In Vitro Performance of Different Universal Adhesive Systems on Several CAD/CAM Restorative Materials After Thermal Aging
To evaluate the microshear bond strength (mSBS) of 10 universal adhesive systems applied on five different CAD/CAM restorative materials, immediately and after thermal aging. Five CAD/CAM materials were selected: 1) feldspathic glass ceramic (FeCe); 2) pre-polymerized reinforced resin composite (ReRC); 3) leucite-reinforced glass ceramic (LeGC); 4) lithium disilicate (LiDi); and 5) yttrium-stabilized zirconium dioxide (ZiDi). For each material, 15 blocks were cut into four rectangular sections (6 × 6 × 6 mm; n=60 per group) and processed as recommended by the respective manufacturer. For each indirect material, the following adhesive systems were applied according to the respective manufacturer’s instructions: 1) AdheSE Universal [ADU]; 2) All-Bond Universal [ABU]; 3) Ambar Universal [AMB]; 4) Clearfil Universal Bond [CFU]; 5) Futurabond U [FBU]; 6) One Coat 7 Universal [OCU]; 7) Peak Universal Bond [PUB]; 8) Prime&Bond Elect [PBE]; 9) Scotchbond Universal Adhesive [SBU]; 10) Xeno Select [XEN, negative control]. After the application of the adhesive system, cylinder-shaped transparent matrices were filled with a dual-curing resin cement (NX3) and light cured. Specimens were tested in shear mode at 1.0 mm/min (mSBS), after 24 hours and 10,000 thermal cycles (TC). All data were submitted to statistical analysis (α=0.05). For FeCe, there was no significant decrease in mean mSBS for AMB, FBU, and SBU after TC when compared at 24 hours. For ReRC, AMB and SBU showed higher mean mSBS when compared to CFU and XEN, after 24 hours and TC. For LiDi, FBU and OCU showed higher mean mSBS when compared to CFU and XEN, after 24 hours and TC. For LeGC, AMB and PUB showed higher mean mSBS when compared to XEN, after 24 hours and TC. For ZiDi, OCU and SBU showed higher mean mSBS when compared to XEN, after 24 hours and TC. In addition, PBE and XEN showed the lowest mean mSBS after TC with higher percentage of bond strength reduction. The mean mSBS among the different universal adhesives varied widely for each CAD/CAM material used. In addition, most universal adhesives underwent a statistically significant bond strength reduction after TC.SUMMARY
Objective
Methods and Materials
Results
Conclusions
INTRODUCTION
Computer-aided design and manufacturing (CAD/CAM) has become an extremely important part of dentistry during the last two decades, as a result of the advances in intra-oral imaging and manufacturing technologies.1–3 CAD/CAM technology has had a dramatic impact on several disciplines especially in prosthodontics and restorative dentistry, making tooth restoration easier and faster. In addition, it provides immediate chairside restorations without the risk of contamination with temporary cements and the need for provisional restoration.4,5
Within the current materials available for CAD/CAM restorations, glass-matrix ceramics and polycrystalline ceramics are the most widely used.3 Nevertheless, new CAD/CAM ceramics known as resin-matrix hybrid materials have been recently introduced. These include pre-polymerized reinforced resin composite materials and polymer-infiltrated ceramic network (PICN) materials,6,7 which have some mechanical properties similar to those of glass-matrix ceramics.7,8
The cementation procedure of a ceramic restoration is an important step in restorative dentistry for the long-term clinical success. This success depends on two interfaces, dental tissues with resin cement, but also resin cement with the specific restorative material.9 The intaglio surface of resin-matrix ceramic restorations can be treated with sandblasting or with hydrofluoric acid, followed by a silane coupling agent.10 On the other hand, sandblasting is not needed for glass-matrix ceramics,11 as adhesion is achieved through a combination of micromechanical retention upon hydrofluoric acid etching and chemical adhesion provided by the silane coupling agent.12 In the case of polycrystalline ceramics or glass-free oxide-based ceramics, hydrofluoric acid does not improve bond strengths, since these substrates do not contain a glass matrix.13 Hydrofluoric acid may be effective on zirconia, but it would require longer application time, higher concentration, and/or a higher temperature, which makes it clinically unfeasible.14,15
