Microplastics and nanoplastics in clinical dentistry and orthodontics: leaching, health implications, and future directions: a narrative review

Abstract

This narrative review critically summarizes that microplastics and nanoplastics have been found in many different environments, including water and food, raising concerns on their possible harm to human health. Previous research indicates that microplastics may cause inflammation and tissue damage; however, the full extent of their health risks remains uncertain. Given the long-term use of plastic-based orthodontic appliances such as aligners, retainers, and widespread usage of adhesives, the potential release of microplastics and nanoplastics during routine wear and mechanical stress warrants thorough investigation to ensure patient safety and long-term biocompatibility. The literature search conducted for this review was structured but non-systematic, with no formal risk-of-bias evaluation. This review aimed to critically evaluate the impact of microplastics and nanoplastics on human health, with a focus on their relevance to orthodontics. The review also aimed to identify possible gaps in current research, particularly regarding the quantification of microplastic leakage from orthodontic appliances and their possible long-term effects. Current evidence highlights a clear need for more targeted research to inform and improve safety standards regarding microplastics and plastic usage in orthodontic and dental practice.

Introduction

A growing body of research highlights the potential health risks posed by microplastics (MPs) on the human body [1–6]. MPs are defined as “synthetic, high-molecular-weight polymeric compounds that have been micronized into plastic particles smaller than 5 mm in size” [1]. MPs can be further categorised into primary microplastics, which are intentionally produced, and secondary microplastics, which arise from the degradation of larger plastic products due to biological, chemical, or environmental factors or influences. Primary MPs tend to be more uniform in size and shape than secondary MPs. MPs are also categorised by type, such as fibres, fragments, beads, and films, and by size: less than 0.1 mm, between 0.1 and 0.3 mm, and greater than 0.3 mm. While there is no global consensus on the precise definitions, most studies refer to MPs as particles smaller than 5 millimetres and nanoplastics (NPs) as particles under 1 micrometre. As previously noted, there are currently no established regulatory limits for microplastic or nanoplastic exposure [2].

Plastics are widely used in dentistry for their durability, aesthetics, and ease of fabrication. Common dental materials such as acrylic resins, polycarbonate, and polyurethane-based compounds are frequently employed in appliances like retainers, mouthguards, and removable prostheses. Vacuum-formed retainers and similar orthodontic devices are typically fabricated using thermoplastic sheets such as polyethylene terephthalate glycol (PETG) or polypropylene. These materials are heated and moulded to fit dental arches precisely [6–8]. Orthodontic aligners, now a popular alternative to fixed appliances, are also composed of advanced thermoplastic polymers. They are designed to be worn almost continuously and replaced every 1 to 2 weeks. While their chemical composition varies slightly across brands, the core components usually include polyurethane-based materials or multilayer copolymers. Despite their clinical benefits, concerns have emerged regarding the potential leaching of plastic constituents, including MPs and NPs (MNPs), during prolonged intraoral use [9–11].

Limited studies [9–11] have focused on the leaching of MNPs from orthodontic appliances in the oral environment and their potential effects on health. This review aims to analyse the existing literature concerning the potential health impacts of MNPs, average human consumption, release of MPs and NPs from a wide range of dental materials and orthodontic appliances, especially orthodontic aligners, alongside methods used to study leaching processes involved to quantitatively or qualitatively assess these micro- and nano-sized particles (MNPs).

Literature retrieval and selection

The primary literature search for this narrative review was conducted in PubMed using the following expanded search strategy:

(“microplastic*“[All Fields] OR “nanoplastic*“[All Fields] OR “plastic particles“[All Fields] OR “plastic pollution“[All Fields]) AND (“dent*“[TW] OR “orthodontic*“[TW] OR “dental material*“[All Fields])

This initial search retrieved 53 records. All titles and abstracts were manually screened for relevance and content. Articles meeting the following inclusion criteria were selected for full-text review: The inclusion criteria were as follows: articles published after 1966, in any language and of any type; studies involving human participants, animals, or in vitro models; research investigating the release, degradation, wear, or leaching of microplastics or nanoplastics from dental or orthodontic materials; and studies providing experimental data, analytical methods, or simulated oral-environment analyses relevant to microplastic or nanoplastic characterization.

