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ORIGINAL RESEARCH ARTICLE  
Year : 2017  |  Volume : 20  |  Issue : 4  |  Page : 260-263
Effect of protective coating on microhardness of a new glass ionomer cement: Nanofilled coating versus unfilled resin


Department of Restorative, School of Dentistry, Islamic Azad University, Tehran, Iran

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Date of Submission04-Aug-2016
Date of Decision03-Oct-2016
Date of Acceptance07-Nov-2017
Date of Web Publication24-Nov-2017
 

   Abstract 

Background and Objectives: EQUIATM is a new gastrointestinal (GI) system with high compressive strength, surface microhardness (MH), and fluoride release potential. This in vitro study aimed to assess the effect of aging and type of protective coating on the MH of EQUIATM GI cement.
Materials and Methods: A total of 30 disc-shaped specimens measuring 9 mm in diameter and 2 mm in thickness were fabricated of EQUIATM GI and divided into three groups of G-Coat nanofilled coating (a), no coating (b) and margin bond (c). The Vickers MH value of specimens was measured before (baseline) and at 3 and 6 months after water storage. Data were analyzed using repeated measures ANOVA.
Results: Group B had significantly higher MH than the other two groups at baseline. Both G-Coat and margin bond increased the surface MH of GI at 3 and 6 months. The MH values of G-Coat and margin bond groups did not significantly increase or decrease between 3 and 6 months.
Conclusion: The increase in MH was greater in the G-Coat compared to the margin bond group in the long-term. Clinically, margin bond may be a suitable alternative when G-Coat is not available.

Keywords: Coating, glass ionomer cements, hardness, resin, surface properties

How to cite this article:
Faraji F, Heshmat H, Banava S. Effect of protective coating on microhardness of a new glass ionomer cement: Nanofilled coating versus unfilled resin. J Conserv Dent 2017;20:260-3

How to cite this URL:
Faraji F, Heshmat H, Banava S. Effect of protective coating on microhardness of a new glass ionomer cement: Nanofilled coating versus unfilled resin. J Conserv Dent [serial online] 2017 [cited 2019 Jul 19];20:260-3. Available from: http://www.jcd.org.in/text.asp?2017/20/4/260/219204

   Introduction Top


Glass ionomer cements have several applications as permanent or temporary tooth-colored restorative materials. They are capable of forming a chemical bond to tooth structure and have cariostatic effects.[1] One important drawback of dental restorations is a change in their surface characteristics such as microhardness (MH), which leads to a reduction in their fracture resistance, and eventual failure of the restoration.[2] Surface MH of gastrointestinal (GI) cements is an important clinical parameter that affects the accumulation of plaque and bacteria, stainability and wear of restorations.[2] In the recent years, GIs have undergone extensive modifications, and several types such as nano-filled GIs have been introduced to the market.[3] GC America recently introduced a new GI restorative system known as EQUIA. The manufacturer claims that this material is a suitable alternative to amalgam and composite for Class I, II, and V restorations due to its improved wear resistance attributed to its surface coating with a nano-filled resin.[4],[5] It is also claimed to have high compressive strength and MH and higher fluoride release potential than other GIs.[4]

In 2013, a study by Bagheri et al. evaluated the effect of aging (through immersion in distilled water) and application of G-Coat Plus on the MH of GI cements and reported that application of G-Coat Plus and aging significantly compromised the mechanical properties of all types of GI cements over time. They also observed that application of G-Coat Plus caused a significant reduction in MH of GI specimens at 4 and 8 weeks.[6]

This in vitro study aimed to assess the effect of aging and type of protective coating on the MH of EQUIA GI cement.


   Materials and Methods Top


In this in vitro experimental study, 30 disc-shaped specimens measuring 9 mm in diameter and 2 mm in thickness were fabricated of EQUIA GI (GC America Inc., IL, USA). Sample size was calculated to be 10 specimens in each group according to a study by Schulz et al.,[7] and using Minitab software considering the difference of 14 between the two means, standard deviation of 8.5, β = 0.2 and α = 0.05.

