INTRODUCTION

The increasing patient demand for esthetic restorations has stimulated the development of tooth-colored restorative materials. The advances in dental materials and adhesive technology have enabled the dentist to make esthetic anterior restorations in a simple and economical way.

Using etch&rinse and self-etch adhesive systems are two main adhesion strategies in current adhesive dentistry. 1 The etch&rinse strategy includes an etching step, followed by the application of a primer and bonding agent. Conventional three-step etch&rinse adhesives have been reported to bond effectively to enamel and dentin.2–4 However, these adhesives have some shortcomings, as it is a time-consuming procedure and there is the risk of over-etching, which may lead to a possible discrepancy between the depth of demineralization and monomer infiltration.5 These shortcomings directed researchers toward developing new adhesives, leading to the eventual development of self-etch systems.

Self-etch adhesives do not require a separate “etch&rinse” phase, because they include acidic monomers that simultaneously condition and prime both enamel and dentin. Elimination of the rinsing step and partial removal of the smear layer and smear plugs with these adhesives leads to less technique-sensitive, time-consuming procedures, as well as a possible reduction of postoperative sensitivity.1 Self-etch adhesives can be subdivided according to their acidity: mild (pH ≥ 2), intermediate (pH ≈ 1.5) and strong (pH ≤ 1).1–2 Strong self-etch adhesives exhibit rather deep demineralization effects and lead to a bonding mechanism and ultra-morphology similar to that produced by etch&rinse adhesives. On the other hand, mild self-etch adhesives partially dissolve the dentin surface, leaving residual hydroxyapatite still attached to collagen. Therefore, specific carboxyl or phosphate groups that have chemical bonding potential to calcium can chemically interact with this residual hydroxyapatite. The combination of the chemical and mechanical bonding in mild self-etch adhesive systems may result in bonding that better resists hydrolytic breakdown and may lead to the production of quite stable bonds.1–2 Unlike bonding to dentin, concern remains regarding the enamel etching-capability of mild self-etch adhesives. While some authors reported a lower bonding effectiveness with these systems when compared to etch&rinse adhesives, 26 others have found a similar bonding effectiveness on enamel for some self-etch adhesives.7–8

Although resin composites accompanied by adhesive systems have been extensively used in Class III cavities, few clinical studies regarding their performance have been published.9–11 Clinical trials regarding the performance of adhesive systems have generally been performed in non-carious Class V restorations.12–14 Although non-carious Class V lesions are suitable for testing bonding effectiveness,15 using other cavity designs may also give important clinical knowledge regarding margin evaluation criteria.

The current study evaluated the effect of additional etching of enamel on the clinical performance of Class III restorations bonded with a mild two-step self-etch adhesive after three years. The null hypothesis tested was that the additional etching of enamel does not improve the marginal quality of Class III cavities restored with a mild two-step self-etch adhesive.

METHODS AND MATERIALS

Patient and Lesion Selection

Thirty-eight patients (10 males and 28 females), ranging in age from 19 to 60 years (with a mean age of 36 years) recruited from the university hospital, were included in this paired-tooth randomized controlled clinical trial. The Committee for Medical Ethics of Suleyman Demirel University approved the study protocol. Each patient signed an informed consent form after the nature and objectives of the clinical trial had been explained at the start of the study. The gender, age, smoking habits and oral hygiene status of patients are presented in Table 1.

Table 1 Baseline Data Regarding the Patients Included in the Study

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The inclusion/exclusion criteria were as follows:

Inclusion Criteria:

1. Good general health.

2. Having at least two Class III carious lesions or existing defective restorations, including proximal surfaces in permanent maxillary anterior teeth, which were asymptomatic or sensitive to cold.

