Introduction

Mechanical and physical properties of denture base materials play a significant role in their clinical performance and longevity. Polymethyl methacrylate (PMMA), widely regarded as the gold standard for denture base fabrication, is associated with advantages, such as ease of processing, aesthetics, and biocompatibility [1]. However, PMMA exhibits inherent limitations, including low impact strength and fracture resistance, which compromise its long-term durability [2, 3].

In recent years, the incorporation of nanoparticles into PMMA has emerged as a promising strategy to enhance its mechanical and physical properties. Among various reinforcements, alumina (Al₂O₃) nanoparticles have gained significant attention due to their excellent mechanical qualities, biocompatibility, and ability to improve the structural integrity of denture base material [4, 5]. These nanoparticles have the potential to enhance properties, such as flexural strength, impact resistance, surface hardness, and dimensional stability [6, 7].

While numerous individual studies have explored the reinforcement of PMMA with alumina nanoparticles, researchers often find the results inconsistent or lacking generalizability. Therefore, this systematic review aimed to synthesize available evidence, evaluate the effect of alumina nanoparticle reinforcement on the physical and mechanical properties of denture base materials, and provide insights into the clinical relevance of these modifications.

Material and methods

Framework and registration

This systematic review and meta-analysis adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, and was structured around a well-defined research question to guide the systematic search strategy. The PICO question employed was whether reinforcing PMMA denture base materials with alumina (Al₂O₃) nanoparticles enhances their physical and mechanical properties compared to unreinforced PMMA. The primary outcomes assessed included flexural strength, impact resistance, surface hardness, and dimensional stability. The review protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO), under registration No. CRD420250655919.

Search strategy and eligibility criteria

A comprehensive literature search was conducted using PubMed, Cochrane Library, Trip, and Google Scholar databases. The search covered studies published from inception to December 31, 2024. Key words, such as “denture base materials,” “PMMA,” “alumina nanoparticles,” “mechanical properties,” “physical properties,” “flexural strength,” “surface hardness,” and “impact resistance” were utilized. Boolean operators, including “AND” and “OR” were applied to refine the search strategy and maximize the retrieval of relevant studies. The detailed search strategies for each database are presented in Table 1.

Inclusion criteria: (1) studies evaluating PMMA denture base materials reinforced with alumina nanoparticles; (2) studies reporting measurable outcomes for physical and mechanical properties (e.g., flexural strength, impact resistance, surface hardness); (3) randomized controlled trials, in vitro studies, or comparative observational studies; (4) articles published in English.

Exclusion criteria: (1) studies using composite reinforcements other than alumina nanoparticles; (2) studies with incomplete methodological details or insufficient quantitative data; (3) review articles, case reports, editorials, and letters to the editor.

Study selection and data extraction

Two independent reviewers conducted the study selection process, starting with title and abstract screening, followed by a full-text review of potentially eligible articles. The PRISMA flow diagram in Figure 1 outlines the selection process. Initial search yielded 387 articles, and after removing duplicates, 373 remained. Title and abstract screening excluded irrelevant studies, leaving 17 records for full-text review, of which 11 met the inclusion criteria.

Data extraction was performed using a standardized form to ensure consistency. Collected data included study characteristics (author/s, year, design, and location), sample details (denture base material type, sample size, and alumina nanoparticle concentration), intervention methods (nanoparticle incorporation and testing procedures), and assessed outcomes (physical and mechanical properties, such as flexural strength, impact resistance, and surface hardness), along with relevant statistical data. Extracted data were cross-verified for accuracy, with discrepancies resolved through consensus or arbitration by a third reviewer. A master spreadsheet was compiled for subsequent analysis.

Quality assessment

Methodological quality of the included studies was assessed using the modified CONSORT checklist for in vitro studies, and results were graphically represented using Review Manager (RevMan) statistical software version 5.4.

Statistical analysis

Meta-analysis was performed using RevMan version 5.4 (the Cochrane Collaboration, 2020). Random-effects models were applied to account for heterogeneity among the studies. Statistical heterogeneity was quantified using the I² statistic, with values above 50% indicating substantial heterogeneity. Effect sizes were calculated as standard mean differences (SMD) with 95% confidence intervals (CIs).

Results

Study characteristics

The selected in vitro studies, published from inception to December 31, 2024, investigated the mechanical, thermal, and tribological properties of PMMA and acrylic composites reinforced with alumina-based nanoparticles. Various reinforcement methods, including surface modifications, different particle sizes, and varying nanoparticle concentrations were employed. Experimental methods used in these studies included mechanical testing (tensile strength, flexural strength, impact resistance, hardness), thermal analysis (glass transition temperature, thermal conductivity, heat capacity), tribological evaluation (wear rate, coefficient of friction), and morphological analysis with scanning electron microscopy (SEM). Each study utilized one or more of these techniques to assess material performance. Control groups consisted of unreinforced PMMA or alternative polymer composites, allowing for a comparative analysis of reinforcement effectiveness. The fundamental characteristics of the included studies [8-18] are summarized in Table 2.

