| Abstract|| |
Objectives: To evaluate the effect of amalgam corrosion products in non-discolored dentin on the bond strength of replaced composite resin.
Materials and Methods: One hundred and sixty-one Class I cavities were prepared on extracted premolars and divided into seven groups. Group 1: Light-cured composite; Groups 2, 3, and 4: Amalgam stored in 37°C normal saline for respectively 1, 3, and 6 months and then replaced with composite leaving the cavity walls intact. Groups 5, 6, and 7: Identical to Groups 2, 3, and 4, except the cavity walls were extended 0.5 mm after amalgam removal. Eighteen specimens from each group were selected for shear bond strength testing, while on remaining five samples, elemental microanalysis was conducted. Data were analyzed using Mann-Whitney and Freidman (α = 0.05).
Results: There was a significant difference between Groups 1 and 4 and also between Group 1 and Groups 5, 6, and 7. However, Groups 1, 2, and 3 showed no significant difference regarding bond strength. Bond strengths of Group 4 was significantly less than Groups 2 and 3. However, Groups 5, 6, and 7 showed similar bond strength. There was no difference among all groups in terms of metal elements at any storage times.
Keywords: Bond strength; corrosion products; EDX analysis
|How to cite this article:|
Ghavamnasiri M, Eslami S, Ameri H, Chasteen JE, Majidinia S, Moghadam FV. Effect of amalgam corrosion products in non-discolored dentin on the bond strength of replaced composite resin. J Conserv Dent 2015;18:25-9
|How to cite this URL:|
Ghavamnasiri M, Eslami S, Ameri H, Chasteen JE, Majidinia S, Moghadam FV. Effect of amalgam corrosion products in non-discolored dentin on the bond strength of replaced composite resin. J Conserv Dent [serial online] 2015 [cited 2019 Jun 17];18:25-9. Available from: http://www.jcd.org.in/text.asp?2015/18/1/25/148884
| Introduction|| |
Since the introduction of amalgam in the nineteenth century, a generation of patients has benefited from its use to restore teeth. However, eventually, many amalgam restorations need to be replaced or repaired due to microleakage, recurrent caries, bulk amalgam fracture, or esthetic demands of the patient. 
At present, the use of composite resin materials in conjunction with dental adhesives is emerging as the material of choice for replacement of existing amalgam restorations. Reliable adhesion to remaining tooth tissue is paramount for successful composite restorations using adhesive techniques. 
Scholtanuset et al. found that discolored dentin adjacent to an amalgam restoration contains amalgam corrosion products that can penetrate deeply into dentinal walls. It has been suggested that there is a relationship between ion penetration, discoloration, and demineralization.  In addition, the result of an scanning electron microscope (SEM) study showing that most dentinal tubules in discolored dentin were open, but the perception of plasma proteins in dentinal fluid adjacent to corrosion products may reduce the permeability of the dentin and interfere with the infiltration of resin monomer.  Therefore, discolored dentin must be considered as a different substrate for clinical procedures when compared with sound, unaltered dentin. ,
Only one recent study found no difference between self-etch and total-etch adhesives with regard to bonding to discolored dentin.  Another study found that the bond strength to darkened dentin was found to be lower than that of intact dentin. , However, Ghavamnasiri et al. showed that after amalgam removal a 0.5 mm extension of cavity walls of non-discolored dentin could improve the dentinal marginal seal to replicate that of the initially placed composite restoration.
The purpose of this study was to analyze the metal elements in the dentin deposited by amalgam corrosion products in 1-, 3-, and 6-month-old amalgam restorations using an energy-dispersive X-ray technique.
This was done to test two null hypotheses:  The aging of the amalgam restorations has no effect on the bond strength of composite restorations using dental adhesives,  Removal of additional dentin after amalgam removal has no effect on the dentin bond strength of composite restorations using dental adhesives.
| Materials and Methods|| |
Sample selection and preparation
One hundred and sixty-one recently extracted sound human maxillary premolars cleaned and stored in 0.1% thymol solution at 4°C.  Each tooth was vertically mounted in a cylindrical mold using polyester resin.
Cylindrical Class I cavities with a depth and diameter of 3 mm were prepared using FGS 010018 (DIA HIR-Italy) bur in a high-speed handpiece with a water spray coolant. A new bur was employed after every five preparations.
