Quality and stability of orthodontic treatment outcomes with clear aligners versus fixed appliances: a systematic review and meta-analysis

BACKGROUND: Clear aligners represent a modern alternative to traditional fixed orthodontic appliances, offering aesthetic and removable treatment options. However, variability in clinical outcomes and treatment protocols has led to inconsistent evidence regarding their effectiveness and stability. OBJECTIVES: To systematically review and synthesize available evidence from randomized and non-randomized trials comparing the quality and stability of orthodontic treatment outcomes using clear aligners versus fixed appliances. SEARCH METHODS: A comprehensive literature search was conducted across seven databases (PubMed®, Google™ Scholar, Embase®, Scopus®, Web of ScienceTM, the Cochrane Central Register of Controlled Trials, ProQuest) up to February 2025, identifying relevant clinical trials assessing treatment outcomes with clear aligners and fixed appliances. SELECTION CRITERIA: Randomized controlled trials and non-randomized clinical trials comparing orthodontic treatment outcomes of clear aligners versus conventional fixed appliances in patients undergoing orthodontic treatment were included. DATA COLLECTION AND ANALYSIS: Two reviewers independently screened titles, abstracts, and full texts, extracted data, and assessed the risk of bias for each included study. The Cochrane Risk of Bias tool (RoB2 tool) was used for randomized controlled trials, while the ROBINS-I tool was applied for non-randomized studies. When appropriate, random-effects meta-analyses were performed. The certainty of the evidence for each outcome was evaluated using the GRADE approach. RESULTS: Fifteen trials involving 1084 patients were included. The risk of bias varied across studies. No significant differences were found in treatment quality or duration between clear aligners and fixed appliances in non-extraction cases, although sensitivity analyses suggested shorter treatment duration with clear aligners. In extraction cases, fixed appliances provided superior treatment quality, attributed to enhanced control of tooth movements. Limited evidence indicated similar post-treatment stability between the two interventions. However, study heterogeneity and short follow-up durations limited the strength of conclusions. CONCLUSIONS: Both clear aligners and fixed appliances effectively achieve orthodontic treatment goals. Case complexity and patient compliance should guide appliance selection. Further large-scale, well-designed trials with long-term follow-up are necessary to define comparative outcomes. REGISTRATION: The protocol for this review was registered with PROSPERO (CRD420251072157) during its initial stages.