Changes of lip morphology following mandibular setback surgery using 3D cone-beam computed tomography images
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 [13–16].
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 [19–21].
Recent stuides reported and discussed about three-dimensional 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.