# Evaluation of the Nasolabbial Angle in Orthodontic Diagnosis: A Systematic Review
## 4 Discussion  
### NLA in Extraction vs. Non-Extraction Cases  
There were 10 articles addressing the variations of the NLA by comparing the extractive cases to the non-extractive cases, with a total of 2845 patients divided into the various study groups, including 428 females and 696 males, with an average age of 14.62 years. Among these, four authors found a statistically significant increase in post-treatment NLA values ranging from 2.4\({}^{\circ}\) to 9.410\({}^{\circ}\) in class I patients treated with extractions of the first four or second premolars, and in patients with maxillary bi-protrusion treated with extractions of all four first premolars. [PERSON] et al. recorded a retraction in the position of the upper and lower lips and the consequent increase in the NLA with an average difference of 4.92\({}^{\circ}\) after the orthodontic treatment of these patients who had extractions. The authors explained that the retraction of the lips, and therefore an increase of the NLA, is due to the retraction of the upper and lower anterior teeth as they move backwards to occupy the extractive spaces. On the other hand, there is no change in the NLA in those cases with extractive orthodontics where the procedure indicated aimed at the resolution of dentoalveolar crowding or a camouflage of mild skeletal malocclusions. The authors stated that a 2 mm change in lip retraction is enough to make the profile look worse or better [3]. Similar results to the previous study were reported by [PERSON] et al., who registered a statistically significant increase both in the NLA and in the length of the upper lip, as related to a retraction of the upper and lower incisors--plus a retro-inclination--in biprotruded patients treated with extractions of the first upper and lower premolars. They also report a decrease of 1.1 mm post treatment in the exposure of the upper incisors [13]. Additionally, [PERSON] et al. reported a significant increase in the NLA in cases involving the extraction of the four upper and lower first premolars compared to non-extractive cases. However, there was no significant increase in the NLA's values obtained in cases where only two premolars were extracted. The retraction of the lips and the increase in the NLA's values were associated with a retraction of the upper or lower incisors. This means that extractions can have a low impact on the facial profile if the retraction of the anterior sector is minimal, as it occurs in cases where the extractive spaces are closed by mesialization of the posterior sectors. There was also a significant association between the NLA values and the patients' age, as a greater increase in the NLA was achieved in older patients than in those youngsters who were still growing. The cause of this association is reported to be related to the growth of the nasal spine or of the lips during the patient's growth [4]. A significant increase in NLA of 9.410\({}^{\circ}\)--with subsequent labial retraction in class I borderline patients--was recorded in the study of [PERSON], [PERSON] and [PERSON], which supports the results reported by the previous authors [5]. Four studies reported no statistically significant values for the NLA by comparing the results obtained among a group of patients undergoing extractions of the four premolars and a non-extractive group, among these: [PERSON] et al. in their study on 20 class I patients, [PERSON] et al. in their study conducted on 195 patients, and [PERSON] et al. in their study on 50 patients [6; 15; 16]. [PERSON] et al., as well, registered no statistically significant NLA values, but in their article they compared the NLA values obtained in two extraction groups, one with the extraction of the four first premolars and one with the extraction of four s premolars [17]. Two more studies reported a statistically significant increase of NLA values in the treatment of extractive cases. One of these studied the results obtained with skeletal anchorage using orthodontic mini screws. [PERSON] et al.