Changes of lip morphology following mandibular setback surgery using 3D cone-beam computed tomography images

Background The aims of this study are to evaluate the lip morphology and change of lip commissure after mandibular setback surgery (MSS) for class III patients and analyze association between the amount of mandibular setback and change of lip morphology. Methods The samples consisted of 14 class III patients treated with MSS using bilateral sagittal split ramus osteotomy. Lateral cephalogram and cone-beam CT were taken before and about 6 months after MSS. Changes in landmarks and variables were measured with 3D software program Ondemand™. Paired and independent t tests were performed for statistical analysis. Results Landmarks in the mouth corner (cheilion, Ch) moved backward and downward (p < .005, p < .01). However, cheilion width was not statistically significantly changed. Landmark in labrale superius (Ls) was not altered significantly. Upper lip prominence angle (ChRt-Ls-ChLt °) became acute. Landmarks in stomion (Stm), labrale inferius (Li) moved backward (p < .005, p < .001). Lower lip prominence angle (ChRt-Li-ChLt °) became obtuse (p < .001). Height of the upper and lower lips was not altered significantly. Length of the upper lip vermilion was increased (p =< 0.01), and length of the lower lip vermilion was decreased (p < .05). Lip area on frontal view was not statistically significantly changed, but the upper lip area on lateral view was increased and change of the lower lip area decreased (p > .05, p < .005). On lateral view, upper lip prominent point (UP) moved downward and stomion moved backward and upward and the angle of Ls-UP-Stm (°) was decreased. Lower lip prominent point (LP) moved backward and downward, and the angle of Stm-LP-Li (°) was increased. Li moved backward. Finally, landmarks in the lower incisor tip (L1) moved backward and upward, but stomion moved downward. After surgery, lower incisor tip (L1) was positioned more superiorly than stomion (p < .05). There were significant associations between horizontal soft tissue and corresponding hard tissue. The posterior movement of L1 was related to statistically significantly about backward and downward movement of cheilion. Conclusions The lip morphology of patients with dento-skeletal class III malocclusion shows a significant improvement after orthognathic surgery. Three-dimensional lip morphology changes in class III patients after MSS exhibited that cheilion moved backward and downward, upper lip projection angle became acute, lower lip projection angle became obtuse, change of upper lip area on lateral view was increased, change of lower lip area decreased, and morphology of lower lip was protruding. L1 was concerned with the lip tissue change in statistically significant way.

Background Improvement in facial esthetics is one of the most important goals of orthodontic treatment and orthognathic surgery [1,2]. To obtain the best results, treatment planning and the assessment of results should be performed on measurable three-dimensional reproductions of the face of the patients [3,4]. However, the soft tissue response after mandibular setback osteotomies is subject to individual variation, and the predictability of soft tissue changes remains an important topic [5,6].
Arnett and Bergman [7], Arnett et al. [2], and Proffit [8] emphasized the importance of esthetics in the frontal view, and orthodontists shifted the focus from the sagittal plane to the frontal plane when evaluating their patients and planning and assessing orthodontic treatment [9].
Researches have used lateral cephalogram to observe the aspect of transition regarding movement of soft tissue after surgery. However, it could result in inaccurate consequences in quantitative evaluation due to distortion of length, angle, and form by reflecting 3D structure in 2D plane [10]. Using laser scanner method can only measure change of soft tissue; therefore, in order to understand the change of skeletal structure, analysis of 2D radiograph should be enforced or 3D CT be re-filmed. Three-dimensional CT data, which can reduce the magnification and distortion errors of 2D radiographs [11,12] and resolve the limitations of the 3D surface scanning system, have been used to analyze and measure the 3D structures [13][14][15][16].
Recent studies reported that in the smiles of the class III pretreatment group, both the upper and lower lips moved to an inferior position, and the upward movement of the upper lip and mouth corners was smaller, compared with those of the control group. And there were previous studies that lip morphology is different in patients with class III malocclusion when resting and smiling, and lip commissure is known to be inferiorly positioned than normal occlusion patient [17]. The morphology of lip and cheilion in class III patients may be shown in less esthetic.
In addition, there is no consensus about the changes of lip commissure related to the amount of mandibular setback. The position of lip commissure is known to be altered after orthognathic surgery. In recent study, upper lip projection angle (ChRt-Ls-ChLt°) became acute and lower lip projection angle (ChRt-Li-ChLt°) became obtuse [1,18]. Change of labrale superius (Ls), cheilion, and labrale inferius (Li) had been debated. Jung et al. reported that the landmark in Ls was not altered significantly and cheilion, Li moved backward [19]. Lim et al. reported that the landmark in Ls, cheilion, and stomion moved backward and downward and Li moved backward [20]. Baik et al. reported that Ls, cheilion, and stomion and Li moved backward and downward [21]. In most papers, the width of the lip and the position of the upper lip were not altered significantly. And, the height of the lower lip was not altered significantly in some study. While, it was significantly decreased in other study [19][20][21].
Recent stuides reported and discussed about threedimensional evaluation of facial soft tissue changes after mandibular setback surgery. They are consistent with the decrease of the upper lip prominence angle, increase of the lower lip prominence angle, and no change of the upper lip length. But, they are not consistent with the commissure position, the lower lip length, and so on. Also, there are few studies about the change of lip morphology according to mandibular setback surgery (MSS), according to the authors' knowledge. We analyzed the lip morphology and change of lip commissure after mandibular setback surgery according to the amount of mandibular setback only (one jaw surgery) with comparison of preoperative and postoperative lip morphology.