In this context, several protocols have been advocated to achieve durable adhesion to polycrystalline ceramics. Some of the newest protocols include primers or silanes mixed with functional monomers, such as 10-methacryloyloxydecyl dihydrogenphosphate (MDP), in order to increase the potential for chemical interaction (ie, Monobond Plus [MB+], Ivoclar Vivadent, Shaan, Liechtenstein).9 In addition, most universal adhesives also contain functional monomers such as MDP. The indications for these adhesive systems has been expanded to glass-matrix ceramics, polycrystalline ceramics, and metal alloys without the need for additional primers. However, a recent study showed that the presence of MDP in universal adhesives did not have a significant influence on the microshear bond strengths of several CAD/CAM materials in the immediate time.12
Another recent development in dental adhesion is the introduction of silane-containing universal adhesives (ie, Clearfil Universal Bond, Kuraray Noritake Dental, Tokyo, Japan) and Scotchbond Universal Adhesive (3M Oral Care, St Paul, MN, USA), in order to bond glass-rich (through silane) and glass-poor ceramics such as zirconia (through MDP). Although a universal adhesive with silane in the same solution is clinically useful, the effectiveness and long-term stability of silane contained in the universal adhesive is controversial.16,17
Accordingly, the effectiveness of universal adhesives with or without MDP, as well as universal adhesives containing silane on different CAD/CAM restorative materials, has not been extensively studied. In addition, there are few studies that investigated the effect of thermal aging on the interface between universal adhesives and different CAD/CAM indirect materials, and how substrates and universal adhesives behave after aging.
Thus, the aim of the present study was to evaluate the microshear bond strength of several universal adhesive systems on five different CAD/CAM indirect materials, after 24 hours of water storage or after 10,000 thermal cycles. The null hypotheses tested were: 1) a given universal adhesive would not result in different bond strengths between immediate time and after 10,000 thermo-cycles, in all five indirect materials; and 2) universal adhesives would not result in different bond strengths for each indirect substrate.
METHODS AND MATERIALS
Sample Size Calculation
To calculate the sample size, we considered the data of Scotchbond Universal (Scotchbond Universal Adhesive, 3M Oral Care), also known as Single Bond Universal in some countries. Means and standard deviations of bond strength of this adhesive system to zirconia as reported in the literature is 29.0 ± 1.1 MPa.18 According to the free website, www.sealedenvelope.com, the minimum sample size required was six ceramic blocks in each group in order to detect a difference of 7 MPa among the tested groups, using a two-sided test with an α of 0.05 and a power of 80%.
Specimen Preparation
Five CAD/CAM materials were selected: 1) feldspathic glass ceramic (FeCe, Vitablocs RealLife, VITA Zahnfabrik; Bad Säckingen, Germany); 2) prepolymerized reinforced resin composite (ReRC, Lava Ultimate CAD/CAM Restorative, 3M Oral Care); 3) leucite-reinforced glass ceramic (LeGc, IPS Empress CAD, Ivoclar Vivadent); 4) lithium disilicate (LiDi, IPS e.max CAD, Ivoclar Vivadent) and; 5) yttrium-stabilized zirconium dioxide (ZiDi, Ceramill Zi, Amann Girrbach, Koblach, Austria).
A total of 75 CAD/CAM blocks, 15 for each material, were used. For each material, the blocks (12 × 12 × 6 mm) were cut into four rectangular sections (6 × 6 × 6 mm; n=60 per group), using a diamond disk in slow speed (Isomet, Buehler, Lake Bluff, IL, USA) under water cooling.12 When applicable, the specimens were fired following the crystallization program recommended by the respective manufacturer. ZiDi specimens were sintered according to the manufacturer’s recommended protocol (Table 1).
