INTRODUCTION

Despite tremendous improvements in the material properties of adhesives and composite resin over the past several decades, the procedures of adhesive restorations are still technique sensitive and need to be meticulously performed to achieve an ideal restoration. Of the many procedures involved in adhesive restoration, finishing and polishing require special care and attention to achieve the desired anatomical contour and lustrous surface. Roughly polished restorations can cause a number of clinical problems, including plaque retention, chronic gingival inflammation, marginal discoloration, and secondary caries.¹ Furthermore, finishing and polishing can influence the long-term clinical performance of composite resin restorations, affecting marginal integrity, wear, and the durability of adhesive restorations.^1-3 Thus, appropriate polishing is a critical clinical procedure to enhance both esthetics and longevity of composite resin restorations.

The various types of tools commercially available for polishing composite resin restorations include abrasive polishing pastes, fluted carbide burs, diamond burs, stones, abrasive discs, and abrasive-impregnated rubber cups, points, and wheels.⁴ Abrasive discs have traditionally been used for planar surfaces, such as anterior tooth restorations and cervical restorations with composite resins. Typically, polishing with abrasive discs is composed of a four-step progression from coarse to superfine, following the traditional polishing procedures.⁵ Recently, abrasive-impregnated rubber instruments have emerged on the dental market. The abrasive-impregnated rubber instruments have a distinct advantage in areas where disc-type polishing instruments do not work properly, such as on the occlusal surface of posterior teeth and on the lingual surface of anterior teeth. These instruments are easy to use because of their availability in various forms and also because they require fewer steps. Thus, since abrasive-impregnated instruments simplify the polishing process, clinicians may prefer to use them even on planar surfaces.⁶

Many studies have reported in vitro comparisons of polishing outcomes, such as surface roughness, gloss, marginal irregularities, and staining susceptibility between traditional multistep abrasive discs and abrasive-impregnated rubber instruments.^7-14 A number of these studies have demonstrated no significant differences between the two polishing systems.^7,8 However, contradictory results (ie, better performance of one or the other method, depending on the experimental design) have also been reported in several other studies.^9-14 Even though the in vitro studies have discussed the importance of performing clinical studies to confirm the clinical efficacy of polishing instruments, to the best of our knowledge, there have been no clinical trials to compare the efficacy of polishing methods on composite resin restorations. Since a clinical trial is considered to provide the most reliable evidence of the effectiveness of clinical materials and methods,¹⁵ a clinical evaluation of the effectiveness of different polishing methods through a randomized controlled trial would be of great worth.

Noncarious cervical lesions (NCCLs) are considered an ideal model for evaluation of the clinical performance of adhesive restorations since NCCLs offer good access for operative procedures and evaluation, operator variability is reduced since there are relatively minimal restorative procedures, and the lesions themselves are widely available in multiple teeth, facilitating patient selection and study design.^12,15 Many clinical trials have compared the clinical effectiveness of adhesives, composites, and operative procedures in composite resin restorations of NCCLs.^16-19 Both multistep abrasive discs and simplified abrasive-impregnated rubber instruments are used for polishing the convex surface in composite restorations of NCCLs. It would be of great interest and significance to compare the effectiveness of these two popular polishing systems in composite resin restorations of NCCLs. Therefore, the objective of this study was to compare the clinical effectiveness of abrasive disc–type and abrasive-impregnated rubber–type polishing systems in class V composite resin restorations. In addition, we have compared the clinical performance of a two-step self-etch adhesive and a one-step self-etch adhesive. Hence, composite resin restorations were placed in NCCLs using two different self-etch adhesives and then polished using either a multistep abrasive disc or a simplified abrasive-impregnated rubber instrument. The clinical performance of the restorations was then evaluated over 24 months using modified FDI criteria.

METHODS AND MATERIALS

Operating Procedure

Four experimental groups combining two polishing systems and two adhesives were compared. The polishing systems used in this study were Sof-Lex XT (Sof; 3M ESPE, St Paul, MN) for the multistep abrasive disc and Enhance/Pogo (EP; Dentsply Caulk, Milford, DE) for the simplified abrasive-impregnated rubber instrument. The adhesives used were Clearfil SE bond (CS; Kuraray, Osaka, Japan) for the two-step self-etch adhesive and Xeno V (XE; Dentsply, DeTrey, Germany) for the one-step self-etch adhesive (Table 1). All NCCLs were restored with light-cured microhybrid resin composites (Z250, 3M ESPE). A total of 164 NCCLs from 35 patients were randomly allocated to one of four experimental groups using a randomization table. Approximately 40 restorations per group were placed in NCCLs of incisors, canines, premolars, and first molars. To minimize patient-related effects that may bias the results, no more than three restorations for one group were allowed in a patient.

