Introduction

The development of new adhesives and the improvement of the physical and chemical properties of composite resins markedly increased the demand for mercury-free, esthetic restorations in restorative dentistry [1, 2]. Because esthetics is a subjective concept, dental procedures should meet the patient’s esthetic expectations while also considering the functionality of the restoration and the characteristics of the material used [3].

Despite significant advances in adhesives technology, two features of dental resin-based esthetic restorations still require improvement: polymerization shrinkage and the development of polymerization shrinkage stress, which degrades the adhesive interface and creates cracks that compromise the quality and longevity of the restoration. Shrinkage stress can lead to marginal microleakage and adjacent caries lesions: the most common cause of failure for composite resin-based restorations [4]. Marginal defects, material and dental fractures, marginal staining, color changes, hypersensitivity, and patient dissatisfaction also cause failures that can lead to restoration replacement or repair [5,6,7,8,9].

The change from dimethacrylate (a base monomer of composite resins) to silorane has been an important strategy in reducing the effects of polymerization shrinkage. Because the polymerization of silorane occurs through a cationic ring-opening reaction, there is less shrinkage [10]. Additionally, the constant search for new adhesive strategies that improve the interfacial contact between teeth and adhesive has also provided solutions for this problem [11].

Different dental tissue treatments have also been tested to increase surface roughness, thereby improving interfacial contact between the tooth structure and adhesives [12]. As an alternative to acid etching, air abrasion with aluminum oxide has also been used for the pretreatment of enamel and dentin surfaces. This pretreatment promotes the mechanical removal of the smear layer and improves the adhesive infiltration in the demineralized dentin, resulting in substantially increased adhesive strength [13,14,15].

The clinical efficacy of air abrasion with aluminum oxide for silorane-based restorations has not been widely evaluated, particularly compared with the attention given to conventional adhesive [16]. Considering that the choice of the ideal surface treatment should be based on clinical trials results, the current study was conducted to investigate the clinical performance of silorane-based composite resin class I restorations with different surface treatments, at baseline and after 1 year. The null hypothesis tested was that different surface treatments do not affect the clinical performance of silorane-based class I restorations.

Materials and methods

Trial design

A prospective double-blind randomized and controlled clinical trial was designed. The experimental unit was the restoration, and the dependent variable was the qualitative categorical ordinal. This study was approved by the Institutional Ethics Committee (CAAE: 17810713.30000.5141) and registered and approved by the Brazilian Clinical Trials Registry (ReBec) (protocol RBR-2xbpff), being conducted according with CONSORT guidelines. Written informed consent was obtained from all participants.

Patient inclusion and exclusion criteria

This study included subjects who had premolars and molars with indications for class I restorations (active and cavitated occlusal caries lesions extending to dentin but free of proximal caries, according to bitewing radiography) and patients with class I restorations that were assessed as failed in at least one of the following clinical criteria: marginal adaptation, anatomical shape, marginal staining, surface roughness, and postoperative sensitivity or secondary caries. The included patients were older than 18 years of age and agreed to participate in all stages of the research.

Subjects with contraindications to dental treatment due to their medical histories, had xerostomia or were taking medication that substantially decreased the salivary flow, had abutment teeth for fixed or removable prostheses, and had teeth without occlusal relationship with natural dentition were excluded. Non-vital teeth and very deep cavity preparations were also excluded. Patients with visible plaque index higher than 33% were excluded. All participants received oral hygiene instructions and guidance for non-cariogenic diets.

Sample size calculation

Twenty-six patients were recruited from the Operative Dentistry Clinic at the FUNORTE Dental School, Montes Claros, Minas Gerais, Brazil. To calculate the sample size, a 50% difference between the groups was adopted, with a statistical power (1 – β) = 0.90 and error type I (α) = 0.05. The minimum number of teeth in each group should be 19. We decided to include 30 teeth in each group to compensate for patient loss to follow-up [17]. As the sampling unit was the tooth, some patients were allocated to more than one treatment group for each tooth that received a different treatment.

