## 1 Introduction
have been introduced: all appliances lead to a predictable maxillary expansion, with low and constant force and predetermined for intensity, direction, and amount of correction. The main difference concerns the two basic appliances:  
1. Reactivable;
2. Preactivated. The reactivable L.E. has a metal structure of chrome-cobalt (Cr-Co) with a central screw whose activation generates the compression of two or more nickel-titanium leaf springs. Figure 1: Leaf expander with 6 mm screw and NiTi leaf springs. Two main reasons differentiating the L.E. from others palatal expanders are:  
1. Active component;
2. Action modalities. The screw action does not act directly on cemented teeth, but generates the compression of a nickel-titanium leaf spring. Later, the screw deactivates the spring by recovering its original size, thus producing a balanced expansion. During early mixed dentition, it is preferable to lean on second deciduous molars (E) through two bands, but if needed, first permanent molars are also accepted. To optimise the expansion and stability of the appliance, it can also have two anterior extensions to the canines. Currently, four reactivable L.E. types are available, which differ in the maximum amount generated (in mm) and the entity of force generated (in gr)  
1. 6 mm screw--450 gr;
2. 6 mm screw--900 gr;
3. 9 mm screw--450 gr;
4. 9 mm screw--900 gr. According to the transverse discrepancy calculation on the models (in mm), a 6 mm screw is used when the deficit is less than 5 mm, while, when the discrepancy is greater, a 9 mm screw is used. Generally, in the case of unilateral cross-bite, 6 mm screws are indicated; in the case of bilateral cross-bite, 9 mm screws will be useful. Usually, in deciduous/mixed dentition 450 gr forces are used, while in permanent dentition 900 gr forces are used. A 6 mm screw has two leaf springs and allows for up to 30 activations; the 9 mm one is characterised by three leaf springs and can be activated up to 45 times. In both, each turn/activation produces a 0.1 mm expansion. The number of leaf springs determines the amount of the expansion while their thickness determines the entity of the force, which remains constant regardless of the degree of compression. Regarding appliance cementation, both on bands and on custom made metal structures, it is essential to know whether the teeth surface is undergoing caries prevention procedures or enamel demineralisation treatment. The application of fluoride varnishes before bonding could significantly reduce the SBS value (Shear Bond Strength). In these cases, it is recommended to use prophylactic agents such as CPP-ACP paste, ozone or nano-hydroxyapatite instead of fluoride varnishes [28; 29; 30; 31]. The L.E. has some advantageous features compared with other palatal expander appliances:  
- No compliance needed;  
- Reduced need for clinical checks;  
- Painless;  
- Controlled, light and graded force production;  
- No risks of overexpansion;  
- Easiness of activation;  
- Vestibular tooth inclination control. Considering that each screw activation generates a 0.1 mm expansion, 10 activations will be required to obtain a 1 mm expansion. In a 6 mm appliance, the maximum number of activations will be 30; in a 9 mm type, it will be 45, generally divided into three sessions. The lateral arms of the screw should remain offset from the palatal mucosa, approximately 2.5 mm, to avoid the possibility of creating decubitus ulcers. Generally, active expansion takes four to six months. After that, L.E. must be maintained in situ for at least three more months. About one year after cementation, as recommended for other palatal expander types, the appliance can be removed without the need for additional retention (Figure 2A-C). **Leaf Self Expander**  
The Leaf Self Expander(r) (L.S.E.) design is similar to that of an L.E. : the differences are the presence of three double pre-activated nickel-titanium springs and the absence of the reactivation screw [2] (Figure 3). The new technology of the L.S.E.