Experimental Design
The specimens (n=60 for each indirect material) were randomly assigned (http://www.sealedenvelope.com) into 10 groups according to the adhesive system used: 1) Adhese Universal (ADU, Ivoclar Vivadent, also known as Tetric N-Bond Universal, Ivoclar Vivadent, Schaan, Liechtenstein); 2) All-Bond Universal (ABU, Bisco, Schaumburg, IL, USA); 3) Ambar Universal (AMB, FGM Prod Odont, Joinville, SC, Brazil); 4) Clearfil Universal Bond (CFU, Kuraray Noritake Dental); 5) Futurabond U (FBU, VOCO, Cuxhaven, Germany); 6) One Coat 7 Universal (OCU, Coltene, Altstätten, Switzerland); 7) Peak Universal Bond (PUB, Ultradent Products, South Jordan, UT, USA); 8) Prime&Bond Elect (PBE, Dentsply Sirona, Milford, DE, USA); 9) Scotchbond Universal Adhesive (SBU, 3M Oral Care, also known as Single Bond Universal in some countries); and 10) Xeno Select (XEN, Dentsply Sirona, also known as Prime&Bond One Select in some countries, Konstanz, Germany). XEN was used as a negative control, as the respective manufacturer does not recommend XEN for indirect restorations due to its low pH. The composition, application mode, and batch numbers are described in Table 2.
Microshear Bond Strength (μSBS)
The specimens were placed inside polyvinyl chloride (PVC) previously filled with acrylic resin (AutoClear, DentBras, Pirassununga, SP, Brazil), leaving a distance of 3 mm between the free surface of the ceramic and the top of the PVC ring. The description of the materials and their respective surface treatments are displayed in Table 1. Monobond Plus (MB+, silane + MDP solution, Ivoclar Vivadent) was used in all substrates prior to the application of universal adhesives, in order to standardize the experimental procedure. Then, the universal adhesives were applied according to the respective manufacturer’s instructions (Table 2). A single operator performed all bonding procedures.
Ten polyethylene transparent Tygon tubes (Tygon Medical Tubing Formulations 54-HL, Saint Gobain Performance Plastics, Akron, OH, USA) with an internal diameter of 0.8 mm and a height of 0.5 mm, which were positioned over the substrate. After that, in order to standardize the experimental procedure, an amine-free dual-curing resin cement (NX3, Kerr, Orange, CA, USA) was used to avoid the possible incompatibility of universal adhesives when in contact with dual-cured resin cements. This dual-curing resin cement was carefully packed inside each tube, and a clear Mylar matrix strip was placed over the filled Tygon tube and pressed gently into place. The resin cement was light cured for 20 seconds using an LED light-curing unit set at 1,200 mW/cm2 (Radii-cal, SDI Limited, Bayswater, Victoria, Australia). A radiometer (Demetron L.E.D. Radiometer, Kerr Sybron Dental Specialties, Middleton, WI, USA) was used to check the light intensity after every five specimens. These procedures were carried out under magnifying loupes.
After storage of the specimens in distilled water for 24 hours at 37°C the Tygon tubes were carefully removed with a blade, exposing the cement cylinders; each specimen was examined under a stereomicroscope at 10× magnification, the bonded cylinder was discarded if there was evidence of porosities or gaps at the interface. Half of the specimens were tested immediately, and the other half tested after 10,000 thermal cycles in distilled water between water baths held at 5 and 55 °C, with a dwell time of 1 minute.19
The specimens were attached to a shear-testing jig (Odeme Biotechnology; Joaçaba, SC, Brazil) and tested in a universal testing machine (Kratos IKCL 3-USB, Kratos Equipamentos Industriais; Cotia, São Paulo, Brazil). Each specimen was mounted in the universal testing machine and a thin orthodontic wire (0.2 mm diameter) was looped around the base of each composite cylinder. The orthodontic wire contacted the composite cement cylinder along half of its circumference. The setup was kept aligned (resin/substrate interface, the wire loop, and the center of the load cell) to ensure the correct orientation of the shear forces.20 The crosshead speed was set at 1 mm/min until failure.
