INTRODUCTION

Resin composites have increased in popularity in the clinical practice of dental professionals. The replacement of amalgam restorations with resin composite has been frequent, especially for esthetic reasons.¹ Despite the development of adhesive and restorative materials, posterior resin composite restorations still require extreme technical accuracy for their best performance. The use of incremental techniques, combined with photoactivation protocols, makes the restorative procedure relatively time consuming. Nevertheless, this care has been determined as essential for achieving restorative success.²

Posterior restorations, especially those involving proximal surfaces, still represent a challenge for the dental professional when re-establishing the contour and interproximal contact to resemble those of natural teeth³ since the most common restorative failures are observed in this region.⁴ Polymerization shrinkage stress is one of the causes of these failures, generating tension at the adhesive interface and compromising the bonding integrity over time.⁵ In addition, polymerization stresses make cavity margins more susceptible to leakage and the development of carious lesions.⁶

Therefore, there is a growing tendency to use materials that generate lower stress at the restorative interface⁷ while allowing restorations to be performed in a considerably shorter time with the advantage of reducing the technical sensitivity of the operative technique. Low-shrinkage flowable resin composites present a main advantage of minimal shrinkage stress during polymerization, with a high level of flow in the cavity, to facilitate handling of the material.⁷ These materials are presented as a restorative alternative for class II cavities because they can quickly replace lost dental tissue by inserting single increments of up to 4 mm thick.^8-10

Clinical studies with conventional resin composites indicate that up to 30% of patients report some discomfort or pain after receiving a restoration in posterior teeth, which is more common in class II restorations.^11,12 This sensitivity has been attributed to the stresses caused by the contraction of the resinous restorative material, leading to possible failures of marginal sealing,^13,14 which may favor the movement of fluids in the dentin-pulp complex or cause pulp injury.¹⁵

Currently, there are a few randomized clinical trials in the literature that have evaluated low-shrinkage resin composites with regard to the occurrence of postoperative sensitivity,^16,17 clinical performance of the material,^17-19 and maintenance of interproximal contact.

Thus, the objective of this study was to evaluate postoperative sensitivity, clinical performance, and interproximal contact after the placement of direct resin composite restorations in class II cavities, using three different restorative strategies. The null hypotheses tested were that there would be no difference among the three restorative systems at each evaluation period for the clinical parameters described above and that there would be no differences for the same restorative strategy at the different evaluation periods.

METHODS AND MATERIALS

Study Design

This clinical trial was a prospective, randomized, double-blind (volunteers and examiners), and split-mouth model. It was carried out after gaining approval from the local ethics committee (approval code: 1,235,100), and it was registered (#RBR-3gg3mg) and conducted according to CONSORT guidelines (Figure 1). Three restorative systems were used: conventional resin composite - considered the control group (PA: Peak Universal+Amelogen Plus, Ultradent, South Jordan, UT, USA), low-shrinkage flowable and nanoparticulate resin composites (ABF: Adper Single Bond 2+Filtek Bulk Fill Flow+Filtek Z350XT, 3M ESPE, St Paul, MN, USA), and low-shrinkage flowable and microhybrid resin composites, (XST: XP Bond+SDR+TPH3, Dentsply, Milford, DE, USA). The two last restorative systems were considered the test groups.

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Restorative Procedures

The cavity preparations were performed using spherical diamond burs (#1015-1017, KG Sorensen, Barueri, Brazil) that were replaced after every three procedures. When there was carious tissue, smooth spherical carbide burs (#1/2-4, Dentsply Maillefer, Ballaigues, Switzerland) were also used in a slow speed hand piece.

Prophylaxis with pumice and water was carried out, and rubber dam isolation was performed in all restorations. In order to evaluate the depth of the preparation in mm, a periodontal probe was used (#6 Satin Steel Handle, Hu-Friedy, Chicago, IL, USA) to measure the greatest depth of the preparation. This was done to limit the use of the low-skrinkage resin composites to the maximum thickness of 4-mm depth as well as to preserve 2 mm of space for inserting the covering resin on the occlusal surface. This measurement was performed in all preparations, even in those that would receive the restorations using the incremental technique.

Then 35% phosphoric acid gel (Ultra-Etch, Ultradent) was used for 30 seconds on enamel and 15 seconds on dentin. Thereafter, the preparation was rinsed with an air/water spray for 10 seconds. Subsequently, adhesive systems and restorative materials were applied, following the recommendations of the respective manufacturers. Table 1 presents the specifications for each group.

