Comparative evaluation of the effect of denture cleansers on the surface topography of denture base materials: An in-vitro study.

AIMS: The aim was to evaluate and compare the effects of three chemically different commercially available denture cleansing agents on the surface topography of two different denture base materials.
MATERIALS AND METHODS: Three chemically different denture cleansers (sodium perborate, 1% sodium hypochlorite, 0.2% chlorhexidine gluconate) were used on two denture base materials (acrylic resin and chrome cobalt alloy) and the changes were evaluated at 3 times intervals (56 h, 120 h, 240 h). Changes from baseline for surface roughness were recorded using a surface profilometer and standard error of the mean (SEM) both quantitatively and qualitatively, respectively. Qualitative surface analyses for all groups were done by SEM. STATISTICAL ANALYSIS USED: The values obtained were analyzed statistically using one-way ANOVA and paired t-test.
RESULTS: All three denture cleanser solutions showed no statistically significant surface changes on the acrylic resin portions at 56 h, 120 h, and 240 h of immersion. However, on the alloy portion changes were significant at the end of 120 h and 240 h.
CONCLUSION: Of the three denture cleansers used in the study, none produced significant changes on the two denture base materials for the short duration of immersion, whereas changes were seen as the immersion periods were increased.

Rough and pitted surface seen on the acrylic denture surfaces acts as a nidus for biofilm formation and colonization of microorganisms.[123] Dentures can be cleaned by mechanical methods, chemical methods or a combination of both. Cleansing with a brush and an abrasive is the most popular mechanical method widely used.[24]

Denture cleansers are the most preferred chemical cleansing methods, which have been suggested for the disinfection of the prosthesis. The best cleanser should fulfill most of the requirements of an ideal cleanser while not causing any kind of alteration in the structure of the prosthesis. The chemically available denture cleansers can be broadly grouped under alkaline peroxides (percarbonate or perborate), alkaline hypochlorites, dilute organic or inorganic acids, and enzymes.[156]

Alkaline peroxides are effective on newly formed plaque and stains, and to be effective it has to stay in contact with the denture surface for a long period.[26] Alkaline hypochlorites are effective on stains, mucin, and other organic substances, as well as against bacteria and fungi.[27] Acids though effective denture cleansers cannot be used on a daily basis due to their corrosiveness.[6] Enzymes act on the glycoprotein, mucoprotein, and extracellular polysaccharide structures, resulting in the breakdown of macromolecules into less adhesive, smaller units, but their use is limited as they are still in the research state.[8]

The disinfectants (e.g., chlorhexidine gluconate) those are not commercially available for denture cleansing have also been experimentally tested and found to cause a significant reduction in the amount of denture plaque when the dentures have been soaked in them. Chlorhexidine gluconate in a concentration of 0.2% was found to be effective on plaque formed on denture surfaces.

Apart from their action on the biofilm accumulation and antimicrobial action, some undesirable properties of these chemical denture cleansers have also been studied and reported; alkaline peroxides are found to cause bleaching of the acrylic resin on extended use.[3] Alkaline hypochlorites are found to cause tarnish and surface discoloration of the metal components such as cobalt chromium alloys used for partial dentures and bleaching of the acrylic resin.[3910] Chlorhexidine gluconate solutions are found to cause heavy discolorations on routine soaking and are unsuitable for everyday use.

Earlier studies have evaluated the efficacy of the different denture cleansers on the biofilm formation and their effects on the surface topography of acrylic resin or partial denture alloy specimens individually.[567891112] Cast partial dentures are a combination of both alloy and acrylic resin materials. Studies focusing on the effect of different denture cleansers on the changes that occur in the surface topography of a combined alloy and acrylic resin samples are lacking.

Hence, the present in-vitro study was conducted to evaluate and compare the effects of three different commercially available denture cleansing solutions (sodium perborate, 1% sodium hypochlorite, and 0.2% chlorhexidine solution) on the surface topography of denture base materials by employing test samples made up of a combination of cobalt-chromium alloy and acrylic denture base resin.

Materials and Methods

The denture cleansing agents, composition, and their manufacturer information are listed in Table 1.

