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This is an open access journal, and articles are distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.
Applying silver diamine fluoride (SDF) is recommended to arrest and prevent dental caries. However, it may jeopardize the bond of the restorative materials to the tooth. The aim of the present in vitro study was to evaluate the effect of the simultaneous use of the SDF and potassium iodide (KI) on shear bond strength to the sound dentin.
In this in vitro study, on the buccal and lingual surfaces of the 48 human third molar teeth, dentinal surfaces with a diameter of 6 mm were created by removing the enamel. Then, the specimens were divided into four groups based on applying or not applying the KI/SDF and adhesive type (etch and rinse [E and R] or universal adhesive). Afterward, cylindrical composite restoration was made on the surfaces. Half of the specimens were subjected to 1000 cycles of thermocycling. Subsequently, the shear bond strength was evaluated by Universal testing machine. Furthermore, the type of failure was determined by a stereomicroscope. Data were analyzed by t-test and Chi-square at a significance level of P < 0.05. In this in vitro study, on the buccal and lingual surfaces of the 48 human third molar teeth, dentinal surfaces with a diameter of 6 mm were created by removing the enamel. Then, the specimens were divided into four groups based on applying or not applying the KI/SDF and adhesive type (etch and rinse [E and R] or universal adhesive). Afterward, cylindrical composite restoration was made on the surfaces. Half of the specimens were subjected to 1000 cycles of thermocycling. Subsequently, the shear bond strength was evaluated by Universal testing machine. Furthermore, the type of failure was determined by a stereomicroscope. Data were analyzed by t-test and Chi-square at a significance level of P < 0.05. There were statistically significant differences in shear bond strength between Groups 1 and 3 (P < 0.05), whereas the bond strength difference between Groups 2 and 4 was not statistically significant (P = 0.609). Failures were predominantly of the mixed type in almost all groups. Based on the results of this study, the superiority of shear bond strength of universal adhesive to the sound dentin compared to E and R adhesive was confirmed. Thus, using this type of adhesive is recommended under the condition that KI/SDF anticaries material is applied.
Many studies have been conducted to introduce a method or a material to limit progression or stop active caries without eliminating sound dental structures. Since centuries ago in Japan, silver-containing compounds such as silver nitrate and combination of silver and fluoride (without stabilizing amine group) have been introduced as antimicrobial materials.
The results of the studies conducted to explore the impact of applying SDF on adhesive's bond strength are inconsistent. Two studies concluded that applying SDF does not negatively affect the bond strength.
However, all studies about SDF examined the initial bond strength. The various methods offered to check the bond strength during a long period of time include application of thermal and mechanical cycling and fatigue test. Thermocycling stimulates temperature changes in clinical conditions. According to some studies showed that thermocycling is able to accelerate the decay process of interface contact between dentin and restoration.
Hence, the purpose of this study was to investigate the effect of simultaneous use of SDF and KI on the shear bond strength of composite restoration to dentin using total etch and universal adhesives after thermocycling. The null hypotheses were: (1) the bond strength of SDF/KI and control groups are similar. (2) The bond strength of the universal and etch and rinse (E and R) adhesives to SDF/KI treated surfaces are similar. (3) The thermocycling process has no effect on the shear bond strength of SDF/KI groups.
Sample preparation
In this in vitro study, based on Lutgen et al.,
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n = 12
Therefore, 48 human extracted third molar teeth without any cracks, decay, restoration, and enamel defects were used. The soft tissue residues were removed by curette and crown surfaces were cleaned with pumice powder, and then, they were rinsed with water. Afterward, the teeth were stored in 1% Coloramine T solution for 72 h. Then, the teeth were rinsed and kept in distilled water at 37°C until the practical phase began.
For the first step, 220 grit sandpaper (Klingspor/Germany) was used to remove the buccal and lingual enamel an evenly expose the dentin layer. To do so, the sandpaper was pulled 10 times clockwise until the surface of the sound dentin became 6 mm diameter. Then, to provide a standard smear layer, the bonded dentin surface was polished with 600 grit silicon carbide paper disk (PS11A, Auto-paper, p600A (Kingspor/Germany)), in the presence of moisture for 1 min. Finally, the specimens were rinsed and the excess moisture was gently wiped off with the wet cotton ball without completely drying their surfaces.
