Evaluation of the Effects of Mouthrinses on the Hardness of Esthetic Restorative Materials (Full)

by Kevin B. Frazier, D.M.D.* and John C. Wataha, Ph.D.**

Introduction

Mouthrinses are used to control a variety of dental conditions including gingivitis, caries, xerostomia and halitosis. Previous studies have shown that alcohol-containing products can affect selected physical properties of resin-based restorative materials. Surface hardness, diametral tensile strength and shear bond strength decrease after exposure to ethanol. A decrease in surface hardness can be expected to affect clinical properties of resin materials such as wear-resistance. A decrease in wear resistance of any dental restorative material may result in premature failure of the restoration, thus requiring its replacement. Alcohol-free products have been developed to minimize the effects of rinsing on restorative materials. The effects of many alcohol-free rinses on resin-based restorative materials has not been reported.

Purpose

One of the most common conditions that mouthrinses are used for is halitosis. A wide variety of commercial products have been introduced for this condition and the effects of many of these rinses on resin-based materials is unknown. The purpose of this project was to evaluate the effects of several commercial mouthrinses (with and without alcohol) on the surface hardness of commercial esthetic resin-based restorative materials.

Materials and Methods

The commercial halitosis-prevention mouthrinses that were used in this study included: Listerine®, Cepacol®, Rembrandt®, Oxygene® (regular and professional strength), and Breath Rx® (Table 1). The esthetic resin-based restorative materials that were tested included: Herculite XRV®, Silux Plus®, Photac-Fil, and Hytac® (Table 2). Forty disc-shaped samples of each restorative material measuring 10.0 mm in diameter and 1.0 mm in thickness were fabricated in a split mold consisting of stainless steel and mylar. The only material requiring mixing was Photac-Fil and it was triturated according to manufacturer’s instructions prior to insertion into the mold. All specimens were light-cured (Optilux 400, Demetron Research, Kerr Corp.) at 600 mWcm2 for a total of 40 sec. on each side to assure complete photo-polymerization. The specimens were removed from the split mold and then stored for 24 hours in deionized water at 37°C in a constant temperature cabinet (Stabil-Therm, Blue M Electric Co, Blue Island, IL).

Following initial storage, five samples of each material were then placed in individual containers with 20 ml of each of the test rinses (Table 1) for 12 hours to simulate a one-year regimen of rinsing for 2 minutes per day (2 min. / day x 365 days=730 min. simulated by 12 hrs. x 60 min. =720 min.). Deionized water was used as a negative control (no affect). The specimen containers were gently agitated (1 cycle / 1.5 sec) and held at a temperature of 37°C during the immersion period in a metabolic shaking incubator (Dubnoff, Precision Scientific, Chicago) to mimic a “swishing” effect at body temperature.

After the 12-hour immersion period, the samples were rinsed in deionized water and subjected to hardness testing. The hardness test was carried out in a Microton Hardness Tester (Model MO, Wilson Instrument Division, American Chain and Cable Co., New York) with a weight of 500 gms. Five readings were made on each sample and the observed filar units were converted to hardness values (Knoop Hardness).

For this experiment, the dependent variable was material hardness and the independent variables were restorative material and type of mouthrinse. Analysis of variance (ANOVA) was used to determine significant differences between treatment groups and material groups. One-way ANOVA was performed on the data to examine the influence of mouthrinse on hardness. Tukey multiple comparison testing was used to determine the grouping of the main effect means (mouthrinse and material type). The significant level was set at alpha = 0.05.

Table 1: Test Rinses
Material Manufacturer Product Type
Listerine Warner-Lambert Alcohol-containing
Cepacol JB Williams Co. Alcohol-containing
Rembrandt Den-Mat Methyl paraben
Oxygene reg. Oxyfresh Worldwide Sodium chlorite
Oxygene prof. str. Oxyfresh Worldwide Sodium chlorite
Breath RX Discus Dental Eucalyptus oil, thymol, ZnCl
75% Ethanol/25% water Fisher Scientific Positive Control
Deionized water none Negative Control
Table 2: Restorative Materials
Material Manufacturer Product Type
Herculite XRV Kerr Hybrid Composite
Silux Plus 3-M Microfilled Composite
Photac-Fil ESPE Light-cured Glass Ionomer
Hytac ESPE Compomer

Results

Figure 1 shows the relative microhardness of the four different restorative materials after immersion in distilled water (negative control). The hardness values that were obtained are representative of values from similar testing procedures that have been previously published and the ranking materials (from hardest Hytac, to softest Photac) was as expected. The large differences (statistically significant according to the letters A through D on the bar graphs) in hardness between material types are due to differences in their composition. In addition, it should be noted that the variation in the test was very low (short error bars at the top of the bar graphs) and the results were statistically significant at the 95% confidence interval.