The prepared teeth were randomly divided into seven groups and restored as follows:
Group 1: Specimens restored with original composite resin restorations. (n = 23)
The cavity preparations were etched with 32% phosphoric acid (Uni-Etch, Bisco, Schaumburg, IL, USA) for 15 s and followed by a 30 s rinse and dry. Two coats of One Step Plus™ adhesive (Bisco, Schaumburg, IL, USA) were applied to the cavity walls and light cured using a Blue phase C 8 (Ivoclar Vivadent, Schaan Liechtenstein) curing light at an irradiance of 800 mW/cm 2 for 20s. Aelite LS™ (Bisco, Schaumburg, IL, USA), a light cured packable composite resin, was incrementally inserted with each increment being approximately 2 mm in thickness and light cured for 40 s.
Group 2: One-month-old amalgam restoration was replaced with composite resin without extending the original cavity walls. (n = 23)
The preparations in this group were first restored using amalgam. The restorative procedure included the application of two layers of Copalite™ copal varnish (Harry J. Bosworth, Skokie, IL, USA) to all cavity walls then restored using a high copper amalgam (SDI™; GS-80-Australia). The samples were stored in normal saline in an incubator at 37°C to facilitate the generation of amalgam corrosion by-products for one month.  At the end of 1 month, the amalgam was carefully removed to avoid cutting the adjacent dentin with the bur. To prevent encroachment on the dentin, the last layer of amalgam was removed with an explorer. The cavities were then restored with composite resin the same manner as Group 1.
Group 3: Three-month-old amalgam restoration was replaced with composite resin without extending the original cavity walls. (n = 23)
The preparations in this group were restored and tested in the same manner as those in Group 2 except that the amalgam restoration was replaced with composite after 3-months storage.
Group 4: Six-month-old amalgam restoration was replaced with composite resin without extending the original cavity walls. (n = 23)
The preparations in this group were restored and tested in the same manner as those in Group 2 except that the amalgam restoration was replaced with composite after 6-months storage.
Group 5: One-month-old amalgam restoration was replaced with composite resin after extending the original cavity walls. (n = 23)
The preparations in this group were first restored using amalgam. Then the samples were stored in 37°C normal for 1 month. At the end of 1 month, all of the amalgam was carefully removed and 0.5 mm of adjacent dentin was removed with a FGS010018 (DIA HIR-Italy) bur. The cavities were then restored with composite resin the same manner as Group 1.
Group 6: Three-month-old amalgam restoration was replaced with composite resin after extending the original cavity walls. (n = 23)
The preparations in this group were restored and tested in the same manner as those in Group 5 except that the amalgam restoration was replaced with composite after 3-months storage.
Group 7: One-month-old amalgam restoration was replaced with composite resin after extending the original cavity walls. (n = 23)
The preparations in this group were restored and tested in the same manner as those in Group 5 except that the amalgam restoration was replaced with composite after 6-months storage.
After polishing all of the restorations with sandpaper discs (Soflex 3M ESPE, St. Paul, MN, USA), composite restorations in all groups were kept in 37°C humidity in an incubatore (Thelco precision model 17) for 24 hours. Eighteen specimens of each group were used for shear bond strength testing and the rest of five samples used for energy-dispersive X-ray (EDX) evaluation.
Shear bond strength analysis
To conduct the shear bond strength analysis, 1-mm-thick slice of mid-crown dentin was prepared.
The filling material was loaded with a cylindrical stainless steel plunger that provided almost complete coverage over the restoration without touching the cavity walls. The plunger was mounted in the upper part of a universal testing machine (Zwick 250, Zwick Company, Germany). A "Push-out test" was then conducted at a cross-head speed of 0.5 mm\min using a 200 kg load cell.
Energy dispersive X-ray (EDX) analysis of dentin
To conduct the EDX analysis, the crown portion of each tooth was mounted in a polyester resin and then separated from the root portion. Next, the crown portion was sectioned in a mesiodistal direction using a KG Sorensen diamond disc (Industria e Comercio, Ltd, Sao Paulo, SP, Brazil). Each section was coated with a palladium-gold layer and mounted in an SEM/EDX; INCAX-sight, (Oxford, England) holder for analysis of the dentin interfaces using an EDX spectrometer (X 1000).
In order to address the lack of normality of the data obtained with a Koromogorov Smirnov test, the statistical analysis of the bond strength data was performed using the spss 11 with Kruskal-Wallis test, followed by Mann-Whitney and then by Wilcoxon tests. The statistical analysis for evaluation of the EDX data was done using the Friedman test. (α = 0.05)
| Results|| |
Bond strength analysis
The mean bond strengths of all groups were shown in [Figure 1] and [Table 1].
|Figure 1: Mean bond strengths of Groups 1, 2, 3, 4, 5, 6 and 7 at different time intervals|
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|Table 1: Mean and Standard deviation of bond strengths among various groups|
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Statistical analysis showed a significant difference between Groups 1 and 4 and also between Group 1 and Groups 5, 6, and 7 (P = 0.014 and P = 0.023 < 0.05 Mann-Whitney). However, Groups 1, 2, and 3 showed no significant difference in terms of bond strength values (P > 0.05 Mann-Whitney).
