Influence of citric acid on the surface texture of glass ionomer restorative materials.

AIM: This study determined the effectiveness of G-coat plus surface protective agent over petroleum jelly on the surface texture of conventional Glass ionomer restorative materials.
MATERIALS AND METHODS: Three chemically cured conventional glass ionomer restorative materials type II, type IX and ketac molar were evaluated in this study. Sixty specimens were made for each restorative material. They were divided into two groups of thirty specimens each. Of the sixty specimens, thirty were coated with G-coat plus (a nano-filler coating) and the rest with petroleum jelly. Thirty samples of both protective coating agents were randomly divided into six groups of five specimens and conditioned in citric acid solutions of differing pH (pH 2, 3, 4, 5, 6 & 7). Each specimen was kept in citric acid for three hours a day, and the rest of time stored in salivary substitute. This procedure was repeated for 8 days. After conditioning, the surface roughness (Ra, μm) of each specimen was measured using a surface profilometer (Taylor & Habson, UK). Data was analyzed using one-way analysis of variance (ANOVA) and Tukey's HSD test at a significance level of 0.05.
RESULTS: The surface textures of all the tested glass ionomer restorative materials protected with G-coat plus were not significantly affected by acids at low pH. The surface textures of all the tested glass ionomer restorative materials protected with petroleum jelly coating were significantly affected by acids at low pH.
CONCLUSION: The effects of pH on the surface texture of glass ionomer restoratives are material dependent. Among all the materials tested the surface texture of Type II GIC (Group I) revealed marked deterioration when conditioned in solutions of low pH and was statistically significant. Hence, a protective coating either with G-coat plus or with light polymerized low viscosity unfilled resin adhesives is mandatory for all the glass ionomer restorations to increase the wear resistance of the restorative materials.

INTRODUCTION

The search for an ideal restorative material to replace natural tooth tissue and the demand for products with good mechanical, adhesive and caries-preventive properties, together with a simple clinical application procedure have led to the development of a number of newer restorative materials. Glass ionomer cement is one among these materials.[1]

Exposure to either extrinsic or intrinsic acids may adversely affect dental restorations, as the low pH level may cause erosion of the glass ionomer based/containing materials and leaching of the principal matrix forming substances.[123]

The increasing use of glass ionomer based/containing restorative materials and the recent trend towards increased consumption of acidic drinks necessitates the study of the effects of citric acid at different pH levels on the surface texture of glass ionomer based/containing restorative materials.[4] Therefore, careful selection of restorative material is important in patients with history of gastric reflux, eating disorders or high consumption of dietary acids.[5]

The purpose of this study was to compare the influence of various concentrations of citric acid on the surface texture of different conventional glass ionomer restorative materials (type II GIC, type IX GIC and ketac molar) with protective coatings petroleum jelly and G-coat plus (a nano filler coating).

MATERIALS AND METHODS

Three chemically cured conventional glass ionomer restorative materials type II, type IX and ketac molar were evaluated in this study. All the glass ionomer cements were mixed according to the manufacturer's directions. The materials were placed in the cubical recesses of 3 mm long × 3 mm wide × 2 mm deep of customized acrylic molds and covered with acetate strips.[5] A glass slide was placed over the acetate strip and pressure was applied to extrude the excess material. Immediately after the setting, the acetate strips were discarded and the specimens were coated with respective protective coatings and stored in salivary substitute for one day at room temperature.

Sixty specimens were made for each restorative material. They were mainly divided into two groups of thirty specimens each. Thirty of the each restorative material samples were coated with G-coat plus a nano filler coating. G-coat plus was applied twice with a micro brush applicator tip and cured for 20 seconds. Remaining thirty specimens were coated with petroleum jelly. The thirty samples of both protective coating agents were randomly divided into six groups of five specimens and conditioned in citric acid solutions of differing pH (pH 2, 3, 4, 5 & 6) levels at room temperature. The pH of the citric acid solutions was adjusted by adding distilled water and was also used as a conditioning agent at pH 7. Each specimen was kept in conditioning solution (citric acid) for three hours a day, and rest of time the samples were stored in salivary substitute (wet mouth). This procedure was repeated for 8 days.[6] Wet mouth is a salivary substitute with a pH of 6.5, which simulates the normal pH of human saliva.

After the conditioning period of 8 days, the samples were rinsed with water and gently dabbed dry with absorbent paper. Surface roughness was then determined using a profilometer with a probe diameter of 0.2 μm. Ra values, for each specimen were taken across the center of each specimen over a standard length of 0.25 mm × 4. The Ra value is the arithmetic mean of the departures of the roughness profile from the mean line calculated by the computer. All statistical analyses were carried out at significance level P value ≤ 0.05 [Table 1]. One-way ANOVA and Tukey's HSD (Scheffe's post hoc test) tests were used to compare the surface roughness of experimental glass ionomer materials after conditioning in citric acid of different pH by using software Statistical Package for the Social Sciences (SPSS version 16.0).

Table 1

Intergroup comparison of mean and S. D values at different pH levels with protective coatings — petroleum jelly and G coat plus

RESULTS

At all pH levels, all glass ionomer restorations with G-coat plus protective coating produced a smoother surface compared to the coating with petroleum jelly. At all pH levels, group III (ketac molar) showed the smoothest surface among all groups, followed by group II and group I respectively. This order of smoothness was same for all the experimental materials irrespective of the protective coatings (both with petroleum jelly and with G-coat plus protective coatings).

