Comparative evaluation of surface microhardness and microleakage after application of commercially available G-Coat Plus, nano silver fluoride resin coat and petroleum jelly over glass ionomer cement restoration: an in vitro study

Introduction

Glass ionomer cement (GIC) is a widely used fluoride releasing tooth coloured material.¹ It has good dimensional stability and coefficient of thermal expansion similar to the tooth structure. However, GIC holds certain undesirable properties like early moisture sensitivity, low strength, poor marginal integrity, and wear resistance.^2–4

In order to overcome the major drawback of moisture sensitivity, various surface coatings like petroleum jelly, varnish, cocoa butter are applied over the GIC restoration.^5–7 The nanofilled, self-adhesive agent G-Coat Plus (GC Corp., Tokyo, Japan) has a single dispersion nano-filler technology for dispersion of silica particles. This gives internal protection to the restorations against cracks and voids for increased fracture toughness. It also reinforces the outer layer for increased wear resistance and protection against acid erosion. G-Coat Plus lamination strengthens, protects and enhances the glass ionomer restorations by giving it a finished superficial surface. However, recent literature evaluating the efficacy of G-Coat Plus states that its application over GIC severely impede its fluoride releasing property. This may predispose it to restoration failure and secondary caries.

Nano silver fluoride is a unique material with amalgamation of silver and fluoride, making it an antibacterial and remineralising agent. The silver nano particles (AgNPs) are claimed to act as fillers and increase the wear resistance of any surface. The incorporation of nano silver fluoride in a resin base was presumed to be a good surface coating over glass ionomer cement and thus, the present study was performed to compare the efficacy of newly formulated nano silver fluoride resin coat with commercially available G Coat Plus and petroleum jelly on microhardness and microleakage of Type IX GIC, under in vitro conditions.

Methods

The present study was carried out in the Department of Pediatric & Preventive Dentistry, Sharad Pawar Dental College, Wardha after obtaining the institutional ethical approval on 29th August 2019 with the reference number DMIMS (DU)IEC/Aug-2019/8298. Sample size was calculated using the following formula: n= $\frac{\left(\mathrm{Z\alpha }+\mathrm{Z\beta }\right)[\mathrm{P}1\left(1-\mathrm{P}1\right)+\mathrm{P}2\left(1-\mathrm{P}2\right)]}{\left(\mathrm{P}1-\mathrm{P}2\right)2}$ and it came down to 15 in each group. Non-carious premolar teeth were taken from healthy patients who underwent extraction for orthodontic purpose.

Results

Table 1: The mean microleakage levels of all the samples in the three groups are compared. In Group 1, all the samples showed dye penetration. Of which, 60% of the samples showed dye penetration of more than 2/3 of cavity depth. Indicating a poor coverage over the GIC surface. 33.3% samples in Group 2 also showed no dye penetration. Remaining samples showed some amount of penetration, however the extent was less than 2/3 depth of the cavity. In Group 3, 46.7% of the samples showed no dye penetration. Remaining 40% and 13.3% samples showed less than 1/3 and 1/3-2/3 penetration respectively. Though Group 3 showed best results, the difference between the all groups is statistically insignificant.

Table 2: The mean micro hardness levels of all the samples in the three groups are compared. The comparison between Group 1 (66.44±4.57) and Group 2 (76.15±3.02) showed no statistically significant difference (p<0.001). The mean microhardness values of Group 2 (76.15±3.02) and Group 3 (79.22±4.03) has no statistical difference (p=0.095). The difference between Group 1 (66.44±4.57) and Group 3 (79.22±4.03) has no statistical difference (<0.001).

