INTRODUCTION

Increased esthetic expectations in dentistry have led to the need for removable partial dentures that expose little or none of the metal supporting structures or retentive clasps. For this reason, non-metal clasp dentures using thermoplastic resins have become alternative treatment options1,2).

Various types of thermoplastic resin for non metal clasp denture have been introduced.

In the 1950s, the use of nylon polymer as a denture base material has been described in the literatures. Nylon is a generic name for certain types of thermoplastic polymer belonging to a class of polyamide3-5). These polyamide resins have advantages of higher elasticity and higher molding precision than conventional heat polymerizing resins6,7). Since they have inherent flexibility, these materials prevent prosthesis fractures and facilitate denture retention by utilizing the undercuts of abutment teeth in the denture base design without metal clasps6).

However, some disadvantages reported in the early form of polyamide resins included high water sorption, discoloration and difficulties in reline and repair1-3).

Existing removable partial denture often requires denture base relines to improve their fitness to the supporting tissue because of gradual changes in edentulous ridges8-10). Successful denture relining depends on the bonding strength between reline resin and denture base resin. Therefore, adequate bonding strength to reline resin is very important factor in the selection of denture base resin11). To compensate the relining problem with polyamide resin, new non-metal

denture base materials are still being developed. Acrytone (High-Dental-Japan, Osaka, Japan) is a

newly introduced thermoplastic acrylic resin in order to maintain advantages, and overcome the shortcomings of conventional heat polymerized acrylic resins and existing thermoplastic resins. According to the manufacturer, it is available for non-metal clasp dentures due to the elastic characteristics, and relining is possible because it is composed of polymethyl methacrylate (PMMA).

The physical characteristics such as flexural strength and modulus of elasticity of Acrytone were reported12), but the studies on the bond strength of reline resins to Acrytone are insufficient.

Even though the bonding strength between reline resins and Acrytone has never been clearly investigated, some practitioners have already begun using them based on their preference and clinical experiences.

The purpose of this in vitro study was to investigate the bond strength of two relining resins bonded to Acrytone, and the results were compared with those of a heat-polymerized acrylic resin and a thermoplastic polyamide resin. In addition, the nature of the fracture surfaces was evaluated. The hypotheses of this study were that the bond strength of reline resins to Acrytone would be different with those of a heat-polymerized acrylic resin and a thermoplastic polyamide resin, and that the type of reline resin may affect the bond strength.

MATERIALS AND METHODS

Three denture base resins and two chairside hard reline resins were selected for the study (Table 1). The denture base resins were a conventional heat-polymerized acrylic resin (Paladent 20; PAL20), a thermoplastic

Evaluation of adhesion of reline resins to the thermoplastic denture base resin for non-metal clasp dentureJi Hye KIM1, Han Cheol CHOE2 and Mee Kyoung SON1

1 Department of Prosthodontics, School of Dentistry, Chosun University, 375 SeoSuk Dong, Dong-gu, GwangJu 501-759, Republic of Korea2 Department of Dental Materials, School of Dentistry, Chosun University, 375 SeoSuk Dong, Dong-gu, GwangJu 501-759, Republic of KoreaCorresponding author, Mee-Kyoung SON; E-mail: son0513@chosun.ac.kr

This study aimed to evaluate the tensile and transverse bond strength of chairside reline resins (Tokuyama Rebase II, Mild Rebaron LC) to a thermoplastic acrylic resin (Acrytone) used for non metal clasp denture. The results were compared with those of a conventional heat polymerized acrylic resin (Paladent 20) and a thermoplastic polyamide resin (Biotone). The failure sites were examined by scanning electron microscopy to evaluate the mode of failure. As results, the bond strength of reline resins to a thermoplastic acrylic resin was similar to the value of a conventional heat polymerized acrylic resin. However, thermoplastic polyamide resin showed the lowest value. The results of this study indicated that a thermoplastic acrylic resin for non metal clasps denture allows chairside reline and repair. It was also found that the light-polymerized reline resin had better bond strength than the autopolymerizing reline resin in relining for a conventional heat polymerized acrylic resin and a thermoplastic acrylic resin.

