# Change of Lip Curvature Through Extraction and Non-Extraction Orthodontic Treatment
## 4 Discussion
The former hypothesis can be examined using patients with entire arch distalization who underwent similar incis retraction as the extraction group. Although the non-extraction group in the present study did not present significant incisor retraction, multiple regression analysis revealed that extraction itself was a predictive variable (Appendix A Table A3), which supports the first hypothesis. Correlation analysis showed that the SAL angle was associated with LL-to-E, which supports the last hypothesis (Appendix A Table A1). Although \(\Delta\)LL-to-E was \(-\)1.8, PBL depth and PBL angle displayed no significant differences before and after treatment, as mentioned above (Tables 2 and 3), which means that while the lower lip maintains its curvature, the upper lips tend not to flatten with the retraction of the upper incisors. Multiple regression analysis demonstrated that \(\Delta\)U1 tip was negative, while \(\Delta\)L1 tip was positively correlated with \(\Delta\)SAL (Appendix A Table A3). The average movement of the upper incisors in the extraction group was \(-\)2.7 \(\pm\) 2.3 in \(\Delta\)U1 tip and 0.61 in \(\Delta\)U1 tip/\(\Delta\)U1 cervical, which means that moderate anchorage and controlled tipping did not worsen lip curvature after treatment (Table 4). Considering the multiple regression formula, when the mean value of \(\Delta\)U1 (\(-\)2.7) and \(\Delta\)L1 (\(-\)3.19) was applied, \(\Delta\)SAL angle = 0.66 \(\Delta\)U1 tip \(-\) 0.98 \(\Delta\)L1 tip \(-\) 1.58, only \(-\)0.25\({}^{\circ}\) of SAL change could be expected. The retraction of the lower lip might support the vertical dimension of the upper lip, which maintained SAL. In addition, the interlabial gap did not change (\(-\)0.40). The reason for this phenomenon may be the upper and lower lip interaction, lip tonicity increment, and buccinator mechanism. Therefore, the general opinion that retraction of the upper incisors flattens the upper lip should be reconsidered. This study has some limitations. To compare similar initial conditions, the study excluded maximum skeletal anchorage cases. Since a previous 3D study reported more lip retraction using TAD [26], the application of this study should be limited to the moderate anchorage cases. A large amount of lip retraction may cause lip flattening and side effects such as root resorption, alveolar bone loss, fenestration, dehiscence and gingival recession [27], and insufficient alveolar bone remodeling [28] must be considered. A recent study examining dentoskeletal and soft tissue changes in patients with and without tooth extractions over a span of 50 years found no significant differences between the two groups. Therefore, the need for excessive retraction of anterior teeth, which can lead to lip flattening, should be reevaluated [29]. Another limitation was that this study sample was skewed toward females. Although the sex distribution of Class I malocclusion is known to be biased toward females, the response patterns to corrective treatment are not different between the sexes [30]. There may still be differences in the muscle tonicity and response. Thus, innate differences between the initial and final states may exist between sexes. Another limitation is that the initial inclination of the upper incisors differed between the extraction and non-extraction groups (U1 FH, 118\({}^{\circ}\) vs. 114\({}^{\circ}\)) although not significantly. Further studies, including similar initial and final incisor positions, will shed light on the correlations among the multiple variables.