Within groups, the bond strengths of Group 4 was significantly less than those obtained at Groups 2 and 3 (P = 0.021 and P = 0.051 <0.05, Wilcoxon). However, Groups 5, 6, and 7 showed similar bond strength.
SEM and Energy Dispersive X-ray (EDX) analysis
The amount of Sn, Cu, Ag, and Hg were measured at a 1000X magnification of the dentin. Tin, copper, and silver were found in the majority of specimens. Mercury was not detected at the interface.
There was no difference in terms of metal elements found between Groups (P > 0.05, Friedman). No metal ion was found in interfacial dentin after refreshing dentinal walls and there was no difference between Groups 5, 6, 7. (P = 0.19 > 0.05, Friedman) [Figure 2] and [Figure 3].
|Figure 2: (a) Scanning electron micrographs of the dentincomposite interface in Group 4, (b) Energy-dispersive X-ray (EDX) micro-analysis of dentin adjacent to the restoration in Group 4|
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|Figure 3: (a) A scanning electron micrograph of the dentincomposite interface in Group 7 (b) Energy-dispersive X-ray (EDX) micro-analysis of dentin adjacent to the restoration in Group 7|
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| Discussion|| |
The findings of the present study showed no significant difference between the bond strength of Group 1, 2, and 3. However, the bond strength at the 6-month storage interval (Group 4) was less than that obtained by Groups 2 and 3. So a significant difference was found between Group 1 and 4. Therefore, the first null hypothesis of study was not accepted for Groups 2, 3, and 4. However, it is accepted for Groups 5, 6, and 7 because the findings of the present study showed that the bond strength values of Groups 5, 6, and 7 were identical.
Degradation of the bonds might result from hydrolysis which occurs either in the adhesive resin, or in the collagen fibers that are not fully enveloped by the adhesive in the hybrid layer. ,, Harnirattisai et al. indicated that after removal of amalgam, the adjacent discolored dentin showed a lesser degree of bond strength compared with the bond strength of these adhesives to the adjacent normal dentin.
Even though the bond strength values of Groups 5, 6, and 7 were identical, they showed lower values than Group 1. This could be due to the structural difference between superficial and deep dentin. This finding lead to the rejection of the second null hypothesis of this study. Previous studies showed that with increasing cavity depth, bond strength decreased as a result of dentin permeability.  Since Groups 1, 2, and 3 were statistically identical regarding bond strength values, this finding might suggest the importance of not extending the cavity preparation into the adjacent dentin unnecessarily after removal of an amalgam restoration if it is not discolored.
The findings of this study showed metal ion precipitations were related to original amalgam. Ghavamnasiri et al. revealed the presence of elements such as copper and silver in the dentin. However, previous studies claimed that these elements are strictly related to discolored and demineralized dentin. They confirmed that tin was consistently found in marginal gaps. , X-ray images of sections of the adjacent dentin in Group 2, 3, and 4 revealed that mercury had not penetrated into the dentin tubules. This is in contrast with the findings of Soremark et al. and Akyüz et al. but is in agreement with studies done by Kurosaki and Fusayama who stated that mercury could not be found inside dentin tubules, but rather it returns to the amalgam structure and reacts with the unreacted alloy core.
Micrographic photographs of Group 1 demonstrated large amounts of Ti and Ba and Si in the adjacent dentin. These elements were related to the application of a dentin adhesive that included a metal opaquer.
For evaluation of metal elements, amalgam fillings, in the present study, were kept in normal saline to generate the corrosion by-products.  Energy dispersive spectroscopic methods produced accurate data in a semi-quantitative manner. EDX detects characteristic X-rays emitted by elements and produces graphic images with peaks representing specific energy levels of the elements. This technique is suitable for determining the relative composition of solid materials. 
As a result, bond strength analysis showed similarity between Groups 4 and 7 after 6-month time intervals.
| Conclusion|| |
On the basis of this in vitro study, there is no need to extend the cavity walls of the cavity preparation by 0.5 mm or more when replacing an orginal amalgam restoration with composite resin to obtain the same level of bond strength that can be achieved with an original composite restoration.
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Dr. Sara Majidinia
Dental Research Center, Department of Operative Dentistry, School of Dentistry, Mashhad University of Medical Sciences, Vakilabad Blvd, Mashhad
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]