INTRODUCTION

For non-professionals, facial aesthetics place more emphasis on the aesthetic degree of facial soft issue. Therefore, patients who seek orthodontic treatment show concern to improve their appearance (^{Kuhn et al., 2016}). The aesthetic standard of facial profile is affected by various factors such as race, ethnicity, gender, age, society and psychology, among which the lip shape is one of the most important factors affecting the aesthetic profile (^{Solem et al., 2013}; ^{Alkadhi et al., 2019}). It is often involved the control of the incisor position to achieve the change of lip morphology in orthodontic treatment. In the case of bimaxillary protrusion, incisor retraction plays an important role in orthodontic treatment, predicting lip soft tissue changes that occur following premolar extractions to some extent (Solem et al.). However, it is a debatable issue whether or not there is an exact relationship in upper lip response to the saggital change of incisor position (^{Leonardi et al., 2010}; ^{Konstantonis, 2012}; Alkadhi et al.).

The lip thickness has been also traditionally given little attention. Therefore, the primary aim of this study was undertaken to explore the response of lip position to incisor position and the association between upper lip thickness and upper lip repositioning upon maxillary incisor retraction in patients with bimaxillary dentoalveolar protrusion who had nearly completed active growth and were treated by extractions of four premolars, aiming to provide a reference for further clinical treatment.

MATERIAL AND METHOD

Establishment of lip thickness classification standard for Han population in Shanxi Province. The study population consisted of 100 adults (50 male, 50 female; 26.38±6.33 years).

Inclusion criteria was Han people in Shanxi. Patients with previous history of trauma, maxillofacial surgery or orthodontics were excluded. Pre- and post-treatment cephalograms were calibrated and digitally traced by one examiner using “Uceph” (Orthodontic Software, Sichuan, China). Secondary analyses were performed by dividing data into three groups based on pre- treatment upper lip thickness using the quartiles as cut- off points (^{Oliver, 1982}).Each group was ranked according to the lip thickness. The samples less than 25 % were in thin lips group, 25-75 % were in normal lips group, and more than or equal to 75 % were in thick lips group. The identified landmarks and measuring projects were displayed in Fig. 1 and described in Table I.

Fig. 1 Measuring of lip thickness. Sn.Subnasale; A’.Holdaway point; Ls. the vermillion margin of the upper lip; Ia. The most labial surface of the maxillary incisor. 

Table. I Cephalometric Analysis Measurements. 

The effect of upper lip thickness on the amount of incisor retraction required to reduce lip procumbency. The sample consisted of 19 consecutive nongrowing Asian females (mean age, 25.4±5.89 years; range, 19-36 years) with a chief complaint of lip protrusion. All were treated with 4 first premolar extractions followed by anterior retraction in the orthodontic clinic at the University of Shanxi, China. The following inclusion criteria were applied:(1) bimaxillary dentoalveolar protrusion; (2) Class I and Class Division 1 molar relationships; (3) the availability of 2 lateral cephalograms of good quality depicting; (4) lips in a relaxed closed position. Patients with craniofacial syndromes, such as cleft lip and palate, facial asymmetry or congenitally missing teeth were excluded. According to the statistical results, 19 female patients were divided into thin lips group, normal lips group and thick lips group, and the ratio between maxillary incisor retraction and upper lip repositioning was calculated, respectively. For the cephalometric analysis, two reference lines were constructed; a horizontal reference line (SN-7°) (Xaxis) through Sella seven degrees inferior to the Sella-Nasion line, and a vertical reference line (Y-axis) through Sella perpendicular to SN-7° (^{Hayashida et al., 2011}). Upper lip protrusion and maxillary incisor position were measured in relationship to the vertical reference plane (Fig. 2, Table I).

Fig. 2 Measuring of incisor and upper lip retraction. S; Sella; N: Nasion; Ls: Labrale superius; Is: Incisor superius. 

