https://orcid.org/0009-0003-1982-6929
Figures
Abstract
Background
Removable orthodontic aligners are widely used due to their aesthetic appeal, removability, and perceived comfort. However, evidence regarding their potential adverse effects remains limited.
Objective
To identify and synthesize the adverse effects associated with removable orthodontic aligner therapy.
Methods
This systematic review with single-arm meta-analysis evaluated adverse effects related to removable orthodontic aligner therapy. Seven electronic databases (Cochrane, Embase, LILACS, Livivo, MEDLINE, Scopus, and Web of Science) and gray literature sources (ProQuest Dissertations and Google Scholar) were searched without language or date restrictions. Two calibrated reviewers independently selected studies. Risk of bias was assessed using RoB 2 for randomized trials and ROBINS-I for non-randomized studies. Due to clinical and methodological heterogeneity, most outcomes were synthesized narratively; meta-analyses were conducted only for pain/discomfort, apical root resorption, and plaque.
Results
Thirty-four studies met the inclusion criteria, including 10 randomized controlled trials, 6 non-randomized clinical studies, and 18 cohort studies. Randomized trials were predominantly at low risk of bias, whereas most non-randomized and cohort studies were at moderate to high risk. Pain and apical root resorption were the most frequently reported adverse effects. Pain peaked within 24 hours, decreased by day 3, and was minimal by 1 week. Apical root resorption was generally small, with a mean linear loss of −0.33 mm (95% CI −0.55 to −0.11) and a volumetric loss of −4.37 mm³ (95% CI −5.51 to −3.24). Findings on plaque at 3 months were inconclusive. Other reported outcomes included periodontal changes, enamel demineralization, white spot lesions, speech alterations, halitosis, open gingival embrasures, temporomandibular symptoms, and awake bruxism.
Citation: Ronchi Lemos C, Aguilar Ventura Fadel M, Polmann H, de Oliveira JMD, Pauletto P, Miron Stefani C, et al. (2026) Adverse effects of removable orthodontic aligners: A systematic review with single-arm meta-analysis. PLoS One 21(7): e0350741. https://doi.org/10.1371/journal.pone.0350741
Editor: Sameh Attia, Justus Liebig University Giessen, GERMANY
Received: January 20, 2026; Accepted: May 18, 2026; Published: July 20, 2026
Copyright: © 2026 Ronchi Lemos et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The minimal data set is available at OSF via https://doi.org/10.17605/OSF.IO/Z9HCG.
Funding: Supported by the Foundation for Research of Santa Catarina (FAPESC nº48/2021) (JMO) and National Council for Scientific and Technological Development (CNPq) (Grant: 307987/2021-3) (GDLC). The funders had no role in study design, data collection, data analysis, interpretation of results, decision to publish, or preparation of the manuscript.
Competing interests: The authors declare that they have no competing interests.
Abbreviations: AB, Awake bruxism; AI, Artificial intelligence; BVS, Biblioteca Virtual em Saúde; CA, Clear aligner; CBCT, Cone beam computed tomography; CI, Confidence interval; DC/TMD, Diagnostic Criteria for Temporomandibular Disorders; GRADE, Grading of Recommendations Assessment, Development and Evaluation; IPR, Interproximal enamel reduction; LILACS, Literatura Latinoamericana y del Caribe en Ciencias de la Salud; MA, Meta-analysis; NRS, Non-randomized studies (study without random allocation); OHIP-14, Oral Health Impact Profile (14-item version); OGE, Open gingival embrasure; PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses; PRISMA-P, Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols; PROSPERO, International prospective register of systematic reviews; QLF, Quantitative Light-induced Fluorescence; RDC/TMD, Research Diagnostic Criteria for Temporomandibular Disorders; RCT, Randomized controlled trial; RoB 2, Risk of Bias 2 tool for randomized trials; ROBINS-I, Risk Of Bias In Non-randomized Studies – of Interventions; SD, Standard deviation; SR, Systematic review; SWiM, Synthesis Without Meta-analysis; VAS, Visual Analog Scale; TMD, temporomandibular disorders; TMJ, temporomandibular joint; VSC, Volatile sulfur compound; WSL, White spot lesion
Introduction
Growing demand for more aesthetic and comfortable orthodontic care has led patients and clinicians to seek alternatives to fixed appliances. Removable orthodontic aligners (clear aligners) have become increasingly popular because they are virtually invisible, are perceived as more comfortable, and are easier to maintain than traditional metal braces [1–3].
