Recently, removable partial dentures fabricated without metal clasps using thermoplastic denture base resins (TDBRs) have become an alternative treatment method due to their superior esthetics.12

As early as 1950s in the United States, polyamide was suggested as the material of choice for denture bases.3 Since then, many types of TDBRs have been approved for use as denture bases in Japan, such as polyesters and acrylic resins. All of these TDBRs exhibit more favorable physical properties such as molding precision and elasticity compared to conventional heat polymerizing resins like polymethyl methacrylate (PMMA).1456 The main advantage of these materials is their flexibility, which inhibits denture fracture, and their ability to engage tooth undercuts to provide removable partial denture (RPD) retention.4 However, previous studies have reported some limitations of the clinical application of TDBRs because some of these TDBRs had a significantly lower flexural strength at the proportional limit and they had issues with repair and relines.178 The flexural strength of polyamide (Valplast) was reported to be 27 – 42 MPa9 and 35 – 41 MPa.1 Another study reported that the flexural strength of polyamide (Valplast) was 13.7 MPa, that of polyester (EstheShot-Bright) was 24.2 MPa, and that of acrylic resin (Acry-Tone) was 17.3 MPa.10

Removable prostheses often require denture base relining to improve their fit due to gradual changes in residual alveolar ridges.111213 Successful denture relining not only relies on the properties of two kinds of materials, but also requires high bond strength between them to achieve a perfectly bonded interface. High bond strength can resist the separation of relined denture bases. On the other hand, weak bond strength will decrease the overall mechanical strength.111213 During mastication, relined dentures have to withstand masticatory forces to prevent fracture. Therefore, knowledge of the mechanical properties (bond and flexural strength) of relined resin base is essential in the selection of denture base resins.

Currently, a new type of TDBR, polypropylene, has also been approved for use as a denture base in Japan. However, not enough research has been performed to evaluate its clinical properties. Furthermore, there are still insufficient studies about the effects of relining materials on the flexural strength of relined TDBRs. Therefore, the aim of the present study was to evaluate the effects of relining materials on the flexural strength of relined TDBRs. The null hypothesis of this study was that denture relining materials has no effect on the flexural strength of relined TDBRs.

The materials involved in this study included four TDBRs: 1. polyester: EstheShot-Bright (ES), I-cast Co. Ltd., Kyoto, Japan; 2. acrylic resin: Acrytone (AC), High Dental Japan, Osaka, Japan; 3. polyamide: Valplast (VA), Valplast International Corp, New York, NY, USA; 4. polypropylene: Weldenz (WE), Weldenz Japan Co. Ltd., Nagoya, Japan). The study also included three Autopolymerizing denture relining resins (ADRRs): 1. Vertex Self-Curing (VE), Vertex-Dental, Zeist, the Netherlands; 2. Tokuyama Rebase II (TO), Tokuyama Dental Corp, Tokyo, Japan; and 3. Ufi Gel Hard (UF), Voco, Cuxhaven, Germany). All materials used are summarized in Table 1 and Table 2.

TDBRs with dimensions of 20.0 × 10.0 × 3.3 mm were fabricated with a stainless steel mold according to the manufacturer's instructions (Table 1) and embedded into autopolymerizing resin using a cylindrical silicone mold with a height of 24.0 mm and a diameter of 23.0 mm. This served as the substrate for shear bond testing. The cylindrical specimens were polished with 600 grit sandpaper to obtain a uniform flat surface. A cylindrical Teflon tube with a height of 3.0 mm and an inner diameter of 6.0 mm was used as a mold to bond the denture relining resins. The ADRRs were mixed accurately with liquid and powder at the ratio according to the manufacturer's instructions (Table 2) then placed into the Teflon tube. The ADRRs were polymerized according to the manufacturer's instructions (Table 2), and the Teflon tube and excess material were removed (Fig. 1). Finally, all specimens were placed in a water bath for 48 hours at a temperature of 37℃.

A total of 12 groups consisting of 10 specimens each were fabricated: a) VA + TO, b) VA + UF, c) VA + VE, d) ES + TO, e) ES + UF, f) ES + VE, g) AC +TO, h) AC + UF, i) AC + VE, j) WE + TO, k) WE + UF, l) WE + VE.

A stainless steel mold was used to fabricate TDBRs and ADRRs into bars with dimensions of 64.0 × 10.0 × 3.3 mm (Fig. 2A), which is in accordance with the International Standards Organization (ISO) 1567 (1999) specifications. A total of 160 bars of TDBRs were created and 30 (10 per group) specimens of ADRRs were also fabricated according to the manufacturer's instructions (Table 1, Table 2).

To fabricate relined TDBRs specimens, 120 bars of TDBRs were further trimmed and polished, using a 600 grit sandpaper to obtain a uniform flat surface to achieve a thickness of 2.0 mm (Fig. 2B). TDBRs specimens were placed into the stainless steel mold for relining. The ADRRs were mixed accurately with liquid and powder at the ratio according to the manufacturer's instructions (Table 2) and then placed into the same stainless steel mold on the top of TDBRs specimens. After polymerization, ADRRs and TDBRs have turned into double layer sandwich specimens and then removed from the stainless steel mold. The thickness of the relined ADRR was 1.3 mm. All specimens were placed in a water bath for 48 hours at a temperature of 37℃.

A total of 19 groups consisting of 10 specimens each were created: a) VA + TO, b) VA + UF, c) VA + VE, d) ES + TO, e) ES + UF, f) ES + VE, g) AC + TO, h) AC + UF, I) AC + VE, j) WE + TO, k) WE + UF, l) WE + VE, m) VA, n) ES, o) AC, p) WE, q) TO, r) UF, s) VE.

