INTRODUCTION

Adhesive systems led to a significant revolution in restorative dentistry, which has provided dentists with the opportunity to offer pleasing, direct esthetic restorations, which are typically able to be completed during a single clinical appointment and with satisfactory mechanical properties.¹

Etch-and-rinse (ER) adhesive systems require previous dissolution of the mineral components in enamel and dentin with phosphoric acid before monomer infiltration into the dental substrates. For most dental adhesives, the depth and pattern of the etching play a significant role in the enamel bond strength values,^2,3 and in dentin, prior dentin demineralization with phosphoric acid is essential to expose collagen fibrils for resin infiltration.^4,5 Three-step and two-step ER adhesive systems have been extensively evaluated in vitro and in vivo, and, in general, the results have been quite satisfactory.^6-8 Regardless of the number of steps, the main disadvantage of the ER system is that there is a risk of collagen fibril collapse during drying of the demineralized dentin, which leads to a consequent decrease in bond strength.¹ To avoid this, demineralized dentin should be kept moist, which is a difficult task clinically since adequate moisture depends on the solvent employed in the material⁹ and on the individual's interpretation of the manufacturer's directions.¹⁰ This complex bonding protocol of ER adhesives may lead to clinical mistakes,^1,9,10 which may be clinically translated as postoperative sensitivity or adhesive failure after some years of function.

On the other hand, self-etch (SE) adhesive systems have a simpler bonding protocol.¹¹ The demineralization of the dental substrates is produced by a non-rinsing acidic primer, and except in the case of some SE systems,¹² the whole extension of the demineralized dentin depth is impregnated by resin monomers. Unfortunately, SE adhesives produce relatively low bond strength values and inferior marginal adaptation to enamel when compared to ER systems.^13,14 SE adhesives do not produce a retentive etching pattern on enamel, such as that produced by phosphoric acid, which may produce restorations with higher rates of marginal discoloration—a clinical problem in anterior restorations. Although selective etching of enamel margins prior to the application of SE adhesives^15,16 can minimize this limitation, an accidental dentin etching may occur and jeopardize bonding efficacy to dentin.^17,18

Considering that both adhesive strategies have advantages and limitations, their selection may vary depending on the clinical scenario. In light of this fact, more versatile adhesives, called multimode or universal adhesives, were developed recently. Theoretically, these universal adhesives are indeed SE adhesives that can be used with or without acid etching in both enamel and dentin.^19-23 Different research centers have shown that universal adhesives applied on dentin in both ER or SE strategies have high bond strength values,^20,21 and this can be attributed to the acidic monomers that promote chemical bonding to a tooth.^21,22

As these materials are relatively new in the literature, few clinical trials have been conducted to evaluate their clinical performance.^19,24,25 Most of the clinical data available are related to 10–methacryloyloxydecyl dihydrogen phosphate10-MDP–containing adhesives. According to an in vitro study,²³ only universal adhesives that contain 10-MDP showed high and stable dentin bonding after six months of water storage, but such a finding warrants clinical validation.

To the authors' knowledge, no clinical study has so far evaluated the clinical performance of a universal adhesive that contains functional monomers other than 10-MDP. Therefore, the objective of this double-blind, randomized clinical trial was to evaluate the influence of different application strategies on the clinical behavior of a new universal multimode adhesive (Xeno Select adhesive system) placed in noncarious cervical lesions (NCCLs) over the course of six months using the evaluation criteria of the World Dental Federation (FDI) and the US Public Health Service (USPHS). The null hypothesis tested was that bonding to NCCLs using the SE strategy—regardless of whether or not it is associated with selective enamel etching or using the ER strategy—applied on dry or moist dentin would result in similar retention levels over six months of clinical service.

METHODS AND MATERIALS

Eligibility Criteria

A total of 80 participants were examined by two calibrated dental students to determine if they met the inclusion and exclusion criteria (Figure 1). Those who qualified for the study were recruited in the order in which they reported for the screening session, thus forming a convenience sample.

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The evaluations were performed using a mouth mirror, an explorer, and a periodontal probe. Participants had to be in good general health, at least 18 years of age, have an acceptable oral hygiene level, and present with at least 20 teeth under occlusion. Participants were required to have at least four NCCLs to be restored in four different teeth. These lesions had to be noncarious, nonretentive, and deeper than 1 mm and had to involve both the enamel and dentin of vital teeth without mobility. The cavo-surface margin could not involve more that 50% of enamel.³²

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.

Interventions: Restorative Procedure

All of the patients selected for this study 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 were initiated.

