Comparative evaluation of the effect of reinforced basalt fibre and glass fibre on flexural strength of heat-cured denture base resin: an in vitro study

INTRODUCTION

Denture base materials have undergone extensive research over the years; however, the ideal material, one that possesses optimum physical and mechanical proper­ties, ensuring the clinical longevity of denture prostheses, remains unfound [1]. Understanding the historical evolution of these materials sheds light on challenges that led to the exploration of contemporary solutions.
In ancient times, materials, such as wood, ivory, and bone have been used for fabrication of denture bases. In the late 18th century, dental polymers with vulcanized rubber derived from plant latex were established. The pivotal moment in the history of denture base materials arrived in 1901, when Otto Rohm introduced a mercantile compound of acrylic acid and a solid transparent polymer of acrylic acid as part of his PhD research. Subsequently, in 1936, Rohm and Hass deve­loped polymethyl methacrylate (PMMA) in the form of a transparent sheet [1].
In 1937, Dr. Walter Wright recognized the potential of acrylic resins for denture bases, owing to their ease of fabrication, cost-effectiveness, lightweight nature, and shade-matching capabilities [2]. However, the inherent drawbacks of acrylic resin denture bases, such as low strength, susceptibility to breakage, and poor thermal conductivity, prompted the exploration of innovative solutions [3, 4].
Denture base resins must withstand diverse loads gene­rated during function in oral cavity, making high flexural strength a crucial attribute [5]. The irregular resorption of the alveolar ridge further emphasizes the importance of flexural strength, given the uneven force distribution on denture base. Flexural strength, also known as bending strength or modulus of rupture, represents the highest stress a material can resist at the moment of rupture.
One common issue observed in PMMA resin dentures is the clinical failure due to flexural fatigue of the denture base, subjected to compressive, tensile, and shear stresses during function [6]. Notably, maxillary dentures exhibit a higher susceptibility to fractures compared with mandibular dentures, with an estimated ratio of 2 : 1.
Reinforcement of acrylic resin with fibers has shown that flexural strength, impact strength as well as fatigue resistance of acrylic resin can be improved. A variety of fibers, including carbon, aramid, polyethylene, and glass fibers have been examined with varying results [7].
Currently, basalt fibers have been increasingly investigated as a possible replacement of traditional glass fibers in fibrereinforced polymer. Basalt fibers are composed of plagioclase pyroxene and olivine. Chemi­cally, they are composed of SiO₂ (51.6-59.3%), Al₂O₃ (14.6-18.3%), CaO (5.9-9.4%), MgO (3-5.3%), FeO + Fe₂O₃, and traces of TiO₂. Density range of basalt fibers range from 2.5 to 2.9 g cm³, which is quite similar to that of glass fibers.
Salloum et al. [8] conducted a study on heat-cured acrylic resin reinforced with basalt powders in different quantity. It was found that flexural strength of denture base acrylic can be increased if reinforced within certain quantity (2 wt.%).

OBJECTIVES

The aim of the current study was to evaluate and compare the flexural strength of PMMA reinforced with glass fibers and basalt fibers.

MATERIAL AND METHODS

This interventional in vitro study was performed at the Department of Prosthodontics and Crown and Bridge and Central Institute of Petrochemicals Engineering and Technology, Jaipur, India (Figures 1 and 2). Ethical clearance was obtained from the Institutional Research Board Committee (approval number: MGDCH/Dental/2020/1303).
Specimen preparation was performed using compression moulding technique. Rectangular aluminum strips (65 × 10 × 3 mm), conforming to ADA specification No. 12, were employed to create wax patterns for specimen fabrication. A thin layer of wax was applied to aluminum strips to create wax patterns. The wax patterns were invested in dental flasks with dental plaster using a two-stage pour technique. Following dewaxing at 100°C for 15 minutes, a thin layer of sodium alginate separating media was applied to each flask half, and allowed to dry.
Heat-polymerized acrylic resin (Dental Product of India, heat-cured acrylic resin) was mixed in a clean porcelain jar according to a manufacturer’s instruction. For fabrication of basalt-reinforced specimens, basalt fibers and PMMA were pre-weighed in order to ensure a fiber concentration of 1%, 2%, and 3% by weight, and the same procedure was performed for glass fibers with 2% concentration. Mixing and blending were done to obtain a regular and uniform mixture. The resultant amalgam was then mixed with a monomer. The mixture reached the dough stage, and was packed into the mold space. Denture flasks were closed and subjected to 2,000 psi pressure for 30 minutes of bench-curing.
After bench-curing, the flasks were clamped and polymerized using a short curing cycle at 74°C for 90 minutes, followed by boiling for 1 hour. The flasks were allowed to bench cool till room temperature be-fore opening. Specimens were retrieved, excess material trimmed, and finishing and polishing were carried out using sandpaper (100 and 80 grit) and a tungsten carbide bur (Table 1, Figures 3-5).
The specimens were preserved in distilled water at 37°C for 5 days before testing. A total of 150 samples were fabricated following the same procedure, and grouped as per Table 2.