However, although orthodontic treatment corrects malocclusions and restores function, it may also result in adverse effects. Enamel demineralization can occur, leading to white spot lesions (WSLs) and an increased risk of caries [4]. Apical root resorption (ARR), associated with the magnitude of applied forces as well as genetic, biological, and systemic factors, remains common [5, 6], and inadequate oral hygiene during treatment may lead to gingival inflammation and attachment loss [7].
With fixed appliances, patients frequently experience discomfort and pain following appliance adjustments, particularly during the early phases of treatment [8]. Allergic reactions to nickel in metal alloys may cause oral irritation or contact dermatitis [9]. Concerns regarding metal ion release during orthodontic therapy have also been raised, although their clinical significance remains unclear [10]. Previous studies suggest that patients treated with clear aligners may experience slightly lower pain intensity and greater comfort, possibly due to reduced soft tissue irritation and the removability of the appliance [11]. Randomized trials comparing aligners and fixed appliances have examined patient-reported outcomes and provide context for interpreting harms reported in single-arm aligner studies [12].
Despite these advantages, clear aligners may also be associated with several adverse effects. Reported outcomes include transient pain and discomfort during the early stages of treatment, ARR, and periodontal changes such as variations in alveolar bone height or thickness and gingival inflammation [13, 14, 15–17]. Dental alterations, including enamel demineralization, white spot lesions, and caries, have also been described during aligner therapy [18–21]. In addition, patient-reported outcomes such as speech alterations, halitosis, open gingival embrasures, and temporomandibular symptoms have been documented in clinical studies evaluating clear aligner treatment [22–24]. However, the magnitude, frequency, and clinical relevance of these adverse outcomes remain uncertain, as available studies vary substantially in design, follow-up duration, outcome definitions, and measurement methods.
As the use of clear aligners continues to expand, it is increasingly important to investigate their potential adverse effects. Recent systematic reviews (SRs) [25–27] have examined specific aspects of aligner therapy, including speech alterations, apical root resorption, and oral health outcomes. However, these reviews have generally focused on isolated outcomes rather than providing a comprehensive synthesis of the full range of adverse effects associated with aligner therapy.
An updated and inclusive synthesis should address the full spectrum of potential adverse effects, including those related to periodontal health, speech, soft tissue responses, biocompatibility, systemic implications, and dental changes. Given the substantial variability in outcome definitions, measurement instruments, and follow-up periods across studies, interpreting the overall burden and clinical relevance of these effects remains challenging.
Therefore, this systematic review aims to provide a comprehensive, evidence-based synthesis of adverse effects associated with clear aligners. By focusing exclusively on adverse outcomes reported in patients treated with aligners, this review seeks to support informed clinical decision-making and the development of strategies to minimize risks while optimizing treatment safety and efficacy.
Materials and methods
Protocol and registration
The protocol of this SR was developed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols (PRISMA-P) and has been registered in the International Prospective Register of Systematic Reviews (PROSPERO) under CRD42023458491. The protocol has also been published previously [28]. As this study is a systematic review based exclusively on data from previously published studies, no new data were collected from human participants, no individual participant data were accessed, and no identifiable personal information was used. Therefore, ethics committee approval and informed consent were not required. This SR was reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement [29] (S1 Checklist – PRISMA checklist) and the Synthesis Without Meta-analysis (SWiM) guidelines (S2 Checklist – SWiM checklist) [30].
