Microbial Adhesion to Poly Methyl Methacrylate (PMMA) Denture Base Resins Containing Zinc Oxide (ZnO) Nanostructures: A Systematic Review of In Vitro Studies

Abstract

Background: Denture stomatitis is an inflammatory condition involving swelling and redness of the oral mucosa beneath a denture. Among various available treatments, zinc oxide nanoparticles (ZnONPs) and nano-wire nanostructures have been suggested as potential future therapies. However, there is a lack of information in the literature about the effectiveness of ZnONPs regarding microbial adhesion to different denture base resins. Here, we review studies on the effect of ZnONP use on microbial adhesion to denture base resins to answer the following study question: “Does incorporating ZnONPs into denture base resins reduce microbial adhesion?” Methods: Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, an electronic and manual search ranging from Jan 2000 to May 2024 was performed using PubMed, Web of Science, and Scopus databases to answer the study question. All full-length English-language articles investigating the effects of ZnO nanostructures on Candida albicans adhesion to polymethyl methacrylate (PMMA) denture base resins were included. The extracted data were tabulated for qualitative and quantitative analysis of the included studies. Results: Of the 479 studies reviewed, 7 studies successfully met the eligibility criteria. All included studies utilized PMMA as the denture base material with different polymerization methods. C. albicans was the most extensively studied microbial species, with various count methods used. Six studies concluded a statistically significant impact of ZnONPs on decreasing C. albicans adhesion to the denture base. However, one study reported the opposite. Conclusions: Incorporating ZnONPs into PMMA denture base resin has a positive impact on reducing C. albicans adherence and could be recommended for denture stomatitis treatment. However, further studies are needed to cover the notable gap in data regarding the safety and effectiveness of ZnO nanostructures.

1. Introduction

The denture base is that part of the prosthesis that carries the artificial teeth and rests on the mucous-bone support. Typically either resin- or metal-based, the denture base should have adequate physical, mechanical, esthetic, and biocompatibility properties [1]. Polymethyl methacrylate (PMMA) is the most used and recommended material for prosthetic fabrication [1,2]. PMMA offers a good esthetic, resembling the natural gingiva, and it is lightweight for patient comfort, cost-effective, easy to fabricate or repair, and highly biocompatible. However, limitations include low strength, risking fractures if dropped or subjected to heavy forces; fluid absorption, causing bad odor, discoloration, and bacterial overgrowth; polymerization-related shrinkage, affecting retention and stability; and, finally, a lack of thermal sensitivity compared to natural oral structures [3,4,5]. Different modification and fabrication techniques are recommended to overcome the disadvantages of traditional acrylic resins. To improve strength, high-impact acrylic (PMMA with additional rubber or fibers) can be used. Incorporating nanoparticles can enhance denture strength and esthetics and reduce bulkiness [2]. New fabrication methods, such as digital fabrication using CAD/CAM (computer-aided design/computer-aided manufacturing) techniques, improve control over denture design; this can lead to enhanced fit, durability, strength, and density while reducing production time and error [1,3].

The surface properties of denture base materials affect the esthetics and success of dentures, ultimately impacting the quality of life of the patient [3,4,5]. Surface roughness is a contributing factor to stomatitis, alongside poor hygiene, medications, and poor autoimmune resistance of the patient [3]. Inflamed and red oral mucosa beneath the denture is called denture stomatitis, and it is usually painless and asymptomatic. However, mucosal bleeding, taste alteration, and a burning sensation can develop [6]. The disease is multi-factorial, and it can develop from poor fit of the denture, poor denture hygiene, and wearing the denture at night [7,8]. Candida albicans is the principal pathogen causing denture stomatitis. Treatment options include anti-fungal agents, such as nystatin or miconazole, and laser therapy [9,10]. Several measures have been suggested to reduce the occurrence of denture stomatitis, including improved fabrication of dentures, improved oral hygiene, and the removal of dentures overnight, as well as the modification of denture bases with antifungal agents [11,12].

