| Abstract|| |
Objectives: The purpose of this study was to evaluate the surface micromorphological changes of conventional glass ionomer cement (GIC) (Fuji II, GC Corporation, Tokyo, Japan) and high strength, high viscosity GIC (Fuji IX GP, GC Corporation) subjected to 1.23% acidulated phosphate fluoride (APF) gel for 4 min. It also evaluated the surface micromorphological changes that occurred in these materials when they were coated with a layer of glass ionomer varnish prior to the application of 1.23% APF gel.
Materials and Methods: A total of 60 uniform glass ionomer cylinders were prepared (30 of each Fuji II and Fuji IX) and stored in distilled water for 48 h, following which they were divided into six groups depending on the surface treatment. Group 1: Fuji II alone, Group 2: Fuji IX alone, Group 3: Fuji II + 1.23% APF gel for 4 min, Group 4: Fuji IX + 1.23% APF gel for 4 min, Group 5: Fuji II + varnish + 1.23% APF gel for 4 min, and Group 6: Fuji IX + varnish + 1.23% APF gel for 4 min. After the required surface treatment had been done, the specimens were mounted, coated, and evaluated under scanning electron microscope.
Statistical Analysis: Kruskal-Wallis test applied to compare the level of degradation among the groups subjected to treatment was highly significant (P < 0.001). Mann-Whitney test was applied to test the association between glass ionomer varnish and level of surface degradation and was also statistically significant (P = 0.001).
Results: The results indicated that APF gel etched the surface of both Fuji II and Fuji IX. However, Fuji IX showed to possess better resistance to the erosive effect of APF than Fuji II. It was also observed that application of glass ionomer varnish prior to APF gel application protected the glass ionomer from the erosive effect of APF gel.
Conclusion: APF gel caused erosive wear of the GICs esp. Fuji II. Hence, it is advisable to avoid the use of APF containing preparations for regular home use especially in children having glass ionomer restoration. If APF gel is indicated or preferred for professional application, it is recommended to apply glass ionomer varnish on the restored teeth prior to the APF gel application.
Keywords: Acidulated phosphate fluoride gel, glass ionomer varnish, glass ionomers
|How to cite this article:|
Khosla E, Kuriakose S, Suderasen C. Surface micromorphological changes of glass ionomer following application of 1.23% acidulated phosphate fluoride: A scanning electron microscope study. Indian J Dent Res 2014;25:493-8
Restoring carious teeth is one of the major treatment needs of children. Today, glass ionomer cements (GIC) (Fuji II, GC Corporation, Tokyo, Japan) are extensively used as a restorative material in the primary dentition for a variety of procedures including occlusal, proximal, labial, lingual restorations and cementation of stainless steel crowns and orthodontic bands. The major advantages of GIC are their ability to release fluoride, recharge ability, chemical bonding to the tooth, biocompatibility, and esthetics. 
|How to cite this URL:|
Khosla E, Kuriakose S, Suderasen C. Surface micromorphological changes of glass ionomer following application of 1.23% acidulated phosphate fluoride: A scanning electron microscope study. Indian J Dent Res [serial online] 2014 [cited 2019 Nov 15];25:493-8. Available from: http://www.ijdr.in/text.asp?2014/25/4/493/142545
The major drawback of conventional glass ionomer is that these materials are sensitive to dehydration and susceptible to moisture contamination, which may result in a decline in their physical and esthetic properties.  This is overcome by allowing the cement to mature under a waterproof protective coating of varnish. 
However, evolution of high strength, high viscosity GICs have allowed wider application of these cements because of their faster hardening, decreased moisture sensitivity and improved wear resistance. ,
Glass ionomers are capable of acquiring further fluoride ions following exposure to fluoridated products; such as solutions, gels and dentifrices, thereby acting as rechargeable fluoride release systems.  However, the high reactivity of the fluoride agents used in topical fluorides may result in the deterioration of the surface properties of esthetic restorative materials. ,,,,, This may affect the clinical durability of the restoration. 
This is of clinical significance because professional fluoride applications are recommended for patients with high caries susceptibility as frequently as 3 months in order to reduce caries. A commonly used topical fluoride for this procedure is 1.23% APF.
