The present study was designed to compare the wear resistance of encapsulated conventional and RM-GICs, and to determine if surface coating of GICs by nanofilled resin based agent has a protective effect against vertical and volume loss of the tested materials. The null hypothesis was partly rejected as the findings revealed no statistically significant difference between the wear resistance of the coated and uncoated GICs. However, there was significant difference between materials of both groups.
An acid-base reaction between the poly acid liquid with the glass powder is the basis of the glass ionomer cement setting reaction. The interaction is formed by the acid attacking the glass network, leading to the discharge of cations like Al3+ and Ca2+ or Sr2+. The early sensitivity of GICs to moisture is due to the formation of calcium polyacrylate, a component that forms within the first 10 minutes of mixing, which is susceptible to hydrolysis. After 24h of mixing, a steadier matrix named aluminum polyacrylate is formed [20–22]. Due to this early water sensitivity, the glass ionomer cement surface should be protected against water contamination during the setting reaction to inhibit the metal cations dissolution. Greater surface corrosion and reduced mechanical and wear properties have been reported in glass ionomer cement restorations contaminated with water during the initial phase of setting .
Wear resistance is considered as one of the most imperative properties for all dental materials in the oral environment. Wear resistance is the capability of the restoration to endure the grinding force of the opposite tooth and food concurrently, whilst upholding its function. Regardless of some favorable properties, GICs have been proven inappropriate for stress-bearing sites because of their poor wear resistance [24, 25]. Therefore, the application of GCP has been recommended to improve mechanical and physical properties of GICs. In a previous study, Bagheri et al  showed that application of GCP has a significant increase on the shear punch strength of Fuji II LC and Fuji IX after 48 h and 8 weeks immersion in distilled water. In another study, they also showed that surface coating significantly increased flexural strength of the most tested GIC materials .
Jafarpour et al  found that coating the GIC restorations decreased water sorption and solubility of almost all tested materials [riva self and light cure (SDI), Fuji II LC and Equia forte Fil (GC)] and reduced their susceptibility to staining. It is speculated that the protective effects of GCP allows complete maturation of the GIC’ reaction, with delayed moisture contamination, preventing water sorption solubility at the same time as creating a stronger material.
Therefore, to examine the effect of coating on wear resistance of GICs, GCP was implemented in the present study. The chewing stimulator was used in this research to simulate mechanical loading and thus increase the clinical relevance of the study. This device is a two-body wear test machine, with its main mechanism in the present study: abrasion in combination with surface fatigue . The specimens were loaded in a biaxial chewing simulator, 120,000 chewing cycles were performed with 700 thermal cycles, which corresponds to a clinical service time of about 6 months.
The application of nanofilled coating in the present study showed no positive effect on wear resistance of the tested materials. Previous studies have been controversial regarding the effectiveness of coating on wear resistance of glass ionomer materials. While Bonifacio et al. , showed significant improvement in the wear resistance of Fuji IX GP Extra when G-Coat Plus was applied, Kielbassa et al.’s  findings on Equia coating supports our outcome by showing a lack of effective long-term protection against abrasive wear for Equia coat. Our finding is also in agreement with that of Rye et al.  who reported no statistically significant differences in the wear resistance between coated and uncoated glass ionomer cements. Moreover, Bertrand et al.  revealed that the application of resin coating caused decreased microhardness of the composite resin's surface. Our results could be justified by the findings of Bagheri et al  which found the application of GCP lead to a significant decrease in the hardness of the GIC restorations. This may be due to the resin-enriched top layer, which is a much weaker phase than the bulk of the cured material and its application cannot reduce the surface wear of GICs. Contrary to our findings, a previous clinical study  proposed that the surface protection with G-coat plus had a protective effect on the clinical wear of GIC approximal restorations in primary molars after 3 years. However, the study did not use an uncoated control group in their methodology, and thereby, their conclusion may not be as reliable. In other work by Ryu et al.  on Equia coat, the authors reported an increase in wear resistance followed by surface protection. The difference in the tested coating as well as fewer chewing cycles implemented in their study may explain the contradictory finding. According to Bonifacio et al. , a micromechanical interlocking was reported between the GCP and the Fuji IX GP Extra under SEM. These findings might suggest that GCP is advantageous in decreasing the early wear when used with Fuji IX GP Extra .
The results of our study revealed a significantly higher wear resistance for one of the RMGICs (Ketac Nano) tested compared to the conventional GICs. Previously, studies have substantiated superior physical properties and high initial strength for RMGICs due to their resin monomer. Croll and Nicholson  demonstrated improved fracture toughness, fracture resistance, and wear resistance for RMGICs. Furthermore, the suitable properties which were fluoride ion hydrodynamics, biocompatibility, favorable thermal expansion and contraction properties, and physiochemical bonding to tooth structure, remain devoid of any degradation. They also proposed that the best mechanical features were reached when the least amount of liquid was used to wet the powder. Daniela S. Rodrigues et al.  also verified the higher wear resistance of RMGICs compared to conventional ones. However, Lohbauer  stated that RMGICs are more disposed to abrasive wear because of a fragile filler matrix coupling and because the added resin monomer and supplementary photo polymerization could not pass the dehydration constraint. This dictates the need for resin coating for at least 1h immediately after restoration. In the present research, Ketac Nano, which is a newly introduced RMGIC, showed the least volume loss values among the tested materials regardless of the coating application. According to the manufacturer’s claims, Ketac Nano is the first nano-ionomer developed in Dentistry. In addition to the classic GIC fluoroaluminosilicate glass, this nano-ionomer comprises silane-treated silica nano-fillers and clusters of single unit nano-sized silica/zirconia, leading to greatly packed filler composition . The findings of the present study substantiate the manufacturer's claims regarding the material’s improved wear resistance compared to that of Fuji II LC, a traditional RMGIC.
As the findings revealed, there was a positive correlation between the volume and vertical loss in all tested materials. Indicating that the alterations observed between the volume loss values of different materials can be attributed to the corresponding vertical losses. The diameter of the abraded geometry was not different between groups following chewing simulation.
One of the limitations of this study was that wear resistance was measured in approximation considering the potential errors using an impression material. Different studies use varying clinical wear measurements making the method unreproducible due to interpersonal variations, making analysis even harder. Although chewing simulator was performed to imitate chewing in the present study, the procedure was still conducted in the laboratory setting where the complete simulation of oral environment was not possible. Thus, further clinical studies are required on this topic. Additionally, the specimens were stored in water for only 24h, therefore the long-term effect of the surface coating should be further evaluated.