Nanotechnology is an area of science concerned with developing and producing extremely small tools and machines through the arrangement of atoms. Nanostructures include nanoparticles (NPs) and nanorods, and they are promising agents for antimicrobial applications [13]. They are mostly considered biocompatible due to the phagocytic activity of human cells against the particles. Nanotechnology could be used to deliver biocompatible therapeutic agents while reducing the development of resistance against regular anti-fungal therapies, which is the major drawback of common antimicrobial agents [14].

The nanostructure of zinc oxide (ZnO) is known for its ability to continuously release metal ions for up to two months; these ions are capable of reducing C. albicans growth [4]. Several studies have demonstrated the anti-fungal activity of ZnO nanoparticles (ZnONPs) added to PMMA in reducing the adherence of C. albicans [15,16]. ZnONPs increase the contact angle when incorporated into acrylic resin, thus increasing the hydrophobicity by increasing the roughness and modifying the surface energy. A hydrophobic surface is beneficial in applications requiring water resistance and in reducing microbial formation and supporting self-cleaning properties [17].

Although previous studies have investigated the capabilities of ZnO nanostructures, there is still a gap regarding the implementation of these nanocomposites as denture base resins, their antimicrobial properties, and their potential for reducing denture stomatitis. In addition, no previous review has been conducted to assess the performance of denture bases containing ZnO nanostructures. There remains a lack of information in the literature concerning the effectiveness of ZnONPs on microbial adhesion to different denture base resins. This review therefore focused on the effect of ZnONPs on microbial adhesion to denture base resins to address the question “Does incorporating ZnONPs into denture base resins reduce denture stomatitis”?

2. Materials and Methods

A systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Figure 1) and structured according to the PICO method: the population consisted of PMMA denture base resins, the intervention was the addition of ZnONPs, the control was unmodified resins, and the outcome was microbial adhesion. This approach produced the following study question: “Does incorporating ZnONPs into denture base resins reduce microbial adhesion”?

To address the study question, the following keywords were used: PMMA, denture base, denture stomatitis, microbial adhesion, C. albicans adhesion and/or ZnONP, nanoparticles, nano-size, or nanostructure. The keywords were used to search the databases according to the inclusion and exclusion criteria. The inclusion criteria were in vitro studies written in English, full-length original articles, use of denture base resins, investigation of microbial adhesion, and use of ZnONPs. The exclusion criteria included documents where only the abstract was available, review articles, and studies not investigating denture base resins (such as those using soft liners).

An electronic search in databases (Web of Science and Scopus) was performed followed by manual searches for any additional articles meeting the inclusion criteria published from January 2000 to May 2024. Data extraction was conducted in three stages: (1) review of titles; (2) review of abstracts; and (3) review of full texts. Two reviewers (N.M.M. and N.S.A.) independently reviewed and analyzed the articles according to the inclusion criteria. Any discrepancies were analyzed by other reviewers (M.S.A. and M.M.G.) to resolve the issue. Data were extracted and tabulated according to items detailed in Table 1. Due to the variations in the included studies in terms of methodology, specimen shape, ZnONP content, salinization processes, testing methods, and aging procedures, it was not appropriate to conduct a meta-analysis. Therefore, the included articles were qualitatively analyzed and described.

Figure 1. PRISMA flow chart of the study selection process.

Figure 1. PRISMA flow chart of the study selection process.

Quality Assessment and Risk of Bias of Included Studies

The included studies were analyzed for quality assessment and risk of bias by two independent reviewers (N.M. and M.A.). Each article was evaluated using a risk of bias tool (the modified Consolidated Standards of Reporting Trials (CONSORT)) consisting of seven parameters and items, described in

Table 2. Quality assessment and risk of bias considering the aspects reported in the Materials and Methods sections.

