ABSTRACT

Objective

To evaluate the effect of silane-free universal adhesives systems on the repair shear bond strength (SBS) of sonic-activated bulk-fill [SonicFill (SF) Kerr, USA] composite applying additional silane in the repair of bulk-fill composite.

Methods

Sonic-activated bulk-fill composite samples (n=40) were prepared using a teflon mold (6 mm x 4 mm) and polymerized. Then all samples were kept in an incubator to simulate the aging process at 37 °C for one month. Aged composite samples were embedded in acrylic resin. For the repair, the specimen surfaces were roughened with a diamond bur. They were divided into 2 groups (n=20) according to the adhesive system used and then into two subgroups (n=10) as additional silane was applied or unapplied.

Adhesive systems Ambar Universal Bond (AUB), (FGM, Brazil), G-Premio Bond (GPB) (GC, USA) and silane (G-Multi primer GC, Tokyo, Japan) were applied according to the manufacturer’s instructions. Samples were subjected to a shear bond test in a universal testing device. For statistical analysis, one-way ANOVA test was performed (p<0.05).

Results

The highest SBS value was obtained in the silane-treated AUB group (21.88±6.4), while the lowest SBS value was obtained in the silane-treated GPB group (16.07±6.2). No statistically significant difference was observed between all groups (p<0.05).

Conclusion

Additional silane application does not affect the SBS of universal adhesives on an SF composite material.

Introduction

In recent times, with the rise in both patients’ and practitioners’ aesthetic expectations, along with advancements in adhesive systems, composite resin has become increasingly common in restoring dental tissue losses (1-3). Especially for endodontically treated teeth and moderate substance loss, using the layering method for composite resin restoration can lead to gap formation between the resin layers due to the prolonged procedure.

Bulk-fill composites can be applied in a single layer up to a thickness of 4 mm, which reduces clinical procedure time, enhances comfort for both the patient and the clinician, and eliminates the risk of moisture contamination and gap formation associated with the layering technique, thereby providing more uniform restorations (4-6). Bulk-fill composites with high and low-viscosity formulations are currently available on the market. They employ various methods to increase the polymerization depth of each bulk-fill composite. These include enhancing the composite’s translucency, utilizing specific polymerization modulators, or employing robust initiator systems (7). Generally, low-viscosity bulk-fillers have a low filler ratio to provide flowability; however, there is also a bulk-fill composite, which has a high filler ratio but increases its flowability with the effect of the sonic stimulator (SonicFill, Kerr, USA). This sonic-activated composite is one of the bulk-fill composites that can be used with an air-driven handpiece, which allows it to be applied to the cavity by reducing the viscosity of the composite with the effect of sonic vibration.

In clinical practice, the failure of composite restorations is a significant concern. Dentists may consider repairing these restorations since replacing the restoration can result in pulp exposure, weakened tooth structure, and loss of healthy tooth tissue (8). Repair is a minimally invasive procedure that preserves the tooth structure and extends the life of the restoration. It is also more cost-effective and significantly prolongs the lifespan of both the tooth and the restoration, making it a common choice over full restoration removal (9, 10). Many methods are used for surface pretreatment of composite resin restorations before the repair process: mechanical roughening with diamond bur or air abrasion, mechanochemical roughening with sandblasting, chemical roughening with phosphoric acid, hydrofluoric acid or acidified phosphate fluoride (11, 12).

Loomans and Özcan (13) stated that the effects of different repair techniques depended on the material, and none of the surface preparation methods were universally accepted.

“Universal” or “Multimode” adhesive systems have been introduced to the market to eliminate the disadvantages of one-step self-etch adhesive systems that provide clinical ease of use. Most acidic functional monomers contain 10-methacryloxydecyl dihydrogen phosphate (10-MDP) and silane. In addition to mechanical and chemical bonding in enamel and dentin, the most important advantage of universal adhesives is the variety of restorative procedures and adhesion strategies (14). The adhesive intermediate layer provides micromechanical retention by chemical bonding to the resin matrix and, if silane-containing, exposed filler particles and monomer penetration into the microporosities on the composite resin (15).

