Shear Bond Strength of Resin Modified Glass Ionomer Cement Using Different Enamel Conditions

Aims: In an effort to improve the shear bond strength of resin modified glass ionomer cement on orthodontic brackets, various enamel conditioning have been evaluated for use with this cement. Materials and methods: A total of 100 freshly extracted human premolars were subjected to two steps of treatment. The first step of treatment involve dividing the teeth into 5 groups each of 20 teeth: (I) treated with 20 second acid etching with 37% phosphoric acid; (II), treated with micro–etching using 50 μ aluminum oxide; (III), treated with air polisher using 45 μ sodium bicarbonate; (IV), treated with coarse finishing disk for 10 second; (V), left the enamel clean without treatment. In the second step each group then subdivided into two subgroups, ten teeth subjected to wetting with tab water and ten left dry. Following, storage for 24 hours at 37°C in distilled water, shear debonding force was measured using a Universal Testing Machine with a cross–head speed of 10 mm/minute. Results: The result indicated that the highest shear bond strength was for acid etched enamel under wet condition with lowest mean for normal dry enamel. the Mann–Whitney analysis estimated a significant difference between wet and dry condition in general with high probability of bond failure for the dry than that of wet conditions. Conclusions: The suitable enamel conditions regarding the shear bond with the mode of bond failure had been shown to be the wet and dry situation of group III and wet situation of group IV. But, it could be concluded that the most suitable enamel condition may be that treated with a coarse finishing disk under wet condition.

the incidence of bond failure (1) .Add to the disadvantages of conventional acid etching resin bonding agent which are the loss of enamel during etching and the remaining resin residue that cannot be easily removed after debonding of the bracket (2) .
The introduction of resin modified glass ionomer cements (RMGICs) which are auto set by the acid-base reaction of glass ionomer cements, have a diffusionbased adhesion between the cement and the tooth surface (1) , combines the advantages of conventional glass ionomer cements with the ability to form chemical bonds with enamel, dentine and metal, significant amount of fluoride release to protect against decalcification (3) , absorb fluoride from other sources, such as fluoride toothpastes and mouth rinses, thus acting as a rechargeable, slow-release fluoride device, they also possess the advantage of easier debond with less potential for damage to the enamel; finally their ability of adhesion even in wet condition (4) .However, RMGICs have lower shear bond strength (SBS) compared to composite resins (5,6) , particularly within the first half hour after bonding (7) , with widely varying bond strengths have been reported, ranging from 5.39 to 18.9 MPa (1) .
Moreover, besides the improvement achieved by the combination of resin composites, the RMGICs still have a lower shear bond strength (8,9) .As a result, The aim of this study is to evaluate various enamel conditions trying to reach to best resin modified glass ionomer cement bond properties (shear bond and mode of bond failure).

MATERIALS AND METHODS
A random selection of 100 freshly extracted human premolars, that had been stored in a 0.1% thymol solution after their debridement from soft tissues remnant.The criteria for tooth selection included intact buccal enamel, no cracks caused by the extraction forceps, no restoration material and no caries.
The roots of teeth were grooved to aid retention and then mounted in plastic ring with stone with their long axes vertical and their crowns protruding.The bonding area was cleaned with a mixture of water and fluoride-free pumice powder, with a rubber polishing cup in a low-speed handpiece for 10 seconds, rinsed with water for 15 seconds, and dried with oilfree compressed air for 15 seconds, then the samples were randomly divided into five groups.Enamel treatment: The enamel treatment composed of two steps: The first step composed of the following groups each of 20 teeth: Group I: the buccal enamel surfaces were acid etched with 37 % phosphoric acid for 20seconds, washed and dried thoroughly (4) .Group II: the buccal enamel surfaces were sandblasted with a micro-etcher (DANVILLE Materials Innovative Dental Product) using 50 µm (Recommended by the manufacture) aluminum oxide for 5 seconds at a distance of 4 mm and then blown with air to remove any residual contamination (10) .Group III: the buccal enamel surfaces were air polished with 45 µm sodium bicarbonate for 10 second then were blown with air to remove any residual contamination (11) .Group IV: the buccal enamel surfaces were treated with coarse finishing disk for 10 second (12) .Group V: the enamel left clean.
The second step involved subdividing each group into 2 subgroups each of 10 teeth.The first subgroups were left as they are and the second subgroup subjected to wetting of the confined area (area to which brackets will attach) with tab water before applying the brackets with the adhesive cement to the buccal surfaces.Therefore the end result was 10 subgroups of different treatment modalities.Bracket bonding: Dentarum (Dentarum, Pforzheim, Germany) standard edgewise orthodontic stainless steel premolar brackets, were used in this study, with an 0.022 × 0.030-inch slot and a base surface area of 10.64 mm 2 .The bonding procedures followed the manufacturer's instructions, which involve mixing the base and catalyst of RMGIC in ratio of 3/1 for powder to liquid respectively after that the paste had been applied on the bracket base then applying the brackets on the confined area on the buccal surfaces after their treatment as shown above then a force of 200 g was applied to each bracket using a surveyor with simple modification to standardize the adhesive thickness.Any excess cement was removed with sharp probe.The bonding material was lightcured on the mesial, distal, incisal, and gingival aspects for 10 seconds for a total of 40 seconds, after that, the brackets were debonded with across head speed of 10 mm/ minute, the shear debonding strength was measured which first recorded in Newton then converted to Mega Pascal, then brackets were examined for adhesive remnant using 10× magnification Microscope (Olympus) (4) .Any adhesive remaining after bracket removal was assessed according to the modified adhesive remnant index (ARI) and scored with respect to the amount of resin material that adhered to the Bracket base (13) .
The criteria for evaluation were: The modified adhesive remnant index scale has a range of 5 to 1 score according to the amount of adhesive remain on the bracket base: Score Definitions: 5: All of adhesive remained on bracket.4: More than 90% of adhesive remained on bracket.3: More than 10% but less than 90% of adhesive remained on bracket.2: Less than 10% of adhesive remained on bracket.1: No adhesive remained on bracket.