Ref.	Sample Size Calculation	Sample Randomization	Control Group	Stating Clear Testing Method	Statistical Analyses Carried Out	Reliable Analytical Methods	Blinding of Evaluators	Risk of Bias
[19]	Yes	No	Yes	Yes	Yes	Yes	No	Medium
[20]	Yes	No	Yes	Yes	Yes	Yes	No	Medium
[21]	No	No	Yes	Yes	Yes	Yes	No	Medium
[22]	Yes	Yes	Yes	Yes	Yes	Yes	No	Low
[23]	Yes	No	Yes	Yes	Yes	No	No	Medium
[24]	Yes	Yes	Yes	Yes	Yes	Yes	No	Low
[25]	No	No	Yes	Yes	Yes	Yes	No	Medium

. A “yes” was assigned when the parameter was reported in the text, and a “no” if the information was absent or unclear. The risk of bias was classified according to the sum of “yes” marks received as follows: 1–3, high; 4–5, medium; 6–7, low risk of bias [18].

Table 1. Content of included studies.

Ref	Denture Base Resin Type/Brand Name	ZnO Size and Brand Name	ZnO%	ZnO Treatment	Addition Method Monomer/Polymer	Specimen Shape and Dimensions/Aging	Sample Size	Microbial Species	Tested Properties	Assessment Method	Results and Outcome
Cierech M, et al. [19]	HP (PMMA)/Superacryl Plus, Spofa Dental, Czech Republic	25–30 nm CHEMPUR, piekary śląskie Poland 25–30 nm	0%wt 2%wt	Sedimented (washed with deionized water, centrifuged, and dried)	Monomer	10 × 10 × 2 mm NS	N = 30 n = 15	C. albicans, ATCC 14055	Density MIC Roughness	CFU	ZnONPs display fungistatic or fungicidal activity and reduce C. albicans adhesion
Cierech M, et al. [20]	HP (PMMA)/Superacryl Plus Spofa Dental, Czech Republic		0%wt 2%wt	Salinization	Monomer	10 × 10 × 2 mm NS	N = 30 n = 16	C. albicans, ATCC 14056	Anti-fungal morphology of cells	CFU	Significantly decreased C. albicans adhesion
Cierech M, et al. [21]	HP (PMMA)/Superacryl Plus Spofa Dental, Jicin, Czech Republic		0% 2.5% 5% 7.5%	Nanowires are produced	Monomer	13 × 13 × 2 mm NS	N = 20 n = 5	C. albicans, ATCC 14057	Zinc ions release Cytotoxicity to human cells	Optical emission spectrometry in inductively coupled plasma	ZnONPs prevent adhesion and biofilm development by C. albican
Kamonkhantikul K, et al. [22]	HP PMMA/Ivoclar Vivadent, Schaan, Liechtenstein	20–40 nm Nano Materials Technology Co.Led. Chonburi, Thailand	0% 1.25, 2.5, 5% wt	NS	Monomer	12 × 2 mm water storage time in 37C deionized water for 48 h or 1 month before testing	N = 98 n = 8	C. albicans, ATCC 90028	Antifungal, optical, and mechanical properties	CFU	ZnONPs cause reduction in C. albicans adherence
Apip C, et al. [23]	HP PMMA (MelioDent Heat Cure, Heareus Kulzer, Hanau, Germany)	25–45 nm NS	0 ppm Study: 250, 500, 1000 ppm	NS	ZnO-NWs were suspended in liquid monomer	10 × 5 × 3 mm Water storage not mentioned	N = 80	C. albicans, ATCC 10231	Anti-biofilm activity	Transmission electron microscopy Raman mapping images and spectra	C. albicans adherence and biofilm formation considerably decreased with increasing ZnO-NWs concentrations in PMMA-ZnO-NW
Anwander M, et al. [24]	AP (Palapress vario, Heraeus Kulzer GmbH, Hanau, Germany)	<100 nm Sigma-Aldrich Co., St. Louis, MO, USA	0% 0.1, 0.2, 0.4, and 0.8 wt	NS	Monomer	7 × 1.5 mm Stored in distilled water for 7 d prior to conducting the experiments	N = 60 n = 15	C. albicans, ATCC 10232	Roughness Biofilm formation Biomass	Energy-dispersive X-ray spectroscopy (EDX)	No statistically significant impact of available ZnO material on decreasing biofilm adhesion proven
Raj I, et al. [25]	PMMA of analytical-grade quality was procured from Alfa Aeser, Haverhill, Massachusetts	60 nm Sigma Aldrich, St. Luis, MO, USA	0 1, 2, 5, 10, and 15 by wt	NS	Polymer	Film, 12 cm length, 8 cm width, and 2 mm thickness Thermal and water storage	Not mentioned 4 experiments and 5 groups	C. albicans, ATCC 10233	Cytotoxicity Crystalline and morphological changes Density and abrasion resistance	Microscopically	ZnONP groups shows decrease in C. albicans adhesion and colonization

