A New Universal Simplified Adhesive: 18-Month Clinical Evaluation

The constant development of dental adhesive materials over the past decades has resulted in the launching of adhesives without reliable clinical validation. Often, a new version of a dental adhesive is introduced to the dental market when clinical studies with the previous version are still being carried out. This is increasingly common, especially with one-step self-etch adhesives. Ideally, once an adhesive is tested in vitro, a clinical trial must follow immediately to evaluate the clinical effectiveness of the tested adhesive.

Bonding to enamel is based primarily on micromechanical interlocking of resin monomers into the enamel microporosities created by chemical dissolution of hydroxyapatite crystallites using phosphoric acid.^1,2  For most dental adhesives, the depth of the etching pattern plays a significant role in the magnitude of the enamel bond strengths.³  Phosphoric acid etching significantly increases the bond strength of one-step self-etch adhesives to enamel.^4,5  However, when enamel is ground or beveled, self-etch adhesives tend to result in enamel bond strengths comparable with those of etch-and-rinse adhesives.^6,7  In class I and class II composite restorations, phosphoric acid–etched margins resulted in tighter marginal integrity and a lower degree of discoloration.⁸ 

Because self-etch adhesives do not etch enamel to the same depth that phosphoric acid does, selective etching of enamel margins has been recommended by some authors prior to the application of self-etch adhesives.^9,10  Manufacturers have also suggested selective enamel etching in the instructions for use of their self-etch adhesives. In noncarious cervical lesions (NCCLs), selective enamel etching did not increase the retention rate in clinical studies when a two-step self-etch adhesive with the ability for chemical bonding was used.^11,12  However, when compared with the self-etch mode, selective enamel etching resulted in an improvement in the enamel marginal integrity at 8 years.¹²  The potential drawback of selective enamel etching is that the clinician may inadvertently etch dentin. In fact, bond strengths may decrease when self-etch adhesives are applied on acid-etched dentin when compared with the same adhesive applied in the self-etch mode.^13,14 

Despite unreliable in vitro studies and clinical longevity results associated with one-step adhesives when compared with that of adhesives that rely on several steps,^15-17  multimode one-bottle universal adhesives have been developed recently to make the clinical procedure more user-friendly. These new adhesives can be used as self-etch or as etch-and-rinse adhesives. The concept behind these adhesives is novel; hence, only short-term clinical and immediate ultramorphological and bond strength studies have been reported.^18-20 

The aims of this randomized double-blind clinical trial were to study the influence of different application strategies on the clinical behavior of a new universal multimode adhesive (Scotchbond Universal Adhesive, SU, 3M ESPE, St Paul, MN, USA) placed in NCCLs, over the course of 18 months, using two evaluation criteria: World Dental Federation (FDI) and United States Public Health Service (USPHS) criteria. The null hypotheses tested were: 1) bonding to NCCLs using the self-etch strategy, associated or not with selective enamel etching or using the etch-and-rinse strategy, applied on dry or moist dentin, would result in similar retention rates over 18 months of clinical service, and 2) different evaluation criteria (FDI or USPHS criteria) would not result in different outcomes for the same data.

The experimental design followed the Consolidated Standards of Reporting Trials (CONSORT) statement.²¹  This was a randomized, double-blind clinical trial. The study was carried out in the clinic of the State University of Ponta Grossa (UEPG) School of Dentistry from January 2011 to November 2011. All participants were informed about the nature and objectives of the study, but they were not aware of what tooth received the specific treatments under evaluation.

The local Ethics Committee on Investigations Involving Human Subjects reviewed and approved the protocol and consent form for this study (protocol 05909/11). Written informed consent was obtained from all participants prior to starting the treatment. Based on preestablished criteria, 39 volunteers were selected for this study (Figure 1).

A total of 82 participants were examined by two precalibrated operative dentistry residents to check if they met the inclusion and exclusion criteria (Figure 1). The qualified patients were recruited in the order in which they reported for the screening appointment, thus forming a convenience sample.