Assessment of methodological quality

The methodological quality of in vitro studies was assessed using the modified CONSORT checklist in RevMan, evaluating structured abstracts, scientific background, objectives, intervention details, and outcomes assessment. As shown in Figures 2A and 2B, most stu­dies had a low risk of bias in structured abstracts, scienti­fic backgrounds, objectives, interventions, and outcomes assessments. However, high risks were noted in rando­mization sequence generation, allocation concealment, implementation, and blinding (80-100% of studies). Sample size reporting was inconsistent, with nearly 50% showing a high risk of bias. While statistical methods and outcome estimations were generally robust, funding sources and protocol transparency were major concerns, with nearly 100% showing a high risk. These findings highlighted the necessity for improved methodological accuracy, standardized reporting, and enhanced reproducibility in future studies evaluating alumina-reinforced PMMA denture base materials.

Meta-analysis

On pooling the in vitro studies, substantial heterogeneity was observed (c² = 48.62, df = 10, p < 0.001, I² = 94%), hence a random-effects model was employed for the analysis. A significant improvement in mechanical properties was noted in the experimental group (Al₂O₃-reinforced PMMA) compared to the control (unmodified PMMA), with SMD = 3.33, 95% CI: 0.85-5.81, and p = 0.009, as illustrated in Figure 3. This high heterogeneity suggested considerable variability among the studies, likely due to differences in sample preparation, testing methodologies, and alumina concentrations.

Discussion

PMMA remains the most widely used denture base material due to its favorable aesthetic properties and ease of processing. However, its inherent limitations, such as low flexural strength, impact resistance, and surface hardness, necessitate modifications to improve its mechanical performance. Over the years, various micro and nanoscale fillers have been tested as reinforcement agents to enhance the physical properties of PMMA. This systematic review primarily focused on evaluating the effect of alumina (Al₂O₃) nanoparticles on the mechanical and physical properties of PMMA denture base materials. While previous reviews have explored different reinforcing agents, no comprehensive systematic review assessed exclusively the role of Al₂O₃ nanoparticles across multiple mechanical parameters.

Nano-Al₂O₃, commonly known as alumina, is widely utilized in both industrial and dental applications. One of its key characteristics is its strong interatomic bonds, which exist in various crystalline forms, particularly in the hexagonal alpha phase. In this phase, alumina exhibits exceptional strength, hardness, thermal stability, and high dielectric properties, making it suitable for numerous dental applications, including reinforcing denture bases and enhancing resin repair materials [19-21]. However, at high concentrations, nano-Al₂O₃ can lead to discoloration and increased opacity in PMMA [22].

The included studies demonstrated a consistent improvement in flexural strength, impact resistance, and surface hardness with Al₂O₃ reinforcement, confirming its potential as an effective reinforcing agent. The meta-analysis, which specifically examined flexural strength, revealed a statistically significant increase in Al₂O₃-reinforced PMMA compared to unmodified PMMA. However, substantial heterogeneity suggests that differences in nanoparticle concentration, dispersion techniques, polymerization methods, and testing protocols contributed to variability in the results. Opti­mizing these factors is crucial for achieving a balance between mechanical enhancement and maintaining structural integrity of PMMA.

One of the key findings of this review is that nanoparticle dispersion and concentration play a critical role in determining the effectiveness of reinforcement. While moderate concentrations (e.g., 2-5 wt.%) improved flexural strength, higher concentrations led to agglomeration, reducing the overall performance of the material. This highlights the importance of precise control over nanoparticle incorporation, as agglomerated particles can act as stress concentrators, resulting in premature failure under mechanical loads [11]. A similar trend was observed in studies evaluating the impact resistance and surface hardness, where proper nanoparticle distribution led to increased toughness and durability [18, 23, 24].

Several studies have demonstrated that the interfacial bonding between Al₂O₃ nanoparticles and the PMMA matrix is critical for enhancing mechanical properties. For instance, research indicates that using a silane coupling agent to modify the surface of nano-Al₂O₃ powder, augments the interface compatibility between Al₂O₃ and polyimide (PI), leading to improved compressive strength and thermal stability of PI/Al₂O₃ composites [25]. Similarly, the application of silane coupling agents has been shown to improve the dispersion of alumina nanoparticles within polymer matrices, resulting in enhanced mechanical properties [26].