For the fabrication of specimens, a metal mold was placed on a glass slab. The mold was filled with EQUIA and a Mylar strip was placed over it. Another glass slab was placed on top of it. Two and a half minutes time was allowed for the specimens to set. Next, the specimens were divided into three groups of G-Coat coating (a), no coating (b) and margin bond unfilled resin coating (c). After applying the G-Coat (GC America, IL, USA) in group A and unfilled resin (Margin Bond, Coltene/Whaledent, Germany) in Group C, the specimens were light-cured according to the manufacturer's instructions for 20 s from each direction by a LED light curing unit (Dentamerica, Taiwan) with an intensity of 600 mw/cm2 using the overlapping technique (a total of 60 s). The specimens were polished by 1400 grit abrasive paper discs (Soflex 3MESPE, USA) and then stored in a dark and humid environment for 24 h. Next, the surface MH was measured in the three groups using Vickers hardness tester (InstronWolpert GmbH, Ludwigshafen, Germany). The specimens were then stored in artificial saliva with a pH of 6.9 at 37°C for 6 months. MH of each specimen was measured at 24 h, 3 months and 6 months.

Data were analyzed using SPSS software version 16 (Microsoft, Chicago IL., USA) and repeated measures ANOVA. Considering the inequality of variances, Tamhane's test was used for pairwise comparison of groups. Level of significance was set at P = 0.001


   Results Top


The MH of the no coating group was significantly higher than the other two groups at baseline (P < 0.001), which decreased after 3 months and the MH value of this group was not significantly different at 3 and 6 months. Irrespective of the baseline MH values, the MH of both groups protected with G-Coat and unfilled resin increased at 3 months. The increase in MH was greater in the unfilled resin group compared to the G-Coat group at 3 months (P = 000). At 6 months, the greatest reduction in MH occurred in the no coating group compared to the G-Coat and unfilled resin groups [Figure 1]. The changes in MH of the G-Coat and unfilled resin groups were not significant between 3 and 6 months (P > 0.001).
Figure 1: Microhardness of groups at 6 months

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At 3 and 6 months, the MH of the G-Coat group was slightly, but not significantly, higher than that of the unfilled resin group. The MH values of G-Coat and unfilled resin groups did not significantly increase or decrease between 3 and 6 months [Table 1].
Table 1: Descriptive statistics of microhardness in the three groups at 24 h, 3 months, and 6 months

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Based on the results of Tamhane's test, the MH of G-Coat and unfilled resin groups was not significantly different at 6 months, but significant differences were noted in MH of G-Coat and no coating and also unfilled resin and no coating groups (P < 0.001). Comparison of the MH of the three groups at 6 months also revealed significant differences (P < 0.001)


   Discussion Top


This study aimed to assess the effect of aging and type of protective coating on the MH of EQUIA GI cement. The results showed that at 6 months, the greatest reduction in MH was noted in the no coating group. The changes in MH of G-Coat and unfilled resin groups were not significantly different, and the MH values of G-Coat and unfilled resin groups did not significantly increase or decrease between 3 and 6 months.

Physical characteristics are very important for the selection of a suitable restorative material in the clinical setting. These characteristics directly affect the durability and survival of the restoration. MH is an important characteristic of restorative materials, which has a direct relationship with their compressive strength and wear resistance.[8]

Glass ionomer cements have several advantages compared to composite restorative materials such as the ability to bond to wet enamel and dentin without requiring a bonding agent, cariostatic properties due to fluoride release potential, biocompatibility, and a coefficient of thermal expansion close to tooth structure.[9],[10] However, these advantages may be compromised by inappropriate surface polishing, highly porous surface and low mechanical properties such as low fracture toughness, brittleness, and high surface wear.[9],[10] G-Coat is a nano-filled coating that contains colloidal silica, methyl methacrylate, and camphorquinone.[2] G-Coat is recommended by the manufacturer (GC America) for application on the surface of GI restorations. Thus, the efficacy of GI as an alternative to amalgam and composite restorative materials is still a matter of debate.

The setting mechanism of GI is though the interaction of polyacid liquid and glass powder in the form of an acid-base reaction. During the formation of calcium poly polyalkenoate, aluminum poly polyalkenoate is also formed. These reactions occur gradually, and the setting is completed over a long period. At the same time, mechanical properties improve within the first 24 h following the setting. Increase in strength may continue for weeks to months.[11] In the first 3–6 min following setting, particularly in the clinical setting, water contamination, and even dehydration interfere with the process of setting[9] leading to subsequent propagation of microcracks and mechanical failure.[12] In the current study, changes in MH were evaluated at 24 h, 3 months and 6 months after setting.