Table 1 Materials Used in This Study

Abbreviations: MDP, methacryloyloxydecyl dihydrogen phosphate; HEMA, hydroxyethyl methacrylate; Bis-GMA, bisphenol A diglycidylether methacrylate; CQ, camphorquinone; UDMA, urethane dimethacrylate; BisEMA, bisphenol A polyethylene glycol diether dimethacrylate; TEGDMA, triethylene glycol dimethacrylate.

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Tooth shade was evaluated prior to the operative procedures by using Vita shade guide (Vita Zahnfabrik, Bad Säckingen, Germany). The lesion was cleaned with plain pumice slurry in a rubber cup and then washed and dried. No surface grinding or enamel beveling were performed for the lesion. The lesion was isolated with a cotton roll and gingival retraction cord (Ultrapak, Ultradent, South Jordan, UT) during the procedure.

Adhesive, either CS or XE, was applied to the lesion according to the manufacturer's instructions listed in Table 1. Composite resin was then built up and light cured with an LED light-curing unit (Bluephase, Ivoclar Vivadent, Schaan, Liechtenstein). The intensity of light was measured by a portable LED radiometer (Bluephase Meter II, Ivoclar Vivadent) prior to each restoration procedure to check for any drop in intensity. For large lesions, two or three increments of resin composite were separately applied. Great care was taken to avoid significant overhanging in the gingival margin during the composite resin buildup. The light curing was performed using a pulse delay cure technique with an initial cure for 2 seconds and final cure for 20 seconds following a waiting period of 3 minutes.²⁰

Excess composite resin on the gingival margin of all restorations was trimmed with a #12 blade during the waiting period for the pulse delay cure. Gross reduction and finishing were performed with a fine diamond point (Mani, Tochigi, Japan) after a final light curing. Further finishing and polishing were then performed using either the Sof system or the EP system. In Sof groups, polishing was performed in a sequence of grit from coarse to superfine with 3/8-inch discs and a mandrel. Each disc step was performed in a dry field for 15 to 20 seconds with a low-speed hand piece. Rinsing and drying were performed before proceeding to the next grit sequence. In EP groups, polishing was sequentially performed with the point shape of Enhance and Pogo using a buffing motion with moderate to light intermittent pressure. Each step was carried out without water for 15 to 20 seconds with a low-speed hand piece. A rinse-and-dry procedure was performed between the use of Enhance and Pogo. One experienced operator performed all the procedures for all the restorations.

Clinical Outcome Evaluation

The class V restorations were evaluated at baseline, 6, 12, 18, and 24 months by two blinded independent examiners. A flow diagram of clinical evaluation is shown in Figure 1. Modified FDI criteria were used to evaluate the clinical performance with respect to the esthetic, functional, and biological properties of the restorations (Table 2). We chose two of the original FDI criteria for each property to evaluate the class V restorations: surface luster and staining for esthetic properties, fracture and retention and marginal adaptation for functional properties, and postoperative sensitivity and recurrence of caries, erosion, and abfraction for biologic properties. Each criterion was evaluated with five scores: all scores ranged from 5 to 1 (with a score of 1 being the poorest). For interexaminer and intraexaminer calibration, the examiners were trained for each criteria using representative sample photographs obtained from previously published literature and from a Web-based tool called “e-calib” (www.e-calib.info).^21,22 If a discrepancy between examiners occurred during evaluation, it was resolved by consensus. All missing cases, including cases involving unannounced nonattendance of the subject at recall, were withdrawn from the study. A restoration with a fracture and a retention score of 1 was counted as missing in the other evaluation criteria. The interexaminer agreement rate ranged from 95% to 100% depending on the criteria.

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Table 2 Modified FDI Criteria Used in This Study

Score designation: 5, clinically excellent/very good; 4, clinically good; 3, clinically sufficient/ satisfactory (minor shortcomings, no unacceptable effects but not adjustable without damage to the tooth); 2, clinically unsatisfactory (but reparable); 1, clinically poor (replacement necessary).

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RESULTS

Of the 164 restorations at baseline, 164 (100%), 154 (93.9%), 143 (87.2%), and 140 (85.3%) were observed at 6, 12, 18, and 24 months, respectively. Descriptive data expressed as percentages of outcomes for the four groups are shown in Table 3. A total of three restorations were dislodged over 24 months: two in the CS/Sof group and one in the CS/EP group. Except for those retention losses, none of the restorations required any repair or retreatment in all criteria over 24 months. The frequencies of below-excellent outcomes (score of <5) at 24 months are shown in Table 4. Significant differences in the proportions of below-excellent outcomes among the four groups were observed for surface luster, surface staining, marginal staining, and marginal adaptation (p<0.05). Polishing with multistep abrasive discs (Sof) was superior to that with simplified abrasive-impregnated rubbers (EP) with regard to surface luster, surface staining, marginal staining, and marginal adaptation (p<0.05). Two-step (CS) and one-step (XE) adhesives did not show any statistically significant differences in any criteria (p>0.05). Representative photographs for clinical evaluation with a score of <5 at 24 months are presented in Figure 2.