The μSBS values (MPa) were calculated by dividing the load at failure by the surface area (mm2). After testing, the specimens were examined under an optical microscope (SZH-131, Olympus; Tokyo, Japan) at 100× magnification to define the location of the bond failure. The failure mode was classified as cohesive failure exclusively in resin cement (CR), cohesive failure exclusively in the ceramic or CAD/CAM indirect resin composite (CC), adhesive/mixed (A/M) failure at the cement/ceramic interface, which included cohesive failure of the ceramic and/or indirect resin composite, resin cement, and adhesive material. Also, the premature failures were recorded.
Statistical Analysis
The data were first analyzed using the Kolmogorov-Smirnov test to assess whether the data followed a normal distribution, and Bartlett’s test for equality of variances to determine if the assumption of equal variances was valid. After confirming the normality of the data distribution and the equality of the variances, the μSBS (MPa) data were subjected to appropriate statistical analysis. The μSBS of all specimens from the same individual indirect specimens were averaged for statistical purposes. Two-way ANOVA was used to analyze the μSBS data for each indirect material (adhesive vs storage time). After that, the Tukey’s posthoc test was used at α = 0.05.
RESULTS
For each indirect material, 30 cylinders were tested at each evaluation time. In the immediate time, the majority of specimens for all indirect materials showed adhesive/mixed failures (Table 3). For some materials there were some cohesive fractures of cement or in the indirect material (Table 3). However, after thermocycling, specimens showed adhesive/mixed failures for all indirect materials, with only a few cohesive fractures of cement or indirect material (Table 3). For ZiDi it is worth mentioning that 16.7% premature failures occurred with CFU, 40% with PBE, and 60% with XEN after thermocycling (Table 3).
For FeCe, the interaction between main factors were statistically significant (p<0.0001; Table 4). The application of ADU, ABU, AMB, and PUB resulted in statistically significant higher mean μSBS when compared with those of CFU, OCU, and XEN in the immediate time (p<0.0001; Table 4). However, after thermocycling, only AMB, FBU, and SBU showed higher mean μSBS when compared to the remainder of the universal adhesives tested (ADU, ABU, CFU, OCU, PUB, PBE, and XEN; p<0.0001; Table 4). XEN showed the lowest mean μSBS after thermocycling; its reduction of more than 40% of bond strength similar only to PBE (p<0.0001; Table 4). There was no significant decrease in mean μSBS for AMB, FBU, and SBU after thermocycling when compared to the immediate time (p>0.05; Table 4).
For ReRC, the interaction between main factors was statistically significant (p<0.0004; Table 5). In the immediate time, statistically higher mean μSBS were observed when ABU, AMB, FBU and SBU were compared with those of ADU, CFU, OCU, PUB and XEN (p<0.0004; Table 5). After thermocycling, AMB, OCU, and SBU resulted in statistically higher mean μSBS when compared to those of CFU and XEN, which showed 49.4% and 56.1% of bond strength reduction, respectively, after thermocycling (p<0.0003; Table 5).
When the universal adhesives were evaluated on the LiDi substrate, the interaction between main factors was statistically significant (p<0.0003; Table 5). In the immediate time, statistically higher mean μSBS were found for AMB, FBU, OCU, and PUB compared to those of CFU, PBE, and XEN (p<0.0003; Table 5). After thermocycling, FBU, OCU, and SBU showed higher mean μSBS when compared to CFU and XEN (p<0.0003; Table 5). CFU and XEN showed the highest percentage of μSBS reduction with 45.5% and 48.3%, respectively (Table 5).
For LeGC, the interaction between main factors was statistically significant (p<0.0004; Table 6). Statistically higher mean μSBS values in the immediate time were observed only for AMB, PUB, and SBU compared to those of OCU and XEN (p<0.0004; Table 6). After thermocycling, AMB, FBU, and PUB showed higher mean μSBS when compared to XEN, which showed the lowest mean μSBS after thermocycling (p<0.0004; Table 6).