Table 1

Chemical composition, product name, manufacturers of chemical disinfectants

A total of 100 samples were made using a custom-made metallic mold, which consists of a base, middle plate, and a lid. Six rectangular slots were present in the 3 mm thick middle plate which is measured in 15 mm × 10 mm. At exactly half the length of the slot, a dip was created to a depth of 2 mm. To accommodate heat cure denture base resin [Figure 1]. Pattern resin was used in these molds to obtain samples, which were then casted with the chrome cobalt alloy used in the study. These samples were polished and were subjected to a layer of heat cure resin on one-half their sizes [Figure 2].

Figure 1

Six rectangular slots 3 mm thick middle plate measuring 15 mm × 10 mm and a slot created to a depth of 2 mm. To accommodate heat cure denture base resin

Figure 2

Completed samples with a layer of heat cure resin on one-half their sizes

The samples were immersed in the custom-made artificial saliva for 24 h to simulate the presence of dentures in patient's mouth. Prior to immersion in the denture cleanser solutions the test samples were subjected to both quantitative and qualitative surface analysis employing surface profilometer and standard error of the mean (SEM), respectively. Among the 100 test samples prepared, 90 test samples were randomly selected and subjected to quantitative surface analysis using a surface profilometer, and the values were recorded to serve as pre-immersion control values. Samples subjected to SEM evaluation need to undergo gold sputtering/layering of their respective surfaces prior to analysis; this procedure renders them unusable for further immersion process after the SEM analysis. For this reason, the remaining 10 samples were subjected to SEM analysis in the present study to obtain the pre-immersion values for the qualitative analysis. The samples were prepared for viewing in an SEM by a critical point drying and by subsequent gold/platinum plating to a thickness of about 200 Å.

The remaining 90 samples were then divided into three groups and assigned to the three denture cleansing agents employed in the study that is, Group II (samples immersed in sodium perborate), Group III (samples immersed in sodium hypochlorite 1%), Group IV (samples immersed in 0.2% chlorhexidine gluconate), which were further divided into sub-groups according to the time intervals at which the study was carried out. There were 3 times intervals (56 h, 120 h, 240 h) used in the study and at the end of each time interval 10 samples were retrieved from the three denture cleanser solutions and were subjected to post-immersion analysis both qualitatively and quantitatively. The values obtained were analyzed statistically using one-way ANOVA and paired t-test.

Surface analysis

The post-immersion surface roughness of both the alloy portion and the acrylic resin were recorded using a surface profilometer for all the groups and sub-groups. With the use of high magnification, data were recorded to the nearest 0.01 μm with a surface analyzer (Surface Roughness Analyzer, Taylor Hobson Talysurf, UK). The mean arithmetic roughness average (Ra) was used to assess surface changes which is the arithmetic average of the absolute values of the measured profile height of surface irregularities measured from a mean line within a preset length of the specimen (traverse length = 5 mm). The mean line is the line, where the sum of the areas contained between it and those parts of the profile that are on either side are equal.

Standard error of the mean analysis

In the present in-vitro study, the surface texture of the two removable denture base materials, (cast metal alloy and acrylic denture base resin) were analyzed for surface topographic changes using the scanning electron microscope (JEOL, ASM 6360, JAPAN). Electron microscopes use a beam of highly energetic electrons (l keV–1 MeV) to examine objects on a very fine scale (0.2 nm upward). As the name suggests, SEM uses a scanned beam rather than a fixed beam and is used primarily for the examination of thick (i.e. electron opaque) samples. The specimens to be magnified may have some conductivity and may get charged up and hence, they are coated with a platinum or gold layer to prevent the charging and in order to increase the secondary emissions.

Results

The Tables 2–4 lists the pre-immersion, post-immersion, and the difference in the roughness values before and after immersion in the three different denture cleanser solution, sodium perborate, sodium hypochlorite, chlorhexidine gluconate, respectively. Ra data collected from the 10 test specimen in each sub-group at each time interval of immersion were averaged to Ramean. The Ramean of each specimen at each time interval of immersion is subtracted from its value before immersion (control) to obtain the change in Ra (ΔRamean) with time.