(PMMA) Poly methyl methacrylate; (CFU) colony-forming unit; (HP) heat-polymerized; (AP) auto-polymerized; (NS) not stated.

Table 1. Content of included studies.

Ref	Denture Base Resin Type/Brand Name	ZnO Size and Brand Name	ZnO%	ZnO Treatment	Addition Method Monomer/Polymer	Specimen Shape and Dimensions/Aging	Sample Size	Microbial Species	Tested Properties	Assessment Method	Results and Outcome
Cierech M, et al. [19]	HP (PMMA)/Superacryl Plus, Spofa Dental, Czech Republic	25–30 nm CHEMPUR, piekary śląskie Poland 25–30 nm	0%wt 2%wt	Sedimented (washed with deionized water, centrifuged, and dried)	Monomer	10 × 10 × 2 mm NS	N = 30 n = 15	C. albicans, ATCC 14055	Density MIC Roughness	CFU	ZnONPs display fungistatic or fungicidal activity and reduce C. albicans adhesion
Cierech M, et al. [20]	HP (PMMA)/Superacryl Plus Spofa Dental, Czech Republic		0%wt 2%wt	Salinization	Monomer	10 × 10 × 2 mm NS	N = 30 n = 16	C. albicans, ATCC 14056	Anti-fungal morphology of cells	CFU	Significantly decreased C. albicans adhesion
Cierech M, et al. [21]	HP (PMMA)/Superacryl Plus Spofa Dental, Jicin, Czech Republic		0% 2.5% 5% 7.5%	Nanowires are produced	Monomer	13 × 13 × 2 mm NS	N = 20 n = 5	C. albicans, ATCC 14057	Zinc ions release Cytotoxicity to human cells	Optical emission spectrometry in inductively coupled plasma	ZnONPs prevent adhesion and biofilm development by C. albican
Kamonkhantikul K, et al. [22]	HP PMMA/Ivoclar Vivadent, Schaan, Liechtenstein	20–40 nm Nano Materials Technology Co.Led. Chonburi, Thailand	0% 1.25, 2.5, 5% wt	NS	Monomer	12 × 2 mm water storage time in 37C deionized water for 48 h or 1 month before testing	N = 98 n = 8	C. albicans, ATCC 90028	Antifungal, optical, and mechanical properties	CFU	ZnONPs cause reduction in C. albicans adherence
Apip C, et al. [23]	HP PMMA (MelioDent Heat Cure, Heareus Kulzer, Hanau, Germany)	25–45 nm NS	0 ppm Study: 250, 500, 1000 ppm	NS	ZnO-NWs were suspended in liquid monomer	10 × 5 × 3 mm Water storage not mentioned	N = 80	C. albicans, ATCC 10231	Anti-biofilm activity	Transmission electron microscopy Raman mapping images and spectra	C. albicans adherence and biofilm formation considerably decreased with increasing ZnO-NWs concentrations in PMMA-ZnO-NW
Anwander M, et al. [24]	AP (Palapress vario, Heraeus Kulzer GmbH, Hanau, Germany)	<100 nm Sigma-Aldrich Co., St. Louis, MO, USA	0% 0.1, 0.2, 0.4, and 0.8 wt	NS	Monomer	7 × 1.5 mm Stored in distilled water for 7 d prior to conducting the experiments	N = 60 n = 15	C. albicans, ATCC 10232	Roughness Biofilm formation Biomass	Energy-dispersive X-ray spectroscopy (EDX)	No statistically significant impact of available ZnO material on decreasing biofilm adhesion proven
Raj I, et al. [25]	PMMA of analytical-grade quality was procured from Alfa Aeser, Haverhill, Massachusetts	60 nm Sigma Aldrich, St. Luis, MO, USA	0 1, 2, 5, 10, and 15 by wt	NS	Polymer	Film, 12 cm length, 8 cm width, and 2 mm thickness Thermal and water storage	Not mentioned 4 experiments and 5 groups	C. albicans, ATCC 10233	Cytotoxicity Crystalline and morphological changes Density and abrasion resistance	Microscopically	ZnONP groups shows decrease in C. albicans adhesion and colonization