The interaction between main factors was statistically significant (p<0.0001; Table 6) when the universal adhesives were evaluated in the yttrium-stabilized zirconium dioxide (ZiDi). In the immediate time, the highest mean μSBS values were measured for ADU, ABU, OCU, and SBU, which were statistically higher compared to those of XEN (p<0.00001; Table 6). After thermocycling, AMB, OCU, and SBU showed higher mean μSBS when compared to those of ADU, ABU, CFU, FBU, PUB, PBE, and XEN. However, two universal adhesives (PBE and XEN) showed the lowest mean μSBS after thermocycling with higher percentage of bond strength reduction (84.6% and 65.5%, respectively; p<0.00001; Table 6).
DISCUSSION
The main objective of the present study was to evaluate whether the mean μSBS of different universal adhesives would decrease after thermocycling when applied on materials used for indirect restorations. Although several universal adhesives are indicated for luting procedures, there are no published studies to our knowledge that have evaluated several universal adhesives applied on a wide number of materials used for indirect restorations. A recent meta-analysis of universal adhesives used for indirect procedures showed that the majority of the studies evaluated up to four universal adhesives (ABU, ADU, CFU, and SBU) applied to one or two indirect substrates.21 However, the majority of studies evaluated universal adhesives against lithium disilicate glass ceramics17,22,23 or yttrium-stabilized zirconium dioxide.17,18,24 Only a few studies have evaluated the bond strength to glass ceramics, leucite-reinforced25 or indirect composite.26 Therefore, it is relevant to test the longevity of several different universal adhesives in vitro applied on the recent CAD/CAM materials for indirect restorations.
Despite some exceptions (AMB, FBU, and SBU bonding to FeCe), all of the universal adhesives showed a reduction in mean μSBS for all indirect restorative materials when submitted to thermocycling. The reduction in mean μSBS after thermal fatigue may be associated with the small molecular size and high molar concentration of water, which allows penetration of the nano-size spaces between polymer chains or clusters around functional groups that are capable of hydrogen bonding.27 This phenomenon could result in a decrease in thermal stability and polymer plasticization.28 In addition, the temperature changes to which the specimens are subjected increase the coefficient of thermal expansion at the adhesive/ceramic interface leading to a premature failure and/or lower μSBS values, due to the loss of chemical retention.28 This could explain the number of adhesive/mixed failures for all indirect materials after thermocycling, with only a few cohesive fractures of cement or indirect material. For ZiDi it is worth mentioning that 16.7% of premature failures occurred with CFU, 40% with PBE, and 60% with XEN after thermocycling (Table 3).
However, the decrease in mean μSBS was not similar for all universal adhesives when evaluated in different indirect materials. Universal adhesives were developed based on the idea of several manufacturers to include acidic functional monomers, such as MDP, in the composition of these adhesives. The addition of acidic functional monomers provides them with a versatility to adhere to dental substrates of different characteristics, such as enamel and dentin, in addition to other substrates including glass-matrix ceramics, oxide-based ceramics, and metal alloys without the need for additional primers.25,29
However, some recent studies showed that the presence of MDP in universal adhesives did not have a significant influence on bond strengths when evaluated in the immediate time.12,30 In fact, several other acidic functional monomers, such as PENTA, MCAP and D3MA (Table 1), could be added to improve the bonding to direct and indirect materials.31
In a previous study, PUB, a MDP-free universal adhesive, was the only adhesive for which the mean μSBS reached the highest ranking of statistical significance among all adhesives for all five CAD/CAM indirect materials after 24 hours of storage in distilled water.12 At that time, the authors attributed these results to the higher viscosity of PUB compared to that of other universal adhesives, which might be responsible for better physical properties. However, in the current study, PUB showed an intermediary behavior in terms of μSBS after thermocycling, with a reduction of 35.8% when all indirect materials are evaluated together. This can be explained because it is not possible to find the presence of any functional monomer within the composition of PUB. Actually, methacrylic acid is a common component of several adhesives without any specific functionality in terms of bonding.32 According to the respective manufacturer, PUB contains diacetate of chlorhexidine in its composition, mainly because the addition of this compound helps prevent dentin bonding degradation in the adhesive interface in vitro.33 However, there is no evidence that the use of chlorhexidine could improve the bonding to indirect materials. Future studies need to be done to evaluate this hypothesis.
