Ethics Approval and Protocol Registration

The Institutional Review Board approved this study. The protocol was registered in the Brazilian Clinical Trials Registry (ReBEC-www.ensaiosclinicos.gov.br). This clinical investigation followed the Consolidated Standards of Reporting Trials statement (CONSORT).⁴⁰

Study Design and Locations of Data Collection

This was a randomized, controlled, parallel, blind (participants and examiners) clinical trial and was conducted between September 2016 and July 2018 at the clinic of the School of Dentistry.

Recruitment and Eligibility Criteria

Individuals seeking treatment in a screening clinic in the Restorative Dentistry Department of the local university were recruited for the study. A total of 128 participants were examined by two calibrated dental students to determine whether they met the eligibility criteria. Participants had to be at least 18 years old and be in good general and oral health, with an acceptable oral hygiene level and with at least 20 teeth in occlusion. In addition, they had to have at least one NCCL to be restored that was deeper than 1 mm in vital canines or premolars without mobility, with an opposing and adjacent tooth, and have an OGD of 1.5 mm or 3 mm (±10%). Patients with poor oral hygiene, severe or chronic periodontal disease, orthodontic appliances, severe bruxism; smokers; or participants undergoing tooth whitening procedures were excluded from the study.

Before measurement of the OGD, a dental screening and dental prophylaxis (rubber cup + pumice + water) was done in all participants. Then, two-step silicone impressions (Express XT Putty Soft + Express XT Light Body Quick; 3M/ESPE, St Paul, MN, USA) were made of each tooth with NCCLs that met the inclusion criteria. Gingival retraction cords (Ultrapak #000 and Ultrapak #00; Ultradent Products, Inc, South Jordan, UT, USA) were used in the gingival sulcus to better visualize the margin of the lesion. The impressions were disinfected and poured with gypsum (Durone IV; Dentsply Sirona, York, PA, USA) 3 hours after removal. The excess material was removed from all surfaces, and the cast was scanned with an extraoral scanner (inEos Blue; Dentsply Sirona, Vienna, Austria). The digitalized data were transmitted to a computer-aided design software program (Rhinoceros 4.0; McNeel North America, Seattle, WA. USA) in which the OGD (distance between the most apical point of the gingival margin to the occlusal margin, tracing a line parallel to the long axis of the tooth) of the lesions, was analyzed. After determining the OGD, the restorative treatment was randomly defined according to the sequence generated.

Sample-Size Calculation

The sample size was estimated by Sealed Envelope online software (Sealed Envelope Ltd. 2012. Power calculator for binary outcome equivalence trial. [Online] Available from: https://sealedenvelope.com/power/binary-equivalence/ [Accessed May 02 2016]) An annual failure rate of 2.2% to two-step self-etch adhesives in NCCLs was considered.¹⁰ Thus, after 2 years, the retention rate of this material will be approximately 95.6%. The minimal sample size was 33 restorations using an alpha of 0.05, a power of 80%, and a two-sided test to detect a difference among the groups of 15%. Considering the estimated dropout rate throughout the experimental period, 35 teeth per group was determined.

Randomization and Allocation Concealment

The randomization lists were prepared using a website (www.random.org) with an equal allocation ratio for all comparison groups: NCCLs with OGD 1.5 mm (±10%) restored with either Filtek Z350 XT [C] or Filtek Bulk Fill Posterior [B] (3M ESPE, St Paul, MN, USA); and NCCLs with OGD 3 mm (±10%) restored with either Filtek Z350 XT [C] or Filtek Bulk Fill Posterior [B], for a total of four groups (n=35): 1.5 mm-B, 1.5 mm-C, 3 mm-B, and 3 mm-C.

For the allocation, opaque and sealed envelopes were used and numbered by a person not involved in the study. Just before the restorative procedure began, the envelope was opened, which ensured allocation concealment. In cases where the participant presented lesions with the same OGD, the tooth located in the quadrant of the smaller number and more mesial in this quadrant was the first to be assigned, continuing similarly until all teeth meeting the inclusion criteria were restored.

Restorative Procedures

Before starting the restorative procedure, participants signed the informed consent form. The dental prophylaxis (rubber cup + pumice + water) was performed and verbal oral hygiene instructions provided.

The shape of the lesions (labeled as saucer-shaped, wedge-shaped, or mixed-shape) were analyzed by a computer-aided design software program (Rhinoceros 4.0). The presence of wear facets was recorded. Preoperative sensitivity was also evaluated by applying air for 10 seconds from a dental syringe placed 2 cm from the tooth surface.

An experienced dentist with more than 5 years of dental practice experience restored all teeth. Participants received restorations in previously selected lesions that met the inclusion criteria.

A predetermined procedure was performed, which included cavity cleaning with rubber cup, pumice and water, rinsing, and drying. A shade guide was used for shade selection. No additional retentive features or bevels were placed in the cavities as per the guidelines recommended by the American Dental Association.⁴¹ The relative isolation method was performed in all procedures. For this, gingival retraction cord (Ultrapak #000; Ultradent Products, Inc), cotton rolls, and a saliva aspirator were used.

The cavities received the two-step self-etch adhesive Clearfil SE Bond (Kuraray America, Inc, New York, NY, USA), according to the manufacturer's instructions (Table 1). One coat of primer was applied on the entire lesion surface for 20 seconds. A gentle air stream was applied for approximately 5 seconds to evaporate the solvent. Then, the adhesive was applied and light cured for 10 s at 800 mW/cm² (Radii cal, SDI, Victoria, Australia).

