Elsevier

Dental Materials

Volume 28, Issue 8, August 2012, Pages 894-902
Dental Materials

Repair of silorane-based dental composites: Influence of surface treatments

https://doi.org/10.1016/j.dental.2012.04.014Get rights and content

Abstract

Objective

The purpose of our study was to evaluate the effects of surface treatment on the microtensile bond strength (MTBS) of repaired silorane-based composites (Filtek Silorane (Silorane), 3M ESPE).

Methods

The surfaces of the aged Silorane blocks (6 mm × 6 mm × 5 mm) were ground with 320-grit SiC paper. The subsequent surface treatments were: no treatment, bonding agent, acetone, acetone + bonding agent, ethanol, ethanol + bonding agent, silane and silane + bonding agent. Another group made from Silorane without repair served as positive control. New Silorane was bonded onto the treated samples. The MTBS test was performed (1 mm/min) with beam-shaped sticks (1.0 mm × 1.0 mm × 10 mm) before and after fatigue simulation (FS). The Weibull moduli for the MTBS data were also calculated. The fracture modes were examined with an optical microscope. The aged specimens after grinding were observed in a SEM and the percentage of uncovered filler surface in relation to the whole surface was calculated.

Results

There were no significant differences between the no treatment group and the acetone only or ethanol only groups. Bonding agent improved the MTBS and the Weibull modulus. Although the SEM image revealed only 5.1% of uncovered filler surface, silane improved the MTBS. The groups with silane and bonding agent had the highest strength values except two groups and showed a fairly large number of cohesive failures. However, after FS, the differences between almost all groups were insignificant. FS increased the MTBS of almost all groups.

Significance

The application of silane and bonding agent enhanced the initial repair strength.

Introduction

Over the years, polymerization shrinkage remains as one of the major drawbacks of composite materials and leads to microleakage which is among the major factors for composite material failures in the oral environment. Silorane-based dental composite materials (Filtek Silorane, 3M ESPE, Seefeld, Germany) are designed to minimize shrinkage and polymerization stress [1]. Silorane derives from siloxanes and oxiranes, which provide the hydrophobic and low-shrinking silorane base of Filtek Silorane. However, despite the continuing development of composite resins with improved properties, several factors, including fracture and discoloration, may still present concerns. As a result, dentists must decide whether to replace or simply repair these restorations. Complete removal of defective composite restorations is not always necessary or desirable [2]. Repairs of composite restorations are frequently accomplished by the addition of new composite to the existing aged composite. Because aged conventional methacrylate-based composite restorations do not retain an unpolymerized surface layer (oxygen inhibition layer) [3], several techniques are suggested to maximize the composite repair strength. The most common suggestions include increased surface roughness [4], [5], silane treatment [6], [7] and the application of a low viscosity bonding agent [7], [8]. Silorane-based composites polymerize by a cationic ring-opening process and are nearly insensitive to oxygen. If the cations are permanently present in the silorane and can be used for repair (in a process called “living polymerization”), this might offer an advantage for silorane repairs over other methacrylate-based composite repairs.

When composite materials with small average filler sizes are ground, only a small area of pure glass surfaces is evident when examined under a scanning electron microscope (SEM). This raises the question of whether the silane treatment could be omitted.

Another aspect to consider is the use of the solvent of the bonding agent such as acetone or ethanol. Solvents enhance resin surface wetting capabilities, quickly vaporize and remove water molecules from the surface [9]. As the hydrophobicity of sirolane-based composites or bonding agents impedes penetration into the smallest crack and gaps of the water-saturated surface on the aged composite, these functions of solvents may play an important role in the repair process. Acetone also might solubilize and soften the surface [10]. The most important function of a repair liquid is considered to be that it solubilizes the base material, penetrates it, and finally leads to chemical bonding with the repair material [5], reported as “liquid etching” [11]. Therefore the application of acetone as a pretreatment may possibly increase the bonding strength. However, the softened layer may also result in reduced bond strength of the dental composite. In addition, solvents such as acetone or ethanol could act as an inhibitor of polymerization for bonding agents or composites [12].

The purpose of our study was to evaluate how to optimize bond strength for the repair of silorane composite materials, which are a new class of compounds for dental use.

Section snippets

Materials and methods

The materials used in this study are shown in Table 1. The experimental design is outlined in Fig. 1.

Results

The results of the microtensile bond strength tests are summarized in Table 2. The tensile strength of unrepaired silorane (positive control group) was 54.7 ± 6.5 MPa. In general, before thermo-cycling and mechanical loading, all repaired groups had only 30–75% microtensile bond strength when compared to the unrepaired control group. All differences between the repaired groups and the unrepaired group were significant (p < 0.05, one-way ANOVA, Scheffé).

Before fatigue simulation, Group 1 (no

Discussion

Several testing methodologies, namely shear [18], [19], [20], tensile [19], and microtensile tests [21], have been suggested for evaluating the bond strength of dental materials. For the test to measure the bond strength values accurately, it is crucial that the bonding interface should be the most stressed region [18], [19]. However, shear tests have been criticized for the development of non-homogeneous stress distributions in the bonding interface, inducing either a misinterpretation or an

Acknowledgment

The authors would like to express their appreciation to 3M ESPE Corporation for providing Filtek Silorane and two kinds of blind pretreatment agents.

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