The degree of sclerotic dentin from the NCCLs was measured according to the criteria described by Swift and others³³ (Table 1). The cavity dimensions in millimeters (height, width, and depth), the geometry of the cavity (evaluated by profile photograph and labeled at <45°, 45°-90°, 90°-135°, and >135°), the presence of an antagonist, and the presence of attrition facets were observed and recorded. Preoperative sensitivity was also evaluated by applying air for 10 seconds from a dental syringe placed 2 cm from the tooth surface and with an explorer. These features were recorded to allow comparison of the baseline features of the dentin cavities among experimental groups.

Table 1 Dentin Sclerosis Scale ³³

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In order to calibrate the restoration procedure, the study director placed one restoration of each group in order to identify all steps involved in the application technique. Then the two operators, who were resident dentists with more than five years of clinical experience in operative dentistry, placed four restorations, one in each group, under the supervision of the study director in a clinical setting. The restoration failures were shown to the operators prior to starting the study. At this point, the operators were considered calibrated to perform the restorative procedures.

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

Before restorative procedures, 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. Then shade selection was made using a shade guide. Following the guidelines of the American Dental Association (ADA),³⁴ no additional retention or bevel was prepared.

A rubber dam was placed, and then the NCCLs received the Xeno Select adhesive system (also known in some countries as Prime & Bond One Select, Dentsply) applied in different modes: an ER approach, keeping the dentin dry (ER-D) or moist (ER-M), and a SE approach with (SE-et) or without (SET) selective enamel etching, which defined the four different groups. The compositions, application modes, and batch numbers are described in Table 2. Some details of the restorative procedures include the following:

Table 2 Materials (Batch No.), Compositions, and Application Mode

Abbreviations: Bis-EMA, bisphenol A dimethacrylate; Bis-GMA, bisphenol A glycidyl dimethacrylate.

^a 

According to the manufacturer's instructions.

^b 

Manufacturer does not indicate application in dry dentin.

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  Etch-and-rinse/dentin dry group (ER-D)—The 37% phosphoric acid (Dentsply, Rio de Janeiro, Brazil) was applied on enamel (30 seconds) and dentin (15 seconds). Then cavities were rinsed thoroughly for 30 seconds and slightly air-dried for five seconds to dry dentin without causing dentin dehydration. The adhesive system was applied sufficiently, wetting all cavity surfaces uniformly, and was gently agitated on the entire enamel and dentin surface for approximately 20 seconds, according to the manufacturer's recommendations (Table 2). Then the adhesive was evaporated by gentle air thinning for five seconds and light-cured (Radii Cal, SDI, Victoria, Australia) for 10 seconds (1200 mW/cm²).

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  Etch-and-rinse/dentin moist group (ER-M)—All of the restorative procedures were similar to those described for the ER-D group. The only difference was that after acid rinsing, dentin was kept visibly moist.

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  Selective enamel etch group (SE-et)—The 37% phosphoric acid (Dentsply) was applied for 15 seconds only on enamel. Then it was rinsed thoroughly for 15 seconds and air-dried for five seconds, until the dentin was dried but not over dried. Following this, the adhesive was applied similarly to the way it was applied to the ER-D group.

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  Self-etch group (SET)—The adhesive system was applied as described in the ER-D group, without any previous acid etching. Then the adhesive was evaporated by gentle air thinning for five seconds and light-cured (Radii Cal, SDI) for 10 seconds (1200 mW/cm²).

After adhesive application, EvoluX (Dentsply) resin composite was used in up to three increments, each one being light cured (Radii Cal, SDI) for 30 seconds. The restorations were finished immediately with fine and extra-fine #2200 diamond burs (KG Sorensen, Barueri, SP, Brazil). Polishing was performed with rubber points (Enhance, Dentsply) one week after placement of the restorations.

Clinical Evaluation

Two experienced and calibrated dentists, not involved with the restoration procedures 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 but were not part of this project. An intraexaminer and interexaminer agreement of at least 85% was necessary before beginning the evaluation.³⁵ After recording the parameters during evaluation using a standardized paper case report form, the evaluation paper had to be sent back to the research staff, so that evaluators were blinded to group assignment during follow-up recalls.

The restorations were evaluated by FDI³⁶ and the classical USPHS criteria (adapted by Bittencourt and others³⁷ and Perdigão and others⁷) at baseline and after six months of clinical service. Only the clinically relevant measures for evaluation of the performance of adhesives were used and scored (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 spontaneous postoperative sensitivity was performed one week after the restorative procedure.

