INTRODUCTION

Direct restorations have been widely employed to restore posterior teeth because of their low cost, less need for the removal of sound tooth substance when compared with indirect restorations, and acceptable clinical performance.^1–3 Recently, tooth-colored direct restorative materials have become very popular with advances in adhesive technology, the development of new dental materials, and increasing esthetic demands. However, there is still uncertainty about which is the ideal material for use in the restoration of posterior teeth in terms of long-term clinical success.

Composite resins have commonly been used for posterior restoration because of their acceptable esthetic qualities and their improved mechanical properties and because they allow more conservative cavity preparation design.^4,5 The main challenge with regard to the use of composite resins is polymerization shrinkage. This might cause negative results such as poor marginal adaptation, marginal discoloration, white line formation around the restoration, tubercule fractures, microleakage, secondary caries, and postoperative sensitivity.^6,7 Furthermore, conventional composite resins commonly need an incremental placement technique to avoid depth-of-cure limitations and to overcome polymerization shrinkage stress.^8,9 This placement technique has some drawbacks, such as voids remaining and contamination risk between layers, difficulty in the placement of layers in small cavities, and increased application time.^8,10 To overcome these problems, a new material class referred to as “bulk-fill composite resins” has been developed. It is claimed that bulk-fill composite resins can be polymerized up to 4-mm thickness in a single step without adversely affecting polymerization shrinkage, the adaptation of the cavities, and the degree of conversion during application. They also exhibit less polymerization shrinkage than conventional composite resins.¹¹

Glass ionomer cement (GIC) can also be used as an alternative to composite resins in the conservative restoration of posterior caries lesions. GIC has some advantages such as physicochemical adhesion to tooth tissues, fluoride release, biocompatibility, low shrinkage, low marginal leakage, anticaries properties on the restoration edges, and increased remineralization in adjacent proximal caries.^12,13 However, conventional GIC also has disadvantages such as low fracture and abrasion resistance, inadequate color stability, moisture sensitivity, and poor esthetic properties. Some of these disadvantages weaken the physical properties of the material and restrict its use in areas exposed to intensive chewing forces.¹⁴ In recent years, to reduce the moisture sensitivity of GIC in the early stages of hardening, to increase their abrasion resistance, and to enable them to be used in areas exposed to chewing forces, the materials have been strengthened by changing the powder/liquid ratio, particle size, and distribution; as a result, highly viscous glass ionomer cement (HVGIC) has been presented to the market.^14,15 The manufacturer suggests that these materials should be applied with surface-coating resins.¹⁶ The application of surface-coating resins to the GIC surface enhances the surface brightness of the material; prevents the reduction in translucency of the material over time; fills the gaps caused by the material, finishing processes, and surface irregularities in such a way as to provide a smooth surface; reduces moisture sensitivity in the early stages of hardening; increases the resistance to fracture and abrasion; and improves mechanical properties.¹⁶

In the literature, several studies have compared the clinical performances of conventional composite and high-viscosity glass ionomer or conventional composite and bulk-fill composite, but to the best of our knowledge, this is the first study to compare the clinical performances of a high-viscosity glass ionomer and a bulk-fill composite resin. For this reason, the aim of this clinical study was to compare the clinical performances of a bulk-fill composite resin, a microhybrid composite resin, and a high-viscosity glass ionomer in Class II cavities using modified US Public Health Service (USPHS) criteria. The null hypothesis of this study was that there would be no difference between the two-year clinical performances of composite resin materials (conventional and bulk-fill) and HVGIC in Class II cavities.

METHODS AND MATERIALS

Before conducting the study, the research protocol was approved by the Faculty of Medicine Ethics Committee, Erciyes University, Kayseri, Turkey (approval 2017/44). In this randomized controlled clinical study, an HVGIC (Equia Forte Fil, GC, Tokyo, Japan), a bulk-fill composite resin (Filtek Bulk Fill Posterior Restorative, 3M ESPE, St. Paul, MN, USA), and a microhybrid composite resin (Charisma Smart Composite, Heraeus Kulzer, Hanau, Germany) were compared. The materials, compositions, and batch numbers are given in Table 1.

Table 1 Materials, Compositions, and Batch Numbers

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Study Design and Patient Selection

Patients attending our clinic for routine dental care were examined clinically and radiographically using bite-wing radiography. In this study, 80 patients were assessed for eligibility for participation, and 26 patients were excluded, either because they did not meet all the inclusion criteria or because they did not agree to attend follow-up visits (Figure 1). A total of 54 patients satisfying the inclusion criteria was selected. The inclusion criteria for the selection of patients for the study were as follows: 1) the patient had no systemic disease, 2) the patient was older than 18 years, 3) the patient had good periodontal status, 4) the teeth to be restored were vital, 5) the teeth to be restored had proximal contacts on both mesial and distal surfaces and were in occlusion with the antagonist teeth, and 6) the teeth had Class II caries lesions in the external and middle third of dentin thickness as determined radiographically. The exclusion criteria were as follows: 1) the existence of xerostomia and bruxism; 2) absence of adjacent and antagonist teeth; 3) extremely poor oral hygiene, severe or chronic periodontitis; 4) pregnant or lactating women; 5) teeth with any restoration, endodontic treatment, or periodontal and periapical pathology; and 6) patients who were undergoing orthodontic treatment. The volunteers participating in the study were informed about the research protocol and possible complications. Finally, an informed consent form was read and signed by the patients.

