Comparative evaluation of surface roughness and impact strength of conventional polymethyl methacrylate and microcrystalline cellulose reinforced polymethyl methacrylate – An in vitro study

Abstract

Aim:

To evaluate and compare the surface roughness and impact strength of conventional polymethyl methacrylate (PMMA) with microcrystalline cellulose (MCC)-reinforced PMMA.

Settings and Design:

An in-vitro experimental study was conducted. Fifty PMMA specimens were fabricated and divided into five groups based on MCC concentration (2% or 5%) and particle size (20 μm or 50 μm).

Materials and Methods:

Specimens (80 mm × 10 mm × 4 mm) were categorized as follows: Group A (control; conventional PMMA), Groups B and D (2% MCC with 20 µm and 50 µm particles, respectively), and Groups C and E (5% MCC with 20 µm and 50 µm particles, respectively). Surface roughness was measured using a contact profilometer, and impact strength was tested with a ZwickRoell impact testing machine.

Statistical Analysis Used:

Statistical analysis was performed using IBM SPSS Version 28.0. One-way ANOVA followed by Tukey’s post hoc test was used to determine intergroup differences, with the significance level set at p<0.05.

Results:

Surface roughness was lower in Groups B (0.89±0.43), C (1.07±0.34), and E (0.77±0.27) compared to the control Group A (1.25±0.42), while Group D (1.84±0.25) showed higher values. Impact strength in Groups C (1.85±0.23), D (1.80±0.17), and E (1.81±0.26) was slightly lower than the control (1.88±0.31), though not statistically significant. However, Group B (1.56 ± 0.20) showed a significant reduction.

Conclusion:

The addition of 20 μm MCC reduced surface roughness at both 2% and 5% concentrations, whereas 50 μm MCC increased roughness at 2% but decreased at 5%. Impact strength remained comparable to the control in all groups except PMMA + 2% MCC (20 μm), which exhibited a significant decline. MCC reinforcement influences PMMA’s mechanical and surface properties, suggesting its potential for denture base modifications.

INTRODUCTION

Any denture base material’s surface qualities should be carefully taken into account because studies indicated a clear link between surface roughness, plaque build-up, and Candida albicans adherence.[1] More Candida species have been found in stomatitis caused by dentures.[2] The synthetic fiber-reinforced denture base resins that are currently commercially accessible can be replaced with microcrystalline cellulose (MCC).[3] Recent trials using MCC which is a partially depolymerized, white, odorless, fine, nonfibrous, and crystalline powder reinforced with polymethyl methacrylate (PMMA) resin revealed enhancements in flexural strength and modulus of elasticity.[3]

The field of biomaterials has seen major advancements, resulting in improved performance and alternatives to conventional synthetic materials.[3] The goal of using these materials with normal human tissues is to make them safe, stable, and comfortable. In dentistry, the loss of teeth is an unavoidable result of dental caries, periodontal disease, trauma, or old age. A patient’s speech, mastication, esthetics, and social life are all impacted by tooth loss, leading in a stronger need for an optimal replacement to restore the patient’s functional and esthetic loss.[3] The complete or removable partial denture is generally made of an acrylic resin denture base material, i.e., PMMA. PMMA has been the preferred material for conventional dentures for many decades. This material offers several advantages, including ease of processing, low cost, and its ability to closely resemble the oral tissues in color and appearance.[4] Nevertheless, they are prone to undergo plaque accumulation on the denture surface which will promote microbial adhesion and fracture leading to prosthesis failure. This is more common on the tissue aspect of denture surface because of being unpolished as compared to the polished surface.[5] The surface properties of denture base materials are of critical importance, as numerous studies have demonstrated a direct association between surface roughness, plaque accumulation, and the adherence of C. albicans.[6] The denture’s rough intaglio surface provides a perfect environment for microorganisms and plaque accumulation intraorally. Candida species have been found to be more prevalent in denture-related stomatitis. Abuzar et al.[7] determined that a surface roughness (Ra) of 0.2 µm represents the clinically acceptable limit, as plaque accumulation does not significantly decrease below this level in prosthetic and dental restorative materials. The relatively low impact strength of PMMA often leads to fractures in acrylic-based dentures.[8] A study conducted by Oliveira et al.[9] found that approximately 63%–68% of complete denture prostheses broke when accidentally dropped onto a hard surface during handling. PMMA denture bases have been reinforced with glass or carbon fibers, as well as chemically modified, to improve their mechanical qualities. MCC has emerged as a potential substitute for commercially available synthetic fiber-reinforced denture base resins. Recent studies have shown improvement in flexural strength and modulus of elasticity of MCC-reinforced PMMA denture base resin.[3] Consequently, it is of prime importance to have a denture base material that possesses the adequate surface roughness and impact strength without compromising on the other physical and mechanical properties. The null hypothesis of this study was that there is no significant difference in surface roughness and impact strength between conventional PMMA and MCC-reinforced PMMA, regardless of MCC concentration and particle size. This study aimed to assess the surface roughness and impact strength of conventional PMMA and PMMA reinforced with MCC.