Keywords: Non metal clasp denture, Thermoplastic acrylic resin, Chairside relining, Bond strength

Color figures can be viewed in the online issue, which is avail-able at J-STAGE.Received Apr 23, 2013: Accepted Oct 17, 2013doi:10.4012/dmj.2013-121 JOI JST.JSTAGE/dmj/2013-121

Dental Materials Journal 2014; 33(1): 32–38

Table 1 Materials used for the study

Brand names(Code)

Main ComponentsManufacturer Processing Method

Powder Liquid Primer

Denture base resin

Paladent 20(PAL20)

PMMA MMA —Heraeus Kulzer, Hanau, Germany

Compression mold technique; heat processed at 80ºC/15 min,

boiling water /20 min

Acrytone(ACT)

PMMA MMA —High-Dental-Japan,

Osaka, Japan

Injection mold technique; 260ºC melting/25 minInjection at 0.7 MPa

Cooling under pressure

Biotone(BT)

Polyamide — —High-Dental-Japan,

Osaka, Japan

Injection mold technique; 300ºC melting

Injection at 0.7 MPaCooling under pressure

Reline resin

Tokuyama Rebase II(TR II)

PEMA AAEM Ethyl acetateTokuyama Dental Corp,

Tokyo, Japan

Tokuyama rebase II primer for 30 s; pour-mixed reline polymer,

Autopolymerizing

Mild Rebaron LC(MRL)

PEMA EMA DichloromethaneGC,

Tokyo, Japan

Mild Rebaron LC primer for 30 s; pour-mixed reline polymer,

10 min light curing

PMMA: polymethyl methacrylate; MMA: methyl methacrylate; PEMA: polyethyl methacrylate AAEM: acetoacetoxyethyl methacrylate; EMA: ethylmethacrylate

Table 2 Test groups

Group 1 Group 2 Group 3 Group 4 Group 5 Group 6

Denture base resin PAL 20 ACT BT PAL 20 ACT BT

Reline resin TR II TR II TR II MBL MBL MRL

acrylic resin (Acrytone; ACT) and a thermoplastic polyamide resin (Biotone; BT). The reline resins were an autopolymerizing type (Tokuyama Rebase II; TR) and a light-activated type (Mild Rebaron LC; MRL).

Six test groups were set up for the tests of tensile and transverse bond strength (Table 2).

Tensile bond strength testNine specimens for each test group (total 54 specimens, n=9) were processed according to manufacturer’s instructions. A brass die, 10 mm in diameter and 43 mm in length, were used. The dies were invested in silicone rubber. The obtained mold was used to prepare the wax blocks, which were used to produce the denture base resin blocks. Denture base resins were polymerized in the molds according to manufacturers’ recommendations. The specimens were then removed from the molds, and 3 mm of the material was cut off from the midsection using a water-cooled diamond

disc (Komet, Gebr Brasseler GmbH & Co. Kg, Lemgo, Germany). The sectioned surfaces of denture base resin were ground with 1,000 grit SiC abrasive paper, and were treated with primer provided by the manufacturer before applying the reline resin. Tokuyama Rebase II was used with an ethyl acetate-based primer and Mild Rebaron LC was used with a dichloromethane-based primer. The primers were applied with a brush and left to dry for 30 s. The specimens were replaced in the molds, and the missing 3 mm sections were repacked with reline resins13). The specimens were polymerized according to manufactures’ recommendations, and trimmed. All specimens were stored in distilled water at 37ºC for 50 h (Fig. 1a).

Tensile bond strength test was performed on each specimen until failure. A universal testing machine (AGS-1000D series, Shimadzu, Tokyo, Japan) at a crosshead speed of 5 mm/min was used for this test (Fig. 1b). The maximum tensile load before failure was

33Dent Mater J 2014; 33(1): 32–38

Fig. 1 Specimen preparation (a) and testing apparatus for tensile bond strength test (b).

Fig. 2 Specimen preparation (a) and testing apparatus for the 3 point bending test (b).

recorded for each specimen. The tensile bond strength was calculated as the load at failure divided by the cross-sectional area of the specimen.