For all the groups, the surface roughness value of the specimens with petroleum jelly coating at pH 2 was more and the value variation gradually decreases towards the pH 7. The value variation for specimens with petroleum jelly coating is more in group I, when compared to the other groups and least variation seen in group III at different pH levels. The difference in values for specimens with petroleum jelly coating in all groups is minimal between pH 3 & 4 and between pH 6 & 7.

The surface roughness value of the specimen with protective coating G-coat plus at pH 2 is more for all the groups and the value variation gradually decreases towards the pH 7. The value variation is minimal. Within the same pH, the roughness value variation among the three groups is minimal.

At pH 7, there is no significant difference in surface roughness values among the three groups with petroleum jelly and with G-coat plus protective coatings.

DISCUSSION

In restorative procedures, surface characteristics such as roughness determine the clinical quality and performance of restorative materials. The esthetics of tooth- colored restorative materials is dependent in part, on the surface smoothness. Surface roughness and irregularities can encourage the accumulation of dental plaque, cause gingival irritation, and reduce the esthetics and the longevity of tooth colored restorative materials.[7] Wear resistance and surface roughness in oral environment are important criteria to determine and predict the clinical deterioration of restorative materials.[8]

Hence, the main objective of this study was the evaluation of surface roughness by profilometer. Commonly used parameter for characterizing surface roughness is the center-line average roughness (Ra), which is the arithmetic mean deviation of the surface height from the mean line through the profile.[9]

All the glass ionomer restorations with G-coat plus protective coating showed the surface roughness values (Ra) lesser than those with petroleum jelly protective coating. Glass ionomers consist of glass particles in a hydro gel matrix, which can easily be leached out by acid attack.[5]

At all the levels of pH, group III (ketac molar) showed the smoothest surface compared with group I (type II GIC) and group II (type IX), probably because of the smallest particle size of the ketac molar (3-50 μm), which showed the smoothest surface compared to the rest (which is similar to the study by Pedrini et al.).[10]

As the pH value decreases, the roughness value has increased for all the group restorations. All the samples at pH 2 showed the maximum roughness (Ra) values and samples at pH 7 showed the least values.

In acid solutions, H+ ions of citric acid diffused into the glass ionomer components and replaces metal cations in the matrix. These free-cations would diffuse outward and be released from the surface. As the metal cations in the matrix decreased, more would be extracted from the surrounding glass particles causing them to dissolve (Fukazawa, Matsuya & Yamane, 1990).[11] Consequently, the set material would present a rough surface with voids and protruded, undissolved glass particles. Prolonged exposure of these glass ionomer materials to acids would result in higher Ra values recorded by the profilometer.[5]

As the pH of the acid increases, the H+ ion concentration decreases. Hence, at pH 2, more number of H+ ions will be readily available for replacing the metal cations from the glass ionomer matrix. So the surface will become rough at this pH. As the pH increases from 2 to 3, the H+ ion concentration decreases by 10 times. As it moves from pH 3 to pH 7, available H+ ions also decreases. Thus, the leaching of glass ionomer cements also decreases. This could be the reason for varying surface roughness values (Ra) of the restorative materials at different pH levels.

The conventional glass ionomer type II material take longer time (24 hrs) to form the insoluble aluminum polyacrylate salts, whereas ketac molar and type IX were formed earlier. This reduction in the time can be achieved at the manufacturing stage by the removal of excess calcium ions from the surface of the glass particles, which will reduce the time taken for the development of the calcium polyacrylate chains, and therefore an early resistance to water uptake and lowered solubility.

Hence, the value variation for specimens with petroleum jelly coating is more in group I compared with other groups and least variation were observed in group III at different pH levels.[12]

It is believed that a threshold surface roughness for bacterial retention (Ra = 0.2 μm) exists below which no further reduction in bacterial accumulation can be expected. An increase in surface roughness above this threshold roughness, however, will result in an increase in plaque accumulation, thereby, increasing the risk for both caries and periodontal inflammation.[13]

The materials with Ra values above the 0.2 μm threshold are, type II GIC, type IX GIC and ketac molar at all pH level (except ketac molar at pH 7) samples with petroleum jelly protective coating and type II GIC (pH 2 to 7) and type IX GIC (pH 2) with G-coat plus protective coating.

This deterioration in the surface texture is more likely to cause increased bacterial adhesion, besides causing a clinically rough and dull surface. Therefore, careful selection of restorative material is important in patients with history of gastric reflux, eating disorders or high consumption of dietary acids.[5]

From the results of the study, it is hypothesized that there is a critical pH unique to individual materials at which the surface texture is significantly affected.

CONCLUSION

Within the limitations of this in vitro study

The surface textures of all the tested glass ionomer restorative materials protected with G-coat plus were not significantly affected by acids at low pH.

The surface textures of all the tested glass ionomer restorative materials protected with petroleum jelly coating were significantly affected by acids at low pH.

The effects of pH on the surface texture of glass ionomer restoratives are material dependent. Among all the materials tested, the surface texture of type II GIC (Group I) revealed marked deterioration when conditioned in solutions of low pH and was statistically significant. Hence, a protective coating either with G-coat plus or with light polymerized low viscosity unfilled resin adhesives is mandatory for all the glass ionomer restorations to increase the wear resistance of the restorative materials.