Comparative Evaluation of Flexural Strength in High-Impact Acrylic, Cobalt–Chromium, and Carbon Fiber-Reinforced Composite Denture Bases: An In-vitro Study

Abstract

Background:

Polymethyl methacrylate is the commonly used denture base material because of its esthetics and handling properties. However, its low flexural strength predisposes it to fractures. To overcome this limitation, reinforcement, such as cobalt–chromium metal frameworks and carbon fiber-reinforced composites (CFRCs), has been proposed.

Materials and Methods:

An in-vitro study was conducted using 33 standardized specimens (65 × 10 × 3 mm³), divided into three groups (n = 11). Group 1—high-impact acrylic resin, Group 2—cobalt–chromium, and Group 3—CFRC. Flexural strength was tested using a three-point bending method on a universal testing machine. Statistical analysis was then done using one-way ANOVA and post-hoc Tukey tests.

Results:

The mean flexural strength values for Group 1, Group 2, and Group 3 were 129.73 MPa, 158.51 MPa, and 246.44 MPa, respectively. One-way ANOVA showed a statistically significant difference among groups (P < 0.001). Post-hoc analysis revealed significant differences between Groups 1 and 3 (P = 0.000) and Groups 2 and 3 (P = 0.005), but not between Groups 1 and 2 (P = 0.511).

Conclusion:

Among the tested materials, the highest flexural strength was exhibited by CFRC, indicating that it is a superior alternative to traditional cobalt–chromium and high-impact acrylic denture bases. Its mechanical advantage, along with its esthetic properties and reduced weight, make CFRC a promising material for complete denture applications. Future clinical studies are necessary to confirm its long-term performance.

INTRODUCTION

Polymethyl methacrylate (PMMA) is a universally accepted material for denture bases due to its lightweight, better esthetics, and ease of handling.[1] The major drawbacks of PMMA are its low flexural and impact strength, which often leads to fractures.[2] To overcome these disadvantages, reinforcement methods like high-impact acrylics, metal frameworks such as cobalt–chromium, and various fibers, including glass and cotton fibers, have been explored in this research.[3] Among these materials, cotton fibers often exhibit superior mechanical properties, including high stiffness, strength, fatigue resistance, and low weight. Even though the black color of the fiber can be unesthetic, it can be masked with opaque or pink composite resins. Another material of choice is carbon fibers, made up of 99.9% pure carbon filaments, and is available in various forms like woven, layered, or strand-based on their application.[4,5] Their effectiveness depends on various factors like fiber orientation, adhesive properties to the resin, and volume within the matrix.

MATERIALS AND METHODS

This in-vitro experimental study involved 33 samples divided into three groups (n = 11 each): Group 1—high-impact acrylic denture base resin (control), Group 2—cobalt–chromium, and Group 3—carbon fiber-reinforced composite (CFRC). Standardized specimens measuring 65 × 10 × 3 mm³ were fabricated by following ADA Specification No. 12. High-impact acrylic samples were processed using traditional compression molding and heat polymerization techniques. Cobalt–chromium samples were designed through Computer aided designing (CAD) after scanning the master cast using the lost-wax technique. Carbon fiber specimens were fabricated by impregnating fiber mats with a 3:1 resin to catalyst ratio, layering with alternating fiber orientations, and vacuum-sealing, postcuring at 80°C. After that, all the specimens were subjected to a three-point bending test using an Instron Universal Testing Machine at a crosshead speed of about 2 mm/min to determine the flexural strength of each material. The mean flexural strength of each group was then calculated and subjected to statistical analysis using a t-test and one-way ANOVA to evaluate and determine the differences among the groups.

RESULTS

The results revealed that Group 3 (CFRC) exhibited the highest flexural strength among all groups, with a mean of 246.45 MPa, followed by Group 2 (cobalt–chromium) with a mean of 158.51 MPa, and Group 1 (high-impact acrylic resin) with the lowest mean of 129.73 MPa. One-way ANOVA revealed a statistically significant difference between the groups (F = 11.134, P < 0.001). Post-hoc Tukey analysis confirmed significant differences between Groups 1 and 3 (P = 0.000) and Groups 2 and 3 (P = 0.005), but no significant difference between Groups 1 and 2 (P = 0.511). The flexural strength values ranged from 54 MPa to 281.77 MPa, with the highest value observed in Group 3 and the lowest value in Group 1. Altogether, based on the results, CFRC demonstrated superior flexural strength compared to the other two materials [Tables 1 and 2].

Table 1.

Mean and standard deviation of each group

Groups		Mean		Standard deviation
Group 1		129.73		77.46
Group 2		158.51		34.49
Group 3		246.45		61.39

Table 2.

Results of statistical analysis using the one-way ANOVA test for flexural strength (MPa) of all groups

Group		Mean flexural strength (MPa)		F		P
Group 1		129.73		11.134		0.000
Group 2		158.51
Group 3		246.44

DISCUSSION

The significant concern in dentistry is tooth loss, which requires various prosthodontic treatments, like removable dentures, to implant-supported prostheses, each varying in cost and suitability. However, fractures of denture bases are a common clinical complication, typically caused by flexural fatigue, midline stress concentration, and impact forces during masticatory functions and accidental drops. PMMA has long been used as a material of choice due to its ease of processing, acceptable esthetics, and biocompatibility. Certain mechanical limitations of PMMA make it susceptible to fractures. High-impact acrylic resins were introduced to overcome these limitations by incorporating rubber phase materials to enhance fracture toughness. Studies done by Chhabra et al.[6] and Jaikumar et al.[7] have shown that even high-impact acrylic resins fail to meet the flexural demands under high functional masticatory stresses.[6,7] Metal-based dentures, especially those fabricated from cobalt–chromium alloys, have been widely recognized for their excellent mechanical properties. The current study found that cobalt–chromium exhibits a mean flexural strength of 158.51 MPa, confirming its mechanical superiority over conventional PMMA, which recorded a lower value of 129.73 MPa. These findings are consistent with the study of Senbagavalli Sagadevan et al.,[8] who reported significantly higher flexural resistance in metal-based dentures compared to acrylic ones. The emergence of fiber-reinforced composite materials, especially CFRC, has potential significance and advancement in dentistry. CFRCs exhibit a high strength-to-weight ratio, excellent fatigue resistance, and dimensional stability, which follows the study conducted by Vallittu.[9] Initially, concerns about their color and potential cytotoxicity were raised, but recent advancements in fiber treatment and masking techniques have largely explored these issues. The present study demonstrated that CFRC denture bases had the highest flexural strength among all the groups tested. These findings corroborate with previous studies conducted by Alla et al.[10] and Vallittu,[9] which reported a substantial increase in flexural strength when carbon or glass fibers were incorporated into the denture base resin.

CONCLUSION

It can be concluded that CFRC denture bases exhibit significantly higher flexural strength compared to both high-impact acrylic and cobalt–chromium metal denture bases. The superior mechanical performance of CFRC makes it a promising alternative for use in complete denture prosthodontics, especially for patients prone to denture fractures or with specific clinical conditions like combination syndrome or opposing natural dentition.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.