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Clinical Evaluation of the Restorations

In this study, all participating researchers were educated for calibration before the study was conducted. After the restoration placement, the patients were recalled after one week (baseline), six months, one year, and two years. The restorations were examined clinically using mirrors and probes, and bite-wing radiographs and intraoral photographs were taken from the patients. Dental floss was used to check the contact points. The restorations were evaluated by an experienced blinded investigator according to the modified USPHS criteria (Table 2). Intraobserver reliability was assessed using Cohen's Kappa, and it resulted in a Kappa value of 0.95.

Table 2 Modified US Public Health Service Criteria Used in This Study

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RESULTS

A total of 109 restorations were placed in 54 patients. Fifty-one of these restorations (46.8%) were placed in premolars, and 58 (53.2%) were placed in molars. Eighty-four restorations were evaluated at the two-year recall with a 77% recall rate, while 103 restorations were evaluated at the one-year recall. None of the restorations showed any change up to six months. The number of evaluated restorations at baseline and at the one-year and two-year recall according to tooth type (premolar/molar) is given in Table 3. There was no significant difference between premolar and molar teeth for all parameters and all evaluation periods (p>0.05). The clinical evaluation scores of the restorations at baseline and at the one-year and two-year recalls are given in Table 4.

Table 3 Distribution of the Materials According to Tooth Type at the Evaluation Periods

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Table 4 Baseline, One-Year, and Two-Year Clinical Evaluation of Restorations According to US Public Health Service Criteria

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At the baseline evaluation, all CSC, FBF, and EF restorations were scored as “Alpha” for all criteria except color match. In the EF group, although the most appropriate material color was selected, eight restorations were scored as “Bravo” and 26 restorations scored as “Charlie” for the color match because of a lack of material translucency.

Retention and Survival Rate

In terms of the retention criteria, all evaluated CSC and FBF restorations were scored as “Alpha” in all the evaluation periods. In the EF group, eight restorations were scored as “Charlie” because of marginal fracture and material loss, which could be seen radiographically in the proximal area as a result of dissolution at the one-year recall. At the two-year recall, six additional EF restorations were scored as “Charlie” in terms of the retention criteria for the reasons mentioned above (Figures 2 and 3). However, a total loss of restorative material was not observed in any of the restorations during the two-year evaluation. In terms of the retention criteria, there was a statistically significant difference between the EF group and the CSC and FBF groups (p<0.05), while there was no statistically significant difference between the CSC and the FBF groups (p>0.05) at both the one-year and the two-year evaluation (Figures 4 and 5).

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After two years, the survival rates of the CSC and FBF groups were 100%, whereas the survival rate of the EF group was 54.3%. In total, 16 EF restorations (10 restorations at the one-year recall, six restorations at the two-year recall) had to be replaced or modified as a base due to marginal fracture and material loss in the proximal area during the two-year follow-up.

DISCUSSION

Bulk-fill restorative materials such as bulk-fill composite resins and HVGICs have recently become very popular materials in operative dentistry because they provide clinicians with an easy and quick application. The bulk application of restorative materials overcomes some challenges such as void formation and contamination risk between the layers and difficulty in the placement of layers in small cavities. In this clinical study, the clinical performances of three restorative systems were evaluated during a two-year period. The null hypothesis of this study was rejected because composite resin materials (conventional and bulk-fill) showed a significantly better clinical performance than HVGIC.

The retention rates represent the most important evaluation criteria to determine the clinical success of restorative materials. The American Dental Association requires a retention rate of at least 90% of the restorations after 18 months to obtain full acceptance.¹⁷ In this study, the two-year survival rate of the CSC and the FBF restorations was 100%, while it was 54.3% for the EF restorations because 16 of 35 EF restorations required replacement at the end of two years. Based on this information, EF with low clinical success was considered unsuitable for the permanent restoration of Class II cavities, whereas bulk-fill and conventional composite resins were found to be clinically acceptable.

In the literature, no previous clinical study has attempted to compare the clinical performances of an HVGIC and a bulk-fill composite resin in Class II cavities. For this reason, it is not possible to make a direct comparison with previous studies. Çolak and others¹⁸ and Bayraktar and others¹⁹ evaluated the one-year clinical performances of Class II restorations made using either bulk-fill composite resin or conventional composite resin. The authors reported that the bulk-fill composite resins showed similar clinical performance when compared with conventional composite resin. The 12-month findings of the present study are consistent with their short-term data. Moreover, in another study comparing long-term clinical performances (10 years) of a hybrid composite resin and a bulk-fill composite resin in Class II cavities, the authors reported that both materials performed quite well clinically with no significant differences.²⁰ Although our evaluation period is two years, similarly, no difference was found between the clinical performances of bulk-fill composite and microhybrid composite. Yazıcı and others²¹ evaluated the 36-month clinical performance of a nanofill composite resin and a bulk-fill composite resin in Class II restorations. According to their study results, at both 24 months and 36 months, the bulk-fill composite resin demonstrated better clinical performance in terms of marginal discoloration and marginal adaptation, while there was no difference between the materials in terms of other parameters. The data of the present study are not in agreement with the study of Yazıcı and others because no significant difference was found in terms of marginal discoloration or marginal adaptation of composite resin materials during the two-year period.