(PMMA) Poly methyl methacrylate; (CFU) colony-forming unit; (HP) heat-polymerized; (AP) auto-polymerized; (NS) not stated.

3. Results

Out of 479 studies identified, 7 [19,20,21,22,23,24,25] met the inclusion criteria and examined the effect of adding ZnO nanostructures to denture base resins on microbial adhesion, as detailed in Table 1. All studies used PMMA acrylic denture bases and employed various polymerization methods [19,20,21,22,23,24,25]. Specifically, six studies used heat-polymerized PMMA [19,20,21,22,23,24,25], while one study used auto-polymerized PMMA [24]. Six studies incorporated ZnONPs, and one study used ZnO nanorods [23]. Nanoparticle sizes ranged from 25 to 60 nm [19,20,21,22,23,25] and were not specified in one study [24].

In all included studies, ZnONP concentrations were represented as percentages [19,20,21,22,24], except for one study that used parts per million (ppm) [23]. All studies used unmodified material as a control [19,20,21,22,23,24,25]. Two studies applied a concentration of 0.2% by weight (wt) [19,20]. For experimental groups, Cierech et al. used 2.5, 5, and 7.5% by wt. [21], while Kamonkhantikul et al. tested concentrations of 1.25, 2.5, and 5% by wt. [22]. Apip et al. used 250, 500, and 1000 ppm [23], and Anwander et al. tested 0.1, 0.2, 0.4, and 0.8% by wt. [24]. Raj et al. used various concentrations: 1, 2, 5, 10, and 15% by wt. [25].

Treatment of the ZnONPs varied across studies, thus potentially impacting the effects on the denture base. In studies by Cierech et al., ZnONPs were treated through sedimentation—washing the material three times with deionized water, and then centrifuging and freeze-drying it [19,20,21]. Kamonkhantikul et al. applied salinization to the ZnONPs [22], while Apip et al. used ZnO nanowires [23]. Two studies did not mention ZnONP treatment methods [24,25,26,27,28].

In six studies, ZnONPs were added to the liquid monomer [18,19,20,21,22,24], while in Anwander et al., they were added to the powder polymer [24]. Specimen sizes also varied: Cierech et al. used 10 × 10 × 2 mm specimens [19,20], and four studies used different sizes [21,22,23,24,25]. For storage conditions, three studies stored samples in deionized water at different temperatures [20,23,24], while the remaining studies did not specify storage environments [19,20,22,25].

For microbial specimens, all included studies used a reference strain of C. albicans [19,20,21,22,23,24,25]. Three studies used C. albicans 14053 [12,13,14], three used C. albicans ATCC 10231 [23,24,25], and one used C. albicans ATCC 90028 [22].

All included studies evaluated four key factors: antifungal properties, surface roughness, density, and morphological changes of the microbes [19,20,22,23,24,25]. To examine nanopowder morphology, a scanning electron microscope (SEM) (Ultra Plus; Carl Zeiss Meditec AG, Jena, Germany) and a sputter coater (SCD 005/CEA 035, BAL-TEC, Switzerland) were used, with an InLens detector for imaging. The InLens detector allowed for the identification of surface contaminants. Density measurements were performed using a helium pycnometer (AccuPyc II 1340, Micromeritics, USA) following an in-house protocol. Surface roughness was measured using a Dektak XT stylus profiler. One study, however, focused on cytotoxicity release rather than roughness or density [21].