Naasan and Watson emphasized that the GI cements should be protected from water contamination during the first phase of setting and from 24 h to 2 weeks after their application. They believed that the surface coating protects the restoration (cement) from wear. Application of a resin coating increases the resistance of cement against water sorption or water loss during the first and final phases of setting.[9]

In the current study, artificial saliva was used as the storage medium for the specimens to better simulate the clinical setting. A previous study showed that the conventional GI without a coating had greater hardness after storage in artificial saliva compared to water after 40 and 80 days.[13] On the other hand, Zoergiebel and Ilie et al. noticed that artificial saliva and distilled water media had no effect on the mechanical and micromechanical properties of GI cements.[2]

Surface micromechanical properties such as Vickers hardness highly depend on the composition of materials. Many previous studies have shown the effects of chemical composition, density, and molecular weight of polycarboxylic acid and glass structure as well as the powder to liquid ratio on surface MH of specimens.[14],[15],[16],[17],[18],[19]

In the current study, we compared the MH of GI specimens coated with G-Coat with those coated with Margin Bond, which is a hydrophobic, unfilled monomer that contains methyl methacrylate. Therefore, lower surface MH in specimens coated with unfilled resin is not far from expectation. The current study results showed that although these two protective coatings had lower MH than the GI cement mass, they well-played a protective role during 3 and 6 months and increased the surface MH of GI specimens. The increase in MH was greater in the group where G-Coat was applied to the specimen surfaces as recommended by the manufacturer. Therefore, in the clinical setting, margin bond may be used as an alternative to G-Coat when G-Coat is not available.

Studies on the effect of different coatings on stainability of conventional and resin-modified GI cements as well as compomers revealed that resin coatings were very effective in decreasing the stainability of conventional and resin-modified GI cements. It has been shown that hydrophobic resins lacking hydroxyethyl methacrylate (HEMA) are more effective for this purpose.[20] Thus, margin bond, which is a hydrophobic resin lacking HEMA was used in the current study because it effectively prevents water sorption.[20]

In our study, the MH of G-Coat group was slightly, but not significantly, higher that of the unfilled resin (Margin Bond) group at 3 and 6 months. The MH values of G-Coat and unfilled resin groups did not significantly increase or decrease between 3 and 6 months. This further confirms the important role of resin coating in maintaining the MH of the restorative material.

The manufacturer claims that G-coat contains nanofillers. Fillers are the strongest phase in composite resins. They are added to strengthen the composite and decrease the percentage of weak resin materials. They eventually lead to an increase in hardness and strength and a decrease in wear.[5] Kim et al.,[21] and Li et al.[22] noticed that increasing the filler content reinforced the mechanical properties of restorative materials. Therefore, the higher MH observed in the G-coat group could be attributed to this incident.

The decrease in MH of both coated groups over time could also be explained by the event of hydrolysis which occurs in the unfilled resin component. Donly et al. showed that hydrolysis of unfilled resin over time could cause defects in restorations.[23] It has been shown that the degradation of unfilled resin is attributed to its high water sorption over time.[24] In general, unfilled resins do not have the required properties of a suitable restorative material. Properties affecting clinical efficacy of restorations including polymerization shrinkage, thermal expansion, modulus of elasticity, water sorption, tensile strength, and wear resistance are mainly attributed to resin components of composites.[25]

It appears that low MH in groups where G-Coat and unfilled resin were applied to the surface of specimens in the first 24 h was due to the weakness of the relatively thick layer of unfilled resin. Over time, this layer undergoes hydrolysis and is somehow eliminated. However, low initial MH in the two groups and wear of coating over 3 months did not prevent its protective effect on GI. At 6 months, only a slight reduction in MH of GI occurred, which is attributed to water sorption, hydrolysis, and wear of the resin coating. It should be mentioned that the manufacturer has guaranteed the positive effectives of G-Coat protective coating for 3 months.