Table 1 Products (Material, Manufacturer, Composition, and Mode of Application) Used in This Study

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To restore the shape of the proximal walls, wooden wedges, preformed metal matrices, and rings (Unimatrix sectional matrix system, TDV Dental Ltda, Pomerode, Brazil) were used. Subsequently, the preparations were restored. In cases where the depth in the proximal box was greater than 6 mm, the low-shrinkage flowable resin composites were inserted in two increments, with the first increment being 4 mm thick. Otherwise, low-shrinkage flowable resin composites were inserted, reserving 2 mm of the restorative depth for the final occlusal resin composite. In order to compare the interproximal contacts of the restorations over time, care was taken to perform restorations so that the point of contact with the adjacent tooth was reconstructed with the bulk-fill resin. Adhesive and resin composites were light cured with a Valo curing light (Ultradent) in the standard application mode and an output of 1000 mW/cm².

After finishing the restorative procedures, the rubber dam was removed, and occlusal adjustments were made using fine and ultrafine diamond burs (#1190F, 3118F, 1190FF, and 3118FF, KG Sorensen). All restorations were finished using polishing points (Jiffy, Ultradent).

Evaluation

Two independent and calibrated examiners, neither of whom placed the restorations, were responsible for the clinical evaluations. The examiners were kept blind at all assessments.

At 24 hours, seven and 90 days, and six months after finishing the restorative procedures, the subjects were evaluated for postoperative sensitivity. The analysis included spontaneous pain as well as that caused by hot and cold thermal stimuli.

Isolation of the operative field during the evaluations was performed using cotton rolls. The cold test was performed with an anesthetic tube filled with frozen water and by placing the ice in direct contact with the restoration for 15 seconds. The hot thermal test was performed using gutta-percha (Dentsply Maillefer) directly heated by a lamp and placed in direct contact with the restoration for 15 seconds (temperature <60°C). Patients received a scale that was used to identify the pain intensity reported according to the modified visual analog scale²⁰ using scores ranging from 1 to 6: 1, no pain; 2, slight pain; 3, moderate pain; 4, a little worse pain; 5, very bad pain; and 6, the worst pain.

The clinical performance of the restorations was performed through visual and tactile inspection, using a flat dental mirror (SS White, Rio de Janeiro, Brazil) and a periodontal probe (#6 Satin Steel Handle, Hu-Friedy).

After 24 hours, six months, and one year, the restorations were evaluated using the modified US Public Health Service (USPHS) criteria,²¹ as described in Table 2. The tightness of the proximal contact was determined based on the resistance to dental floss (Sanifill, São Paulo, Brazil) between the restored surface and the adjacent tooth. The following scores were used: 0, no contact; 1, minimum contact; 2, ideal contact; 3, tight contact; and 4, very tight contact.²² In cases where more than one proximal surface was involved, the worst score of the two contacts was recorded.

Table 2 Modified US Public Health Service Criteria Rating System for Clinical Evaluation of the Restorations

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Statistical Methods

The kappa index was used to measure the degree of agreement between the two evaluators. The Friedman and Wilcoxon tests were used to evaluate postoperative sensitivity and interproximal contacts within each group, and the Kruskal-Wallis and Mann-Whitney tests were used within the same evaluation period. The equality test of two proportions was used to evaluate clinical performance.

Logistic regression analysis was performed to predict the probability of success of the clinical performance results at one year, using the characteristics cited in Table 3. Afterward, the Hosmer-Lemeshow test was performed to evaluate the efficacy of the logistic regression model. “A” and “B” scores were considered because there were only two observations for the “C” score (color and gingival tissue). All tests were performed at a significance level of 0.05%.

Table 3 Characteristics of the Cavities and Restorative Procedures

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RESULTS

The mean age of the 53 subjects was 48.3 years (±10.0), and all participants attended all of the evaluations. The oral health characteristics were (in percentage) VPI, 22.92 ± 20.3; GBI, 14.52 ± 18.8; and DMFT, 22.71 ± 3.91. A total of 65 molars and 94 premolars were restored. The characteristics of the preparations and the restorative procedures are described in Table 3.

There was a statistically significant agreement among the evaluators at the periods analyzed (p<0.001), showing that there was an excellent concordance of kappa (baseline=0.79, six months=0.91, one year=0.89).

Data regarding postoperative sensitivity are shown in Table 4. No statistically significant difference was observed at any of the evaluated periods among all groups (p>0.05). There was a statistically significant reduction in comparison to spontaneous sensitivity between the periods for all study groups when comparing the initial evaluation to the subsequent periods (p<0.05). Regarding cold stimulus, ABF was the only group that did not present a reduction in cold sensitivity over time (p>0.05). No statistically significant differences between periods were found for the hot stimulus (p>0.05).