The evaluations were performed using a mouth mirror, an explorer, and a periodontal probe. Participants had to be in good general health, be at least 18 years old, have an acceptable oral hygiene level, and present at least 20 teeth under occlusion. Participants were required to have at least four NCCLs in four different teeth that needed to be restored. These lesions had to be noncarious, nonretentive, deeper than 1 mm, and involve both the enamel and dentin of vital teeth without mobility. The cavo-surface margin could not involve more than 50% of enamel.²² 

All patients were given oral hygiene instructions before the operative treatment was performed. Patients with extremely poor oral hygiene, severe or chronic periodontitis, or heavy bruxism habits were excluded from the study.

All of the volunteer participants received dental prophylaxis with a suspension of pumice and water in a rubber cup and signed an informed consent form two weeks before the restorative procedures.

The features of the NCCLs were evaluated prior to the placement of the restorations. The degree of sclerotic dentin was measured according to the criteria described by Swift and others²³  (Table 1). The cavity dimensions in millimeters (height, width, and depth) and the geometry of the cavity (evaluated by profile photograph and labeled at <45°, 45°-90°, 90°-135°, >135°) were also recorded. Other features, such as the presence of attrition facets, were also observed and recorded. Preoperative sensitivity was evaluated by applying air for 10 seconds from a dental syringe placed 2 cm from the tooth surface. For the calibration procedure step, the study director placed one restoration of each group in order to identify all steps involved in the application technique. Then, all four operators, who were resident dentists with more than five years of clinical experience in operative dentistry, placed four restorations of each group under the supervision of the study director in a clinical setting. The restoration deficiencies were shown to the operators prior to starting the study. At this point, the operators were considered calibrated to perform the restorative procedures.

The same calibrated operators restored all teeth under the supervision of the study director. All subjects received a minimum of four restorations, one of each experimental group, in different lesions previously selected according to the inclusion criteria.

The randomization process within patients was performed using computer-generated tables by a staff member not involved in the research protocol. Details of the allocated group were recorded on cards contained in sequentially numbered, opaque, sealed envelopes. These were prepared by a staff member not involved in any of the phases of the clinical trial. The allocation assignment was revealed by opening the envelope on the day of the restorative procedure. The operator was not blinded to group assignment when administering interventions; however, participants were blinded to the group assignment.

Before placing the rubber dam, the operators anesthetized the teeth with a 3% mepivacaine solution (Mepisv, Nova DFL, Rio de Janeiro, RJ, Brazil) and cleaned all lesions with pumice and water in a rubber cup, followed by rinsing and drying. Using a shade guide, the proper shade of the composite was determined. Following the guidelines of the American Dental Association (ADA),²⁴  the operators did not prepare any additional retention or bevel.

The NCCLs received the SU adhesive system applied in different modes: an etch-and-rinse approach, keeping the dentin moist (ERm) or dry (ERd), and a self-etch approach with (Set) or without (SE) selective enamel etching. The compositions, application modes, and batch numbers are described in Table 2.

In the ERm group, dentin was kept visibly moist, while in the ERd group, dentin was air dried for five seconds but not overdried. In the Set group, the lesion was air dried after rinsing the etchant from the enamel. Dentin was kept dry in both the Set and SE groups. The adhesive was vigorously agitated on the entire dentin surface in all groups for approximately 20 seconds, according to the manufacturer's recommendations (Table 2). The brush was scrubbed on the dentin surface under manual pressure (equivalent to approximately 45 g or more) followed by gentle air thinning for five seconds and finally light curing (Radii Cal, SDI, Victoria, Australia) for 10 seconds (1000 mW/cm²).

Filtek Supreme Ultra (3M ESPE, St. Paul, MN, USA) resin composite was used in up to three increments, each one being light cured (Radii Cal, SDI, Victoria, Australia) for 30 seconds. The restorations were finished immediately with fine diamond burs (KG Sorensen, Barueri, SP, Brazil). Polishing was performed with rubber points (Astropol, Ivoclar Vivadent, Liechtenstein) one week after placement of the restorations.