Transverse bond strength test Nine specimens for each test group, total 54 specimens (64 mm×14 mm×2.8 mm in size), were prepared for the transverse bond strength test. The specimens were prepared in the same manner as previously described in the tensile bond strength test. A 10 mm section was removed from the center of each specimen. The primers were applied to the sectioned denture base surfaces with a brush and left to dry for 30 s. The samples were replaced in the molds, and the missing 10 mm sections were repacked with reline resins. The specimens were polymerized, removed from the molds, and trimmed. All specimens were kept in distilled water at 37ºC for 50 h (Fig. 2a).

The transverse bond strength of the specimens was measured using a 3 point transverse flexural test in a universal testing machine at a crosshead speed of 5 mm/min (Fig. 2b). The transverse bond strength of

each specimen unit was determined using the formula: S=3WL/2bd2, where W is the flexural load, L is the distance between supports (50.0 mm), b is the specimen width, and d is the specimen thickness.

Statistical analysisThe results of the tensile and transverse bond strength (MPa) were analyzed with 2-way analysis of variance (ANOVA) using statistical software (SPSS version 17.0; SPSS Inc, Chicago, USA). The variables were denture base resin and reline resin. One-way Anova and the Tukey HSD post hoc comparison were applied when appropriate (α=0.05).

Failure analysisThe failure site of all specimens after tensile test were gold sputtered and examined by means of scanning electron microscopy (SEM, S-4800, HITACHI) at 15.0 kV to analyze the mode of failure. The SEM images were developed with ×800 magnification for visual inspection. The failure mode was recorded as adhesive, cohesive or mixed.

34 Dent Mater J 2014; 33(1): 32–38

Table 4 Effect of the denture base polymer type and reline resin type on the tensile bond strength compared by 2-way ANOVA

source Type III Sum of square df Mean square F Sig.

Denture base 704.257 2 352.129 51.418 0.000

Reliner 188.701 1 188.701 27.554 0.000

Denture base × Reliner 16.717 2 8.358 1.221 0.305

Error 301.327 44 6.848 — —

Total 5194.541 50 — — —

a. R square=0.753 (Corrected R square=0.725)

Table 5 Mean transverse bond strength (MPa) between the denture base resin and reline resin with 1-way ANOVA using a Tukey HSD

PAL20 ACT BT

Mean SD Mean SD Mean SD

TR II 15.08a 2.21 17.68a 1.89 5.03b 0.00

MRL 24.93c 2.82 27.54c 6.34 10.42d 1.40

Group with the same superscripts were not significantly different (p>0.05)

Table 3 Mean tensile bond strength (MPa) between denture base resin and reline resin with 1-way ANOVA using a Tukey HSD

PAL20 ACT BT

Mean SD Mean SD Mean SD

TR II 9.30a 2.53 8.39a 1.84 1.42b 0.33

MBL 13.48c 3.92 13.76c 3.49 3.81b 0.52

Group with the same superscripts not significantly different (p>0.05)

RESULTS

Tensile bond strengthThe results of tensile bond strength with multiple pairwise comparisons using the Tukey HSD are shown in Table 3. Two-way ANOVA results revealed that significant differences existed as functions of the denture base resin type (p<0.001) and the relining resin (p<0.001), whereas the interaction term (Denture base×Reliner) was not significant (p=0.305) (Table 4). For this reason, One-way ANOVA and the Tukey HSD post hoc comparisons were applied to the denture base resin-reline resin combination. The tensile bond strength of groups 1 (9.30 MPa) and 2 (8.39 MPa) was significantly higher than that of group 3 (1.42 MPa) (p<0.05). The values for groups 4 (13.48 MPa) and 5 (13.76 MPa) were significantly higher than those of group 6 (3.81 MPa) (p<0.05). There was no significant difference between groups 1 and 2 (p>0.05), and between

groups 4 and 5 (p>0.05).