Studies unrelated to dentistry or orthodontics were excluded from the review, including studies examining environmental or ecological aspects of microplastics, such as degradation in soil, water, or by microorganisms (e.g., role of earthworms or marine species), as well as those addressing water and waste management, environmental monitoring, or plastic pollution in non-clinical settings.

To ensure comprehensive coverage, supplementary searches were conducted in Google Scholar and general Google Search, using similar terms with an emphasis on orthodontic appliances, aligner materials, and polymer-based dental devices. This approach was suggested following our preliminary review meeting and allowed the identification of additional relevant literature not indexed in PubMed.

In total, 11 studies were selected for detailed review and synthesis. These included both laboratory-based and review articles that investigated MNP release from orthodontic devices (e.g., clear aligners and retainers) and other dental materials, as well as studies employing simulated oral-environment experiments, such as mechanical wear, cyclic loading, and degradation testing. No PRISMA diagram or formal risk-of-bias assessment was conducted. The selection of articles emphasized thematic relevance and conceptual depth.

Potential effect of microplastics on human health

Plastics can enter the human body through three primary routes: inhalation, ingestion, and dermal contact (Fig. 1) [12]. A recent review by Blackburn and Green [3] addressed the limitations of definitive evidence linking MPs to human health, but suggested potential adverse effects based on correlative human studies, as well as animal and cell culture experiments [3].

Fig. 1.

Diagram showing the methods of uptake and mobilisation of microplastics in the human body

The potential health impacts may vary depending on the size, shape, type, and concentration of MPs. Sharp MPs may cause direct physical damage to the human body, leading to inflammation of the gastrointestinal tract and an imbalance of the intestinal microbiome. This could result in various unfavourable gastrointestinal symptoms, which the individual may experience (Fig. 2). Chemicals used in synthesising plastic polymers may act as endocrine disruptors, causing reproductive system disorders. Exposure to airborne MPs is associated with respiratory diseases and may cause oxidative stress in the lungs. Furthermore, an association between NPs and mitochondrial damage in respiratory cells has been suggested [1] (Fig. 2).

Fig. 2.

Diagram showing the effects of microplastics on the environment and different parts of the human body

In vitro experiments with human cells and in vivo studies with mice have both shown potential adverse effects, including inflammation, disturbances in lipid metabolism, and neurotoxicity [1]. In addition to their intrinsic toxicity, MPs may act as carriers for other toxic substances, such as organic pollutants or bacteria, which could further harm the body. Moreover, because of their small size, NPs can penetrate the capillaries of the human body and enter the bloodstream, potentially causing various side effects [1]. However, more research is needed to elucidate the cellular and molecular mechanisms underlying the toxicity of MNPs and to establish a threshold exposure level for harm to human health.

A common plasticiser called phthalates, which are more likely to be released due to their lack of bonding to polymers, is associated with asthma and allergies [3]. Bisphenol A(BPA) is a chemical compound that is used primarily in the production of polycarbonate plastics, typically used in orthodontic applications. BPA exposure is nearly universal and may impair reproductive function in both men and women, affecting hormone balance, semen quality, and fertility outcomes. Although epidemiological evidence on pregnancy outcomes is limited, its widespread use and mechanistic data justify reducing BPA exposure, especially during pregnancy [3]. Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardants used in plastic products. These have been shown to accumulate in humans and wildlife. However, their impact is unknown, although some studies have suggested neurological and endocrine effects [3, 4]. Interestingly, MNPs could be carriers of such compounds and other potentially harmful organic pollutants and heavy metals [3].