Research question
This SR aimed to address the following question: “Among orthodontic patients wearing orthodontic removable aligners, what are the potential adverse effects?”. The research question was structured according to the PIOS framework (Population, Intervention, Outcomes, and Study Design) (Fig 1).
Eligibility criteria
The review included cohort studies and clinical trials, whether randomized or not, involving orthodontic patients of any age or sex. Eligible studies evaluated clear aligners and reported adverse effects during or after treatment. For studies with multiple arms, only data from the group treated with clear aligners were included for analysis. No language or publication date restrictions were applied. Studies were excluded for the following reasons:
- Ineligible population;
- Intervention not limited exclusively clear aligners (e.g., hybrid with fixed/anchorage, other removable devices) or where an aligner-only sample could not be isolated.
- Ineligible outcomes (e.g., studies reporting only treatment effectiveness or patient satisfaction, without assessing adverse effects such as ARR, periodontal changes, pain, or speech impairment);
- Non-primary research and ineligible designs: opinion pieces, conference materials, technical articles, clinical guidelines, reviews, descriptive studies, and case reports.
As pre-specified, studies without available full text or with missing data were to be excluded after three attempts to contact the corresponding authors by email. Full texts were obtained for all eligible studies.
Information sources
Searches were conducted in Cochrane (CENTRAL), Embase, Latin American and Caribbean Health Sciences Literature (LILACS, via BVS [from Portuguese: Biblioteca Virtual em Saúde]), Livivo, MEDLINE (via PubMed), Scopus, and Web of Science. Grey literature sources were also searched, including Google Scholar and ProQuest Dissertations & Theses Citation Index, to reduce potential publication bias and identify relevant studies not indexed in traditional bibliographic databases. This approach is particularly important in SR of adverse effects, where harms may be incompletely reported in published literature. Reference lists of included studies and experts on the topic were also consulted. The search was conducted by the first author on May 4, 2025.
Search strategy and study selection
A comprehensive search strategy, developed in collaboration with a health sciences librarian, was applied to seven key databases (S3 Table – Search strategy), with syntax adapted for each platform. References were managed in EndNote-Web™ (Clarivate, USA) to remove duplicates before import into Rayyan® (Qatar Computing Research Institute) for further deduplication and selection [31, 32]. Two reviewers (CRL and MAVF) independently screened titles and abstracts in phase 1 and full texts in phase 2 according to predefined eligibility criteria. Prior to formal screening, the reviewers conducted a calibration exercise using a pilot sample of studies to ensure consistent interpretation of the eligibility criteria. Discrepancies were discussed until consensus was reached and the screening criteria were refined when necessary. During the selection process, disagreements were resolved through discussion and, when necessary, consultation with a third reviewer (HP). Studies published in non-Latin scripts were translated using the Google extension Sider AI [33], and fully rendered into English via ChatGPT [34], including figures and tables. Studies published online ahead of print or available as early online versions at the time of the search were considered eligible and included when they met the predefined criteria. Publication year was recorded according to the final journal assignment when available.
Data items and collection
Two reviewers (CRL and MAVF) independently extracted data into Excel® (Microsoft, USA), with discrepancies resolved through discussion or consultation with a third reviewer (HP). When necessary, data transformations were performed, including conversion of median to mean [35, 36], and extraction of data from graphical representations using WebPlotDigitizer (https://automeris.io/WebPlotDigitizer) [37]. A summary table presents key study characteristics, including authors, country, year, sample size, intervention type, treatment duration, measurement methods, outcomes, adverse effects, frequency, and follow-up. Additional comprehensive tables were developed to organize and detail all extracted data.