The sample size calculation was based on the retention rate of the simplified etch-and-rinse Adper Single Bond (3M ESPE), the predecessor of this multimode adhesive from the same manufacturer. The retention rate was reported to be 94% at 18- to 24-month follow-ups.^25-30  Using an α of 0.05, a power of 80%, and a two-sided test, the minimal sample size was 50 restorations in each group in order to detect a difference of 20% among the tested groups.³¹ 

Two experienced and calibrated dentists, not involved with the placement of the restorations and therefore blinded to the group assignment, performed the evaluation. For training purposes, the examiners observed 10 photographs that were representative of each score for each criterion. They evaluated 10 to 15 patients each on two consecutive days. These subjects had cervical restorations, and they did not participate in this project. An intraexaminer and interexaminer agreement of at least 85% was necessary before beginning the evaluation.³² 

All parameters during evaluation were recorded using a standardized paper case report form. The evaluation paper was sent to the research staff after each observation, so that the evaluators were blinded to group assignment during follow-up recalls.

The restorations were evaluated by two criteria: the FDI criteria^33,34  and the classical USPHS criteria adapted by Bittencourt and others³⁵  and Perdigão and others³⁰  at baseline and after 6 and 18 months of clinical service.

For either of the two criteria, only the clinically relevant measures of performance for adhesives were evaluated. Therefore, wear and color match were not evaluated (Tables 3 and 4). The primary clinical endpoint was restoration retention/fractures, but the following secondary endpoints were also evaluated: marginal staining, marginal adaptation, postoperative sensitivity, and recurrence of caries. The evaluation of the postoperative sensitivity was performed one week after the restorative procedure by applying air for 10 seconds from a dental syringe placed 2 cm from the tooth surface.

These variables were ranked according to the criteria in the following scores: 1) FDI criteria (clinically very good, clinically good, clinically sufficient/satisfactory, clinically unsatisfactory and clinically poor) and USPHS criteria (alfa, bravo, and charlie). In the case of marginal staining and marginal adaptation, the semiquantitative criteria (SQUACE) proposed by Hickel and others was used.^33,34  Each evaluator outlined the extent of the observed event on a sketch of each restoration using a pen and according to defined criteria (marginal staining and marginal adaptation); after that, each margin was assessed quantitatively as a proportion of the total length of the margin.

Both examiners evaluated all the restorations once and independently. When disagreements occurred during the evaluations, they had to reach a consensus before the participant was dismissed.

The restoration retention rates were calculated according to the ADA guidelines.²⁴  Cumulative failure percentage = [(PF + NF)/(PF + RR)] × 100%, where PF is the number of previous failures before the current recall, NF is the number of new failures during the current recall, and RR is the number of currently recalled restorations.

The statistical analyses followed the intention-to-treat protocol according to CONSORT suggestion.²¹  This protocol includes all participants in their originally randomized groups, even those who were not able to keep their scheduled recall visits. This approach is more conservative and less open to bias.

Descriptive statistics were used to describe the distributions of the evaluated criteria. Statistical analysis for each individual item was performed, as well as for each overall parameter (FDI criteria). The differences in the ratings of the four groups after 18 months were tested with the Friedman repeated-measures analysis of variance by rank (α=0.05), and differences in the ratings of each group at baseline and after 18 months were evaluated using the McNemar test (α=0.05). Data from SQUACE was categorized into three scores: 1) marginal discrepancies involving less than 10% of the total length of the restoration, 2) between 10% and 30%, and 3) more than 30%, and the groups were compared with Kruskall-Wallis and Mann-Whitney nonparametric tests (α=0.05). Cohen's kappa statistic was used to test interexaminer agreement.

Forty-three of 82 patients were not enrolled in the study because they did not fulfill the inclusion criteria. Thus, 39 subjects (28 patients with four restorations and 11 patients with eight restorations) were selected. All baseline details relative to the research subjects and characteristics of the restored lesions are displayed in Table 5. The overall Cohen's Kappa statistics showed excellent agreement between the examiners at the 6-month (0.94) and 18-month (0.92) follow-up. All research subjects were evaluated at baseline and at six months, and only one patient did not attend the 18-month recall (moved to another city; Figure 1).