Additionally, the size, shape, and orientation of nanoparticles significantly influenced the reinforcement effect in polymer nanocomposites. Studies have found that smaller nanoparticles with a high specific surface area increase interfacial interactions, indicating improved mechanical properties [27]. Moreover, the orientation of two-dimensional nanoparticles affects the mechanical properties of nanocomposites, with proper alignment in enhanced performance [28].

While alumina reinforcement has demonstrated significant improvements, it is essential to analyze these findings by comparing them with other materials incorporated into PMMA to improve its performance. Zirconia (ZrO₂) nanoparticles are one of the extensively studied fillers for their ability to enhance fracture toughness, flexural strength, and wear resistance of denture base materials. Silica (SiO₂) nanoparticles are also often incorporated to improve surface hardness and resistance to wear, but may reduce translucency at higher concentrations [7, 19]. Titanium dioxide (TiO₂) nanoparticles have shown promise in increasing flexural strength and imparting antimicrobial properties, which could benefit denture hygiene. Carbon nanotubes, though less commonly studied in dentistry because of concerns regarding biocompatibility, offer exceptional reinforcement potential due to their high tensile strength and elastic modulus. Similarly, glass fibers have been shown to significantly improve impact resistance and overall toughness of PMMA denture bases [29, 30]. These comparisons emphasize that each filler material contributes unique advantages, and alumina stands out particularly for its balance of mechanical reinforcement, biocompatibility, and availability. However, the optimal choice of reinforcement may depend on the clinical requirement, whether the priority is strength, toughness, aesthetics, or antimicrobial activity.

Despite these promising results, methodological limi­tations were evident in the included studies. The modified CONSORT checklist assessment revealed a high risk of bias in areas, such as randomization, allocation concealment, blinding, and sample size justification. Many studies lacked proper sample power calculations, raising concerns about the reliability of the reported results. Additionally, inconsistencies in polymerization techniques, testing standards, and outcomes reporting, further contributed to variability in the findings. Future studies should adhere to standardized testing protocols, including ISO guidelines for denture base materials, to improve reproducibility and ensure clinically relevant outcomes.

While in vitro findings suggest that Al₂O₃ reinforcement enhances PMMA’s mechanical properties, the clinical implications remain uncertain. Factors, such as oral environment conditions, thermal cycling, and long-term fatigue resistance are critical considerations, which require further investigation. Additionally, evaluating biocompatibility, wear resistance, and long-term performance under simulated clinical conditions would provide valuable insights into the real-world applicability of Al₂O₃-reinforced PMMA.

This systematic review underscores the potential of Al₂O₃ nanoparticles as a viable reinforcement for PMMA denture base materials, improving flexural strength, impact resistance, and surface hardness. However, variability in research methodologies and lack of standardized nanoparticle incorporation techniques highlight the need for future high quality studies. Optimizing nanoparticle concentration, dispersion, and surface treatment will be crucial for achieving consistent and reproducible improvements. Further clinical trials and long-term in vivo studies are necessary to validate these findings, and to establish the practical applicability of Al₂O₃-reinforced PMMA in prosthodontics.

Conclusions

The findings of this systematic review and meta-analysis provide valuable insights into the reinforcement of PMMA denture base materials using Al₂O₃ nanoparticles. The included studies indicate that nanoparticle incorporation influences material behavior, with variations in research methodologies contributing to differences in the reported outcomes. While the improvements in mechanical properties are evident, the lack of standardized protocols limits the ability to draw definitive conclusions. Addressing inconsistencies in sample preparation, polymerization techniques, and material characterization is essential for future research. Moreover, exploring the long-term stability, biocompatibility, and wear resistance of Al₂O₃-reinforced PMMA under simulated oral conditions will help determine its clinical applicability. Also, establishing clear guidelines for nanoparticle integration in dental materials will be crucial in translating these laboratory findings into practical applications in prosthodontics.

Disclosures

Author contributions: Conceptualization: R.K.A., R.K.B.; Methodology: D.B.V., R.K.B., R.K.A.; Questionnaire adaptation: Not applicable; Investigation and data collection: D.B.V., S.K.K., S.M., A.K.; Formal analysis: D.B.V., R.K.A., S.M.; Data curation: D.B.V., S.K.K., A.K.; Writing – original draft: D.B.V., S.K.K.; Writing – review and editing: S.M., A.K., R.K.B., R.K.A.; Supervision: R.K.A., R.K.B.; Project administration – R.K.A., R.K.B., D.B.V. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board statement: Not applicable.

Informed consent statement: Not applicable.

Data availability statement: The data presented in this study are available within the article. Additional information is available from the corresponding author and can be provided upon reasonable request.

Acknowledgments: None.

Conflicts of interest: The authors declare no conflicts of interest.

AI use statement: No artificial intelligence tools were used in the preparation of this manuscript.

References