INTRODUCTION

The development of new adhesive systems has been prompted by the need to simplify single-bottle adhesives and provide faster, more user-friendly, and less technique-sensitive products.¹ Recently, multimode simplified adhesive systems with fewer steps, known as universal adhesives, have been launched as viable options for enamel and dentin bonding,^2,3 These adhesives can be used with the self-etching or etch-and-rinse strategy^3,4 and act as silane agents for ceramics and metals.⁵

Differences in composition and structure between enamel and dentin make the bonding strategy decision challenging. The etch-and-rinse strategy with phosphoric acid remains the gold standard for enamel. Studies on self-etching adhesives^6,7 indicate that the adhesives are usually not able to form a proper and uniform layer, resulting in lower bond strengths compared with etch-and-rinse systems.⁸ Dentin, however, has a higher organic content than enamel, and etching with phosphoric acid can increase collagen degradation by activating metalloproteinases and cathepsins. The use of self-etching adhesives could decrease this degradation phenomenon.^9-11

Recent studies with universal multimode adhesives have shown that the bonding strategy used—self-etching or etch-and-rinse—does not influence short-term bonding performance.^3,12,13 However, at a three-year follow-up, some marginal degradation was found for the self-etching strategy when used in noncarious cervical lesions (NCCLs).¹⁴

NCCLs are characterized by cervical dental tissue loss without the presence of caries. Their etiology is multifactorial and is often associated with dentin sensitivity and poor esthetics.¹⁵ Bonding to NCCLs is challenging because of the general absence of macromechanical retention and the presence of both enamel and dentin margins requiring different adhesive strategies. Therefore, this clinical study aimed to evaluate the bonding performance of a universal adhesive used according to the self-etching and the etch-and-rinse protocols in NCCLs and to compare the two protocols to their respective gold standard techniques. The null hypothesis tested was that no difference would be found between the two protocols after a two-year follow-up.

METHODS AND MATERIALS

This randomized, double-blind clinical study followed the CONSORT statement.¹⁶ The experimental design was a controlled, double-blind, randomized clinical trial and was conducted after approval from the local Institutional Review Board. This trial was registered in the Brazilian Clinical Trials Registry (ReBEC - www.ensaiosclinicos.gov.br) under the identification number RBR-4rw55d.

Participants

Fifty participants from the Department of Restorative Dentistry triage center were examined by four precalibrated operative dentistry professionals to determine whether they met the inclusion criteria (Figure 1). Teeth with NCCLs were selected by using a mouth mirror, an explorer, and a periodontal probe. Prior to participation in the study, all participants signed a written informed consent and received oral hygiene instructions.

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The study inclusion criteria were that the patient had to be in good general health, be at least 18 years old, have an acceptable oral hygiene level, and present with at least 20 teeth in occlusion. Participants were required to exhibit NCCLs with a depth of at least 1 mm and have both enamel and dentin margins with the cavosurface margin of at least 50% enamel. Also, the selected teeth had to be vital and not mobile.^12,13,17 Individuals with poor oral hygiene, severe bruxism, and severe or chronic periodontitis were excluded.

Restorative Procedures

Before placement of the restorations, the characteristics of the NCCLs were evaluated. The cavity dimensions in millimeters (height, width, and depth) and the geometry of the cavity (labeled at <90°, 90% to 135°, >135°) were evaluated from a photographic profile. Each profile was measured by computer software (Image J v1.52, Wayne Rasband, National Institutes of Health, Bethesda, MD, USA). The amount of sclerotic dentin was measured according to the criteria described by Swift and others¹⁸ (Table 1). Preoperative sensitivity was also evaluated by applying air for 10 seconds from a dental syringe placed 2 cm from the tooth surface (Table 1).

Table 1 Dentin Sclerosis^a and Preoperative Sensitivity Scales

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Participants who met the inclusion criteria received restorations bonded with one of the three adhesives and one of the two bonding techniques tested. Randomization was achieved by using a random list generated by a website (www.random.org) and allocated using sealed envelopes. The operators were not blinded to group assignment when performing the restorations; however, participants and examiners were blinded to the adhesive and technique used.

