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I. INTRODUCTION
Composite resin restoration has been increasingly used in restorative dentistry with patient's demands, and also composite resin materials as well as bonding agents have been developed continuously. Despite of these developments, several factors, such as inadequate form, wear, color mismatch, chipping or bulk fracture may still present concerns1). In case of failure, the clinician must determine whether to replace the whole restoration or repair the part of restoration.
When the restoration is completely replaced, it is inevitable that further tooth loss lead to the weakening of tooth structure or pulp damage. Therefore, if we could obtain adequate bond strength between old and new composite resin, the repair instead of the replacement could be better treatment option from minimally intervention approach.
Gordan et al.2) studied about two-year clinical evaluation of repair versus replacement of composite restoration. They concluded that resinbased composite restorations that present lessthan-ideal marginal adaptation and stained margins were better off being repaired.
Many laboratory studies3-13) reported that the aged composite resin restoration provided sufficient bond strength by adequate surface treatment and bonding agent. Various pre-treatment methods have been suggested to produce adequate bond strength between old and new composite 3-8,10-12). These methods include the aluminum oxide sandblasting, roughening with diamond instrument, acid etching, silanization, application of bonding agent, etc. In several in vitro studies 3,4,9-11), it was concluded that aluminum oxide sandblasting, followed by application of a bonding agent, proved to be a reliable method for enhancing composite repair strength.
Aluminum oxide sandblasting is surface treatment that causes microretentive feature and increasing the surface area available for wetting by adhesive resin3). By penetrating the adhesive resin into the surface microirregularities, micromechanical interlocking is produced.
Some researchers13,14) investigated about the effect of different bonding agents on bond strength between old and new composite resin. Cavalcanti et al.4) reported that Clearfil SE Bond as a self-etching system showed higher repair bond strength than Single Bond as a 3 step total-etching system. They explained that it was because the Clearfil SE Bond contained the proprietary acid phosphate monomer 10-methacryloyloxydecyl dihydrogen phosphate and the acidic monomer might have a role in the higher capacity to wet the composite surface. Teixeria et al.14) studied about shear bond strength of various self-etching bonding systems. In their study, Optibond Solo Plus SE showed the higher bond strength compared to some other self-etching systems. Shahdad et al.15) said that long-established bonding agents based on chlorphosphate esters of bis-GMA have proven to be acceptable choices as intermediate materials. Lucena et al.7) described that low-viscosity filled adhesives have been demonstrated to have a high capacity to wet composite surfaces and penetrate the organic phase of the composite.
However, there were few studies about the difference of repair bond strength among various bonding systems. Currently available bonding systems include 3 step total-etch system, 2 step total-etch system, 2 step self-etch system and 1 step self-etch system. Presumably, 2 step total-etch system and 1 step self-system might have lower repair bond strength because of their priming components.
Therefore, the purpose of this study is to compare the shear bond strength of repaired composite resin with different bonding systems and evaluate the effect of bonding systems on composite repair.
II. MATERIALS AND METHODS
1. Specimen Preparation
The materials used in this study are listed in Table 1. Forty composite resin discs were made from hybrid composite resin (Z-250, Shade A3; 3M ESPE, St. Paul, MN, USA). Cylindrical teplon mold with an inner diameter of 8 mm and a height of 2 mm was used. Composite resins were inserted into the mold, and then they covered with glass cover plate placed perpendicular to the long axis of the cylinder and cured for 20 s at 90 degrees to the top surface with a light curing unit (Bluephase; Ivoclar Vivadent, Shann, Liechtenstein). After the specimens were removed from the mold, they were cured for a further 20 s on the portions of the specimens that were in contact with the mold, in order to ensure uniform and complete polymerization.
Aging was achieved by thermocycling for 1 week between 5℃ and 55℃ with a dwell time of 30 s.
2. Surface Treatment
All specimens were air abraded for 10 s from a distance of approximately 5 mm perpendicular to the specimen surface using an intraoral air abrasion device (Danville Engineering Inc., Danville, CA, USA) filled with 50 µm aluminum oxide particles. Then they were rinsed with distilled water, and dried with oil-free compressed air.
The specimens were randomly assigned to four groups for different bonding agents (n = 10). Each group was following:
SB group: Scotchbond Multi-Purpose (3M ESPE, St. Paul, MN, USA)
SE group: Clearfil SE Bond (Kurary Co., Osaka, Japan)
XP group: XP Bond (Dentsply Caulk, Milford, DE, USA)
XE group: XenoIII (Dentsply Caulk, Milford, DE, USA)
Directions for bonding procedure of each group are described in Table 2.
Ten more specimens were prepared for control group and these were not treated with bonding agent.
3. Application of the Repair Composite
After completion of bonding procedure as above, each specimen was positioned into the mold, and fresh resin, same as respective composite substrate (Z-250, Shade A3), was applied in 2 mm increments to the substrate surface using a polyethylene mold with a diameter of 3 mm. Then they were cured for 20 s.
The specimens were removed from the mold and thermocycled for 1week between 5℃ and 55℃ with a dwell time of 30 s.
4. Shear Bond Strength Test
Following this aging process, shear bond test was performed using a shear bond test machine (R&B Inc., Daejeon, Korea) (Figure 1). The specimens were fixed in a mounting jig, and then they were loaded exactly at the intermaterial interface. The load at failure was recorded in newton (N) and divided into cross section area (mm2). Then, shear bond strength was expressed in MPa.
