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ORIGINAL RESEARCH
Year : 2022  |  Volume : 13  |  Issue : 3  |  Page : 99-102

How Many Microwave Disinfection Cycles is Safe for the Adaptability of Polymethyl Methacrylate (PMMA) Denture Base Materials?: An In Vitro Study


1 Department of Conservative Dentistry and Prosthodontics, Universiti Sains Islam, Nilai, Negeri Sembilan, Malaysia
2 Ministry of Health, Putrajaya, Malaysia

Date of Submission12-Jul-2021
Date of Decision27-Apr-2022
Date of Acceptance05-Jul-2022
Date of Web Publication19-Sep-2022

Correspondence Address:
DDS Wan Nor Syariza Wan Ali
MClinDent (London), MPros RCS (Edin), Faculty of Dentistry, Universiti Sains Islam Malaysia, Tower B, Bangunan MPAJ, Pandan Indah, 55100 Kuala Lumpur
Malaysia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/denthyp.denthyp_97_21

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  Abstract 


Introduction: We aimed to determine the maximum microwave disinfection cycles that is safe for the adaptability of polymethyl methacrylate (PMMA) denture bases under power–time setting of 630 W for 3 minutes. Materials and Methods: Thirty PMMA maxillary denture bases were constructed and divided into two groups, control (no disinfection) and test (microwave disinfection at 630W for 3 minutes in water immersion). Coltene Rapid Liner (Coltene/Whaledent AG, Altstatten, Switzerland) silicone was coated onto the internal surface of the samples followed by placement of 8 kgf of axial load. The silicone film was removed, and weighed using analytical balance. One day interval was given between each cycle and samples were stored in 37°C water bath. Tests were carried out until a statistically significant silicone film weight change detected. The data were analyzed on SPSS version 21.0 (IBM Corp, Armonk, NY). Results: Statistically significant change of the denture base was detected after the tenth cycle of microwave irradiation. Conclusion: The maximum microwave disinfection cycles that is safe for the adaptability of PMMA denture bases under power–time setting of 630 W for 3 minutes is only up to 10 cycles.

Keywords: acrylic denture base, denture adaptability, microwave disinfection, polymethyl methacrylate


How to cite this article:
Ali WW, Ahmad NF, Yussof SS. How Many Microwave Disinfection Cycles is Safe for the Adaptability of Polymethyl Methacrylate (PMMA) Denture Base Materials?: An In Vitro Study. Dent Hypotheses 2022;13:99-102

How to cite this URL:
Ali WW, Ahmad NF, Yussof SS. How Many Microwave Disinfection Cycles is Safe for the Adaptability of Polymethyl Methacrylate (PMMA) Denture Base Materials?: An In Vitro Study. Dent Hypotheses [serial online] 2022 [cited 2022 Sep 28];13:99-102. Available from: http://www.dentalhypotheses.com/text.asp?2022/13/3/99/356347




  Introduction Top


Removable prosthesis in the oral cavity may lead to plaque formation on the denture surface which was found to be related to surface irregularities and microporosities of the denture surface, design of the prosthesis, health of the mucosa, salivary flow, denture wearing habits, and oral hygiene of the patients.[1] Plaque accumulation in partial denture wearers will lead to several harmful effects such as dental caries, periodontal diseases, infection of the underlying mucosa, and denture stomatitis. Denture hygiene can be conducted by using mechanical and chemical methods; however, not all methods guarantee disinfection of the dentures.

Microwave disinfection of the dentures has been proven to disinfect dentures as it is effective toward several microorganisms especially Candida albicans.[2] Proper use of microwave disinfection is important to avoid distortion or changes in the surface characteristics and hardness of the denture bases, thus compromising its longevity. Nevertheless, up to present time, there is no standard irradiation cycle protocol for denture disinfection using microwave oven. Therefore, the aim of this study is to find the maximum number of microwave irradiation cycles that is safe to be conducted for acrylic denture bases, using the suggested power–time setting of 630 W for 3 minutes, which was found to be the lowest power setting and time length that is proven to be effective for bacterial and fungal sterilization.[3]