Four restorations were lost at six months (one for ERm and three for SE). One restoration was lost between the six- and the 18-month recall (one for Set). According to FDI and USPHS criteria, the 18-month retention rates (95% confidence interval) were 98% (90%-100%) for ERm, 100% (93%-100%) for ERd, 98% (90%-100%) for Set, and 94% (84%-98%) for SE. There was no statistical difference between any pair of groups at 18-month recall and for each group when baseline and 18-month times was compared (p>0.05; Tables 6 and 7).

Eight restorations had postoperative sensitivity at the 18-month recall using both the FDI and USPHS criteria (three for ERm, three for ERd, and two for Set) with no statistically significant difference when comparing different pairs of groups and for each group when baseline and 18-month times were compared (p>0.05; Tables 6 and 7).

Seventy-nine restorations were considered to have minor discrepancies in marginal adaptation at the 18-month recall using the FDI criteria (19 for ERm, 20 for ERd, 18 for Set, and 22 for SE). No significant difference was detected between any pair of groups at the 18-month recall for either criteria (p>0.05). However, a significant difference was detected when baseline and 18-month data were compared within each group (p<0.05). Despite these minor discrepancies, only nine restorations were considered to have clinically relevant discrepancies in marginal adaptation (one for ERm, five for ERd, and three for SE, p>0.05; Table 6). When the USPHS criteria were used, nine restorations were scored as bravo for marginal adaptation (one for ERm, five for ERd, and three for SE, p>0.05). No significant difference was detected between any pair of groups at the 18-month recall for either criteria and for each group when baseline and 18-month times were compared for both criteria (p>0.05).

When SQUACE^33,34  was used, there was a statistical difference among groups at the 18-month evaluation (p<0.007; Table 8). SE resulted in a significantly greater number of restorations, with more than 30% of the total length of the interface showing marginal discrepancy (14 restorations) in comparison with the other groups (four for ERm, three for ERd, and four for Set). Also, when baseline vs 18-month results were compared, SE was the only group for which a significantly greater percentage of the interface showed marginal discrepancy at 18 months (p<0.05).

For the USPHS modified criteria, nine restorations were classified as bravo for marginal adaptation (one for ERm, five for ERd, and three for SE). No significant difference was found between any pair of groups at the 18-month recall. Likewise, no significant difference was found for each group when baseline and 18-month marginal adaptation data were compared (p>0.05; Table 7).

Marginal staining was observed in 15 restorations (four for ERm, five for ERd, two for Set, and four for SE) according to FDI criteria. No significant difference was found between groups at 18 months and within each group when baseline and 18-month data were compared (p>0.05). For the USPHS-modified criteria, 11 restorations were classified as bravo for marginal staining (three for ERm, three for ERd, two for Set, and three for SE), and no significant difference was found between any pair of groups at the 18-month recall. No statistical difference was measured when the baseline and 18-month results were compared within each group (p>0.05; Table 7).

No restoration had clinical problems related to fracture and recurrence of caries at 18 months for either the FDI or the USPHS criteria. When the FDI criteria for “acceptable” vs “not acceptable” restorations were applied, only the five lost restorations were ranked as “not acceptable” (Table 9).

New criteria for evaluating dental restorations were published in 2007 and named the “FDI criteria,” as a result of the efforts of the FDI to organize them.^33,34  Nonetheless, only a few publications have used the FDI criteria since then.^20,36-38  Most clinical studies reporting clinical evaluation of NCCL restorations still use the USPHS criteria.^{23,25-30,39-42}  One study,³⁶  published as an abstract, concluded that the FDI criteria were more sensitive for identifying differences in the restorations than the USPHS criteria. A more recent publication²⁰  compared the six-month clinical behavior of several adhesion strategies using both FDI and USPHS-modified criteria. The findings suggested that the FDI criteria are more sensitive than the USPHS-modified criteria to small variations in the clinical outcomes when evaluating restorations of NCCLs. This finding was corroborated in the present study, as the marginal discrepancies were more frequently measured in the FDI criteria in relation to USPHS criteria.

Under the USPHS criteria, only nine restorations were scored as bravo after 18 months of clinical service, and no significant difference was detected among groups. This percentage is very similar to what was previously reported in clinical trials that used the USPHS-modified criteria after 18 months of evaluation.^{23,25,35,43,44}  On the other hand, marginal integrity was statistically worse for SE when assessed with the FDI criteria, specifically when SQUACE was used.

