occur due to the presence of acidic monomers within the ED Primer composition.4 However, as light activation was performed immediately upon luting, this effect is probably negligible.1 When Clearfil SE Bond was used prior to the application of the Panavia F system, increased bond strengths
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were observed. This observation can be attributed to the hydrophobic, filled adhesive layer that is applied over the self-etching primer and polymerized before application of ED Primer and the luting agent itself. This hydrophobic layer can reduce permeability of dentinal fluids between dentin and the luting agent.6 In addition, direct light activation of the adhesive system probably resulted in a better monomer conversion within the hybrid and adhesive layers, resulting in higher bond strengths. In the PS group, ED Primer was applied over Clearfil SE Bond to assist in the chemical polymerization of the luting agent. It might be expected that the acidity of ED Primer would not be buffered, as it did not contact the mineralized dentin surface; however, the presence of aromatic sodium sulphinate salts probably reduced the concentration of acidic monomers.4 All of the other self-adhesive cements presented significantly lower values than the multistep systems RX, PS, and PF. The self-adhesive cements RelyX U100 (UC) and RelyX Unicem (UN) were developed by the same manufacturer and are marketed under the same name in some countries. According to the manufacturer, the only difference between these products is the delivery system. While UN requires an activator, triturator, and applicator, UC can be hand mixed. Bond strength values were not signifi-
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November 2009 cantly different between UC and UN; however, UN µTBS values were significantly lower than those of the multistep systems RX, PS, and PF. The quality of the dentin-adhesive-cement interface is closely related to the extension of monomer infiltration into the demineralized collagen network.7 Despite the low initial pH of UN (pH<2 in the first minute, according to the manufacturer), almost no demineralization and hybrid layer formation was observed on the dentin surface.1,16,29 This finding might be attributed to the high cement viscosity, which hinders the wetting and infiltrating of the dentin surface by the luting agent.1 These factors, along with the cement thixotropic properties, may also explain the better fitting of the indirect restoration to the tooth when such a luting agent is subjected to seating pressure.16 This fact is clinically relevant, considering that most indirect restorations must be cemented under slight digital pressure. At that moment, the multifunctional monomers having acidic phosphate groups are supposedly capable of demineralizing and infiltrating the substrate simultaneously. The primary polymerization can be initiated by either light exposure or an autopolymerizing reaction. The polymerization mechanism results in a highly crosslinked polymer with high molecular weight.15 Furthermore, an increase in pH from 1 to 7 is observed as a consequence of the reaction between phosphate groups and both alkaline filler particles and hydroxyapatite from enamel and dentin, to neutralize resin acidity.2,30 The pH neutralization results in water formation and a more hydrophilic cement, which enhances the cement’s wetting ability on the dentin surface and the cement tolerance to water. Water is crucial for self-adhesive luting agents to release hydrogen ions required for smear layer demineralization,8 and is also reused in the reaction between multifunctional acidic phosphate monomers and alkaline filler particles. Such a phenomenon is speculated to be
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responsible for a change in the nature of the cement from hydrophilic to hydrophobic, which is thought to improve adhesive stability. The favorable bond strength observed for UN has been attributed to the micromechanical retention and chemical interaction between monomer acidic phosphate groups and dentin/enamel hydroxyapatite.15 A large number of specimens failed prematurely during preparation for the microtensile bond strength test in the groups that presented the lowest bond strength values, mainly when SmartCem 2 and Maxcem were used. This finding corroborates the observations by Goracci et al,16 who also showed a great number of fractures occurring prematurely before testing. In the present investigation, pretest failures were not included in the statistical analysis. It is known that when calculations are based on specimens that survived preparation procedures, there is an overestimation of the bonding potential. However, if zero values are attributed, bond strengths are underestimated, as some amount of stress was probably necessary to produce the pretest failure.14,31,32 The low bond strengths recorded for the self-adhesive cements are probably related to the cements’ limited ability to demineralize and infiltrate dentin substrate.1,14,29 Despite their initial low pH, the higher viscosity of the self-adhesive cements, if compared to self-etching primers, may explain why no true hybrid layer is formed when applied to dentin.1,29 To promote a micromechanical interlocking with dentin collagen fibrils, these cements should be able to etch the substrate in a relatively short time, requiring optimal wetting properties to ensure a fast interaction with dentin.9 Luting of porcelain veneers with self-adhesive cements is not recommended by manufacturers. Despite the shorter working time, luting of a nonretentive preparation by use of self-adhesive cements should be avoided with these systems. The bonding ability of self-adhesive ce-
ments can be attributed, in part or primarily, to their ability to chemically interact with dentin hydroxyapatite.10,25 This observation can explain the high number of adhesive failures for the self-adhesive materials. Failure mode analysis revealed that for the self-adhesive cements, and for the self-etching system PF, the predominant type of failure was adhesive between the luting agent and dentin. For the multistep systems RX and PS, which presented the highest bond strengths, cohesive failures in dentin and cohesive failures in the luting agent were commonly observed. The self-adhesive cement GC presented favorable bond strengths, which were not significantly different from PF. The bonding mechanism of GC has been reported by the manufacturer to be based on the glass ionomer technology modified by exchanging polyacrylic acid with the acidic functional monomers 4-MET and phosphoric acid esters.19 Water in the cement composition of GC is expected to aid the conditioning reaction, reducing the time needed for interacting with the substrate. However, the relatively weak chemical bonding potential of 4-MET and the high molecular weight of the functional monomer are expected to contribute inadequately, within a clinically relevant time, to the supposed chemical reaction.10 The lowest bond strengths were observed when the self-adhesive cements MC, SC, and SET were used. According to the manufacturers, the self-etching capacity is attributed to the presence of different monomers in the luting agent formulation, such as GPDM in Maxcem, the hydrophilic monomer 4-MET in SmartCem 2, and methacrylated phosphoric esters in SET. Han et al30 reported low pH values for GC, MC, SC, and UN a few seconds after manipulation. However, after 48 hours, only UN presented a neutral pH (pH 7.0). According to the authors, the pH reported 48 hours after polymerization was 2.4 for MC, 3.6 for GC, and 4.0 for SC.
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Volume 102 Issue 5 Even though an initial low pH is important for etching of enamel and dentin, if a low pH is maintained for a long time, it can adversely influence the adhesion of the mixed cement to dentin.11,30 Most self-adhesive luting agents evaluated in the current study provided low bond strength values on dentin surfaces. Therefore, further studies evaluating the ultramorphological features of bonding interfaces created by such products over short and long terms are required to explain these unfavorable results.
CONCLUSIONS Despite the favorable technique associated with their use, self-adhesive cements should be avoided when bonding indirect restorations, such as veneers, to nonretentive preparations.
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The Journal of Prosthetic Dentistry
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