Evaluation of the Effect of Household Cleansers on the Color Change of Two Types of Permanent Soft Liners: An In Vitro Study

Removable dentures are alternatives for missing teeth, and their success depends on aesthetics, function, and patients’ comfort. In edentulous patients, denture’s pressure is directly delivered to the underlying mucosa by means of acrylic resin, which causes tissue damage and bone resorption (1). Soft liners are substances introduced to the market in order to solve this problem and have been used extensively in removable dentures for over a century (2). Due to their elastic property, these substances are widely used in patients who cannot tolerate the prosthesis, or patients with ridge resorption, sensitive mucosa, bone resorption, and bruxism (3). Moreover, these substances can equally distribute the pressure of the prosthesis and reduce occlusal forces (4).

Soft liners can be classified into resin or acrylic, self-cured, or heat-cured. In addition, considering the time used, they are divided into two categories, namely permanent or provisional. Provisional soft liners are soft liners that can maintain their softness and elasticity for a maximum of 30 days, whereas permanent soft liners can be utilized for more than 30 days even for months or years as some studies suggest (5). These substances require some properties, such as compatibility with the oral tissue, shape and colour stability, resistance to wearing, strong bond to the denture, and also resistance against the dynamic situation of the oral cavity. These properties should not be affected by cleaning methods (6).

The liners' color stability is an important criterion to assess their functional ability; moreover, color change indicates aging and damage which have a negative impact on patients’ satisfaction (7).

Since the structure and surface roughness of these substances lead to the colonization of microorganisms (8), patients should regularly perform the cleaning methods which can cause alteration in some of the liners properties, such as discoloration, swelling and water absorption, imbibition, and microorganism’s growth (9).

The most common and effective way to remove the biofilm from the denture’s surface is the utilization of the chemical cleansers besides the mechanical methods (10). Household cleaners are among the cleansers which are used by many patients due to their accessibility. These cleansers are cheap and can normally be found at homes (11). Daily use of these cleaners affect the physical and mechanical properties of these liners and can lead to discoloration and esthetics defects which eventually results in spending more time and expense on their repair.

Sometimes, the patient is satisfied with the prosthesis and feels comfortable with it; however, the issue of aesthetics is the reason behind the patient’s dissatisfaction (12, 13). Therefore, the color stability of the liners is of great importance, and it is necessary to choose the right and ideal cleansers to reduce possible alterations (14). Although these cleaners are used by the majority of people, there is a dearth of research investigating the effects of cleansers on the liners. Furthermore, it is of utmost importance to find a suitable therapeutic method to disinfect the dentures without harming their mechanical properties (15, 16). Therefore, this study was conducted to determine the effect of household cleansers on the color change of soft liners which was an important indicator for their replacement. To this end, this study also attempted to enhance the people's knowledge about the effect of household cleansers on the quality of the prosthesis.

Materials and Methods

Table I presents the materials used in this study (i.e., 40 heat-cured and 40 self-cured silicon liners). In order to make the samples, 65×40×6 mm brass mold with 3 separate and attachable plates was made using screws

(Figure 1). The upper and lower plates were flat in order to prepare a smooth surface for liners, and the middle plate had six 15×2 mm cylindrical spaces for soft liners (1).

Figure 1. The dimension of a brass mold

Fabrication of heat-cured soft liners is as follows:

The inner surface of the mold was greased by Vaseline to facilitate the extraction of soft liners to make Molloplast-B samples. Then, the soft liners were placed in the spaces using a clean plastic spatula. Three plates of the mold were attached to each other, the excess liners were removed using a scalpel, and the molds were placed under a pressure of 100 KP for 10-15 min. For polymerization, the molds were placed in the cold water inside the curing units (Polymerizer FOR MUFLAS M-9 MESTRA, SPAIN). In this unit, the temperature gradually increases up to 100C/121F and remains in this situation for 2 h. Subsequently, the samples cool down slowly. It should be noted that the process of curing the soft liner was followed according to the manufacturers’ instructions.

