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Table of Contents   
ORIGINAL RESEARCH  
Year : 2020  |  Volume : 31  |  Issue : 2  |  Page : 282-290
Evaluation of chemical disinfection and microwave irradiation on denture base materials: An in vitro study


1 Department of Prosthodontics, Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education (MAHE), Manipal, Karnataka, India
2 Department of Public Health Dentistry, Manipal College of Dental Sciences, Mangalore, Manipal Academy of Higher Education (MAHE), Manipal, Karnataka, India

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Date of Submission02-Nov-2017
Date of Decision27-Nov-2019
Date of Acceptance03-Mar-2020
Date of Web Publication19-May-2020
 

   Abstract 


Objective: This study evaluated the effect of chemical disinfection and microwave irradiation on the surface hardness and roughness of two commercially available hard relining materials (Ufi Gel hard, GC Kooliner) and one denture base resin (Trevalon). Materials and Methods: A total of 72 specimens (n = 24) were divided into four groups. C: Not disinfected, Cl: disinfected with 4% chlorhexidine solution, Gl: disinfected with 2% glutaraldehyde solution, Mw: disinfected with microwave irradiation (650 W; 6 min). Hardness and roughness measurements were made after polymerisation, 1st day, 14th day and 28th day. Results: Ufi Gel hard showed an increased roughness after 1st day (P = 0.021) following chemical disinfection and GC Kooliner showed similar results after 14th day (P < 0.05). Microwave irradiation showed a significant increase in surface roughness value after 1st day (P < 0.05) for both Ufi Gel hard and GC Kooliner. Hardness of both Ufi Gel (12.131 to 7.333 VHN) and Kooliner (9.133 to 5.276 VHN) was significantly reduced by chemical disinfection, while microwave irradiation resulted in an increased surface hardness of Kooliner (from 9.126 to 12.713 VHN) and Ufi Gel hard (from 11.698 to 14.940VHN). Results for Trevalon were not significant for both the disinfection methods. Conclusions: Microwave irradiation increased the surface roughness and hardness of Ufi Gel hard and Kooliner, while chemical disinfection resulted in a decreased hardness and increased roughness of both hard relining materials. There was no effect of either of the disinfection methods on Trevalon.

Keywords: Chemical disinfection, hard relining material, microwave disinfection, surface hardness and roughness

How to cite this article:
Kabra R, Rodrigues SJ, Pai U, Shenoy R, Shetty TB, Hegde P, Mahesh M, Saldanha S. Evaluation of chemical disinfection and microwave irradiation on denture base materials: An in vitro study. Indian J Dent Res 2020;31:282-90

How to cite this URL:
Kabra R, Rodrigues SJ, Pai U, Shenoy R, Shetty TB, Hegde P, Mahesh M, Saldanha S. Evaluation of chemical disinfection and microwave irradiation on denture base materials: An in vitro study. Indian J Dent Res [serial online] 2020 [cited 2020 Aug 11];31:282-90. Available from: http://www.ijdr.in/text.asp?2020/31/2/282/284578



   Introduction Top


The prime objective of an oral rehabilitation procedure is the preservation of residual oral tissue, maintenance of oral health and the ability of patients to wear prostheses comfortably.[1] The use of complete dentures over a prolonged period may lead to damage of supporting tissue resulting in chronic soreness, pathologic changes and bone loss. These changes result in loss of retention, which can be eliminated by relining with hard or soft denture relining materials.[2]

Direct relining can also be used as a substitute for the laboratory relining to reproduce the morphological features of oral soft tissues directly to the denture base.[2],[3] Chairside relining of a dental prosthesis with hard or resilient relining materials is a faster and easier method to improve stability and retention of the prostheses and to redistribute the masticatory forces transmitted to the underlying tissues.[4]

Large quantity of Candida species and some gram-positive and gram-negative bacterial species associated with systemic diseases have been found on dentures.[5] Proper maintenance of removable dentures will result in an aesthetic, odour free prosthesis as well as good oral health of the denture wearer.[6] Inadequate or improper denture hygiene incites oral infections like denture stomatitis because of biofilm formation on the tissue bearing surface of the prostheses.[7] In addition, respiratory tract infection can also occur in the elderly population because of the presence of microorganisms in the oral cavity.[8] Therefore, denture disinfection is recommended as it prevents cross-contamination and helps in maintaining healthy oral mucosa.