When it comes to physically inhaling MPs, this also has adverse effects on respiratory health, such as inflammation and lesions, especially from synthetic fibres within these particles. Research suggests that MPs can accumulate in the lungs and cause irritation. Some studies regarding the micro-leakage of MPs from prostheses found in the body have shown minimal amounts of micro-leakage and possible inflammatory responses. One such study had found a promoting effect on osteoclasts [4].

MPs typically have a hydrophobic surface, which is ideal for biofilm formation and can create a ‘plastisphere’. Although some biofilms may degrade plastic, there is little evidence of this happening in natural environments. It is also notable that MPs can have high counts of antibiotic-resistant bacteria and may also contain pesticides and fungicides that could alter the gastrointestinal tract microbiome when ingested [3]. Many such studies that have been performed are lab-based and not reflective of the real world [3].

A recent autopsy study found MNPs in atheromatous plaques, suggesting a potential link between plastic particle pollution and atherosclerosis [5]. The authors proposed that these particles may contribute to inflammatory processes within the vascular system, stimulating immune cells (particularly macrophages), which can exacerbate plaque formation and increase the risk of rupture. This finding highlights a possible link between MP exposure and an elevated risk of myocardial infarction and stroke. The study also points to the potential mechanisms of harm, where these particles can penetrate biological barriers, circulate in the bloodstream, and lodge in tissues, triggering oxidative stress and inflammation.

A study at the University of Rochester School of Medicine and Dentistry, New York, US, on younger boys, measured the anogenital distance (ADG) of those born to women exposed to everyday levels of phthalates during pregnancy [13]. This research has identified the phthalate’s ability to mimic the female hormone oestrogen, which can disrupt the development of ‘baby boys’ [13]. This study revealed that high concentrations of four out of the nine phthalate metabolites found in the urine of pregnant women were associated with shorter AGD in their sons. None of the boys exhibited abnormal genital development, but those born to mothers with the highest phthalate exposure were significantly more likely to have shorter AGDs compared to those with lower exposure.

A recent review had identified possible hazards in relation to the polymer compositions of MPs [6]. Authors have identified that harmful effects resulting from MNP leakage may be occurring due to the polymers within these plastic particles, rather than the plastic as a whole itself. They found that polyethylene (PE) polymers can cause drowsiness, and polyamide (PA) polymers may have negative effects on the respiratory system, skin and eyes [6]. A very recent study [7] found that environmental MNPs, predominantly PE, have been identified in human kidney, liver, and brain tissues, with increasing concentrations over time. Brain tissues exhibit higher MNP burdens, characterized by nanoscale shard-like fragments. Enhanced MNP accumulation in dementia cases suggests potential neurovascular and immunological implications warranting further mechanistic investigation.

Average consumption of microplastics in humans

A literature review of 26 studies to assess human MPs consumption in foods, beverages, and air evaluated MP structures and chemical identification methods and categorised intake by gender and age (male/female, children and adults) [14].

The study estimated ingestion of approximately 41,000 MPs per year for male children, 52,000 for male adults, 39,000 for female children, and 46,000 for female adults. Daily inhalation added roughly 110, 170, 97, and 132 MPs respectively, corresponding to annual inhalation totals of about 40,000, 62,000, 35,000, and 48,000 MPs, respectively. Overall, combined annual exposure was estimated at 81,000 for male children, 121,000 for male adults, 74,000 for female children, and 98,000 for female adults, with male adults experiencing the highest exposure. Inhalation contributed most to MP intake, followed by bottled water and seafood.

The average daily intake of particles from drinking water varies widely depending on the source. Studies have shown significant variability in MNP content in both bottled and tap water, with average across demographic groups showing annual MP intake from drinking water is around 90,000 from bottled water and 4000 if only tap water is consumed, representing more than a 20-fold difference.

These estimates are constrained by the limited number of available studies, variability in reported concentrations, and differences in the methods used to identify and quantify MNPs, which may lead to under- or overestimation of exposure. In addition, key dietary groups such as meat, grains, and vegetables were excluded due to insufficient data, meaning that total intake may be underestimated, and the results should be interpreted with caution.

