An Evaluative In Vitro Study of the Effect of the Three Commercially Available Denture Cleansers on Physical Properties of Heat-cured Resin

INTRODUCTION

As age advances, the rate of complete and partial edentulism is likewise expanding. In this way, the utilization of removable prosthesis has expanded among the older adults, who are essential wearers of dentures. Oral health and general well-being of the patient is affected by edentulism. The placement of denture produces huge changes in the oral environment and adversely affects the integrity of the oral tissues. The patient experiences reduced chewing efficiency and denture-related oral lesions such as angular cheilitis, traumatic ulcer, and denture stomatitis.¹

The majority of the patients wearing partial and complete denture leaves the dental office with next to no learning about cleaning and maintenance of dentures. This happens because of inability of the clinicians to teach their patients regarding availability and accessibility of various denture-cleaning techniques and aids.

The post-insertion instructions given to the patients are most critical for maintaining oral mucosal health and longevity of the prosthesis. As we know, the denture surface itself can produce a number of favorable changes for the accumulation of bacteria and yeast. The intaglio surface of the dentures usually show micropits and microporosities, which makes it possible for the yeast to nest.

Therefore, the mechanical methods were employed for cleaning the denture but were found to be ineffective in removing the microorganisms completely from the denture surface because of lack of compliance and poor motor coordination due to age.² So, mechanical methods were utilized in conjugation with magnetic stirrers, agitators, sonic vibrators, and ultrasonic devices, yet at same time failed to demonstrate total adequacy in cleaning dentures.³

This led to use of the chemical cleansers as adjunct to mechanical cleaning methods. Chemical cleaners are available in form of creams, pastes, gels, solutions, and tablets. They may include one or combination of various chemical agents such as sodium hypochlorite, enzyme, chlorhexidine, alkaline peroxidase, and diluted acids as the immersion medium for denture cleaning.²

But, some chemical agents have been reported to damage acrylic resin by altering the surface properties of the acrylic resin if used for a longer time without following manufacturer’s instructions.⁴

The prime physical properties affected by the use of denture cleansers are the flexural strength of acrylic resin. They result in clinical failure of the prosthesis by producing flexural fatigue⁵ and increase the risk of intraoral and extraoral fracture of dentures.

Another important property influenced by use of denture cleansers is the surface roughness. It commonly aids in biofilm formation by providing rough irregular areas for retaining the debris and microorganisms. Increased surface roughness can make removal of the biofilm more difficult.⁶ Surface color of the prosthesis is also affects by use of denture cleansers, disturbing the patient the most. It is an indicator of ageing or damage to dental materials. It provides information on serviceability of the material.⁷

This study was undertaken to investigate how different denture cleansers with different compositions and immersion times affect above-mentioned properties of the heat-cured resin as most of them were readily available over the counter. Also to determine the most suitable denture cleanser for heat cure-based denture bases. The null hypothesis for the current study was that there were no effects on physical properties of heat-cured resin bases associated with commercial denture cleansers.

MATERIALS AND METHODS

In the present study, a total of 80 heat-cured resin samples were taken as per statistician’s recommendation and were divided into four equal groups. Group I was immersed in distilled water (control group). Group II, III, and IV were immersed in three different denture cleansers.

The whole study protocol was divided into eight parts:

RESULTS

Effects of three commercially available denture cleansers (Clinsodent, Fittydent, and Secure) were evaluated and compared with the control group. There was significant mean difference between the groups post-immersion with p value less than 0.001. For heat-polymerized acrylic resin, the maximum mean value of flexural strength was noticed in control group (100.71 MPa) and minimum change was observed in the secure denture cleanser (97.72 MPa) and maximum change was observed in the Fittydent denture cleanser (72.32 MPa) as seen in Figure 4. The highest mean of color change was observed in the Clinsodent denture cleanser (56.63 NBS unit) while least change was observed in the Fittydent denture cleanser (48.00 NBS units) as seen in Figure 5. With respect to surface roughness, the minimum mean value was noticed in Fittydent (0.54 μm) while maximum change was observed in the Clinsodent denture cleanser (0.71 μm) as in Figure 6.

DISCUSSION

The oral prosthesis is provided to replace the lost part and also to restore the lost or impaired function due to the missing part or organ of the body. To make it more meaningful, the care and maintenance of the prosthesis is of paramount importance including maintaining hygienic condition.⁹ Inadequate post-insertion home care can compromise the clinical longevity of the prosthesis fabricated utilizing excellent material and techniques.

