Comparative Evaluation of the Effectiveness of Denture Cleansers on Surface Hardness of Permanent Silicone Denture Liners - An In vitro Study.

AIM: This study aimed to determine the effect of three commercially available denture cleansers on surface hardness of two different silicone denture liners at various time intervals.
MATERIALS AND METHODS: A total of 120 cylindrical specimens of permanent silicone-based soft liners were prepared from a custom made metal mold with dimensions of 15 mm × 10 mm, of which 60 specimens were fabricated using Voco™ and the remaining 60 specimens with Molossil™ permanent denture liners. All specimens were divided into four major groups based on cleansing treatments, comprising 30 specimens each. And then, all cleansers dissolved in 250 mL water as recommended by the manufacturer. Specimens of the first group were cleansed daily by rinsing with water and second group with Periogard™ denture cleansing solution and third group with Secure™ denture cleansing solution and the fourth group with Polident™ denture cleansing solution. Specimens of all groups were tested on 1st week, 1st month, and 6th month time interval using a Shore A Durometer. Three readings were noted for each sample and the mean of those readings was taken.
RESULTS: Surface hardness of all the samples was significantly higher after a period of 6 months irrespective of the cleansing treatment. Statistically significant changes were observed between Polident™, Secure™, and Periogard™ denture cleanser groups over control with time. Minor changes were observed in surface hardness of Voco™ denture liners as compared to Molossil™ soft liners for all groups, as time progressed.
CONCLUSION: Voco™ soft denture lining material can be performed significantly better than Mollosil™ soft denture lining material in all cleansing treatments and Polident™ denture cleanser has the least effect on surface hardness of soft liners than Periogard™ and Secure™ denture cleansing agents. Copyright:

INTRODUCTION

Removable dentures are extensively used in rehabilitating partially or completely edentulous patients. Most of the removable dentures which rely upon tissue support transmit the load to the underlying bone through the mucous membrane.[1] Some areas of the alveolar ridge are very sensitive to the pressure of hard prosthesis due to the thin overlying mucous membrane.[2] When the shock-absorbing behavior of mucus membrane is diminished, masticatory forces are directly transmitted to the underlying bony tissues. As a result, there is an increased stress on the residual ridges.[3] Soft lining materials are able to form an absorbing layer on the part of denture in contact with the oral mucosa and this allows less traumatic occlusal force transmission.[4]

The resilient soft lining materials present problems during clinical use, such as weakening of bond between lining and denture, loss of resiliency, color alterations, and porosity. The loss of resiliency may be due to the leaching out of the plasticizer and other components.[5] Simultaneously, water is absorbed until equilibrium is reached thereby, increasing surface roughness of the resilient denture liners.[6]

Denture plaque control using mechanical and chemical methods is essential for the maintenance of good oral hygiene of denture wearers. However, mechanical cleansing is not advisable for soft denture liners since it can damage the resilient lining. Chemical cleansing by denture cleansers is the first choice for denture plaque control of denture liners. A denture cleanser is used to clean dentures when they are out of the mouth. The main use is to control the growth of microorganisms on the dentures, especially Candida albicans, thereby preventing denture-related stomatitis.[7]

The purpose of this study was to determine the effect of three commercially available denture cleansers on surface hardness of two different brands of permanent silicone denture liners. The selected brands of silicone liners for this study were Voco™ and Mollosil™, and denture cleansers were Polident™, Secure™ and Perioguard™. The surface hardness of these materials was determined by using a surface hardness tester Shore A Durometer.

MATERIALS AND METHODS

The present study was conducted in the Department of Prosthodontics at Kannur Dental College and testing procedure was carried out in the technical laboratory of MIT Engineering College, Kannur, using a surface hardness tester – Shore A Durometer.

The procedure involved in the present study was divided into four steps:

Step 1: Specimen preparation

Two commonly used silicone-based resilient liner materials were selected for the study [Table 1]. A total of 120 cylindrical specimens were prepared from a custom-made metal mold with dimensions of 15 mm × 10 mm (According to ASTM: D-2240-64T) to prepare the specimen and the metal mold was placed over glass slab applied with petroleum jelly for easy removal of specimens. Soft denture liners were manipulated according to the manufacturer's instructions and were loaded into mold. The mold was then covered from the top by another glass slab applied with petroleum jelly. The two glass slabs were then pressed tightly against the mold to remove excess material and to shape the specimen according to dimensions of the mold. After material was set, the specimens were then removed from mold and excess was trimmed using a BP blade. All specimens were stored in artificial saliva throughout the study [Figure 1].