In elderly patients, denture hygiene protocol should include mechanical cleaning, the use of chemical solutions or microwave irradiation ensuring complete eradication of microorganisms from the surface of prosthesis.[2],[9]

The previous studies have shown that denture disinfection can be achieved by scrubbing the denture with either 4% chlorhexidine or 2% glutaraldehyde followed by immersion in the same solution for 10 min.[10],[11],[12] Similarly microwave irradiation for 6 min at 650 Wattage (W) with the specimen submerged in 200 millilitre (ml) of water resulted in effective disinfection.[13],[14]

Surface smoothness of acrylic resins is vital, as smooth surfaces of resilient liners and denture base are significantly less prone to adherence of microorganisms and plaque formation.[15],[16]

Surface hardness is the property of acrylic materials which indicates its capability to withstand occlusal forces as well as its resistance to scratching during disinfection procedures.[17]

Studies have evaluated the effect of chemical disinfectants like glutaraldehyde, sodium hypochlorite,[12] chlorhexidine gluconate[18] and sodium perborate[14],[19],[20] on the properties of denture base acrylic resins and relining materials such as colour, water sorption, solubility, hardness, porosity and surface roughness. Alterations in surface hardness and roughness of hard relining materials due to various chemical disinfectant solutions have been reported.[21] The degree of changes was dependant on the type of disinfectant and duration of immersion.

With regard to microwave irradiation, the studies have evaluated hardness,[16] residual monomer content and[22],[23] bond strength of acrylic teeth and denture base resin and flexural strength[24] of acrylic resins. Microwave irradiation may cause a breakdown of the surface layer of acrylic resins due to high temperature attained during microwave disinfection.[25] It was established that microwave irradiation considerably reduced the surface hardness of 5 brands of artificial teeth[9] and altered the surface roughness of a microwave-cured denture base acrylic resin.[26] In addition, the rise in temperature in microwave irradiation facilitates residual monomer release, enhancing further polymerisation[27],[28],[29] consequently influencing the surface layer properties of the resin.[22],[30] The surface layer is identified to have a high level of residual monomer content.[31]

While several previous investigators have studied the effect of chemical and microwave disinfection on denture base acrylic resins for one to two cycles or for 7 days, little has been known about the effect on denture base materials and hard relining materials after repeated cycles of chemical and microwave irradiation.

The purpose of this study was to evaluate and compare the effect of two chemical disinfectants and microwave irradiation after repeated cycles of disinfection on the hardness and surface roughness of two hard chairside relining materials and one heat polymerising acrylic denture base resin.


   Materials and Methods Top


The materials used in this study are presented in [Table 1].
Table 1: Materials used in study

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Specimen preparation

Seventy-two samples (n = 24) were made in the form of modelling wax blocks of dimension (12 mm × 12 mm × 3 mm). The thickness of the material was selected based on the Study of Pavarina AC et al.[14]

For preparing the specimens, the block was made of self cure acrylic resin of size 12 mm × 12 mm × 3 mm. Putty impression material was used to make the impression of self cure acrylic resin block to get the mould. The modelling wax was melted and poured into the putty mould. Once the modelling wax was hardened, the wax block was carefully retrieved from the putty impression. In this manner, seventy-two wax blocks of size 12 mm × 12 mm × 3 mm were prepared. The wax blocks were invested in type II dental plaster in the standard metal dental flask using compression moulding technique followed by dewaxing of the specimens [Figure 1].
Figure 1: Specimens used in the study

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The powder and liquid components of all the materials were mixed according to the manufacturer's instruction [Table 1], inserted into the moulds and packed. The specimens of the hard reline resins were kept under pressure at room temperature according to the recommended time by the manufacturer [Table 1]. For the hard relining materials, surface hardness and surface roughness measurement were performed soon after polymerisation. These values were used as baseline values due to the fact that patients wear this relined denture immediately after polymerisation.[4] For the resin (T), the specimens were stored in distilled water for 48 h before hardness and roughness measurement. This time represented the time elapsed between laboratory relining and denture insertion.[4]

Surface hardness measurement

Surface hardness was measured with Vickers Hardness Tester (Otto Wolpert-Werke, Germany). The Vicker hardness test is used for evaluating hardness, viscoelastic properties of rigid polymers.[13] Vicker hardness test has been used by some of the previous investigators in their studies to detect a change in surface hardness of denture base resins.[2],[32] The test involves the use of square base shaped pyramid diamond indenter point using a 30 g load for 30 s.

The difference between making and reading indention was very brief (15 s). The viscoelastic recovery of diagonals was minimal because of the short time period involved.

Three indentations were made at different points on each specimen and the mean of the individual was averaged.

