|Year : 2010 | Volume
| Issue : 3 | Page : 193-199
Comparative evaluation of the effect of topical fluorides on the microhardness of various restorative materials: An in vitro study
NC Gill1, A Pathak2
1 Senior Lecturer, Department of Pedodontics & Preventive Dentistry, Dr. HSJ Institute of Dental Sciences & Hospital, Panjab University, Chandigarh, India
2 Professor & Head, Govt. Dental College & Hospital, Patiala, India
|Date of Web Publication||11-Dec-2010|
N C Gill
House no. 1511, Phase 10, Sector 64, Mohali, Punjab
| Abstract|| |
Context: Topical fluorides can recharge the fluoride content of exhausted glass ionomer cements, converting them into fluoride reservoirs. However, the high reactivity of fluoride agents used in topical fluorides may result in the deterioration of surface properties of these restorations. Aim: To evaluate and compare the effect of topical fluorides on the microhardness of conventional glass ionomer cements (Fuji II,GC Corporation,Tokyo, Japan and Ketac Fil Plus,3M ESPE, Seefeld, Germany), high viscosity conventional glass ionomer cements (Ketac Molar Easymix, 3M ESPE, Seefeld, Germany and Fuji IX GP, GC Corporation, Tokyo, Japan), and resin-modified glass ionomer cements (Vitremer, 3M ESPE, St.Paul, MN, USA and Fuji II LC, GC Corporation, Tokyo, Japan). Materials and Methods: Twenty-one pellets were made of each material and stored in distilled water at 37°C for 48 h. These were then randomly divided into 3 subgroups of 7 pellets each. One subgroup was treated by 4 min application of 1.23% acidulated phosphate fluoride (APF) gel, other subgroup with 2% sodium fluoride (NaF) gel, and the third subgroup was used as control. Thereafter, all the pellets were subjected to microhardness testing (load = 100 g for 15 s). Results: APF gel of 1.23% produced a statistically significant decrease in microhardness (P<0.05) of all the restorative materials as compared with restorative materials used as control. The decrease in the microhardness was more pronounced in conventional glass ionomer cements and least pronounced in resin-modified glass ionomer cements. No statistically significant difference (P > 0.05) in microhardness was found after NaF treatment in all the restorative materials tested as compared with control subgroups. Conclusion: The use of 1.23% APF gel may be detrimental to the long-term durability of glass ionomer restorations.
Keywords: APF gel, glass ionomer cements, microhardness, NaF gel
|How to cite this article:|
Gill N C, Pathak A. Comparative evaluation of the effect of topical fluorides on the microhardness of various restorative materials: An in vitro study. J Indian Soc Pedod Prev Dent 2010;28:193-9
|How to cite this URL:|
Gill N C, Pathak A. Comparative evaluation of the effect of topical fluorides on the microhardness of various restorative materials: An in vitro study. J Indian Soc Pedod Prev Dent [serial online] 2010 [cited 2013 Jun 19];28:193-9. Available from: http://www.jisppd.com/text.asp?2010/28/3/193/73784
| Introduction|| |
Restoring carious teeth is one of the major treatment needs of young children. Glass ionomer cements are extensively used as restorative materials in the primary dentition because of their esthetics, biocompatibility, ability to release fluoride, rechargeability, and chemical bonding to enamel and dentine. ,,,
High viscosity conventional glass ionomer cements and resin-modified glass ionomer cements have allowed wider application of these cements because of their properties of decreased moisture sensitivity and improved wear resistance. ,
Restorative filling materials used in dentistry are required to have long-term durability in the oral cavity. One of the most important physical properties of restorative filling materials is surface hardness, which correlates well to compressive strength and abrasion resistance. 
Glass ionomer cements are capable of acquiring further fluoride ions following exposure to topical fluorides thereby acting as rechargeable fluoride release systems.,,, However, the high reactivity of fluoride agents used in topical fluorides may result in the deterioration of surface properties of esthetic restorative materials. ,,,,, This may have implications on the clinical durability of restorations. ,
The present study was undertaken in vitro to evaluate and compare the effect of a 4 min application of 1.23% acidulated phosphate fluoride (APF) gel and 2% sodium fluoride (NaF) gel on the microhardness of conventional glass ionomer cements, high viscosity conventional glass ionomer cements, and resin-modified glass ionomer cements.
| Materials and Methods|| |
The materials used in the study were
Restorative materials [Figure 1].
Conventional glass ionomer cements
- Fuji II (GC Corporation, Tokyo, Japan )
- Ketac Fil Plus (3M ESPE, Seefeld, Germany)
High viscosity conventional glass ionomer cements
- Ketac Molar Easymix (3M ESPE, Seefeld, Germany)
- Fuji IX GP (GC Corporation, Tokyo, Japan )
Resin-modified glass ionomer cements
- Vitremer (3M ESPE, St.Paul, MN, USA)
- Fuji II LC (GC Corporation, Tokyo, Japan)
Topical fluorides [Figure 2]
1.23% APF gel
- Topex (Sultan Dental Products, Englewood, NJ, USA)
2% NaF gel
- pH 7 Neutral Gel (Pascal International Inc., Bellevue, WA, USA).
