Since its introduction, polymethyl methacrylate (PMMA) resin has been extensively used in denture fabrication due to its favorable features such as ease of handling and repair, satisfactory esthetic characteristics, low cost, low water sorption and solubility, and adequate strength. ¹ However, it has some drawbacks such as dimensional instability, poor mechanical properties, and residual monomer content. In addition, this material is an insulator, and thus, does not transmit temperature changes, which is important for gustatory sensation. ² , ³ , ⁴ The weak mechanical properties of PMMA can cause fracture in denture base material. The fracture of this material is usually due to its low impact, transverse, and fatigue strength. ⁴ , ⁵

Prosthodontists have previously pursued numerous methods for reinforcement of PMMA denture base material. ¹ , ³ , ⁶ , ⁷ Several researchers have employed a stainless steel mesh to enhance the mechanical properties of acrylic denture base material. ⁸ , ⁹ However, none of these metal reinforces have gained wide acceptance among the dentists due to the recurrence of denture base fractures. Moreover, stainless steel mesh improves the transverse strength of PMMA resin only slightly. Besides, the fractured denture base which is reinforced with metal wires would be more difficult to repair because of the wires bulging from the fractured site. ⁷

High-performance polymer (BioHPP) is a thermoplastic polymer based on polyetherketone (PEEK), which has been created and optimized for dental use. PEEK is a synthetic polymeric material that was used in medical orthopedics for years. ¹⁰ The high-mechanical strength of BioHPP is due to the special ceramic filler with a grain size of 0.3–0.5 μm. ¹¹ Ease of preparation and use, biocompatibility, measurable resistance of approximately 600 MPa, white color and acceptable esthetics, lack of abrasive effects on the remaining tooth structure, high polishability, and least environmental contamination are among the favorable properties of this thermoplastic polymer. ¹² , ¹³ , ¹⁴

BioHPP is among the materials that have been recently implemented in dentistry, especially for dental implants as a framework material in implant-based bridges. ¹⁵ This novel material is superior to metal mainly because of its white and esthetically satisfying appearance. Besides, this substance does not pose a risk of galvanic shock. Moreover, BioHPP can be applied in delicate thicknesses; it is easily polished, and does not abrade the natural teeth. This material is also as elastic as a natural bone and quite cost-effective. ¹¹ , ¹⁵

In the study by Vosshans et al., ¹⁵ the application of BioHPP in dental restorations showed favorable advantages such as stress-free primary framework, fixedness of prostheses, convenient insertion/removal of the removable prostheses for patients, good hygiene, plaque resistance, color stability, and low weight. In another study, Bechir et al. ¹¹ assessed the properties of BioHPP as a superstructure material in oral implantology. They found that BioHPP possesses favorable features for the fabrication of fixed prosthetic restoration such as superstructure on dental and implant abutments.

One of the major advantages of this material for the implant-based prosthesis is its lower modulus of elasticity compared with metal and its cushion effect to reduce the forces implemented on the implant. ¹⁶ BioHPP can also be used for the framework of partial dentures, fabrication of removable dentures-obturators, as well as the crowns and bridge restorations. ¹⁴ , ¹⁷ , ¹⁸

Controversies still exist regarding the effectiveness of metal reinforcement, and no definite solution is known to increase the strength of acrylic resin denture base materials. No study has ever evaluated the effect of BioHPP material on the flexural strength (FS) of PMMA denture base material. Hence, the present study was designed to assess the effect of a novel material called BioHPP and metal reinforcement on the FS of heat-cured PMMA. The null hypothesis was that neither the BioHPP nor the metal reinforcement material could enhance the FS of PMMA denture base material.

In this experimental study, a stainless steel metal mold (64 mm × 13 mm × 3 mm) was prepared based on ASTMD790 (American Society for Testing and Materials) (2010) Figure 1. ¹⁹ Ten perforated special trays were made by using self-cure acrylic resin (GC Unifast III, GC Corp., Tokyo, Japan). From the metal mold, 30 impressions were made with silicone impression material (Speedex, Coltene, Swiss) and poured with dental stone (Fujirock, GC Dental Industrial Corp., Tokyo, Japan) Figure 2. Figure 1

A stainless steel metal mold.

