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 Table of Contents  
Year : 2019  |  Volume : 16  |  Issue : 1  |  Page : 25-28

Influence of nano-silver fluoride, nano-hydroxyapatite and casein phosphopeptide-amorphous calcium phosphate on microhardness of bleached enamel: in-vitro study

Department of Operative Dentistry, Faculty of Dentistry, Al Azhar University, Cairo, Egypt

Date of Submission31-Jul-2018
Date of Acceptance11-Dec-2018
Date of Web Publication13-Jun-2019

Correspondence Address:
Mostafa S.M. Ata
Department of Operative Dentistry, Faculty of Dentistry, Al Azhar University, Cairo
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/tdj.tdj_29_18

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This study investigated the influence of nano-silver fluoride (NSF), nano-hydroxyapatite (N-HAP), and casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) paste treatments on the enamel surface microhardness (SMH) after dental bleaching in vitro.
Materials and methods
A total of 60 samples were randomly divided into four groups (n = 15) according to the surface treatment material as the following: (a) control (no treatment), (b) NSF, (c) N-HAP, and (d) CPP-ACP paste. Baseline SMH test of the samples in each group was tested by Vickers microhardness tester. The samples were bleached with 40% hydrogen peroxide for three times (20 min each time). Then NSF, N-HAP, and CPP-ACP paste were applied for 10 days and 2 min/day. The SMH was measured after 24 h and the data were statistically analyzed by one-way analysis of variance.
Postbleaching SMH values decreased in all groups (P ≤ 0.00.1). Post-treatment SMH values significantly increased in comparison to postbleaching values (P ≤ 0.001) except for control group which was not different statistically (P = 0.68). The highest SMH values were observed in NSF group (mean: 238.84 ± 20.31). Statistically significant differences were shown between all groups (P ≤ 0.05).
The use of the remineralizing materials is important in clinical practice to reduce the undesirable changes of enamel surface after bleaching procedures.

Keywords: bleaching enamel, casein phosphopeptide-amorphous calcium phosphate, hardness, nano-hydroxyapatite, nano-silver fluoride

How to cite this article:
Ata MS. Influence of nano-silver fluoride, nano-hydroxyapatite and casein phosphopeptide-amorphous calcium phosphate on microhardness of bleached enamel: in-vitro study. Tanta Dent J 2019;16:25-8

How to cite this URL:
Ata MS. Influence of nano-silver fluoride, nano-hydroxyapatite and casein phosphopeptide-amorphous calcium phosphate on microhardness of bleached enamel: in-vitro study. Tanta Dent J [serial online] 2019 [cited 2021 Dec 7];16:25-8. Available from: http://www.tmj.eg.net/text.asp?2019/16/1/25/260276

  Introduction Top

Tooth Bleaching is the easiest, most widespread, least invasive and is one of the most common requested in dental procedures by the people. Teeth bleaching includes the use of a chemical agent that oxides the organic pigmentation of tooth structure. Although the bleaching procedure removes the pigment molecules, it alsoreduces the hardness of enamel[1]. The loss of hardness of enamel after the use of bleaching agents could be regained by incorporation of agents that can compensate for mineral loss[2]. Recently, synthetic nano-hydroxyapatite (N-HAP) has been considered as bioactive material as it contains calcium nanophosphate organized in a crystalline form of hydroxyapatite and has some potential to repair enamel[3]. A new technology based on casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) that aims to create calcium and phosphate reservoir which bind stably to dental surfaces and so able to prevent negative changes of hardness to enamel[4]. Recently, other alternatives for improving enamel remineralization are metal ions. Silver has different applications in medicine and dentistry due to its anticaries, antimicrobial, and antirheumatic potentials[5]. A recent review reported that silver diamine fluoride can prevent and arrest caries even when applied once a year[6]. Despite its efficacy, silver diamine fluoride has some disadvantages such as black staining of carious tissue, slightly painful lesions which may disappear within 48 h and the metallic taste[7]. More recently, Targino et al.[8], have introduced nano-silver fluoride (NSF). This material's safety has been approved. Also, it does not cause black staining. Based on my search, none of the studies have investigated the postbleaching remineralization effect of NSF, N-HAP and CPP-ACP paste on surface microhardness (SMH) of enamel. Therefore, the goal of this study was to investigate the effects of NSF, N-HAP and CPP-ACP paste treatments to the change of enamel SMH after dental bleaching in vitro. The null hypothesis was that there is no difference in remineralizing effect of these materials.

