INTRODUCTION

Defective dental amalgam restorations are usually treated by replacement with a restorative material.¹ As a viable conservative technique, the repair of defective areas has been suggested because it is simple and quick.^1‐3 Repair also preserves the tooth, as it is difficult to remove a restoration without removing an integral part of the tooth itself. If not necessary, sound enamel and dentin removal is avoided, strength is maintained, and longevity of the tooth is increased.¹ Cusp fractures or fractures within amalgam restorations are usually due to impact load, fatigue load during mastication, microdefects in the amalgam, secondary caries, technical errors during cavity preparation, incompatible thermal expansion coefficients, history of endodontic treatment, insufficient sound tooth material available such as undermining cusps, number of surfaces restored, or occlusal prematurity.^2,3

A few reports in the literature describe the incidence of tooth fracture.^2,4,5 In one study, the rate of cusp fracture was reported to be 5 teeth per 100 adults per year.⁴ In another study, the incidence of cusp/incisal edge fracture per 100 subjects was reported as 14 teeth over the course of 2 years.⁵ Cusp fractures are a significant dental health problem. In many cases, they are caused by the conventional preparation technique required for amalgam restorations, wherein large undercuts are needed to attain macromechanical retention. Continuous occlusal loading of such weakened cusps could result in horizontal crack formation and cusp fractures.⁶ In a previous study on the incidence of cusp fracture over a 3-month period, molars were more frequently registered with cusp fractures than were premolars.² Although maxillary molars presented a greater number of fractures of buccal cusps, mandibular molars presented more fractures of the lingual cusps. Mastication was most frequently reported as the cause of fracture.

The more surfaces that were restored and/or the wider the isthmus, the greater was the risk of cusp fracture.^2,7 Thus, it is likely that the restorative status of the tooth has an influence on the incidence of fracture. Most fractures were observed in the supragingival location, suggesting that fractures could be restored easily.³ Although sparse literature on the subject has been published, repair of a restoration is more cost beneficial than total replacement, wherever appropriate.⁸ A properly and skillfully manipulated amalgam restoration that replaces one or two cusps or restores an entire occlusal surface may serve for many years. However, in visible areas of the mouth, an amalgam restoration may present an esthetic problem.⁹ Fracture and the esthetic appearance of amalgam restorations present a clinical problem and have created a demand for the development of a simple, efficient intraoral repair method that does not necessitate removal and remaking of the entire restoration.

Recently, veneering the amalgam restoration with resin materials was done in an attempt to solve the problem.¹⁰ Various repair techniques have been suggested in the literature, many of which are based on mechanical and/or chemical means.^9,11 Mechanical means require threading of pins to provide retention for resin composite veneers.¹¹ Chemical means, on the other hand, use multipurpose adhesive agents.^1,12 Several studies have shown that air-borne particle abrasion modifies the metal surface and provides micro-roughness that is essential for mechanical bonding,^1,10,11,13 while primers and adhesives form ionic bonds with metal oxides or with the active metal compounds of the amalgam.¹³ However, the existence of a true chemical bond between amalgam and resin composites is controversial.⁹

The literature contains many reports on bonding of resin composites to alloy surfaces,¹⁴ but these methods have not been investigated for the purpose of bonding composites to amalgam. Although the concept of veneering composites to amalgam restorations is not new to restorative dentistry, there seems to be no consensus in the literature regarding the best method for repairing amalgam restorations. Therefore, the objective of this study was to evaluate the effects of various surface conditioning methods, silane coupling agents, metal primers, and fibers, in combination with various opaquers, on the bond strength of resin composite to amalgam.

MATERIALS AND METHODS

The brand, manufacturer, chemical composition, and batch number of materials used in this study are listed in Table 1.