Methods
The samples consisted of 14 class III patients (seven males and seven females, mean age 20.3 ± 3.1 years, range 18 to 29 years) with Alphard-3030 cone-beam CT system (Asahi Roentgen Ind. Co., Ltd.) with the following parameters: 17-s scan time and a voxel size of 0.39 mm. And they had mandibular prognathism (ANB <0°, N-perpendicular Pog > 5 mm) and were conducted only MSS with bilateral sagittal split ramus osteotomy (BSSRO) by two surgeons (Table 1). Patients who had two jaw surgery or orthodontic treatment with premolar extraction or significant mandibular asymmetry (chin deviation >5 mm from facial midline) or lip incompetency or functional muscular problem were excluded. To evaluate the effect of the hard tissue changes on the soft tissue changes more accurately and to exclude the effect of the maxillary surgery on the soft tissue changes, the samples need to be limited to cases with MSS.
Cone-beam CT was taken immediately before (T1) and 6 months after MSS (T2) with centric occlusion, reposed lip, and natural head position and no face movement. We chose a period 6 months after surgery as the T2 stage because we reasoned that after this period, the facial soft tissue would be stable and any subsequent changes would be small enough to be ignored [22].
The cone-beam CT raw data were reformatted into 3D views using 3D software program Ondemand™ (Cybermed Inc., Seoul, Korea). Reference plane was set by using landmark on orbitale, nasion, porion, and frontozygomatic suture that were not changed after operation. It was reoriented based on this landmark. Axes of X, Y, and Z was set in the horizontal, vertical, and anterior-posterior axis. Landmarks and reference planes are defined in Table 2 and Fig. 1. The 3D positions of the landmarks and five linear and seven angular variables at T1 and T2 stages were measured by a single operator using Ondemand 3D™ (Table 3 and Figs. 2 and 3). The extent of changes in the landmark and the linear and angular variables between the T1 and T2 stages were measured by a single operator using Ondemand program (Tables 3 and 4).

Statistical analyses
We evaluated the clinical data of retrospective cohort study about 14 patients. The targets were patients who visited the Department of Oral and Maxillofacial Surgery at the Dental Hospital of Wonkwang University during January 2012~October 2015. An independent t test was used to compare the amounts of soft tissue changes with the amounts of hard tissue changes in T1 and T2. The correlations between the soft tissue and hard tissue movements in the horizontal and vertical planes were examined by Pearson   Coronal plane A plane constructed with the right frontozygomatic suture (FSZ) and left FZS, perpendicular to the FH plane Sagittal plane A plane constructed with nasion, perpendicular to the FH and coronal planes correlation analysis. The horizontal ratios of the soft tissue to hard tissue changes in the groups were also compared. We conducted Shapiro-Wilks test to identify this normality test, and all variables had been normally distributed in significant probability over 0.05. A paired t test was performed to analyze the changes of 3D coordinate values and measurements between before and after the surgery. Statistical analysis was performed using Fisher's exact test and Pearson correlation analysis and with SPSS ver. 12.0.

