Cephalometric features associated with the mentolabial angle and lower lip eversion in young adults: A cross-sectional study

Background: Understanding the cephalometric factors that affect the mentolabial angle and lower lip eversion is essential for orthodontics. The objective was to evaluate the influence of various skeletal and dentoalveolar cephalometric features associated with the mentolabial angle and lower lip eversion in young adults. Material and Methods: This cross-sectional study assessed lateral head radiographs of individuals between the ages of 15 and 40 years. Two trained and calibrated evaluators performed angular and linear cephalometric measurements utilizing BlueSky Plan 4 software. The variables evaluated in this study included the presence of the mentolabial angle, labial eversion, overjet, and the position and inclination of the lower central incisors (measured by I-NB, I.NB, IMPA) as well as the upper central incisors (measured by I-NA, I.NA, UIPP). Additionally, we assessed the sagittal positions of the maxilla and mandible using SNA and SNB measurements, along with the sagittal and vertical skeletal relationships determined by the ANB and FMA angles. We applied multiple linear and binary logistic regression tests for statistical analysis (p<0.05). Results: 138 radiographs were evaluated (73 females and 65 males). The mentolabial angle is, on average, 7.68° greater in women than men (p=0.001). An increase in the IMPA and overjet decreases 0.40° (p=0.012) and 2.02° (p=0.003) in the mentolabial angle, respectively. Likewise, females are 82% less likely to experience lip eversion than males (exp B = 0.18, 0.05 to 0.60 CI 95%; p=0.006). Furthermore, for each degree increase in lower incisor inclination (I. NB) or overjet, the risk of developing lip eversion increases by 1.17 times (1.02 - 1.34 CI to 95%, p=0.023) and 1.85 times (1.23 - 2.78 CI to 95%, p=0.003), respectively. Conclusions: The inclination of the lower incisors and the overjet primarily influences the mento-labial angle. Women tend to have a greater mento labial angle, meaning it is more retrusive, compared to men. Additionally, the likelihood of lip eversion is higher in males. For each degree of increased lower incisor inclination or overjet, the risk of developing lip eversion also rises.