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Microtensile Bond Strengths of Two Adhesive Resins to Discolored Dentin after Amalgam Removal
1 Department of Operative Dentistry, Faculty of Dentistry, Mahidol University, 6 Yothee Road, Rajthewee, Bangkok 10400, Thailand; and Correspondence: * corresponding author, dtchn{at}mahidol.ac.th
Studies have reported the discoloration of dentin beneath amalgam restorations. The purpose of this study was to test the hypothesis that the bond strengths of adhesive resins to this discolored dentin are lower than those to normal dentin, and are related to the presence of metallic ions or corrosion products. Amalgam-filled extracted human teeth were used. After the removal of amalgam, the discolored dentin and surrounding normal dentin were bonded with Single Bond or Clearfil SE Bond and tested for microtensile bond strengths. The bond strengths of Single Bond and Clearfil SE Bond to normal dentin were greater than to discolored dentin. Clearfil SE Bond demonstrated higher bond strength to normal dentin than did Single Bond. However, no differences were found between the bond strengths to the discolored dentin of both adhesives. Elemental micro-analysis revealed various amounts of tin in all discolored dentin.
Key Words: discolored dentin • corrosion products • amalgam • dentin bonding • hardness • tin
One of the shortcomings of amalgam is corrosion. In some restorations, by-products of corrosion from amalgam penetrate the dentinal walls and cause black discoloration. The discolored dentin occurs as a two-way exchange of materials between the restoration and the surrounding tooth structure. An early study reported that amalgam discolored, but did not noticeably soften, the dentin. The dentin structure remained intact, with a normal tubular structure (Massler and Barber, 1953). However, other studies have suggested that corrosion in the gap between an amalgam filling and the cavity wall results in the formation of a chemical solution of highly acidic, highly concentrated dissolved metallic ions (Marek and Hochman, 1974). This condition was thought to cause localized demineralization of tooth structure and precipitation of metal ions (Jorgensen, 1965; Marek, 1992). The precipitation of amalgam elements, mainly tin (Sn) and zinc (Zn), was found to have accumulated in the superficial part of demineralized dentin (Kurosaki and Fusayama, 1973; Van der Linden and Van Aken, 1973). The Sn and Zn might diffuse from Ca-P-Sn or Ca-P-Sn-Zn complexes, which were formed at the tooth-amalgam interface and on the cavity floor in the dentin (Hals and Halse, 1975; Sarkar et al., 1981; McTigue et al., 1984). The amounts of Sn and Zn were found between the range of 5 and 10% in the tubular areas of discolored dentin (Halse, 1975). However, for high-copper amalgam, Cu and Zn were found in higher amounts than was Sn (Sarkar et al., 1981). Frequently, many amalgam restorations are being replaced with adhesive resin composites, due to marginal leakage, recurrent caries, bulk fracture, or patients’ esthetic concerns (Mjör, 1981; Letzel et al., 1989). In some cases, the corrosion products penetrate deeply into dentinal walls and appear as small to larger areas over the cavity floors. Even though this discolored dentin is slightly demineralized, it is not infected, as in dentin caries, and need not be removed (Fusayama, 1981). The presence of metallic ions may inhibit bacterial infection and modify caries activity (Hals and Halse, 1975), since it has been found that polyvalent cations such as Sn, Zn, and Cu cause reduction in acid formation of dental plaque in vivo (Skjorland et al., 1978; Afseth et al., 1980). Currently, two adhesive resin systems are commonly used in resin composite restorations. The first system, termed ‘total-etching’, uses an acid for complete removal of the smear layer and demineralization of superficial dentin. After the etched substrate is rinsed, a self-priming adhesive is applied. The second system, termed ‘self-etching primer’, consists of an acidic primer applied to smear-layer-covered dentin. An adhesive resin is applied to the treated, unrinsed dentin (Van Meerbeek et al., 1998). In many clinical situations, these adhesive resins are bonded to ‘abnormal’ dentin—e.g., sclerotic dentin (Pashley and Carvalho, 1997). Bond strengths of adhesive resins to ‘abnormal’ dentin were found to be lower than or similar to those to normal dentin (Nakajima et al., 1995; Yoshiyama et al., 2000). However, the adhesion of resin to the dentin discolored by amalgam is still unclear. The purpose of this study was to test the hypothesis that the microtensile bond strengths of two different adhesive systems to discolored dentin after the removal of amalgam restorations is lower than that to normal dentin, and is related to the presence of metallic ions in the dentin.