While inhalation and ingestion are the primary pathways for MP exposure in humans, dermal contact may also contribute, though it is less studied. Most research focuses on specific contexts and environments, making it difficult to estimate total global exposure [14]. These data describe environmental background exposure and should not be directly extrapolated to orthodontic settings.

Microplastics and nanoplastics leakage from dental materials

A few studies have explored micro-leakage from dental-related products and appliances, and even fewer have focused on orthodontic appliances specifically.

A recent study in India examined MP leakage from oral health products [6]. These products included common toothbrushes, toothpaste, mouthwash, and floss. Five different brands were tested, and ninety samples were examined.

Out of 4042 floating particles, 1200 were confirmed as MPs. Toothbrushes leached the most MPs (30–120 per brush), followed by toothpaste, while mouth rinses had the lowest emission. The most common MPs were colourless fragments smaller than 0.1 mm, although toothpaste, tooth-powder, and mouthwash showed the greatest variety. FTIR-ATR revealed PE as the dominant polymer.

A further study had explored MNP leakage in Another study explored MNP leakage from resin-based composites (RBC). It also considered their impact on environmental pollution through the procedures in dentistry [15].

The results in this study showed that the direct RBC commercial and control materials were in the MP particle size range from 6.4 to 10 μm. It was also found that increased nanofillers in the composite also lead to a smaller particle size, but their aggregation leads mainly to MPs. There was also evidence of surface change over time with FTIR, suggesting the biofilm colonisation or elution of monomers.

One study emphasised the increasing concern of MPs in different industries and the harmful impact these could have on the environment and the human body [12]. The authors emphasised that when dental materials, such as composites, are drilled and exposed into the mouth, these chemicals could leach into the body by inhalation, ingestion, or infiltration into the dental pulp. Debonding of orthodontic fixed appliances may also expose these resins in the oral environment [16].

Triethylene glycol dimethacrylate (TEGDMA) is also a monomer that can be used to formulate plastics. This monomer is released from dental adhesives. Uptake of TEGDMA in the body may result in adverse DNA effects and oestrogen mimicking [15].

Microplastics and nanoplastics leakage from orthodontic appliances

Appliances such as elastomeric ligatures and chains, aesthetic brackets, and lip bumpers are composed of different polymers, including polyurethanes, polycarbonates, and polyamides [9]. These said polymers have the potential to leach MNPs into the oral environment, which may subsequently enter the body through various pathways.

Adhesives used in the bonding of orthodontic appliances, such as brackets and retainers, may also release MNPs. One study aimed at the detection and identification of microplastics in four commonly used light-cure orthodontic adhesives [9]. These light-cure adhesives included: Transbond XT (3 M Unitek, Monrovia, CA), Ormco Enlight (Ormco, Orange, CA), Orthofix SPA (Orthofix, Verona, Italy), and Aqualine LC (Tomy International Inc., Tokyo, Japan).

Using SEM, the researchers identified a range of MPs, with fibres, fragments, and pellets being the most predominant types. Each pack of dental adhesives contained fewer than 150 MPs on average, with the highest concentration of MPs detected in the M3-Aqualine LC adhesive. While the study did not investigate the potential release of MPs during the functional use of these adhesives in the oral environment, the presence of MPs in the adhesives suggests a possibility that some may leach out over time.

In orthodontics, rubber bands and elastomeric ligatures are commonly used to correct misaligned teeth. A recent study investigates the release of MNPs from new and used rubber bands during treatment [10]. SEM imaging revealed that new orthodontic elastomeric ligatures have smooth surfaces with some small pores due to the nature of the material. In contrast, used rubber bands exhibit significant changes in surface morphology, becoming rougher and more porous. Release of MNPs was also observed. These findings are likely due to the biting, friction, and etching from saliva, which may generate and release these particles or debris directly into the digestive system.

