This in vitro study was performed at the Dental Faculty of Tabriz University of Medical Science at Tabriz, East Azarbaijan, Iran, from May 2018 to Jun 2019.

A total of 112 disk-shaped samples were prepared from Acrosun (Betadent, Iran) and TDV dental (Brazil) acrylic resins (n=56 for each type of acrylic used), measuring 20 mm in diameter and 2 mm in thickness. Each acrylic resin type was further divided into seven groups of eight samples each.

A special mould was designed to fabricate the acrylic blocks. After mixing, the acrylic resin was packed into the mould according to the manufacturers’ instructions and placed in boiling water. After 24 hours at room temperature, the acrylic samples were retrieved, and the excess resin was trimmed.

All the samples were polished until a glossy and shiny surface was achieved. Special kits (Microdent Polident, Brazil) and special 400-grit- and 600-grit polishing sandpaper (Sankyo Rikagaku Co., Ltd., Saitama, Japan) were used two times for three minutes for polishing. The samples were polished by one operator, and a new paper was used for each sample. Finally, the samples were washed and towel dried. The acrylic samples were measured before and after placement in experimental solutions in terms of surface roughness, using Ra and Rz indices, and discolouration using a*, b*, and L* indices.

In the present study, Swiss made Bonyf professional cleansing effervescent tablets (sodium bicarbonate and potassium monopersulfate) and NaOCl solution (MEHTAJ Co., Iran) were used as cleaning agents, and Golestan teabags (Iran) were used as a staining solution. Distilled water (Pars Shimi Company, Tehran, Iran) was used in the control group.

The colour characteristics of the samples were measured using a UV160 spectrophotometer. The International Commission on Illumination (CIE) l*a*b* system is used commonly in dentistry to determine colour change in dental materials. Colourimeter instruments measure three parameters of colour; l* (lightness), a* (red/green), and b* (yellow/blue). ΔE is the colour difference between two specimens, as calculated by the following formula [9]:

The roughness of sample surfaces was measured using a profilometer (Surtronic, Taylor Hobson Company, England).

The samples were placed on the table, and the device’s probe was moved on the surface of the sample. The surface roughness was displayed as a value on the screen of the device; then, the data were recorded. The device was moved in the direction of the sample’s diameter [8-12].

Ra measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length.

Rz measures the vertical distance from the highest peak to the lowest valley within five sampling lengths and averages the distances.

The groups in this study were the following:

Group 1: Immersion in 2% NaOCl solution for 10 minutes and then stored in distilled water for one month.

Group 2: Immersion in 2% NaOCl solution for eight hours and then stored in distilled water for one week.

Group 3: Immersion in the cleansing tablet solution for eight hours and then stored in distilled water for one week.

Group 4: Immersion in tea for two hours and then stored in distilled water for one month.

Group 5: Immersion in tea for two hours and then in 2% NaOCl solution for 10 minutes and distilled water for one month.

Group 6: Immersion in tea for two hours and then in the cleansing tablet solution for eight hours and distilled water for one month.

Group 7: Immersion in distilled water for one month (control group).

Data were analysed with Statistical Package for the Social Sciences (SPSS) version 21.0. The statistical significance level was set at p-value <0.05. Data distribution was first investigated using the Kolmogorov-Smirnov test. One-way Analysis of variance (ANOVA) was used to compare the mean surface roughness and colour changes between the groups due to the normal distribution of data. Post-hoc tests were used to evaluate the mean difference between the paired groups. When the variances were homogeneous, the Tukey’s HSD (Honestly Significant Difference) test was used, and when the variances were not homogeneous, the Dunnett T3 test was used. Finally, the independent sample t-test was used to examine the mean difference between the acrylic resin groups of the two manufactures (two-by-two comparisons) due to the normal distribution of data.

Comparison of colour changes (ΔE) in different groups of acrosun acrylic samples showed that the mean changes in group 4 (acrylic resin samples exposed to tea solution for two hours every day for a month) were higher compared to other groups; in the control group, the changes were lowest compared to the other groups. One-way ANOVA showed that the differences between the groups were significant (p-value <0.001) [Table/Fig-1].

Comparison of colour changes (ΔE) in different groups of TDV acrylic samples showed that the mean changes in group 4 (acrylic resin samples exposed to tea solution for two hours every day for a month) were higher compared to other groups; in the control group, the changes were lowest compared to the other groups. One-way ANOVA showed that the differences between the groups were significant (p-value <0.001) [Table/Fig-2].