Transverse bond strengthThe results of transverse bond strength with multiple pairwise comparisons using the Tukey HSD are depicted in Table 5. Two-way ANOVA indicated significant differences according to the denture base polymer type (p<0.001) and relining resin (p<0.001), whereas the interaction term was not significant (p=0.056) (Table 6). The Tukey HSD post hoc comparison was applied to the denture base resin-reline resin combinations (α=0.05). The results of transverse bond strength were similar to those of tensile bond strength. The bond strength of groups 1 and 2 were significantly higher than those of group 3, and the values of groups 4 and 5 was significantly higher than those of group 6 (p<0.05). There was no significant differences between groups 1 and 2, and between groups 4 and 5 (p>0.05) (Table 5).

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Table 7 Failure mode analysis in the tensile bond strength test

Relining material Denture base Pure adhesive Mixed adhesive Cohesive

TR II

PAL 20 1 7 1

ACT 1 8 —

BT 9 — —

MBL

PAL 20 — 9 —

ACT 1 8 —

BT 7 — —

Table 6 Effect of the denture base polymer type and reline resin type on the transverse bond strength compared using 2-way ANOVA

source Type III Sum of square df Mean square F Sig.

Denture base 2274.459 2 1137.230 116.481 0.000

Reliner 945.015 1 945.015 96.794 0.000

Denture base × Reliner 59.675 2 29.837 3.056 0.056

Error 468.633 48 9.763 — —

Total 3747.782 53 — — —

a. R square=0.875 (Corrected R square=0.862)

Fig. 3 SEM images of the fracture sites of denture base after the tensile bond strength test. The arrows indicate the border between denture base resin (D) and reline resin (R).

(a); group 1, (b); group 2, (c); group 3, (d); group 4, (e); group 5, (f); group 6.

Fracture analysisThe mode of failure results are shown in Table 7. Different failure types were observed among the groups. Most of the failures in groups 1, 2, 4 and 5 were

mixed failure, whereas groups 3 and 6 presented pure adhesive failures. The SEM images of fracture surfaces are shown in Fig. 3. The remnants of relining resins adhered to the denture base resin are presented in

36 Dent Mater J 2014; 33(1): 32–38

groups 1, 2, 4 and 5. Pure adhesive failure modes are presented in groups 3 and 6.

DISCUSSION

The bond properties of acrylic resin combinations have been the subject of many investigations. This property has been examined in several studies using tensile9,14), shear15,16), and transverse tests8,10). In the present study, the bond strength between reline resins and denture base resins were measured by using tensile test and 3 point bending test. The tensile strength test method was preferred because it applies a simple tensile load to the joint, which allows for comparison among different materials15). And the provided fracture surface can offer information about the structure of the boundary layers and the site of failure14). However, the tensile test requires a precise loading direction to the specimen in order to get correct results, and this factor considerably affects the values, and it is very difficult to control17). The 3 point transverse flexural test has been widely used to evaluate the bond strength between different materials as well as the transverse strength of the acrylic resins itself. The validity of the 3 point flexural test is closely correlated with the failure mode of the specimens18,19). Therefore, in this study, two experimental methods were used to increase the reliability of the bond strength measurement.

The hypothesis that the bond strength of reline resins to Acrytone would be different with those of a heat-polymerized acrylic resin and a thermoplastic polyamide resin was partially accepted. The bond strength of Acrytone did not show a significant different with that of a heat-polymerized acrylic resin. However, thermoplastic polyamide resin showed significant different.

The bond strength between the existing denture base resin and added reline resin is affected by the chemical composition of these two resins20). Paladent 20 is a heat-polymerized acrylic resin with excellent physical or clinical properties. Another favorable property of acrylic resin is its bonding ability with new resin. The autopolymerizing and light-polymerized chairside reline resins are based on either PMMA or its copolymer polyethyl methacrylate (PEMA). Because of the almost identical chemical composition, the autopolymerizing and light-polymerized reline resins are considered to actively bond to the acrylic denture base resins. In addition, the bonding of reline resins to denture base resin may be achieved by penetration and diffusion of monomer into denture base resin. Thus, the difference of molecular weight of monomer in reline resin could affect bonding ability21).