The shear bond strength tests were performed in a universal testing machine (AG-10KNX, Shimadzu Co., Kyoto, Japan) at a cross-head speed of 1 mm/min until failure occurred. Shear bond strength was expressed as follows: F = N/A where N = maximum force (in N) exerted at the specimen and A = area of bonding (in mm²).

For flexural strength test, all the specimens were placed in a universal testing machine (AG-10KNX, Shimadzu Co., Kyoto, Japan) for three-point bending test at a cross-head speed of 5.0 mm/min and a distance of 50.0 mm between jig wedges. Flexural strength was computed from the equation: FS = 3Fl / 2bh² where: F = maximum load (in N); l = distance (in mm) between the supports; b = width of the specimen (in mm); and h = thickness of the specimen (in mm).

Data was analyzed by ANOVA with Tukey's test as post hoc analysis (P = .05) and Spearman's correlation coefficient analysis (P = .05) using SPSS software (SPSS ver. 20.0, IBM, Chicago, IL, USA).

AC groups demonstrated significantly higher bond strength and WE groups showed the lowest shear bond strength compared to other groups (P < .05). Groups where ES was bonded with VE resin showed higher bond strength compared to other groups (P < .05, Table 3).

Between the TDBRs groups, ES exhibited significantly higher flexural strength compared to all the other groups (P < .05). WE demonstrated showed lower flexural strength than all the other groups (P < .05, Table 4).

Among the ADRRs groups, the VE group showed significantly higher flexural strength than the TO and UF groups (P < .05, Table 4).

Between the relined TDBRs groups, TDBRs relined with VE resin exhibited significantly higher flexural strength compared to the other groups (P < .05). The relined ES groups showed significantly higher flexural strength and relined WE groups exhibited lower flexural strength compared to the other groups (P < .05, Table 4).

The flexural strength of relined TDBRs exhibited a positive correlation with the flexural strength of TDBRs (P = .001) and shear bond strength (P = .013, Fig. 3).

The results obtained in the present study demonstrated a positive correlation between the flexural strength of relined TDBRs and shear bond strength. Therefore, the null hypothesis that the denture relining materials has no effect on flexural strength of relined TDBRs was rejected.

This study explored the effects of relining materials on the shear bond strength and flexural strength of relined TDBRs. Bond strength between denture base resins and relining resins can be affected by the chemical structures of the two different materials.14 In our study, AC bonded with VE exhibited the highest bond strength because the chemical structure of AC and VE are the same as PMMA. Polyesters were reported to bond well with ADRRs, while polyamides showed low bond strength with ADRRs.91516 In our study, the results were similar to those in the previous studies. Polyesters are polycondensates of polyfunctional carboxylic acid and polyalcohol. Because they adhere strongly to ADRRs, dentures made of these materials are also easy to repair.8 Polyamide are linear polymers composed of repeated amide bonds (–CONH–). Insolubility of nylon materials in methyl methacrylate (MMA) means that they do not adhere directly to ADRRs due to the hydrogen bonds between amide groups (–CONH₂–), making denture fractures difficult to repair.15 If the bond strength is weak, delamination of the interface can occur.17 In this study, delamination occurred in the WE groups during flexural strength test of relined TDBRs. This may have been caused by the low bond strength, since the WE groups showed the lowest bond strength, which was around 1 MPa.

According to the ISO 1567 (1999), the flexural strength of TDBRs should be more than 65 MPa. In this study, our results indicated that only ES met the standard value requirements and mostly exhibited low flexural strength, especially in the WE groups. Hamanaka et al. demonstrated that four types of TDBRs had significantly lower flexural strength at the proportional limit compared to the conventional PMMA.10 In another study, six thermoplastic resins and a conventional acrylic resin were examined and the results exhibited that they had lower flexural strengths than conventional PMMA.1 Polyamide was reported to have low flexural strength,1 while polyester has moderately high flexural strength.10 Similar results were found in our study. Therefore, caution is needed when using TDBRs in clinical settings because there is a possibility of permanent deformation or fracture of the removable prosthesis after loading. In the flexural strength test of relined TDBRs, the relined ES groups showed significantly higher flexural strength and the relined WE groups exhibited significantly lower flexural strength. This indicates that the flexural strength of relined TDBRs can be affected by the flexural strength of the original material.

Although most studies showed that the flexural strength of relining materials has an effect on the flexural strength of relined denture bases,18192021 a previous study evaluated the resistance to plastic deformation of a relined denture base with different relining materials and reported that relined denture bases exhibited a significant decrease in flexural strength.18 In addition, a recent study investigated the ability of the relining materials to strengthen denture base resins and found that the strength of relined denture bases is related to the strength of the original two materials.11 Different results were found in the current study compared to previous studies.

According to the Spearmen's correlation coefficient, only the flexural strength of TDBRs has a positive effect on the flexural strength of relined TDBRs (Fig. 3A). The flexural strength of relining materials had no statistical effect on the flexural strength of relined TDBRs (Fig. 3B). This may be caused by the varying thickness of the resins (TDBR = 2 mm, relining materials = 1.3 mm). Bond strength can also affect the flexural strength of relined thermoplastic denture base resins (Fig. 3C).

This in vitro study had some limitations such as difficulty in the simulation of the oral environment and a need for long-term study. Further studies should be done on how different surface treatment on TDBRs may affect the shear bond strength.

In summary, the results for shear bond strength and flexural strength, from the three-point bending test, suggested that ES has the most suitable mechanical properties for use in clinical practice.

Within the limitations in this study, it can be concluded that flexural strength of relined TDBRs was affected by the flexural strength of the original denture base resins and the bond strength between denture base resins and relining materials. Also, EstheShot-Bright was concluded to have the most suitable mechanical properties that could be used in the clinical practice.