MATERIALS AND METHODS

The study protocol was approved by the institutional ethics committee (Approval number: IEC 926/2019). The sample size was calculated based on the findings of Rahaman Ali et al.[3] for flexural properties of MCC reinforced denture base resins. The sample size was estimated using G*Power software (G*Power version 3.1.9.7). Based on the findings, an effect size of 0.83 was obtained. Substituting this value, with a power of 95% and confidence interval of 95%, the sample size was estimated to be 7 per group. Considering the possibility of attrition or loss of samples during the experiment, the sample size was inflated and rounded to 10 per group. Thus, a total of 50 samples were fabricated which were divided into five groups based on the particle size of the reinforcement material and percentage of MCC - Group A conventional heat polymerized PMMA, Group B PMMA + 2% MCC (20 µm), Group C PMMA + 5% MCC (20 µm), Group D PMMA + 2% MCC (50 µm), and Group E PMMA + 5% MCC (50 µm). The product name, manufacturer details, and proportions of the materials used in this study are listed in Table 1 (product name and manufacturer details) and Table 2 (proportions of the materials used in this study), respectively.

Table 1.

Product name and manufacturer details

Material used	Product information	Manufacturer
Trevalon heat cure acrylic resin powder	Conventional heat cure acrylic denture base powder	Dentsply, India Pvt. Ltd.
Trevalon universal liquid	Conventional heat cure acrylic denture base liquid	Dentsply, India Pvt. Ltd.
Microcrystalline cellulose	Microcrystalline cellulose of two different particle sizes, i.e., 20 μm and 50 μm	Sigma-Aldrich, Ireland

Table 2.

Proportions of the materials used in this study

Group	Test group	Powder (g)	Liquid (mL)	MCC (g)
A	Conventional heat polymerized PMMA	12.00	4.00	Nil
B	Conventional PMMA reinforced with 2% (20 μm) MCC; same percentage of powder decreased	11.76	4.00	0.24
C	Conventional PMMA (A) reinforced with 5% (20 μm) MCC; same percentage of powder decreased	11.40	4.00	0.60
D	Conventional PMMA (A) reinforced with 2% (50 μm) MCC; same percentage of powder decreased	11.76	4.00	0.24
E	Conventional PMMA (A) reinforced with 5% (50 μm) MCC; same percentage of powder decreased	11.40	4.00	0.60