Randomization

The treatments were assigned to each tooth according to a sequence of random numbers generated by MS Excel software (Microsoft Corp., Redmond, WA, USA). After completing 30 random assignments to specific treatments, each group was excluded from subsequent allocations. A professional who was not involved in the intervention or clinical evaluations conducted the randomization process.

Treatment groups

To assess the effect of surface treatments on the clinical performance of silorane restorations, the following groups were tested: control 1/G1, silorane specific self-etching adhesive (P90 self-etch primer and bond) + silorane-based composite resin (Filtek™ P90 low shrink posterior restorative); G2, 37% phosphoric acid + silorane specific self-etching adhesive + silorane-based composite resin; G3, blasting with aluminum oxide + silorane specific self-etching adhesive + silorane-based composite resin; and control 2/G4, self-etching adhesive (Adper™ SE plus self-etch adhesive) + dimethacrylate-based composite resin (Filtek ™ P60 posterior restorative). Table 1 lists the chemical composition and manufacturers of the materials used in this study. Group G2 used 37% phosphoric acid prior to the silorane specific self-etching adhesive system according to the manufacturer recommendations (3M/ESPE); this protocol is optional.

Table 1 Chemical composition and manufacturers of materials used in the restorations
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Restorative procedures

Table 2 lists the clinical sequence of surface treatment and restorative procedures. To minimize operator bias, all restorative procedures were performed by only one operator, in accordance with the manufacturer’s instructions. Oblique increments (1.5 mm) of composite resins were inserted and light-cured (40 s). Coarse finishing was made using 3168 FF® diamond tips (KG Sorensen) immediately after completing the restorations, and the polishing was performed with a 9714 FF® carbide drill (KG Sorensen) and Enhance® system (Dentsply) 7 days after the restorative procedure.

Table 2 Clinical sequence of the surface treatment and restorative procedures
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Examinations

At baseline (after polishing) and after 1 year, two trained examiners (kappa ≥ 0.78) evaluated the restorations using the FDI World Dental Federation clinical criteria related to direct restorations [18]. The following parameters were evaluated: smoothness and brightness, surface staining, marginal staining, material fracture and retention, marginal adaptation, contour form and occlusal wear, patient opinion, postoperative hypersensitivity, secondary caries, and tooth integrity. For each criterion, the restorations were classified as one of the following conditions: clinically excellent; clinically good, does not need treatment; clinically satisfactory, acceptable effects; clinically unsatisfactory but repairable; and clinically bad, needs replacement.

Statistical analysis

The normality of errors and homoscedasticity of data were assessed for each response variable. Kruskal-Wallis tests compared the treatment groups for each evaluation (baseline and after 1 year), considering all listed criteria. Wilcoxon tests compared the groups among themselves, evaluating the classifications at baseline and after 1 year. The statistical tests considered all classifications in each criterion, but the restoration condition was characterized showing only the percentage of “clinically excellent” cases (the best condition). SPSS® version 20.0 (Statistical Package for the Social Sciences, IBM Company, Hong Kong, China) was used in these calculations, and a significance level of 5% was adopted.

Results

Thirty patients were invited to participate in the trial. However, after applying the inclusion and exclusion criteria, 26 patients were included in the study (18 women and 8 men). The patients were 18–62 years of age (mean = 32.15 years, standard deviation = 11.81). The number of restorations performed matched the number of restorations evaluated at baseline (141 restorations). However, 5 patients did not respond to the 1-year follow-up, and 18 sampling units were lost (Fig. 1).

Fig. 1
figure 1

A flowchart of the clinical trial

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The distribution of premolars and molars included in each treatment group at baseline is shown in Table 3. Table 4 shows the clinical performance of the restorations at baseline, according the surface treatment groups for each evaluation criterion. No significant difference was observed between the treatment groups (p > 0.05). Similarly, after 1 year, no significant difference (p > 0.05) was observed between the treatment groups for all evaluated clinical criteria (Table 5).