Table 1

Silicone denture liners and their information are listed below

Product	Materials	Manufacturer
Ufi gel P	A-silicone-based, nonhardening relining material	Voco GmbH Anton-Flettner-Straße 1-3, 27,472 Cuxhaven, Germany
Mollosil	A-silicone-based, permanently soft, relining materials	DETAX GmbH and Co. KG P.O. Ettlingen Germany

Figure 1

(a) Metal mold, (b) materials used, (c) measurement of surface hardness using Shore A Durometer

Step 2: Grouping of specimen

All specimens were divided into four major groups based on cleansing treatments. Group A (Control), Group B (Periogard™), Group C (Secure™), and Group D (Polident™) [Table 2], comprising of 30 specimens each. Each group was divided further into two subgroups: Subgroup 1 and Subgroup 2, each consisted of 15 specimens and each subgroup was again divided into three minor subgroups: Minor Subgroup A – to be tested 1st week, Minor Subgroup B – to be tested on 1st month, and Minor Subgroup C – to be tested on the 6th month [Tables 1 and 2].

Table 2

The denture cleansing agents and their manufacturer information are listed below

Product	Main ingredients	Manufacturer
Polident	Potassium monopersulfate	Polident D.O.O
	Sodium percarbonate	VolcjaDraga 42
	Tetraacetylethylenediamine	Slovenia
Secure	Sodium bicarbonate	Secure
	Trisodium phosphate	Grandinetti drive
	Sodium perorate monohydrate	Ghent, NY 12075 - USA
Periogard	Chlorhexidine gluconate Propylene glycol, glycerin	Colgate Palmolive India Ltd
		Thousand Lights, Chennai, India

Step 3: Placement of specimen into cleanser

All cleansers were dissolved in 250 mL water as recommended by the manufacturer. Specimens in Group A were immersed in water and specimens in Group B were immersed in Periogard™ cleansing solution and specimens in Group C were immersed in Secure™ denture cleansing solution and specimens in Group D were placed in Polident™ denture cleansing solution for 8 h daily. After 8 h, all specimens were taken from cleansing solutions and rinsed with running water and then rest of the hours hey were stored in artificial saliva (wet mouth, ICPA health solutions) at room temperature.

Step 4: Placement of specimen on Shore A Durometer

Specimens of three minor subgroups were tested at 1st week, 1st month, and 6th month time interval at MIT Engineering College, Kannur. The specimens were tested using a Shore A Durometer.

Hardness was measured using a Shore A Durometer which was calibrated in accordance with ASTM D-2240 under the spring force of 822 gf (8.06 N). The instrument consists of a blunt-pointed indenter attached to a scale by lever arrangement with a recording scale from 0 to 100 Shore A units. The more the indenter penetrates the specimen, the lower the hardness value was. The shaft was lowered onto the middle of the specimen, and the highest reading was noted.

Statistical analysis

Three readings were noted for each sample and the mean of those readings was taken. These readings were then subjected to one-way ANOVA, post hoc test, and pair t-test for statistical analysis.

RESULTS

On applying one-way ANOVA in Mollosil™ soft liners, it was found that there was a statistical significant difference in mean surface hardness and the mean value of surface hardness was highest with Periogard™ denture cleanser and lowest with Secure™ denture cleanser.

When compare the group interactions of the Mollosil™ soft liner with post hoc Tukey test, for the 1st week, there was no significant difference in the mean surface hardness in all the comparisons. And for the 1st month, there was no significant difference in the mean surface hardness except for Periogard™ and Secure™. And for the 6th month, there was a significant difference in the mean surface hardness except for control and Polident™.

After comparing Bothvoco™ and Mollosil™ groups using unpaired t-test, control group shows that surface hardness is higher in Voco™ group, and during 1st month and6th month, it was higher for Mollosil™ group and it was statistically nonsignificant. In Periogard™ group, 1st week, 1st month, and 6th month results showed that it was higher in Mollosil™ which was statistically significant only during the 6th month. In Secure™ group, it was higher in Mollosil™ group and it was statistically nonsignificant. In Polident™ group, it was higher in Mollosil™ group, and it was statistically significant [Table 3].