Fabrication of self-cured soft liners is as follows:

Soft liners were injected into the molds. The molds were put under pressure so that the excess liners would be evacuated to make Mucopren soft samples. Then, excess liners were removed using a scalpel. For completion of polymerization, molds were placed in the 45-50 °c water according to the manufacturers’ instruction. After polishing the samples, 80 samples of soft liners with a size of 15×2 mm were made in this study. All samples were immersed in distilled water for 24 h (2).

Colour measurement of the samples is as follows:

After the determination of the initial color by a colorimeter device (Konica Minolta CR-400, JAPAN), each of the 40 self-cured and heat-cured samples of silicone soft liners were randomly divided into 4 groups. Each group was immersed in either distilled water, 2.5% ­white vinegar, 2.5% bleaching hypochlorite sodium solution, or 2.5% hand washing liquid for 8 h at the room temperature (Figure 2), and for the rest of the day, they were placed in distilled water (13,17,18) (Table I). Solutions were changed daily, and samples were washed by distilled water and dried in the open air (2). The color of soft liners was measured after 7 and 30 days using a colorimeter device.

Figure 2. Samples distilled in cleansers

Colorimeter device determines the color variations using three indices, namely L*, a*, and b*. The L* indicates lightness and has a range between 0 (black) and 100 (white). Moreover, the positive and negative values of a* show red and green colors, respectively. Regarding b*, positive and negative values signify yellow and blue colors, respectively. Color change in samples was calculated using the following formula:

^1/2I[ ²(b*∆) +a*)²  ∆( +L*)²  ∆)] = E∆

The data were analyzed using SPSS software (version 23). In addition, the Shapiro-Wilk test was used to evaluate the normal distribution of the data. Since the data were distributed normally, the analysis of variance (ANOVA) was applied for the analysis of the four sample groups. Furthermore, Tukey's post hoc and independent-sample t-test were employed for the paired comparison of the groups. A P-value less than 0.05 was considered statistically significant.

Results

    According to the results obtained from ANOVA to study the discoloration, an association was observed between the type of the liner and that of the cleanser (P<0.001). In addition, there was a statistically significant difference between the cleansers regardless of the soft liner type (P<0.001). Moreover, a statistically significant difference was found between the two types of soft liners regardless of the cleansers type (P<0.001) (Figure 3).

The following results are obtained from independent t-test after one week and one month:

The mean color change using distilled water, white vinegar, and hypochlorite sodium was significantly lower after one week in the Molloplast-B group, compared to the Mucopren soft group (P=0.001, P<0.001, and P<0.001, respectively). However, regarding the hand washing liquid, the Molloplast-B group obtained a higher number of discoloration, compared to Mucopren soft liner, which was not statistically significant (P=0.331, TableII).

Furthermore, the mean discoloration using distilled water, white vinegar, and hypochlorite sodium cleansers after a month was significantly lower in the Molloplast-B group, compared to Mucopren soft liner (P=0.004, P<0.001, and P=0.002, respectively). However, the mean discoloration was lower in the Molloplast-B group, compared to Mucopren considering hand washing liquid (Figure 4). It is worth mentioning that this difference was not statistically significant (P=0.780, Table III)

Table I. Commercial name, materials, and manufacturer used in the study

Table II. Comparison of discoloration between soft liners after a week sorted by the cleansers

Table III. Comparison of discoloration between soft liners after a month sorted by the cleansers

According to the results obtained from two-by-two comparisons of cleansers using Tukey’s post hoc test for the Molloplast-B group, it was shown that the mean color change in sodium hypochlorite group was significantly higher than that in other cleansers. However, no significant difference was observed among the other cleansers. With regard to Mucopren soft liner, the results of Tukey’s post hoc test suggested that the mean color change was significantly higher in the sodium hypochlorite group, compared to others. It should be noted that the difference between distilled water and handwashing liquid was not statistically significant (Table IV).

Table IV. Pairwise comparisons of the cleansers regarding the discoloration of the soft liners after one month using Tukey's post hoc

               *The difference between the two groups is significant.

Figure 3. Effect of cleansers on the discoloration of Molloplast-B soft liner based on time

Figure 4. Effect of cleansers on the discoloration of Mucopren soft liner based on time