Table 3 World Dental Federation (FDI) Criteria Used for Clinical Evaluation³⁶

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Table 4 Modified US Public Health Service (USPHS) Criteria According to Perdigão and Others⁷ and Bittencourt and Others³⁷

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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 others³⁶ was used. Each evaluator outlined the extent of the observed event on the sketch of each restoration using a pen 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 of 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 guidelines³⁴ 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.

RESULTS

The restorative procedures were implemented exactly as planned, and no modification was performed. Forty-nine out of 80 patients examined for eligibility were not enrolled in the study because they did not fulfill the inclusion criteria. Thus, a total of 31 subjects (15 men and 16 women) were selected. One hundred and twenty-four restorations were placed, 31 for each group (Figure 1).

All baseline details relative to the research subjects and characteristics of the restored lesions are displayed in Table 5. The overall Cohen kappa statistics showed excellent agreement between the examiners in the six-month (0.94) follow-up. All research subjects were evaluated at baseline and at the six-month recall.

Table 5 Characteristics of the Research Subjects and the Noncarious Cervical Lesions (NCCLs) per Group

Abbreviations: ER-D, etch-and-rinse, dry dentin; ER-M, etch-and-rinse, moist dentin; SE-et, self-etch with selective enamel etching; SET, self-etch without selective enamel etching.

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Retention/Fracture

Fifteen restorations were lost or fractured after six months (one for ER-D, three for ER-M, five for SE-et, and six for SET). According to both evaluation criteria, the six-month retention rates (95% confidence interval [CI]) were 96.8% (83.8%-99.4%) for ER-M, 90.3% (75.1%-96.7%) for ER-D, 83.9% (67.4%-92.9%) for SE-et, and 80.7% (63.7%- 90.8%) for SET, with no statistical difference identified between any pair of groups (p>0.05; Tables 6 and 7).

Table 6 Number of Evaluated Restorations for Each Experimental Group Classified According to the World Dental Federation (FDI) Criteria³⁶

Abbreviations: ER-D, etch-and-rinse, dry dentin; ER-M, etch-and-rinse, moist dentin; SE-et, self-etch with selective enamel etching; SET, self-etch without selective enamel etching.

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Table 7 Number of Evaluated Restorations for Each Experimental Group According to the Modified US Public Health Service (USPHS) Criteria^7,37

Abbreviations: ER-D, etch-and-rinse, dry dentin; ER-M, etch-and-rinse, moist dentin; SE-et, self-etch with selective enamel etching; SET, self-etch without selective enamel etching.

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When the data from the six-month results from each group were compared with their baseline findings, a significant difference was found only for SET (p=0.03; Tables 6 and 7). Also, when the overall retention rate of the ER approach (93.6%, 95% CI 84.5%-97.5%) was compared with the overall retention rate of the SE approach (82.2%, 95% CI 70.9%-89.8%), significant differences in the retention rates were detected after six months (p=0.001; Tables 6 and 7).

Marginal Staining

Marginal staining was observed in three restorations (one for ER-M and two for SET) in both evaluation criteria. No significant difference was found between groups at six months or when the overall ER data were compared to the SE data. It was not detect any significant differences within each group when baseline and six-month data were compared for both criteria (p>0.05; Tables 6 and 7). When SQUACE³⁶ was used, we did not detect a statistical difference among groups at the six-month evaluation (p>0.05; Table 8).

Table 8 Number of Evaluated Restorations for Each Experimental Group According to the Semiquantitative Score (SQUACE)³⁶

Abbreviations: ER-D, etch-and-rinse, dry dentin; ER-M, etch-and-rinse, moist dentin; SE-et, self-etch with selective enamel etching; SET, self-etch without selective enamel etching.

^a 

Different letters indicate significant differences between groups (Kruskal-Wallis, p>0.05).

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Other Parameters

No restoration showed recurrence of caries after six months for either criteria. One restoration had clinical problems related to fracture at six months for both criteria. When the FDI criteria for “acceptable” vs “not acceptable” restorations were applied, 16 restorations were ranked as “not acceptable” (14 restorations were lost, one restoration fractured, and one showed severe lack of marginal adaptation). Among these 16 restorations, only five were placed with the ER approach, while the remaining 11 restorations were placed with the SE approach (Table 9).

Table 9 Restorations Acceptable or Not Acceptable According to the World Dental International (FDI) Criteria After Six Months³⁶

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DISCUSSION

Most of the studies using universal adhesives found in the literature were laboratory tests, although it is known that the validity of bond strength testing is questionable.^38,39 Clinical studies in NCCLs provide the ultimate proof for the evaluation of the performance of adhesive systems, mainly because the most important parameter for evaluation of restorations in NCCLs is retention.⁶ This is considered a true endpoint, because if restorations are lost, none of the other parameters can be evaluated.

