Pure Ormocer vs Methacrylate Composites on Posterior Teeth: A Double-blinded Randomized Clinical Trial

INTRODUCTION

The demand for dental materials with high esthetics and longevity led to the research and development of new composites. Composition, size, shape, distribution, and content of filler particles are paramount for the composite's properties.^1,2 Great efforts in recent years have been made to improve the filler technology, increasing the mechanical and esthetic properties of these materials, resulting in the current nanohybrid and nanoparticle containing composites.³ However, few changes were performed in relation to the organic matrix, and many traditional dimethacrylate monomers are still in use.

Bis-GMA (bisphenol A-glycidyl methacrylate) has been the main monomer used in composite formulations since its development in 1956 by Bowen.⁴ Due to its high viscosity, it is necessary to add low molecular weight monomers in the blend to achieve the appropriate viscosity on the final formulation for clinical use.⁵ However, these diluent monomers increase the polymerization shrinkage and water sorption of the composites.⁶ In addition, unreacted monomers are eluted from the cured material, increasing its cytotoxicity to the pulp cells.⁷ Thus, aiming to improve the properties of the composite restorative materials, new monomers have been investigated.

Ormocer is the acronym for organically modified ceramic. They are produced by hydrolysis and polycondensation reactions (sol–gel processing) to form a molecule with a long inorganic silica chain backbone and organic lateral chains.⁸ Compared with Bis-GMA, the ormocer molecule has more methacrylate groups available to set bonds.⁹ The composites with ormocer are expected to demonstrate higher degree of conversion and increased wear resistance and toughness due to the formation of a polymer network more highly crosslinked.¹⁰ Another advantage of ormocer would be higher biocompatibility, because the increased number of chemical bonds between the methacrylate groups would reduce the amount of unreacted free monomers in the polymer network.¹¹

However, the first generation of ormocer composites contained, besides the ormocer molecules, regular low molecular weight dimethacrylate monomers acting as diluents. The presence of such diluents may have hampered the expected results, and no clear advantages were observed when using the first-generation ormocer-based fillings in comparison with conventional composites.^12,13 Recently, a pure ormocer composite was developed. According to the manufacturer, there is no diluent methacrylate monomer in the composition, because special ormocer molecules of various viscosities were created.

In comparison with methacrylate composites, the new ormocer material may offer advantages of lower polymerization shrinkage and water sorption.^14,15 In addition, this new material was reported to present higher microhardness and degree of conversion.¹⁶ However, there is still a lack of information regarding its clinical performance. Thus, the aim of this study was to investigate the clinical performance of a pure ormocer and a methacrylate-based resin composite in class II restorations. The null hypothesis tested was that the monomer composition (ormocer × methacrylate) does not influence the restoration clinical behavior in relation to esthetic, functional, and biological properties.

METHODS AND MATERIALS

The description of the experimental design followed the Consolidated Standards of Reporting Trials (CONSORT) statement.¹⁷

Eligibility Criteria

A total of 92 participants were examined to form a group with 30 patients that attended to the inclusion and exclusion criteria (Figure 1). Patients were required to have good general health, be older than 18 years old, and present at least 20 teeth in occlusion. Patients with extremely poor oral hygiene, severe or chronic periodontitis, or heavy bruxism habits were excluded from the study. Patients had to present at least two teeth with class II cavities to be restored (two molars or two premolars). The cavities had to present size of the isthmus being no more than two-thirds of the intercuspal distance; the antagonist and the adjacent tooth had to make contact; there needed to be vital pulp; and there needed to be an absence of painful symptoms.

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Interventions: Restorative Procedure

Three operators, each with at least three years of clinical experience, performed all interventions. Shade selection was performed using a VITA Classical shade guide (VITA Zahnfrabrik, Bad Säckingen, Germany). All patients received local anesthesia before the tooth preparation, which was performed using a high-speed handpiece fitted with a round diamond bur under water cooling. When present, carious dentin was removed with a round carbide bur at low speed. The outline shape of the preparations was limited to the removal of caries/defective restoration, without beveling. Isolation with rubber dam and clamps was performed. Each patient received at least two restorations, one using a pure ormocer composite and another using a methacrylate composite, subjecting the different materials to the same clinical conditions and enabling the comparison between them. Table 1 presents the specifications of the materials used.

