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.
The purpose of this study was to evaluate the microtensile bond strengths (μTBS) produced by different self-adhesive 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 mm 2 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).
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.
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)
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.
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. 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.
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. This material aims to combine the favorable properties of conventional (zinc phosphate, glass ionomer, and polycarboxylate cements) and resin luting agents. 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 “mistake-free” application technique. Thus, several manufacturers developed self-adhesive 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. 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 self-adhesive systems that were introduced subsequent to RelyX Unicem (3M ESPE), following the same intent. Despite the favorable dentin bond strength behavior reported for RelyX Unicem, bond strength can vary among materials due to differences in composition.
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 etch-and-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 ).
|Type||Manufacturer (Lot Numbers)||Delivery System (Cement)||Composition||Application Technique|
|Dual-polymerizing resin cement + 2-step etch-and-rinse adhesive system||RelyX ARC (GEHG) + AdperSingle Bond 2 (8RW) 3M ESPE, St. Paul, Minn||Automatic dispenser, 2 pastes, hand mixed for 10 s||
||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||
||h (A+B) (leave undisturbed for 60 s); mix cement; apply mixture; i (40 s)|
|Dual-polymerizing resin cement + 2-step self-etching adhesive system||
||f (20 s); e; g; i (10 s); h (ED Primer); e; mix cement; apply mixture; i (40 s)|
|Dual-polymerizing self-adhesive resin cement||G-Cem (0702191) GC America, Inc, Alsip, III||Capsules, mechanically mixed 10 s||
||Automix cement; apply mixture; i (40 s) orj (5 min)|
|Dual-polymerizing self-adhesive resin cement||
||Clicker dispenser 2 pastes, hand mixed||
||Mix cement; apply mixture; i (40 s) orj (5 min)|
|Dual-polymerizing self-adhesive resin cement||
||Paste/paste dual syringe, direct dispensing through mixing tip||
||Automix cement; apply mixture; i (20 s) orj (3 min)|
|Dual-polymerizing self-adhesive resin cement||
||Paste/paste dual syringe, direct dispensingth rough mixing tip||UDMA, di- and tri-methacrylate resins, phosphoric acid modified acrylate resin, barium boron fluoroaluminosilicate glass, organic peroxide initiator, camphorquinone photoinitiator, phosphene oxide photoinitiator, accelerators, butylated hydroxytoluene, UV stabilizer, titanium dioxide, iron oxide, hydrophobic amorphous silicon dioxide||Automix cement; apply mixture; i (40 s) orj (6 min)|
|Dual-polymerizing self-adhesive resin cement||SeT (50711292) SDI Ltd, Bayswater, Australia||Capsules, mechanically mixed for 10 s||Methacrylated phosphoric esters, UDMA, photoinitiator 67 wt% (45 vol%), fluoroaluminosilicate glass, pyrogenic silica||Automix cement; apply mixture i (20 s) orj (5 min)|
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/mm 2 ) 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 mm 2 using a diamond saw (IsoMet 1000; Buehler Ltd, Lake Bluff, Ill). 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 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. 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 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 classified 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.
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-and-rinse system RX and the 2-step self-etching technique PS produced the highest bond strength values and 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).
|Product Type||Material (Number of Teeth)||Mean (SD)||Tukey||Pretest Failure/Number of Beams|
|Two-step etch-and-rinse adhesive/resin cement||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|
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.