The results of the present study showed that after six months of clinical service a total of 15 restorations failed as a result of debonding—11 bonded with the SE approach and four bonded with the ER approach (ER-M and ER-D)—which highlighted a poor bonding efficacy of the Xeno Select when used in the SE strategy. This poor bonding ability may be related to the kind of chemical bond produced by this adhesive with the dental substrates.

In general, SE adhesives differ from one another in many aspects, especially in their resin monomer composition, water content, and acidity.⁴⁰ They usually dissolve the smear layer and do not remove the dissolved calcium (Ca) phosphates.⁴¹ According to the manufacturer of the Xeno Select, two acidic monomers are responsible for the interaction of this adhesive with the dentin surface: an “inverse” functionalized phosphoric acid ester and an acryloylamino alkylsulfonic acid, which was also present in the predecessor Xeno V.⁴²

The manufacturer claims that the presence of a phosphoric acid ester group in the functionalized monomer makes it very similar to dipentaerytritolpentacrylate–phosphoric acid–monomer (PENTA). There is evidence (by Raman spectroscopy findings) that the ester group of PENTA can establish a covalent bond with the Ca from dentin and enamel.⁴³ However, not only chemical bonding is essential to provide a strong bonded interface; the Ca salt produced by this chemical interaction should also be stable in an aqueous environment,^44,45 which ultimately depends on the stability of the formed bond to Ca, according to the “adhesion-decalcification” concept.^41,46-48

Different functional monomers, such as “inverse” functionalized phosphoric acid ester and acryloylamino alkylsulfonic acid from Xeno Select, initially bond to Ca of hydroxyapatite, but they readily debond.^41,49 This debonding was recently confirmed by Zhou and others⁵⁰ Through the use of attenuated total reflection (ATR) spectroscopy, the authors evaluated the chemical interaction of functional monomers with dentin. The authors did not observe any signal of chemical bonding of the Xeno V monomers to dentin surfaces, indicating low affinity of Xeno V (the predecessor of Xeno Select) to this substrate.⁵⁰

It is also known that during interaction with the hydroxyapatite surface mild and ultramild SE adhesives leach little Ca, as they bind strongly enough to mineral content. On the other hand, intermediary strong or strong SE adhesives also bind to Ca ions from the hydroxyapatite, but they also demineralize the substrate more and tend to produce more soluble Ca salts.^46,47 According to the Xeno Select data, this adhesive system can be considered to be of intermediate strength (pH<2.0, manufacturer instructions), along with the predecessor Xeno V, which was classified as a strong SE adhesive system (pH=1.38 for Xeno V).^50,51

The PENTA ester group is considered a weak acid, but this low pH is likely obtained after addition of the acryloylamino alkylsulfonic acid to the adhesive formulation. According to the manufacturer's instruction, this second acidic monomer was added to the formulation to adjust the acidity of Xeno Select.⁴² Compared to other mild SE adhesives, the Xeno V produced cleaner dentin surfaces with more exposition of dentin tubules, likely due to the lower pH of Xeno V in comparison to that of the other mild SE adhesives tested.⁵⁰ Using field-emission scanning electron microscopy and ATR findings, these authors⁵⁰ also observed that the mild SE adhesives contained functional monomers with a higher affinity to dentin, likely due to the type of functional monomers presented in their composition.

This was also confirmed by a systematic review⁵¹ of clinical studies in NCCLs. The authors showed that the mild and ultramild SE systems contained functional monomers with the chemical potential to produce restorations with higher quality and durability in cervical lesions.⁵¹ This was also confirmed by the few clinical studies^19,24,25,52 that evaluated ultramild universal adhesives in NCCLs. Mena-Serrano and others²⁴ showed that after six months of clinical service, only three restorations were lost (94% retention rate), and this feature was very similar to the more recent publication of Lawson and others⁵² (one restoration lost; 97% retention rate). Even after two and three years of clinical service, retention rates as high as 89% to 93% were observed.^25,52

The good performance of composite restoration in cervical lesions from these studies^19,24,25,52 is likely due to the presence of the acidic functional monomer 10-MDP in the SE adhesives.^41,46-48 This functional monomer is responsible for the formation of a stable salt with the Ca in hydroxyapatite. The stability of this Ca salt has been correlated⁴⁰ with the high bond strength of MDP to enamel and dentin immediately and after long-term water storage.

When the six-month data of this material are compared with that of other 10-MDP–containing universal adhesives in the SE strategy, we clearly show that 10-MDP adhesives show higher retention rates (94%-97%)^19,24,25,52 than are reported in this clinical trial (80.7%), which reinforces that MDP might have an important role on the bonding performance of universal adhesives when used in the SE mode. Future studies comparing universal adhesives with and without MDP should be conducted.