Table 1:  Brand, Manufacturer, Chemical Composition, and Batch Number of Materials Used in This Study

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Specimen Preparation

Eight experimental groups (n=12) used one type of fresh amalgam (Cavex Amalgam, Cavex Holland BV, Haarlem, The Netherlands). The amalgam used was a non-gamma 2, high-copper, dispersed phase amalgam alloy composed of a mixture of lathe-cut particles and microfine eutectic spherical particles (69.2% Ag, 18.6% Sn, 11.9% Cu, 0.3% Zn). It was triturated according to the manufacturer's recommendations. The control group (n=12) consisted of natural central incisors. The amalgam was condensed with a hand instrument into a cylindrical cavity (diameter, 6 mm; thickness, 2 mm) prepared in autopolymerizing polymethylmethacrylate (PMMA) (Palapress, Vario, Hereaus Kulzer, Wehrheim, Germany) until it was slightly overfilled (N=108). One day after condensation, the exposed surface of each specimen was ground finished to 400-grit silicone carbide abrasive (Struers RotoPol 11, Struers A/S, Rodovre, Denmark), except for the control group, which was ground finished with 800-grit silicone carbide abrasive.

The specimens were randomly assigned to one of the combinations of silicacoating (Sc), sandblasting (Sb), coupling agents, metal primers, adhesives, fibers (Fb), and opaquers (O) as shown in Table 2. The control (clinical) group contained natural central incisors that had cavities on their labial surfaces filled with amalgam. The preparations and undercuts for retention were made with a diamond bur (1014-KG Sorensen, Barueri, SP, Brazil) with an air turbine. Then the amalgam was condensed in the same way as in the other groups.

Table 2:  Experimental Groups and Sequence of Applied Surface Conditioning Methods

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RESULTS

Shear bond strengths (SBSs) were significantly affected by the conditioning methods used (p<0.05). Mean bond strength values of the experimental groups are shown in Figure 1.

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In groups 1, 2, 3, and 4, different opaquers were used, and group 9 was the control. In group 1, Sinfony Opaquer (S) exhibited the highest results (35.5 ± 4.1), and these results were significantly higher than those of groups 4 (19.4 ± 8.9), 6 (19.1 ± 7.8), and 8 (20.6 ± 4.1) (p<0.05). The difference between groups 2 (Clearfil St Opaquer-CstO) (27 ± 12.5) and 3 (Miris Opaquer-M) (24.4 ± 5.1) was not significant (p>0.05).

In group 5, Alloy Primer (AP), and in group 6, after AP, Porcelain Bond Activator (PP) were used. No significant differences were noted between groups 5 and 6 (22.6 ± 4.3 and 19.1 ± 7.8, respectively) (p>0.05). Group 9, containing enamel at the borders of the amalgam restoration, exhibited significantly lower SBSs (8.3 ± 3.4) than were seen in groups 1 to 3 (35.5 ± 4.1, 27 ± 12.5, 24.4 ± 5.1, respectively) (p<0.05). In group 7 (16.4 ± 5.9), where fiber sheets were used, SBSs were significantly lower than in group 1 (35.5 ± 4.1) (p<0.05).

DISCUSSION

The repeatedly removed amalgam restoration is accompanied by associated loss of tooth tissue through progressive cavity enlargement and repeated insults to the pulp.⁸ Because the addition of amalgam to amalgam restorations is not reliable,¹⁵ experience indicates that whatever the circumstances of the amalgam restoration to be repaired, an adhesive approach should be considered. Moreover, for the repaired restoration to withstand functional loads, a strong and durable bond is desirable, to maintain the remaining restoration in function.¹³

In this study, different types of surface conditioning methods were used to adhere a composite resin to dental amalgam. Techniques used included surface roughening by silicacoating with 30 μm SiO₂ or sandblasting with 50 μm Al₂O₃. Then the silicacoated specimens were silanized, or Alloy Primer was applied to the sandblasted surfaces. Opaquers, bonding agent, and then composite were applied. One group was fiber-treated, and in another group, an extending border of enamel was present.

