INTRODUCTION

Restorative materials used in the posterior region of the mouth include amalgam, composites, compomers and glass ionomer cements. However, none of these materials has been found to be fully acceptable or ideal. In 1998, a new ion-releasing resin composite material (Ariston pHc, Ivoclar Vivadent, Schaan, Liechtenstein), based on different kinds of dimethacrylates and containing a mixture of inorganic fillers, was introduced. The advantage of this restorative material is that it releases fluoride, calcium and hydroxyl ions to the filling margins, providing additional caries protection.1 The physical properties of Ariston pHc restorative polymer are comparable to those of fine-particle composites.1–2

Ariston pHc releases fluoride, hydroxyl and calcium ions immediately adjacent to the restorative material, depending on the pH value. With a lower pH value caused by active microorganisms in dental plaque, the release rate of the functional ions increases. The ion effects are as follows: fluoride ions help to inhibit demineralization, promote remineralization and inhibit bacterial growth,3–4 while hydroxyl ions neutralize the acid produced by cariogenic bacteria and inhibit bacterial growth.2–7 This phenomenon is based on recently developed alkaline glass fillers and is expected to reduce the formation of secondary caries at restoration margins.8

The monomer matrix of this new material consists of a mixture of dimethylmetacrylates and inorganic fillers, including alkaline glass, Ba-Al fluorosilicate glass, ytterbium trifluoride and highly dispersed silicon dioxide. It also contains a catalyst and stabilizers. It has a white color, so that, for aesthetic reasons, it is more acceptable than amalgam.

To determine which materials release fluoride and are capable of aiding in resisting decay, it is necessary to determine the concentration as time elapses during the release of fluoride.9–11 A study performed by Xu and Burgess, with materials that released fluoride, found that Ariston pHc released 40 μg/cm²/day of fluoride.2

Studies carried out in vitro strongly suggest that the development of decay is also caused by the formation of a high plaque index and a reduction in the pH or acidogenic ability. Additionally, the influence of pH in demineralization also indicates that decreasing the pH may favor increases in the cariogenic process. Reversion of this tooth demineralization can be induced when the pH of the oral cavity is found to be at 5.5.1012 At this critical pH value, human saliva is unable to be saturated with calcium and phosphate ions.13

Therefore, it is reasonable to consider that the chemical process that focused on the development of new materials to be used in dentistry must take into account the possibility of extensive fluoride release, following reduction of the pH to lower than 5.5. Physiochemical surface components found to be involved with bacterial adhesion are surface charge (zeta potential),14–15 hydrophobicity/hydrophilicity,16–17 surface-free energy (SFE)17 and surface roughness.18–20 Consequently, pellicle formation and bacterial adherence are highly dependent upon the reactivity of the surface, which necessitates precise measurements of the physicochemical characteristics of the surfaces.20

The current study investigated the influence of pH on the wettability and fluoride release of an ion-releasing resin composite used as a restorative material—Ariston pHc. The null hypotheses tested included: 1) pH does not influence the wettability of Ariston pHc, 2) pH does not influence the fluoride releasing ability of Ariston pHc, 3) finishing procedures do not influence the wettability of Ariston pHc and 4) finishing procedures do not influence the fluoride releasing ability of Ariston pHc.

METHODS AND MATERIALS

Specimen Preparation, Surface Treatment and Groups Designation

The set-up used in this study is schematically represented in Figure 1. The current study assayed the ion-releasing resin composite Ariston pHc (Vivadent), whose main characteristics are described in Table 1.

View Figure

• Download as Powerpoint
• Download Figure

View Fullscreen

Fifty-four specimens were made in an articulated metal circular matrix (Ø 10 mm x 2 mm) with a fitting system to facilitate removal of the specimens after polymerization.

The material was inserted into a matrix following the manufacturer's instructions using a Centrix syringe (Vivadent). After filling, the matrices were compressed by two glass slides to standardize the surface contour and distance from the source of light over the material. The light source used was an Optilight 600 (Gnatus, São Paulo, SP, Brazil), with the light intensity adjusted to 400 mW/cm², monitored with a radiometer for 40 seconds on both sides of the circular matrix. The matrix was then opened and the samples removed after polymerization.