Risk of bias/quality assessment in individual studies
The risk of bias of the included studies was assessed using the RoB 2 tool [38] for randomized controlled trials (RCTs) and the ROBINS-I tool [39] for non-randomized studies (NRS) and cohorts. ChatGPT (OpenAI) [34] was used solely to assist in identifying relevant text excerpts to inform domain-specific judgments through predefined prompts (S4 - AIs Risk of Bias Prompts). All risk-of-bias assessments and data extraction were performed independently by two reviewers (CRL and MAVF), with AI outputs used only as supportive material and critically appraised before any decision-making. Disagreements were resolved by consultation with a third reviewer (HP). Prior to formal assessment, a calibration exercise was conducted using one study from each design category to ensure consistent interpretation of the RoB 2 and ROBINS-I domains. Results were reported according to each tool’s guidance, and visualizations were generated using robvis [40].
Synthesis methods
Given the absence of comparison groups, meta-analyses of single-arm means (and change scores, when applicable) were conducted only when outcomes were consistently defined, measured, and reported across studies. Quantitative synthesis was restricted to outcomes assessed using comparable instruments and reported at similar follow-up intervals. For most outcomes, substantial heterogeneity in measurement instruments, diagnostic criteria, data formats, and follow-up precluded meta-analysis. Such heterogeneity is expected in systematic reviews of adverse effects, reflecting variability in outcome definitions, populations, and measurement approaches [41]. In line with Cochrane guidance, statistical heterogeneity alone does not preclude meta-analysis when studies address a common clinical question; therefore, pooled estimates were interpreted as average effects across heterogeneous contexts rather than as a single underlying effect. Where heterogeneity could not be explained, findings were interpreted with caution and certainty of evidence was downgraded accordingly.
Outcome harmonization procedures were applied prior to synthesis using a predefined stepwise approach to ensure comparability across heterogeneous outcomes, in line with methodological guidance for adverse effects. Measurement scales were standardized (e.g., pain outcomes on different Visual Analogue Scales [VAS] were converted to a common 0–10 scale), and summary statistics were harmonized by estimating means and standard deviations (SDs) from medians and ranges using established methods [35, 36]. Units of analysis were aligned by aggregating tooth- or site-level data to the study level. When only baseline and follow-up values were available, change scores were derived and SDs were estimated assuming baseline–follow-up correlations. A correlation coefficient of r = 0.5 was assumed for the primary analyses, and sensitivity analyses were conducted using r values ranging from 0.3 to 0.7. When multiple reporting formats were available, post-intervention means were prioritized and change scores were analyzed separately. These procedures ensured transparent and clinically justified handling of heterogeneous outcome measures and are described in detail, with worked examples, in the Supplementary Material (S5 - Outcome harmonization procedures and worked examples).
Random-effects models with restricted maximum likelihood estimation and Hartung–Knapp adjustment were applied. Statistical heterogeneity was quantified using I² and τ². Analyses were conducted in R version 3.4.4 (The R Foundation for Statistical Computing, Vienna, Austria). Publication bias could not be formally assessed due to the small number of studies per outcome; this limitation was considered in the interpretation. Selective outcome reporting was assessed by comparing Methods and Results sections of each study. A structured assessment of selective outcome reporting for harms was conducted (S6 - Assessment of selective reporting and completeness of adverse effects). Although most studies reported at least one adverse effect, the evaluation was typically restricted to a single domain, such as pain, ARR, periodontal parameters, or enamel demineralization. Only a minority of studies assessed multiple adverse outcomes. Overall, the evidence indicates consistent incomplete reporting of harms, as many clinically relevant adverse effects were not assessed or reported, suggesting a moderate to high risk of selective outcome reporting across the included studies. Previous meta-epidemiological work highlights inconsistent harm reporting in orthodontic trials, underscoring the need for standardized adverse-event protocols [42].
For outcomes not suitable for meta-analysis, narrative synthesis was conducted following SWiM guidelines [30], considering direction and magnitude of effects across study design, sample characteristics, interventions, follow-up, and measurement methods. Results are presented in structured tables and figures to highlight patterns and inconsistencies.