Figure 2 shows the effects of different test rinses on the hardness of Herculite XRV Composite. As expected, the hardness in distilled water (Dw) was high and the hardness in ethanol (75Et) was low indicating a softening effect. Oxygene mouthrinse was similar to distilled water (similar letters indicate no statistical difference) as indicated by the letter C. Listerine and Breath Rx had a significantly greater softening effect on Herculite Composite than did Oxygene professional strength, whereas the effect from immersion in Oxygene regular strength was found to be statistically equivalent to Listerine, Breath Rx, Cepacol, and Rembrandt.

Figure 3 shows the effects of different test rinses on the hardness of Hytac Compomer. Hytac is similar in composition to Herculite and, therefore, it is not surprising that the results of this test were similar to those for Herculite. Ethanol and Listerine had a greater softening effect on Hytac than the rest of the mouthrinses and were not significantly different than distilled water.

Figure 4 shows the effects of different test rinses on the hardness of Photac Light-cured Glass ionomer. The hardness value of the specimens in water (negative control) of this material is approximately one half (30 vs. 55-60 KHN) that of the previous two materials. Although the range in hardness for the various rinses appears to be smaller on an absolute scale, the percentage difference between the worst result (Rembrandt) and the best result (Oxygene professional) was similar to the previous two materials. In this case, Rembrandt is worse than the other rinses, which are statistically similar to distilled water.

Figure 5 shows the effects of different test rinses on the hardness of Silux Composite. All rinses had a minimal effect on the hardness of this material (all bars are labeled with the letter A).

Discussion

This project showed that there were significant differences in the hardness values of the selected commercial resin-based restorative materials (Fig. 1). The hardness values were representative of published values and they reflected differences in composition. These materials represent four common categories of direct-placement esthetic restorative materials that contemporary dentists use on a daily basis.

The rinses produced a different softening effect in each of the tested materials (Fig. 2-5). The only material that did not show a significant softening effect from mouthrinse immersion was Silux Composite (Fig. 5). The only rinses that did not produce a significant softening effect as compared to water on any of the four restorative materials were Oxygene Professional Strength, Oxygene Regular Strength, and Cepacol. The active ingredient in the Oxygene products is sodium chlorite which is unique from the other rinses and may account for the reported results. Cepacol contains alcohol, although the concentration is approximately one-half that of Listerine and one-fifth as much as the positive control (75% Ethanol). Apparently the relatively low concentration of alcohol in Cepacol is insufficient to produce a significant softening effect in the test materials.

This project was relatively narrow in scope to facilitate timely completion. The immersion technique that was selected (continuous for 12 hours) represents an extreme test condition. However, there is no better way to duplicate the effects of a one year exposure that has any more clinical relevance other than conducting a project that takes one year to complete.

There are other related areas that need to be investigated regarding the effects of commercial mouthrinses on esthetic restorative materials. A wider range of both restorative materials and mouthrinses should be evaluated. In addition to surface hardness, several other physical properties including flexural and tensile strength, bond strength to treated tooth surfaces, color stability and wear-resistance should be evaluated for their response to exposure to various mouthrinses.

Conclusions

Within the specified parameters of this project, it can be concluded that:

  1. There were significant differences in the hardness values of the selected restorative materials.
  2. There were significant differences in the effects of the rinses on hardness that were material dependent.
  3. Silux was the only material that did not show a significant softening effect from any of the rinses.
  4. When considered as a group, the only rinses that did not appear to be significantly different from water (no effect on hardness) for all four tested materials were Oxygene Professional strength, Oxygene Regular strength, and Cepacol.

References

Gurgan S, Onen A, Koprula H. In vitro effects of alcohol-containing and alcohol-free mouthrinses on microhardness of some restorative materials. J of Oral Rehabilitation 24(3): 244-6, 1997.

Settembrini L, Penugonda B, Scherer W, Strassler H, Hittelman E. Alcohol-containing mouthrinses: effect on composite color. Oper Dent 20(1):14-7, 1995.

Lee SY, Greener EH, Mueller HJ, Chiu CH. Effect of food and oral stimulating fluids on dentine bond and composite strength. J of Dentistry 22(6):352-9, 1994.

Penugonda B, Settembrini L, Scherer w, Hittelman E, Strassler H. Alcohol-containing mouthwashes: effect on composite hardness. J of Clinical Dentistry 5(22):60-2,1994.

Sneed WD, Draughn RA. Effect of alcohol on the strength of a composite resin. Oper Dent 5(2):47-8, 1980.

*Assistant Professor, Medical College of Georgia, School of Dentistry, Augusta, GA
**Associate Professor, Medical College of Georgia, School of Dentistry, Augusta, GA