Table of Contents    
ORIGINAL ARTICLE
Year : 2018  |  Volume : 9  |  Issue : 2  |  Page : 263-267  

An In vitro evaluation of tensile bond strength of commercially available temporary soft liners to different types of denture base resins


1 Department of Prosthodontics, Malla Reddy Institute of Dental Sciences, Hyderabad, Telangana, India
2 Department of Prosthodontics, Manipal College of Dental Sciences, Manipal, Karnataka, India
3 Department of Dental Materials, Manipal College of Dental Sciences, Manipal, Karnataka, India
4 Department of Conservative Dentistry and Endodontics, Malla Reddy Institute of Dental Sciences, Hyderabad, Telangana, India

Date of Web Publication20-Jun-2018

Correspondence Address:
Meghna Chauhan
Department of Prosthodontics, Malla Reddy Institute of Dental Sciences, Hyderabad, Telangana
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jnsbm.JNSBM_38_18

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   Abstract 

Introduction: Optimal bond between denture base and the soft liner is necessary as adhesion failure is the most common problem encountered clinically. Hence, this study evaluated the tensile bond strength of two commercially available temporary soft liners to different types of denture base resins. Materials and Methods: Prepared wax specimens were heat cured to get 16 samples of each type of denture base resin specimens (DPI, Trevalon, and Trevalon HI). A 3 mm section of acrylic denture base material from the center of each specimen was removed, and the two parts of the fragmented specimens were then repositioned into the aluminum die for the application of temporary soft liners (Group A, Viscogel and Group B, GC tissue conditioner). The specimens were tested for bond strength using universal testing machine and debonded surfaces were observed under a stereomicroscope for analysis of failure. Results: Viscogel showed the highest tensile bond strength with all three different types of denture base resins. Viscogel was significantly superior over GC tissue conditioner. Trevalon exhibited better bond strength results with both Viscogel and GC temporary soft liners as compared to other two types of denture base resins. Conclusion: Viscogel, a temporary soft liner, showed better bond strength compared to GC soft liner in bonding conventional and modified denture base resins.

Keywords: Denture base resins, temporary soft liners, tensile bond strength


How to cite this article:
Chauhan M, Aparna I N, Ginjupalli K, Kumari S, Sandhya P S, Mitra N. An In vitro evaluation of tensile bond strength of commercially available temporary soft liners to different types of denture base resins. J Nat Sc Biol Med 2018;9:263-7

How to cite this URL:
Chauhan M, Aparna I N, Ginjupalli K, Kumari S, Sandhya P S, Mitra N. An In vitro evaluation of tensile bond strength of commercially available temporary soft liners to different types of denture base resins. J Nat Sc Biol Med [serial online] 2018 [cited 2018 Oct 22];9:263-7. Available from: http://www.jnsbm.org/text.asp?2018/9/2/263/234729


   Introduction Top


Transmission of occlusal forces on the denture-bearing area by denture bases leads to considerable strain on the soft tissue. Hence, ill-fitting dentures lead to inflammation of underlying soft tissue. Earlier hard and rigid polymers were used for the management of ill-fitting dentures.[1] In 1961, Chase [2] introduced the use of materials designed to rehabilitate abused denture supporting tissues and to restore them to health. Subsequently, numerous studies have led to introduction and use of different products designed to recondition denture-bearing tissues. Their function is to spread the forces uniformly to underlying soft tissues and also act as “shock absorber” of the strain created during mastication.[3],[4] Conventional heat-cured poly methyl methacrylate (PMMA) resins are source for most of the removable denture bases as they have fracture toughness and rigidity. Modifications in PMMA materials such as high impact strength resins, rapid heat-polymerized materials, and butadiene-styrene rubber were introduced to improve flexural, tensile, and impact strength, reduced water sorption, and for effective and efficient manufacturing.[5],[6]

Optimal bond between denture base and the soft liner is necessary to make the denture to work effectively. Adhesion failure between the soft liner material and denture base is the most common clinical problem. When inserted into the oral cavity, soft denture liners undergo (1) leaching out of plasticizers and other soluble ingredients and (2) absorption of water and saliva. The swelling of the denture base leads to stress between the bonding surfaces, which changes the viscoelastic properties of the resilient liners and makes it brittle and transfers the external loads to the bond area. These problems are further fortified by the deposition of dental plaque, calculus, and bacteria growth. Together these decrease the strength of the denture and predispose them to fractures.[7] Hence, rigorous evaluation of tensile bond strength between soft liners and denture base resins is necessary. This in vitro study hence evaluated and compared the tensile bond strength of two commercial brands of temporary soft liners to different types of denture base resins.