It is also interesting to observe that even when the SE was applied after selective enamel etching (SE-et), the retention pattern did not improve significantly.^53,54 The clinical trials^53,54 that compare the benefits of selective enamel etching before application of SE adhesives do not report improved retention rates of composite resin restorations in NCCLs, and this finding has also been observed for one universal adhesive.^19,24,25 On the other hand, selective enamel etching with SE adhesives (SE-et) can reduce marginal discoloration at the restoration interface after medium/long-term clinical service.^53,54

The better performance of Xeno Select with the ER approach may be related to the fact that after phosphoric acid etching, the adhesive is no longer dependent on the chemical bonding produced by the acidic monomers with the dental substrates.⁵⁵ In this case, micromechanical bond is responsible for the good retention of the adhesive so long as the material produces a well-impregnated hybrid layer⁵⁵ and a strong polymer inside the hybrid layer.¹

In the present investigation, the results of Xeno Select when applied in wet (ER-M) and dry (ER-D) conditions showed similar results in terms of retention rates, as well as all other parameters evaluated. The similar results of the ER-M and ER-D groups were not surprising, as other clinical trials that compared wet and dry bonding with ER systems did not report higher retention rates for the wet bonding protocol,^56,57 mainly if the adhesive was applied actively,^57,58 as was done for Xeno Select (manufacturer's recommendation).

In the case of Xeno Select, this new adhesive system contains a special solvent named tert-butanol (2-methyl-2-propanol), which is able to re-expand collapsed collagen fibrils produced by air-drying. Indeed, during microscopic analysis of resin-dentin interfaces produced by an adhesive system (XP-Bond) containing tert-butanol, the formation of very similar hybrid layers and resin tags in dentin under dry and moist conditions was observed.⁵⁹ Although tert-butanol has a higher molecular weight than ethanol, the evaporation rate is almost the same, with a latent heat of vaporization of 41 kJ/mol for tert-butanol and of 42 kJ/mol for ethanol.⁶⁰ The vapor pressure of the different kinds of solvents at 20°C is given as 2330 Pa for water, 4133 Pa for tert-butanol, 5900 Pa for ethanol, and 23,300 Pa for acetone.⁶¹

Regardless of the bonding strategy used, the present study observed a significant deterioration of the marginal adaptation even at the short-term evaluation. These results are similar to those of earlier published articles^19,24,25 that compared universal adhesives using ER and SE approaches using a more sensitive criteria for evaluation (SQUACE). Although different clinical trials have shown that marginal discrepancies of a composite restoration usually develop rather rapidly,^19,24,25,52 most of the marginal defects were small and clinically acceptable.⁵¹

While there is a general consensus that marginal defects can affect the final performance of resin composite restorations, to the extent of our knowledge no study has so far observed an association between marginal defects and loss of retention. These defects can cause early marginal discoloration, which may jeopardize the restoration esthetics. Fortunately, restoration re-polishing can amend these discrepancies without causing any damage to the integrity of the restoration.⁶²

As a result of the efforts of the FDI, new criteria for evaluating dental restorations were published in 2007 and named the “FDI criteria.”³⁶ Since then, only a few publications have used the FDI criteria,^{19,24,25,63,64} as most clinical studies on NCCL restorations continue using the USPHS criteria.^6-8,52 Recent publications^19,24,25,63 that compared the six-month clinical behavior of several adhesion strategies using both FDI and USPHS-modified criteria concluded that the FDI criteria are more sensitive than the USPHS-modified criteria to small variations in the clinical outcomes 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. We opted to keep both criteria in the present study to allow comparison of the present results with those of studies that used both criteria.

Clinical trials have greater value when published after long-term follow-ups. However, the large number of failures in such a short-term follow-up justifies the publication of these data, as this can provide clinicians with further evidence before selecting a universal adhesive for purchase and use in their clinical offices. Additionally, these findings may also stimulate further studies based on this material, improving when they use the SE approach in their formulation, as Xeno Select did not fulfill the criteria for provisional acceptance according to the ADA guideline of having a failure rate of less than 5% after six months of clinical performance.³⁴ Future clinical recalls are already scheduled in order to assess the long-term performance of Xeno Select in NCCL restorations.

CONCLUSIONS

Within the limitations of this study, the six-month clinical behavior of Xeno Select Universal Adhesive (Dentsply) depends on the bonding strategy used. The universal adhesive did not fulfil the ADA criteria for full approval when used in the SE mode.