Treatment was performed according to a predetermined procedure, which included prophylaxis of the cavities using a rubber cup with pumice and water; shade selection with VITA Classical shade guide, and local anesthesia (Mepivalem AD 2%, DLA Pharmaceutical Ltda, Dentsply, Catanduva, SP, Brazil). No additional retention or bevels were made in the cavities. All procedures were carried out using the relative isolation method with cotton rolls, saliva aspirator, and gingival retraction cord (Ultrapak #000, Ultradent Products, Inc, South Jordan, UT, USA) in the gingival sulcus.

The teeth were etched with 37% phosphoric acid (Condac 37, FGM, Joinville, SC, Brazil) applied for 30 s to the enamel and 15 seconds to the dentin (depending on the technique used), followed by air-water spray for 30 seconds to remove the acid. Excess water was removed with absorbent paper, leaving a moist dentin surface. Subsequently, the cavities received the adhesive system applied using the two different techniques, etch-and-rinse or self-etch, according to the manufacturer's instructions. The compositions and application mode are described in Table 2.

Table 2 Compositions and Application Mode of the Adhesive Systems

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The cavities were restored with a direct restorative nanocomposite resin (Filtek Supreme, 3M ESPE, St Paul, MN, USA). Increments up to 2 mm in thickness were placed and light polymerized (750 mW/cm²; Emitter A, Guilin Woodpecker Medical Instrument Co, Guilin, China) for 20 seconds each. Final contour was achieved with a fine diamond rotary instrument (KG Sorensen, SP, Brazil) immediately after the restorative procedure. The restorations were polished with rubber points (Astropol F, Ivoclar Vivadent AG, SP, Brazil) seven days after placement.

Clinical Assessment

The restorations were evaluated using the FDI World Dental Federation criteria^19,20 as shown in Table 3. The most relevant items for testing the adhesive performance were selected. The primary measurable variable was restoration retention and fractures, followed by the secondary measurable variables: marginal staining, marginal adaptation, postoperative sensitivity, and recurrence of caries. Those measurable variables were ranked as (1) clinically very good, (2) clinically good, (3) clinically sufficient/satisfactory, (4) clinically unsatisfactory, and (5) clinically poor.

Table 3 FDI Criteria Used for Clinical Evaluation ²⁰

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The two calibrated examiners conducted the evaluations according to the periods: baseline (one week after the restorative procedures) and 6, 12, and 24 months after the restorative procedures. If there were disagreements between the two examiners in the individual analysis, they discussed the situation to reach a consensus. Both participants and examiners were unaware of the material used to perform the restorations, accordingly resulting in a double-blind study.

RESULTS

In this study, 152 restorations were placed in 34 participants, with a median of four restorations per participant (minimum one and maximum 10 restorations) and reevaluated after six months, one year, and two years. In the recall after two years, 29 participants were evaluated with 133 restorations. Details regarding the research participants and characteristics of the restored cavities are presented in Table 4.

Table 4 Baseline Characteristics of the Subjects and the Noncarious Cervical Lesions Included in the Study Groups

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None of the characteristics of the cavity influenced retention of the restoration (Table 5). Also, no statistically significant differences in esthetics or functional and biological evaluated criteria were found among adhesive systems and techniques (Table 6).

Table 5 Number and Percentage of Success for Each Studied Variable (Kruskal-Wallis)

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Table 6 Comparison of the Tested Adhesive Systems According to the FDI Criteria Compared by Kruskal-Wallis Test at p<0.05 for Each Recall Time ^a

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Table 6 Comparison of the Tested Adhesive Systems According to the FDI Criteria Compared by Kruskal-Wallis Test at p<0.05 for Each Recall Time ^a (ext.)

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The comparisons of each group at baseline and after each recall visit (6, 12, and 24 months) for each criterion are presented in Table 7.

Table 7 Percentage of Acceptable Restorations (Scores VG, GO, and SS) for Each Recall Time According to the Experimental Group Compared by Friedman Test and Least Significant Difference Test at p<0.05

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Table 7 Percentage of Acceptable Restorations (Scores VG, GO, and SS) for Each Recall Time According to the Experimental Group Compared by Friedman Test and Least Significant Difference Test at p<0.05 (ext.)

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DISCUSSION

The formation of NCCLs has been associated with occlusal factors²¹ and biocorrosive processes that alter dentin morphology and structure,²² directly influencing the adhesion and therefore the survival rates of these restorations. The success of restorations involving cavities without additional mechanical retention, such as NCCLs, is directly related to the adhesion strategy used.