Results
The samples consisted of 14 class III patients treated with MSS using bilateral sagittal split ramus osteotomy. The average periods when follow-up cone-beam CT was taken were about 10 months (range = 6 to 24 months) after operation. Five patients had not undergone genioplasty, three patients had undergone reduction genioplasty, and six patients had undergone advance genioplasty.    (Table 4 and Fig. 4).

Change of cheilion, cheilion width, and lip prominence angle
Landmarks in the cheilion moved backward and downward (respectively 1. 8

Change in lip height, vermilion length, and the lip area
The height of the upper and lower lips was not altered significantly. The length of the upper lip vermilion was increased (3.5 mm, p < .01) and the length of the lower lip vermilion was decreased (1.9 mm, p < .05) ( Table 7 and Fig. 2). The lip area on frontal view was not statistically significantly changed, but the upper lip area on lateral view was increased and lower lip area decreased (upper lip area 6.3 cm 2 increased, lower lip area 15.6 cm 2 decreased, p > .05 and p < .005) ( Table 9 and Fig. 5).  (Table 8 and Fig. 6).

Correlation between the hard and soft tissue variables
The following is the result of Pearson coefficient analysis regarding the soft and hard tissue. The most influential landmark is L1, and it was related with cheilion's backward and downward movement (R 2 = 0.67 and R 2 = 0.60; p < .05 and p = .05) and lower lip prominence angle (Stm-Y-Li°) (p =.057). The upper movement of menton is concerned with increase in stomion prominence angle (ChRt-Stm-ChLt°) (R 2 = 0.59; p < .05) (Table 11).

Discussion
The lip is a structure positioned in the central part of the face, which has important influences on esthetic impression. After orthognathic surgery, the morphological change is inevitable so that accurate understanding about the alteration of soft tissue around lips is required. Three-dimensional CT may be an effective tool for investigating the 3D changes in hard and soft tissues simultaneously in terms of direction and amount of movement information that 2D radiographs and 3D surface scanning systems cannot provide. If a better algorithm to combine the 3D laser or optical surface scanning and CT without distortional error is developed, it would be a great advance for the clinical research.
In our studies, these showed the same results as the previous studies. There was no change of lip width and decrease in upper prominence angle and lower lip vermilion, but increase of the lower prominence angle and the upper lip vermilion length showed the same results like the previous research [19][20][21].

Change of cheilion, cheilion width and lip prominence angle
There have been controversies over mouth corner (cheilion position) in each study. In our study, landmarks in the mouth corner (cheilion, Ch) moved backward and downward after MSS (respectively 1.8 and 1.2 mm, p < .005 and p = .005). Lim [19][20][21]. The result of cheilion moving posteriorly is shown among the three studies. However, regarding the present study and the inferior movement of Lim and Baik's one, the superior movement of Rafiqul after the surgery was the   opposite result [17]. Further study is required for the inferior movement of cheilion. Cheilion width was not statistically significantly changed. Landmark in labrale superius (Ls) were not altered significantly. It implies that muscle tonicity might not be changed within a short period of time [19]. Also, this means that an increase in the lip length between CBP and Ch was produced by stretching the soft tissues. Posterior and inferior movements of Ch were also observed in the 3D coordinates.
Before MSS, soft tissue stretching of the chin in the prognathic mandible can pull the upper and lower lips downward. When normal lip posture is achieved after MSS, normal convexity of the upper and lower lips can be restored. 3D soft tissue changes in class III patients after MSS occurred more in the Li than Ls with an increasing gradient. Because there was more backward movement of Li than Ls (4.3 vs. 0.2 mm, Table 6), the positions of the upper and lower lips were affected by the position of lower incisors in class III patients before MSS. However, after MSS, the upper and lower lips were under the influence of the upper incisors. The lower lip tension was reduced after MSS allowing for the formation of a better environment for lip sealing and eventually increase in lower lip prominence angle [20].