Microtensile Bond Strength Test Twenty-five recently extracted teeth with old amalgam restorations were collected after informed consent had been obtained from the donors under a protocol approved by the Committee on Human Experimentation, Faculty of Dentistry, Mahidol University. Amalgam was removed by means of a round carbide bur in a high-speed handpiece. During cutting of the amalgam, copious water spray with high-velocity evacuation was used to prevent contamination of the cavity floor with amalgam dust. The inclusion criteria were that, after amalgam removal, a black discolored dentin floor was visible, with sufficient normal dentin beneath the surrounding walls to be used as a control bonding area, and that no dental caries was detected. To confirm that discolored dentin without caries was obtained, we used a caries-detecting solution (CDS Kuraray Co., Ltd., Osaka, Japan). If stainable dentin was observed, the tooth was excluded. After the removal of amalgam, the surrounding cavity wall was removed by means of a model trimmer and 600-grit silicon carbide paper (Struers, Ballerup, Denmark), to expose the flat, discolored dentin floor and surrounding normal dentin at the same level (Fig. 1  ).
After preparation, dentin surfaces, both discolored and normal, were bonded with either a total-etching adhesive (Single Bond, 3M ESPE Co., St. Paul, MN, USA), which consists of acid etchant (Lot 9LU) and adhesive resin (Lot 9CL), or a self-etching primer adhesive (Clearfil SE Bond, Kuraray Co., Ltd., Osaka, Japan), consisting of SE primer (Lot 103A) and adhesive resin (Lot 033A). Bonding procedures were performed according to the manufacturers’ instructions. After the adhesive resin was light-cured, a resin composite block (Clearfil APX, Lot 0598, Kuraray Medical, Osaka, Japan) 4 mm high was incrementally built up and light-cured for 20 sec per increment. After 24 hours’ storage in water at 37°C, bonded specimens were serially sectioned perpendicular to the bonded surfaces with a diamond saw under water spray (Acutome-50, Struers Co., Copenhagen, Denmark). From 2 to 5 slices, 1 mm thick, sectioned through the discolored dentin and normal dentin, were obtained from each tooth. This resulted in 25 slices for each group. These slices were trimmed to form an hourglass shape, with the narrowest portion of approximately 1 mm at the bonded interface, providing 1 mm2 of bonding area. Trimmed specimens were attached to a Bencor-Multi-T-testing apparatus (Danville Engineering Co., Danville, CA, USA) with a cyanoacrylate adhesive (Zapit, DVA, Anaheim, CA, USA) and tested for microtensile bond strength in a universal testing machine (EZ-test 500N, Shimazu Co., Kyoto, Japan) at a crosshead speed of 1 mm/min. After specimens were tested, fracture modes of the debonded specimens were determined by laser scanning microscopy (1LM21, Lasertech Co., Yokohama, Japan).
Microhardness Measurement and Scanning Electron Microscopy Observations Following the hardness measurement, specimens were lightly re-polished with 1 µm diamond paste for SEM observation and EDS analysis. We used a SEM (JSM-5310V, JEOL, Tokyo, Japan) to observe the type of dentin (dentin with open tubules or sclerotic dentin with occluded tubules). Hybrid layers were also observed on the fractured specimens, in which remnants of resin composite were sometimes found attached to the dentin.
EDS Micro-analysis of the Discolored Dentin
Statistical Analysis
The bond strengths of Clearfil SE Bond and Single Bond to discolored dentin were significantly lower than those to normal dentin (p < 0.05) in the same teeth. Even though the bond strength of Clearfil SE Bond to normal dentin was significantly higher than that of Single Bond, this difference was not found when they were bonded to the discolored dentin (Table 1 ).
For each material, the hardness values of the discolored dentin were not different from those of the normal dentin (p > 0.05) (Table 1  ). Most failures were adhesive (Table 2). A few specimens exhibited mixed failure or cohesive failure within the dentin. For each material, no difference was found between failure modes of normal and discolored dentin (p > 0.05).
The SEM observations of the discolored dentin revealed open dentinal tubules in all specimens. In the Single Bond group, a hybrid layer of approximately 10 µm thick was clearly visible on the superficial discolored dentin and seemed to be no different from that found in normal dentin (Figs. 2A, 2B ). For the Clearfil SE Bond group, a thin hybrid layer, less than 1 µm, was observed in both normal and discolored dentin (Figs. 2C, 2D). For the same bonding group, hybrid layers and resin tags appeared similar in both normal and discolored dentin.
From the results of the elemental micro-analysis of the discolored dentin, Sn was found in various amounts in all discolored dentin, and traces of Zn and Cu were found in some specimens (Figs. 2B, 2D ). In normal dentin, Sn, Zn, and Cu were not detected (Figs. 2A, 2C).
The results of this study support the hypothesis that the microtensile bond strengths of two different adhesive systems to discolored dentin under amalgam restorations were lower than those of normal dentin. The dentin used in this study was from shallow cavity preparations filled with amalgam where base or lining materials were not required. The pulpal floors of these preparations were usually no deeper than the mid-coronal dentin.
