Liners are soft materials that are used to improve the adaptation of dentures and the condition of traumatized tissues. In general, these materials are classified into two categories: provisional soft liners (tissues conditioning materials) and permanent soft liners. ¹ In another classification, the currently available liners are divided into two broad categories: silicone and acrylic, with each group being subdivided into self-cured and heat-cured subgroups. ¹ These materials are usually marketed as a powder and a liquid, which are mixed to produce a gel that is soft and viscoelastic. ² Permanent soft liners are usually used as a therapeutic intervention in patients who cannot tolerate stress produced by dentures. These liners distribute functional and parafunctional forces more properly due to their elastic behavior, resulting in the relaxation of the underlying tissues. ³ The use of these liners improves mastication and articulation and increases the retention and stability of the denture. ⁴ , ⁵ , ⁶ Soft liners can easily and quickly be colonized by microorganisms; however, antimicrobial mouthwashes can be used to prevent microbial colonization of denture surfaces. The selection of these mouthwashes should be based on their effect on microorganisms and the absence of any detrimental effects on the soft liners. ⁷ When the denture liners continually contact water and other liquids, they lose their mechanical and physical properties over time for two reasons: (1) due to the elimination of the plasticizer and other water-soluble agents and (2) due to the absorption of waster by polymers. These changes give rise to various problems with the use of soft liners, including the failure of the bond between the liner and the resin base of the denture (which will make it ineffective), loss of softness and elastic properties, Candida albicans colonization, and an increase in surface roughness (SR) (leading to microbial plaque accumulation). ⁸ , ⁹ , ¹⁰ A systematic review (2019) found that different mouthwashes have different effects on the mechanical properties of soft liners. ¹¹ Therefore, selection of an appropriate disinfectant is necessary to decrease such changes. ¹²

The present study aimed to evaluate the effect of chlorhexidine (CHX) mouthwash, which is useful in eliminating Gram-positive, Gram-negative, and Candida species ¹³ and a new antifungal herbal mouthwash containing ginger on the shear bond strength (SBS) and SR of denture liners.

The null hypothesis of this study was that the CHX and ginger mouthwash had no effect on SBS and SR of two soft liners.

Shear bond strength

Each sample consisted of two Triplex Hot acrylic resin blocks (Ivoclar, Vivadent, Liechtenstein) (40 mm × 10 mm × 10 mm) that had been bonded together with an exact thickness (3 mm) of the soft liner. The blocks were fabricated using the compression mold technique with a special flask that could provide the desired dimensions. In another custom-made flask, the liner was injected between the two blocks ¹⁴ Figure 1and Figure 2. Figure 1

Packing of the acrylic resin into the special flask for preparation of the acrylic resin blocks.

Table 1presents the descriptive data and statistical analysis on the SBS test. According to the Table, Mollosil liner exhibited the highest SBS in the distilled water group (100.4 ± 9.76 N); the CHX group exhibited the lowest SBS (91.31 ± 13.22 N). In addition, the Molloplast liner exhibited the highest (177.05 ± 19.82 N) and lowest (154.78 ± 23.25) SBS means in the distilled water and CHX groups, respectively. The independent t-test was used to compare the mean SBS values between the liners separately in each solution. The results showed that the liner type resulted in significant differences in the SBS values in each group of three solutions (P < 0.001). The mean values were compared between the different solutions separately in each liner with the use of one-way ANOVA. The results showed that the solution type did not result in a significant difference in the mean SBS between the two liners (P = 0.423 for Mollosil group and P = 0.093 for Molloplast group) Table 1. [Graph 1]a and [Graph 1]b present the mean SBS values in terms of the groups and different liners.{Table 1}[INLINE:1]

Table 2presents the descriptive data and statistical analysis of the SR in all groups of liner/solutions at 1 day and 90 days. According to the results of one-way ANOVA analysis, differences of mean SR values in each liner soaked in three solutions were not statistically significant in both time intervals (P values: Mollosil/day 1 = 0.138, Molloplast/day 1 = 0.821, Mollosil/day 90 = 0.978, Molloplast/day 90 = 0.159).{Table 2}