Various methods were used for microbial count assessment, including colony-forming units (CFU) [19,20,22], transmission electron microscopy (TEM) [23], energy-dispersive X-ray (EDX) microscopy [23], and light microscopy [25]. All studies concluded that ZnONPs enhanced antimicrobial properties against C. albicans [19,20,21,22,23,25], except one, which could not statistically demonstrate the impact of commercially available ZnO material on reducing biofilm adhesion [24].

• Quality assessment

This review consists of two articles with a low risk of bias and five with a medium risk, as illustrated in Table 2 and Figure 2. None of the studies used blinded evaluators, but all stated their methods clearly.

Different supportive findings, such as SEM and TEM [19,20,21,23,25], were used to analyze existing colonies on the specimen surface. Some showed colonies with different morphologies and a fully mature biofilm with a multilayered network of microbes and hyphae, while others showed damage to cells [23,25].

4. Discussion

ZnONPs are widely recognized for their multifunctionality, transparency, and stability, making them effective inorganic fillers [19,20,21]. When incorporated into a polymer, ZnONPs significantly enhance mechanical properties, such as tensile and impact strengths, as well as optical properties, contributing to a more esthetically favorable material [21]. Additionally, ZnONPs exhibit potent antimicrobial activity effective against a broad spectrum of microorganisms, including both Gram-negative and Gram-positive bacteria [22]. Owing to these attributes, ZnONPs have been selected as a reinforcing agent for denture base materials. Their incorporation results in improved mechanical strength, abrasion resistance, and antimicrobial properties, particularly anticandidal effects, while maintaining the biocompatibility of the denture base. These properties highlight the significant promise of ZnONPs in the development of advanced dental materials [25].

To counter C. albicans adhesion to PMMA surfaces, various approaches can be employed. These include the addition of bioactive glass, which decreases the C. albicans count at a concentration of 5% [26]. Other studies have investigated different nanoparticles and their antimicrobial effects, such as silver nanoparticles (AgNPs), titanium dioxide nanoparticles (TiONPs), and zirconium dioxide nanoparticles (ZrO₂NPs) [27,28]. Additionally, the application of topical cleaning agents or antifungal medications can help reduce biofilm formation. Various oral antifungal agents, including fluconazole, nystatin, amphotericin B, miconazole, ketoconazole, itraconazole, and clotrimazole, are recommended for treating denture stomatitis [26].

The findings of this review confirm the anti-adhesive activity of ZnO against C. albicans at a minimum inhibitory concentration of 0.75 mg/mL [19]. There was no difference in the morphology of HeLa cells treated with ZnONPs reported in a study by Cierech et al. and the structure of the cell monolayer between the control group and the cells treated with lower ZnONP concentrations (1–30 mg/L). However, in higher concentrations (50 mg), morphological changes (polygonal, flat cells turned spherical) were observed, but the structure of the monolayer remained unaffected (no gaps between adjacent cells, cells adhering to each other). Adherence was varied in cells treated with 100 mg/L of ZnONPs [21]. The use of ZnONPs reduces C. albicans adhesion, which in turn decreases the incidence of denture stomatitis [19,20,21,22,23,24,25]. In another study of a mono-species biofilm consisting of C. albicans, significantly fewer adherent cells (measured by relative absorbance values) were identified after 44 h compared to 20 h of biofilm formation in the denture [24]. Less adherence indicates a lesser risk of C. albicans infection.