Change in lip height, vermilion length and the lip area
The height of the upper and lower lips was not altered significantly. These results are in accordance with Jung et al. and Baik et al. [19,21]. But the height of the lower lip is reported to change statistically significantly. They explain that the position of the lower lip would be influenced by the position of the upper incisors after   [19,20,24].
The length of the upper lip vermilion was increased (3.5 mm, p < .01) and the length of the lower lip vermilion was decreased (1.9 mm, p < .05). After MSS, there is a possibility of decrease in lower lip tension, which would allow for replacement of the upper lip over the lower lip and eventual opposing change in the vermilion border length [20].
On frontal view, the lip area was not statistically significant changed, but the upper lip area on lateral view was increased and the lower lip area decreased after MSS (6.3 cm 2 , p = .078, -15.6 cm 2 , p < .005). This is due to the cheilion which moved backward and downward and labrale inferius which moved backward. These changes make the facial profile more esthetic in class III patients (Table 5, 9 and Fig. 5).

Change in the 3D linear angles of lip
On lateral view, the upper lip prominent point moved downward (1.24 mm, p < .05) and stomion moved backward and upward (1.79 and 0.63 mm, p < .005 and p = .103) and the angle of Ls-UP-Stm (°) was decreased (−9.36°, p < .05) ( Table 8). The lower lip prominent point moved backward and downward (−2.7 and −2.1 mm, p < .05 and p < .005) and the angle of Stm-LP-Li (°) was increased (6.43°, p .065) ( Table 8). Li moved only   backward (−4.32, p < .001) and not downward. Finally, morphology of the lower lip was protruding (Table 8 and Fig. 6). The eversion of the lower lip became distinct, and a more natural lip line was formed. After the surgery, the outcome of the lower lip eversion is shown in other studies as well [17,[19][20][21]. Three-dimensional optical surface scans could measure the magnitude of change and show a significant eversion of the lower lip, which was reported by Hershey and Smith [23].
In that study, using the thin-plate spline analysis, the forward movement of stomion inferius was shown, while labrale inferius was moving backward, displaying the eversion of the lower lip and increasing the lower vermilion border in most class III cases. This was because the relaxation of the lower lip after the tension caused by the lower teeth and the alveolar bone was eliminated with a mandibular setback, eventually eliminating lip incompetence and forming a better lip seal.

Change in the lower incision tip and stomion
Landmarks in the lower incisor tip (L1) moved backward and upward but, in stomion moved downward. After surgery, L1 was positioned superiorly than stomion (−1.97 mm, p < .05) ( Table 10 and Fig. 6). According to Robinson, Ls and Li moved inferiorly, even though almost all hard tissue landmarks had moved superiorly. This was because the free ends of the upper and lower lips interfered with the contacting incisor surface and the opposite lips recovered by moving downward [24].

Correlation between the hard and soft tissue variables
The following is the result of Pearson coefficient analysis regarding the soft and hard tissue. The most influential landmark is L1, and it was related with cheilion posterior movement (p < .05 and p = .05) and lower lip prominence angle (Stm-Y-Li°) (p = 0.057). The upper movement of menton is concerned with increase in stomion prominence angle (ChRt-Stm-ChLt°) (p < .05) ( Table 11). These changes may be a contributing factor that the lower lip makes eversion and is more esthetic. Perhaps, decrease of muscle tension might change the prominent angle of the upper lip and lower lip at lateral view. It might make the form of the lip esthetically and eversion.
Lip commissure was seemed to be positioned higher in postoperative phase than in preoperative phase. It may be due to increase of lip thickness. Finally, lip morphology seems to be improved in the view of esthetics after orthognathic surgery (one jaw) for mandibular prognathism.
After mandibular setback, the position of the lip commissure tended to be higher, and the amount of 1.27 mm was not a few changes. This amount of changes was statistically significant.
Because of the limited number of the included patients in this study, authors could not analyze how much genioplasty affects the changes of lip morphology. Afterwards, there would be a research more focus on the change in lip morphology according to genioplasty or non-genioplasty group based on significant number of patients. Two-jaw surgery is more influencing on the muscle and soft tissue than one-jaw surgery, and further research on the lip change is needed. Further researches are needed to be followed on this topic, and it would help us to predict the changes of lip morphology according to the orthognathic surgery.  ΔCh Rt-Stm-Ch Lt (°) 0.646 0.032* *Statistically significant difference between the groups (P <0.05) **Statistically significant difference between the groups (P <0.01) Correlations and ratios of the anteroposterior and vertical changes between the hard and soft were examined. Pearson correlation analysis was done. Ratio (R 2 ) means amount of change in the soft tissue/amount of change in the hard tissue. Positive (+) value, the change with the same direction; negative (−) value, the opposite direction