Using a computer algorithm (http://www.random.org), the specimens were randomly assigned to one of the following surface treatment groups: finishing (G1) and non-finishing (G2) (n=27 per group). Finishing procedures were accomplished with sequential silicon carbide sandpapers (400, 500 and 600 grit), with the specimens washed with deionized water. Polishing was accomplished with a paste polisher of fine granulation (Proxyt, Vivadent) using a disc felt.

Each sample was stored in individual and sterile plastic vials and placed into a Lab-line CO₂ incubator at 37°C until the moment of measurement (seven days). All of the specimens (G1: finishing and G2: non-finishing) were randomly assigned using the same computer algorithm to one of the following three pH subgroups (n=9 per subgroup):

The samples of the first subgroup (G1A and G1B) were immersed in 5 ml of deionized water (pH=6.8). The second subgroup (G2A and G2B) was stored in 5 ml of lactic acid at pH 5.0 and the third subgroup (G3A and G3B) was immersed in 5 ml of lactic acid at pH 5.5. Lactic acid was chosen, since Ariston pHc presents fluoride release at pH levels at or below the critical value. The liquids were not changed during the storage time.

RESULTS

Wettability Results

The wettability results (means and standard deviations) and statistical comparisons are summarized in Table 2. Based on the contact angle data, the following can be observed: the contact angles obtained by both the finishing and non-finishing groups were influenced by the pH value (p<0.05). There was a positive correlation between pH and the contact angle. Generally, all samples presented larger contact angles when glycerol was used as the testing liquid. The comparison between the experimental groups and the control group demonstrated no statistically significant differences (p<0.05) for both water and glycerol measurements.

Table 2 Contact Angles Formed at the Interface of Both Ariston pHc and Enamel

View Fullscreen

Fluoride Release Results

Figure 2 summarizes the fluoride release results (means). The maximum fluoride release occurred in materials at pH 5.0 and 5.5, without taking into account the surface treatment. At pH 6.8, the finished and non-finished samples showed a significant decrease in fluoride release when compared with 5.0 and 5.5 pH values (p<0.05). Non-finished samples showed less fluoride release than finished samples.

View Figure

• Download as Powerpoint
• Download Figure

DISCUSSION

Fluoride is a very important material in the prevention of dental decay, since it directly interferes with the discrepancies of enamel demineralization.8 Since fluoride is released as part of a dentifrice or mouthwash and from restorative materials, the main purpose of the current study was to investigate the influence of the fluoride ion on the surface wettability of a widely used restorative material—Ariston pHc.8

Based on the results of the current study, the first and third null hypotheses tested were rejected. Figure 2 demonstrates that, under the same experimental conditions, both the pH and finishing procedures influenced the wettability of Ariston pHc.

The influence of pH on the fluoride releasing ability of Ariston pHc was also noted. Therefore, the second null hypothesis was rejected.

Table 2 shows the comparison between the conditions with and without finishing when glycerol, which has a medium level of hydrophobicity, was used as a testing medium for determining contact angles. All of the angles found while using glycerol had increased values when compared with those values found with water. When the pH fell, the contact angles were reduced with and without finishing.

Glycerol confirmed the hydrophilic tendency of Ariston pHc. The wet polymer, when dripped with glycerol, “expelled” the glycerol, increasing the contact angles. The interaction with glycerol was very low, while the finished surfaces had a higher interaction with water, as that medium was polar.

The topography of the surface is an important factor that can increase the accumulation of dental plaque on restorative materials. It is accepted that the bacteria adheres to surfaces that are abraded and wrinkled to a greater extent than on flat surfaces.21–22

Table 2 demonstrates the contact angles obtained with the two different liquids (water and glycerol) in the two conditions of finishing and in the three levels of pH. The values of the contact angles between water and glycerol and at the various levels of pH are significant in the two conditions of finishing. The significance of those values indicates that the contact angles measured with glycerol are greater, confirming the hydrophilic characteristics of the material.