Therefore, patients with glass ionomer restorations, who receive topical fluoride treatments, could be at a risk of increased surface roughness, which in turn decreases surface hardness. This would make glass ionomer restorations susceptible to erosion and eventual degradation, thereby significantly shortening the life span of such restorations.
Hence, the present study was undertaken to evaluate the surface micromorphological changes of conventional glass ionomer (Fuji II) and high strength, high viscosity glass ionomer (Fuji IX) when subjected to 1.23% acidulated phosphate fluoride (APF) gel for 4 min. It also evaluated the surface micromorphological changes that occurred in these materials when they were coated with a layer of glass ionomer varnish prior to the application of 1.23% APF gel.
The study hypothesis is that:
- There exists a difference in the level of surface micro-degradation between Fuji II and Fuji IX upon application of 1.23% APF gel
- A layer of glass ionomer varnish applied to the surface of Fuji II and Fuji IX prior to the application of 1.23% APF gel will reduce the level of surface micro-degradation.
| Materials and methods|| |
- Conventional glass ionomer GC Fuji II
- High strength, high viscosity GIC GC Fuji IX
- Glass ionomer varnish; Shofu, Tokyo, Japan
- 1.23% APF gels (Nupro).
- Scanning electron microscope, model: JSM 5600 LV (JEOL Ltd., Tokyo, Japan)
- Ion sputtering unit JEOL JSM-1200 fine coater.
Preparation of glass ionomer cylinders
Thirty cylinders each of Fuji II and Fuji IX were prepared using plastic moulds of the following dimensions-length 8 mm, height 3 mm, and width 3 mm. This ensured the standardization of shape and size of each specimen. The molds were coated with a thin layer of Vaseline on all sides. The materials were mixed according to the manufacturer's instructions and condensed into the Vaseline coated moulds in 3-4 small portions. The moulds were overfilled, and a cellophane strip was stretched over the surface and left undisturbed for 8 min at room temperature. The strips were removed, and excess glass ionomer trimmed off. The surface was finished and polished with fine polishing burs. The specimens were then stored in distilled water for 48 h.
Mounting of specimens
After 48 h, the specimens were dried using oil free water free syringes and divided into 6 groups depending on the surface treatment they received. Each group consisted of 10 cylinders.
- Group 1: Fuji II alone (control group)
- Group 2: Fuji IX alone (control group)
- Group 3: Fuji II + 1.23% APF gel application for 4 min
- Group 4: Fuji IX + 1.23% APF gel application for 4 min
- Group 5: Fuji II + varnish + 1.23% APF gel application for 4 min
- Group 6: Fuji IX + varnish + 1.23% APF gel application for 4 min.
(Note: In Group 5 and 6, chemical cure glass ionomer varnish was applied to the surfaces of glass ionomer cylinders and left to dry for 15 min before application of APF).
After the required surface treatment, the specimens were thoroughly rinsed with distilled water, dried, mounted and coated for examination under SEM.
Specimen coating was done in JEOL fine coater ion sputtering unit (JSM-1200) (JEOL Ltd., Tokyo, Japan). The mounted specimens were fixed in the sputter unit. The pressure of its vacuum chamber was reduced. A constant volt of 1 KV was maintained for 3-4 min. Thus, the heated coating material was rapidly evaporated into monatomic state. The vapor molecules, essentially unimpeded by air molecules, left the source and moved in a straight line until they impinged on the surface of specimens where they condensed to form a thin film. Using specimen holder, the specimens were mounted on the rotary device, which facilitated presenting all surface to evaporated source. Coating of all surfaces was carried out. The stub with the specimen was placed inside the vacuum chamber of scanning electron microscope (SEM). The accelerating voltage, angle of tilt and aperture were adjusted to suit the specimen and optimize the quality of micrograph. The surface was scanned at different magnification, observed on the screen and rated according to the following criteria:
- Surface intact: Surface appears undisturbed, glass particles in level with matrix, no visible etched surfaces
- Mild degradation: Surface appears roughened, glass particles in level with matrix, no visible cracks or voids
- Moderate degradation: Surface irregular with particles partly protruding, a limited number of cracks or slight pitting of glass particles
- Severe degradation: Little or no matrix around particles, the excessive number of voids, severe cracking or pitting present.
| Results|| |
Control groups of Fuji II and Fuji IX did not show any signs of glass particle degradation or matrix dissolution. Narrow cracks and surface crazing could be seen. Agglomerates of unreacted particles were also seen. Glass ionomer lacks the resistance to dehydration, and the crazing is most likely due to high vacuum necessary for SEM.