Statistical Analyses. All statistical analyses were performed with SPSS version 24 (SPSS, IBM©, Armonk, NY, USA) to test the significance of the differences between the change values. Based on their skewness and kurtosis, the distributions were all normal. Independent-sample t-tests were used to evaluate the significance of the differences between males and females. Differences between the cephalometric variables before and after treatment were analyzed by using paired-t test. ANOVA was applied to compare the differences of upper lip thickness among the three groups. Post hoc tests were done using the LSD-t method and correlation analysis was conducted between the ratio of maxillary incisor retraction to upper lip retraction and upper lip thickness. Bivariate regressions were performed to determine the relationships between the upper lip thickness and the ratio of hard to soft tissue movement commonly used for predicting soft tissue changes. The linear regression model was used to investigate possible associations between different ranges of lip thickness and the average ratio of maxillary incisor to upper lip repositioning. P-values less than 0.05 were considered statistically significant.

RESULTS

Establishment of lip thickness standard for Han population in Shanxi Province. Means and standard deviations of males, females cephalometric values, and the observed changes between them were displayed in Table II. A statistically significant difference between the sexes was found. The upper lip thickness of the male was greater than that of the female (P<0.001). To further investigate this primary finding, we divided our sample in three groups, based on their lip thickness. For males, first group fell within the lower 25 percent (<13.9 mm; n=14) and was considered to have thin lips, while third group fell within the upper 75 percent(>15.9 mm; n=12) and was considered to have thick lips; the rest was the normal group (13.9-15. mm; n=24). For females, first group fell within the lower 25 percent (<12.3 mm; n=15) and was considered to have thin lips, while third group fell within the upper 75 percent(>14.2 mm; n=12) and was considered to have thick lips; the rest was the normal group(12.3-14.2 mm; n=23). We applied ANOVA to investigate statistical significance for different lip thickness groups, as shown in Table III.

Table II Comparison of upper lip thickness between males and females (n=50). 

Table III Analysis of variance for different lip thickness groups. 

The ratio between maxillary incisor retraction and upper lip repositioning with different lip thickness. This study demonstrated that a significant retraction of the upper lip following premolar extractions in bi-protrusive patients occurred with treatment. In thin lips group, the position of the maxillary incisor relative to SN-7°perpendicular was 82.20 mm (SD,10.30) at T1. At T2, its position was 75.20 mm (SD, 12.38), a change of -7.00 mm (SD,3.06). The position of the upper lip in relationship to the vertical reference plane was 95.10 mm (SD,10.53) at T1. At T2, its position was 90.30 mm(SD,13.47), a change of -4.80 mm (SD,3.13). The results of normal lips group and thick lips group were similar to those of thin lips group (Tables IV, V and VI). Furthermore, thick lips would experience less lip retraction. A significant correlation was found between changes in maxillary incisor position following premolars extraction and changes in upper lip position (r=0.959, P<0.001) (Fig. 3). The bivariate association between upper lip and the underlying hard tissue retraction was moderately strong (r=0.735, P < 0.001) (Fig. 4). The difference of upper lip thickness had a negative correlation with the retraction of upper lip. When we divided our pretreatment sample (females) into 3 groupsthin (n=5), normal (n=7), and thick (n=7), there were significant differences in the change of the measurements (P < 0.001). The ratio between retraction of the upper incisal edge and labrale superioris was 1.6:1, 1.9:1 and 2.2:1 in thin lips group, normal lips group and thick lips group, respectively.

Typical cases. The facial profile of this patient was improved primarily by the retraction and retroclination of maxillary incisors to decrease upper lip procumbency and convexity (Figs. 5 and 6). The pre- and post-treatment lateral cephalometric analysis was compared, as shown in Fig. 7 and Table VII.

Table IV Table IV Descriptive statistics of pre- (T1) and post-treatment (T2) measurements in the thin lips group. 

Table V Table V Descriptive statistics of pre- (T1) and post-treatment (T2) measurements in the normal lips group. 

Table VI Table VI Descriptive statistics of pre- (T1) and post-treatment (T2) measurements in the thick lips group. 

Fig. 3 The correlation between maxillary incisor retraction and lip retraction. 

Fig. 4 The relationship between upper lip thickness and the ratio of maxillary incisor retraction to upper lip retraction. 