Table 1shows the mean ± SD of the FS values in the study groups. As displayed in Figure 6, the highest FS was observed in the metal-reinforced group, which was significantly higher than that in the BioHPP-reinforced (P < 0.001) and control groups (P < 0.001). The FS values were not statistically significantly different between the BioHPP-reinforced and the control group (P = 0.614).{Table 1} Figure 6

Mean ± standard deviation of flexural strength of different groups.

This in vitro study evaluated the effect of various reinforcement techniques on the FS of PMMA. The null hypothesis was rejected as the findings revealed significantly higher FS in the metal-reinforced group compared with the BioHPP-reinforced and nonreinforced groups.

Studies have previously examined the effect of various forms of metal reinforcement on the mechanical strength of PMMA. ⁹ , ²¹ In a study by Vallittu and Lassila, ⁹ the thick wires which were subjected to sandblast showed better adhesion to the acryl and significantly increased the resin fracture resistance. In line with the present study, Polyzois concluded that the addition of a metal mesh to PMMA would increase its fracture strength. ²² In another study, Lee et al. ²³ detected that the incorporation of metal wire in poly (ethyl methacrylate), significantly improved its FS.

The favorable properties of metal reinforcement could be related to the higher modulus of elasticity of the metal mesh compared with the acryl, which results in the force absorption by the metal without any significant deformation. Although the metal could compromise the integrity of the acrylic structure, as a solid foundation, it protects the remaining acrylic structure from deformation and its consequent potential failure. ²¹ However, in contrast to our findings, Vallittu ²⁴ claimed that the stainless steel mesh could not significantly increase the transverse strength of PMMA resin.

Studies on glass fibers have also shown similar promising reinforcement effects. The application of these fibers has been reported to reinforce the acryl, probably due to their high-elastic modulus. ¹ However, most of these investigations did not consider the clinical conditions such as the presence of saliva, body temperature, and the presence of cyclic forces, and they often tested static forces.

A major problem with these metal reinforcements is the dark metal shadow, which is visible from the underlying acryl in the esthetic areas. Therefore, several alternatives of metal such as glass, Aramide, Kelvar, and nylon have been proposed and evaluated; the results of which are quite different and diverse. ⁶ , ²⁵ , ²⁶

Concerning the metal-resin-related problems, previous clinical studies reported that BioHPP could be an alternative framework material for full-arch restorations. ¹⁴ , ¹⁶ The present study used this novel substance as an esthetic substitute of metal mesh for strengthening the PMMA. However, the findings revealed that BioHPP could not improve the FS of heat-cured PMMA; thus, it cannot be a suitable alternative to metal reinforcement. A possible reason could be related to the low thickness of BioHPP mesh (1 mm), which results in the bending and fracture of the frame and acryl.

In the present study, three-point bending test was used for measuring the FS of specimens, since this test can precisely simulate the type of force applied on the denture through mastication. ⁸ The bending test not only assesses the strength but also gives an estimate of the material rigidity. ²⁷

The limitation of the present study was that the effect of thermomechanical cycling was not measured. Moreover, these reinforcing materials are likely to diminish the mechanical properties of acrylic resins over time, and consequently the denture longevity. Thus, there is an urge to simulate the clinical conditions in further investigations.

With respect to the findings of this study, it can be concluded that using metal mesh to reinforce the PMMA, considerably enhances the FS of heat-polymerized denture base resin; while, BioHPP does not have such a significant effect. Accordingly, BioHPP is not a suitable substitute for metal reinforce to enhance the FS of PMMA denture base material.

Acknowledgments

This article was based on the thesis by Amir Hashemi Hosseini and Marzieh Moayedi for partial fulfilment of DDS degree. The authors would like to thank the Vice-Chancellery of Research, Shiraz University of Medical Sciences, for supporting this research (Grant No. 1778). Appreciations are also expressed to Ms. Farzaneh Rasouli for improving the use of English in this manuscript.

Financial support and sponsorship

Vice-Chancellery of Research of Shiraz University of Medical Sciences (Grant No. 1778).

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.