Glass ionomer + 1.23% APF gel application for 4 min
Considerable dissolution of the matrix was observed in both the groups. Significant etching effect of this gel on glass particles was evident as pitting and cracking. Large numbers of voids-both rounded and irregular were also observed.
Glass ionomer + varnish + 1.23% APF gel application for 4 min
Only localized areas of etching lying beneath the folds of varnish were seen in both Fuji II and Fuji IX. These represent areas were the varnish was absent and thereby got exposed to the etching action of APF gel. The data were further analyzed to see the effectiveness of varnish in each group.
Nonparametric test was applied to compare the levels of degradation among the four groups which were subjected to treatment. Kruskal-Wallis test revealed that the difference between four groups was statistically highly significant with Group 6 (Fuji IX + varnish + 1.23% APF gel) showing least degradation (Mean rank 10.20) and Group 3 (Fuji II + 1.23% APF) highest degradation (34.30) [Table 1] and [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6].
|Figure 1: Fuji II after immersion in water for 48 h. No other surface treatment done|
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|Figure 2: Fuji IX after immersion in water for 48 h. No other surface treatment done|
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|Figure 3: Fuji II after application of 1.23% acidulated phosphate fluoride gel for 4 min|
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|Figure 4: Fuji IX after application of 1.23% acidulated phosphate fluoride gel for 4 min|
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|Figure 5: Fuji II protected with varnish prior to application of 1.23% acidulated phosphate fluoride gel. Showing etching in areas where varnish was absent|
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|Figure 6: Fuji IX protected with varnish prior to application of 1.23% acidulated phosphate fluoride gel. Shows etching in areas where varnish was absent|
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|Table 1: Comparison of level of degradation among APF group and varnish protected APF group |
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Nonparametric Mann-Whitney test was used to analyze and compare the level of degradation between Fuji II and Fuji IX subjected to 1.23% APF gel for 4 min. It showed that Fuji II and Fuji IX were significantly different with Fuji II showing more degradation than Fuji IX (P = 0.001) [[Table 2], Graph 1a and b].
|Table 2: Statistical analysis of level of degradation among the four groups which were subjected to treatment |
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Mann-Whitney test also analyzed the association between varnish and level of micro-degradation. It was found that application of varnish prior to applying 1.23% APF gel, caused highly significant reduction in the level of surface degradation in both Fuji II and Fuji IX groups (P < 0.001) [[Table 2] and Graph 2].
However, both Fuji II and Fuji IX coated with varnish showed a similar level of degradation with P > 0.05; that was not statistically significant [Graph 3].
| Discussion|| |
Fluorides are undoubtedly an important part of preventive dental therapy. They are capable of enhancing the remineralization of partially decalcified tooth structures. Topical fluorides can also recharge the fluoride content of exhausted GICs, thus converting them into fluoride reservoirs. Three types of fluoride preparations are being used for professional applications-APF, stannous fluoride and sodium fluoride. 1.23% APF gel is extensively used in dentistry especially pediatric dentistry because of their ease of application and clinical efficacy.
However, due to the high reactivity of fluoride agents used in topical fluoride treatments, there can be some potential adverse effects on various restorative materials.
Glass ionomer cements are clinically accepted preventive restorative materials because of their fluoride releasing property besides their esthetics, biocompatibility and chemical adhesion to enamel and dentin. Since they release fluoride ions, they can reduce enamel solubility and plaque formation by bacteria that initiate dental caries, thereby preventing enamel demineralization along with preventing secondary caries and caries on adjacent teeth. The evolution of high strength, high viscosity conventional GICs with improved physical properties has led to increasing clinical acceptance of these materials. The setting reaction of these materials is rapid, with the early moisture sensitivity considerably reduced and solubility in oral fluids becoming very low.