Exclusion Criteria:

1. Absence of adjacent and antagonist teeth.

2. Severe periodontal diseases and poor oral hygiene.

3. Symptoms of pulpitis, such as spontaneous pain or sensitivity to pressure.

The mild two-step self-etch adhesive system, Clearfil SE Bond (Kuraray, Osaka, Japan), was used in the current study following two different protocols (Table 2). While the enamel cavity margins were additionally etched with 35% phosphoric acid (SS White, Gloucester, England) for 15 seconds prior to the application of Clearfil SE Bond in the experimental protocol (C-SE etch), Clearfil SE Bond was applied to enamel and dentin without additional etching in the control protocol (C-SE non-etch). The same restorative material (Clearfil AP-X PLT, Kuraray) was used for all restorations.

Table 2 Composition and Application Procedures of the Materials Used

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Following a paired-tooth design, a total of 51 pairs of restorations (right central incisor-left central incisor, right lateral incisor-left lateral incisor and right canine-left canine) were placed. The selection of protocol for a pair of teeth was randomized with the help of flipping a coin to select the first protocol, then the tooth on the right side received the selected protocol first. The tooth on the left side received the other protocol. Each patient received a maximum of six restorations, three per group. Table 3 shows the distribution of restorations according to their location, the liner used and the presence of preoperative sensitivity.

Table 3 Baseline Data Regarding the Lesions Included in the Study

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Evaluation Procedure

Two dentists other than the operator who placed the restorations performed an independent evaluation of the teeth. The evaluators were blinded to the experimental protocol used for any Class III cavity and were calibrated prior to the study. The test of intra-examiner and inter-examiner agreement resulted in a Cohen's Kappa statistic of 0.87 and 0.91. All the restorations were evaluated at baseline and after six months, one year, two years and three years of clinical use. The restorations were examined using the modified Ryge criteria (USPHS)16–18 for retention, marginal discoloration, marginal adaptation, postoperative sensitivity and secondary caries (Table 4). The restorations were scored as follows: Alpha represented the ideal clinical situation; Bravo indicated that the restoration was clinically acceptable and Charlie indicated that the restoration was clinically unacceptable and the restoration had to be replaced.16 When disagreement occurred during the evaluations, the restorations were re-evaluated by both dentists and a consensus was obtained.

Table 4 Modified Ryge Criteria Rating System 16

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Postoperative sensitivity was measured by blowing a stream of compressed air for three seconds at a distance of 2–3 cm from the restoration and by moving the probe over the restored tooth surface. At each recall, vitality tests were recorded.

The overall success rate was determined using the parameters of retention, marginal adaptation, marginal discoloration and secondary caries. Retention loss, severe marginal defects, discoloration that needed repair or replacement and the occurrence of caries along the restoration margins were considered to be clinical failures.19

RESULTS

There were no significant differences between study groups in terms of the number of cavities restored due to untreated carious lesions or previously treated lesions. At the three-year follow-up, 80 of the 102 restorations were evaluated (78% recall rate). One patient (six restorations) did not attend the recall appointments after six months and seven patients (16 restorations) did not attend the three-year recall. Reasons for not being available at each recall were checked. These patients were unavailable for recall appointments because some moved away from the city and there were unknown reasons for others; however, no patient reported any negative comments for the restorative procedures done. The results for the evaluated restorations are displayed in Table 5.

Table 5 Results for Different Parameters Evaluated in This Study. For the Modified Ryge Criteria, Data Shown is n of Examined Restorations/n of Ratings (% of ratings)

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Retention: None of the restorations presented at the three-year recall was lost, resulting in a retention rate of 100% for both the C-SE non-etch and etch groups.

Marginal Discoloration: While six C-SE non-etch and three C-SE etch restorations scored Bravo for marginal discoloration at one year (p=0.572), the number of restorations showing superficial marginal discoloration (Bravo) was five for the C-SE non-etch and three for the C-SE etch groups at two years (p=0.486). In addition, one C-SE non-etch restoration exhibited deeper, more defined marginal staining and scored Charlie (Table 5). However, the number of restorations with superficial marginal discoloration increased to 15 in the C-SE non-etch group at three years, whereas only four restorations showed superficial marginal discoloration in the C-SE etch group. The difference between the C-SE non-etch and C-SE etch groups was statistically significant at three years (p=0.001). No significant differences were found between patients with good oral hygiene and those with moderate gingival inflammation and plaque accumulation (p=0.745 for C-SE non-etch, p=0.172 for C-SE etch) and between smokers and non-smokers (p=0.065 for C-SE non-etch, p=0.640 for C-SE etch) in terms of the presence of marginal discoloration.