ZnONPs have been examined for their effects on C. albicans adhesion. The studies included here conclude that there is an action of these nanoparticles on cell adherence to the acrylic surface. Specifically, the papers focus on the effect of different concentrations of ZnONPs incorporated into PMMA and how this influences microbial biofilm integrity and pathogen adherence [21,22,23,24,25]. Their results indicate that lower concentrations (30–50 mg/L) do not affect cell adhesion, while higher concentrations (100 mg/L) do [21]. This indicates that ZnONPs can enhance the surface properties of acrylic resins by inhibiting bacterial adherence and preventing biofilm formation at certain concentrations [21].

The antimicrobial effects of the materials can be affected by different factors, including nanoparticle concentration, shape (e.g., nanoparticles versus nanowires), and the method of introduction to the resin [24]. PMMA loaded with nanoparticle concentrations ranging from 100 to 250 ppm suggests an inverse relationship between microbial adhesion and nanoparticle concentration, as confirmed through SEM. This imaging method provides visual confirmation of the reduction of adherence, showing fewer fungal cells adhered to the surfaces of the ZnO-modified PMMA compared to controls. Furthermore, ZnO nanowires (ZnO-NWs) at high concentrations (up to 500–1000 ppm) show a role in impairing fungal adherence. Reports suggest a dose-dependent relationship, with greater reductions in adherence associated with higher concentrations of ZnO [23].

The anti-fungal effect of nanoparticles can be mediated through several mechanisms of action [29]. However, studies investigating the mechanisms of action of ZnONPs in particular are limited. However, the main reported mechanism of action excreted by ZnO on C. albicans is the release of metal ions and the formation of reactive oxidative species (ROS). The interaction of these two substances with the cell membrane leads to the inhibition of cell wall synthesis, cell signaling, enzyme activities, ribosome distribution, and DNA damage, the inactivation of protein synthesis, and molecular changes in cell proteins [15].

ZnONPs have the ability to reinforce the mechanical properties of denture bases [30]. Augmenting any material brings the advantages of the added material. However, adverse effects can also be associated with augmentation [30]. This can include brittleness in the case of high concentrations of ZnONPs. In addition, the color stability of denture bases can be influenced by nanoparticle incorporation as a result of the interaction between ZnONPs and the polymer, thus decreasing PMMA translucency [31]. The clinical use of ZnONPs in denture bases requires further study to ensure homogeneity, microbiological efficacy, mechanical strength, and biocompatibility [19]. Assessing ZnO’s potential for mucosal irritation and its long-term effects is essential [20,30,31]. Despite ZnO’s promise in preventing denture stomatitis, clinical trials are required to validate its safety and effectiveness [19,20,23,24].

The effect of ZnONPs on denture base wettability is often measured based on the contact angle. This angle affects how easily microbes can adhere to and spread across a surface. When ZnONPs are added to a PMMA, they often increase the surface roughness and alter the material by increasing the hydrophobicity through a higher contact angle. A higher contact angle therefore means a reduction in microbial adhesion and a surface less prone to retaining the moisture that helps plaque accumulation. Higher surface roughness tends to trap air between the surface and moisture, which increases the contact angle and reduces the area in contact with water (and, by extension, microorganisms). ZnO nanorods embedded in materials can achieve superhydrophobicity, with contact angles exceeding 160°, leading to reduced microbial adhesion and overgrowth [19,24,32].

The number of studies addressing this topic is limited, making it hard to write a high-quality comprehensive review. The quality of the review was also limited by two of the included publications having a high risk of bias. There is a lack of data regarding the long-term impact of ZnONPs on pathogens and human health. No clinical studies have been conducted to give more reliable information. This study focused only on the addition of ZnONPs to PMMA; all included studies were in vitro, with no clinical trials. Further studies are therefore needed to investigate ZnONP addition in different dental materials and the long-term related effects, and in vivo studies are recommended.

5. Conclusions

This review suggests that adding ZnONPs to denture base resins can help reduce denture stomatitis. The concentration and even distribution of ZnONPs are essential for effectiveness due to their impact on antimicrobial properties. However, all of the reviewed studies were conducted in vitro, with no clinical trials available. Future research, especially using clinical trials, is needed to confirm the effectiveness and safety of ZnONPs in real-world use with different dental materials.