Surface roughness measurement [Figure 2]

The surface roughness (Ra, μm) was analysed with profilometer (Hommel Tester T) with a diamond stylus which is connected to a unit that processes and analyses the information by software (Turbo Datawin 1.34). A reading was obtained by the needle passing across 0.8 mm length at 0.5 mm/s. 3 readings were made at different positions resulting in a total reading length of 2.4 mm. Ra average of 3 readings for individual specimens was noted.
Figure 2: Armamentarium for recording surface hardness and roughness

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Grouping of specimen:

24 specimens of each material were divided into 1 control group and 3 experimental groups of 6 specimens each.



For all groups, individually the specimens were immersed in 200 ml volume of water. This is the minimum volume of water required to cover a maxillary complete denture in a denture pot.[18] The surface hardness and roughness of each specimen were again measured at 1st, 14th and 28th day as described above.

The results were analysed for statistical comparison and significance using one way ANOVA with Bonferroni's test. For each material, changes in hardness and roughness at different time intervals within each group were analysed using Student's t-test. Statistical analyses were conducted at 95% level of confidence and 80% power.


   Results Top


[Table 2] shows the results from the student t-test for the analysis of the difference between the comparison of surface roughness measurements obtained at three evaluation periods (1st, 14th and 28th day) and the baseline readings for the three groups analysed in this study.
Table 2: Comparison of surface roughness measurements obtained at three evaluation periods (1st, 14th and 28th day) and the baseline readings for the three groups analysed in this study

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No significant change was found for the material Trevalon.

Ufi gel hard showed a statistically significant increase in roughness (P- value < 0.05) between the three evaluation periods with respect to baseline irrespective of the disinfection method used or water as a control.

GC Kooliner showed a significant increase in roughness after 14 and 28 cycles for glutaraldehyde and chlorhexidine group, while for microwave irradiation, a significant increase was observed for all the three evaluation periods.

[Table 3] represents the comparison of surface roughness values of Ufi Gel hard within each group (C, Gl, Ch, Mw) at baseline, 1st, 14th and 28th day using Bonferroni's test.
Table 3: Comparison of surface roughness values of Ufi Gel hard within each group (C, Gl, Ch, Mw) at baseline, 1st, 14th and 28th day using Bonferroni's test

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The surface roughness values at 14th day showed a statistically significant difference between control and microwave (P = 0.003), chlorhexidine and microwave (P = 0.041), glutaraldehyde and microwave (P = 0.043) group indicating that microwave irradiation results in a rougher surface as compared to chemical disinfection or immersion in water. Similar results were found for 28th day.

[Table 4] represents the comparison of surface roughness values of GC Kooliner within each group (C, Gl, Ch, Mw) at baseline, 1st, 14th and 28th day using Bonferroni's test.
Table 4: Comparison of surface roughness values of GC Kooliner within each group (C, Gl, Ch, Mw) at baseline, 1st, 14th and 28th day using Bonferroni's test

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The surface roughness values at 14th day showed a statistically significant difference between control and microwave (P = 0.003), chlorhexidine and microwave (P = 0.041), glutaraldehyde and microwave (P = 0.043) group indicating that microwave irradiation results in a rougher surface as compared to chemical disinfection or immersion in water. Similar results were found for 28th day.

[Table 5] shows the results from the student t-test for the analysis of the difference between the comparison of surface hardness measurements obtained at three evaluation periods (1st, 14th and 28th day) and the baseline readings for the three groups analysed in this study.
Table 5: Comparison of surface hardness measurements obtained at three evaluation periods (1st, 14th and 28th day) and the baseline readings for the three groups analysed in this study

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No significant change was found for the material Trevalon.

Ufi Gel hard and GC Kooliner showed a statistically significant decrease in hardness (P < 0.05) between the three evaluation periods with respect to baseline for chemical disinfection method used or water as control, while for microwave irradiation, there was a significant increase in hardness.

[Table 6] and [Table 7] represent the comparison of surface hardness values of Ufi Gel hard and GC Kooliner, respectively, within each group (C, Gl, Ch, Mw) at baseline, 1st, 14th and 28th day using Bonferroni's test.
Table 6: Comparison of surface hardness values of Ufi Gel hard within each group (C, Gl, Ch, Mw) at baseline, 1st, 14th and 28th day using Bonferroni's test

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Table 7: Comparison of surface hardness values of GC Kooliner within each group (C, Gl, Ch, Mw) at baseline, 1st, 14th and 28th day using Bonferroni's test

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The surface hardness values at 1st day showed a statistically significant increase in between control and microwave (P < 0.05), chlorhexidine and microwave (P < 0.05), glutaraldehyde and microwave (P < 0.05) group indicating that microwave irradiation results in a significant increased surface hardness as compared to chemical disinfection or immersion in water. Similar results were found at 14th and 28th day.