Confidence in cumulative evidence
The overall certainty of evidence was assessed by outcome using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. Two reviewers (CRL and MAVF) independently evaluated certainty across the five GRADE domains, with disagreements resolved by consensus or consultation with a third reviewer (HP). Certainty ratings started at high for randomized evidence and at low for non-randomized evidence and were subsequently downgraded or, when justified, upgraded. Summary of findings tables were generated using GRADEpro GDT, with explicit outcome-level justifications based on pooled estimates or, when meta-analysis was not feasible, on the overall body of evidence [43].
Results
Study selection and characteristics
A total of 9,464 records were identified, of which 5,062 duplicates were removed. Following screening of the remaining 4,402 records, 178 were selected for full-text screening. The study selection process is summarized in a PRISMA 2020 flow diagram (Fig 2). Ultimately, 34 unique studies (35 reports) were included: 10 RCTs, six NRSs, and 18 cohort studies; one study was published in two separate reports. Excluded studies and reasons for exclusion are provided in S7 - Excluded studies and reasons for exclusion.
The included studies were conducted in 15 countries across four continents: Asia [13, 14, 18, 22, 44–50], the Americas [15, 19, 23, 51–53], Europe [11, 24, 54, 55], and Africa [56], involving 1,636 participants. Sample sizes ranged from 10 [54–56] to 244 [19] including both males and females, with mean ages ranging 13.9 [57] to 45 [46] years. Additional details on the included studies are provided in Tables 1, 2, and 3.
Risk of bias within the studies
The risk of bias assessment indicated that most RCTs were at low risk of bias, supported by robust methodological approaches. Among NRSs, five were assessed as having a moderate risk of bias, and one as having a high risk, primarily due to selection bias and lack of blinding. Cohort studies generally demostrated a moderate to high risk of bias, with concerns related to participant selection, control of confounding, and blinding; however, one study was rated as having a moderate risk with robust outcome measurement. Further details are presented in Fig 3.
(B) Non-Randomized Clinical Trials assessed by ROBINS-I tool. (C) Cohort Studies assessed by ROBINS-I tool.
Synthesis of results
Based on the predefined synthesis criteria described in the Methods, meta-analyses were conducted for outcomes with sufficient methodological consistency (pain/discomfort, ARR, and plaque), while all other outcomes were synthesized narratively due to substantial heterogeneity in definitions, measurement methods, and follow-up periods.
The studies were categorized according to the study design (RCTs, cohort studies, and NRSs), assessed outcomes, measurement methods, and follow-up periods. The analyzed outcomes included awake bruxism (AB) [58], chewing performance [59], enamel changes (Caries/filling, enamel demineralization, and WSL) [18–20, 57], halitosis [24], open gingival embrasures [22], pain (discomfort, general pain, periodontal pain and type of pain) [11, 13, 46–50, 52, 53, 55, 60], periodontal health status (bone height/thickness, bleeding on probing, gingivitis, probing pocket depth) [16, 17, 44, 54, 61, 62], plaque (plaque and plaque index) [18, 54], ARR (apical and volumetric) [14–16, 44, 45, 51, 56, 59, 62, 63], speech (perception, evaluation, and performance) [23, 53], and temporomandibular joint disorders (TMD) [24] (Fig 4).
Meta-analysis was feasible for only three outcomes: pain/discomfort, ARR, and plaque. A total of 10 studies assessed pain or discomfort associated with removable aligner therapy [13, 46–50, 52, 53, 55, 60]. Apical root resorption was evaluated in six studies [14–16, 44, 59, 62], while three used three-dimensional (3D) techniques to measure volumetric loss [51, 56, 62]. Plaque was evaluated in three studies [17, 54, 59]. The following sections present the quantitative effect estimates across different follow-up intervals. All R scripts used for the statistical analyses are available in S8 - R Scripts.
Quantitative synthesis
Pain and discomfort meta-analysis.
Pain peaked at 24 hours (mean 3.06, 95% CI: 2.15 to 4.37) (Fig 5a), decreased to 1.55 (95% CI: 1.09 to 2.21) at three days (Fig 5b), and further declined to 1.08 (95% CI: 0.33 to 3.51) by one week (Fig 5c). High heterogeneity (I² > 80%) was observed at all time points.