Raman spectroscopy was also used for analysis in this study, and confirmed that the particles released consist mainly of rubber MPs. The study estimated that approximately 5.2 million microplastic particles could be released from each rubber band per day, based on the surface area measurements of the tested rubber bands. This is a model estimate under laboratory conditions. However, this raises concerns about the potential ingestion or inhalation of these particles into the body. This estimate is based on surface area measurements and does not account for the exact conditions in the mouth during treatment.

Leaching of microplastics from orthodontic aligners

Aligners, increasingly popular as an alternative to traditional braces, are primarily made from thermoplastic polymers. These devices are intended for long-term wear and are typically changed out every 1 to 2 weeks. Although different manufacturers may use slightly different components, most aligners are constructed from polyurethane derivatives or layered copolymer materials [14, 17, 18].

A recent study [11] showed evidence of secondary MPs detachment from orthodontic aligners. This study aimed to assess the potential release of MPs due to mechanical friction in seven different orthodontic clear aligners. These aligners included: Alleo, FlexiLigner, F22 Aligner, Invisalign^®, Lineo, Arc Angel, and Ortobel Aligner [11].

The study was conducted over a 7-day protocol in artificial saliva. SEM and Raman Microspectroscopy were used to measure the number, size, and shape of the detected MPs (Fig. 3, modified from data acquired from Quinzi et al. [11])

Fig. 3.

Manufacturer, mean number, smallest and largest size of the MPs detected in three replicates of the following aligners: Alleo, FlexiLigner, F22 Aligner, Invisalign^®, Lineo, Arc Angel, and Ortobel Aligner (data acquired from Quinzi et al. [11])

Invisalign^® showed the lowest average number of MPs, while Arc Angel aligners showed the highest number of MP detachments. No significant difference was found among groups related to the size of the MPs (p > 0.05); however, the smallest MPs detected were found in both Invisalign and Alleo, with measurements of 3.1 μm and 3.4 μm, respectively. Smaller size MPs (< 5 μm) could cross membranes and gut epithelium, which may potentially cause more harm than larger MPs (≥ 20 μm), which would likely be excreted from the gastrointestinal tract. Some of the representative MPs from each group were also examined under a light microscope (100x magnification) and reported that almost all cases showed MPs as irregular fragments with different shapes and sizes.

Since the study was conducted under in vitro simulated conditions, the leaching protocol may not accurately represent the actual degradation of clear aligners in the oral environment, where factors such as enzymatic activity, mechanical abrasion, chemical attrition, and temperature fluctuations play a role. Given that clear aligners are worn for 20–22 h daily, continuous frictional contact between the occlusal surfaces further contributes to material wear. Although the in vitro model could not fully replicate these dynamic oral conditions, the authors acknowledge that their findings likely underestimate the extent of MNP detachment that occurs in vivo.

This study concludes that clear aligners used for a short duration are generally considered safe; however, further research is essential to evaluate new biopolymer materials for clear aligners and determine the most effective wearing protocols [10, 19].

Oral parafunctional habits, such as bruxism, can also impact the leaching of MPs. Aligners are often used for patients with bruxism to minimise tooth attrition. However, the grinding and clenching of teeth with high bite forces for extended periods on these plastic aligners may at least in principle, trigger MPs release within the oral environment [11].

A recent in vitro study using Invisalign aligners demonstrated that mechanical friction significantly increases MP release. The aligners were immersed in artificial saliva and subjected to friction in a linear reciprocating tribometer for varying durations for 7 days (3, 5, and 7 h). Samples were then analysed with Raman microspectroscopy (RMS) and scanning electron microscopy (SEM) to characterise particle morphology and size. These analytical methods are comprehensive, offering high spatial resolution while also distinguishing between different chemical structures. Results showed that friction markedly increased MP release compared with controls, which exhibited no detectable MPs. After 3 h of friction, only a small number of MPs were detected, whereas samples exposed to 5 and 7 h showed the highest release, with particles as small as 5 μm. This study highlights how mechanical friction increases MP release, a finding particularly relevant for orthodontic aligners given their prolonged wear in the oral cavity [20].