Table 3 Distribution of the teeth included in each treatment group at baseline (n/%) n = 141
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Table 4 Comparison among the treatments groups for each clinical criterion at baseline, with the frequency and percentage (n, %) of the “clinically excellent” classification in each criterion
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Table 5 Comparison among the treatments groups for each clinical criterion after 1 year, and frequency and percentage (n, %) of “clinically excellent” classification for each criterion
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In the comparison of the clinical performance of restorations between baseline and after 1 year, only the 37% phosphoric acid + silorane specific self-etching adhesive (G2) showed a significant reduction (p < 0.05) in marginal adaptation. No significant difference was observed in the other treatment groups and evaluation criteria (p > 0.05) (Table 6).

Table 6 Statistical analyses (Wilcoxon test) of the comparison within the same group during different periods (baseline and after 1 year) for the clinical criteria
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Discussion

In general, the different adhesion strategies tested exhibited similar clinical performance for the silorane-based composite in class I restorations, accepting the null hypothesis tested. However, a reduction in marginal adaptation was found when 37% acid phosphoric etching was used prior to the silorane specific self-etching adhesive. Approximately 50% of restorations in this group were classified as “clinically excellent.” Most restorations received clinically favorable ratings, demonstrating an acceptable clinical performance for all groups tested after 1 year. None of the restorations evaluated throughout the study required replacement, and only five needed to be repaired. Independent of the surface treatment used; these results may be due to the quality of the silorane-based composite, which exhibited good clinical performance [19].

The loss to follow-up was approximately 12%. This rate was comparable to those found in similar clinical studies, which usually recorded up to a 15% loss in 1-year recalls [20,21,22]. Even taking into account the loss to follow-up, the minimum sample size was exceeded in each group (19 restorations).

Despite advances in adhesive dentistry and improvements in the physical-chemical properties of dental composites, including an increase in the diversity of the types and brands of composites available on the market, it is necessary to determine their clinical performance. For this reason, clinical studies are the methodological gold standard for generating clinical evidence that is reliable and consistent with practice. This study demonstrated that all the surface treatments tested can be used for restorations of posterior teeth with silorane composites. After a 1-year follow-up, all treatment groups exhibited favorable clinical performance in terms of the evaluated criteria. However, a longer follow-up is required to confirm the long-term clinical performance of these treatments.

Moreover, a reduction in marginal adaptation was found for the group treated with 37% phosphoric acid prior to silorane specific self-etching adhesive. Resin composite direct restorations in posterior teeth are likely to develop cracks in the dental interface because the polymerization shrinkage of the resin and the stress generated by polymerization shrinkage promoting cuspal deflection [23, 24]. Thus, to minimize risks and avoid interpretation bias, the polymerization procedure was carefully performed. The restorations were made using the incremental layering technique, and the quality and intensity of the light emitted by the light-curing device were properly maintained. Therefore, these failures have been attributed to the polymerization shrinkage of the composite and to long-term adhesive degradation [13, 25]. Although this study is still in the initial phase, similar results have been reported by previous studies that detected imperfections in the marginal integrity and function of restorations evaluated from the first year of follow-up [20, 26]. Nevertheless, it is not plausible that the presence of dental caries related to the restoration may have affected the marginal integrity and marginal adaptation, as the short follow-up may not have been sufficient to allow the development of these lesions. Additionally, the lack of injuries can be attributed to the fact that patients with poor oral hygiene (IPV > 30%) and those with a low production of saliva were excluded from the study.

The clinical performance of dental restorations also depends on the patient behavior to maintain proper diet and hygienic habits, as well as the meticulous operative technique and professional skills that play a decisive role in the success of the restorative treatments [27]. Moreover, there is no consensus in the scientific literature regarding the criteria used to evaluate the clinical features of dental restorations or regarding the subjectivity of the process [29]. In this study, we opted to use the criteria proposed by Hickel et al. (2007) because these criteria are a consolidated tool composed of clinical and biological parameters that can be selected based on the specific objective of the study [18, 28,29,30].

Despite the satisfactory clinical results found for all clinical parameters evaluated, future studies and evaluations must be conducted in the medium and long term to affirm the effect of the surface treatments tested, primarily in deep cavity preparations. Overall, the surface treatments and the base monomers tested provided clinically acceptable resin composite restorations in posterior teeth achieving high rates of excellence. However, 37% phosphoric acid etching prior to the silorane specific self-etching adhesive can compromise the marginal adaptation.