PMMA: Polymethyl methacrylate, MCC: Microcrystalline cellulose

Specimen preparation

The denture base resin for Group A (control group) was prepared using a powder-to-liquid ratio of 3:1, as per the manufacturer’s instructions (Trevalon, Dentsply India Pvt. Ltd.). For Groups B, C, D, and E, both PMMA and MCC powders were precisely weighed using a digital balance with an accuracy of ±0.01 mg. A total of five groups of test specimens were fabricated in rectangular form, with 10 mold cavities prepared for each group and coded accordingly. The test group B (PMMA + 2% MCC) and C (PMMA + 5% MCC) were fabricated by incorporating 2% and 5% MCC by %wt with a particle size of 20 µm, respectively, while the Group D (PMMA + 2% MCC) and E (PMMA + 5% MCC) were fabricated by incorporating 2% and 5% MCC by %wt with a particle size of 50 µm, respectively. Metal molds of desired inner dimensions measuring 80 mm long, 10 mm wide, and 4 mm thick were fabricated using aluminum. Mold space was created precisely using milling machine. Modeling wax was melted and poured into the aluminum molds which after cooling resulted in wax patterns. These wax patterns were then invested in dental stone by placing them in the lower part of the flask followed by dewaxing process. The molds thus obtained were used for the fabrication of specimens with dimensions of (80 × 10 × 4) mm according to ISO 179-1:2010 specifications using conventional flasking technique and were processed in denture flask (Hanau Varsity Flask, Ejector Type) according to the manufacturer’s recommendations and processed specimens were finished and polished, as shown in Figure 1.

Figure 1.

Specimens after finishing and polishing

Testing of samples for surface roughness

The samples were placed on a flat surface and were subjected to surface roughness measurement by using Surface Profiler (Form Talysurf Intra, Taylor Hobson, Ametek Inc. UK), as shown in Figure 2. The diamond stylus (tip radius = 1.5 µm) of the profiler runs for 3 mm on the surface of the sample with a force of 1 mN and calculates the surface roughness using the following formulae R_a = 1/L ₀ ʃ^L |z (x)| dx

Figure 2.

Measurement of surface roughness

Three readings were recorded from each sample and their mean was taken as the surface roughness (Ra and Rt).

Testing of samples for impact strength

Charpy impact test was performed according to ISO 179-1:2010. V shaped notch was formed on all the specimens with the Motorized Notch Cutter (Advance-Equipments Ltd., Thane, India). The notch was made of 0.25 notch root radius and 2 mm depth with 45 degrees notch radius as per the ISO specification for Charpy impact test, as shown in Figure 3. The specimens were subjected to testing using Digital Charpy Impact Testing Machine (ZwickRoell, Germany). For this the specimen was placed on the jig in such a way that the notch face toward the pendulum hammer so that the pendulum energy imparts at the center of the notch side, as shown in Figure 4. The net energy absorbed was calculated by the computer associated to the machine by the formula:

Figure 3.

ISO specification for charpy impact test

Figure 4.

Measurement of impact strength

IS = (Energy absorbed/[Effect width × Thickness]) × 1000

RESULTS

Data were analyzed using IBM SPSS Statistics version 28.0, with the level of significance set at P < 0.05. Descriptive statistics were computed to determine the mean and standard deviation for each group. The Shapiro–Wilk test was applied to assess the normality of data distribution. Inferential statistics were conducted using the one-way analysis of variance (ANOVA) to evaluate the differences among groups, followed by Tukey’s post hoc test for pairwise group comparisons.

Table 3 (descriptive analysis of surface roughness) and Table 4 (descriptive analysis of impact strength) present the mean and standard deviation values for surface roughness and impact strength across the experimental groups. A one-way ANOVA revealed statistically significant differences among the groups for both parameters. Tukey’s honestly significant difference (HSD) post hoc test [Table 5] identified significant differences in surface roughness (P < 0.05) between the following group pairs: Control versus 50 µm 2% MCC + PMMA, Control versus 50 µm 5% MCC + PMMA, 20 µm 2% MCC + PMMA versus 50 µm 2% MCC + PMMA, 20 µm 5% MCC + PMMA versus 50 µm 2% MCC + PMMA, and 50 µm 2% MCC + PMMA versus 50 µm 5% MCC + PMMA [Table 5]. For impact strength, Tukey’s HSD post hoc analysis [Table 6] revealed a statistically significant difference (P < 0.05) only between the control group and the 20 µm 2% MCC + PMMA group.