  Materials and Methods Top


Thirty samples in the form of maxillary denture base plates of the same thickness and sizes were constructed by using simulated edentulous maxillary arch study models. Conventional flasking and dewaxing procedures were carried out. Polymethyl methacrylate (PMMA) acrylic resin (Simplex Rapid Powder, Kemdent, Swindon, UK) was mixed and packed into the flasks and put under 1250 kgf/cm2 of final packing pressure. Strain clamps were placed and cured for 9 hours at 74°C in water bath.[4] Deflasking was conducted after bench cooling and the samples were trimmed and polished according to the standard denture making procedures. The final thickness of the samples was standardized at 15 mm.

The prepared samples were randomly divided into two groups: control with no disinfection procedures (n = 15) and test for microwave disinfection (n = 15) following the protocol of 630 W for 3 minutes. The denture bases were immersed in water during microwave disinfection and after disinfection the denture bases were removed and dried.

Low viscosity silicone lining material, Coltene Rapid Liner (Coltene/Whaledent AG, Altstatten, Switzerland) was coated onto the internal surface of all denture bases (control and tests) and placed between the denture base and study model. Pressure of 8 kgf of axial load was applied to the samples for 15 minutes. After removal of load, excess silicone lining was removed using sharp scalpel blade and remaining silicone film was peeled from the denture base. Weighing of the silicone films was done by an independent laboratory assistant who was blinded to the type of silicone films’ group, to avoid bias. An analytical balance (Fisher Science Education Precision Balance, MA) was used to determine the silicone weight change. The weight change of the low viscosity silicone will represent the change in adaptation of the PMMA denture base.[5]

One-day interval was given between each disinfection cycle and all samples were stored in water bath at 37°C. Disinfection cycles were carried out until a statistically significant distortion was detected. The weight of the silicone films was analyzed using paired t test via SPSS 21.0 (IBM Corp, Armonk, NY).


  Results Top


Fifteen silicone films from each group were obtained from every disinfection cycle. The silicone films were weighed using analytical balance every day, and the mean of the silicone weight was compared with the mean of the initial reading at day 0. The silicone films weight was found to be increasing over time when compared with the initial reading at day 0 for the test group. There was no significant weight change in the control group until the end of the study (P ˃ 0.05).[6] However, significant change of silicone film weight was noted starting at cycle 10 until cycle 12 in the test group (P < 0.05) [Figure 1].[6]
Figure 1 Pattern of the weight changes of silicone films. The vertical axis showed the means of the weight subtraction between the baseline and after disinfection cycles. Dotted lines showed linear forecast trend analysis.

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  Discussion Top


Cleaning of the complete dentures must be quick, efficient, and easy to perform, and must not affect the properties of the denture base materials. Mechanical denture hygiene requires high manual dexterity, and abrasiveness will affect the surface characteristics and properties of the denture base. Chemical denture hygiene care can be carried out by soaking dentures in chlorhexidine, sodium perborate, or normal saline. Chemical denture soaking methods are considered time consuming and inappropriate for chairside use.[7]

One of the methods for denture disinfection is by using microwave. Suggestions were made by several studies on the disinfection protocols using microwave oven. A few studies recommended the use of microwave oven under the power of 650 W in 3 minutes[3],[8],[9] while the other study suggested 6 minutes[10] to kill the microorganisms. It was also shown that irradiation of dentures in water environment is more effective than in dry specimens.[2]

Microwave disinfection was proven to be as effective as antifungal therapy in treating denture stomatitis.[11] A study showed that combination between microwave disinfection with brushing and denture cleanser would effectively disinfect the dentures and remove denture biofilms, thus reducing denture stomatitis among denture wearers.[12] The use of microwave disinfection once a week for 2 weeks was found to be as effective as topical antifungal therapy in treating denture stomatitis.[13]

