|Year : 2022 | Volume
| Issue : 1 | Page : 57-63
Effect of surface treatments on the tensile bond strength of heat cure silicone soft liner to an acrylic resin denture base in a simulated oral environment - A comparative SEM study
Nithisha Brahmandabheri, Chalapathi Rao Duggineni, Ravi Kumar Chitturi, Harilal Guguloth, Mahinder Dubasi
Department of Prosthodontics, Mamata Dental College, Giriprasad Nagar, Khammam, Telangana, India
|Date of Submission||21-Jan-2021|
|Date of Acceptance||26-Mar-2021|
|Date of Web Publication||23-May-2022|
Dr. Nithisha Brahmandabheri
Plot No 15, 16. SY 185, Varaalaksmi Habitats, Pragati Nagar, Near Shubhankari Subrahmanyeshwara Swamy Temple, Hyderabad, Telangana State - 5000990
Source of Support: None, Conflict of Interest: None
Aim: To assess and compare the effect of surface treatments in a simulated oral environment through thermocycling on tensile bond strength of silicone soft liner with acrylic resin base.
Settings and Design: In Vitro Comparative and SEM analytical study.
Materials and Methodology: 120 samples of heat cured acrylic resin (DPI) and silicone soft liner (Molloplast B) with desired dimensions were fabricated using customized brass flask. 120 samples were divided into 1 control group (untreated) and 3 test groups (treated) consisting of 30 samples each. 3 test groups were divided based on surface treatments of resin samples to adhere to silicone soft liner to them via acid etching (36% phosphoric acid), air abrasion (50 μm Al2O3) and laser treatment (Er-YAG). All the 4 groups (1 control and 3 test groups) were subdivided into two subgroups based on their subjection to thermocycling i.e., 15 samples of each group were not subjected to thermocycling (before) and remaining 15 samples of each group were subjected to thermocycling (after).
Statistical Analysis Used: All the samples were evaluated for tensile bond strength using universal testing machine. Obtained values were statistically analyzed with student paired T test, one way ANOVA and post HOC Tukey B test to compare, identify significant differences and also to arrive at order of groups. The effects of surface treatments and silicone liner on the surface of denture base resin were examined with scanning electron microscopy.
Results: The tensile bond strength was significantly different between control and test groups and also among test groups before and after thermocycling (P < 0.05). The specimens of acid group had higher bond strength values followed by laser treated and abraded groups. Thermocycling had decreased bond strength values. SEM observations also revealed that, surface treatments modified the surface of the denture base resin with variability.
Conclusions: Surface treatments increased the bond strength values while thermocycling had decreased them. Among all the groups tested, acid etching group of samples exhibited higher bond strength values of all test groups before and after thermocycling followed by laser, abrasion and control groups. Bond strength values of all test groups before and after thermocycling were still higher than 0.45 MPa, which is clinically acceptable bond strength level.
Keywords: Heat cure acrylic denture base resin (DPI), silicone soft liner (Molloplast B), surface treatments, tensile bond strength, thermocycling
|How to cite this article:|
Brahmandabheri N, Duggineni CR, Chitturi RK, Guguloth H, Dubasi M. Effect of surface treatments on the tensile bond strength of heat cure silicone soft liner to an acrylic resin denture base in a simulated oral environment - A comparative SEM study. J NTR Univ Health Sci 2022;11:57-63
|How to cite this URL:|
Brahmandabheri N, Duggineni CR, Chitturi RK, Guguloth H, Dubasi M. Effect of surface treatments on the tensile bond strength of heat cure silicone soft liner to an acrylic resin denture base in a simulated oral environment - A comparative SEM study. J NTR Univ Health Sci [serial online] 2022 [cited 2022 Nov 30];11:57-63. Available from: https://www.jdrntruhs.org/text.asp?2022/11/1/57/345809
| Introduction|| |
Liners have been widely used in the clinical management of prosthodontic patients predominantly to form all or part of the intaglio surface of a denture and help to condition traumatized tissues providing interim or permanent cushion like effect.,,,, These liners may be classified as short term or long term, room temperature or heat temperature vulcanized and also as acrylic and silicone liners based on their composition.,
Silicone liners are superior to acrylic resin liners in terms of resiliency and maintaining cushioning effect for prolonged periods as they are devoid of plasticizer. However, they lack adhesion and resulting in bond failures which is their major drawback.,,, To improve adhesion and resultant bond strength of silicone soft liners, various researchers had suggested/employed air particle abrasion, acid treatment and laser treatment of denture base preceding placement of a liner by providing an irregular surface and mechanical retention.,,,
Oral environmental variables being, flexural cyclic loading and thermal stresses enhance the degradation and curtail the clinical life of relined denture bases.,,, This study was conducted to evaluate and compare the influence of surface treatments of denture base in a simulated oral environment on bond strength of silicone soft liner and also to visualize, categorize and correlate through thermocycling.
| Materials and Methodology|| |
240/120 blocks/samples of heat cured PMMA (control & test groups) were fabricated using customized brass flask with 5 mould spaces (63 mm × 10 mm × 10 mm) separated by 3 mm wide brass strap for uniform thickness of silicone soft liner, thus producing 10 acrylic blocks of dimensions 30 mm × 10 mm × 10 mm. Hence, 2 acrylic blocks along with attached soft liner forming one sample.