Table 1 Materials Used in the Restorative Procedures

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In deep preparations, a glass-ionomer cement liner (Meron R, Voco, Cuxhaven, Germany) was applied on the pulpal wall. In very deep preparations, calcium hydroxide cement (Dycal, Dentsply, Rio de Janeiro, Brazil) was applied, followed by a thin layer of glass ionomer cement.

Futurabond M+ (Voco) was used in self-etching mode according to manufacturer's instructions for all preparations. A thin layer of adhesive was actively applied for 20 seconds, followed by a gentle blow of air for five seconds and light curing for 10 seconds. The light curing was performed with an LED device having an emittance of 700 mW/cm² (Emitter A, Schuster, Santa Maria, RS, Brazil).

Restorative procedures were performed using a precurved metallic sectional matrix (Unimatrix System, TDV, Pomerode, SC, Brazil), associated with a separating ring and a wooden wedge. The composite was applied using an incremental oblique technique. Each increment of 2 mm was light cured for 20 seconds. Finishing was performed with fine-grain diamond burs (KG Sorensen, São Paulo, SP, Brazil). At the proximal surface, any excess was removed with abrasive strips (3M ESPE, St Paul, MN, USA). After seven days, the polishing procedures were performed with abrasive silicone tips (Dimanto, VOCO, Cuxhaven, Germany) and at the proximal surfaces with fine-grained strips.

RESULTS

The restorative procedures were implemented exactly as planned, and no modification was performed. Sixty-two of 92 subjects were not enrolled in the study because they did not fulfill the inclusion criteria. Thus, 30 subjects were selected. Details regarding the characteristics of the research subjects and the restored cavities are shown in Tables 2 and 3.

Table 2 Number of Lesions According to Sex and Age of Patients

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Table 3 Characteristics of Restored Cavities

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The percentage of patients that attended recall evaluations was as follows: 100% (7 days), 100% (6 months), 87% (12 months), and 77% (24 months). The qualitative evaluation according the FDI guidelines is presented in Table 4. The overall Cohen's κ statistics showed excellent agreement between the examiners in the 7-day (0.93), 6-month (0.96), 12-month (0.96), and 24-month (0.91) follow-up.

Table 4 Number of Evaluated Restorations and Classification According to the FDI Criteria

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After 24 months, only one restoration made with pure ormocer composite and one made with methacrylate composite presented small fractures, which did not indicate the need for replacement of the restoration. Only one tooth presented fracture of the remaining dental structure (one nonsupporting cusp). The fractured areas were restored with the same technique and materials applied at the baseline. Fisher's exact test detected no significant difference (p>0.05) among evaluated FDI criteria for the two materials after 24 months.

In general scores for esthetic, functional, and biological properties, the pure ormocer composite presented, respectively, 100%, 95.66%, and 100% of clinically acceptable scores and the methacrylate composite presented, respectively, 100%, 91.31%, and 95.66%.

DISCUSSION

The results of this study showed that the different composites did not influence the clinical performance of class II restorations after 24 months. Thus, the null hypothesis tested was accepted.

Minimizing polymerization shrinkage is still the main goal of composite science development, because it remains as the principal reason for restorations failure. Shrinkage occurs during polymerization as weak van der Walls forces are converted into covalent bonds, reducing the distance between monomer molecules while forming a polymer.²³ The shrinkage stress at the tooth-restoration interface can produce cuspal deflection, fracture of the remaining tooth structure, and formation of marginal gaps, which have been associated with marginal staining and postoperative sensitivity.²³ A previous study found lower polymerization shrinkage in the latest version of ormocer composites compared with traditional methacrylate-based materials.¹⁴ This may be due to the fact that ormocer molecules are larger than the Bis-GMA,⁹ promoting a lower volumetric reduction of the material. However, the results of the current study showed no significant differences between the pure ormocer and the conventional methacrylate composite, regarding the marginal stain and postoperative sensitivity. This may be due to the incremental technique that may have reduced the effects of the polymerization shrinkage²⁴ and masked the differences between the composites.