Discussion

Conventional glass ionomer cement (GIC) is a fluoride releasing cement which sets by acid base reaction.^8,9 Water content in GIC is critical in attaining optimal properties. It acts as a medium for the dissolution of the polymeric acid, and allows it to ionise and donate protons, thereby behaving as a Bronsted-Lowry acid. The cations are released from the glass and are hydrated unbound water. The unbound water hydrates the cations released from the glass to form a sheath of water around aluminium, magnesium and silica ions. The bound water sheath matures in a centripetal fashion to polymerise the cement.⁸ Maintaining the water balance early in the life of the cement is important. Loss of water leads to formation of microscopic cracks by local contraction as the water escapes.¹⁰ Similarly, contamination with excessive water leaches out the ions causing dissolution of the cement. To prevent this, clinicians are advised to cover the surface of newly placed glass ionomer cement with a protective coating. Various studies are in agreement with this fact that surface protective coatings over Glass ionomer cement are helpful to avoid loss and gain of water during the setting reaction.^5,7,11,12

In the present study, G-Coat plus group showed better results when compared to Petroleum jelly group with respect to both microhardness and microleakage. This is in congruence with studies conducted by Mandikos MN et al.¹³; Tantbirojn D et al.¹⁴ and Arthilakshmi et al. who assessed the microhardness.¹⁵ A study done by Tyagi S. et al. assessed microleakage of dye using spectrophotometer and showed results analogous to the present study.¹⁶ This can be attributed to the even dispersion of nanofillers (30 nm) that have a ‘micro-lamination effect’ giving it an uniform flow over the cement forming a protective coating of about 35-40 μm.^17,18 Though Petroleum jelly is the most commonly used surface protective agent, studies depict its limited efficacy as it gets easily washed away. It may also leave its residues in between the restoration and cavosurface margins leading to persistence of the nanogaps.

In the present study, efficacy of a novel preparation containing nano silver fluoride, resin and tartaric acid was assessed.^19,20 The microhardness of GIC was highest when coated with NSF resin followed by G Coat Plus and Petroleum jelly. This is accredited to the addition of d-tartaric acid which acts as an accelerator aiding in the extraction of ions from the aluminosilicate glass and facilitates their binding to the polyanion chains.²¹ Tartaric acid, also acts as a crosslinking agent in the matrix formation of GIC acting as a base in the setting reaction, thereby accelerating early maturation. This action depends on its concentration. Low concentrations accelerate the development of viscosity of the cement paste, while high concentrations retard it. Literature suggests that enhancement of physical properties of GIC occurs on the addition of 10% (w/w) tartaric acid.²¹ At intermediate concentrations, tartaric acid has an interesting, uniquely favorable effect on setting characteristics. First, it induces a lag period in the setting process during which the viscosity of the cement paste remains constant. This lag period is followed by a sharp, almost exponential, increase in viscosity. Thus, tartaric acid is found to have a dual effect on setting, first inhibiting gelation and then accelerating it. The practical effect is to prolong working time and sharpen setting. A study conducted by Ahmadi Jaya Permana et al. found that addition of tartaric acid enhanced compressive strength of GIC and quoted similar reasons for it.²²

On assessing microleakage, it was found that NSF resin coat had the best results in the present study. However, the difference between G Coat plus group and NSF resin coat group was not statistically significant. NSF resin coat is a self-cured adhesive, hydrophilic resin which polymerises within 20 seconds of application to form a thin film over the restored GIC. It provides convenience of handling and prevents the probability of contamination. The low viscosity aids in sealing the microgaps in between the margins and restoration. The uniformly dispersed nano silver particles present in the coat may also protect the resinous coat from abrasive wear.

Researchers claim that AgNPs are more potent in having bactericidal and bacteriostatic action than silica nano particles present in G Coat Plus.²² The indigenious product used in the present study has additive antibacterial and fluoride releasing property. Application of nano silver fluoride resin coat will aid in halting the initiation secondary caries in the restored teeth by having an anti-plaque action.²³ A study conducted by Rekhalakshmi K. et al. compared the fluoride release after application of Petroleum jelly and G Coat plus, and stated that there was a remarkable decrease in the fluoride release after coating the tooth with G Coat Plus.¹⁶ However, nano silver fluoride resin coat overcomes this shortcoming because of the presence of fluoride in it. It can provide a sustained fluoride release after application.

The present study concluded that nano silver fluoride resin coat over Glass ionomer cement restoration demonstrated better mechanical properties than G Coat Plus and Petroleum jelly, within the limitations.

Conclusion

The novel surface protecting agent containing nano silver fluoride is a self-cured adhesive possessing antibacterial and fluoride releasing properties is thereby intended to augment the properties of glass ionomer cement.