Clinical wear of approximal glass ionomer restorations protected with a nanofilled self-adhesive light-cured protective coating

Abstract High viscous glass ionomer cement (GIC) has gained popularity as a restorative material; however, high wear is pointed as one of the major drawbacks of this material. Protective surface coatings were developed to protect GIC from water contamination with the additional advantage of occluding any surface cracks or porosities commonly found in this material, possibly resulting in an increased wear resistance of the restorations. Objective: The objective of this study was to investigate the clinical wear of GIC approximal restorations in primary molars protected either with a nanofilled self-adhesive light-cured protective coating (NPC) or with petroleum jelly. Material and Methods: Approximal caries lesions in primary molars from 32 schoolchildren previously enrolled in another clinical trial were included in this investigation. GIC restorations were performed according to the Atraumatic Restorative Treatment approach and protected with either petroleum jelly or a NPC. Impressions of the restored hemiarch were done after 1 day and 6, 12, 24 and 36 months. The impressions were scanned in a 3-D appliance and the obtained images were superimposed using an appropriate computer software. Two-way ANOVA for repeated measures and Tukey's post-hoc test were used to analyze the wear of restorations (α=5%). Results: A significant difference was found between the two groups, with a wear protection offered by the application of a NPC. Conclusion: These results suggest that the application of a NPC has a protective effect on the clinical wear of approximal GIC restorations in primary teeth.


Introduction
High viscous glass ionomer cement (GIC) has gained popularity in contemporary dentistry, especially in pediatric dentistry, where this material is considered a viable option to restore dental caries lesions. 3 The main advantages attributed to this material are the ease of use due to its bulk application and acceptable physical-mechanical properties, the last property is a result from its high powder/liquid ratio (3:1 to 4:1).
Additionally, the GIC presents good biocompatibility and chemical adhesion to tooth structures, 3,32 as well as the ability of fluoride release and recharge with external sources, which is believed to benefit the restored tooth, 26,32 and possibly the surrounding restored surfaces, 30 which could be considered an advantage, especially for high caries-risk patients.
However, GIC presents some peculiarities that must be respected to ensure its best properties. Essentially, high viscous GIC is composed of a basic (ionleachable) aluminosilicate glass powder that is mixed with an aqueous solution of polymeric acid, resulting in a viscous paste. The setting of GIC occurs in the presence of water, as the polymeric acids needs this medium to releases protons, therefore, starting the acid reaction and setting of the material. During this reaction, the hydrated protons of the acid interact with the glass particles and release metal ions. As a result, some soluble salts such as calcium polyacrylates are formed and gradually replaced by insoluble aluminum polyacrylate salts, leading to the hardening of the cement. This process occurs 24 h after the material's mixing and during this period the GIC is sensitive to water exchanges, which may interfere in the material's mechanical properties. 27 If prematurely exposed to moisture, it may lose substance, which is clinically perceived as surface wear and reduced translucency. If the setting reaction happens in a dry environment, the GIC tends to lose water, which results in dimensional changes, adhesion problems and the formation of internal cracks, reducing the material's strength. 20,28 Some materials are indicated to overcome the issues related to water sensitivity and protect the GIC surface. Varnishes, adhesive systems and petroleum jelly can be used as GIC coatings and among those, petroleum jelly is considered a good option due to its safety and biocompatibility; 7 however, petroleum jelly can be easily washed away 13 and a surface coating that lasts longer is desired, as the GIC should be isolated from moisture during the entire setting period.
Given this context, a new generation of nanofilled self-adhesive light-cured protective coating (NPC) for GIC was developed, claiming to isolate the GIC from saliva contamination during the complete maturation of the material, as well as occluding any surface cracks and porosity of the GIC, reinforcing the strength of this material and therefore, increasing its wear resistance, 17 without compromising the fluoride release when compared to petroleum jelly application 21 nor the caries-preventive effect of GIC. 31 Some laboratory studies have simulated the wear of specimens protected with this NPC and contradictory results were found. 6,24 However, investigating the clinical wear of GIC coated restorations is relevant, as the dental material is exposed to the intraoral environment, in which a complex process involving masticatory loading in the presence of a chemically active environment contributes to the degradation of the restoration. 12 Attempting to answer this question,

Study design
This is an in vitro investigation that is nested to another randomized controlled trial with a parallel group design 19

Clinical procedures
The main study consisted of four protocols for treating primary molars according to the Atraumatic Restorative Treatment (ART) premises. The implementation of the study has been described in detail in another study. 19 Shortly, 208 children (6-7 years old) that presented approximal cavitated caries lesions were included in the main study. Only one cavity per child was selected and four trained operators were responsible for conducting the operative phase in the main study. 19 In this study, a total of 32 restorations were evaluated. The treatments were performed according to the ART guidelines 15 and the cavities were restored using the high viscous GIC Fuji IX (GC Europe, Leuven, VB, Belgium). After inserting the GIC in the cavity, the press-finger technique using a thin layer of petroleum jelly was performed in both groups. Half the sample    GIC + petroleum jelly 16 15 1 (bulk fracture of restoration) 10 2 (patients moved)

Discussion
High viscous GIC has been widely used in pediatric dentistry and considered an effective material to treat dental caries, especially within the ART approach. 1,14 In general, single surface restorations have shown better results than multi-surface ones 14     showed that the coated restorations presented higher survival rates when compared to those protected with petroleum jelly. 19 Therefore, we believe that the bonding of the coat to the GIC was sufficient to protect the material during its maturation period, thus, preserving its mechanical properties. The immediate mechanical properties of GIC are known to be low, and even considered insufficient to withstand mastication forces, but they increase over time. 2,33,34 As already mentioned, GIC is susceptible to water content changing during the setting period, which can decrease the mechanical properties of this material, 7,22 including the FS values. 6  with the GIC, 6 it probably lasts longer on GIC's surface.
Therefore, surface protection is achieved during the entire setting reaction of GIC, resulting in an increased FS of the material 6,24 and a possible protection against clinical wear, which was actually observed in our study.
Given this context, the presence of surface pores and cracks in the GIC are also considered a drawback of this material, since there reports that the propagation of these cracks may result in internal fragility and reduced wear resistance, leading to restoration failures. 24 As previously mentioned, the NPC may promote a micro-mechanical interlock while filling the superficial cracks and pores of GIC 6 and we speculate that, as a consequence of the porosity reduction the propagation of cracks also decreases, possibly resulting in a higher wear resistance of the material, as confirmed by our results. Thus, the NPC could be advised as a surface protection material to be used together with GIC restorations to increase the mechanical properties of the later.

Conclusion
In conclusion, the results of our study suggest that the application of a NPC provides protection against the clinical wear of approximal GIC restorations in primary teeth when compared to the restorations protected with petroleum jelly.