III. RESULTS
The mean and standard deviation of the shear bond strength of all tested group were presented in Table 3. Different bonding agents had no significant effect on shear bond strength between old and new resin. And all experimental groups were not significantly different with control group.
Fracture mode was presented in Table 4. Only in control group, 100% cohesive failure was occurred. In SB and XP group, only one specimen respectively presented adhesive failure, all other specimens showed cohesive or mixed failure.
IV. DISCUSSION
Repairing failed composite restorations could be considered as a minimal invasive and cost-effective treatment option. During composite resin repair, two critical factors must be considered; surface treatment and intermediate agent. While surface treatment promotes mechanical interlocking, the intermediate agent improves surface wetting and chemical bonding with the new composite.
Subject of adequate surface treatment was suggested by many investigators3-12) in various methods, such as aluminum oxide sandblasting, roughening with diamond bur and silanization. It was demonstrated that aluminum oxide sandblasting of aged composite resin surface was most effective surface treatment method3,4). It was said that aluminum oxide sandblasting caused microretentive feature and increasing the surface area available for wetting by adhesive resin. So, in this study, aluminum oxide sandblasting was selected for surface roughening.
The primary focus of this study was to evaluate the effectiveness of different adhesive systems for repairing aged composites. Several previous studies 3,4,9-11) have recommended application of intermediate materials to improve bond strength for repairs. Bonding agents are used for penetrating into the microirregularities and increasing bond strength. There are several bonding systems available in dentine bonding procedure. They are divided into 3 step, 2 step, and 1 step, and 2 step is subdivided into self-etching primer and self-priming adhesive. Generally, it have been known that the 3 step etch and rinse adhesives remained the 'gold standard'in terms of adhesion durability in dentine bonding and any kind of simplification in the clinical application procedure resulted in a loss of bonding effectiveness. To investigate that the general concepts about dentine bonding systems are also applicable to composite repair, we used four different adhesive systems for experimental group in this study. ; Scotchbond Multipurpose (3 step total-etch system), Clearfil SE bond (2 step self-etch system), XP bond (2 step total-etch system), XenoIII (1 step self-etch system). In this study, it was concluded that different bonding systems had no significant effect in repair bond strength.
In XP and XE group, we assumed that they had lower shear bond strength than SB and SE group because of their priming components containing both hydrophilic and acidic resin monomer. Like our expectation, it was resulted that XP and XE group had lower shear bond strength than SB and SE group, but no significant difference found in each group. From this result, priming components of XP and XE group might not be critical factor in composite resin repair bond strength.
For measuring of repair bond strength, shear bond test was used in this study. Shear bond test is a method used in many studies to investigate the bond between adhesive materials and dentin or enamel. In this study, large percentage of all groups showed a cohesive failure. And the cohesive shear strengths of the control group had not significant difference with the shear bond strength of the experimental groups. From this result, it is thought that the repairing procedure used in this study was adequate for aged composite resin repair. If a composite repair tends to fracture within the original composite (cohesive fracture), one can assume the selected protocol to be appropriate to bear the occlusal loads. The location of the repair failure within the repaired material itself, rather than at the adhesive surface, suggests a better bond16). Nevertheless, shear bond strength tests may cause cohesive fractures of the substrate. This must be taken into account in the interpretation of the results.
Tezvergil et al.13) reported that storage conditions did not show any significant effect. On the other hand, Ozcan et al.8) reported that thermocycling seemed to be more effective in degradation of the composite tested. In addition to the weakening effect on physiochemical properties, temperature alterations could decrease the number of unreacted double bonds on the surface or within the composite which in turn may affect the composite-composite repair strength. Therefore, for producing the similar clinical condition, specimens were submitted to a themocycling in this study.
According to the Söderholm's study17), it has been suggested that the greatest residual free radical activity of the substrate can be found on the surface of the substrate during the first 24h after polymerization. But, Boyer et al.18) reported that the adhesion between aged composite resin and repairing composite resin gradually decreased up to 24 hours, and when comparing the adhesion after 1 day and 7 days, those after 7 days showed little decrease from the adhesion than those after 1 day. This decrease in adhesion is because of decrease in number of un-reacted monomers on the surface of restored resin as polymerization occurs18). So, this experimental study was tested after 1 week thermal cycling. However, Sau et al.19) reported that there were general increases in the repair shear bond strengths at 1 week and general deteriorations at 4 weeks. Hence, further studies are required to address the effect of long-term storage in a moist environment on repair shear bond strength.
Consequently, repairing procedure selected in this study, air abrasion followed by the bonding agent, was provided the sufficient bond strength for aged composite resin repair. But, there was no significant difference between all experimental groups. From this result, bond strength of repaired composite resin seems to have no difference regardless of bonding system used.
For both dentine bonding and re-bonding for repairs, it is crucial that the intermediate system wet the substrate surface and employ a solvent that encourages bonding system monomer to penetrate into the substrate. Both of these key factors are influenced by the application protocol that might include multiple applications, longer dwell times and more careful solvent drying.14) Therefore, it is required that the clinician should apply the bonding agent carefully according to the manufacturer's instruction.
From the result of this study, the bonding system had no significant effect on the composite repair bond strength, but clinically, it should be also considered the enamel and dentine bonding. It is because that enamel and dentine wall often might be contained with prepared cavity during the repair of restoration. Therefore, the bonding system should be selected considering enamel and dentine bonding as well as composite repair strength.
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