120 samples were divided into 1 control group (untreated) and 3 test groups consisting 30 samples each. 3 test groups (30 × 3 = 90) were divided based on surface treatments of resin samples to enhance adhesion/bonding of silicone soft liner to them via acid etching, air abrasion and laser treatment.
Acrylic blocks/specimens (180/90) of test groups were subjected to surface treatments and as follows; acid etching (test group 1) - applied 36% phosphoric acid (Detray conditioner) for 30 seconds, washed for 30 seconds and dried for 20 seconds, air particle abrasion - (test group 2) done for 10 seconds with 50 μm aluminium oxide particles at pressure of 0.2 MPa by working tip at distance of 10 mm in a sand blasting unit and cleaned in ultrasound cleaner for 4 minutes to remove traces and laser treatment (test group 3) – with Er-YAG Laser of wavelength 2940 mm, pulse frequency 10 Hz, pulse energy 300 MJ for very short duration by holding laser tip at distance of 10 mm to surface of the specimens and irrigated for 20 seconds.,
Samples of untreated (control) and treated (test groups) were reassembled in the flask to add and bond silicone soft liner of 3 mm thickness between them, closed and cured.
All the groups were again subdivided into two subgroups based on their subjection to thermocycling, i.e., 15 samples of each group were not subjected to thermocycling (subgroup A) and remaining 15 samples of each group were subjected to thermocycling (subgroup B).
All the cured samples were stored in de-ionized water at room temperature and 15 samples of control and test groups were subjected to 3000 thermal cycles between water baths of 5 0C and 55 0C with a dwell time of 1 minute in each bath.
All the samples were evaluated for tensile bond strength using universal testing machine. All these samples were gold sputtered and visually inspected using scanning electron microscope (Hitachi S-3700N)) under magnification ×2000 at 15 KV to evaluate and correlate with obtained bond strength values and also to categorize failure mode (adhesive, cohesive& mixed) [Figure 1],[Figure 2],[Figure 3].
|Figure 2: Images of de-bonded samples of all groups before thermocycling|
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Obtained bond strength values were averaged; student paired T test was used to compare the bond strength values of all the groups with and without thermocycling. Further, one way ANOVA was used to analyze the effects of surface treatments and find out differences between the groups. Post Hoc Tukey B test was then used to determine the order of groups. The statistical analyses were performed with SPSS software (version 23). The ethical committee is obtained and it has been approval on 23.12.2015.
| Results|| |
Obtained bond strength values of controll and test groups before (witthout) and after thermocycling were averaged and depicted in [Table 1]. Student paired T test was used to compare (intra group) the differences between the subgroupsb (before and after thermocycling) of control and test test groups. Though variations exist with all the groups before and after thermocycling but found to be significant only with laser treated samples (P < 0.05) [Table 2].
One way ANOVA F test was done to compare the effect of surface treatments along with thermocycling on bond strength values of control and test groups found to be significant (P < 0.05) but did not specify the order of groups based on the results [Table 3].
Post Hoc Tukey B test determined the order of groups as per obtained bond strength values are presented in [Table 4].
Overall observations revealed that surface pretreatments enhanced/increased adhesion/bond strength of silicone soft liner when compared to untreated samples while thermocycling had decreased them.
SEM (scanning electronic microscope) observations of untreated (control), acid, abrasion and laser treated samples in terms of failure mode and percentage of distribution of failure are presented in [Table 5].
|Table 5: Failure Mode Distribution of Control and Test Groups (Sem Observations)|
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| Discussion|| |
A prosperous relining depends on the bond strength between the liner and resin base. The most common reason for failure of a soft lined denture is failure/lack of adhesion between the liner and denture base in particular to the use of silicone soft liners and their service., Bond failures between the liner and denture base evoke a micro-leakage leading to local unhygienic conditions and often causing functional failures are dependent majorly upon chemical composition of materials involved and thermal stresses in the oral environment.,,
Distinct chemical nature of silicone soft liners and resin denture bases is more vulnerable to bond failures, unlike acrylic liners.,
To overcome the incompatibility between the silicone liner and acrylic resin, Craig and Gibbons initially advocated and suggested roughening of resin base surfaces, thus increasing the surface area to be bonded.
Various investigators had employed/suggested acid treatment (36% phosphoric acid), air particle abrasion (50 μm Al2O3) and laser treatment (Er-YAG) of resin surfaces prior to placing a silicone liner individually and also in combination, thereby increasing the bond strength between them.,,
Factors that may influence the bond between resilient liners (silicone) and denture bases consist of ageing in water, thermal stresses, flexural loading, primer application, thickness of liner and nature of resin base materials.,,,
The present investigation evaluated and compared the effect of surface treatments in a simulated oral environment (thermocycling) on the bond strength of silicone soft liner to acrylic resin base. Heat cured acrylic resin (DPI) and silicone soft liner (Molloplast B) materials were chosen as they are superior to auto polymerized forms in terms of properties and their service.