There is limited literature on studies evaluating the shear bond strength (SBS) in the repair of sonic energy-activated bulk-fill materials using both silane-containing and silane-free universal adhesives. The purpose of this study is to evaluate the repair bond strength (RBS) of two silane-free universal adhesive to a sonic-activated bulk-fill (SABF) composite resin after additional silane application. The null hypothesis of the study is that the additional application of silane in the repair of SABF composites would not affect the RBS of silane-free universal adhesives.

Methods

Forty SABF (SonicFill, Kerr USA) composite resin specimens (6 mm x 4 mm) were prepared using a teflon mold and polymerized for 40 seconds with a light emitting diode light device (Demi Ultra Capacitor, Kerr, USA, 450-470 nm wavelength). The prepared composite specimens were subjected to 10,000 cycles of thermocycling over the course of one month to simulate the aging process (16). 2 mm of the aged composite specimens were embedded in acrylic resin (İntegra, İstanbul, Türkiye). They were divided into 2 groups (n=20) according to the adhesive system used and then into two subgroups (n=10) as additional silane was applied or unapplied. The specimen surfaces were roughened with a green band diamond fissure bur (G&Z instruments Gmbh 6890 Lustenau/Austria) (17). Adhesive systems Ambar Universal Bond (AUB), (FGM, Brazil), G-Premio Bond (GPB) (GC, USA) and silane (G-Multi primer GC, Tokyo, Japan) were applied according to the manufacturer’s instructions (18). The specimens were divided into four groups based on the type of bonding agent applied and the presence of a silane agent.

- Grup AUB

- Grup AUB + silane

- Grup GPB

- Grup GPB + silane (GPBS)

The main components of the materials used in this study are shown in the table below (Table 1). Specimens were subjected to a SBS test using a universal testing machine (Shimadzu AGS-X Universal, Tokyo, Japan) (crosshead speed: 1 mm/min) with a wedge-shaped tip aligned parallel to the longitudinal axis of the interproximal surfaces of the restorations until failure occurred. Failure modes were evaluated under a stereomicroscope. The obtained values were recorded as MPa.

Repair Procedures

A 2 mm x 2 mm silicone mold was used. Afterward applying the relevant surface treatments and adhesive procedures to the surface of the acrylic-embedded and aged bulk-fill composite specimen, a 2 mm restoration was performed with a bulk-fill (SonicFill, Kerr USA) composite using each specimen mold.

Statistical Analysis

In calculating the specimen size, the probability of type 1 error (a=0.05) and the power of the test (1-b) were considered 0.95. Using the G Power 3.1.9.2 program, it was calculated that the total specimen size should be at least 10. Therefore, the specimen size was determined as 10.

The data were analyzed using IBM statistical package for social sciences (SPSS) 26 Software for Windows. By performing the Shapiro-Wilks test, it was determined that the obtained data showed a normal distribution. Parametric tests were conducted among the groups for pairwise comparisons using ANOVA. The results were evaluated with a significance of p<0.05.

Failure Modes

After bond strength testing, the failure mode of all specimens was assessed under a light microscope at a magnification of x15 (Stemi 305, ZEISS, Germany) and classified into the following categories: Failures were classified as “adhesive” if the fracture occurred at the interface between the restorative material and the repair material. A “cohesive” failure was identified if the fracture occurred within either the restorative material or the repair material. If the fracture occurred both within the materials and at their interface, it was classified as a “mixed” failure.

Results

The descriptive statistics values of the SBS of the SABF composite material are shown in Table 2, 3. The highest SBS value was obtained in the AUB group (21.88±6.4 MPa) without silane application, while the lowest SBS value was obtained in the silane-treated GPBS group (16.07±6.2 MPa). No statistically significant difference was observed between all groups (p<0.05).

In the GPB group without silane application, only adhesive failures were observed. In other groups, adhesive failures were generally observed.

