Sample Preparation

A total of 120 cylindrical samples made of a micro-hybrid composite (Clearfil AP-X, Kuraray, Japan) were fabricated according to the manufacturer's instructions using a polyethylene mold 6 mm high with an internal diameter of 5 mm. Composite increments (2 mm thin) were light-cured (Coltolux 75, Coltene/Whaledent, Cuyahoga Falls, OH, USA) vertically from a 1 mm distance for 40 seconds. Light intensity was calibrated at 800 mW/cm² with a radiometer (Optilux Radiometer Model 100, SDS Kerr, Danbury, CT, USA) after curing of each of five specimens. After polymerization, the polyethylene molds were gently removed.

Aging

To age the composite, the substrates were placed in distilled water and stored in an oven (WTE Binder, Tuttlingen, Germany) at 37°C for three weeks.

Surface Treatments of the Aged Composite

Specimens were randomly divided into four groups (each, n=30) (Table 2):

Table 2:  Surface Treatment Protocols for Materials Used in This Study^a

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• • Control: No surface treatments were performed on control specimens.

• • Group 1 (Self-etching Bonding): Composite surfaces were roughened for 20 seconds with a diamond bur (No. 881-012, Diatech Dental AG, Swiss Dental Instruments, Charleston, SC, USA) attached to a water-spraying high-speed handpiece (KaVo K9, Handpiece Type 950, KaVo, Biberach, Germany). A new set of burs was used after every five preparations. Afterward, the available primer (Table 1) was applied to the composite surface for 20 seconds (using an applicator sponge) and was gently air thinned. The bonding agent (Table 1) was applied to the surface (with an applicator) and light-cured for 20 seconds.

• • Group 2 (Silane Bonding): Composite surfaces were roughened as described previously. Then the primer and the silane (Table 1) were blended 1:1, applied to the composite surface for five seconds, and air thinned. Afterward, the bonding agent was applied to the surface and light-cured for 20 seconds.

• • Group 3 (Etching/Silane Bonding): After surface roughening with diamond burs, the surface was etched (40% phosphoric acid, Table 1) for five seconds and then was rinsed and dried thoroughly, according to the manufacturer's instructions. Afterward, the primer, the silane, and the bonding agent were applied as described in group 2.

Repairing the Aged Composite

After the previously described surface treatments were applied with the use of a translucent polyethylene mold 3 mm in diameter and 6 mm high, fresh composite resin (Clearfil AP-X) was placed by the same operator on the conditioned flat surface of the aged composite cylinder, and then was vertically photopolymerized (40 seconds) in three increments of not more than 2 mm.

Aging the Repaired Composite Setup

After polymerization, specimens were removed from the mold and were stored in distilled water placed in an oven (WTE binder) at 37°C for 48 hours; they then were thermocycled (Dorsa, Behsaz, Iran) for 2000 cycles at between 5 ± 2°C and 55 ± 2°C with a dwell time of 30 seconds and a transfer time of five seconds.

After thermocycling, to consider the stability of composite repairs in water, the specimens in each group were randomly assigned to two subgroups (each, n=15) (subgroups new and old). Before the SBS was tested, specimens in the subgroups labeled new were stored in distilled water in an oven (WTE binder) at 37°C for one week, while specimens in the subgroup old were stored in the same conditions (37°C) for six months.

Repair SBS

A shearing rod attached to a universal testing machine (Zwick Model 1494, Zwick GmbH & Co KG, Ulm, Germany) exerted the shear force to the samples (parallel to the flat bonding area) at a crosshead speed of 0.5 mm/min until fracture (Figure 1). The SBS was calculated in megapascals (MPa) by dividing the fracture load (Newton) by the repair surface area (7.065 mm²). A stereomicroscope (Olympus SZX-12, Olympus, Tokyo, Japan) was used to visualize the fractured surfaces of specimens at 40× magnification to determine the fracture type (adhesive [at the intermediate layer], cohesive [within the old or the repairing resin composite], or mixed [a combination of cohesive/adhesive failure modes]).

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Statistical Analysis

Descriptive statistics were calculated. Data were analyzed using two-way analysis of variance (ANOVA) with focus on factors of treatment type and storage time, and the Tukey post hoc test was completed. The level of significance was set at 0.05. In addition, Weibull analysis of data pertaining to the experimental groups was carried out, and stress rates at 5% and 90% fracture probabilities were predicted.

Reliability of Bond Strength

Results for Weibull analysis are presented in Table 4 and Figure 3. Characteristic bond strengths and stresses at 5% and 90% failure probabilities for group 2 were higher than those of the other groups. Weibull analysis gives the Weibull modulus characteristic. At 90% probability of failure, bond strength was lower in the old subgroups compared with the new subgroups. High Weibull moduli were obtained in the groups, including silane application (groups 2 and 3), after six months of water storage (Table 4, Figure 3).

Table 4:  Weibull Analysis of Experimental Groups

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Site of Bond Failure

The mode of failure was predominantly adhesive for the control group and group 1. Group 2 tended to exhibit cohesive failure inside the aged composite material. In group 3, the mode of failure was mainly mixed. The mode of failure for each group remained unchanged after six months of water storage (Table 3).