The McNemar test revealed significant differences between the baseline scores and that of the recalls in the C-SE non-etch group, while no significant differences were observed in the C-SE etch group. The number of Bravo or Charlie scored restorations in the C-SE non-etch group significantly increased after three years when compared to the baseline scores (p=0.031).

Marginal Adaptation: A number of small detectable enamel marginal defects (Bravo) were observed for both groups (6% for C-SE non-etch and 2% for C-SE etch) at two years. However, the number of restorations with small detectable enamel marginal defects increased to 14 in the C-SE non-etch group at three years; whereas, this number was only four in the C-SE etch group. The difference between the C-SE non-etch and C-SE etch groups was statistically significant at three years (p=0.002).

There were significant differences between the baseline and three-year recall in the C-SE non-etch group, while no significant differences were observed between the baseline and recall periods in the C-SE etch group. The number of Bravo restorations in the C-SE non-etch group significantly increased after three years when compared to the baseline scores (p=0.000).

Postoperative Sensitivity: None of the restorations were postoperatively sensitive to air or tactile contact.

Secondary Caries: No secondary caries was observed after three years of clinical service.

DISCUSSION

In the current study, the effect of additional enamel etching on the clinical performance of Class III composite restorations was evaluated in a paired-tooth randomized controlled clinical trial. The reason for studying Class III restorations was that the presence of marginal defects and discoloration as a result of using a mild self-etch adhesive could lead to pronounced aesthetic shortcomings in such restorations. In addition, there are a limited number of clinical studies regarding the performance of self-etch adhesives in Class III restorations.9–11 C-SE Bond was chosen as a mild two-step self-etch adhesive because of its proven excellent performance in laboratory and clinical studies. 3720–21 For additional etching, 35% phosphoric acid was applied to enamel margins before the application of C-SE Bond. The idea behind the current study was that the etch&rinse approach still provides the most effective bond to enamel, and the bonding effectiveness of mild self-etch adhesives to enamel can be improved with the additional etching of enamel using phosphoric acid.

Several patients were unavailable for recall appointments because they had moved or were unreachable based on the phone numbers they provided; the recall rate of the current study at the end of three years was 78%, however, no patient reported any negative comments for the restorative procedures done.

No restorations were lost during this three-year clinical trial, resulting in a retention rate of 100% for both groups. In previous studies, 100% retention rates for C-SE were also reported at 12–36 months in Class III and Class V restorations.1012–14 That result may be attributed to its two different bonding mechanisms: micromechanical and chemical bonding. C-SE interacts with dentin and produces a hybrid layer approximately 1 μm thick by simultaneous demineralization and infiltration of dentin with monomers, resulting in uniform and stable resin-infiltrated dentin.5 Moreover, its functional monomer 10-MDP has been shown to chemically interact with residual hydroxyapatite around the exposed collagen fibrils.22

In the current study, the increase in marginal defects and marginal discoloration from baseline to three years was significant for the C-SE non-etch group; whereas, throughout the same period, no significant differences were found for the C-SE etch group. The mild self-etch adhesive system has a less stable bonding capacity to enamel, probably because of its more shallow etch pattern.23–24 Previous in vitro studies also reported that the etching potential of mild self-etch adhesives was lower than that of phosphoric acid, and the bond strength values of mild self-etch adhesives could be improved by the adjunctive use of phosphoric acid.25–27 Moreover, in a literature review regarding marginal integrity, significantly better marginal adaptation in enamel margins was indicated in vitro and in vivo with etch&rinse adhesives when compared to self-etch adhesives.28 The reason for the compromised enamel marginal quality in the self-etch systems was attributed to the lack of observable resin tags.3

