# Change of Lip Curvature Through Extraction and Non-Extraction Orthodontic Treatment
## 1 Introduction  
Orthodontic treatment holds the promise of enhancing mastication, pronunciation, and aesthetic appeal for both doctor and patients. Subjective satisfaction is the standard when patients are evaluating these functions; notably, in matters of aesthetics, individual preferences must be well reflected in the completed treatment. In the past, it was considered that an excellent occlusal relationship automatically led to the aesthetics of soft tissue, but due to paradigm shifts based on studies by several predecessors [1; 2; 3], consideration of the soft tissue is essential in establishing treatment plans. Various systematic analyses of soft tissue morphology have been introduced to address morphological issues comprehensively [4; 5; 6; 7; 8; 9; 10; 11]. When confronted with instances of lip protrusion, extractions are frequently employed as a remedy. The objective is to create space through extraction and retract the incisors to promote the retreat of soft tissue, especially the lip area. Nevertheless, the advent of temporary anchorage devices (TADs) ushered in clinical trials centered on full arch distalization, enabling significant posterior movement of the dental arch without extraction [12]. However, as the correlation between dental posterior movement and lipmovement is not invariably constant, studies have delved into diverse factors influencing this relationship. When attenuating lip protrusion accompanied by extracting the maxillary incisor, the ratio of movement between the anterior teeth and lips approximately ranges from 1:0.75 to 1:0.7, depending on the tension of the lips during mouth closure [13]. Notably, among the measurements concerning anterior teeth, the backward movement of the cervical position is most significantly related to the posterior movement of the lips [13; 14]. The anteroposterior position of the lips and their morphological differences affect the facial profile. While the nose and lip line angles (nasolabial angle) have been measured [15; 16], the contour formed by the upper and lower lips has yet to undergo analysis. One of the significant anxiety factors when considering extraction is the potential flattening of the lip curvature as incisors recede post-extraction. An excessively flattened lip curve can confer an aged appearance, which appears worse than that before orthodontic treatment. The influence of first premolar extraction on lip position has been extensively studied, but findings remain inconsistent. Several studies, including [PERSON] et al. [17], [PERSON] [18], [PERSON] et al. [19], and [PERSON] et al. [20], reported varied outcomes on the retraction of the upper and lower lips following extraction treatments. A recent systematic review supported the general observation that extraction typically results in lip retraction relative to the E-plane, though ethnic differences, age, gender, and initial lip protrusion have been highlighted as significant factors that complicate direct comparisons between studies [21]. Recent advancements in imaging techniques have significantly contributed to this research. While CBCT-based 3D analysis is commonly used, it is often limited by less defined soft tissue images [22]. In contrast, face scanners have emerged as a promising tool for acquiring detailed three-dimensional digital information about soft tissues. For instance, [PERSON] et al. [23] demonstrated that orthodontic treatment involving extractions generally leads to significant lip retraction, while non-extraction treatments maintain or slightly enhance lip fullness. [PERSON] et al. reported similar trends, in hyperdivergent Class II patients, where extraction treatments resulted in a more pronounced lip retraction [24]. [PERSON] et al. [25] highlighted that orthodontic treatment, regardless of whether extractions were performed, could lead to reductions in facial fullness.