Functional appliances and airway outcomes in Class II malocclusion: an umbrella review of evidence certainty and clinical credibility.

BACKGROUND

Functional appliances are widely used for Class II correction in growing patients, and their potential influence on upper-airway dimensions has been examined in several systematic reviews. However, methodological quality, overlap of primary studies, and credibility of evidence remain uncertain.

OBJECTIVES

To synthesize evidence from published systematic reviews and meta-analyses on functional appliances and airway dimensions in growing Class II patients, and to evaluate methodological quality, overlap, and credibility of evidence.

SEARCH

METHODS

Comprehensive electronic searches of PubMed, Embase, Cochrane Library, Scopus, Web of Science, and supplementary sources identified eligible systematic reviews up to August 2025.

SELECTION CRITERIA: Systematic reviews or meta-analyses assessing airway changes after functional-appliance therapy in normal growing Class II patients with true external or growth-matched controls were included.

DATA COLLECTION AND ANALYSIS: Four systematic reviews encompassing 39 unique primary studies were retained. Methodological quality was assessed using the Joanna Briggs Institute (JBI) checklist and the Risk of Bias in Systematic Reviews (ROBIS) tool; study overlap was quantified by the Corrected Covered Area (CCA); credibility was evaluated using Ioannidis' classification; and evidence certainty was graded with the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) tool.

RESULTS

Meta-analyses revealed significant oropharyngeal enlargement: superior pharyngeal space + 1.73 mm (95% CI 1.13-2.32), middle + 1.52 mm (95% CI 1.09-1.96), and inferior + 1.25 mm (95% CI 0.66-1.84). Oropharyngeal airway volume increased by 2356 mm³ (95% CI 1276-3436) and nasopharyngeal volume by 382 mm³ (95% CI 141-623). Changes in posterior airway space (+0.52 mm, 95% CI -0.20 to 1.24) and upper-airway thickness were not significant. ROBIS classified four reviews as low-risk and two as high-risk. Study overlap was moderate (CCA = 6.8%). By Ioannidis' criteria, most outcomes were weak (Class IV) or non-significant. GRADE certainty was low to very low, mainly due to risk of bias, heterogeneity (I² up to 90%), and imprecision.

CONCLUSIONS

Functional appliances yield modest yet statistically significant enlargement of oropharyngeal airway dimensions in growing Class II patients, with the effect being more pronounced with removable appliances. Nevertheless, evidence credibility remains weak, and overall certainty is low. Results should be interpreted with caution, and future studies should employ prospective, volumetric, growth-controlled, and standardized protocols.

REGISTRATION: This umbrella review was registered with PROSPERO (CRD42023408859).