Table 2 analyzes the contact angles of the enamel surface with the two liquids (water and glycerol). There were no differences between the two liquids when those structures were measured at a pH of 6.8. The values that were found for enamel showed a tendency toward hydrophilic characteristics, potentially explaining the lack of significance between water and glycerol contact angles.

Evidence from in vivo and in vitro studies state that a continuous presence of fluoride at low concentrations promotes a quicker remineralization processes.23–24 Dijkman showed that a monthly cumulative F release, consisting of 200–300 mg/cm², was sufficient to completely inhibit enamel demineralization.25 DeSchepper and others26 reported that 20 ppm fluoride released from restorative materials seemed to kill bacteria directly, although this kill rate seems to be a function of low pH (below 5) and fluoride release. Others have reported that a minimum inhibitory concentration of 100–200 μg/ml of NaF is required to inhibit the growth of oral streptococci,27 while 30-fold concentrations were necessary for a bactericidal effect. Naturally occurring fluoride, at concentrations as high as 21 μg/ml, did not produce any obvious effects on the composition of supragingival plaque. Additionally, no glass ionomer materials maintained their acidity for periods longer than 48 hours.2

Fluoride ions enrich the oral fluids with fluoroapatite, enabling remineralization, while intervening in the metabolism of the bacterial plaque. This phenomenon happens more intensely with pH levels around 5.0 than at pH levels around 6.8.6 In the current research, the values of fluoride release for Ariston pHc are in agreement with other studies.4 The current results (Figure 2) demonstrate that fluoride release was between 7.9 and 8.6 mg/l at a pH of 6.8. Additionally, the current study confirms that Ariston pHc liberates a larger volume of fluoride at ph levels of 5.0 and 5.5 (10.1 mg/L) compared to when the material was unfinished and at a pH level of 6.8.

The contact angle data obtained in the current study shows that the wettability of Ariston pHc is altered when taking into account the surface treatment. Thus, based on the current results, the third null hypothesis that was tested was rejected.

While these mechanisms explain some of the general characteristics of adhesion to roughened surfaces, they may also introduce physio-chemical changes that affect surface energy and wettability.28

Overall, independent of the pH values, some studies29 that evaluated Ariston pHc showed results of inhibition of secondary caries at greater levels than other such products, such as composites, compomers and glass ionomer cements.

The differences in the studies may be that the adhesive-treated dentin surface was acidic and some resin composites have initiator systems that are more compatible with an acidic environment than other systems.30 Another possibility for the difference is that the surface energy parameters of the monomers of the various resin composites differed in such a way that they interacted with the surface energy parameters of the adhesive-treated dentin, providing different levels of adhesion.2831–33

The pH of 5.0 has a tendency to convert the material to a hydrophilic condition. The contact angles were reduced in a more acidic pH, although it was not statistically different with and without finishing. At the 5.5 pH level, the values of the contact angle were statistically different in the two surface finishing conditions. The most acidic pH caused the hydrophilic material to have moderate hydrophilicity, due to the large organic groups found on the enamel surface.34

Finally, the finishing procedures have an influence on the fluoride releasing ability of Ariston pHc. Therefore, the fourth null hypothesis that was tested was rejected.

The bacterial biofilm on restoration margins is one of the main causes of secondary caries. Therefore, it is important to develop methods that prevent the occurrence or decrease the formation of biofilm on restorations.2935 Several studies132936–40 have demonstrated that fluoride interferes with the dynamics involved in the development of caries and could present an anti-microbial effect and provide demineralization inhibition or dental remineralization.29

Independent of the surface finishing, as displayed in Table 2, Ariston pHc is lightly hydrophilic at a pH of 5.0. This study verified that pH interacts with the material, making ions available on the surface, including fluoride, thus altering the values of the contact angles. Additionally, the material liberates fluoride on finished surfaces at pH levels as high as 5.5, which is the critical pH value for enamel demineralization. An important point to note is the correlation of contact angle and fluoride release. Fluoride and contact angles interfere with bacterial adhesion, because the fluoride released acts as an anti-microbial. It is known that, at low pH levels, the two main cariogenic microorganisms, S mutans and S sobrinus, multiply more quickly.4641 The adhesion of bacteria to oral surfaces is one of the key factors for survival of the microorganisms in the oral environment.27