Poor denture hygiene is considered to have a strong association with oral candidial colonization. In the presence of poor oral hygiene, Candida can stick, penetrate, and aggregate with bacterial communities. And to accelerate this process of adhesion and colonization, surface roughness of denture bases is considered to play a key role. Therefore, it is important to look for measures that completely remove microorganism from colonizing into the denture prosthesis.¹⁰

Unfortunately, mechanical methods failed in complete removal of microorganisms from dentures. Thus, it is proposed that chemical cleansers are more effective and indispensable in daily denture care, in which hypochlorites, peroxides, enzymes, and acids act as chemical immersion solutions for denture cleaning. However, some chemical agents tend to damage acrylic resin and metal alloys by corroding or staining these alloys as a result of their contact with chlorine or oxygen present in the commercial denture cleanser.

Ideally, immersion in the chemical denture cleanser should not alter the physical, mechanical, and chemical properties of the denture-based acrylic resin. But on contrary, use of these agents do result in changes in surface morphology and flexural strength of denture resins.¹¹

However, several studies revealed that the physical, mechanical, and chemical properties of the acrylic resin are affected by denture cleanser.^12,13

The primary reason for clinical failure of the prosthesis is flexural fatigue of the denture.¹⁴ The flexural three-point bending test is performed on denture resin because it generates similar type of stresses as applied to the denture during mastication.¹⁵ As reported by various authors,^16,17 after complete polymerization reaction, some amount of residual polymethyl methacrylate monomer content remains, which acts as a plasticizer and reduces the interchain forces and yield in early deformation under load which result in reduced flexural strength of denture base acrylics.

To remove this residual monomer, Harrison and Huggett¹⁷ recommended use of long water bath cycles with terminal boil as it will reduce the levels of the residual monomer by approximately three times. For residual monomer elimination, the specimens of this study were immersed in distilled water. Also the standard acrylization procedure was followed to prepare all acrylic specimens.¹⁸

Among all the three denture cleansers used, group III showed statistically minimal reduction in values of flexural strength in comparison to other two groups. Smith et al.¹⁹ explained that if polymerized acrylic resins are immersed in the immersion medium for a longer period, they tend to absorb water continuously which act as a plasticizer reducing the strength of the resin. However, in the above study group III was immersed for the least immersion period of 5 minutes hence producing least reduction in flexural strength.

Beyli et al.²⁰ also stated that water sorption by acrylic resin resulted in dimensional instability and fatigue of acrylic resin leading to crack formation and subsequent fracture of the denture. On immersion in Clinsodent, Fittydent, and Secure denture cleansers, reduced flexural strength of the heat-cured acrylic resin was observed in comparison to distilled water immersion in the present study.

The color stability of the prosthesis is the most important factor for determining patients’ acceptance for it.^21–23 In the present study, we have evaluated color change by using the CIEl*a*b* colorimeter system.^24,25 For the comparison of color and quality control parameters, NBS units were used in the study.

In the present study conducted, it was observed that color change in the control group (distilled water) showed statistically insignificant increase in values the after immersion, similar result was found out in group I which were explained by Pinto et al.²⁶ According to him, that water with repeated sorption and desorption cycles produces microcracks, which cause irreversible damage to the acrylic resin. This damage occurs in the form of hydrolytic degradation of the polymer causing damage to ester linkage and weakening the infrastructure of the polymer. Hence creating acrylic zones with different optical properties²³ that can be esthetically undesirable and can be detected visibly.

According to Robinson et al.,²⁷ Saraç et al.,²³ and Purnaveja et al.,²⁸ it was concluded that the solvent present initially penetrates into the intermolecular polymer network and causes expansion of the intermolecular spaces due to water sorption in acrylic resin materials, facilitating leaching out of the intrinsic pigments and penetration of extrinsic colorant in denture cleaning solution which can also cause color change. This could be the probable cause that the acrylic resin used in the study tends to change color as a result of leaching out of the coloring material from the acrylic surface with long-term immersion in distilled water. Later opacity of acrylic resin occurs as a result of monomer leaching out and water getting absorbed on to the surface.²⁹

Whereas among groups II and III, group II showed significantly less color change in comparison to group III. Whitening of all acrylic resin occurs due to qualitative difference or pH difference of the cleaning solution.²⁸ Unlü et al.³⁰ also observed the whitening effect of the denture base resin, which tends to occur due to the presence of deleterious combination of oxidation and strong alkaline solution present in the sodium perborate type of denture cleanser.²⁹