Twenty-one pellets were made from each material resulting in a total of 126 pellets. The pellets prepared from these restorative materials were mixed and/or cured as per the manufacturer's instructions. Sufficient amount of the material was placed into a brass mold with a specification of 6.5 mm diameter Χ 2 mm thickness, which ensured the standardization of shape and size of each pellet [Figure 3]. The material was pressed between Mylar strips supported by glass slabs on either side. For the light cure materials, glass slabs were replaced by glass slides during curing.  The pellets were allowed to set at room temperature for 15 min.  Finishing procedures were not incorporated as surfaces were cured against matrix strips, which resulted in a satisfactory finish. 
Twenty-one pellets of each material prepared to make a total of 126 pellets were grouped as follows:
Group I-21 pellets of Fuji II
Group II-21pellets of Ketac Fil Plus
Group III-21 pellets of Ketac Molar Easymix
Group IV-21 pellets of Fuji IX GP
Group V-21 pellets of Vitremer
Group VI-21 pellets of Fuji II LC
Each pellet was immersed in 2 mL of distilled water in a color-coded plastic vial and stored in an incubator at 37C for 48 h. 
After 48 h, each group was further divided into 3 subgroups of 7 pellets each [Figure 4] and treated as follows:
In each group
Subgroup A-Each pellet was individually immersed in 2 mL of APF gel for 4 min.
Subgroup B-Each pellet was individually immersed in 2 mL of NaF gel for 4 min.
Subgroup C-Pellets of this subgroup were not treated with any topical fluoride and were used as control.
After topical fluoride treatment, the pellets were thoroughly rinsed with distilled water and were blot dried.
|Figure 4: Pellets in color-coded plastic vials. (a) Pellets immersed in distilled water (Control); (b) Pellets immersed in 1.23% APF gel (Topex); (c) Pellets immersed in 2% NaF gel (pH 7 neutral gel)|
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The microhardness tester (Buehler Micromet II, Lake Bluff, IL, USA) standardized for Vickers hardness test was used in this study [Figure 5]. The testing parameter of 100 g for 15 s was used and the test was carried out (loading and unloading) in an automated sequence. Vickers hardness number (HV) was obtained using the following formula:
HV = 1854.4P/d2
Where P equals the applied load in grams and d equals the average length of the diagonal of indentation measured in microns.
Two indentations were made on each pellet to obtain the average Vickers hardness number (HV) of that pellet.  The procedure was repeated for all the pellets.
| Results|| |
The mean microhardness values were calculated and subjected to statistical analysis.
Wilcoxon's signed-rank test was used for control and after topical fluoride treatment comparison. All the restorative materials tested showed a statistically significant decrease in microhardness (P < 0.05) after APF treatment as compared with control subgroups [Table 1], [Figure 6]. No statistically significant difference (P > 0.05) in microhardness was found after NaF treatment in all the restorative materials tested as compared with control subgroups [Table 2],[Figure 6].
|Figure 6: A comparison of the effect of topical fluoride treatment on microhardness of various restorative materials|
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|Table 1: The mean difference in microhardness (in HV) of various restorative materials between control subgroups and 1.23% acidulated phosphate fluoride gel-treated subgroups |
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|Table 2: The mean difference in microhardness (in HV) of various restorative materials between control subgroups and 2% NaF gel-treated subgroups |
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After APF treatment, the decrease in microhardness was more pronounced in conventional glass ionomer restorative materials (Fuji II and KetacFil Plus) and least pronounced in resin-modified glass ionomer cements (Vitremer and Fuji II LC).
Mann-Whitney U test was used for APF-NaF comparison. All the restorative materials tested showed a statistically highly significant (P < 0.005) difference in microhardness [Table 3].
|Table 3: The mean difference in microhardness (in HV) between 1.23% APF gel- and 2% NaF gel-treated restorative materials |
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One-way analysis of variance (F test) and Newman-Keul range test [Table 4] were used for the comparison of pre- and posttreatment values between different restorative materials tested. Fuji II, Ketac Fil Plus, Ketac Molar Easymix, and Fuji IX GP showed a statistically highly significant (P ≤ 0.0001) decrease in microhardness when compared with Vitremer and Fuji II LC after APF treatment whereas a statistically nonsignificant difference (P > 0.05) in microhardness was observed between these materials.
|Table 4: A comparison of difference in microhardness (in HV) between pre- and post treatment values among the restorative materials |
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Vitremer and Fuji II LC showed a statistically nonsignificant difference (P > 0.05) in microhardness with each other, whereas a highly significant difference (P ≤ 0.0001) was observed with Fuji II, Ketac Fil Plus, Ketac Molar Easymix, and Fuji IX GP after APF treatment.
| Discussion|| |
Topical fluorides are without doubt an important part of preventive dental therapy. Fluoride inhibits demineralization and promotes remineralization, thus encouraging repair or arrest of carious lesions. Topical fluorides can also recharge the fluoride content of exhausted glass ionomer cements, thus converting them into fluoride reservoirs. However, due to the high reactivity of fluoride agents used in topical fluoride treatments, there can be some potential adverse effects on various esthetic 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. Because they release fluoride ions, they are particularly useful where cariostatic action is needed. Recent introduction of high viscosity conventional glass ionomer cements and resin-modified glass ionomer cements has allowed a wider application of these cements.