Table 1 Information About Restorative Materials Used

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The cavities were restored with two different resin composites (Table 1). Filtek Bulk Fill Posterior was placed in a single increment and light cured for 40 seconds using a calibrated light-curing unit (Radii cal, SDI) at 800 mW/cm² with a radiometer (Demetron; Kerr, Orange, CA, USA) for half the lesions according to the predetermined OGD (1.5 mm±10% and 3 mm±10%). Filtek Z350 XT was used in up to three increments for cavities with an OGD of 3 mm (±10%) and in a single increment for cavities with an OGD of 1.5 mm (±10%). Light-curing of each increment was performed for 20 seconds at 800 mW/cm².

Final contouring was carried out with 12-fluted tungsten carbide burs (FG Bur; KG Sorensen, Barueri, SP, Brazil) immediately after the restorative procedure. Polishing was performed with disks (Sof-Lex, 3M ESPE) 7 days after placement.

Calibration Procedures

For calibration, 10 representative photographs of each score for each criterion were examined by two independent experienced dentists. Also, 10 patients, not involved in this study, were evaluated on two consecutive days. Before the evaluations, an intra-examiner and interexaminer agreement of at least 85% was obtained.⁴²

Blinding

The two examiners and all participants were blinded to the group allocation during all recalls, resulting in a double-blind trial design.

Clinical Evaluation

Two examiners evaluated all the restorations independently. When disagreements occurred, a discussion led to a consensus. The parameters evaluated by each examiner were individually recorded in a standardized form during all recall times.

The restorations were evaluated at baseline (7 days) and at 6 and 12 months after restoration placement using the modified US Public Health Service (USPHS) criteria (Table 2). The primary measurable variable was restoration retention or presence of fractures, followed by the secondary measurable variables: marginal staining, marginal adaptation, recurrence of caries, anatomic form, postoperative sensitivity, and surface texture. Seven days after the restoration placement, postoperative sensitivity was evaluated. Air from a dental syringe positioned 2 mm from the tooth surface was applied for 10 seconds. These variables were ranked with the following scores: Alfa (acceptable restoration), Bravo (minor change of the restoration), and Charlie (unacceptable restoration).

Table 2 Category and Grade Descriptions for the Clinical Evaluation According to the Modified US Public Health Service Criteria

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Statistical Analysis

R statistical language R Studio (version 3.4.4, R Studio Team, Boston, MA, USA) was used for statistical analysis of each criterion. The descriptive statistical analyses included the evaluated restorations, not considering the dropouts. The inferential statistical analyses followed the intention-to-treat protocol suggested by CONSORT,⁴⁰ which involved all teeth initially randomized, including those that were not evaluated at the specified time. In this case, the missing data were included with the score of the last evaluation.

The Kruskal-Wallis test was performed to analyze all evaluated criteria among the groups at each evaluation time. The Friedman repeated analysis of variance assessed the difference in the performance of each group among three recall times (baseline, 6 months, and 12 months), followed by the least significant difference test for multiple comparisons, when applicable. The Cohen kappa statistics were used to test interexaminer agreement. A significance level of 5% was adopted in all tests.

Characteristics of Included Participants

In this study, 128 individuals were examined. Fifty-one did not fulfill the inclusion criteria, so they were not included. Thus, 77 participants were enrolled in the clinical trial. A total of 140 restorations were placed, 35 in each of the four groups (Figure 1). Details about the participants and characteristics of the NCCLs are presented in Table 3.

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Table 3 Details Regarding Research Participants (Sex and Age), Characteristics, and Distribution of NCCLs

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At baseline (7 days) and after 6 months, all participants were evaluated (Figure 1). After 12 months, the recall rate was 97.1% (136 restorations of 140). Four restorations were not evaluated because the participants did not attend during the follow-up period; of these, 2 restorations were in the 1.5 mm-C group, 1 restoration in the 1.5 mm-B group, and 1 restoration in the 3 mm-C group.

Overall Analysis

Two restorations were lost, one after 6 months and another after 12 months. According to the USPHS modified criteria, the 12-month retention rates (95% confidence interval) were 100% (90% to 100%) for 1.5 mm-C; 97% (85% to 99%) for 1.5 mm-B; 100% (90% to 100%) for 3 mm-C; and 97% (85% to 99%) for 3 mm-B, with no statistical difference among the groups (p=0.570) (Table 4). Also, no significant differences were found in marginal staining, marginal adaptation, recurrence of caries, anatomic form, postoperative sensitivity and surface texture among experimental groups.

Table 4 Number (Percentage) of Evaluated Restorations for Each Recall Time According to Experimental Group, Classified According to Modified US Public Health Service Criteria Compared by Kruskal-Wallis Test at p<0.05

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The only significant difference among the follow-ups for the same group was found in marginal staining. In this parameter, the score Bravo increased significantly with time for 1.5 mm-C (p=0.049) and 1.5 mm-B groups (p=0.049). However, the only significant difference was found between the baseline and the 12-month follow-up. For the 3 mm-B group, three restorations had Bravo scores after 12 months of follow-up, showing a significant increase compared with the other recall times (p=0.049). In the 3 mm-C group, although more restorations rated as Bravo at the 12-month follow-up compared with those at the 6-month follow-up, there was no significant difference between them, only for baseline (p<0.01).