   Materials and Methods Top


The present study was conducted in the department of prosthodontics in 2015. In this in vitro study, 48 test specimens were fabricated to evaluate tensile bond strength of two commercially available and commonly used temporary soft liners and three different types of heat-cured denture base resins. Tensile bond strength was analyzed using a universal testing machine (Computerized Universal Testing Machine, Naugra Export Group of Companies, Ambala, Haryana, India) equipped with a computer control, data acquisition, and data analysis software.

A machine milled aluminum die with a dumbbell-shaped mold (dimensions, 20 mm × 6 mm × 3 mm) was used to prepare test specimens. Modeling wax was poured into the mold, and wax patterns were obtained which were invested separately for conventional compression molding procedure and dewaxed to obtain plaster molds. Heat-cured denture base resin of each brand (DPI, Trevalon, and Trevalon HI) was mixed and packed into the mold space following which curing cycle recommended by the manufacturers for each type of heat-cured denture base resin was adopted. After completion of curing cycle, the cured specimens were retrieved, cleaned, and finished and stored in distilled water at room temperature until the application of temporary soft liners (Group A, Viscogel and Group B, GC tissue conditioner) for the testing of tensile bond strength [Figure 1]a.
Figure 1: (a) Finished and polished denture base resin specimens. (b) Fragmented specimen reassembled in the aluminum mold. (c) Injecting the liner in the 3 mm space created between the fragmented ends of resin specimen. (d) Evaluation of tensile bond strength using Instron machine

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Using fissure burs, 3 mm sections of acrylic denture base material from the center of each specimen were removed to create a uniform space for the application of temporary soft liners [Figure 1]b. Fragmented end of each specimen to be bonded was then abraded with 1000 grit silicon carbide abrasive paper (FlexiCut Abrasive Barabazar, Kolkata, West Bengal, India) for 1 min to standardize the surface roughness. A separator containing mineral and Spearmint oil (Viscogel, Dentsply Detrey GmbH, Konstanz, Germany) was applied on the acrylic denture base surfaces that were not involved in bonding to prevent adhesion of the temporary soft liner. The two parts of the fragmented specimens were then repositioned into the aluminum die, and the application of soft liner was done in the created space [Figure 1]c. After 30 min of setting, specimens were retrieved from the mold and any excess was cut off. Prepared specimens were stored in 200 ml of distilled water at 37°C in a thermostatically controlled water bath for 24 h, to simulate oral conditions. These specimens were then held in the tensile jigs of the universal testing machine, and tensile bond strength was measured at a crosshead speed of 20 mm/min [Figure 1]d. The peak load recorded was taken as the maximum load to induce a failure. The tensile bond strength was calculated as:

F = N/A, where F (MPa) is the ultimate bond strength, N (Newton) is the maximum load to failure, and A (mm 2) is the bonding area.

Stereomicroscope (Motic SMZ-168 Stereo zoom microscope, Ted Pella Inc., CA, California) images at a magnification of ×10 were used to observe the debonded surface for analysis of failure [Figure 2]. The results were tabulated and subjected to statistical analyses using statistical software IBM SPSS Statistics for Windows, Version 20.0., IBM Corp., Armonk, NY, USA, and using post hoc Tukey's honest significant difference and two-way ANOVA tests with significance set at P < 0.05.
Figure 2: Stereomicroscopic picture of bonding interface showing adhesive failure

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   Results Top


The composition of commercially available soft liners and denture base resins used in the study is summarized in [Table 1] and [Table 2], respectively. Viscogel had the highest tensile bond strength with all three different types of denture base resins with a mean of 0.44 ± 0.09 MPa while GC temporary soft liner gave a mean of 0.37 ± 0.11 MPa [Table 3]. The tissue conditioner, Viscogel (Group A), showed bond strength of 0.42 ± 0.03 MPa and GC (Group B) showed a mean of 0.28 ± 0.02 MPa with DPI denture base resin. Viscogel showed highest bond strength of 0.47 ± 0.10 MPa and GC showed 0.41 ± 0.14 MPa when tested with Trevalon. Viscogel group showed a mean of 0.43 ± 0.07 MPa as compared to a mean of 0.42 ± 0.06 MPa with GC when both were compared with Trevalon HI. The difference in tensile bond strength observed among the different tissue conditioners and among the various resins was, however, not statistically significant (P = 0.161) [Table 3]. A statistically significant (P< 0.05) difference in tensile bond strength between the temporary soft liners and denture base resins was observed [Table 4]. Trevalon DPI denture base group was significantly (P > 0.01) superior compared to other groups [Table 5].
Table 1: List of commercially available temporary soft liners used in the study

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Table 2: List of commercially available denture base resins used in the study

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Table 3: Mean and standard deviation of bond strength in Group A and B with three different types of denture base resins

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Table 4: Tensile bond strength of soft liners with denture base resins

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Table 5: Multiple comparisons among the denture base resins