Metal elements from the corrosion products of amalgam were confirmed to be associated with the dentin discoloration beneath the amalgam restorations. Tin was found in the majority of specimens. The other elements detected were Cu and Zn, which are in agreement with results reported from previous studies (Wei and Ingram, 1969; Kurosaki and Fusayama, 1973; Halse, 1975). Since this experiment was conducted on teeth of unknown origin, it was comparable with the clinical situation in which the replacement of an amalgam filling with an adhesive resin is frequently performed without the clinician’s knowledge of the type of amalgam being replaced. The Sn detected was probably from the corrosion products of the Previous studies (Massler and Barber, 1953; Kurosaki and Fusayama, 1973) have reported slightly softer discolored dentin beneath old amalgam restorations, though the hardness was not determined in those studies. The hardness of the discolored dentin found in this study was not significantly different from that of the normal dentin, but it was slightly softer and had greater variation. The hardness of the discolored dentin varied from 20.13 KHN, which is still in the range of the upper part of the inner carious dentin, to 86.4 KHN, which is similar to the hardness of ‘normal’ dentin (Fusayama et al., 1966). The result of the SEM study, showing that most dentinal tubules in the discolored dentin were open, was perhaps surprising, but in agreement with results from a previous study, with light microscopy, which revealed a normal tubular structure in the dentin discolored by amalgam (Masser and Barber, 1953). Due to the precipitation of metal ions in the dentin, it is interesting to speculate as to whether the permeability of the discolored dentin by corrosion products with open dentinal tubules differs from that of normal dentin. Further investigation is needed, since this change may affect the penetration of adhesive resin monomers into the dentin. Bond strengths of Clearfil SE Bond and Single Bond to the discolored dentin were lower than those to normal dentin. This situation is similar to that of caries-affected dentin, in which the dentin was softer (e.g., KHN 25 vs. 57 for normal dentin) and bond strength was lower than that to normal dentin (Nakajima et al., 1995). Those authors attributed the lower bond strength to the absence of resin tags, due to the occlusion of dentinal tubules with calcium crystals. However, in the current study, most tubules at the interface were patent, and resin tags were present. Moreover, the discolored dentin in this study, though slightly softer than normal dentin, was much harder (i.e., KHN 52–57 vs. 25 for caries-affected dentin) than caries-affected dentin (Fusayama et al., 1966). Thus, the reason for lower bond strength to this discolored dentin was not due to the morphological or physical changes of the dentin substrate. It might be due to the precipitation of plasma proteins in dentinal fluid by corrosion products that may reduce the permeability of the dentin and interfere with the infiltration of the resin monomer. Another possible explanation was that the metal elements from corrosion products of amalgam were found to be bound on the surfaces of collagen fibrils (Ellender et al., 1979). These metallic elements might affect polymerization of the resin monomer. It was reported that the particles of Cu, Zn, and Sn could initiate the polymerization of UDMA-based monomer at room temperature in the absence of tertiary amine when moistened with a small amount of water. The reason is that a small amount of the released metal ions acts as a reductant of redox polymerization (Miyagawa et al., 2000). However, the initiation mechanism might depend on the amount of metal particles used, since it was reported that high amounts of Zn ions tended to retard the setting reaction of a resin monomer (Wanichacheva et al., 2000). A third possible explanation is that these heavy metals reduced the acid solubility of smear layers, making them less ‘etchable’ than normal smear layers. The result that most failures were adhesive may be due to the microtensile testing method, allowing for uniform stress distribution at the interface. Mixed failure was observed more in the discolored dentin for both materials, and was probably a result of minor irregularities of the surface of discolored dentin compared with the normal dentin. These irregularities might have served as focal points of stress, leading to mixed failure. When an old amalgam restoration is replaced with a resin composite, the discolored dentin may show through the overlying composite material, thus affecting the esthetic appearance of the restoration. To solve this problem, the discolored dentin can be masked with an opaque resin. It is interesting to speculate whether the use of an appropriate opaquing metal primer prior to the application of the adhesive may restore the lower bond strength to this dentin with metallic compound precipitation. This requires further study.
This study was supported by the JSPS-NRCT core university program in Dentistry FY2000. A preliminary report was presented at the IADR 79th General Session and Exhibition, June 27–30, 2001, Chiba, Japan. Received for publication February 10, 2006. Revision received November 3, 2006. Accepted for publication November 14, 2006.
Journal of Dental Research, Vol. 86, No. 3,
232-236 (2007)
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ABSTRACT
INTRODUCTION
2-containing amalgam, which, on the dentin surface, consists mainly of Ca-P-Sn compounds, whereas Zn and Cu may be from the corrosion products of high-copper amalgam (