Conventional glass ionomer cements (Fuji II and Ketac Fil Plus) consist of basic glass and an acidic polymer, which sets by an acid-base reaction between these components. 
High viscosity conventional glass ionomer cements (Ketac Molar Easymix and Fuji IX GP) are also set by acid-base reaction. The setting reaction of these materials is rapid, with the early moisture sensitivity considerably reduced and solubility in oral fluids becoming very low. These properties have been obtained by altering the particle size and particle distribution of the glass powder, so that setting occurs more rapidly. The relative high viscosity of the materials has been achieved by the addition of polyacrylic acid to the powder and finer particle distribution. 
In resin-modified glass ionomer cements (Vitremer and Fuji II LC), polymerizable functional groups are added to impart more rapid curing when activated by light or chemicals to overcome the inherent drawbacks of moisture sensitivity and low early strength of conventional glass ionomer cements. Resin-modified glass ionomer cements allow the acid-base reaction to take its course long after polymerization is complete. 
APF gel 1.23% (pH 3.5) and NaF gel 2% (pH 7) were used in this study since these are the American Dental Association recommended specifications for professionally applied topical fluorides.  Four-minute treatment time of topical fluoride application was used as this time has been recommended for professionally applied topical fluoride solutions, gels, or foams. ,
Surface hardness is defined as resistance to indentation and is influenced by the material's strength, proportional limit, ductility, malleability, and resistance to abrasion and cutting.  One of these properties that have clinical relevance is its resistance to abrasion or wear. Hardness measurements allow relative determination of this behavior. 
Vicker's microhardness test was used in this study as this test is suitable for determining the hardness of brittle materials. 
In the present study, 1.23% APF gel applied for 4 min produced statistically significant (P < 0.05) reduction in the microhardness values of all the tested materials as compared with control subgroups.
One of the components of the APF gel is phosphoric acid, which is meant to etch the enamel and thereby enhance fluoride uptake. A number of studies have shown that phosphoric acid significantly alters the surface morphology of various restorative materials leading to changes in the physical properties, such as hardness, roughness, and erosion resistance. ,,,,,,
Smith  showed that the glass ionomer surface integrity was essentially destroyed after 1 min of phosphoric acid etching and that individual particles dissociated from each other as the gel matrix dissolved.
The decrease in microhardness was more pronounced in conventional glass ionomer restorative materials (Fuji II and Ketac Fil Plus) and least pronounced in resin-modified glass ionomer cements (Vitremer and Fuji II LC) after APF treatment in this study. These findings are in concurrence with studies of El-Badrawy and McComb,  Abate et al.,  and Setty et al.
In resin-modified glass ionomer cements, the acid-base curing reaction is supplemented by a light-/chemical-activated second reaction, which is accomplished by the addition of monomers and/or pendant methacrylate groups, small quantity of resin components, such as Bis-phenol A glycidyl methacrylate (Bis-GMA) or hydroxy ethyl methacrylate (HEMA).  The presence of these resin components could be the reason for a greater resistance of resin-modified glass ionomer cements to acidic challenge.
Difference in microhardness between conventional glass ionomer cements and high viscosity conventional glass ionomer cements after APF gel treatment was statistically nonsignificant (P > 0.05). This can be attributed to the fact that both conventional and high viscosity glass ionomer cements set by the same acid-base reaction. 
Neutral NaF gel 2% (pH 7) applied for 4 min produced no statistically significant (P > 0.05) reduction in microhardness values of all the tested materials as compared with the control subgroups in the present study. El-Badrawy et al. reported no statistically significant degradation of the ionomer matrix even after intermittent 24 h exposure to neutral NaF. However, APF (pH 5) and a nonproprietary NaF (pH 5.8) had a significant effect on glass ionomer matrix and particles.
Statistically highly significant (P < 0.005) difference in microhardness was observed in all the restorative materials treated with APF gel as compared with NaF gel in this study. Diaz-Arnold  reported that neutral NaF recharges glass ionomer cements, while maintaining the integrity of the restorations, although the amount of fluoride release after neutral NaF treatment is not high as compared with APF treatment.
The results of the present study suggest that 1.23% APF gel applied for 4 min produces more detrimental effect as compared with 2% NaF gel applied for 4 min on the microhardness of the restorative materials tested.
The hardness loss of a restorative material may contribute to the deterioration of the material in a clinical environment leading to increased surface roughness, increased plaque retention, loss of anatomical form, and discoloration. This may significantly shorten the lifespan of restorations. ,
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3], [Table 4]