The clinical paradox of clear aligners: between hygiene and biological impact

Conventional fixed orthodontics, although proven, imposes major periodontal constraints by creating plaque retention areas that promote gingival inflammation and enamel demineralization. In this context, Clear Aligner Therapy (CAT) has emerged as an aesthetic alternative perceived as more hygienic. However, this transition to removable polymer devices raises new biological and material issues often overlooked in daily practice.

Study objectives and scope

This narrative review aims to synthesize current evidence regarding the composition of aligners (polyurethanes, polyesters), their behavior towards intra-oral aging, and their overall impact on oral health. The authors explore the precarious balance between immediate clinical benefits and long-term consequences, including the growth of bacterial and fungal biofilms on thermoplastic surfaces.

Hypotheses and research areas

The work is based on the analysis of the interaction between oral fluids and synthetic materials. The authors examine the hypothesis that masticatory load cycles and thermal fluctuations alter the tribological properties of polymers, thereby promoting microbial adhesion and the potential release of microplastics. Finally, the study evaluates the ecological burden of this therapy, whose serial aligner replacement model poses a major challenge in terms of sustainability and polymer waste management.

Narrative review methodology

This publication consists of a multidisciplinary narrative review synthesizing data on the historical evolution, oral health effects, material properties, and environmental impact of clear aligner treatments (CAT). Unlike a single experimental study, this work compiles evidence from various scientific sources up to 2025.

Le protocole de recherche s'est appuyé sur les paramètres suivants :

• Databases searched: PubMed/MEDLINE, Scopus and Web of Science, supplemented by manual screening of the reference lists of relevant articles.
• Search keywords: The strategy combined precise terms such as "clear aligner therapy", "orthodontic aligners", "oral microbiome", "thermoplastic polymers", "polyurethane", "PET-G", "microplastics" and "life cycle assessment".
• Selection criteria: The authors prioritised peer-reviewed original articles, systematic reviews and meta-analyses dealing with the biological, physico-chemical, clinical and ecological aspects of CAT.
• Exclusion criteria: Publications without a direct link to aligners, lacking sufficient scientific support, or not providing a substantial contribution to the interdisciplinary objectives of the journal were excluded.

The analysis covers the complete life cycle of the device: from polymer production and manufacturing (thermoforming vs 3D printing) to intra-oral wear and the issue of microplastics.

Periodontal and microbiological impact: an advantage under conditions

The authors of this review report that clear aligner treatment (CAT) allows for better dental plaque control and increased periodontal health compared to fixed multi-bracket appliances. However, this clinical superiority is not absolute and depends on strict behavioral variables.

• Biofilms: Aligner materials promote the growth of bacterial and fungal biofilms.
• Caries risks: Phenomena of demineralization or enamel erosion are observed, correlated with the patient's initial caries risk and the duration of treatment.
• Adherence: Periodontal benefits are directly linked to compliance with hygiene protocols and dietary guidelines.

Behavior of polymers in the intra-oral environment

Data synthesis shows that aligners, primarily composed of polyurethane and polyester polymers, undergo significant structural degradation once in the mouth. This intra-oral aging process modifies the physical properties of the device:

This surface degradation is not only mechanical: it directly influences microbial adhesion, creating a vicious cycle between polymer wear and biofilm accumulation.

Environmental assessment and microplastics release

The life cycle assessment (LCA) presented in this review highlights that the ecological impact of CAT extends far beyond the simple issue of solid waste. The compiled data indicate that polymer production, energy-intensive manufacturing, packaging, and distribution logistics constitute a major environmental burden.

Notable fact: the review reports that aligners release plastic microparticles during clinical use. This phenomenon is accentuated by the mechanical stress and cyclic loading experienced by the trays. The authors specify that even small amounts of materials per patient generate, on a global market scale, significant plastic pollution, exacerbated by the low biodegradability and limited recycling options of saliva-contaminated thermoplastics.

Historical evolution of removable devices

The review traces the key stages that led to current systems, marking the transition from craftsmanship to digital design:

Clinical analysis and material challenges

The findings of this narrative review highlight that while clear aligner therapy (CAT) theoretically promotes better plaque control and superior periodontal health compared to fixed appliances, these benefits are not systematic. They depend closely on patient compliance and their initial caries risk profile. The study highlights an intra-oral "aging" phenomenon: under the effect of saliva, temperature cycles, and mechanical stress, polymers (polyurethanes, polyesters) undergo morphological degradation. This modification of surface roughness facilitates bacterial and fungal adhesion, which can lead to enamel demineralization or erosion if hygiene and device maintenance are neglected.

Environmental limits and perspectives

One of the major contributions of this work is linking clinical performance to environmental sustainability. The authors note that the release of microplastics during use, induced by cyclic loading and tribological wear, constitutes an emerging concern still under evaluation. Although this review synthesizes recent data up to 2025, it highlights the complexity of recycling thermoplastics contaminated by saliva. The evolution towards direct 3D printing is identified as a major technological turning point, allowing for the elimination of intermediate physical models and thus reducing material consumption and waste.

Implications for daily practice

For the practitioner, these results indicate that the aligner should not be considered an inert device. Biofilm management must be personalised and aligner maintenance rigorously taught to counteract the effects of polymer degradation. The study suggests that an integrated strategy, taking into account the complete life cycle of the product — from production to waste management — is now essential for modern orthodontics, which is both effective and eco-responsible.

Synthesis

This narrative review demonstrates that clear aligner therapy (CAT) improves periodontal health compared to fixed appliances, although its effectiveness remains correlated with compliance and initial caries risk. The analysis highlights the vulnerability of polymers (polyurethanes, PET-G) to intra-oral aging, leading to microbial colonization and the release of microplastics under mechanical stress.

Concretely, for the practitioner:

• Personalise follow-up: Adapt your hygiene recommendations based on the baseline caries risk, as aligners can promote demineralisation if poorly maintained or worn while consuming sugary drinks.
• Secure the protocol: Impose strict maintenance of the device (regular cleaning) to prevent the formation of fungal and bacterial biofilms on polymer surfaces degraded by tribological wear.
• Streamline your workflow: Prioritize, whenever possible, direct 3D printing of aligners to eliminate the manufacturing of intermediate physical models, thereby significantly reducing plastic waste production in the practice.

Technical lexicon of the study

Thermoforming: Conventional method of manufacturing aligners consisting of heating a thermoplastic polymer sheet to mold it over a physical 3D model (working model), then cutting and polishing it.

Direct 3D printing: Technological innovation using biocompatible photopolymer resins to manufacture the aligner without an intermediate model stage, optimizing the digital workflow and potentially reducing material waste.

Tribological wear: Degradation of the polymer surface morphology caused by mechanical friction (insertion-removal cycles, occlusal forces), which can increase roughness and promote bacterial and fungal adhesion.

Microplastics: Polymer particles released by aligners during clinical use under the effect of mechanical stress and chemical degradation, raising environmental and biological concerns.

Life Cycle Assessment (LCA): Global evaluation framework measuring the environmental impact of aligner treatment, from polymer production and energy-intensive manufacturing to distribution and the management of saliva-contaminated waste.

Dysbiotic changes: Alterations in the oral microbiota balance. The study indicates that fixed appliances promote these changes, while the removable nature of clear aligners (CAT) may limit this risk by facilitating biofilm control.

Thermoplastic polymers (PET-G / Polyurethane): Constituent materials of aligners chosen for their transparency, dimensional stability, and their ability to exert controlled orthodontic forces while resisting the stresses of the oral environment.

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