A study in 2025 aimed to simulate the mechanical stresses that clear aligners undergo and to assess microplastic (MP) release. Three brands were tested: Essix Ace, Ghost Aligner, and Invisalign. Aligners were exposed to 22,500 cycles of compressive loading over 15 days. The samples were analysed using microscopy. Results showed that all three brands released MPs during loading: Essix Ace and Ghost Aligner released larger MPs, generally greater than 20 μm, whereas the MPs released from Invisalign were smaller than 20 μm. However, there are notable limitations, including the complexity of simulating oral conditions such as body temperature. Another significant limitation is the use of microscopy, which does not chemically characterise MPs and can lead to inconclusive results [21]. The variations in number of MPs amongst different studies may be a result of variations in testing conditions. There is a need for standardized testing conditions (medium composition, temperature, friction cycles, filtration control).

Furthermore, recent review articles have also highlighted the degradation of aligner materials and have shown release of MNPs with surface topography analysis demonstrating an increase in toughness and microcrack formations [22, 23]. A recent systematic review investigated the potential health concerns associated with microplastic release from clear aligners, though the clinical consequences of such exposure remain unclear. The review primarily included in vitro studies examining intraoral aging of clear aligners made from commonly used thermoplastic polymers, such as PET, PETG, and TPU. These studies consistently found that the surface roughness of aligners increases gradually over time, with morphological and topographical changes occurring after intraoral use. The review demonstrated that clear aligners can release microplastic particles during routine wear, typically in the size range of 5–20 μm [24]. Most studies focused on the mechanical properties and surface morphology of the materials, highlighting how aging, mechanical wear, and exposure to the oral environment contribute to polymer degradation. This degradation, in turn, facilitates the dispersion of microplastics into saliva, raising potential concerns about ingestion or systemic exposure. A 2024 systematic review highlights concern over clear plastic orthodontic aligners, particularly TPU and PET-G, which can release microplastics. These particles may have direct and systemic health effects, while improper disposal contributes to environmental pollution [25]. Aging and wear increase aligner roughness and microfractures, accelerating microplastic release, influenced by friction and temperature. The study emphasizes the need for standardized methods to quantify microplastic release and assess long-term health impacts.

A 2023 review article reports that orthodontic aligner materials are exposed to extreme thermal changes, mechanical wear, pH fluctuations, and enzymatic degradation from bacterial and salivary enzymes, which can lead to microplastic leaching. This article notes that the potential toxicity of microplastics depends on their size, shape, and chemical composition. Size is a key factor for uptake: very small particles can passively cross cell membranes, whereas larger ones typically require active endocytosis. Particle shape also influences toxicity by modifying interactions with cells and tissues; for example, microfibers interact differently than microspheres, fragments, or films. The review recognizes that characterizing microplastics and nanoplastics remains challenging. Although recent advances allow testing of most particles, accuracy and certainty need further improvement, and additional research is required to better assess associated risks [26].

With a lot of environmental sources contributing to microplastics, isolating them individually for analysis is highly challenging. The studies reviewed above primarily rely on visual detection methods, which often produce imprecise data. Furthermore, the analytical techniques used to examine MNPs vary considerably, ranging from SEM to FTIR and RMS, creating limitations when comparing results across studies. Moreover, the in-vitro studies discussed do not account for salivary enzymes and other biological factors, highlighting the need for further investigation.

Material properties of orthodontic clear aligners

Clear aligners from different commercial brands were examined in a comprehensive review by Bichu et al. [19] for their composition and material characteristics. Commonly used aligner polymers are identified and summarised in Table 1, each selected for their distinct material properties. Thermoplastic polyurethane (TPU) demonstrates high mechanical strength and durability but is prone to water absorption and swelling. Polyethylene terephthalate glycol (PETG) offers excellent transparency, impact resistance, and stiffness, while polyethylene terephthalate (PET) provides a balance of ductility and structural strength. Recognising these characteristics allows for a more informed assessment of both the performance versus potential biological risks, such as leaching.