Factors inherent to dentin, such as the presence of humidity, which varies according to the depth, presence of caries-affected tissue, degree of demineralization, and lack of retention provided by high tissue loss, have been associated with failure at the adhesive interface. Also, external factors, such as pH challenges, saliva, and thermal-mechanical stress, contribute to reduced longevity of the restorations.^23-25 Therefore, longitudinal clinical studies are preferable in evaluating the adhesive longevity of restorations as the materials are exposed to all the oral conditions and can be tested in similar conditions.

The adhesive strategy frequently used for dentin differs from that for enamel due to the inherent characteristics of the tissues. Often, the adhesives used in dentin are self-etching and provide less aggressive acid conditioning; this minimizes the collapse of collagen fibrils after drying, which leads to decreased bond strength.²⁶ Enamel, however, requires etching with phosphoric acid to increase retention.⁶ and currently, the so-called selective etching technique has been adopted for restoration of cavities involving both enamel and dentin. The adhesive systems chosen for this study, both etch-and-rinse and self-etching, represents the most popular systems and have been designated as gold standards in the literature. The universal system, however, was used in both adhesive strategies to evaluate its bonding performance longitudinally. The results of this study showed findings with regards to the evaluated criteria of retention, esthetics, and functional and biological factors. The null hypothesis was not rejected for a two-year follow-up of the two protocols.

The retention rates comparing the adhesion strategies and the adhesive systems used (Table 6) were not significantly different, showing that the universal system tested is a viable option for restoring NCCLs over the two-year term. However, most of the teeth restored in this study presented minimal or no dentin sclerosis. In addition, no significant differences (Table 5) were found between the success rate and the study variables (degree of dentin sclerosis, depth, height, width, angle of lesion), findings that were consistent with those of previous studies.^13,14,27

The action mode of universal adhesives is entirely different from that of the conventional self-etching and etch-and-rinse systems and can be applied over wet or dry dentin. The hybrid layer may not represent the primary mechanism of action in long-term dentin adhesion.²⁸ The success of universal adhesives regarding bonding is attributed to the presence of 10-methacryloyloxydecyl dihydrogen phosphate monomer, which is responsible for the chemical bonding between the adhesive and the dentin, creating a stable interface even without micromechanical retention.²⁹ This monomer also contains a phosphate radical in its molecule that, together with the carboxylate radical from the polyalkenoate copolymer in the universal adhesive, reacts with the calcium from hydroxyapatite to create stable bonds, termed “hydroxyapatite nanolayering.”^3,29 This nanolayering is essential for bonding, not only to dentin, but also to enamel, and might explain the good performance of this adhesive on enamel even without selective etching, as shown by the results of retention and marginal adaptation and discoloration (Table 6). The pH of the universal adhesive tested is claimed to be around 2.7 to 3.0.^14,29 This is claimed to be ultramild, and it is not strong enough to provide adequate etching of the enamel. This effective bond to enamel at two years has been previously reported.^3,13,27,29 The retention rate and marginal adaptation have been acceptable immediately and after 18 months. However, differences between the techniques have been reported after 36 months.¹⁴

The participants in this research were predominately women, aged between 41 and 60 years old, and with dentin sclerosis category 1 (Table 4). Canines and premolars were the most affected teeth, which is consistent with a previous report.³⁰ Due to its small occlusal surface and location in the arch, the premolar undergoes load concentration during mastication. This overload can be transmitted to the cervical region of reduced enamel volume, promoting microcracks and initiating the NCCL. Also, hypersensitivity was rarely reported, the lesion height was less than 4 mm, and the lesion angle was between 90° and 135°. The data do not confirm the evolution of the lesion or a relation to patient lifestyle.

The restoration of NCCLs should include an analysis of the etiologic factors and their removal, when possible. Not removing unwanted occlusal forces on the restored teeth will also decrease the success rate, factors clinicians should be aware of when treating patients. The Single Bond Universal, in both bonding strategies used in this study, proved to be useful compared with the etch-and-rinse and self-etch conventional adhesives for a two-year period. However, more extended evaluations are necessary to establish its long-term performance.

CONCLUSION

This two-year clinical evaluation demonstrated that the universal adhesive system tested, Scotchbond Universal, performed similarly in restoring NCCLs with minimal dentin sclerosis in both strategies used compared with the etch-and-rinse (Single Bond) or self-etching (Clearfil SE Bond) systems.