Two other MDP-free universal adhesives were evaluated in the present study: PBE and XEN. Both showed one of the highest reductions in mean μSBS (49.8% and 47.4%, respectively). Both materials contain PENTA or a phosphoric acid ester group in the functionalized monomer very similar to PENTA.34 Chen and others35 observed that primers containing 15 and 20 wt% PENTA increased the binding affinity with ZiDi, via the formation of Zr-O-P bond, when compared to a primer containing MDP. Unfortunately, the exact concentration of PENTA or derivates in PBE and XEN are a property of the respective manufacturer. However, when commercial universal adhesives containing PENTA were compared to commercial universal adhesives containing MDP, lower bond strength to Lidi and ZiDi were observed for the former.18,22,24 According to Elsayed and others,24 this could be related to the increased viscosity of the PENTA-containing primer as a result of the presence of five vinyl groups, which may hinder the ability of the primer to establish a strong chemical bond, mainly to ZiDi.35,36 This mechanism may explain the worst behavior of PBE and XEN in terms of bond strength reduction (84.6% and 65.5%, respectively) when evaluated on ZiDi. In general, intermediary results were obtained for MDP-free universal adhesives when compared to MDP-containing adhesives.
It is worth mentioning that PBE is the only acetone-based universal adhesive. Ethanol, water, or a mix of them are commonly used as solvents of universal adhesive. Acetone has a higher vapor pressure than ethanol and water, which may reduce the time required for evaporation compared to ethanol.32 A recent study showed that a longer evaporation time than that recommended by the respective manufacturer was required for PBE.37 This may occur because of the high acetone content in PBE, which may hinder an adequate solvent evaporation after application. This can leave residual solvent in the adhesive resin, which results in porosities in the cured adhesive layer,38 which has been corroborated by several authors when acetone-based universal adhesives were applied under indirect substrates.18,22,24
Also, the pH of universal adhesives seems to play an important role in the bond strengths to different substrates. XEN has the lowest pH among universal adhesives used in this study (pH=1.6)39 while PBE has a pH around 2.5.37 These factors might be responsible for the lower adhesion capability of PBE and XEN. However, it is worth mentioning that the respective manufacturer does not recommend XEN for indirect restorations.
We should also point out that an MDP-containing silane was applied on the indirect materials surface prior to the universal adhesives tested. Several studies have shown that the use of an MDP - containing silane improves the chemical interaction when associated with MDP-containing universal adhesives applied under glass ceramics, leucite-reinforced,25 lithium disilicate22,23 and yttrium-stabilized zirconium dioxide surfaces.18,40 For glass ceramics, methacrylate groups within the adhesive can copolymerize with silane molecules41 and silanol groups produced by the corresponding methoxy groups can react with the glass ceramic surface.42,43 In the case of yttrium-stabilized zirconium dioxide, MDP may also react with zirconia through hydroxyl groups present both on the MDP molecule and the zirconia surface.44–46 On the other hand, the application of a second adhesive coating may protect the surface of the MDP-containing silane, maintaining the bonding of silane to lithium disilicate22 and yttrium-stabilized zirconium dioxide.18
Despite the presence of MDP in the majority of universal adhesives evaluated, several differences were observed between MDP-containing adhesives evaluated. These differences could be explained based on the composition of the different adhesives. For example, considering the concept of versatility to different substrates, at least two silane-containing universal adhesives (ie, CFU and SBU) were launched in the market. According to the respective manufacturers, the silane helps improve the bonding to glass-matrix ceramics, such as feldspathic, leucite-reinforced, and lithium disilicate.