Fig. 5 Pre-treatment frontal (A) and profile (B) photographs. 

Fig. 6 Post-treatment frontal (A) and profile (B) photographs. 

Fig.7 Pre-(A) and post-(B) treatment lateral cephalograms. 

Table VII Cephalometric Measurements at Pretreatment and Posttreatment. 

DISCUSSION

Orthodontic treatment has reshaped facial aesthetics as one of its main goals. It is widely agreed that the position of incisors has an important effect on the shape of facial soft tissue, which can affect and reconstruct facial aesthetics by changing the structure of 1/3 lower face, especially the position and shape of lips (^{Kirschneck et al., 2016}). In this respect, due to the ever-increasing demands for orthodontic treatment in nongrowing patients for esthetic reasons, and the need on the other hand for clinicians to somehow forecast therapy outcomes, it seems beneficial to forecast the changes of perioral soft tissue that occur following premolar extractions in biprotrusive patients, thereby establishing a reliable ratio between incisor retraction and lip retraction.

The relationship between lip response and incisor movement has received noticeable research focus; it is expressed as the ratio of maxillary incisor retraction to posterior movements of labrale superioris, with previously reported ratios ranging from 1.2:1 to 3.2:1 (^{Kusnoto & Kusnoto, 2001}; ^{Ramos et al., 2005}; ^{Brock et al., 2005}). It is a widely-concerned issue whether or not there is an exact relationship between the changes in hard and soft tissue (Leonardi et al.; Konstantonis; Alkadhi et al.). In truth, several factors may have adversely affected the results as follows.

Previous studies have shown that racial differences have an impact on the retraction of lip following the movement of incisor. In the investigation carried out by ^{Sodagar et al. (2010}) the ratio of maxillary incisor to upper lip retraction was 2:1 in Iranian girls, which was in disagreement with ^{Arumugam’s study that indicated the ratio was 3:1 in South Indian females (Arumugam et al., 2012}). In fact, the nose and the chin move forward gradually, leading to a lesser protrusive profile. we should isolate the effects of treatment from growth in the evaluation of profile changes. It should be clear that this assumption can be true only when described changes are observed during the post treatment period until the completion of growth. The intertegmental profile can be accomplished with consideration of patients who had nearly completed active growth, so as to reduce growth effects (Kuhn et al.). In addition to age effects, sex differences in growth need to be taken into consideration, because women have less growth potential and reach maturity years before men. There is a difference between sexes regarding the lipincisor relation at this age. Males presented thickening of soft tissue and week correlation between the movement of incisor and soft tissue both cervical and incisal point. In female subjects, there was a strong correlation between the retraction movement and soft tissue (^{Paranhos et al., 2013}). Considering the above, there are, to my mind, plenty of contributing causes, among which the following one that is the object of this experiment, lip thickness, is particularly worth. Thicker lips appear to respond less to incisor movement and provide better supports to lip morphology (Alkadhi et al.). It also needs to be noted that extraction treatments tends to flatten the profile, whereas nonextraction treatment has a tendency to make it more protrusive. The study by ^{Bills et. al., (2005}) al showed that premolar extractions can be successful in reducing lip procumbency in patients with bimaxillary protrusion. On average, the upper lip retracted by 3 mm and the lower lip showed a 2.4 mm retraction (^{Petrén et al., 2003}; Bills et al.). As has been shown (Konstantonis). The extraction group terminated treatment with more retracted lips, a more obtuse nasolabial angle, and a thicker upper lip. Whereas the nonextraction group showed significant upper lip retraction but lower lip protraction. Last but not least, one should bear in mind that individual variation in soft tissue response is great. In short, We were obliged to carefully design this study in the selection of included samples, so as to reduce many of the variables that could adversely affect the results.

To ideally rebuild lip shape, the exact prediction of retraction extent of incisor is a wide-ranging discussion. This study, obviously, would be of help for the clinicians who predict soft tissue changes attributed to incisor retraction. This, in turn, would assist the clinicians in making extraction decisions and determining the amount of incisor retraction required to reduce lip procumbency and convexity.