Table 3

Comparison of the Voco™ and Mollosil™ groups using unpaired t-test

	Subgroup	n	Mean	SD	t	df	P
Control
1st week	Voco™	5	20.4	0.548	0	8	1
	Mollosil™	5	20.4	1.14
1st month	Voco™	5	25.2	1.304	−0.866	8	0.412
	Mollosil™	5	25.8	0.837
6th month	Voco™	5	31.2	1.483	−0.728	8	0.488
	Mollosil™	5	31.8	1.095
Periogard
1st week	Voco™	5	21.2	1.095	−0.283	8	0.784
	Mollosil™	5	21.4	1.14
1st month	Voco™	5	26	0.707	−0.667	8	0.524
	Mollosil™	5	26.4	1.14
6th month	Voco™	5	32	2	−3.087	8	0.015
	Mollosil™	5	35.6	1.673
Secure
1st week	Voco™	5	20.6	0.548	−2.309	8	0.05
	Mollosil™	5	21.4	0.548
1st month	Voco™	5	22.8	1.095	−2.263	8	0.053
	Mollosil™	5	24.4	1.14
6th month	Voco™	5	27.2	0.837	−0.73	8	0.486
	Mollosil™	5	27.6	0.894
Polident
1st week	Voco™	5	20.2	0.837	−2.683	8	0.028
	Mollosil™	5	21.4	0.548
1st month	Voco™	5	22.6	0.894	−4.017	8	0.004
	Mollosil™	5	24.8	0.837
6th month	Voco™	5	26.8	0.837	−6.532	8	<0.001
	Mollosil™	5	30	0.707

SD: Standard deviation

DISCUSSION

Soft denture lining materials lead to a more uniform distribution of stress at the mucosa/lining interface. Their effectiveness in handling such cases has been attributed to their inherent elasticity and viscoelastic property that enables them to control the distribution of stresses over the denture-bearing mucosa and they also provide a cushioning effect to the cyclic forces of mastication.[4] Singh and Gupta[8] done a case study and quoted that fabrication of new dentures and relining of new or old dentures with permanent silicone soft liner is a novel way to minimize the trauma of compromised underlying denture-supporting tissues.

Soft denture liners have several problems associated with their use such as a loss of softness, increasing of hardness, water sorption, colonization of Candida albicans, and adhesion failure between liner and denture base. Therefore, frequent clinical evaluation of soft denture liners is needed. Surface irregularities cause microbial adhesion which is the first step for the colonization and development of an oral infection in denture wearers. Gedik and Ozkan[9] conducted a study to evaluate surface roughness and adherence of C. albicans and found that all tested silicone-based soft lining materials showed adherence of C. albicans after contamination procedures.

To prevent this bacterial colonization and denture plaque, denture cleansing agents were found to be the best solution. Denture plaque control using mechanical and chemical methods is essential for maintenance of good oral hygiene of denture wearers. Mechanical plaque control is not advisable for resilient denture liners because it can damage the resilient lining. Furthermore, ultrasonic treatment is not effective for the removal of denture plaque.[5] This is supported by Hermann et al.[10] who observed in their study that mechanical brushing promoted wear abrasion of soft liners. Only chemical treatment by denture cleansers can be applied to such soft material.

Mollosil™ soft liners showed a highly significant increase in surface hardness than Voco™ soft liners with Periogard™, Secure™, and Polident™ groups at 1st week and 1st month. After 6th month, among both the soft liners tested in the study Voco™ soft liners performed significantly better than Mollosil™ soft liners for all groups. This result is supported by Pahuja et al.[11] in vitro study which revealed that silicone-based soft liners showed better compatibility with cleansing solutions and maintained their resiliency better thereby, proving to be more promising for long-term usage.

CONCLUSION

Within the limitations of this study, it can be concluded that

Chemical denture cleansers can be used daily to cleanse soft denture liners, but in long use, it has an adverse effect on their surface hardness

Voco™ permanent soft liners showed better compatibility with cleansing solutions and Polident™ denture cleanser showed minimal effect on surface hardness of resilient denture liners, proving it to be more promising for long-term usage.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.