The decrease in contact angle is caused by the reduction of line tension, due to the redistribution of charges on the three-phase contact line.42 The surface free energy, contact angle, hydrophobicity and wettability are important properties for enamel and restorative materials affecting the initial adsorption of the bacterial microorganisms. Those parameters, mainly wettability, can be changed throughout the course of a 24-hour period.43–48 As the contact angle is affected by many factors, the measurement of a contact angle for a liquid in flow must be very complex.49–51

The microorganisms that develop in dental plaque are hydrophobic. It is possible to establish a positive correlation with the adhesion of S sanguis to the biomaterials and the hydrophobic properties of the microorganisms.52 Altering the hydrophobicity of the surface can influence the adhesion of the bacteria.2153

At a pH of 5.5 (Table 2), significant differences were verified for the contact angle with and without finishing. It is important to note that, in the comparison of fluoride release and contact angle, at a specific pH, there was adequate fluoride release, independent of surface finishing.

Finishing restorative materials is an important step in improving the aesthetics and longevity of a restoration. The finish of the restoration, the superficial roughness and its integrity, as well as the physiochemical properties of the material, can influence the build-up of dental plaque.1854–56 For the samples with surface finishing, the contact angle data showed higher wettability values. As shown in a previous study,57 the topography of the material surface, including the surface of Ariston pHc, is noticeably different with and without finishing, including superficial roughness.

In the current study, the wettability of the examined material was increased when the surface received finishing and the amount of fluoride released was superior. However, finishing only increased the fluoride release at the lowest pH (5.0). Wettability was increased when the material received surface finishing for all pH levels tested. This phenomenon is explained due to exposure of particles, such as silicon dioxide and barium fluorosilicate, which are extremely small and irregular, improving the quality of roughness of that surface and lowering the contact angles for exposure with water.58–60

The organic constituents are polymerized with a photo-curing unit at an irradiation exposure of 450 nanometers, which may release residual monomers. When the organic molecules are excited at a high energy level by the curing light, camphorquinone absorbs an appropriate quantum energy, and its reaction, resulting in free radicals, starts the polymerization process.60 Therefore, the monomer conversion in polymers depends on the material composition, light source, time of radiation and distance of the light source from the object. In general, the conversion occurs in approximately 43.5% to 73.8% of the volume of the material being irradiated.62

Ariston pHc, at acidic pH levels, releases not only fluoride but also calcium and hydroxyl ions, modifying the wettability capacity of the material, explaining the hydrophilic aspect of the material at low pH and with surface finishing. The fluoride recruits calcium and hydroxyl ions to the surface of the material, increasing interactions with the liquid used for contact angle measurements. The contact angles are dependent on the liquids applied for measurements. This relation is governed by two factors. The first is a the adsorption of the liquid and the other involves the surface acid–base interactions, as indicated by Fowkes and others.63 The hydrophobic effect is important with regard to bio-adhesion. There is a tendency for water to form orderly structures when in proximity with polar structures, while water can also attract residues that are not polar.64 At a pH of 6.8, Ariston pHc exhibited less hydrophilicity without surface finishing, which was statistically different from the material with surface finishing. It is important to note that Ariston pHc liberated a smaller amount of fluoride when the pH level and contact angles were greater.

CONCLUSIONS

• The contact angles obtained for both the finishing and non-finishing groups were influenced by the increase in pH value (p<0.05);

• There was a positive correlation between pH and contact angle;

• Generally, all samples presented larger contact angles when glycerol was used as the experimental liquid;

• The comparison between the experimental groups and the control group demonstrated no statistically significant differences (p<0.05) for both water and glycerol measurements;

• At a pH of 5.5, significant differences were verified for the contact angle with and without finishing.