Table 2

Pre-immersion, post-immersion and difference in roughness values (ΔRamean) for the alloy portion and acrylic resin of group IIa, group IIb, group IIc test samples immersed in sodium perborate solution

Table 4

Pre-immersion, post-immersion and difference in roughness values (ΔRamean) for the alloy portion and acrylic resin of group IVa, group IVb, group IVc test samples immersed in chlorhexidine gluconate solution

The corresponding values and standard deviations for the three disinfectants at the 3 times intervals are presented in Tables 5 and 6. One-way ANOVA test was used to calculate the P value and Tukey honestly significant difference procedure was employed to identify the significant groups at 5% level.

Table 5

Comparison of the mean change between pre- and post-immersion roughness values among different sub-groups in each study group for the alloy portion

Table 6

Comparison of the mean change between pre- and post-immersion roughness values among different sub-groups in each study group for the acrylic resin portion

The pre-immersion SEM evaluation for the qualitative analysis were carried out for the Group I samples to serve as a control. The post-immersion evaluation were performed for all the Group II, Group III, and Group IV samples at each time interval that is, 56 h, 120 h, 240 h and compared with the pre-immersion photomicrographs to evaluate any changes in the surface topography of the two denture base materials used in this study. The post-immersion photomicrographs for the qualitative analysis using SEM at × 1000 magnification showed a gradual increase in roughness in all the three groups at the 3 times intervals tested when compared with the pre-immersion photomicrographs.

The size and distribution of the voids observed in the photomicrograph of the resin portion shows that the mean change in roughness for sodium perborate (Group II) was higher followed by sodium hypochlorite (Group III) and was lowest in chlorhexidine gluconate solutions (Group IV), but there was no significant difference in mean values among the three groups (P > 0.05) [Figure 3]. In the alloy portion, there was an increase in the size of micro-porosities in Group II samples followed by Group III samples and Group IV samples. The difference between Group III and Group II, Group IV was statistically significant, but there was no statistical significance between Group II and Group IV roughness values [Figure 4].

Figure 3

SEM photomicrograph of the resin portion showing the change in roughness for sodium perborate (Group II), sodium hypochlorite (Group III), and chlorhexidine gluconate solutions (Group IV) at the end of 240 h

Figure 4

SEM photomicrograph of the alloy portion showing the change in roughness for sodium perborate (Group II), sodium hypochlorite (Group III), and chlorhexidine gluconate solutions (Group IV) at the end of 240 h

Discussion

The longevity of any dental prosthesis is mainly dependent on the maintenance/cleanliness of the prosthesis, which in turn relies on the proper homecare procedures carried out by the patient. Inadequate cleaning of the denture leads to accumulation of food debris, which is in turn harbors bacteria and salivary mucin resulting in malodor. On long-term effects, it leads to degradation of mechanical properties of the denture material and affects the oral mucosal health of the patient.[121314]

The most routinely followed method for cleaning the dentures were overnight soaking in any commercially available denture cleansing solutions. Most proprietary immersion cleansers can be divided into alkaline peroxides (percarbonate or perborate), alkaline hypochlorites, dilute organic or inorganic acids, and enzymes.[125]

Alkaline peroxides dissolved in water forms an alkaline solution of hydrogen peroxide which produces an effervescent action resulting in mechanical loosening action between the denture surface and the debris.

Alkaline hypochlorites are acting directly by causing dissolution of the polymer structure in the organic matrix.[271215] Acids cannot be used on a daily basis due to their corrosiveness.[2] About 0.2% chlorhexidine gluconate is effective on plaque formed on denture surfaces. They have a bacteriostatic effect at lower concentrations and a bactericidal effect at higher concentrations.[12] It is considered as the best choice among antiseptics for dental biofilm control.[2]

Alkaline peroxides cause bleaching of the acrylic resin for extended use.[4] Alkaline hypochlorites cause tarnish and surface discoloration on the metal components of cobalt chromium alloys used for partial dentures and also bleaching of the acrylic resin portions.[371015] Chlorhexidine gluconate solutions cause heavy discolorations on routine soaking and are unsuitable for everyday use.[2]