   Discussion Top


The surface roughness of polished acrylic resin ranges from 0.03 to 0.75 μm.[33] Surface roughness of acrylic resins directly affects the health of oral tissues which are in direct contact with prosthesis.[34] The previous investigations have shown that the frequency of denture stomatitis in complete denture patients ranges from 10% to 67%.[35] Therefore, it is recommended that the prosthesis should be as smooth as possible to prevent biofilm formation and mucosal infection. The results of this study showed that the surface roughness of the denture base acrylic resin (Trevalon) is neither significantly affected by immersion in water nor by the chemical or microwave disinfection. These results were in compliance with those of the study conducted by Harrison et al. who studied the effect of denture cleansers on surface roughness of conventional denture base acrylic resin.[36] Similar conclusions were drawn by Machado et al. who evaluated change in roughness by microwave irradiation and chemical disinfection on Lucitone 550 whose composition is similar to that of Trevalon and found that there was no significant difference in roughness values measured after 1st, 3rd and 28th day of disinfection.[4]

The results may be attributed to the type of polymerisation methods. Heat-polymerised materials, such as Trevalon, present a higher monomer-to-polymer conversion rate and lower residual monomer content.[22],[23] In addition, Trevalon contains cross-linking agents in its composition (namely, ethylene glycol dimethacrylate), which may have contributed to the formation of more dense and stable polymer structures.[37],[38] Concerning the hard relining materials, the results showed that there is a significant increase in surface roughness with both of the disinfection methods used. This result was in compliance with the result of the study performed by Machado et al.[16] who studied the effect of surface roughness and hardness of hard chairside reline materials and found that both chemical and microwave disinfections increase the surface roughness. Similar results were found by Pinto Lde R et al.[2] who studied repeated cycles of chemical disinfection on surface roughness and hardness of hard reline acrylic resins in which specimens were subjected to 30 disinfection cycles.

The increase in the surface roughness of Ufi Gel hard and GC Kooliner may be attributed to the fact that the level of residual monomer in autopolymerising acrylic resins is higher in the surface layer.[39] Although Ufi Gel hard has a high concentration of cross-linking agent, namely, 1,6 hexanediol dimethacrylate, it has been established that Ufi Gel hard showed significantly higher residual monomer content and release than a reline resin with similar composition, namely, Tokuso Rebase Fast.[23] The release of this residual monomer molecules might have contributed to alter the surface characteristics of the material, thus increasing the roughness at the initial stage.[40] When the hard relining materials were subjected to chemical disinfection along with water, some components of the solution penetrates the acrylic resin which results in micro-crack formation causing irreversible damage to the material. The formation of micro-cracks at the superficial layer causes an increase in the surface roughness at a later stage.

When the hard relining materials are subjected to microwave disinfection, the increase in the surface roughness may be attributed to the high water temperature attained during microwave irradiation. Acrylic resins on boiling exhibit a breakdown of surface layer because of micro crazing of the surface.[25] During microwave disinfection, water starts to boil after 1 min 30 sec and the disinfection procedure is completed only after 6 min. The temperature attained during microwave disinfection explains the more significant alterations observed as compared to chemical disinfection and control.[41]

The hard relining materials exhibited the highest roughness mean value of 0.53 μm for Ufi Gel and 0.51 μm for GC Kooliner at 14th day followed by the decrease in surface roughness value at 28th day. This trend may be due to the cessation of diffusion of monomer into the water and also the continuing polymerisation, which improves the mechanical properties of the reline resins.[27] Rate of monomer diffusion from the resin into water decreases continuously and this process usually gets completed by 14th day,[42] thus explaining the decrease in the surface roughness in due course of time. Similar results were found by Izumida FE et al.[43] in their study on surface roughness of reline resins after long-term chemical disinfection with chlorhexidine. Surface roughness was measured at different time intervals up to 365 days and the result showed the highest mean surface roughness at 15th day.

Surface roughness is not the only factor involved in the adhesion of microorganisms to polymer-based materials.[4] The presence of salivary pellicle and the surface free energy were also reported to influence the adhesion of microorganism.[44] Others factors such as wettability, hydrophobicity and electrostatic interactions of materials used in prosthodontics may also influence the microbial retention.[45]

A small and significant decrease in the hardness values of Ufi Gel hard and GC Kooliner when subjected to chemical disinfection or immersion in water was observed. In contrast to chemical disinfection, microwave irradiation results in a significant increase in surface hardness. Neither chemical disinfection nor microwave irradiation showed detectable changes in the hardness of Trevalon. Similar results were found by Luciana de Rezende Pinto et al.[2] who evaluated the effect of repeated cycles of chemical disinfection on the surface hardness of hard reline acrylic resins. Devlin and Kaushik[46] in their study concluded that the water sorption on the acrylic surface was associated with a reduction in surface hardness.

