Factors Associated with Lip-Closing Strength in Children Undergoing Orthodontic Assessment: A Cross-Sectional Study

Background/Objectives: Lip-closing strength (LCS) is an objective measure of perioral muscle function, yet its associations with sagittal and vertical skeletal patterns and mouth-breathing screening status in school-aged children have not been well characterized. This study aimed to investigate the associations between LCS and demographic characteristics, sagittal and vertical skeletal patterns, and mouth-breathing screening status in children undergoing orthodontic assessment. Methods: A cross-sectional study was conducted on 379 children aged 7–12 years presenting for initial orthodontic evaluation. LCS was measured using a custom-built dynamometer with a lip-button attachment. Sagittal and vertical patterns were classified by lateral cephalometry (ANB, SN–MP angles), and mouth-breathing screening status was assessed using a validated questionnaire (Habitual Mouth-Breathing Score ≥ 4 defined as screen-positive). Multivariable linear regression was performed with LCS as the outcome. Results: The mean LCS was 4.61 ± 1.58 N. Skeletal Class II (4.41 ± 1.33 N) and screen-positive mouth breathing (median 3.97 N) were associated with significantly lower LCS compared to Class I and screen-negative participants, respectively. Age, sex, and vertical pattern showed no significant association. In adjusted analysis, skeletal Class I (B = 0.575, β = 0.180, 95% CI: 0.268–0.882) and Class III (B = 0.867, β = 0.188, 95% CI: 0.416–1.317) were associated with significantly greater LCS relative to Class II (p < 0.001 for both), whereas screen-positive mouth breathing was associated with significantly lower LCS (B = −0.808, β = −0.267, 95% CI: −1.099 to −0.517, p < 0.001). Conclusions: Lower LCS was associated with skeletal Class II pattern and positive mouth-breathing screening status. The observed differences were modest, and their clinical significance remains uncertain. LCS should not be used as a standalone diagnostic or treatment-planning indicator.