Nonsignificant differences were detected between the composites in the parameters related to the esthetic properties. They presented only “clinically excellent” and “clinically good” scores. Both composites tested contain the same nanohybrid particles and similar filler content (w/w). The filler size and distribution are the main determining factors for surface properties such as roughness and gloss after polishing.²⁵ In addition, the nanoparticles present size below the wavelength of visible light (0.1-100 nm) that provides translucency and opalescence to these materials, enhancing their esthetic properties.²⁶

Nonsignificant differences were detected between the composites in the parameters related to the functional properties. After 24 months, only one restoration made with pure ormocer and one made with methacrylate composite presented small fractures with partial loss (less than half of the restoration). A previous review showed that the incidence of fractures is higher in the first three years after the placement of the restoration.²⁷ Thus, we may speculate that in the next follow-ups, the fracture rate of the restorations will tend to remain low, indicating an excellent performance of the composites.

Both composites showed similar biological properties results. No caries recurrence was observed in the present study, which may be an indication that the restorations with both composites present an excellent polishability, reducing the biofilm retention. However, it should be highlighted that secondary caries trends occur later, being detected in longer-term follow-up periods.²⁷ Thus, a longer follow-up time may be necessary to confirm the observed results.

Soft tissue response to dental materials may be detected by effects on the periodontium and adjacent mucosa. These effects may be related to presence of residual unreacted monomers, roughness, and biofilm accumulation.²¹ A previous in vitro study showed that the methacrylate and the ormocer composite tested present a low surface roughness, even after abrasive episodes.¹⁶ The Bis-GMA molecule has two polymerizable units, whereas the ormocer has numerous organic polymerizable ones that increases the probability of interaction and chemical bonding with neighbor molecules and reduces free monomers after curing.⁹ Thus, pure ormocer composites are expected to present better biological properties. However, within the limits of this study, it was not possible to demonstrate better clinical behavior of the improved ormocer restorative material in relation to the conventional nano-hybrid composite over two years. A longer period of evaluation may be necessary to show potential relevant differences.²⁸

Previous studies demonstrated the role of the adhesive system on the clinical performance of a dental restoration, mainly related to postoperative sensitivity, marginal staining, retention, and secondary caries.^28,29 In the current study, all restorations were performed with a self-etching adhesive containing 10-MDP acidic monomer, which is incorporated in different adhesive formulations by various manufacturers. Besides micromechanical retention provided by the hybrid layer and tag formation, 10-MDP showed chemical bonding to the tooth structure, contributing to the bonding durability.^30,31 The adhesive system used was compatible with the pure ormocer restorative material formulation. According to the manufacturer, the adhesive system tested can be used with or without total or selective acid etching. They claim that the formulation is capable of effectively etching enamel and dentin without previous acid treatment, which was demonstrated in previous in vitro studies.^32,33 Therefore, it was decided to test the clinical performance of the material in association with ormocer material in its more challenging situation, based only on its self-etching properties. Previous clinical trials demonstrated adequate performance of this adhesive in the self-etching mode only, in association with methacrylate-based composites.^19,34

According to the guidelines proposed by the American Dental Association, adhesive-based materials can be considered clinically acceptable if, after a six-month follow-up, they present less than 5% failure rate.^35,36 For full acceptance, the restoration losses cannot exceed 10% after a period of 18 months. Thus, the materials tested in this study could obtain complete acceptance, because the fracture rates of the pure ormocer and the methacrylate composites were both 3.33% (one failure in 30 restorations). The advantages of the ormocer technology that has been demonstrated in vitro^14-16,37 offers potential clinical advantages that may be proven with continued clinical studies.

CONCLUSION

After a 24-month follow-up, ormocer and methacrylate-based composites showed acceptable clinical performance in class II restorations. Nonsignificant differences were detected between both materials.