These materials can also be converted to fluoride reservoirs by subsequent topical fluoride application. ,
APF gel 1.23% (pH 3.5) was used in this study since it is recommended by American Dental Association for professionally applied topical fluoride and also is the most widely used one today. Four-minute treatment time was used as this time has been recommended for professionally applied topical fluoride gels, solution and foams.
The possible adverse effects of topical fluoride treatments on these esthetic restorative materials have been the subject of many studies during the recent past. Considerable work has been done on the effect of professionally applied APF and sodium fluoride (NaF) agents on Ceramics, composites and sealants, which revealed important structural alterations that were dependent on the fluoride agent used. [ 8],,, The damage caused by APF on glass ionomer has been shown by Kramer et al.  who reported that an APF mouth rinse caused greater solubility of glass ionomer compared with other types of fluoride rinses following 72 h of immersion.
It has been suggested that the application of APF gel could recharge the fluoride content of exhausted GICs.  However, APF gel contains hydrofluoric and phosphoric acids. Phosphoric acid has the ability to etch glass particles, and hydrofluoric acid is more destructive than phosphoric acid because it can etch glass at lower temperatures. The pH of the APF gel results in the chemical erosion of the restorative materials by acid etching the surface and leaching the principle matrix-forming cations, Na, Ca, Al, and Sr ions, which also increases its fluoride release. 
However, erosion of GICs depends not only on the pH of the gel, but also on the complex forming ability of the particular acid present.  el-Badrawy et al.  showed that Neutral sodium fluoride had no significant effect on glass ionomer, whereas APF and nonproprietary sodium fluoride, containing phosphoric acid and citric acid were shown to cause significant dissolution of the matrix of glass ionomer. Phosphoric acid was capable of forming stable complexes with metal ions in the ionomer, resulting in greater surface erosion. Glass ionomer surface integrity is essentially destroyed due to the dissociation of individual particles from each other as the gel matrix dissolves. Neuman and Garcia-Godoy also showed that glass particles were left protruding from the cement surface after APF gel application. 
One of the most important physical properties that affect the long term durability of the restorations in the oral cavity is surface roughness. The increased roughness could become an area to harbor the colonization of Streptococcus mutans. Increased plaque formation has been found on conventional GICs in situ. This could potentially increase the risk of periodontal disease and dental caries, especially sub gingivally although pathogenicity of the plaque may be low. The increased roughness may be a reflection of the deterioration of the materials. Protection of the glass ionomer restorations by adding fresh cement or resin composite overlays may be considered if the restorations are not replaced. Loss of hardness of the material may contribute to the deterioration of the material in a clinical environment, including loss of anatomical form and discolorations. ,,
In the present study, single application of 1.23% APF gel to glass ionomers for 4 min resulted in significantly increased surface roughness that was particularly obvious with conventional glass ionomer (Fuji II) and less pronounced with high strength high viscosity glass ionomer (Fuji IX).
Earl et al. tested a number of varnishes and concluded that immediate sealing of GIC surface with an unfilled light cured bonding agent with very low viscosity appears to be most effective method of protecting the surface of newly placed GIC.  Serra et al. in 1994  tested various surface treatment agents and concluded that nail varnish provided the best results . Unfilled resin glaze (ketac glaze) has been shown to protect glass ionomer from the erosive effect of APF. 
In this study, chemical cure glass ionomer varnish (Shofu, Tokyo, Japan) was used to protect the GIC surface. It was applied with a brush and allowed to dry for 15 mins following which the surface of glass ionomers were treated with 1.23% APF gel for 4 mins. The surfaces were then rinsed, dried and scanned using JEOL JSM-5600 LV SEM. The surfaces examined were found to be smooth. They showed no signs of matrix dissolution or degradation of glass ionomer particles. However areas where the varnish was absent got exposed to the APF and showed etching, thus establishing the protective effect of this varnish.
| Conclusion|| |
APF gel caused erosive wear of the GIC esp. Fuji II. Hence, it is advisable to avoid the use of APF containing preparations for regular home use especially in child with glass ionomer restoration. If APF gel is indicated or preferred for professional application, it is recommended to apply glass ionomer varnish on the restored teeth prior to APF gel application.
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Department of Pedodontics and Preventive Dentistry, Mar Baselious Dental College, Kothamangalam
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2]