G-Coat is the coating recommended by the manufacturer for application on the GI restorations. Since margin bond, similar to G-Coat, had a protective effect on GI and increased its MH, it may be concluded that margin bond (unfilled resin) may be used as an alternative protective coating when G-Coat is not available. However, the slightly lower efficacy of margin bond in comparison with G-Coat in the long-term should be noted.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
   References Top

1.
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2.
Zoergiebel J, Ilie N. Evaluation of a conventional glass ionomer cement with new zinc formulation: Effect of coating, aging and storage agents. Clin Oral Investig 2013;17:619-26.  Back to cited text no. 2
    
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Ghinea R, Ugarte-Alvan L, Yebra A, Pecho OE, Paravina RD, Perez Mdel M, et al. Influence of surface roughness on the color of dental-resin composites. J Zhejiang Univ Sci B 2011;12:552-62.  Back to cited text no. 3
    
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Friedl K, Hiller KA, Friedl KH. Clinical performance of a new glass ionomer based restoration system: A retrospective cohort study. Dent Mater 2011;27:1031-7.  Back to cited text no. 4
    
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Abdullah Saleh AJ. In-vitro Wear and Hardness of New Conventional Glass Ionomer Cement Coated with Nano-Filled Resin. Thesis Accepted by the Faculty of the Department of Restorative Dentistry, Indiana University School of Dentistry, in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry; May, 2011.  Back to cited text no. 5
    
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Bagheri R, Taha NA, Azar MR, Burrow MF. Effect of G-coat plus on the mechanical properties of glass-ionomer cements. Aust Dent J 2013;58:448-53.  Back to cited text no. 6
    
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Schulze KA, Marshall SJ, Gansky SA, Marshall GW. Color stability and hardness in dental composites after accelerated aging. Dent Mater 2003;19:612-9.  Back to cited text no. 7
    
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Naasan MA, Watson TF. Conventional glass ionomers as posterior restorations. A status report for the American journal of dentistry. Am J Dent 1998;11:36-45.  Back to cited text no. 9
    
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Kato K, Noguchi T, Nakaseko H, Akahane S. The influence of coating for the maturation of glass-ionomer cement. IADR General Session & Exhibition. 2006.  Back to cited text no. 13
    
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Mitsuhashi A, Hanaoka K, Teranaka T. Fracture toughness of resin-modified glass ionomer restorative materials: Effect of powder/liquid ratio and powder particle size reduction on fracture toughness. Dent Mater 2003;19:747-57.  Back to cited text no. 14
    
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Wilson AD, Hill RG, Warrens CP, Lewis BG. The influence of polyacid molecular weight on some properties of glass-ionomer cements. J Dent Res 1989;68:89-94.  Back to cited text no. 15
    
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Fonseca RB, Branco CA, Quagliatto PS, Gonçalves Lde S, Soares CJ, Carlo HL, et al. Influence of powder/liquid ratio on the radiodensity and diametral tensile strength of glass ionomer cements. J Appl Oral Sci 2010;18:577-84.  Back to cited text no. 16
    
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Crisp S, Lewis BG, Wilson AD. Characterization of glass-ionomer cements 3. Effect of polyacid concentration on the physical properties. J Dent 1977;5:51-6.  Back to cited text no. 17
    
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Pires RA, Nunes TG, Abrahams I, Hawkes GE. The role of aluminium and silicon in the setting chemistry of glass ionomer cements. J Mater Sci Mater Med 2008;19:1687-92.  Back to cited text no. 18
    
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Griffin SG, Hill RG. Influence of glass composition on the properties of glass polyalkenoate cements. Part IV: Influence of fluorine content. Biomaterials 2000;21:693-8.  Back to cited text no. 19
    
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Kim KH, Ong JL, Okuno O. The effect of filler loading and morphology on the mechanical properties of contemporary composites. J Prosthet Dent 2002;87:642-9.  Back to cited text no. 21
    
22.
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23.
Donly KJ, Keprta M, Stratmann RG. An in vitro comparison of acid etched vs. Nonacid etched dentin bonding agents/composite interfaces over primary dentin. Pediatr Dent 1991;13:204-7.  Back to cited text no. 23
    
24.
Drummond JL. Degradation, fatigue, and failure of resin dental composite materials. J Dent Res 2008;87:710-9.  Back to cited text no. 24
    
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Correspondence Address:
Foad Faraji
Department of Restorative, School of Dentistry, Islamic Azad University, Noor No18, Shamsabad 9821, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/JCD.JCD_83_16

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