**2. Rapid maxillary expansion**

RME is a widespread orthodontic practice [3]. It is a dentomaxillofacial orthopedic treatment process commonly adopted in young patients to treat narrowed maxilla (**Figure 1**) [4]. This procedure is used for handling structural and functional problems in the middle of the face [5]. It involves applying an intraoral screw mechanism anchored on selected teeth (**Figure 2**) or bone that produces orthopedic forces to the mid palatal suture, with the forces dispersing through the circum-maxillary and cranial sutures [6].

In particular, RME enlarges the nasal cavity base and the maxilla [7, 8]. RME is used to fix the constricted transverse diameter [9], expanding the arch perimeter that will deliver more space for the correct positioning of crowded teeth and permit crossbite correction [10].

#### **Figure 1.**

*Severely narrowed maxilla and first ERM appliance. Anterior and posterior crossbite before and after the first RME appliance.*

#### **Figure 2.**

*After the first ERM appliance, a diastema appeared between central incisors, showing that the midpalatal suture opened.*

When we open the midpalatal suture, we get a diastema between central incisors (**Figure 2**) that is closed per se without an appliance procedure.

The RME device is a fixed appliance that does not require from parents or children intense cooperation. It is easily cleaned and works in a short period, between two and four weeks [11].

Palatal enlargement in children increases nasal flow and diminishes nasal resistance [12].

The treatment of mouth breathing is multidisciplinary, and the patient should be sent to the otorhinolaryngologist as treatment should be initiated after the triggering condition of oral breathing is resolved. RME is the recommended therapy in the interception phase [13].

After RME, improved resistance and nasal flow were documented in patients in the supine position, who had a posterior and anterior obstruction. Smaller changes were observed in isolated forms of obstruction and in the orthostatic position. In nasal airway obstruction with maxillary constriction, RME has proved effective for improving nasal breathing in children via an enlargement effect on the nasopharynx [14]. Pharyngeal airway pressure during inspiration is decreased with the reduction of nasal resistance [15].

In intermaxillary, internasal, frontonasal, frontomaxillary, and maxillonasal sutures, RME produces significant width increases, whereas the zygomaticotemporal, frontozygomatic, pterygomaxillary, and zygomaticomaxillary sutures showed nonsignificant changes; forces elicited by RME affect mainly the anterior sutures (maxillary frontal nasal and intermaxillary interfaces) compared with the posterior (zygomatic interface) craniofacial structures [16]. Cephalometry showed increased maxillary width after RME [10].

The RME substantially increases interglenoid chamber distance and mandibular condyle displacement at six months in growing patients compared to a control group. RME is effective during growth, enlarging the interglenoid fossa space and the lateral positions of the condyles and eventually expanding the nasal cavity without producing asymmetry [17].

After RME, substantial improvement in the transverse dimensions of the maxillary basal bone, the nasal cavity, and the midpalatal suture opening happened, with the highest growth in the midpalatal suture followed by nasal cavity and basal bone [18].

Considerable alterations in the space of the pharynx may be obtained in Class II patients through both RME and mandibular advancement devices with the capacity of palatal expansion [19].

RME improved the mucociliary clearance in children with maxillary narrowness, positively affecting nasal physiology and improving nasal cavity volume [20].