Table 4 Means (Standard Deviation) of the Postoperative Sensitivities Experienced by Patients for All Groups and Evaluation Periods Using a Modified Visual Analog Scale^a

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The data from the USPHS criteria are presented in Table 5. When the analysis among groups was carried out, a statistically significant difference was observed for the following criteria: color, marginal discoloration, and superficial staining. A statistically significant color alteration was observed for PA when compared to ABF and XST at six months and one year. After one year, a low percentage of color alteration was observed for XST, which was statistically different from the other groups. Regarding marginal discoloration, the only difference observed was for ABF at six months, which presented a greater number of restorations with the bravo score when compared to the other groups.

Table 5 Clinical Evaluation of Resin Composite Restorations (US Public Health Service). Percentage Values of “A” Score and Numbers of “A,” “B,” and “C” Scores in Parentheses, Respectively^a

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When comparing the evaluation periods for each of the groups, a statistically significant difference was observed for marginal discoloration, marginal integrity, and superficial staining. With regard to marginal discoloration, a statistical difference occurred in both PA and XST after one year when compared to the other periods. ABF presented statistically significant differences in marginal discoloration between the baseline and the subsequent evaluations. Initial degradation of marginal integrity was observed for all groups; PA presented continuous and increased degradation over time, while ABF and XST presented differences only at the one-year follow-up. With regard to superficial staining, PA and ABF presented alterations over time; ABF staining was observed starting at the six-month evaluation period, while PA presented alterations only after one year.

All XST restorations presented an A score for superficial staining at six months, which was statistically different from the other groups. For the one-year evaluation, the highest number of bravo scores for superficial staining was observed for ABF, which was statistically different from the other groups.

The interproximal contacts data are shown in Table 6. When the performance of each group was compared over time, a significant reduction in the intensity of contacts for XST was observed after one year. There was no significant difference among the groups.

Table 6 Means (Standard Deviation) of the Interproximal Contacts for Groups and Evaluation Periods^a

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The probability of success after one year was observed in the following variables: number of surfaces for the retention, operator for the marginal integrity, sclerotic dentin for marginal discoloration, and mandibular molar for surface texture (p<0.05).

DISCUSSION

The present study represents a prospective, randomized, double-blind, split-mouth model, which provides the best evidence for a clinical trial.²³ With the split-mouth design, it is possible to analyze the test and control groups under the same conditions, where the three analyzed groups are present in the same subject, increasing the statistical efficiency and decreasing the amount of patients required for the study.²⁴ The method used for the analysis of postoperative sensitivity was the visual analog scale, which has been the most frequently used scale for this type of clinical trial.²⁵ According to the results found in our study, no differences were found among groups for postoperative sensitivity. These results are in accordance with another clinical study in which the sensitivity risk was not affected by the type of adhesive or the restorative strategy used.^16,26

The highest mean of sensitivity was found during the first 24 hours after the restorative procedure, which leads us to reject the first null hypothesis. The higher intensity of spontaneous sensitivity found in this period may be associated not only with the restorative materials used but also with the trauma generated during the cavity preparation and restorative procedures.^11,16,27

When the data obtained for cold sensitivity testing were analyzed, only one restorative system with a low-shrinkage flowable resin composite (ABF) maintained the same intensity of discomfort at all evaluation times. On the other hand, the other restorative system that used a low-shrinkage flowable resin composite (XST) resulted in a reduction of this type of sensitivity after 90 days. These data corroborate the findings of another clinical study that observed low postoperative sensitivity after 30 days in restorations performed with the same low-shrinkage resin used in the present study.¹⁷

The distinct performance of the two restorative systems containing low-shrinkage flowable resin composites is possibly a result of the different monomers present in the composition of these resin composites. Although the materials had similar percentages of filler loading (Filtek Bulk Fill Flow 64.5%, SDR 68% by weight), the monomers of Filtek Bulk Fill Flow present a similar structure to conventional resins,²⁸ while Surefil SDR presents a patented monomer (SDR-UDMA), according to the manufacturer. Furthermore, Surefil SDR showed a significantly higher degree of conversion when compared to Filtek Bulk Fill Flow.²⁹

No differences in sensitivity were observed when the hot stimulus was applied since reversible or irreversible pulpal inflammation is required for a response to this test.³⁰ Other studies in the literature that assessed postoperative sensitivity did not evaluate restorations with the hot stimulus test;^16,17 therefore, it is not possible to compare the results found in this study with other studies.

With regard to the clinical analysis of the restorations, the modified USPHS criteria have been used to evaluate the survival of restorations in several clinical studies,^18,19,31 although there are other criteria for the clinical evaluation of restorations, such as those used by the World Dental Federation.¹⁶ It is worth mentioning that the kappa test revealed excellent agreement among the evaluators. This was possible due to the calibration carried out prior to the evaluations using projected images of the different criteria that would be analyzed.