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   Discussion Top


A soft material is placed under the fitting surface of a denture to enable the traumatized soft tissues to recover before recording an impression for a new denture. Patients who cannot tolerate a “hard” denture base are provided with a “permanent” soft cushion on the fitting surface of the dentures base.[8] Soft liners are made from acrylic, silicone, or polyphosphazine fluoroelastomers. Silicone-based liners are frequently used on a long-term basis because they are more resilient and resistant to aged deformation than acrylic-based denture liners. In contrast, acrylic-based soft denture liners exhibit better viscoelastic properties and add marked improvement to masticatory function. Hence, in this study, acrylic-based soft liners and commonly used denture base in clinical practice were used.[9],[10]

Ideally, a 2–3 mm thickness of soft liner is required for adequately cushioning the residual hard acrylic base, as considerable variations are observed when the thickness of soft liners is increased over 3 mm.[11] Hence, in this study, 3 mm thick temporary soft liners were used. Consistent with previous report,[12] we observed significantly (P = 0.011) higher tensile bond strength with all three different types of denture base resins in Group A as compared to Group B.

Bonding compatibility of denture base material with soft liner material is an important factor to be considered in studying failure strength. Plasticized PMMA temporary soft liners and PMMA denture base materials are similar in chemical structure. Application of a bonding agent before the use of a liner is considered unnecessary for acrylic-based liners; therefore, bonding agents were not used, as the denture base materials and temporary soft liners used (PMMA based) in the study were similar in chemical composition.[13] Several studies have attempted to improve bond strength by mechanical roughening using sandblasting or lasers and treatment with denture base monomers.[14],[15],[16] In this study, untreated surfaces of acrylic resin specimens were bonded to temporary soft liners after standardizing the surface roughness by abrading the fragmented end of each specimen with 1000 grit silicon carbide abrasive paper.[14],[15] The impact of roughened surface on adhesiveness is contradictory, with studies reporting both improvement and weakening effect on the bond.[16],[17] Such a variability may potentially be attributed to the difference in the quality of the polymers used in the dental liners and base.

The failure in bond strength invariably increases on water exposure, indicating that the materials became more brittle and probably less viscoelastic. In the current study, all the bonded specimens were hence stored in distilled water at 37°C for 24 h as described in the ISO specification 11405:2003 for short-term water storage. The tensile strength of lining material increased significantly with increasing rate of deformation to a limit of 40 mm/min, beyond which the strength of the bond decreased. In the current study, the testing was done at a crosshead speed of 20 mm/min as reported in the literature.[11],[18],[19] Comparative evaluation of the denture bases revealed that Trevalon denture base resin showed highest bond strength values as compared to other two types of resins, with both Viscogel and GC groups, indicating a significant difference in tensile bond strength between the different types of resins. Such differences in the performance of dental materials are previously reported for PMMA versus Urethan Dimethcrelate (UDMS) specimens, GC Reline Ultrasoft versus GC Reline,[20] and Super-Soft liners versus Luci-Sof liner.[21]

We observed that Viscogel–Trevalon (0.47 MPa) combination had highest 24-h bond strength whereas GC–DPI (0.28 MPa) had lowest 24-h bond strength values [Graph 1]. Since the recommended adequate adhesive value for soft lining materials is 4.5 kg/cm 2 or 0.44 MPa,[16],[22] only Viscogel–Trevalon combination was found to be acceptable for clinical use. In all other temporary soft liner-resin group specimens, the adhesion was found to be lower than the recommended value.



The bond failures were classified as adhesive, cohesive, and mixed types. Adhesive failures occur when tensile strength of the soft liner is greater than its bond strength to PMMA. Cohesive failures occur when tensile strength of the soft liner is less than bond strength to PMMA. Mixed type of failures occur when the bond strength to PMMA is nearly equal to the tensile strength of the liner.[17] These failures are characterized by the interrelationships between the properties, chemical characteristics, and compatibility of the liners and denture base materials.[11],[18] In the visual inspection, the debonded surface was examined to confirm the mode of failure in this study using a stereomicroscope. The nature of bond failure could be categorized as cohesive, adhesive, or mixed.[19] Stereomicroscopic analysis confirmed adhesive failures which were seen in all the groups of specimens, which clearly indicates that the cohesive or tensile strength of Viscogel and GC temporary soft liners is greater than their bond strength to different types of resins used in the study.


   Conclusion Top


Viscogel temporary soft liner showed better bond strength as compared to GC temporary soft liner with conventional as well as modified denture base resins. Trevalon exhibited better bond strength results with both Viscogel and GC temporary soft liners as compared to other two types (conventional and high impact strength) of denture base resins. This study emphasizes the need to consider the compatibility between temporary soft liner and denture base resin for optimum results.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
   References Top

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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