  Materials and Methods Top

Sample preparation

A total of 60 freshly extracted maxillary premolar teeth indicated for orthodontic reasonsatisfied the inclusion criteria of the study were collected. Teeth with no visible caries or structural defects on enamel surface were used. The teeth were, cleaned, and disinfected in a 0.1% thymol solution at room temperature. The extracted teeth used in this research were obtained from the human tooth bank of the Faculty of Dental Medicine, Al-Azhar University with the requirements of local ethical committee. All patients were informed about using their extracted teeth in this study and a signed consent was given from them according to the instructions of ethics committee of the Faculty of Dental Medicine, Al-Azhar University.

The crowns were separated from roots 2 mm below the cemento-enamel junction. They were embedded in epoxy resin with their buccal surfaces exposed. The teeth were wet polished with Silicon Carbide papers (800 and 2400-grit, respectively), to obtain a flat surface and then washed with distilled water for 20 s. In order to limit the surface to an exposed area, 4 × 4 mm label sticker was placed on the center of the exposed buccal surface and the remaining surface was covered with a colored varnish using applicator brush. Samples were stored in normal saline during the whole experimentation.

Baseline microhardness test

The samples were randomly divided into four groups (n = 15) according to the surface treatment material as the following: (a) control (no treatment), (b) nano-silver fluoride paste (Sigma-Aldrich, Munich, Germany): NSF, (c) nano-hydroxyapatite paste (PrevDent International BV, Amesterdam, The Netherlands): N-HAP, and (d) casein phosphopeptide-amorphous calcium phosphate paste (MI paste, Tooth Mousse; GC Corp., Tokyo, Japan): CPP-ACP. Baseline SMH test of the samples in each group was tested by Vickers microhardness tester (SCTMC, MHV 10002; Weiku teste rmachine Co, Changhai, China) under a 100 g load for 10 s, placing its indenter in the labial enamel surface, and repeated at three random points. The first measurement was performed at the center of the sample and the other ones were at a distance of 300 μm from the first indentation and then the mean for SMH was calculated[9].


The samples in each group were bleached with 40% hydrogen peroxide (Opalescence Ultradent Products Inc., South Jordan, Utah, USA): hydrogen peroxide (HP), three times, 20 min each time.

Postbleaching microhardness test

After 24 h, SMH testing of samples in each group was performed again after bleaching with the same microhardness testing machine and the same operator as mentioned before.

Surface treatment

After the postbleaching microhardness test, the samples were treated according to each groups' manufactures' instructions as following:

  1. Group 1 (control): samples were left untreated.
  2. Group 2 (NSF): samples were treated by application of NSF paste with a brush for 10 days and were left in contact with the enamel surface for 2 min/day.
  3. Group 3 (N-HAP): samples were treated by application of nano-HAP cream with its sponge for 10 days and were left in contact with the enamel surface for 2 min/day.
  4. Group 4 (CPP-ACP paste): samples were treated by application of CPP-ACP paste with its brush for 10 days and were left in contact with the enamel surface for 2 min/day.

Post-treatment microhardness test

The samples were stored between treatments in artificial saliva and incubated at 37°C and humidity of 100%. The samples in each group were then tested with the same condition (load, time, and the machine) and with the same operator after 24 h of the last treatment.

The surface microhardness recovery (SMHR) percentage was assessed as follows: % SMHR equals 100 (post-treatment − postbleaching)/baseline − postbleaching)[3].

  Results Top

The significance level was set at 0.05. The SMH values and the SMHR percentages were compared between groups with one-way analysis of variance and the Tukey HSD post-hoc test.

Baseline, postbleaching and post-treatment SMH values and also % SMHR for each group are shown in [Table 1]. At baseline, SMH values of all samples ranged from 320.38 to 324.12 and no statistically significant differences were seen between groups. Postbleaching SMH values ranging from 112.70 to 121.60, were significantly lower than baseline SMH values (P ≤ 0.001). Post-treatment SMH values ranging from 103.94 to 238.84 and were significantly different from postbleaching ones (P ≤ 0.001) except for control group which was not statistically different. The highest SMH values were observed in NSF (238.84) group followed by CPP-ACP paste group, N-HAP group and control group. However, CPP-ACP paste and N-HAP groups were not different statistically. Recovery of SMH was observed in all groups but % SMHR was negative in control group.
Table 1 Surface microhardness values (mean±SD) and percentage of surface hardness recovery of groups at baseline, postlesion and post-treatment