The increase in small enamel marginal defects over time in the C-SE non-etch group may be attributed to degradation of the bonding resin. Small marginal defects may have caused marginal discoloration as a result of miroleakage through the degraded resin-tooth interface as indicated in previous clinical trials.29–31 Some studies reported that etch&rinse adhesives or additional etching exhibited higher percentages of gap-free margins in enamel after thermomechanical loading when compared to two-step self-etch adhesives.332 After fatigue loading, self-etch adhesives have been found to be less effective when compared to etch&rinse adhesives,333 although most of the adhesives bonded well to cut enamel prior to functional and thermal stresses.234 It has been proposed that the presence of air voids trapped along the resin-enamel interface may cause a more rapid in-plane crack propagation in self-etch adhesive systems. On the other hand, the incorporation of resin tags in acid-etched enamel reduces crack propagation and improves fracture toughness of the interface.3

In previous studies, the clinical performance of C-SE with and without the additional etching of enamel using phosphoric acid was evaluated in Class III and Class V restorations.1012–1335 In one study, it was reported that additional etching of the enamel margins of Class III cavities did not influence the marginal adaptation and marginal discoloration scores of C-SE Bond after a one-year clinical evaluation.10 However, in three- and five-year clinical evaluations of Class V cavities, the number of small incisal enamel marginal defects gradually increased and became significantly higher in the C-SE non-etch groups when compared with the C-SE etch group at two years.1335 In the current study, although the number of small detectable enamel marginal defects and superficial marginal discoloration in the C-SE non-etch group was higher than that of the C-SE etch group at each recall, the differences between the two groups became statistically significant only at the three-year recall.

One of the reasons for the pronounced increase at the three-year recall may be attributed to operator-dependent factors. The interpretation of evaluation criteria can differ in clinical trials, especially when the deterioration is small and the staining is slight. The shortcomings of the current evaluation criteria and a need for evaluating marginal integrity and marginal discoloration in a more standardized way were emphasized and the use of new, more detailed evaluation criteria was recommended by Hickel and others.36 The nutritional habits of patients, especially the frequency of consuming products that stain the teeth, may also contribute to the different results from the different studies.37 In addition, differences in cavity designs and chewing forces affecting the teeth examined may be the reason for differences in the results of these studies. Although more small defects and superficial marginal discoloration in the current study were obtained in the C-SE non-etch group compared to the additionally etched group, this was not critical for the overall clinical performance of this adhesive. Similarly, problems in marginal adaptation and marginal discoloration of the self-etch adhesive system were slight in previous studies and did not affect the overall clinical performance of this adhesive.1012–1335 However, how these criteria will be affected by time-dependent factors should be observed at further evaluation periods.

One of the other parameters evaluated in the current study was postoperative sensitivity. Twenty teeth (10 for each group) had preoperative sensitivity. Moreover, four cavities (two for each group) were relatively deep, so a calcium hydroxide liner was used over the deep portion of the preparations in these restorations. However, patients who received C-SE restorations with or without additional etching of the enamel margins reported no postoperative sensitivity. This criterion remained optimal during this three-year clinical trial. This result indicated that the dentinal tubules were adequately sealed with this mild two-step self-etch adhesive system, which is consistent with the findings that have exhibited a reduction in postoperative sensitivity with this self-etch adhesive.38–40

CONCLUSIONS

The clinical performance of the mild two-step self-etch adhesive in Class III cavities was found acceptable after three years. Although additional etching of the enamel margins improved the marginal quality of this adhesive by preventing small marginal defects and superficial marginal discolorations, it was not critical for the overall clinical performance of this adhesive. Further recalls are planned to follow-up on the clinical performance of these restorations, because the differences between the study groups regarding marginal adaptation and discoloration might become more apparent at later periods, indicating negative environmental effects on the clinical performance of resin-based materials, which may increase with time.