Acrylic resin sample fabrication, surface treatments, bonding of silicone soft liners and subjection of samples to thermal cycles were performed in a standardized in vitro protocol.,
All the samples were evaluated for tensile bond strength for determining the bonding mechanism and failure. Bates and Smith and Kawano F reported tensile test as the reliable one to determine the bond strength of liners as it provides information on the strength of the bond in comparison to the strength of the material.
In the present study, thermocycling (3000 cycles) was conducted similar to the study of Pinto et al., by alternatively immersing the samples in cold and hot water baths for 1 minute each. 1000 cycles in this type of test perhaps the reasonable appraisal of the maximum temperature changes expected during a year, therefore 3000 cycles would relate to roughly 3 years of use.
Clinical studies pointed out that the lining layer must be of adequate thickness (2 to 3 mm), hence 3 mm thickness of liner is employed in the present study.,
Mean bond strength values obtained with respect to all the groups when not subjected to thermocycling were as follows, acid group (1.12 MPa), laser group (1.01 MPa), abrasion group (0.92 MPa) and control group (0.85 MPa).
Though surface treatments enhanced adhesion, varied bond strength values obtained with them could be attributed to size and uniformity of irregularities created by them, thus increasing the surface area to bond for flow of material into them.
Higher bond strength values obtained with acid treatment were in line with the conclusions of the study conducted by Gundodgdu et al., as well as laser enhancing adhesion as per Akin et al. and lower bond strength values obtained with air abrasion were similar to the observations of Jacobson et al. and Amin et al.
Obtained bond strength values of control, acid etching, air abrasion and laser treatment groups after thermocycling were 0.80, 0.97, 0.85 and 0.82 MPa respectively and it was revealed that it had weakened the bond between liner and denture.
The decrease in the bond strength values of all the groups after thermocycling could be attributed to water uptake at the interface between liner and denture base affecting the dimensional stability of the material.
However, bond strength values of control, acid treatment, abrasion group with and without thermocycling were found to be statistically insignificant, except with the laser group. This could be due to the acrylic resin and the soft lining material type, sample shape, type of surface treatment, number of cycles, thermocycling temperature, and type of test and speed of force application.
Craig and Gibbons and Kawano F et al. claimed that an adequate bond strength value for a resilient liner is 0.44 MPa. In the present study, though, bond strength values for all the groups decreased after thermocycling, but they were still higher than 0.44 MPa, which is clinically acceptable bond strength level.
Visual observation of SEM images of acid group, abrasion group and laser group samples revealed that they produced irregularities of varying nature when compared to control group. It was also found that irregularities were more with the acid group followed by laser and abrasion respectively in enhancing the bond strength of resilient liner with denture base resin.
The mode of failure was more of adhesive in nature with respect to control, abrasion and laser groups except the acid group wherein it was of cohesive in nature suggesting bond strength was enhanced greatly by acid treatment when not subjected to thermocycling [Figure 1],[Figure 2],[Figure 3].
Images of de-bonded specimens of all groups after thermocycling also revealed that it had affected the bond strength by interfering the resin-liner interface. Mode of failure observed with all groups was more of adhesive in nature after thermocycling [Figure 1],[Figure 2],[Figure 3].
In clinical instances requiring the use of silicone soft liners, acid etching is to be performed on a routine basis prior to application irrespective of polymerization method.
Limitations of the study were
- Only one denture base and one silicone soft liner is been tested.
- Limited surface treatments were done among the surface treatments, for sandblasting only 50 microns particles were used for abrasion, only one concentration of acid was been used that is 36% phosphoric acid, for laser different pulse durations, different power and energies and combination of surface treatments were not employed.
- The dimension of the bonding site tested in this study was short, in comparison to entire intaglio surface of the complete denture, which is generally greater. Therefore, further investigations are required to evaluate bond strength under more closely simulated clinical conditions.
- In the oral cavity, denture base is exposed to forces of varying magnitudes acting in different directions. The same situation could not be simulated in this in-vitro study.
- In-spite of limitations mentioned earlier, if the protocol of this study is followed; surface modifications will certainly improve the bond between silicone liners and denture bases in clinical studies. SEM images confirmed the same.
- Based on the present study results, further in vivo investigations with randomized controlled trials can be conducted to compare the efficacy of surface treatments on the bond strength of resilient liners for clinical use.
| Conclusions|| |
Within the limitations of this study, the following conclusions were drawn:
- Significant increase in the bond strength values was observed with surface treated specimens when compared to untreated specimens (P < 0.05).
- Thermocycling had weakened the bond strength of untreated and treated samples.
- Among the surface treatments, acid etching found to be superior followed by laser and abrasion.
- SEM observations also confirmed the effect of surface treatments and thermocycling increasing and/or decreasing the bond between silicone soft liner and acrylic denture base.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]