Comparison of the SR values at 1- and 90-day intervals with paired t-test in each liner/solution group showed that only Mollosil liner exhibited a significant difference in the ginger extract solution group (P = 0.04). Data of SR in different solutions and liners at 1- and 90-day intervals are presented in [Graph 2] and [Graph 3].[INLINE:2][INLINE:3]

Soft liners are added to the inner surface of dentures to homogenously distribute forces and decrease pressures at certain points. Conventionally, disinfectants are used to prevent and manage microbial colonization in soft liners; however, an agent should be selected that does not disrupt the mechanical properties of soft liners, while it decreases or prevents changes due to its use. Therefore, the present study was undertaken to (1) compare the SBS of Mollosil Plus and Molloplast B soft liners to heat-cured acrylic resin after immersion in different solutions for 20 days, including distilled water, 0.2% CHX, and ginger extract solution, and (2) compare the surface of the two liners immediately and 90 days after immersion for 15 min daily in the above solutions.

According to the results, in both soft liners (heat cured and intraoral), the highest SBS was recorded in the distilled water group (the control), with the lowest in the CHX group. In the Mollosil Plus soft liner, the comparison of SBS between the ginger extract solution and CHX groups and the control group did not reveal any significant differences (P = 0.42). In the Molloplast liner, too, such a comparison with the control group showed a nonsignificant decrease in SBS (P = 0.09). The soft liner type resulted in a significant difference in the SBS (P < 0.001).

Concerning SR in the Mollosil liner group, the ginger extract solution resulted in a significant increase in this variable from day 1 to day 90 (P = 0.04). In addition, CHX mouthwash resulted in a nonsignificant decrease in SR in both liners (P > 0.05).

Several studies have evaluated the effects of different mouthwashes on the SR of soft liners and the bond strength of soft liners to the acrylic resin base. Mahboub et al. evaluated the effects of NaOCl and Corega on the shear and tensile bond strengths of GC soft liner. NaOCl significantly decreased the TBS and SBS of the GC soft liner compared to the control groups; however, Corega did not significantly affect the SBS and TBS compared to the control group. ¹⁰

Geramipanah et al. evaluated the effects of 2.5% NaOCl and Corega on the tensile bond strength of Acropars, Molloplast, GC, and Mollosil soft liners and reported that there was a significant relationship between the soft liner type and the tensile bond strength: Acropars > Molloplast > GC > Mollosil. However, the effect of the solution type was not significant on the tensile bond strength of the samples. ¹⁵ In the present study, too, the liner type had a significant effect on the bond strength; however, immersion in different solutions did not result in significant differences in the bond strength of different liners.

Abdul-Razaq et al. evaluated the effects of 0.2% CHX and Solo solutions on the SBS, hardness, and SR of two soft liners (Mollosil and Viscogel). None of the solutions significantly affected the SBS and hardness of soft liners. In addition, 0.2% CHX significantly decreased the SR of Viscogel soft liner and nonsignificantly decreased the SR of Mollosil soft liner. The Solo solution, too, significantly decreased the SR of Mollosil soft liner. The results of the present study on the effect of CHX on the SBS and SR of soft liners are consistent with those of the study above. ¹⁹ Izumida et al. (2014) also found a reduction in roughness associated with brushing and disinfection with sodium perborate and/or CHX gluconate and related it to cross-linked agents that reduce the acrylic resin solubility in organic solvents. ²⁰

In general, differences in the behaviors of different soft liners in different solutions are factors that determine changes in the SR.

Pavarina et al. (2003) evaluated the effects of CHX solution and microwaves on the SR of five relining materials and reported that CHX increased the SR of the liners. The differences in the results of the study above and the present study might be explained by the different nature of the liner, the difference in the CHX concentrations, and the difference in the protocol of the use of CGX between the two studies. ¹⁷

The SBS of the soft liners evaluated after immersion in CHX and ginger extract solution was not significantly different from that after immersion in distilled water

Therefore, 0.2% CHX mouthwash is recommended for the disinfection of both liners evaluated in the present study. However, the use of the ginger extract-containing mouthwash is recommended for the disinfection of heat-cured soft liner.

Financial support and sponsorship

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.