The probable cause of color change observed after immersion in the denture cleanser was that these contain sodium perborate which forms an alkaline peroxide solution of pH ranging from 9 to 11. Nikawa et al.^31,32 stated that reason of damage to denture resin when immersed in peroxide-containing denture cleanser was increased the peroxide content and the accelerated level of oxygenation in highly alkaline solution. They have effervescent component such as sodium perborate or sodium bicarbonate, which when dissolved in water form alkaline peroxide solution that decomposes to produce oxygen that loosens the food debris via mechanical means.^23,33 Therefore, its use results in hydrolysis and decomposition of polymerized acrylic resin itself. This is the reason why the sodium perborate denture cleanser exhibits greater influence on color stability in comparison to any other type of denture cleansers.

Therefore, care must be taken to not expose the denture resin to the peroxide-type denture cleanser for a longer period of time and should instruct the patient to follow the instructions properly.

To increase the adherence of food debris and microorganisms, surface roughness plays a key role as it accelerates the biofilm formation by providing retentive areas and makes it difficult to remove the biofilm from the denture surface. Surface roughness was estimated by value of R_a, which gives the average of peaks and depressions on the surface and enables us to evaluate the possibility of bacterial adhesion.

Quirynen and Bollen³⁴ also emphasized on importance of having the smooth denture surface because having rough surfaces attracts bacteria, which will adhere to these surfaces for a long period of time making them difficult to be removed by regular hygiene methods. Therefore, recommended surface roughness should be ≤2 μm. Williams and Lewis³⁵ also supported the fact. The cited literature also states use of the smooth acrylic surface having roughness of 0.12 μm, which is well below the critical value of 2 μm.³⁶

These rough surfaces can be avoided by proper finishing of dentures, which involves both abrading and polishing. Finishing abrasives advocates use of hard and coarse abrasives to remove gross irregularities from the surface, whereas polishing abrasives advocated use of finer particle sizes to smoothen surfaces that had been roughened by finishing abrasives. Therefore, the sample was sandpapered in a unidirectional motion using three different grits of sandpapers (grit sizes 40-coarse, 60-medium, 100-fine) and polished using a rag wheel and pumice slurry. Bollen et al.³⁷ reported a positive corelation of the polishing grit to the surface roughness of acrylic resin.

As observed by Harrison et al.,³⁸ these chemical‐based denture cleansers did not contain abrasive particles. Effervescent peroxide or sodium hypochlorite was the main cleansing agents among them. The oxygen is released effectively dislodging debris and creating a plaque-free surface. Duyck et al.³⁹ stated that reduced total bacterial count on acrylic removable dentures results if dentures are immersed in denture-cleansing tablets overnight rather than denture storage in water overnight.

It was demonstrated that there was significant less reduction in the surface roughness values observed before and after immersion in group I, where Clinsodent denture cleanser was used as an immersion medium. In the current study, it was observed that surface roughness values reduced after immersion in denture cleansers, which was verified by a study conducted by Garcia et al.⁴⁰ who reported reduced surface roughness of acrylic samples when immersed in a commercial cleanser due to its inability to completely remove pellicle formed on the surface of acrylic.

Peracini et al.⁴¹ on the other hand stated that the sodium perborate type of denture cleanser did not alter the surface roughness of acrylic resin. However, this inferences varies in studies that have shorter durations and immersion periods.^42,43 Therefore, it was concluded that while using alkaline peroxides, the time of immersion along with the type of denture base resin used was considered important.

There were some limitations to the above study as there was only one type of heat-cured denture-based acrylic resin used and the parameters like pH of saliva were not simulated as that of normal saliva. Also the study did not simulate clinical behaviors as it was conducted at room temperature. To overcome this limitation, the in vivo study should be conducted in future along with exploring other mechanical properties of heat cure-resins as well.

CONCLUSION

Heat-cured acrylic resins specimen on immersion in distilled water and denture cleanser tablets for 6 days and 30 immersions simulating 180 days of cleanser use by the patient demonstrated noticeable change in surface color, surface roughness, and flexural strength. The change observed in surface color of all denture base resins was within the clinically accepted range and was maximum in Secure followed by Fittydent denture cleansers; least was seen among Clinsodent denture cleansers. The change in surface roughness observed was least in the Fittydent denture cleanser while Clinsodent followed by Secure denture cleanser showed maximum change in surface roughness values. In regard to flexural strength, significant decrease in strength was observed with all three denture cleansers with least reduction observed among Secure denture cleansers while maximum was seen among Clinsodent followed by Fittydent denture cleansers.

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