Table 3.

Descriptive analysis of surface roughness

Group	Control (A)	PMMA + 20 μm 2% MCC (B)	PMMA + 20 μm 5% MCC (C)	PMMA + 50 μm 2% MCC (D)	PMMA + 50 μm 5% MCC (E)
Mean±SD	1.25±0.42	0.89±0.43	1.07±0.34	1.84±0.25±0.25	0.77±0.27

PMMA: Polymethyl methacrylate, MCC: Microcrystalline cellulose, SD: Standard deviation

Table 4.

Descriptive analysis of impact strength

Group	Control (A)	PMMA + 20 μm 2% MCC (B)	PMMA + 20 μm 5% MCC (C)	PMMA + 50 μm 2% MCC (D)	PMMA + 50 μm 5% MCC (E)
Mean±SD	1.88±0.31	1.56±0.20	1.85±0.23	1.80±0.17	1.81±0.26

PMMA: Polymethyl methacrylate, MCC: Microcrystalline cellulose, SD: Standard deviation

Table 5.

Intergroup comparison of surface roughness

Comparison	P-value
P (one-way ANOVA)	0.0001*
P (Tukey’s HSD post hoc test)
A versus B	0.16
A versus C	0.77
A versus D	0.004*
A versus E	0.02*
B versus C	0.77
B versus D	0.0001*
B versus E	0.93
C versus D	0.0001*
C versus E	0.32
D versus E	0.0001*

*P<0.05 is statistically significant. HSD: Honestly significant difference, ANOVA: Analysis of variance

Table 6.

Intergroup comparison of impact strength

Comparison	P-value
P (one-way ANOVA)	0.03*
P (Tukey’s HSD post hoc test)
A versus B	0.03*
A versus C	0.99
A versus D	0.94
A versus E	0.96
B versus C	0.06
B versus D	0.18
B versus E	0.15
C versus D	0.98
C versus E	0.99
D versus E	0.99

*P<0.05 is statistically significant. HSD: Honestly significant difference, ANOVA: Analysis of variance