A significant difference was observed between the Sinfony opaquer applied group and the control group, and between the Sinfony group and the experimental opaquer. Opaquers have been advocated to mask the amalgam prior to composite bonding to simulate clinical situations wherein fracture of amalgam involved esthetics, such as in some visible areas of the mouth.^1,3 In this study, one powder-liquid opaquer (Sinfony) and three paste opaquers were used. It is likely that bond strengths produced by opaquers, besides being dependent on chemical composition, can depend on consistency and thus handling characteristics. When they were mixed in proportions specified by manufacturers, the consistency of the opaquers showed slight differences from one another; this may have contributed to differences in thickness between the resin and the conditioned amalgam, thereby affecting bond strength. High viscous paste opaquers possibly did not wet the surfaces evenly.¹⁶ For this reason, a powder-liquid mixture with its low viscosity showed better results.

It might be difficult to get an equally thin layer in paste opaquers because the paste comes out of a tube, and air bubbles can be entrapped. The powder-liquid opaquer is easier to handle in terms of amount, consistency, and thickness.^1,3 Probably as a result of the thickness of the paste opaquer applied, the deepest part of this layer was not fully polymerized.^13,16-18 On the other hand, a powder-liquid opaquer can be easily controlled during application by using the tip of a probe. The experimental opaquer in the present study could not mask the metal surface; therefore it could not fulfill the requirement of masking amalgam.

The woven fiber–applied group showed lower bond strength than group 1, in which a bonding agent was also applied. However, in a previous study, higher bond strengths were attained when a preimpregnated glass fiber was used.¹³ This might have resulted from use of another type of composite for repair. Although a small portion of fibers was extending at the borders, this did not increase bond strength.

In Group 9, simulating the possible clinical situation, the adhered area contained a small portion of enamel at the margins of the amalgam. Although it could be postulated that this would lead to an increase in bond strength due to enamel etching, group 9 showed significantly lower bond strengths when compared with groups 1, 2, and 3. This might be attributed to the small area of enamel left at the margins. Larger areas of enamel surrounding amalgam may enhance these results.

In groups 5 and 6, Alloy Primer was used, but no significant differences were noted between these groups and the other groups. In this study, higher SBS results (22 MPa) were attained than in a previous study (2.2 MPa)¹³; however, lower SBS results were attained from another study (27 MPa), wherein bond strengths of composite resins to cast titanium alloy with different surface conditioning methods were studied using this alloy primer.¹⁷ In those studies, different types of amalgam or cast metal alloy substrates were used, and an opaquer was not included. Significant differences between groups 1 and 6 could be due in part to differences in surface roughening (silicacoating or sandblasting). The particle morphology of these two sand types is different. Particles used for sandblasting were ordinary alumina particles with 50 μm particle size, whereas silicacoating was achieved using alumina particles coated with silica, with particle size of 30 μm. The rough surfaces may not allow for good wettability of the silane or the primer in the pores.¹⁶ One other reason could be greater hydrolytic degradation of methacryloyloxydecyl dihydrogen phosphate (MDP)-containing primers and adhesives versus methacrylate-based silanes and adhesives.^12,16,17 In fact, Alloy Primer is claimed to enhance the bond strength to both noble and base metal alloys because it contains both 6-(4-vinylbenzyl-n-propyl) amino-1,3,5-trizine-2,4-dithio (VBATDT), the functional monomer for noble alloy, and MDP, the functional monomer for base metal alloy.¹⁸ Long-term hydrolytic durability of such adhesive promoters needs further investigation.

A change in the surface roughness of high-copper amalgams occurs over time because of the formation of Cu₆Sn₅ crystals that provide micro-roughness essential for mechanical bonding to occur.⁹ Future studies should investigate the effects of aging of amalgam and surface roughness on adhesion; this could be considered as the limitation of this study.

Shear bond testing is one convenient method of testing adhesive interfaces that is well established in biomaterials research.^19,20 However, the actual mode of disruption of the interface is virtually never a pure shear deformation. Nevertheless, the resultant data could be used for ranking materials and protocols within a single study.

CONCLUSIONS

From this study, the following could be concluded:

1. Bond strength of the resin composite to amalgam substrates varies in accordance with the surface conditioning methods used.

2. The Sinfony opaquer used to mask the amalgam showed significantly higher bond strength than the experimental opaquer.

3. Leaving a small border of enamel around the amalgam restoration led to significantly lower bond strengths.