Water sorption initially causes swelling of the polymer network reducing the frictional forces between the polymer chains resulting in softening of the acrylic resin. Absorbed aqueous disinfectant solution acts like a plasticiser, decreasing the glass transition temperature (Tg).[2] Below the Tg, methyl methacrylate is immobilised in the glassy polymer leading to its limited polymerisation, ensuing the high residual monomer content on the surface layer, thus decreasing the hardness of the resin.[47]

Studies have evaluated the effect of immersion in water on the hardness of auto polymerised acrylic resin for different time periods and noticed an increase in hardness after 7 days,[48] 32 days[32] and 60 days.[42] It is likely that due to further polymerisation and residual monomer release mechanisms, which probably overcame the plasticising effect of water uptake, the increase in the hardness was observed.

For the microwave disinfection method, the water in which the materials were placed reached the boiling temperature. The Tg of autopolymerising resin ranges from 67°C–78°C.[49] Hence, when the temperature reaches above the Tg of the resin, the monomers in the resin polymerise due to mobilisation of the methyl methacrylate in the glassy polymer at high temperature.[47] This induces further polymerisation and residual monomer release process. Faltermerier et al.[50] also observed that heat used as an additional polymerisation method considerably improved the hardness of autopolymerising acrylic resins.

Trevalon was not affected by either chemical or microwave disinfection method. This is in agreement with other studies, which also found that repeated microwave disinfection[51],[52] or treatment with warm water (40°C) and alkaline peroxide for 30 days[46] did not result in any noticeable alterations in the hardness of the heat-polymerised resin evaluated.

Hardness is proportional to the lower residual monomer content[53] as seen in Trevalon. These factors may explain the difference between the hardness value of Trevalon, Ufi Gel hard and GC Kooliner specimens.

Among the hard relining materials, the lowest hardness values were observed for GC Kooliner specimens. Kooliner does not contain cross-linking agents, whereas Ufi Gel hard contains a high percentage of crosslinking agent 1,6 HDMA favouring monomer to polymer conversion. In addition, Kooliner has the lowest powder-to-liquid ratio among the materials tested in this study.[38] It has already been demonstrated that lower the powder liquid ratio, higher is the content of residual monomer present resulting in lower hardness value.

Although the results of this study revealed a small, but significant decrease in hardness in hard relining materials after repeated cycles of chemical disinfection, these findings (5.745 Vicker Hardness Number (VHN)) were not considered clinically significant, since it did not cross the lower threshold value (4.246VHN) of hardness for hard reline acrylic resin as reported in the literature.[54]

It is suggested that the hardness exhibited by all the hard reline materials before and after repeated cycles of disinfection procedure is acceptable for safe use by wearers of complete acrylic dentures. Therefore, in complete denture wearers with stomatitis, it is possible to implement a daily immersion protocol for up to one month without any adverse effect on the hardness and roughness with any of these chemical disinfection solutions or microwave irradiation to complement mechanical cleansing.


   Conclusions Top


Within the limitations of this in-vitro study, the following conclusion can be drawn:

  1. Hardness and roughness of the denture base acrylic resin were not adversely affected by either the chemical or microwave disinfection methods.
  2. Microwave irradiation resulted in a significant increase in surface roughness of both the hard relining materials and showed maximum roughness value at 14th day.
  3. Microwave irradiation resulted in a significant increase in surface hardness of Ufi Gel hard and GC Kooliner.
  4. Immersion in chlorhexidine or glutaraldehyde solution resulted in a significant decrease in surface hardness of Ufi Gel hard and GC Kooliner.
  5. Chemical disinfection resulted in an increase in surface roughness of Ufi Gel hard and GC Kooliner with maximum roughness value on 14th day of disinfection cycle.
  6. Microwave irradiation resulted in a more adverse effect on surface roughness as compared to chemical disinfection at 14th day and 28th day of disinfection cycles of Ufi Gel hard and GC Kooliner.
  7. No significant difference was found between 2 chemical disinfectants for all the three materials evaluated in the study.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Correspondence Address:
Dr. Shobha J Rodrigues
Professor and Head, Department of Prosthodontics, Manipal College of Dental Sciences, Lighthouse Hill Road, Mangalore - 575 001, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijdr.IJDR_622_17

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