Considering that two buccal and lingual surfaces were prepared for each tooth, a total of 96 surfaces were provided for testing. Then, the teeth were embedded in autopolymerized resin. The prepared specimens were divided into four groups (n = 24), based on their adhesive types and applying or not-applying KI/SDF
Group 1: On the prepared dentin surface, two drops of 38% SDF (advantage arrest SDF/elevate oral care/USA) were applied with microbrush for 10 s. Immediately afterward, KI was added to the surface by means of a microbrush, and consequently, a white precipitate is produced. Adding KI continued until the white precipitate was no longer produced. Afterward, rinsing the dentin surface was done for 30 s and the excess moisture was air-dried thoroughly for 5 s to evaporate the extra water. For the next step, the single bond Adper bond 2 (SB) (3M ESPE-ESPE/USA) adhesive was applied to the surface according to the factory instructions. To do so, first, the dentin surface was etched by 35% phosphoric acid/Ultra-Etch (Ultradent products, Inc.) for 15 s and rinsed for 10 s, and then, adhesive was applied in two layers and cured by the LED (Kerr/USA) device with the intensity of 900 mw/cm2 for 15 s Group 2: On dentin surface, 35% phosphoric acid and SB were applied, as described in Group 1 Group 3: Similar to Group 1, SDF and KI were applied on the surface, and then, with a microbrush, a drop of single bond universal adhesive (SBU) (3M ESPE/USA) was applied to the dentin surface for 20 s, and a light flow of air was blown for 5 s to evaporate the solvent. Then, the bonding agent was cured for 10 s Group 4: SBU was applied to the dentin surface in the same way which was described for Group 3.
It should be noted that in each tooth, the same adhesive system was used on the buccal and lingual surfaces, expect that on buccal surface, KI/SDF was applied but on lingual surface, no conditioning was considered.
In all groups before adhesive curing, the silicon mold (4 mm diameter and 3 mm length) was placed on the dentin surfaces, and it was fixed during the adhesive curing. The Z250 composite A2 shade (3M ESPE/USA) was applied in two increments and each layer cured for 40 s.
In each group, half of the specimens (n = 12), were kept inside the incubator for 24 h at 37°C. The other half of the specimens (n = 12) underwent thermocycling process. The specimens were subjected to 1000 thermal cycles at 5 and 55 with the dowel time of 30 s in the thermocycling device (Vafaei/Iran). Then, the specimens were dried and their shear bond strength was measured.
Shear bond strength test
To measure the shear bond strength, Universal testing machine Zwick/Roell Z050 (Zwick GmbH and Co. KG, Ulm. Germany) was used. To apply the shear force around the composite cylinder, the blade was inserted in a way that it was in the interface between the tooth and the composite and in contact with the dentin. The force was increasingly at a speed of 1 mm/min, perpendicular to the longitudinal axis of the composite cylinder until the bond failure occurred. In a case that before loading a specimen was deboned, a number of zero would be considered for it. Bond strength was reported as an amount of nominal stress value (in Mpa), which is the amount of load applied at the moment of failure (in N) divided by the total bonded surface (in mm2) (bond strength = F/A).
Evaluation of failure pattern
A stereo microscope (Olympus/Japan) with ×40 magnification was used to examine the failure mode of the specimens. Debonded surfaces were classified according to the mode of failure into one of the following three groups: (1) cohesive (completely in dentin or in resin composite substrate), (2) adhesive (in dentin resin interface), and (3) mixed (the combination of adhesive and cohesive failure).
Statistical analysis
Data were uploaded into SPSS17 software (Chicago, USA). The mean and standard deviation for each group were calculated. Data were subjected to Kolmogorov–Smirnoff analysis to examine normal distribution, in which case since the data distribution was normal, t-test was run to evaluate the shear bond strength, Chi-square was run to analyze the mode of failure. All analyses were performed at the statistical significance level of α =0.05.
Shear bond strengths in MPa (means and standard deviations) for the study groups are represented in Comparison of the shear bond strength in different groups. KI: Potassium iodide, SDF: Silver diamine fluoride, SB: Single bond Adper bond 2, SBU: Single bond universal adhesive.
The t-test revealed that there was statistically significant difference in shear bond strength between Group 1 and 3 (P < 0.05), whereas the bond strength of Groups 2 and 4 was not statistically significant (P = 0.609). Furthermore, the effect of thermocycling process on the shear bond strength of all groups was insignificant.