FLEXURAL STRENGTH TESTING

Transverse strength of each specimen was tested using an universal testing machine (Shimadzu, Japan). The spe­cimens were marked at the center, and a span length of 40 mm was maintained. Force was applied at a cross-head speed of 5 mm/min in the middle of the specimen, until fracture occurred. The load at which fracture occurred was documented, and flexural strength (FS) was calculated using the following formula:
FS = (3PL)/(2BD²),
where FS is in N/mm², P is the peak load in N, L is the span length, B is the specimen’s width, and D is the specimen’s thickness.

RESULTS

The FS of different denture base groups (A-E) was assessed, and notable variations were observed (Table 3). The highest mean FS was noted in group E (62.93 MPa), while the lowest in group C (45.36 MPa). Analysis of variance (ANOVA) test indicated a statistically significant difference in the mean FS among the five groups (F = 339.846, p ≤ 0.01). Significant differences identified by ANOVA and confirmed by post hoc Tukey test, emphasized the distinct performance of each group (Table 4).

DISCUSSION

A diverse array of polymers finds extensive applications across various disciplines in prosthodontics. Among these, PMMA is the base material for dental prosthetics, serving in the creation of artificial teeth, denture bases, and orthodontic appliances. Additionally, PMMA plays a pivotal role in crafting temporary crowns, occlusal splints, and customized dental casts, while also facilitating repairing of prosthetic devices. Its distinct characteristics, including lightweight properties, esthetic appeal, economic viability, ease of manipulation, and customizable physical attributes, render it the preferred biomaterial for such multi-faceted dental applications [9].
However, the disadvantage lies in its sub-optimal physical and mechanical properties [3]. This study investigated the realm of reinforcing PMMA acrylic resins with fibers, specifically exploring the impact of basalt and glass fibers on FS.
Over the last three decades, researchers have sought ways to enhance the physical properties of acrylic resins, not only in dentistry, but across diverse industries. This study aligns with this pursuit, emphasizing the need for proper adhesion between reinforcement and acrylic resin in order to achieve the desired improvements.
Johnston et al. [5] revealed a substantial percentage of acrylic resin dentures breaking within a few years, which underscores the urgent need for addressing the issues of fatigue and impact failure. To fortify PMMA’s mechanical properties, three approaches are commonly employed: (1) reinforcing with materials, such as glass, woven glass, polyethylene, aramid, and Kevlar fibers;
(2) adding filler particles, such as aluminum oxide or zirconium oxide nano-particles; and (3) chemically modifying or replacing PMMA [10, 11]. Several studies have shown that FS of heat-activated PMMA resin rein-forced with glass fibers was more than heat-activated PMMA resin reinforced with polyethylene and aramid fibers [12, 13]. PMMA strengthened with carbon fibers showed the highest improvement in the FS compared with other reinforced fibers. Carbon, polyethylene, and aramid fibers increased FS, but showed difficulty in polish­ing and were unesthetic due to blackish color.
The enhancement of denture base polymer strength is typically achieved through cross-linking agents or reinforcement of resin with fibers, rods, metal wires, or nets. Earlier studies demonstrated the superior FS of PMMA resin reinforced with glass fibers compared with polyethylene and aramid fibers. However, the esthetic and polishing challenges associated with these materials prompted the exploration of alternative reinforcing agents.
A study by Tacir et al. [14] demonstrated that the FS of PMMA was improved by addition of 2 wt.% glass fibers. Studied showed that there was significant increase in the FS of conventional denture base material with addition of glass fibers, when comparing with conventional denture base acrylic material. A study by Nayak et al. [15] reported that the FS of PMMA resin was the highest in specimen with basalt incorporated in transverse framework pattern resin specimen, followed by glass fibreincorporated specimen.
Also, a study conducted by Yerliyurt et al. [16] revealed that all three fibers, i.e., carbon fibers, glass fibers, and polypropylene fibers, exhibited reinforcement of FS, regardless of their length and concentration.
The introduction of glass fibers mitigated various challenges, but posed adhesion issues with acrylic resin. This led to the introduction of silanated glass fibers to optimize the bond between the reinforcing material and PMMA. In comparison, basalt fibers emerged as a pro­mising alternative due to their higher tensile strength, better chemical stability, and cost-effectiveness.
The current study sought to evaluate and compare the impact of basalt and glass fibers on the FS of heat-cured PMMA denture base material. The results revealed that group E (PMMA + basalt fibers; 1 wt.%) demonstrated the highest FS, with 62.93 MPa, while group C (PMMA without reinforcement) exhibited the lowest value, with 45.56 MPa. The comparison across the groups highlighted that PMMA with 1% basalt fiber showed the most significant increase in the FS.
The rejection of the null hypothesis reveals that mechanical properties of denture base resins can be enhanced by reinforcing agents in denture base materials.

CONCLUSIONS

The findings of this study strongly indicate a substantial enhancement in the FS of heat-cured PMMA with the addition of basalt fibers at different concentrations (1%, 2%, and 3% by weight) and glass fibers (2% by weight). This not only holds promise for improved denture longevity, but also paves the way for advancements in prosthodontic materials.

DISCLOSURES

1. Institutional review board statement: The study was approved by the Institutional Research Board Com-mittee of the Mahatma Gandhi Dental College and Hospital, India, with approval number: MGDCH/Dental/2020/1303.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

References