RESULTS AND DISCUSSION
For the mean and Duncan grouping of shear bond strength, as presented in Tables (1and 4), it was observed that the highest mean and Duncan grouping are for acid etched enamel under wet condition with the lowest mean is for normal enamel under dry condition, with the more or less the same reading for the remaining groups, these results may ordinarily be due to the fact that the use of acid etching result in highest bond to enamel as explained before in literature (10,19,20,21) (acid etching of enamel result in deep resin tag which may reach to a depth of 5-25 µ with the diameter of the defect ranging from 5-6 µ comparing to a uniform roughness of the enamel up to 5 µ in depth as produced by micro-etching technique for example).Although the manufacturer of Fuji Ortho LC (FOLC, GC Corporation Tokyo), reports that RMGICs can be used in a moistened environment with no acid etching and obtain clinically acceptable bond strengths.This was verified by Silverman et al., (17) in a clinical study.These features would save chair time and allow a safe debonding without enamel damage.A previous study by Cacciafesta et al., (18) using RMGIs showed that saliva contamination actually improved shear bond strength.and combining the results presented in Tables (1, 2 and 3) had been proved to be the same that of manufacturers instruction and direction, in that the use of RMGIC in wet condition could result in higher shear bond strength than in dry condition.Referring to the results presented in Table (5) it had been shown that the Mann-Whitney analysis estimated a significant difference between wet and dry condition in general with high probability of bond failure for the dry than that of wet conditions.
But, reference to the Kruskal-Wallis analysis as it was presented in the same table, it had been shown that better mode of bonds failure are for normal enamel in dry and wet situation than the remaining enamel treatment approaches, add to acceptable mode of bond failure could be seen in the same table for the air polishing and coarse finishing disk treatment approach under dry situation and coarse finishing disk treatment approach under wet situation.Furthermore, the coarse finishing disk treatment approach had been shown to produce a suitable SBS in both wet and dry situation referring to minimum and maximum value of the SBS readings, according to the standardization of SBS that presented by Reynolds (22) (the preferable SBS is 7.00 MPa which could withstand orthodontic forces, force of mastication add to their preferable mode of bond failure which is usually adhesive failure at bracket enamel interface).Thus, all above could explain that the preferable enamel condition for bonding brackets using RMGIC Fuji Ortho LC is enamel treatment with coarse finishing disk under wet situation according to the combination of suitable bond strength and acceptable mode of bond failure that result in acceptable bond strength with nearly intact enamel surface after debonding.

CONCLUSIONS
It had been shown that the best enamel condition with regard to bond strength is that treated with 37% phosphoric acid for 20 seconds with the wet better than dry situation which come similar with the manufacturer recommendation.The best enamel condition in regard to the mode of bond failure as it had been presented with the enamel left clean.The suitable enamel conditions regarding the bond strength and the mode of bond failure are those air polished with 45 µm sodium bicarbonate for 10 second and wet situation of those treated with coarse finishing disk for 10 second.But, it could be concluded that the most suitable enamel condition is: Its treatment with coarse finishing disk under wet condition.

Table ( 1
): Descriptive statistics of shear bond strength in MPa.

Table ( 2
): One way ANOVA of shear bond strength for wet / dry.

Table ( 5
): Mode of bond failure for all groups.Mann-Whitney U= 932.500, p= 0.000, Significant AE: Group treated with acid etching; ME:Group treated with microetching; AP: Group treated with air polisher; CD: Group treated with coarse finishing disk; N: Normal enamel condition.