Table 1.

Common composition of clear aligners and their properties

Aligner material	Description of material	Properties
Thermoplastic Polyurethane (TPU)	Di- and tri- isocyanates and polyols	Chemical and Abrasion resistance High tear resistance with wide resiliency [19] Highest crystallinity, mechanical strength and glass transition temperature compared to PETG aligner materials. PU absorbs more water and swells more significantly, with the surface changing from wrinkled to smooth as temperature increases in SEM analysis [27]
Polyethylene terephthalate glycol (PETG)	Non-crystallising amorphous copolymer of PET comprised of 1,4-cyclohexane two methanol, ethylene glycol and terephthalic acid	High impact Strength and resistance to chemical changes Good transparency, adequate flow property [19] Greater stiffness than aligners combined with thermoplastic polyurethanes [28] PETG displays similar crystallinity, mechanical resistance and thermal stability. PETG disks have better transparency, with lower UV-VIS absorbance compared to polyurethane. PETG also shows hyperbolic saturation that increases with temperature [27]
Polyethylene terephthalate (PET)	Combination of ethylene glycol with terephthalic acid in amorphous and crystalline forms	Good hardness, stiffness and strength for crystalline structure [19]

Previous research has examined a variety of clear aligner types and brands, focusing on their reaction to different environments and stimuli. Investigated properties include colour stability, cytotoxicity, and other properties [29]. The findings highlight that performance may vary largely across brands, even for the same aligner composition, which may be ascribed to manufacturing variations (Table 2). Table 2 illustrates the variability in compositions and properties across different aligner brands, which may eventually influence the MNPs released from these aligners.

Table 2.

Aligner brands and composition of aligners

Aligner Brand	Composition of Aligner	Properties
Invisalign	Multi-layered aromatic thermoplastic polyurethane from methylene diphenyl diisocyanate and 1,6-hexanediol plus additive	Comparable microhardness compared to Eon, SureSmile and Clarity other systems and under electron microscopy and EDX analysis demonstrated a more homogenous and smoother surface [29] More colour changes exhibited than Smartee and Angelalign, especially after 7 days in the coffee solution. Rougher surface with more pore formation with SEM analysis [30] Invisalign displayed significantly more staining compared to Zendura and PETG aligners after 7 days in coffee and red wine after a cleansing cycle [31] Invisalign displays the highest water absorption after 1 day and 14 days as well as a low rate of hygroscopic expansion in a 2006 study completed by Ryokawa et al. [32]
Eon	Thermoformed Polyurethane Resin	Comparable microhardness compared to Invisalign, SureSmile and Clarity other systems [29]
SureSmile	Thermoformed Polyurethane resin	Comparable microhardness compared to Invisalign, Eon and Clarity other systems [29]
Clarity	Thermoformed Polyurethane resin	Comparable microhardness compared to Invisalign, Eon and SureSmile other systems [29]
Zendura	Thermoformed Polyurethane resin	Slight colour changes after 7 days in staining solutions. All in the clinically acceptable range for the staining solutions studied [31]
Biolon	Polyethylene terephthalate glycol (PETG)	Displayed highest cytotoxicity of all clear aligners in a study of four different aligner materials; Duran, Zendura and SmartTrack—still within range of being slightly cytotoxic [33]
Duran	Polyethylene terephthalate glycol (PETG)	Displayed lowest cytotoxicity of all clear aligners in a study of four different aligner materials; Duran, Zendura and SmartTrack—still within range of being slightly cytotoxic [33]
Smartee	Polycarbonate based	Slight colour changes after 7 days in staining solutions [30]
Angel align	Polyethylene terephthalate glycol (PETG)	Slight colour changes after 7 days in staining solutions [30]