After the one-year follow-up, the restorations in all groups showed similar clinical performance. In three- and five-year follow-ups, 27.6% and 35.9%, respectively, of class II restorations with minimal marginal discoloration were found when the low-shrinkage Surefil SDR resin composite was used.^18,19 Similar results were obtained in the present study, in which 22.6% of the restorations presented bravo scores for this criterion after one year. These data also show that, up to the one-year evaluation period, the restorative system is not responsible for the degree of marginal discoloration of the restorations.

When considering superficial staining, only the XST restorations remained without a statistical difference after the one-year follow-up, rejecting the second null hypothesis. On the other hand, the beginning of superficial staining of the ABF restorations began at the six-month evaluation, with a greater number of B scores at the last evaluation. The good performance of XST over time may be related to the absence of triethylene-glycol dimethacrylate in the coating resin, leaving this restorative option less susceptible to liquid absorption when compared to the other options evaluated.³² Additionally, the decreased behavior of ABF when compared to PA may be related to the greater sorption capacity of the nanoparticulated resin in ABF when compared to the microhybrid resin in PA.³³

When the criterion of marginal integrity was analyzed over time, the teeth that received restorations with the incremental technique presented alterations starting at the six-month evaluation and became worse at the one-year evaluation. On the other hand, the groups that received the low-shrinkage flowable resin composites presented marginal degradation only after one year. In a recent study comparing the same low-shrinkage resin composites used in this study, Surefil SDR and Filtek Bulk Fill Flow showed similar polymerization shrinkage when evaluated using microtomography in class II cavities.³⁴ These materials differ from conventional resin composites because they have increased polymerization depth, which can be attributed to an increase in translucency and specific monomers. However, the literature is inconsistent in determining the depth of polymerization, although it is reported that these materials are more suitable for narrow preparations with depths greater than 4 mm since they have a greater potential for adaptation.³⁵ This fact can probably be explained by the low modulus of elasticity of these materials, reducing the stresses generated by the polymerization contraction and, thereby, maintaining the marginal integrity.³⁶ This information may justify the similarity found among the groups evaluated after one year of clinical evaluation.

In order to evaluate the interproximal contact of the restorations over time using the new low-shrinkage materials, the restorations restored the area of contact with the adjacent tooth. The literature is scarce in clinical work evaluating the intensity of the interproximal contacts of resin composite restorations. Teich and others²² attempted to standardize the evaluation of the thickness of the contact areas using different brands of dental floss. They carried out a study assigning scores to measure the intensity of the interproximal contacts in natural and restored teeth. The same scores were used in this study to evaluate the interproximal contacts re-established by the restorations using the low-shrinkage resin composites in comparison to the incremental technique.

The current study found no difference among the groups tested, which can be justified by the standardization of the matrix used since the use of different types of matrices can influence the intensity of the interproximal contact as opposed to the technique or material used.³ A decrease in the intensity of contact was observed only for XST. However, it should be noted that not all patients had a harmonious and standardized occlusion, and dental movement may have occurred in some cases. Furthermore, a one-year period is not sufficient time to develop variations in most categories. These are potential limitations of this study.

According to the logistic regression analysis, some factors influenced the results of the present study after one year of evaluation. A lower number of surfaces restored increased the retention of restorations, with a smaller interface area being subject to failure. The operator factor influenced the marginal integrity results, where the most experienced operator obtained a higher number of A scores. There are reports in the literature that the most experienced operators influence the success of noncarious class V restorations and restorations using the active release technique.^37,38 Other studies have reported that the clinical success of restorations depends on several factors, including tooth type and location, operator, socioeconomic status, demographics, and behavioral elements.³⁹ The properties of the material showed a smaller effect on the longevity of the restorations.³⁹ The presence of sclerotic dentin influenced the present results; it was observed that most of the teeth restored in this study had sclerotic dentin (78.6%), which may have caused a bias in the interpretation of the results.

It is important to emphasize that a new generation of low-shrinkage resin composite has been released in the market, the “full-body” bulk-fill resin composites, which contain a high filler load and are preferable in more extensive restorations.³⁵ It is believed that clinical trials evaluating this new generation of resin composite are necessary. In addition, a longer evaluation of this study can also elucidate the differences between the restorative systems used.

CONCLUSIONS

The different restorative systems did not influence spontaneous postoperative sensitivity, and after 90 days, there was a reduction of this sensitivity for all study groups. ABF maintained the same degree of cold sensitivity over time. Marginal discoloration was identified earlier for ABF, although marginal discoloration was observed for all study groups after one year. A reduction in interproximal contact was observed for XST after one year of evaluation. Furthermore, other factors should be considered during a clinical evaluation, such as current composite chemistry enhancements and the C-factor.