Biotone is a thermoplastic resins belonging to the class known as polyamides. These polyamides are produced by condensation reactions between the diamine and dibasic acid3). Since thermoplastic resins for nonmetal clasp dentures have a low modulus of elasticity and are easily manipulated, these materials make it possible for larger undercuts to be used for

retention compared to acrylic resin1). However, polyamide polymers are highly chemical-resistant materials owing to their high degree of crystallinity. Therefore, it is difficult to react with the monomers and resin primers of reline resins6). In the present study, the bond strength of reline resins to Biotone showed the lowest value.

Acrytone is a thermoplastic acrylic resin produced by an injection molding technique, rather than by polymerization of the polymer and monomer. According to the manufacturer, Acrytone has 82 MPa of flexural strength and 2,500 MPa of elastic modulus. Other test report12) also showed average 89 MPa of flexural strength and 2,072 MPa of elastic modulus. The flexural strength is similar to that of heat-polymerized acrylic resin, and the elastic modulus is intermediate between polyamide resin and heat-polymerized acrylic resin. The flexibility of Acrytone is superior to the heat-polymerized acrylic resin, but is approximately half of polyamide resin. The impact strength of Acrytone is double than that of conventional heat-polymerized acrylic resin12). Thermoplastic polyamide resin is sensitive to temperature, and curves gently in hot water due to loosening of the molecular structure, while Acrytone maintains the existing rigidity.

In this study, the bond strengths of reline resins to Acrytone were similar to Paladent 20. It is because that Acrytone has the same chemical composition (PMMA) with conventional heat polymerized acrylic resin.

The second hypothesis that the type of reline resin may affect the bond strength to denture base resin was accepted.

It has been reported that the type of relining material may affect the bonding properties of different hard reline resins14,15). The mechanisms for adhesion of reline resins to acrylic denture base resins are dependent on swelling of the PMMA surface by the monomer or solvent, diffusion of monomers in the swollen denture base material, polymerization, and the formation of an interpenetrating polymer network (IPN)14,21). Therefore, adhesion between the denture base and reline resins can be improved by wetting the surfaces with methyl methacrylate (MMA) based monomers in reline resins and applying appropriate solvents to the acrylic resin surfaces. Some studies reported that these organic solvents, such as chloroform, acetone, and dichloromethane increase the bond strength of reline resins to the denture base resin16,22).

Surface preparation with dichloromethane can cause the surface to swell, allowing diffusion of the polymerizable material. Such preparation can create surface pores, approximately 1 µm, in the acrylic denture base resin19,23,24). Ethyl acetate also has the ability to swell the surface and allow the diffusion of the denture base resin. Shimizu et al. reported that a 120 s application of ethyl acetate was effective as a 5 s application of dichloromethane at preparing the surfaces of denture base resin19). This means that, in the same application time of 30 s, dichloromethane is more effective than ethyl acetate at preparing the surfaces of

37Dent Mater J 2014; 33(1): 32–38

denture base resin.The present study also showed that Mild Rebaron

LC with dichloromethane-based primer groups presented higher bond strength than Tokuyama Rebase II with ethyl acetate-based primer groups in all three denture base resins.

Another possible reason for this result includes that autopolymerizing reline resin have higher residual monomer after polymerization and can allow the inflow of air bubbles during relining, and then causes weak bond strength25,26).

The limitations of this in vitro study include that the bond strength were measured only after 50 h storage of specimens. Denture base and reline resins have different solubility and water sorption, therefore, the changes in bond strength might be occurred by storage time. As further studies, the long-term storage or thermal cycling experiments to simulate intraoral conditions will be needed.

CONCLUSION

Within the limitations of present study, the following conclusions were drawn:

1. The bond strength of reline resins to a thermoplastic denture base acrylic resin used in non metal clasp denture base was similar to that of a conventional heat-polymerized acrylic resin.

2. Among three denture base resins, a thermoplastic polyamide resin showed the lowest value in the bond strength.

3. For a conventional heat-polymerized acrylic resin and a thermoplastic acrylic resin, the light-polymerized reline resin had better bond strength than the autopolymerizing reline resin. However, there was no difference for a thermoplastic polyamide resin.

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