Morphological Study of the Characteristics of Lips in Children With Mouth Breathing

This study aimed to explore the association between long-term mouth breathing habits and the morphological characteristics of children's lips, providing a basis for clinical diagnosis and early intervention. Given the cross-sectional design and reliance on parental questionnaire-based exposure classification, causal inference is not warranted; findings are strictly associative.

A total of 210 children aged 6-12 years were enrolled, comprising 105 children clinically classified as long-term mouth-breathing children and 105 nasal-breathing children. Classification was based on a validated parental questionnaire with defined major and minor signs; objective airway assessment was not performed, which is acknowledged as a limitation. Lip morphological characteristics were analyzed from standardized lateral cephalograms. Quantitative parameters included protrusion of the subnasal region, upper lip, lower lip, and chin; length of the upper lip, lower lip, and chin; the ratio of upper lip length to lower lip length; nasolabial angle; mentolabial angle. Comparisons between groups used independent-samples t-tests with covariate adjustment (analysis of covariance [ANCOVA]) for age, sex, and BMI, and the Benjamini-Hochberg false discovery rate (FDR) correction for multiple comparisons. Effect sizes (Cohen's d) and 95% confidence intervals (CIs) are reported.

Significant positive correlations were found among Ns to N' Vert, UL to N' Vert, LL to N' Vert, Pog' to N' Vert, UL-L, LL-L, and Cn-L. Compared with nasal-breathing children, mouth-breathing children showed decreased protrusion of the subnasal region and upper lip, retraction of the chin, a sharper nasolabial angle, and a more obtuse mentolabial angle. No significant difference in lower lip protrusion (LL to N' Vert) was found between groups (adjusted p = 0.225). Because all participants were recruited from an orthodontic clinic, these findings may not be generalizable to the general pediatric population.

Mouth breathing is associated with significant differences in lip and facial soft-tissue morphology in children, particularly in the protrusion and length of the subnasal region, upper lip, and chin, as well as in nasolabial and mentolabial angles. Early identification and interdisciplinary management of mouth breathing may be important for preventing further morphological changes; however, longitudinal studies are needed to establish temporal relationships and causal mechanisms.