Stereophotogrammetry to reveal age‐related changes of labial morphology among Chinese women aging from 20 to 60

Abstract Background The lip is of important aesthetic value and highly subjected to aging. Collecting anthropometric baseline data and understanding age‐related changes of labial morphology can help with diagnosis of deformity, assessment of aging, and planning of cosmetic procedures. Many studies have focused on Caucasians, while there is a lack of anthropometric data on Chinese women. Methods A total of 169 women were enrolled in this cross‐sectional study and divided into four consecutive age groups. Linear distances, angles, and surface area data were obtained via stereophotogrammetry. Intergroup comparisons between different age groups were performed to find age‐related differences. Results Lip width significantly increased with age while philtrum width seemed to show no obvious change. Cutaneous upper and lower lip height increased, lengthening the lip in the vertical dimension. Decrease of upper vermilion height and changes in angles indicated that aging process shortened the upper vermilion and flattened the vermilion border. Surface area also showed age‐related changes. Intergroup comparison showed no statistical significance in most variables between 20s and 30s or 30s and 40s, while age‐related changes in some variables were significant between 40s and 50s. Conclusion This study provided anthropometric data of labial morphology across a wide age range. Aging process affected a variety of labial anthropometric variables. Age‐related changes accelerated after 40 among Chinese women.

aging lips. 7,8 Magnetic resonance imaging (MRI) suggested upper lip complex experienced a combination of soft-tissue lengthening and volume loss. 9 These age-related changes are often reversed by surgical rejuvenation, usually a combination of a precise lip left and upper lip augmentation. 10 Before these procedures being practiced, a standard of youthful lip should be set and how much to correct should be known. Many studies have given their answers in Caucasians while there is a lack of data on Chinese women. 11 It has been pointed out that cosmetic surgery on Asians relying on Western standards might lead to inharmonious facial proportions and a loss of ethnic features, so data depicting lip aging on Asians should be collected. 12 Since it takes too long to track one subject to observe age-related changes, cross-sectional study is a common choice that efficiently provides normative data. 13 However, many such studies only compared two age groups (usually young subjects in their 20s and elder subjects around 60), while few analyzed several consecutive age groups. The authors believe it is of importance to study a large consecutive age range to give more comprehensive anthropometric data to guide cosmetic procedures.
This study is designed to provide baseline anthropometric data on labial morphology and analyze age-related morphological changes of the lip of Chinese women . Normative results could help with diagnose of deformity, preoperative assessment, and planning of cosmetic procedures.

| Study sample
This cross-sectional study was approved by Institutional Review

| Data collection
The subjects sat still in natural head position with neutral expression and mouth gently closed. VECTRA H1-270 camera (Canfield Scientific Inc) was operated by one of the authors strictly according to instructions to capture 3D images under the same brightness.
Three shots were taken from right oblique, front, and left oblique views. Three images were automatically stitched into a 3D facial image to allow further modification.
As shown in Table 1 Figure 1A). The horizontal plane was set to pass the origin and parallel to the true horizontal plane ( Figure 1B). The sagittal plane was defined as the plane perpendicular to both Camper's plane and true horizontal plane ( Figure 1C). Finally, the coronal plane was set to pass the origin and to be perpendicular to both horizontal plane and sagittal plane ( Figure 1D).

| Method error
To evaluate method error, intra-observer and inter-observer reliability were evaluated by intra-class correlation coefficient (ICC). After the landmark identification, ten 3D photographs were randomly selected from the database for reliability test. Landmarks onto these photographs were removed. Two weeks later, the main observer and an assistant observer identified landmarks onto them independently. All measurements were compared for intra-and inter-observer reliability.

| Statistical analysis
Linear and angular measurements are presented as mean ± standard deviation (SD). For each variable, Tukey's post hoc test was performed to analyze difference between two age groups if ANOVA or Kruskal-Wallis test revealed statistical significance among four groups. Statistical analysis was conducted using SPSS 23.0 (IBM). P value < 0.05 was considered statistically significant.

| RE SULTS
The study population was divided into four age groups at 10   Notably, the intergroup comparison showed that age-related changes on most anthropometric variables were subtle between 20s and 30s or between 30s and 40s and became more significant between 40s and 50s, indicating that lip aging accelerated after 40 among Chinese women.

| D ISCUSS I ON
Of great aesthetic value, the lip is an important structure in determining one's attractiveness. The aging lip brings a frustrated and lifeless look, thus becomes a main target for facial rejuvenation.
Many attempts have been made to lip rejuvenation by, for example, dermal filler injection, botulinum toxin application, and lip lift. 15,16 When giving these treatment, many surgeons lack objective numeric reference and relay on subjective impressive, which could lead to unproper correction. 17  an impression of less feminization. 16 Bisson et al have examined these angles in their study comparing models from fashion magazine to ordinary people. 18 They found the right and left bow angles (angles 1 and 2 in this study) were smaller in models and that the upper lip angle (angle 3 in this study) was larger in models.
Results of their study and ours confirmed the instinct that aging process made the upper lip less aesthetically pleasing.
With regard to lower lip, age-related changes were similar . The  Figure 3A).
Meantime, hydraulic acid is widely used for lip augmentation that can last for 4-8 months. Reestablishment of a fine vermilion border is the key for aesthetic purposes, so the injection is often given along the vermilion border at the red roll. 25 These surgical or nonsurgical procedures should not be done without orientation value. This cross-sectional anthropometric analysis aims to deepen the understanding of how aging process affects labial morphology to help with pre-and post-operative assessment so that under or over correction of aging lips could be avoid.
This is a stereophotogrammetry study on aging lips, taking advantage of 3D photography, and measuring technology. In recent years, 3D has been increasingly applied in anthropometric studies due to its good reliability and high accuracy. 26 In the authors' opinion, 3D photography is especially suitable for labial morphology analysis where surface area difference and volume loss are important variables. Stereophotogrammetry could further show its strength in the evaluation of lip rejuvenation. Future study is on the way to validate the assessment of lip cosmetic procedures via 3D anthropometry.
One limitation is that the study population comes from 12 provinces of Northern China. The results could not represent the morphological characteristics of the lip of Southern Chinese women.
Future study is expected to focus on Southern Chinese population and the North-South differences in labial morphology.

| CON CLUS ION
This study provided baseline anthropometric data for lip morphology and demonstrated age-related changes in Chinese women across in height and got increasingly flatten with age. These changes were significant after 40 years old. Aging process also brought significant surface area decrease to both upper and lower vermilion.

ACK N OWLED G EM ENT
The authors declare that they have no conflict of interest.