Failure modes of the study groups are shown in
The results of the present study showed that the use of KI/SDF reduces the shear bond strength compared to the control group. Furthermore, universal adhesive had higher shear bond strength with the prepared surfaces using KI/SDF. Therefore, the first two null hypotheses of the study were rejected. On the other hand, thermocycling did not have a significant effect on the shear bond strength of the studied groups. Hence, the third null hypothesis of the study was confirmed.
In the present study, the shear bond strength test was done to evaluate the bond strength of the specimens. While some believe that microtensile test is the preferred method for bond strength testing, shear bond testing provides more reliable results in examining the effect of dental materials on bonding. It is also easier and less time-consuming to prepare a specimen for a shear test.
The present study showed that KI/SDF reduces the bond strength of both types of adhesives compared to the control group. SDF can interfere with the penetration of primer and bonding into the intertubular and peritubular dentin which leads to the less hybrid layer formation with the lower collagen matrix.
The types of adhesive used in the aforementioned studies were different. Furthermore, a possible reason for the inconsistency among the results of the different studies is the lack of standard sampling method and the SDF application protocol. In some studies, the dentin surface was rinsed immediately after applying SDF. While in other studies, SDF was allowed to be air dried. However, it should be noted that in clinical conditions, neither of these two methods is used. The patient is only asked to avoid eating and drinking for up to half an hour after the SDF application.
With the use of electron microscope, Lutgen et al. showed that when the surface was not rinsed after applying SDF, a thick layer of SDF covered the dentin surface and inside the tubules to a depth of 20 micron. Rinsing the SDF, the extra amount of it which has not been absorbed by the tooth was removed from peri and intertubular dentin. Observating the surface treated by SDF with scanning electron micrograph (SEM) shows that due to the small size of the SDF, it is possible for it to penetrate into the dentin to a depth of 200 μ. Studies have also shown that silver and fluoride ions penetrated to a depth of 450 nm into demineralized dentin. Therefore, it seems that rinsing only removes superficial SDF, so even by rinsing its therapeutic effect still exists. However, this layer can be mechanically removed with the help of 600 grit silicon carbide disc. However, there is a concern that the treated surface with the SDF and formed fluoridated hydroxyapatite may be eliminated.
The highest SDF density is in the most superficial dentin layer, and therefore, the negative effect of SDF on bonding is greater in this area. Among the aforementioned studies, for those that rinsing was not done, the bond strength of the groups that etched by the phosphoric acid was higher.
In previous studies on the use of SDF or KI/SDF, the impact of aging on the bond strength has not been evaluated, so the available information is related to the initial bond strength. The durability of the resin-dentin interface depends on the formation of a compact and hemogen hybrid layer. The acceleration of hybrid layer degradation is stimulated by the thermocycling process. In the present study, the effect of aging on prepared specimens with anticaries material was investigated by applying 1000 thermal cycles. The result of this study revealed that the thermal aging process in groups applying KI/SDF reduces the dentin bond strength of both type of adhesives. Although the reduction was not statistically significant, it was higher in E and R adhesive than in universal adhesive. One study showed that although bond strength was higher in SDF-treated groups, their SEM images showed that limited resin tags were formed in dentin tubules. This contradiction negatively affects the reliability of the bonding result. In fact, in larger cavities, the bond strength is good in some areas and weak in others. Therefore, in the long time, the bonding consistency is compromised
In the present study, failure mode in most cases was mixed and in the second place was adhesive. Furthermore, two cases of cohesive failure were reported. Cohesive failure may happen for some reasons such as an error in the specimen's orientation relative to the longitudinal axis of the testing devices, microcracks in the specimens created during their preparation, and the fragility of the examined material. Therefore, data related to adhesive and mixed failure are more valuable.
This study was conducted on sound dentin. It has been shown that the adhesion to the carious dentin was less successful than to the sound dentin.
This study showed that applying SDF/KI reduces the shear bond strength to the sound dentin. Furthermore, according to the results of the present study, the superiority of universal adhesive over E and R adhesive was confirmed since it had better performance in terms of thermocycled and nonthermocycled shear bond strength. Therefore, in a case of applying KI/SDF, the use of universal adhesive is recommended.
Financial support and sponsorship
This study was financially supported by the Department of Operative Dentistry, Shahid Sadoughi University of Medical Sciences.
Conflicts of interest
The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.