Aging

When a restoration is restored in the oral cavity for a long time, this means that it has been subjected to humidity, and therefore its free radicals have been eliminated.³⁴ In this study, to stimulate the aging process, composite resins were stored in 37°C distilled water for three weeks.^21,22,35 This approach was based on a study of Pillar and others, who found that water storage for less than 28 days did not affect fracture resistance of the composite.³⁶ In this study, boiling the specimens was not used as a method of stimulating the aging process, because Brendeke and Ozcan showed that microcracks and filler dissolution will happen following the boiling process.³⁷ All repaired specimens were placed in water and thermocycled to simulate the stress placed on interfacial bonds.³⁸ After thermocycling, to consider the stability of composite repairs in water, half of the specimens were stored in distilled water for a period of six months.

Surface Roughening

In the experimental groups, all aged composites were roughened by diamond burs to obtain a higher bond before any additional treatments were provided. The phenomenon that aged composites roughened with a diamond bur might exhibit higher bond strengths was shown by Bonstein and others.²¹ Also, Bouschlicher and others found that surface roughening is necessary for creating macro and micro retention. They showed that increasing exposed filler particles besides providing silane treatment could produce a higher SBS.²⁰ Crumpler and others concluded that surface roughness might enhance the ability of fresh composite to interlock mechanically into the substrate, because increased surface area is available for micro-mechanical bonding.³⁹ Söderholm and Roberts went further and concluded that surface abrasion was the single most important factor during composite repair.⁶

Treatment With 40% Phosphoric Acid Etchant

In this study, group 2 (silane bonding without etching) showed the highest SBS before and after long-term storage. This value was statistically significantly different compared with that of group 3 (etching/silane bonding). Also, significant differences in the bond strengths of these two groups were detected after six months of storage. Previous studies showed no increase in bond strength when repaired composites were treated with phosphoric acid.^{11,19,21,40,41} Also, Fawzy and others reported that acid etching exercises provide only a superficial cleaning effect, which removes debris and grinding dust from the resin-based composite surface.⁴² Not only was the application of phosphoric acid etchant ineffective in this study, it also had a negative effect on the SBS (compared with group 2). These differences might be attributed in part to the manufacturing and chemical composition details and the quality of different brands used in these studies.¹⁵

Effects of Silane and Bonding System

Application of silane and the primer followed by the bonding resin showed the highest SBS values after short-term or long-term water storage times; this finding was in agreement with the findings of several previous studies.^20,22,43,44 Both groups that provided silane treatment had higher SBS values when compared with groups lacking the silane application. This may be due to the formation of siloxane bonds between fillers and the polymer matrix. These results supported the findings of several previous studies showing that the application of silane and unfilled resins can improve bond strength,^{22,36,41,45-47} and contrasted with the results of some other studies reporting that the addition of silane to the bonding system might not significantly improve bond strength.^15,16 Controversy might be rooted in the different protocols and materials employed in these studies.

However, acid etching of the composite surface contributed to reduction of repair SBS in the group containing silane. In addition, Weibull analysis showed that treatment of the aged composites with silane and the bonding agent without acid etching produced the most reliable bond strength.

Extended Water Storage

In this study, a trend of decreasing bond strength was noted when repaired specimens were stored for six months in distilled water in all groups. This phenomenon was dominant in groups treated with the silane coupling agent and the bonding resin. These groups exhibited less difference in SBS when compared with group 1 (bonding). This phenomenon is consistent with the findings of Söderholm and others, who showed that water diffusion might weaken the bonded area.⁴⁸ The adhesive/silane system has potential water uptake caused by the presence of hydroxyl, carboxyl, and phosphate groups; this may cause the monomer to be more hydrophilic, and hence more prone to water sorption. This leads to reduction of the mechanical properties of the polymer.⁴⁹

Technique Sensitivity

Weibull analysis provides information about the variability of results, reflecting the structural reliability of materials or bonded assemblies.^26,50,51 The Weibull modulus (m) is an experimental constant related to the defect size distribution.⁵² The probability of failure, calculated from the Weibull distribution, has been considered a suitable alternative for analyzing SBS data and may be useful in selecting techniques or materials that are less technique sensitive.^53-55 In this study, the lowest probability of failure was obtained in the groups repaired with silane and the bonding agent, and the highest probability of failure was noted in the group treated with the bonding agent only, both before and after long-term storage. However, the bond strengths of all experimental groups fell within the acceptable range for bearing occlusal load.

Fracture Type

In general, failure modes indicate that those groups with high bond strengths exhibit cohesive failure inside the composite. However, low bond strength groups tend to exhibit adhesive failure rather than cohesive failure. Fractures within the composite resin (cohesive failure) seem to be more appropriate for bearing occlusal loads.²⁰ In the present study, failure mode analysis revealed that the SBS of group 2 (silane bonding) was significantly different from that of acid-etched composites. No difference in failure modes was noted after six months of aging in three groups, but in the acid-etched group, a slight improvement in the failure mode was not significant.

However, it should be noted that it cannot be certainly determined using a light microscope whether failure was truly adhesive. Therefore, more mixed failures than those observed may have occurred, and results should be cautiously interpreted.

Limitations

This study was limited by some factors. Findings regarding a specific brand should be cautiously generalized to other brands.¹⁵ However, consistency between the results of this study and those of several other studies might confirm the advantage of silane usage regardless of the brand used. Moreover, during long-term water storage, specimens were not subjected to any occlusal loadings. Finally, surface roughening was performed in all experimental groups. Future studies should include some experimental groups in which the effects of surface roughening without subsequent chemical treatments could be observed. In addition, some control groups should be included, in which only the effects of chemical treatments (without surface roughening) could be assessed. These measures exceeded the available time and budget projected for this study but might be considered in further studies.