Discussion

It was determined that the application of additional silane to the repair process performed with two silane-free adhesives did not alter the RBS of the SABF composite. Thus, the null hypothesis was rejected. Composite resin restorations may deteriorate due to mechanical, thermal, and chemical stresses in the oral environment. In this respect, thermal cycling of bonded specimens is the one of the best method to mimic the aging of interfacial bonds. Additionally, the success of composite repair procedures depends on several factors, including surface properties, wettability of chemical bonding agents, and the chemical composition of composites (19-21). Surface roughness is very important for composite repair and the roughening process can be achieved mechanically with the diamond burs used for this purpose (19, 22-24). Chemical bonding is also an ideal method for enhancing the bond strength of restored composite materials, apart from micromechanical interlocking (25). Silane agents chemically bond the fillers of the old composite resin to the organic resin matrix of the new composite resin (26, 27). For this reason, the use of a silane coupling agent is recommended for composite resin repair (19).

An essential factor for the successful restoration of dental composite is to retain a strong bond with restorations (28). Regarding the repair of composite resins, consensus has not been reached on the optimal protocol or materials for surface preparation of existing and aged composite resin surfaces in clinical practice. In this study, as stated in the literature, roughening of the surface with a bur was used in the surface preparation for the repair process (17). The main factor influencing the RBS has been proposed to be the type of composite resin (29) and some research has indicated that the repair should be done with composite resin of the exact origin as the composite used to perform the original restoration (30).

In this study, the same composite material was used for repair according to the literature. The application of the same bulk-fill composite, which may be explained by the presence of similar monomers in its composition, is expected to increase the effectiveness of the repair procedure that allows adequate copolymerization of methacrylate groups from new and aged composite materials (31). There was no statistically significant difference between the two silane-free universal adhesives used in this study and the repair SBS obtained by applying additional silane.

Furthermore, the absence of any favorable effect of silane in universal adhesives can be clarified by its low stability in aqueous acidic adhesive solution, where the silanol groups formed by hydrolysis can undergo dihydroxylation and condensation to form an oligomer that cannot adhere to glass (32). While some studies (33, 34) found no difference in RBS using silane-containing and silane-free universal adhesive, another study showed that previous silane application improved immediate RBS. Nevertheless, once the bottle is opened, the hydrolyzed silane solutions become less and less reactive and prevent optimal adhesion in the long run (26). Cuevas-Suárez et al. (35) claimed that pre-treatment with a silane coupling agent and application of a hydrophobic resin could increase the bond strength of bulk-fill restorations. Contrary to the findings of Cuevas-Suárez et al., (35) it was found that additional silane application did not affect the bond strength in this study. A major factor influencing the bond strength of repair is considered to be the type of resin (36) and some research has suggested that the repair be done with the same composite resin used to perform it (30). It has also been observed that using the same bulk-fill composites in the repair procedure can increase the system’s effectiveness. We can explain this with the existence of similar monomers, which allows for the same copolymerization between methacrylate groups in new and existing composite materials (31).

In this study, stereomicroscope observations indicated that the failure modes were predominantly adhesive. In addition to adhesive failures, cohesive failures in the restorative material were also observed, and the application of additional silane did not reduce the adhesive failure rate. This is consistent with our SBS test results.

In this regard, it is worth noting that when performing a composite restoration, it is essential for the operator to accurately document the type of material used so that the repair can provide adequate bond strength if needed. In this study, it was found that there was no statistically significant difference between the groups with additional silane application and the groups without silane application. Therefore, it can be concluded that applying silane may not be necessary for the repair process of the bulk-fill composite resin. The results of this study show that the bulk-fill composite can be repaired with the same bulk-fill composite without the need for additional silane application. Including silane in universal adhesives may have questionable importance in clinical practice. Silanes in acidic conditions can become unstable due to self-condensation. The reaction of silanol groups (37) causes bond degradation over time (31). Therefore, further studies are needed to evaluate other roughening methods and materials.

Study Limitations

This study was limited to the use of a single composite resin. Only two silane-free adhesive systems were evaluated in this study. The surface treatment was restricted to bur-roughening. The SBS test was limited by the negative characteristics and inhomogeneity of stress distribution at the bond interface.

Conclusion

When repairing a SABF composite material with the same bulk-fil material, it can be repaired with silane-free universal adhesives without the need for an additional silane application.