Association of Body Mass Index (BMI) with Lip Morphology Characteristics: A Cross-Sectional Study Based on Chinese Population

Abstract

Background: Lip morphology is essential in diagnosis and treatment of orthodontics and orthognathic surgery to ensure facial aesthetics. Body mass index (BMI) has proved to have influence on facial soft tissue thickness, but its relationship with lip morphology is unclear. This study aimed to evaluate the association between BMI and lip morphology characteristics (LMCs) and thus provide information for personalized treatment.

Methods: A cross-sectional study consisted of 1185 patients from 1 January 2010 to 31 December 2020 was conducted. Confounders of demography, dental features, skeletal parameters and LMCs were adjusted by multivariable linear regression to identify the association between BMI and LMCs. Group differences were evaluated with two-samples t-test and one-way ANOVA test. Mediation analysis was used for indirect effects assessment.

Results: After adjusting for confounders, BMI is independently associated with upper lip length (0.039, [0.002-0.075]), soft pogonion thickness (0.120, [0.073-0.168]), inferior sulcus depth (0.040, [0.018-0.063]), lower lip length (0.208, [0.139-0.276]), and curve fitting revealed non-linearity to BMI in obese patients. Mediation analysis found BMI was associated with superior sulcus depth and basic upper lip thickness through upper lip length.

Conclusions: BMI is positively associated with LMCs, except for nasolabial angle as negatively, while obese patients reverse or weaken these associations.

Keywords: BMI; body mass index; cephalometric; lip morphology; multivariable regression.

Figure 1

Study flow chart demonstrates the analysis process and content of the research.

Figure 2

Schematic diagrams of the eight lip characteristics used in the present study and representative patients’ profiles. (A) The description of eight lip characteristics. 1: Nasolabial angle, 2: Superior sulcus depth, 3: Basic upper lip thickness, 4: Upper lip thickness, 5: Upper lip length, 6: Soft pogonion thickness, 7: Inferior sulcus depth, 8: Lower lip length. (B) A representative underweight patient profile. (C) A representative overweight patient profile.

Figure 3

Data description of distributions for lip characteristics and BMI. (A) Data distributions of eight lip characteristics. (B) Correlations among the eight lip characteristics. (C) Data distribution of BMI characteristics. (D) Data distribution of four BMI categories. (E) Univariate analysis comparing BMI and eight lip characteristics and results demonstrated by forest-plot. NLA: Nasolabial angle, SSD: Superior sulcus depth, BULT: Basic upper lip thickness, ULT: Upper lip thickness, ULL: Upper lip length, SPT: Soft pogonion thickness, ISD: Inferior sulcus depth, LLL: Lower lip length, BMI: Body mass index. *: p < 0.05, **: p < 0.01, ***: p < 0.001.

Figure 4

The relationships between the adjusted eight lip characteristics and BMI. The solid blue line fitted by the Loess function reveals the true variation tendency of lip characteristics according to the BMI, and the grey dashed lines fitted by the lm function reveal the general trend. NLA: Nasolabial angle, SSD: Superior sulcus depth, BULT: Basic upper lip thickness, ULT: Upper lip thickness, ULL: Upper lip length, SPT: Soft pogonion thickness, ISD: Inferior sulcus depth, LLL: Lower lip length, BMI: Body mass index.

Figure 5

The comparations of multivariate regression analysis adjusted for eight lip characteristics among four BMI categories. NLA: Nasolabial angle, SSD: Superior sulcus depth, BULT: Basic upper lip thickness, ULT: Upper lip thickness, ULL: Upper lip length, SPT: Soft pogonion thickness, ISD: Inferior sulcus depth, LLL: Lower lip length, BMI: Body mass index. ns: p >= 0.05, *: p < 0.05, **: p < 0.01, ***: p < 0.001, ****: p < 0.0001.

Figure 6

Mediation analysis among the four un-independent upper lip characteristics, ULL and BMI. (A) Mediation analysis model results of BMI, ULL and NLA (adjusted). Total effect (−0.056, p > 0.05) of BMI towards NLA consisted of direct effect (−0.081, p > 0.05) and indirect effect (0.025 = 0.036 × 0.679, p > 0.05). (B) Mediation analysis model results of BMI, ULL and SSD (adjusted). Total effect (0.014, p > 0.05) of BMI towards SSD consisted of direct effect (0.005, p > 0.05) and indirect effect (0.009 = 0.046 × 0.191, p = 0.018). (C) Mediation analysis model results of BMI, ULL and BULT (adjusted). Total effect (0.031, p = 0.014) of BMI towards BULT consisted of direct effect (0.020, p > 0.05) and indirect effect (0.011 = 0.053 × 0.207, p = 0.010). (D) Mediation analysis model results of BMI, ULL and ULT (adjusted). Total effect (−0.007, p > 0.05) of BMI towards ULT consisted of direct effect (0.000, p > 0.05) and indirect effect (−0.008 = 0.041 × (−0.197), p > 0.05). NLA: Nasolabial angle, SSD: Superior sulcus depth, BULT: Basic upper lip thickness, ULT: Upper lip thickness, ULL: Upper lip length, BMI: Body mass index. *: p < 0.05, **: p < 0.01.