Legend: (a) Mean and SD of Pain/Discomfort after 24 hours. (b) Mean and SD of Pain/Discomfort after 3 days. (c) Mean and SD of Pain/Discomfort after 1 week.
Apical root resorption meta-analysis.
Meta-analysis of ARR showed a mean loss of −0.33 mm (95% CI: −0.55 to −0.11) (Fig 6a). At six months, the estimate was −0.20 mm (95% CI: −0.45 to 0.05) (Fig 6b). Subgroup analyses by follow-up duration showed that resorption ranged from −0.06 mm at six months (95% CI: −0.35 to 0.22) to −0.51 mm at 12 months (95% CI: −0.54 to −0.48). Apical root resorption was also observed at 18 months (−0.31 mm; 95% CI: −0.53 to −0.08) and 26 months (−0.31 mm; 95% CI: −1.10 to 0.48). Moderate heterogeneity was present (I² = 62.8%), and subgroup differences by follow-up duration were statistically significant (p = 0.0059). At 18 months, no heterogeneity was detected (I² = 0%), and the mean reduction was −0.69 mm (95% CI: −0.96 to −0.41) (Fig 6c). Volumetric analysis showed a mean ARR of −4.37 mm³ (95% CI: −5.51 to −3.24) by the end of follow-up (Fig 6d).
Legend: (a) Apical root resorption at 6 months; (b) at 18 months; (c) at end of treatment; (d) volumetric root resorption (mm³).
Plaque meta-analysis.
Of the three included studies, one RCT [46] assessed two Ramfjord sites and showed a reduction in Plaque Index at 3 months (Δ −0.10; 95% CI −0.44 to 0.24). In contrast, increases in Plaque Index were observed in the multicenter study [59] and in the study with imputed statistics [51] (Fig 7). Substantial heterogeneity was observed (I² = 95–96%).
Legend: (a) Conservative meta-analysis using imputed means and SDs; (b) Optimistic meta-analysis. Mertoglu et al. included based on imputed data from medians.
Narrative synthesis.
The findings of the included studies for outcomes not suitable for meta-analysis are summarized in Table 4.
Certainty of evidence
Table 5 presents, for each outcome, the effect estimates (absolute and/or relative), the number of studies and participants, and the certainty of the evidence (high, moderate, low, or very low) assessed using the GRADE approach. Certainty ratings were based on the five GRADE domains: risk of bias, inconsistency, indirectness, imprecision, and publication bias. Explicit justifications for each decision to downgrade or upgrade the certainty are provided in the “Reasons for rating decisions” column. When quantitative synthesis was feasible, certainty assessments were informed by the meta-analysis; otherwise, they were based on the narrative synthesis of the body of evidence.
Discussion
This SR identified several adverse effects associated with removable clear aligners, many of which also observed with fixed appliances and should be considered during treatment planning and informed consent. Pain is expected at treatment initiation and is generally mild–moderate, peaking at 24–48 h, decreasing by day 3, and reaching minimal levels by week one. Differences across studies largely reflect variation in measurement instruments (0–100 mm vs 0–10 cm VAS) and assessment methods. The removability of clear aligners may also influence perceived discomfort [47, 64, 65]. Our meta-analyses confirmed the temporal pattern, with pain highest at 24 h, lower by day 3, and minimal by week one, although heterogeneity was very high (I² ≈ 83–99%). This level of heterogeneity is expected in single-arm syntheses and supports the need to explore, rather than exclude, variability [41]. The high heterogeneity observed across several analyses should be interpreted with caution but is not unexpected in SR of adverse effects, where variability in outcome definitions, measurement instruments, treatment protocols, and follow-up duration is common [66]. Methodological guidance indicates that statistical heterogeneity alone does not preclude meta-analysis when studies address a common clinical question; in such cases, random-effects models can be used to account for between-study variability [41]. Accordingly, pooled estimates should be interpreted as average effects across heterogeneous clinical contexts rather than as precise treatment effects. Risk of bias was generally low among RCTs, although one trial was rated as high risk. Non-randomized studies ranged from moderate to critical risk, primarily due to confounding and selection bias [11, 46, 47, 52, 59]. The use of standardized pain measures and longer follow-up periods is warranted to improve comparability and strengthen the evidence base [11, 51, 65, 67].