Thermoforming is a common way of manufacturing orthodontic aligners. This process involves polymers such as polyethylene (PE), polyurethane (PU) or co-polyester, polypropylene (PP), polycarbonate (PC), ethylene vinyl acetate, and polyvinyl chloride (PVC), or a combination [34, 35]. Another way of manufacturing—which is becoming increasingly popular—is 3D printing (also known as additive manufacturing) [36, 37]. This process tends to use different materials, such as polylactic acid, polyamide (and glass-filled polyamide), epoxy resins, silver, titanium, and so on [34]. 3D printed, Direct print aligners (DPA) produced approximately tenfold higher microplastic concentrations than thermoformed aligners, with an average particle size nearly 1000 times larger (216.23 ± 44.5 μm² vs. 0.24 ± 0.09 μm²) [38]. These findings indicate that DPA manufacturing parameters markedly influence micro- and nanoplastic generation during simulated mastication.

It is important to understand that the MNP leakage is more due to the polymer/s that make up the appliance, rather than the appliance itself, potentially affecting various human body systems. Differences in polymer composition and structure across brands may influence MNP release, as dental polymers undergo physical and chemical changes in the oral environment. Hence, a comparative analysis of various aligner brands is essential to understand their role in MNP.

Future directions

This review highlights significant gaps in our current understanding of MNPs in dentistry, particularly in orthodontics, regarding the heterogeneity in MNP release across different polymer formulations and the lack of evidence on long-term biological effects. Most available studies are limited by in vitro settings, short durations, and inconsistent testing protocols, making it difficult to assess real-life clinical impact. A consensus minimum dataset for in-vitro testing (media composition, temperature profile, cycling parameters, particle capture protocols) should be developed. Multidisciplinary collaboration between orthodontists and materials scientists is essential to improve comparability and clinical relevance.

The long-term fate of MNPs within the human body, their potential for systemic accumulation, and their interactions with cellular and immune mechanisms remain poorly understood.

To address these shortcomings, future research must prioritise: (1) comparative studies evaluating MNP release from various aligner brands using realistic intraoral simulations; (2) comprehensive in vivo toxicological investigations into how MNPs affect multiple organ systems over time; and (3) the development and biocompatibility assessment of alternative polymeric materials with minimal leach potential. A multidisciplinary approach integrating orthodontics, materials science, and biomedical toxicology is essential to ensure patient safety and to better understand the long-term implications of polymer-based appliances in clinical orthodontics.

Conclusion

• Plastic dental appliances, particularly aligners and retainers, can be a source of micro- and nanoplastic release due to prolonged oral exposure, mechanical forces, and temperature fluctuations.

• Accumulation of MNPs in human tissues may trigger inflammation, oxidative stress, and other biological responses, though systemic effects from dental appliances remain unclear.

• Parafunctional habits like bruxism can increase appliance wear and particle release, highlighting the need for patient screening and guidance.

• Material selection, proper cleaning, and storage are key to minimizing potential MNP exposure in clinical practice.

• Interpret in-vitro particle counts with caution, considering the differences between laboratory conditions and the actual oral environment.

• Responsible disposal or recycling of used appliances can reduce environmental impact and support broader public health efforts.

These insights could help shape guidelines to reduce MNP exposure, addressing a broader public health and environmental challenge.

Author contributions

K.U, A.L, E.G & MC prepared the original draft of the manuscript. M.F & A.A supervised the project and reviewed the manuscript. A.V. conceptualised the project, reviewed and revised the manuscript.

Funding

Open access funding provided by Dr. DY Patil Vidyapeeth, Pune (Deemed to be University). The authors declare that no funding was received for the writing or publication of this manuscript.

Data availability

No datasets were generated or analysed during the current study.

Declarations

AI Use Declaration

Artificial intelligence tools were used to assist with grammar checking, language refinement, and rephrasing during the manuscript preparation process. No content, ideas, data interpretation, or conclusions were generated by AI. All intellectual content and critical analysis remain the sole responsibility of the authors.

Competing interests

The authors declare no competing interests.