SU is considered an ultra-mild self-etch adhesive because its pH is relatively high (3.0; data not shown). This high pH may explain the significant deterioration of marginal adaptation from baseline to 18 months for SE, especially when compared with Set. The less pronounced etching pattern of SE when compared with the results of previous studies in which phosphoric acid was applied prior to self-etch application^45-47  may explain these findings. It is worth mentioning that the marginal discrepancies were typically observed in the enamel margins, and this occurrence was not deemed as a clinical failure as it can usually be solved by repolishing the restoration.^48,49 

Despite the use of two clinical evaluation criteria, the most important parameter for the evaluation of NCCL restorations has been retention rate. If the restorations are lost, all of the other criteria cannot be evaluated. In general, the clinical behavior of SU at 18 months in this study, regardless of the bonding strategy, was very good and comparable to that of the three-step etch-and-rinse adhesive Adper Scotchbond Multi-Purpose (3M ESPE) and that of the two-step etch-and rinse adhesive Adper Single Bond Plus (3M ESPE).³⁰ 

SU differs from Adper Single Bond Plus adhesive primarily by the incorporation of the 10-MDP monomer in the former, to provide acidity for its self-etching capability. Chemical bonding between 10-MDP and enamel/dentin may have resulted in stable interfaces even without micromechanical retention from etching in the SE group.^50,51  The success of 10-MDP has been reported in the literature. Its intrinsic chemical bonding, combined with the good mechanical properties and high conversion rate of a filled hydrophobic resin,^52,53  resulted in very good clinical behavior of Clearfil SE Bond (CSE, Kuraray, Osaka, Japan) at eight years.¹² 

The clinical behavior of SU in our study was also similar to that of Adper Easy Bond (AEB, 3M ESPE), a one-step self-etch adhesive, in a recent 18-month clinical study,⁴³  regardless of the application of a hydrophobic resin layer over AEB. This similarity in clinical behavior between these two adhesives from the same manufacturer may have to do with the comparable concentration of the polyalkenoic acid copolymer in both materials (1%-5%).^54,55  This copolymer may also provide chemical bonding derived from its spontaneous bonding to hydroxyapatite.⁵⁶  The polyalkenoic acid copolymer was first used in the composition of Vitrebond (3M ESPE) and therefore is also known as Vitrebond copolymer, or VCP. For self-etch adhesives, chemical bonding between polycarboxylic monomers (such as VCP) and hydroxyapatite plays a crucial role in their bonding mechanism.^57,58  More than 50% of the carboxyl groups in the polyalkenoic acid copolymer are capable of bonding to hydroxyapatite.⁵⁷  Carboxylic groups replace phosphate ions on the substrate and make ionic bonds with calcium.⁵⁷ 

Taking into account that SU contains two molecules with a potential for chemical bonding (10-MDP and VCP), the clinical behavior of SE and Set in our study may have been a result of: 1) the formation of a submicron micromechanical interlocking at the dentin surface by SU,⁵¹  2) the chemical bonding of both the 10-MDP monomer and VCP to hydroxyapatite, or 3) the protective effect of the Ca-MDP salt, which is a very hydrolytically stable salt.⁵⁹  However, it is worth mentioning that, although CSE resulted in nano layering within the hybrid layer and into the adhesive layer, SU resulted in nano layering only at the tubule orifices, where the adhesive infiltrated the residual smear layer.⁵¹  The difference between these two adhesives may rely not only on the higher concentration of 10-MDP in CSE, but also on the presence of hydroxyethyl methacrylate (HEMA) in SU, which may prevent interfacial self-assembled nanolayering.⁶⁰  Therefore, further data from subsequent clinical recalls should be compared with those of CSE¹²  to validate this compositional difference between SU and CSE. In fact, nano layering of two 10-MDP molecules may result in an adhesive interface more resistant to degradation.⁶⁰ 