The main concern for microwave disinfection is distortion of the acrylic resin which will affect the adaptation and fitting of the denture base. Studies have shown that 604 W for 10 minutes of microwave disinfection will produce greatest discrepancies in the denture base adaptation.[14],[15],[16] Contrarily, irradiation under 500 W for 3[17] and 15 minutes[15],[16] and 650 W for 3 minutes[3] did not promote clinically relevant alteration of the acrylic resins, nor alter their fitting/stability as well as no detrimental effects on the hardness, flexural, and impact strength of denture resins. Repeated microwave disinfection of 650 W for 3 minutes for up to five cycles also did not show any statistically significant changes in the surface hardness of the PMMA denture base resin.[17]

In this study, irradiation for 630 W for 3 minutes was chosen as the standard protocol as this was found to be the lowest power setting and time length that is proven to be effective for bacterial and fungal sterilization[4],[8] and did not alter the properties of the acrylic denture bases and[18] hardness value of the acrylic resin,[14] nor affect its transverse strength, surface hardness, and surface roughness.[18]

Rapid Liner from Coltene/Whaledent is a C-silicone polyvinyl siloxane material and has high tear resistance property, making it is easier to peel than conventional light body silicone. The difference in silicone weight from day 0 to day 12 was used to represent the changes in adaptation accuracy of acrylic denture bases. This method was adapted from a study by Lee et al.,[5] who showed positive correlation between weight and the adaptation/border gap of a denture base.

In this study, the PMMA denture bases used were in the form of maxillary dentures instead of acrylic resin rectangles to replicate actual clinical situation. In between the test cycles, denture bases were stored in water bath at 37°C to replicate the body temperature and were immersed in water during microwave disinfection cycles. Immersion in water during microwave disinfection cycles has been found to diminish the dimensional change in PMMA when compared to irradiation in dry conditions[19] and to improve disinfection effectiveness.[10]

Microwave disinfection heats water up to the temperature of >72°C which leads to further polymerization in the polymer chains and thus, increase in linear dimensional changes in the acrylic denture bases. In this study, linear dimensional change in acrylic denture was reflected by the change in silicone weight. Dimensional changes in PMMA denture base will lead to distortion and impair the adaptability of the denture base to the study model, which leads to changes in silicone weight. In this study, statistically significant change of Rapid Liner silicone film weight was detected after the tenth cycle of microwave disinfection. This finding was similar to findings by Basso et al.,[20] who found no significant difference in the occlusal vertical dimension (OVD) of dentures if irradiated under 650 W for 3 minutes, once a week for 4 weeks, but significant change in OVD was noted if the irradiation frequencies were increased to three times a week for 4 weeks (12 cycles). However, our finding was in disagreement with a few other studies[14],[17] who found no distortion after microwave disinfection cycles, this might be due to the fact that they stopped only at the maximum of seven cycles, and did not proceed until distortion is detected.[14],[17]

Disagreement and disparities in results could also be attributable to differences in study methodologies, such as disinfection time, microwave oven power, material type, and distortion measurement methods.[21]

Even though microwave disinfection procedures are proven to disinfect dentures, however, there is no standard protocols for the use of microwave in denture cleaning.[22] Recommendations to the patients for the use of microwave disinfection are still controversial due to its effect on the properties of the denture bases. Therefore, it is crucial for the patients to be liable to use the minimum of 630 W for 3 minutes power–time setting concept for microwave disinfection as this has shown to have the lowest power setting and time length that is proven to be effective for bacterial and fungal sterilization.[4],[8] However, if the denture was to be disinfected at 650 W for 3 minutes, the maximum number of cycles would be only three cycle, as recommended by Klironomos et al.[3]

Microwave disinfection is not recommended to be used as daily, weekly, or monthly routine since, it will shorten the lifespan of the dentures. However, microwave disinfection is still beneficial to disinfect the dentures as to prevent cross-contamination from patients or dentures to clinicians or dental technicians and vice versa, especially in high-risk patients and clinicians.[3] Nevertheless, we would suggest that submission to microwave disinfection at 630 W for 3 minutes must not exceed 10 cycles to prevent unwanted distortion and adaptability the PMMA denture bases.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Polychronakis N, Yannikakis S, Zissis A. The effect of repeated microwaving disinfection on the dimensional stability of acrylic dentures. Acta Stomatol Croat 2014;48:279-84.  Back to cited text no. 19
    
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