Table 3 Descriptive Data (Percentage) of Clinical Outcomes Evaluated at Baseline and 6, 12, 18, and 24 Months According to the Modified FDI Criteria^a

Abbreviations: SL, surface luster; SS, staining surface; SM, staining margin; FR, fracture and retention; MA, marginal adaptation; PS, postoperative sensitivity; RC, recurrent caries; Sof, Sof-Lex; EP, Enhance/Pogo; CS, Clearfil SE bond; XE, Xeno V.

^a 

The data of biologic properties including postoperative sensitivity (PS) and recurrent caries (RC) were omitted because those criteria showed the highest score (score of 5) in all experimental groups and evaluation time points, except one restoration of each CS/Sof (score of 3) and CS/EP (score of 4) for PS criteria in 18 months, respectively.

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Table 4 Frequencies (Percentage) of Below-Excellent Outcomes (Score of <5) for Each Criterion at 24 Months According to the Groups, Polishing System, and Adhesives^a

Abbreviations: SL, surface luster; SS, staining surface; SM, staining margin; FR, fracture and retention; MA, marginal adaptation; PS, postoperative sensitivity; RC, recurrent caries; Sof, Sof-Lex; EP, Enhance/Pogo; CS, Clearfil SE bond; XE, Xeno V.

^a 

Different letters (a and b) represent significant differences among groups at an alpha level of 0.05.

^b

 p-values by the Fisher exact test.

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GEE analysis considering the five repeated measurements from baseline to 24 months is shown in Table 5. In the evaluation of marginal staining, there was no significant difference among the four groups, though the XE-EP group showed the highest percentage (6.5%) of worse outcomes. There was no significant difference both between the two polishing systems and between the two adhesives. In the evaluation of marginal adaptation, the XE-EP group showed a significant difference in the odds ratio (OR=4.07, p=0.001) compared to the CS-Sof group and also showed the highest percentage (24.6%) of worse outcome over 24 months. Use of the simplified abrasive-impregnated rubber polishing system resulted in a significantly inferior performance compared to the multistep abrasive disc polishing system (OR=3.22, p<0.001).

Table 5 Odds Ratio and Adjusted Proportion of Below-Excellent Outcomes (Score of <5) Under Consideration of Repeated Observation for 24 Months^a

Abbreviations: SM, staining margin; MA, marginal adaptation; Sof, Sof-Lex; EP, Enhance/Pogo; CS, Clearfil SE bond; XE, Xeno V.

^a 

Different letters (a and b) represent significant difference among groups at an alpha level of 0.05.

^b 

Interaction term was omitted as insignificant in SM (p=0.615) and MA (p=0.157).

^c

 p<0.05 by generalized estimating equation considering repeated observations for 24 months.

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In the analysis for the below-good outcomes (score of <4), frequencies at 24 months and GEE analysis considering the repeated measurements did not show any significant differences both between polishing systems and between adhesives.

DISCUSSION

To the best of our knowledge, this is the first study employing a randomized controlled clinical trial to clinically compare multistep abrasive discs with simplified abrasive-impregnated rubber instruments in terms of polishing effectiveness in class V composite resin restorations. The main finding of the present study is that Sof, a multistep abrasive disc system, showed better clinical performance than EP, a simplified abrasive-impregnated rubber system, in all the esthetic properties (surface luster, surface staining, and marginal staining) and one functional property (marginal adaptation) in composite resin restorations at 24 months. According to GEE analysis considering repeated observations over 24 months, polishing with multistep abrasive discs also presented a superior outcome in marginal adaptation compared to that with simplified abrasive-impregnated rubber instruments.

The polishing effectiveness of abrasive-coated discs and abrasive-impregnated rubber instruments has been previously compared in many in vitro studies. Among the various polishing systems, abrasive-coated discs have been reported to be the most effective polishing instruments available and are reported to produce the highest gloss and smoothest surface with composite resins.^9-11 On the other hand, in a number of recent studies, abrasive-impregnated rubber instruments showed comparable or even better performance in reducing the surface roughness when compared to abrasive discs.^12-14 These contradictory results from in vitro studies might be due to differences in the experimental conditions, such as initial roughness state, combinations of finishing and polishing protocols, and polishing time and pressure. Despite the use of a variety of in vitro experimental conditions, in vitro studies might not completely reflect the clinical efficacy of polishing instruments. Most in vitro studies evaluate the polishing performance on a flat and homogeneously finished composite specimen for experimental standardization, while, in clinical practice, the desired surface of composite restorations is rather convex or concave and the finishing state not as homogeneous as that observed in the experimental setting. Finishing with diamond point, the procedure implemented in the present study, delivers a comparatively good finish on curved surfaces and feather edges but has also been reported to leave a relatively irregular surface.^25,26 Finishing using abrasive disc instruments begins with a coarse disc before polishing proceeds with gradually finer-grained abrasive discs. Therefore, abrasive discs may have been less affected by the irregularity of the finished surface and hence may have produced a smoother final polishing surface than the abrasive-impregnated rubber instruments. This may explain the better clinical performance score in surface luster, surface staining, and marginal staining at 24 months.