At 18 months, there were no differences in postoperative sensitivity between any pair of groups in the present study. Nevertheless, it is noteworthy that SE did not result in any restoration with postoperative sensitivity. Other clinical studies in NCCLs have shown no difference in postoperative sensitivity between self-etch and etch-and-rinse adhesives.^30,61,62  Self-etch adhesives use part of the smear layer as the bonding substrate; therefore, the monomer-impregnated smear plugs serve as a barrier to prevent the fluid shift inside the tubules. It has been reported that self-etch adhesives result in a simultaneous demineralization and infiltration of the dentin substrate.¹²  Nevertheless, incomplete infiltration of dentin by mild self-etch adhesives has been reported,⁶³  because these adhesives have a reduced etching potential toward the base of hybrid layers.⁶³  On the other hand, etching dentin with phosphoric acid has been associated with reduced bond strengths with self-etch adhesives.^13,14  However, more recent studies have demonstrated that the effects of intentional dentin etching with phosphoric acid prior to the application of self-etch adhesives are material dependent.^64,65 

One-step self-etch adhesives are highly hydrophilic; therefore, they attract water and may increase the potential for degradation,^66,67  as water sorption of adhesive resins is proportional to their hydrophilic characteristics.⁶⁸  The self-etching ability of self-etching primers is achieved by incorporating sufficient water in the solution for adequate ionization of the acidic monomers without lowering the monomer concentration to a threshold that would compromise the bonding efficacy. Water is an important ingredient because it ionizes the acidic groups, allowing the formation of hydronium ions (H₃O⁺), which etch hydroxyapatite.⁶⁹  Water also facilitates solubilization of the reaction products resulting from the etching process. Increasing the water concentration dilutes the concentration of the acidic monomer and may decrease its bonding effectiveness. The presence of such hydrophilic moieties may induce increased water sorption and water uptake,⁷⁰  in turn jeopardizing the stability of the polymer network with time.

With this in mind, further studies should include the use of SU as a two-step self-etch adhesive by adding a hydrophobic resin layer as a second step in the bonding sequence. The laboratory and clinical success of mild two-step self-etch adhesives might also be a result of the presence of a hydrophobic bonding layer.^44,71  Nonetheless, adding a hydrophobic layer to one-step self-etch adhesives may not result in improved clinical performance for all simplified adhesives. A recent 18-month clinical trial reported that the clinical behavior of AEB did not change with the application of an extra layer of a bonding resin.⁴³  Therefore, the effect of the hydrophobic layer may depend on the specific composition of the one-step self-etch adhesive.

In addition to further recall evaluations already planned for this same project, bond strengths of aged dentin-resin interfaces with the same experimental groups are needed, as they may shed some light on the clinical behavior of SU over five years. While laboratory studies cannot always predict the clinical durability of bonded restorations, the dentin bond strengths of aged specimens seem to correlate with five-year clinical data.^17,72 

This clinical study has limitations, as 18 months is still a short period for evaluating the long-term clinical behavior of any dental adhesive. Nevertheless, SU belongs to a novel generation of simplified adhesives that are indicated for use under different application strategies, although they lack clinical data. Another limitation of the present study is that more than four restorations were placed in several patients, which may have caused a clustering effect. Despite being a common situation in the dental literature,^35,73  the influence of clustering on the data was not computed.

SU fulfilled the ADA guidelines²³  on the basis of the 18-month recall data, as this clinical study demonstrated no greater incidence of clinical failures than 10%, regardless of the bonding strategy used. We failed to reject the first null hypothesis, as there were no statistical differences in the clinical retention rates at 18 months for the different bonding strategies tested in this study. We have to partially reject the second null hypothesis, as significant differences were measured for marginal integrity when using the FDI criteria.

Within the limitations of this study, the 18-month clinical behavior of Scotchbond Universal Adhesive (3M ESPE) does not depend on the bonding strategy used. The new multimode adhesive fulfilled the ADA criteria for full approval when using all of the bonding strategies suggested by the manufacturer. The FDI evaluation criteria are more sensitive to small variations in the clinical outcomes than the USPHS criteria are, when evaluating restorations of NCCLs.

The materials were provided by 3M ESPE.

The authors of this article certify that they have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company that is presented in this article.