Groups 1, 2, 3, 4, 5, and 6 of Acrosun acrylic resin were compared to investigate the colour changes in different disinfection techniques and the effect of these techniques after exposure to tea solution. The independent samples t-test showed significant difference in comparisons between groups (1 and 3), (5 and 6) and (2 and 3) and (4 and 6) [Table/Fig-3].

Groups 1, 2, 3, 4, 5, and 6 of TDV acrylic resin were compared to investigate the colour changes in different disinfection techniques and the effect of these techniques after exposure to tea solution. The Independent-samples t-test showed significant difference in comparisons between groups (3 and 6) and (4 and 6) and (4 and 5) [Table/Fig-4].

In groups 1 and 2, the colour changes in the Acrosun acrylic group were greater compared to that in TDV dental acrylic resin. The independent samples t-test showed that the difference between the two groups was significant p-value=0.002* and p-value=0.03*, respectively. In group 3, the colour changes of Acrosun acrylic were greater compared to TDV dental acrylic resin. However, the independent samples t-test showed that the difference between the two groups was not significant [Table/Fig-5].

Group 4 consisted of acrylic resin samples exposed to tea solution. Colour change in the Acrosun acrylic resin group was more severe compared to that in TDV dental acrylic resin. However, independent samples t-test showed that the difference between these two groups was not significant, p-value=0.797 [Table/Fig-5].

In group 5, the colour changes in the Acrosun acrylic resin were greater compared to the TDV dental acrylic resin. Independent-samples t-test showed a significant difference between these two acrylic resins, p-value=0.017* [Table/Fig-5]. In group 6, Acrosun acrylic resin samples exhibited more colour changes. However, independent samples t-test showed no significant difference between them, p-value=0.326 [Table/Fig-5].

Ra index shows the overall surface roughness.

Comparison of the Ra indexes in acrosun acrylic resin samples showed that the average Ra index in group 6 was higher than that in other groups. In group 1, it was lower than that in other groups. One-way ANOVA showed that the differences between the groups were significant, p-value=0.025* [Table/Fig-6].

Comparison of the Ra indexes in TDV acrylic resin samples showed that the average Ra index in group 6 was higher than that in other groups. In group 1, it was lower than that in other groups. One-way ANOVA showed no significant differences between the groups [Table/Fig-7].

Overall comparison of the Ra indexes (regardless of acrylic resin type) showed that the average Ra index in group 6 was higher than that in other groups. In group 1, it was lower than that in other groups. One-way ANOVA showed that the differences between the groups were significant [Table/Fig-8].

Comparison of Rz index in acrosun acrylic resin samples and all samples regardless of acrylic resin type showed that the mean Rz index in group 6 was higher compared to other groups, with the lowest value in group 1. One-way ANOVA test showed that the differences between the groups were not significant (p-value=0.874) [Table/Fig-9].

Comparison of the Rz indexes (regardless of acrylic resin type) using one-way ANOVA test showed that the difference was not significant (p-value=0.258) [Table/Fig-10].

Comparison of Rz index in TDV acrylic resin samples showed that the mean Rz index in group 6 was higher compared to other groups, with the lowest value in group 7. One-way ANOVA test showed that the differences between the groups were not significant (p-value=0.329) [Table/Fig-11].

In groups 1 and 2, surface roughness (Ra and Rz) in the Acrosun acrylic resin was higher compared to that in the TDV acrylic resin. However, the independent sample t-test showed that the difference between the two groups was not significant in either case. In group 3, the surface roughness (Ra and Rz) in the TDV dental acrylic resin was higher compared to that in the Acrosun acrylic resin. However, the independent samples t-test showed that the difference between the two groups was not significant in either case [Table/Fig-12]. In groups 4, 5 and 6, the independent samples t-test showed that the difference between the two groups was not significant in either case [Table/Fig-12].

This study was performed to compare the effect of different irrigation methods on acrylic resin surface roughness. The results of one-way ANOVA showed that the surface roughness indices of Ra and Rz were not significantly different in any of the study groups [Table/Fig-13].

Denture disinfection might result in changes in the physical properties of the denture base. A previous study had suggested different materials and durations for denture cleaning and disinfection [13]. It is of clinical significance to determine whether denture detergents alter the acrylic resins’ properties. If the disinfection methods are not used properly, changes are possible in the colour of the denture base polymers [13]. Denture fading also occurs due to the high temperature of the water used. Also, discolouration could result from aging or damage to the denture [14].

Irregularities and porosities on the denture surface collect stains and microbial plaque [15]. Surface roughness is considered a critical clinical factor in the formation of biofilms and makes it difficult to eliminate them [16].