Gurgan and others²² investigated the long-term clinical performance of the Equia restorative system on permanent posterior teeth in Class I and Class II caries lesions according to USPHS criteria and compared it with a microhybrid composite resin. The researchers reported that both restorative materials showed a clinically successful performance after 6 years. In another long-term clinical trial, the clinical performances of two different HVGICs (Equia Fil and Riva SC) were assessed using USPHS criteria.²³ The authors stated that Equia Fil showed acceptable clinical performance at the end of six years. However, Türkün and Kanik²³ did not assess the restoration as a fail since the restoration did not require a replacement, although it needed to be repaired. In our study, if the restoration needed to be repaired, it was evaluated as a fail. The restorations that had only a small degree of chipping that did not cause food impaction and did not require repair were maintained and monitored throughout the study.

In the literature, there are studies that evaluated the clinical and radiographic performances of Class II restorations involving HVGIC, and it was reported that concavity was seen radiographically on the proximal wall of the restorations at the 18-month recall.^24,25 In the present study, we also evaluated the clinical and radiographic performances of the restorations, and when any loss of restorative material (fracture and/or material loss in the proximal wall) was noted, these restorations were scored as “Charlie” in terms of the retention criteria. At the end of two years, the loss of material in the proximal wall that could be observed on radiographs was seen in a total of nine restorations (25.7%). We agree with previous studies on the cause of the occurrence of this material loss in the proximal wall of the restorations. It may be related to the inability of the protective resin to be applied effectively to the proximal wall of the glass ionomer restoration because it is not easy to access the proximal area. If the surface-coating agent cannot be applied effectively, the proximal area is unprotected from moisture contamination during the initial hardening phase, and the GIC may dissolve.^24,25 The use of metal matrices during the restorative procedure can also be another cause. GIC can adhere to metals chemically, and micro cracks may occur in GIC because of the force applied during the removal of the matrix. These micro cracks may make the material more susceptible to chemical attack.²⁵ Furthermore, it has also been reported that surface-coating agents wear over time.²⁴ In our study, deterioration in the surface texture was observed at the one-year and two-year evaluation of the EF restorations. We think that the wear of the surface-coating agent and the glass ionomer material may cause this deterioration.

In this study, the color match with the surrounding dental tissue of the restoration was a problem in the EF group. HVGICs have more translucency than conventional GICs. The HVGIC used in this study, also known as the glass hybrid system, has many color options. Even so, the color and translucency properties of HVGIC restorations were still not enough, and its color match was not as good as that of composite resin restorations up to one year. However, since the restorations were in the posterior region, and the patients were not disturbed by the appearance of the restorations, the replacing of the restoration was not considered in order to remedy the color mismatch. Also, it was known that the translucency of GICs would improve due to continuous maturation.²⁶ Our findings support this information. At the two-year evaluation, a slight improvement was observed in terms of the color match of the HVGIC restorations. We believe that this improvement in the color match is associated with increasing cement maturation. Consistent with our findings, Diem and others²⁷ reported that the color match of HVGIC restorations improved over the three years of the study (about 25% “good” at baseline, steadily increasing to about 80% “good” at three years) with improving translucency over time as the cement matures.

Secondary caries and postoperative sensitivity were not observed in any restoration during the two-year follow-up period. Chlorhexidine is commonly used as an antimicrobial agent for cavity disinfection, and it was used for cavity disinfection before the placing of the restorations in this study. Besides, EF is a fluoride-releasing restorative material, and it may have prevented the formation of caries under the restoration in this group. The absence of secondary caries could also be related to the good oral hygiene status of the patients. Cavity depth plays an important role in determining postoperative complications. The deeper the cavity, the greater the likelihood of postoperative sensitivity. In addition, operative trauma is another factor that may cause postoperative sensitivity.²⁸ In the present study, operative procedures (both cavity preparation and placement of the restoration) were done carefully by an experienced operator. The depth of cavities was mostly moderate in this study, and Ca(OH)₂ cavity liner material was applied in deeper cavities. In addition, a universal bond was used in the self-etch mode in this study, and acid etching that removes the smear layer and requires technique sensitivity was not applied. All factors mentioned above might have had a favorable effect on postoperative sensitivity.

CONCLUSIONS

During a two-year follow-up period, a conventional microhybrid composite resin and a bulk-fill composite resin showed similar clinical performance, and these materials were found to be clinically more successful than a high-viscosity glass ionomer material. Moreover, within the limitations of this study, it can be concluded that the use of high-viscosity glass ionomer material as a permanent restorative material in Class II cavities is not appropriate.