Changes in plaque were small, with wide confidence intervals crossing the null and inconsistent directions across studies and measurement approaches (whole-mouth assessments vs Ramfjord sites), resulting in very high heterogeneity (I² = 95–96%) and low certainty of evidence. Periodontal pain tended to peak at 24 hours, particularly in the mandibular arch [11, 67].
Although ARR remains a concern, particularly in the maxillary incisors, pooled estimates indicated a minimal mean reduction by the end of treatment (approximately −0.33 mm; 95% CI −0.55 to −0.11), with small volumetric losses. The magnitude of these changes is unlikely to be clinically significant over the long term [14, 15, 68, 69]. Variability appears to be related to factors such as age, sex, malocclusion, bone density, treatment duration, and biomechanics, as well as differences in outcome assessment methods [15]. While some studies relied on linear measurements obtained from periapical radiographs or CBCT-derived lengths, others used volumetric analyses based on three-dimensional segmentation, which may provide a more comprehensive evaluation of structural changes. These methodological differences limit direct comparability and may contribute to heterogeneity in the pooled estimates. CBCT-based assessments provide higher sensitivity for volumetric root loss and alveolar bone changes and should be considered when available for precise quantification [70]. Study quality ranged from low to high risk of bias across designs. Current guidance supports the use of light forces and staged tooth movements, with caution during intrusion and rotational movements [14, 15, 56, 59, 71].
Aligner therapy was primarily associated with mild TMD symptoms, with no reports of severe manifestations. However, one study was assessed as having a critical risk of bias (selection, confounding, and reporting), which limits confidence in these findings despite a low risk of bias in outcome measurement [72, 73].
One RCT assessing AB found no difference between aligners and fixed appliances; an initial decrease returned to baseline at six months (low risk across RoB 2 domains) (59). Other studies have reported a higher prevalence of AB among aligner users, highlighting the need for individual risk assessment [74].
Aligners were associated with increased occurrence of OGEs in one study (overall moderate risk; low risk in key domains), with supporting external evidence indicating notable incidence in anterior teeth. Interproximal reduction may help prevent OGEs by repositioning the contact point apically [22, 75].
Three studies reported buccal bone reduction (−0.80 to −3.05 mm; 6.3–12% volume/density), which may increase the risk of alveolar bone dehiscence or fenestration in susceptible patients. These defects may compromise periodontal stability and predispose teeth to gingival recession when orthodontic movement exceeds the biological limits of the alveolar bone housing. Careful treatment planning is therefore essential, particularly in cases involving dental expansion or significant buccal tooth movement. Assessment of the patient’s periodontal phenotype, control of orthodontic force magnitude, and individualized biomechanics should be considered to minimize the risk of alveolar bone defects. When clinically indicated, imaging modalities such as cone-beam computed tomography may aid in evaluating the buccal bone plate and guiding safer treatment decisions. The broader orthodontic literature reports a higher prevalence of buccal dehiscence and fenestrations associated with no-extractions expansion, particularly in patients with thin periodontal phenotypes or limited alveolar bone support [76–78], supporting the need for preventive planning and periodontal monitoring during treatment [16, 17, 44, 79]. The risk of bias across these studies was mixed, with cohort designs generally presenting moderate risk or some concerns, warranting cautious interpretation of the findings [16, 44, 62].
Transient speech alterations (e.g., /ch/, /s/, /z/) were frequently reported, with recovery typically occurring within 1–3 months, although longer adaptation periods may be observed in indivuduals with speech-sensitive professions. Referral may be beneficial in persistent cases [23, 61, 80, 81]. Methodological quality varied across studies with RCT generally at low risk of bias and cohort studies at moderate risk) [23, 61].