In the present study, Sof also showed better marginal adaptation than EP at 24 months. An earlier in vitro study reported that the hardness of the aluminum oxide abrasive used in abrasive discs is higher than that of most filler particles in the composite resin formulation; thus, this abrasive cuts filler and resin matrix equally to produce a smooth surface.²⁷ Because of the hardness of the abrasive particle and the thin flexible backing, Sof appears to work better than EP in convex cervical composite restorations by blending smoothly with the contours of the tooth being restored. Moreover, EP also appears to be too bulky to reach the interproximal and cervical margin of class V composite restorations, regardless of their shape.

Although analysis of the percentages below score 5 (the clinically excellent outcome) indicated that the polishing performance of Sof is better in class V composite restorations, EP did not show any significant differences from Sof, in any criteria, in the analysis of the percentages below score 4 (the clinically good outcome). Accordingly, EP is also considered to be an effective instrument to obtain a clinically good polishing performance in class V composite restorations. The use of EP, which is an abrasive-impregnated rubber polishing system, is more advantageous when access to the anatomically contoured occlusal surface is required since the various forms of the instrument (cup, point, and wheel) offset the access limitations of the abrasive disc.^13,28 Additional significant advantages of abrasive-impregnated rubber instruments over multistep abrasive discs include the relative ease of handling and the significant time savings.

It is not clearly understood why the XE-EP group showed a significantly inferior outcome to the CS-Sof group with regard to marginal adaptation. The observed difference between the polishing systems presented a high odds ratio showing significance, while the observed difference between the adhesives did not (Table 5). Therefore, the polishing system is assumed to be a variable with significantly more effect than the adhesive system. In previous studies, one-step self-etch adhesives have been reported to show lower bond strength to enamel relative to two-step self-etch adhesives.^29-31 The lower bond strength of one-step self-etch adhesives is likely to be more evident for uncut enamel without additional acid etching, as in the present study.^32,33 The possibility of leaving a thin overhang of composite resin on the uncut enamel is greater when polishing with EP than when polishing with Sof, which shows superior finishing ability by starting the procedure with a 100-μm-grain-sized coarse disc. This thin overhang might be more easily fractured out from restorations bonded with XE due to the relatively weak bond strength in comparison to the bond strength with CS. As a consequence, fracturing of a thin overhang may be reflected as inferior performance in the marginal adaptation in the XE-EP group in comparison with the CS-Sof group.

The FDI criteria that were recently introduced for clinical evaluation of dental restorations were slightly modified in the present study. From the 16 categories of original FDI criteria for evaluation of class V restorations, we appropriately chose representative categories that we could easily assess. For the sake of convenience, each criterion was scored from a high of 5 (clinically excellent) to a low of 1. This was in contrast to the original FDI criteria, where 1 was best and 5 was worst.¹⁵ In addition, in several criteria, the original subscores were integrated to present one score for simplification of the evaluation. The FDI criteria appear to be more sensitive and more precise in the evaluation of composite resin restorations in comparison with the US Public Health Service criteria system, which had been generally used in many clinical trials, because of the use of a detailed scoring system.

The present study does not address all clinical aspects of polishing systems utilized in composite resin restorations. A number of studies have reported that the polishing outcome depends on the filler size, shape, and loading of composite resin.^32,33 The present study used only one type of microhybrid composite resin. The dependence of clinical polishing effectiveness on the different filler types of composite resin would be an intriguing issue for future studies. Another limitation of this study is that the observation period of 24 months may be relatively short to confirm the clinical polishing performance over the whole service span of composite resin restorations. Further observations with longer follow-up are required.

CONCLUSIONS

In conclusion, multistep abrasive discs (Sof) were clinically superior in clinical polishing performance to simplified abrasive-impregnated rubber instruments (EP) with regard to criteria such as surface luster, surface staining, marginal staining, and marginal adaptation for class V composite resin restorations over a 24-month follow-up period. Although EP showed inferior performance to Sof, this instrument still presented clinically good outcomes. Considering the advantage of EP in ease of handling and time saving, abrasive-impregnated rubber instruments may also be recommended for the polishing of class V composite resin restorations. XE, one-step self-etch adhesives showed clinically equivalent performance to CS, two-step self-etch adhesives.