DISCUSSION

The MCC selected in this study was in accordance with the study by Rahaman Ali et al.[3] who evaluated different test groups with different concentrations of MCC such as 5% and 2% and concluded that all groups with added MCC showed enhanced flexural strength and flexural modulus, both before and after thermocycling, compared to the conventional resin without MCC. Verran and Maryan in their study[10] proved that rough surfaces are prone to higher cell attachment and biofilm formation when compared to smooth surfaces. The present study was conducted to evaluate the surface roughness and impact strength of conventional PMMA and MCC reinforced PMMA as the addition of any nano particles or natural fibers to denture base materials have shown an increase in the accumulation of plaque and the adherence of C. albicans[11] with increase in surface roughness. In this in vitro study, it is observed that the results of the Groups PMMA + 20 µm 2% MCC, PMMA + 20 µm 5% MCC, and PMMA + 50 µm 5% MCC are not in accordance with some of the previous studies where Wady et al.[12] reported an increase of surface roughness due to the addition of nanoparticles or fillers. Anwander et al.[13] reported the biofilm formation on denture base resin including ZnO, CaO, and TiO₂ nanoparticles. This laboratory study evaluated the effects of incorporating ZnO, CaO, and TiO₂ nanoparticles into conventional acrylic denture base resins on the formation of C. albicans biofilms. The surface roughness (Ra) values ranged from 0.04 to 0.07 µm, with no significant differences observed between the modified denture base resins and the unmodified control group. In this study, it is observed that the surface roughness values of group PMMA + 20 µm 2% MCC, PMMA + 20 µm 5% MCC, and PMMA + 50 µm 5% MCC are comparatively lower than the control group, whereas the Ra values of group PMMA + 50 µm 2% MCC are higher than the control group. The difference between the PMMA + 50 µm 2% MCC and PMMA + 50 µm 5% MCC could have been due to the improper distribution of the cellulose material added. Because of variances in the experimental protocols, methods for polishing and measuring the surface roughness, and various PMMA materials utilized, it is challenging to directly compare Ra values with those from other investigations. Operator variability might potentially exist, although this study did not examine it. The conventional polishing method and contact profilometers used to obtain Ra values in this study have also been employed by previous researchers. The results obtained for the PMMA + 50 µm 2% MCC group are broadly comparable and fall within the range reported in the literature.[14,15] In an in vitro study conducted by Begum et al.,[16] the impact strength and dimensional accuracy of heat-cured denture base resin reinforced with ZrO₂ nanoparticles were evaluated. The study concluded that impact strength decreased with increasing concentrations of ZrO₂, with the lowest value observed at 7 wt% among the tested concentrations (3%, 5%, and 7%). The amount of reinforcement employed in the polymer matrix, as well as the adherence or wettability of the material during resin impregnation, were reported to affect the strength of reinforced polymers.[17,18] The impact strength values of groups PMMA + 5% MCC 20 µm, PMMA + 2% MCC 50 µm, and PMMA + 5% MCC 50 µm are lower than the control group in this investigation, although the difference is not statistically significant. However, group PMMA + 2% MCC 20 µm demonstrated significant decrease in the impact strength than the control and other groups. The lower impact strength values of group PMMA + 2% MCC 20 µm compared to the other groups could be due to the lower concentration of MCC and the particle size used. The controlled laboratory conditions may not fully replicate the complex oral environment, potentially affecting the applicability of the results to the clinical settings. The null hypothesis, which stated that MCC reinforcement would not significantly affect the surface roughness and impact strength of PMMA, was partially supported as impact strength differences were generally nonsignificant except for one group (PMMA + 2% MCC, 20 µm), which showed a notable reduction, while it was rejected for surface roughness due to significant variations among different MCC concentrations and particle sizes, particularly with larger particles increasing roughness.

This study has several limitations that should be considered. Since it was conducted in vitro, the controlled laboratory conditions may not fully replicate the complex oral environment, which could affect the clinical applicability of the results. The study also did not evaluate the long-term effects of MCC reinforcement, such as aging, wear, and degradation over time. Furthermore, the study focused only on surface roughness and impact strength, neglecting other critical mechanical properties such as flexural strength, hardness, and fatigue resistance. Another potential concern is that MCC, being an organic material, might promote microbial growth, including Candida species, which was not explored. Addressing these limitations in future research would provide a more comprehensive understanding of MCC-reinforced PMMA as a denture base material. Long-term studies assessing aging effects, wear resistance, and material degradation would provide better insights into its durability. Further optimization of MCC composition, including variations in concentration and particle size, could help determine the most effective formulation. Comparative studies with alternative reinforcements, such as nanoparticles or natural fibers, may also provide insights into superior material enhancements. Investigating different polymerization techniques and processing conditions could improve material consistency, while surface modification techniques, such as coatings or treatments, may help reduce roughness and enhance microbial resistance.

CONCLUSIONS

Based on the findings of this study, the following conclusions can be drawn:

1. The addition of 20 µm MCC particles resulted in a reduction in surface roughness at both 2% and 5% concentrations

2. In the group with 50 µm MCC particle size, surface roughness was significantly higher in the 2% MCC group compared to the 5% MCC group, possibly due to the nonhomogeneous mixing of PMMA powder and MCC

3. The incorporation of 20 µm MCC particle size led to a significant reduction in impact strength in the 2% MCC group, while in the 5% MCC group, the impact strength was nearly equivalent to the control group but not statistically significant

4. In the group with 50 µm MCC particle size, impact strength decreased at both 2% and 5% concentrations, although the reduction was not statistically significant.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.