Evidence suggests that WSLs may increase with aligner therapy, particularly in the presence of suboptimal oral hygiene, pre-existing lesions, or attachments. Although some comparisons with fixed appliances report a lower incidence, lesions may be larger and less mineralized. Overall, findings are inconsistent and of very low to low certainty [18–21, 57, 82].
Perceptions of halitosis and dry mouth may increase transiently during the first months of aligner therapy, particularly within the first 3–4 months, with minimal impact on volatile sulfur compounds (VSCs) or inflammatory parameters and only temporary benefit from chlorhexidine; oral hygiene remains a key factor [24]. The cohort study addressing halitosis presented a serious risk of bias, primarily due to confounding and participant selection, which limits the reliability of its findings [24]. These short-term perceptions are generally mild and tend to decrease over time [24, 54]. Evidence regarding longer-term changes remains limited; however, available data suggest that these effects may stabilize during treatment. Persistent alterations may occur in the presence of inadequate oral hygiene or sustained biofilm accumulation on aligner surfaces. Orthodontic appliances, including thermoplastic aligners, can modify oral microbial ecology and plaque accumulation patterns, partly due to surface irregularities and microabrasions that facilitate bacterial adhesion and biofilm formation [82, 83]. These biofilms may contribute to oral malodor when hygiene is suboptimal, as demonstrated by studies reporting bacterial colonization of removable orthodontic appliances [84, 85]. Together, these findings support the importance of strict oral and aligner hygiene to minimize both short- and longer-term adverse effects.
Many adverse effects are minor and/or transient. Clinicians should assess individual risks factors (e.g., bruxism, susceptibility to ARR), monitor bone and periodontal health (with imaging when indicated), provide guidance on pain and speech adaptation, consider IPR to mitigate OGEs, and reinforce strict oral hygiene to reduce the risk of WSLs, halitosis, and related complications. These clinical considerations are consistent with general orthodontic practice principles, although the present review did not include direct comparisons with fixed appliances. Based on the findings of this review, the following clinical considerations may assist in interpreting potential adverse effects in practice (Table 6).
Limitations
Substantial heterogeneity in measures, study populations, and follow-up durations, along with incomplete representation of malocclusion complexity, limits comparability and generalizability. Standardized methodologies and consistent follow-ups protocols are needed to strengthen the evidence base.
Conclusion
The main adverse effects of clear aligners include pain and discomfort, typically most intense during the first 24–48 hours, and mild ARR, particularly in incisors. Less frequently reported but clinically relevant effects include enamel demineralization, white spot lesions, caries in areas with poor hygiene, open gingival embrasures after prolonged treatment, and mild, transient speech alterations. Periodontal health compromise is generally mild. Bruxism and temporomandibular disorders, though often mild to moderate, remain a concern. Although this review analyzed aligner arms only and direct comparisons with fixed appliances cannot be made, evidence from randomized trials comparing aligners and fixed appliances provides important context for interpreting patient-reported outcomes and adverse effects.
Supporting information
S5. Outcome harmonization procedures and worked examples.
https://doi.org/10.1371/journal.pone.0350741.s005
(DOCX)
S6. Assessment of selective reporting and completeness of adverse effects.
https://doi.org/10.1371/journal.pone.0350741.s006
(DOCX)
S7. Excluded studies and reasons for exclusion.
https://doi.org/10.1371/journal.pone.0350741.s007
(DOCX)
Acknowledgments
The authors thank the librarians, MSc. Karyn Munyk Lehmkuhl (Federal University of Santa Catarina, Brazil, E-mail: karyn.lehmkuhl@ufsc.br) for her support in developing the search strategy.
Generative AI statement
The authors used ChatGPT (OpenAI) to assist with translation and formatting of extracted data. All outputs were reviewed and verified by the authors.
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