Davenport et al., reported that the denture surface roughness could traumatise the oral tissues [17]. Williams DW and Lewis MA concluded that surface roughness was in favor of microorganism colonisation and caused indirect tissue damage [18].

Since various methods used to provide denture hygiene have been shown to affect the physical and mechanical properties of acrylic resins, this in vitro study was undertaken to investigate the effects of tablets and 2% NaOCl solution used to cleanse complete dentures in association with a tea solution on heat cured acrylic surface irregularities and colour changes.

Azevedo A et al., investigated the effect of 1% hypochlorite detergents and 4% chlorhexidine on surface roughness of acrylic resins and showed that disinfectant solutions do not have a significant effect on acrylics surface roughness, consistent with that, in this study hypochlorite and professional tabs had no significant effect on acrylics surface roughness [19]. On the other hand, Firouz F et al., showed that sodium 5.25% hypochlorite concentration significantly increases the level of surface roughness compared to lower concentrations [20]. However, surface roughness from sodium hypochlorite with lower concentrations was clinically acceptable. Da Silva FC et al., also studied the effect of disinfectant solutions on the surface of acrylic resins and showed that the hypochlorite solution could have some effect on the acrylic resin surface roughness [12]. Disinfectants in higher concentrations could have a softening effect on the matrix of acrylic resin and also cause the dissolution of free monomers and the decomposition of the matrix [11].

Comparison of the surface roughness of Acrosun acrylic resin and TDV dental acrylic resin under the effect of detergents showed that Acrosun acrylic resin is comparable to the TDV dental in terms of surface roughness changes.

Finally, regardless of the type of acrylic resin, group 6 samples exhibited the highest surface roughness, and group 1 exhibited the lowest value. Colour change (ΔE) evaluation in the study groups showed that the mean colour changes in group 4 (tea solution) were higher compared to the other groups, and in the control group, the changes were lower compared to the other groups. Therefore, individuals wearing dentures can decrease the consumption of these staining drinks and help stabilise the colour of dentures in the long term. Keskin S reported that staining drinks leave layers on the surface of acrylic resin, which changes its colour [21]. Colour changes might also be due to the presence of pigmented particles in the porosities and surface irregularities of acrylic resins.

Comparison of groups 1, 2 with the control group showed that sodium hypochlorite could have a significant effect p-value <0.05* on colour change. Davi LR et al., examined the effect of immersion in several detergent solutions on the colour stability of acrylic resins [11]. They showed that the highest discolouration occurred in 1% NaOCl and reported that immersion in sodium hypochlorite solution for eight hours a day for 180 days had a definite effect on the colour stability of the denture, consistent with the current results. Peracini A et al., studied the colour change of acrylic resins after immersion in detergent solutions (corega tabs and bony plus) and reported that the colour changes were not significant clinically [8]. In the current study similar results were obtained (comparison of group 3 and control group was insignificant). Sato S et al., reported no colour change in acrylic resins immersed in detergent solutions (corega tabs, bony plus and efferdent plus) [22]. The result was similar to the present study (comparison of group 3 and control group was insignificant).

Examination and comparison of colour changes of Acrosun and TDV dental acrylic resins in tea solution (group 4) showed that the colour changes in Acrosun acrylic resin group were greater compared to TDV dental acrylic resin. However, the difference was not significant. This indicates that the rate of discolouration in both types of acrylic resin was the same.

The results showed that TDV dental acrylic resin exhibited more colour stability (less colour change) in NaOCl disinfectant solution (group 1, 2 and 5), and the difference was significant, However, the professional cleansing tablet solution had almost the same effect on both acrylic resins. Although the colour changes were less in TDV dental acrylic resin, it was not significantly different from Acrosun acrylic resin (group 3 and 6).

There were certain limitations in the present study like true simulation of oral conditions, composition and pH of saliva was not done and time period of the study was limited, denture cleansers may be used for much longer time.

Acrylic resins exposed to tea exhibited more surface roughness compared to those not exposed to tea; therefore, it can be claimed that tea increases the surface roughness and colour change in acrylic resins. Acrosun (Betadent Co., Iran) and TDV dental (Brazil) acrylic resins did not exhibit a significant difference in surface roughness after the tests. However, TDV dental acrylic resin exhibited better colour stability. The results showed that NaOCl solution exhibited the highest potential to remove stains. The professional cleansing tablets were weaker in removing tea stains compared to NaOCl solution. The results showed that TDV dental acrylic resin exhibited more colour stability (less colour change) in NaOCl disinfectant solution. However, the professional cleansing tablet solution had almost the same effect on both acrylic resins.