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  Discussion Top

Reduced enamel surface hardness and increased hypersensitivity as a result of bleaching treatments are matters of concern[10],[11]. In this in-vitro study, three different remineralizing agents were compared to evaluate their effect on bleached enamel. A bleaching agent containing high concentration (40%) of HP (pH: 3.7) was used to obtain the maximum negative effect on the enamel surface followed by evaluation of microhardness[12]. In natural condition, demineralization starts at a critical pH of hydroxyapatite, that is, 5.5, when the oral environment is under saturated with mineral ions, relative to mineral content of teeth[13]. Remineralization helps in regaining the lost calcium, phosphate, and fluoride ions of the tooth structure and is replaced in the form of fluorapatite crystals, which are more resistant to acidic dissolution and substantially larger than the original crystals[14]. In order to overcome the possible mineral loss from enamel due to bleaching procedures, different methods and materials of restructuring bleached dental enamel have been suggested[15],[16]. Bleached enamel showed initial enamel lesion in form of an erosion-like roughened surface that formed on the surface and had significantly more porosity than the sound enamel[17].

The null hypothesis was rejected. The results showed that NSF had the highest SMH values and %SMHRCPP-ACP and N-HAP paste also increased the enamel SMH but they were not significantly different.

In this study, N-HAP in form of paste that contained aqua, xylitol, N-HAP, potassium chloride, menthe-piperita oil, linalool, and limonene were used. N-HAP paste was applied once daily for 10 days. It was found that daily application of 10% N-HAP as its efficacy was similar to 1000 ppm fluoride[18],[19]. N-HAP and CPP-ACP paste enhance the remineralization of enamel surface, which had been demineralized after tooth bleaching[18]. Remineralization of enamel surfaces occurs by occluding the surface microporosities, which results in prevention of stain absorption and increasing SMH[20].

It has been reported that the application of CPP-ACP paste either before or before and after in-office bleaching protocols was able to prevent negative changes of roughness and hardness to enamel[21]. Remineralizing potential of CPP has been attributed to the ability of CPP to localize ACP at the tooth surface. CPP maintains super saturation of calcium and phosphate ions thus modulating the bioavailability of calcium phosphate levels and finally leading to an increase in remineralization[3]. Thus, CPP-ACP has shown to reduce demineralization and enhance remineralization of the enamel subsurface demineralized areas. CPP-ACP has remineralizing effect on artificial subsurface enamel lesions and the remineralization effect increased with increase in the usage of the paste on first, fifth, and 10th day, respectively[22]. There are studies comparing the enamel remineralization potential of CPP-ACP and N-HAP and it was clearly shown that samples had plugs that sealed the fissures formed by demineralization and hence it was concluded that both N-HAP and CPP-ACP were capable of remineralizing early carious lesion and CPP-ACP had the potential to remineralize to a greater extent when compared to N-HAP. In another study, it was considered that both the materials are effective in repair and prevention of demineralization and CPP-ACP showed better results initially, but eventually both had similar remineralizing potential[23].

In this study, NSF paste showed the highest SMH values and % SMHR. Zhi et al.[24]demonstrated that, both silver and fluoride ions were responsible for enamel remineralization. In addition, silver ions could infiltrate into areas of demineralized enamel and precipitate there, which result in enamel hardening. Furthermore, they found that, the small size of the NSF nanoparticles (3.2–1.2 nm) and their spherical shapes potentiate the antimicrobial effect by increasing the contact surface. There are several theories explaining the mechanism of action of fluoride, which invoke its actions towards demineralization, enhancement of remineralization and inhibition of bacterial activity in the plaque. Thus, Zhang et al.[25]suggest the use of silver nanoparticles in adhesives, composites, cements, and sealants to inhibit biofilms and increase remineralization. They found that NSF had the advantage of not staining the dental tissue black, do not form oxides when contacting oxygen in the medium and had no metallic taste as occurs in sodium fluoride treatment. NSF application is inexpensive and thus can be afforded by most communities. The treatment procedure is simple and requires no full dental equipment or a clinical setting. Because this process requires non-invasive procedures, the risk of cross-infection is significantly reduced. Under the conditions of this in-vitro study, 40% hydrogen peroxide bleaching gel application decreased the hardness of enamel. However, bleaching followed by NSF, ACP-CPP paste and N-HAP application led to an increase in the hardness of bleached enamel.

  Conclusion Top

  1. Based on the results of this in-vitro study, significant reduction of hardness occurred after whitening treatment. NSF could have the greatest remineralization capability in comparison to other materials evaluated.
  2. The use of remineralizing material can be useful step in clinical practice to reduce negative changes of enamel surface after bleaching procedures.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

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