Surface texture analysis using surface profilometer

The samples that were immersed in sodium perborate denture cleanser showed a gradual increase in the roughness values for both alloy and acrylic resin as time duration increased [Table 2] but not statistically significant. At the end of 120 h, a statistically significant change in roughness was seen on alloy portion. The exact mechanism of sodium perborate affecting the surface of the alloy and the possibility of removal of superficial metal layer needs to be evaluated in further studies. Furthermore, operator variability in the polishing of samples could have resulted in this change as there was no replication of the polishing effect in all the alloy samples. Roughness in acrylic portion can be attributed to the higher peroxide content and level of oxygenation that can cause hydrolysis and decomposition, which can be damaging to the denture base materials especially acrylic resin.[1617] These results were similar to the studies conducted by da Silva et al.[6] and Machado et al.,[18] and Rudd et al.[19]

Samples that were immersed in sodium hypochlorite 1% also showed an increase in the roughness in the alloy and acrylic portion and it was statistically insignificant, and the results were similar to studies done by Lima et al.,[10] Jagger,[20] Keyf et al.,[9] At the end of 120 h, there was a decrease in roughness in the alloy and acrylic portion compared to 56 h. It may be due to operative variability in polishing of samples, and further studies are needed to evaluate the exact mechanism. After 240 h, the alloy portion showed a statistically significant change in surface roughness. The mean difference of roughness, however, was less for the present study due to the use of 1% concentration instead of the 5.25% concentration used in the previous studies. Studies have shown that the concentration, pH, and the duration of immersion had a direct effect on the surface roughness of the alloy portions.[19] Sodium hypochlorite solution showed a statistically significant change in the alloy portion when compared to the other two solutions.

Samples immersed in 0.2% chlorhexidine showed the least change in roughness values for both the alloy and acrylic resin portion at all the time intervals, which was statistically insignificant. Budtz-Jørgensen et al,[2] showed similar results where they described discolorations of dentures on prolonged use, but no change in the roughness of the denture base materials.

Surface texture analysis using standard error of the mean

The pre-immersion SEM evaluation on Group I samples serves as a control for qualitative analysis. The post-immersion results showed a gradual increase in roughness in all the three groups at the 3 times intervals tested compared with the pre-immersion photomicrographs. The size and distribution of the voids observed in the photomicrograph for chlorhexidine gluconate solutions (Group IV) were smaller and widely distributed, when compared to the other two. The size of voids increased from chlorhexidine gluconate (Group IV), sodium perborate (Group II), and sodium hypochlorite (Group III) for the alloy portion indicating an increase in roughness at all-time intervals.

The selection of a good denture cleanser not only depends upon its safe use on the denture base materials, but also on their effectiveness to remove surface debris and micro-organisms. The effectiveness of sodium perborate was done in several studies, but their cleansing action was considered to be lesser than sodium hypochlorite solutions and chlorhexidine gluconate.[13613] About 0.2% chlorhexidine gluconate, when compared to hypochlorites are less effective in cleansing activity, but are more effective than alkaline perborates as stated by da Silva et al.[6]

Conclusion

At the end of 56 h, all the three denture cleansers produced changes on the surface of alloy and acrylic resin portion but the changes were not statistically significant. At 120 h, sodium perborate solution showed a statistically significant change only in the alloy portion when compared to 0.2% chlorhexidine gluconate solution and 1% sodium hypochlorite solution. At the end of 240 h, surface statistically significant changes were seen in the alloy portion of 1% sodium hypochlorite solution.

Hence, it can be concurred from the present study that, all three denture cleanser solutions showed no statistically significant surface changes on the acrylic resin portions at 56 h, 120 h, and 240 h of immersion. All the three denture cleanser solutions did not exhibit any statistically significant surface change in the alloy portion at the end of 56 h, but at the end of 120 h and 240 h statistically significant surface changes was seen among, which 0.2% chlorhexidine gluconate solution exhibited the least surface change. This was in correlation with the results obtained by the quantitative surface analysis using SEM.

Table 3

Pre-immersion, post-immersion and difference in roughness values (ΔRamean) for the alloy portion and acrylic resin of group IIIa, group IIIb, group IIIc test samples immersed in sodium hypochlorite solution