# Evaluation of the Nasolabbial Angle in Orthodontic Diagnosis: A Systematic Review
## 1 Introduction  
The evaluation of the facial profile is an important factor in any current orthodontic diagnosis, considering that an extreme advancement or retreat in the position of the upper lip or chin can determine the worsening of the patient's profile and aesthetic results. There are many factors to consider to preserve the aesthetics of the facial profile: the nasolabbial angle (NLA), the nasal prominence, the position of the upper and lower lip and the depth of the chin-labial sulcus. The NLA is one of the key factors to be considered in an orthodontic diagnosis as guidance for the aesthetics of the nose and facial profile. It is defined as the angle formed by the two lines passing through the lower edge of the nose (the columella) and the edge of the upper lip (as shown in Figure 1). As described in literature, the ideal nasolabbial angle ranges from 90\({}^{\circ}\) and 95\({}^{\circ}\) for males and 95-115\({}^{\circ}\) for females, although these values may vary among the various phenotypical groups (races). In general, the NLA values are increased in Asian populations, showing a flatter profile and a more obtuse angle in comparison with Caucasian populations or with African populations [1]. Dental extractions in orthodontics are often recommended in order to gain space in the case of severe or moderate crowding or for the camouflage of consolidated skeletal malocclusions. However, the orthodontic diagnosis and decision of whether or not to treat a patient with extractions remains controversial. As for the NLA modification, many theories exist. Some orthodontists believe that extractions worsen the patient's softtissue profile by making it too flat. Therefore, they prefer to follow a conservative approach, creating minimally invasive treatments that allow the recovery of space within the arch, such as: dental stripping, expansion of the dental arches and distalization [2]. On the other hand, there are orthodontists who believe that extraction represents the right therapeutic strategy. According to this approach, extraction does not worsen the profile, indeed in some cases it may improve it, by increasing the NLA values [3]. Furthermore, extractions often have a strong positive impact on factors such as: vertical dimension, treatment stability, width of the arches, facial convexity and period tissues [4]. The drawback of extractions is that they may result in a bi-retruded or flat profile, narrow dental arches, or an increase in the width of buccal corridors, which, in non-extractive cases, may result in an instability of the occlusal contacts with the risk of relapse. Additionally, in borderline cases, this may lead to a biprotruded profile [5]. Therefore, the customary extraction of four premolars is contraindicated in cases of a deep bite and horizontal growth pattern, as it will lead to the loss of the vertical dimension, further retrusion of the lower third of the face and an increase in NLA values [6]. Among non-extractive protocols that may influence the NLA, there are the distalization of the upper molars and the use of mandibular advancement devices (MAD), for the treatment of class II malocclusions [7,8]. Treatments such as the Delaire face mask for maxillary advancement, orthodontic camouflage and orthognathic surgery represent valid non-extractive therapeutic options for the treatment of patients with skeletal class III [9, 10, 11]. Rapid palatal expansion (RPE) is an effective treatment option for the correction of transverse maxillary discrepancies, which produces changes in the soft tissues related to an increase in the size of the nasal soft tissues and bone bases.