However, some concerns have been raised in the literature regarding this issue. It is generally understood that acidic pre-hydrolyzed silane coupling agents have a relatively short shelf life.47 This occurs due to the hydrolysis and self-condensation of silane being affected by the pH value of a solution. Generally, the silane used in dentistry has a pH between 4 and 5.48 On the other side, the pH of CFU is 2.3 and the pH of SBU is 2.7.12 Therefore, it is unlikely that the incorporation of silane increases the bond strengths for these universal adhesives.17 In fact, this reinforces the idea that a previous application of a silane-based primer is crucial for current silane-containing universal adhesives.18,22,43 According to Cuevas-Sanchez and others,21 there are currently three universal adhesives for which the respective manufacturers indicate that the use of a separate primer for adhesion to silicate ceramics is not necessary (CFU, FBU, and SBU).
However, a closer view of the results in our study showed a lower performance of CFU when compared with SBU in four of the five substrates after thermocycling, which is supported by several studies.42,49 This may be explained by the presence of silane and a high concentration of Bis-GMA in the composition of CFU (15%–35%) in comparison with the concentration of Bis-GMA in SBU (15%–25%). The two components coexisting in one bottle may have a negative influence on the efficiency of a universal adhesive. Chen and others50 reported that the incorporation of Bis-GMA monomer significantly inhibited the action of silane-containing porcelain primers and inhibited the chemical reaction between silane primer and glass ceramic.
In a recent study,17 nuclear magnetic resonance analyses showed that the spectra peaks of 9.90 ppm (Si-O-Si- group) indicates the formation of silane oligomers over time, which potentially impair the bonding performance. In this context, another study51 showed that low Si-O-Si peaks were registered in SBU, probably due to increased propensity for intermediate reactions between silane and the variety of -OH sources in SBU (2-HEMA, MDP, VP-copolymer, water, etc). Thus, even if the silane in SBU does not help improve the adhesive properties, the mixture of all other components is likely to maintain a good chemical interaction of SBU with indirect substrates. This might be the reason why SBU showed better results than CFU in four of the five indirect substrates in our study, with the exception of LeGC.
Although MDP-containing universal adhesives showed higher mean μSBS after thermocycling when compared to MDP-free universal adhesives, there is no consensus on which is the better universal adhesive to be used in all substrates, as per the results of our study. Unfortunately, the exact amount of MDP is a manufacturer’s trade secret. For instance, AMB, as well as SBU, showed higher mean μSBS even after thermocycling for four of the five indirect substrates, with the exception to LiDi. According to the manufacturer of AMB,52 MDP has a higher reactivity, which results from redistributing the concentrations between solvents, water, and acidic monomers. Nevertheless, this concept has not been proven. In addition, the ideal amount of MDP to enhance its interaction with indirect materials is not consensual.
In fact, there are some studies that evaluated the effect of different concentrations of MDP on bonding to zirconia. While Yoshida and others53 and Nagaoka and others46 reported increasing bond strengths to zirconia for higher concentrations of MDP up to 1 wt%, Chen and others54 showed that concentrations of MDP up to 10% showed higher bond strengths. However, the chemical affinity of MDP for zirconia is optimally achieved using 10 wt% MDP. On the other hand, Llerena-Icochea and others55 evaluated experimental adhesives containing 3 to 15 wt% MDP when applied to zirconia. The results showed that there was no significant correlation between the concentration of MDP in the experimental adhesives and mean bond strengths to zirconia. Future studies need to be carried out to evaluate the exact concentration of MDP needed in the primers and universal adhesives to improve the bonding interaction to indirect resin composite, glass-matrix ceramics, and yttrium-stabilized zirconium dioxide.
Thus, the first null hypothesis was rejected, because all universal adhesives underwent a statistically significant bond strength reduction between the immediate time and after 10,000 thermal cycles, except AMB, FBU, and SBU for FeCe substrate. The second null hypothesis was rejected, as the mean microshear bond strengths among the different universal adhesives varied widely for each CAD/CAM material used.
CONCLUSION
Factors such as pH, type of solvent, and the presence of silane and/or MDP in the composition of each adhesive seem to be important in choosing a universal adhesive system for each particular substrate.
Contributor Notes
Clinical Relevance
The performance of universal adhesives varied for different CAD/CAM materials. The results of this study may help clinicians elect the best adhesive system for each specific clinical case.