# Maximally Deficiency: Treatments and Applications for Adolescents
## 2 Orthopedic Approaches
### Preadolescents in Late Mixed Dentition and Early Adolescents
#### 2.2.1 Tooth-Borne Expander  
The HYRAX is a tooth-borne expander featuring a jackscrew within an all-wire frame. The expander has heavy-gauge wire extensions that conform to the palatal contours, either soldered to metal bands on teeth or embedded into acrylic on the buccal dentition (Figure 1a). It is capable of achieving a sutural separation rapidly by activating the jackscrew. Each activation of the screw produces approximately 0.2 mm of lateral expansion. Thesedevices can exert up to 10 kg of force, with initially 80% of the expansion being skeletal, mainly in the anterior region. Over time, significant relapse occurs, retaining only about 50% of the skeletal expansion. Expansion with this appliance can effectively achieve up to 10 mm of maxillary expansion [16]. The bands of the HYRAX expander are affixed to the first premolars and first molars, or exclusively to the first molars with extension arms. Research indicates that compared to two-band devices, four-band devices enhance transverse expansion and arch perimeter, particularly in individuals over 12 years of age when the suture is more calcified, so banded first premolars are recommended when available [74]. Additionally, second primary teeth can be used as anchorage in two-band devices by substituting the first molars with bands, thereby mitigating premature stress on the first permanent molars and preventing their buccal tipping [60; 75; 76]. A retrospective multicenter radiographic study highlighted that the HYRAX expander anchored on deciduous molars achieved significant anterior and posterior skeletal expansions but minimized unwanted dental effects on maxillary permanent teeth compared to those using permanent molar anchorage [76]. A randomized clinical trial also demonstrated that the HYRAX expander anchored on deciduous molars produced more anterior expansion than the Leaf Expander [60]. In instances where first premolar anchorage is not available, bonded HYRAX expanders can be attached through acrylic or metal cast splint bonding on the buccal dentition (Figure 1b,c). The expanders with an acrylic cap cover the occlusal, facial, and lingual surfaces of the posterior teeth, raising the bite for patients with anterior occlusal interference [77]. The study found that there is no significant difference in molar tipping and extrusion between banded and bonded HYRAX expanders [78]. In contrast to conventional expanders, which employ a centrally positioned parallel-opening screw to dissociate the mid-palatal suture uniformly, the fan-type expander is engineered to focus expansion effects primarily in the anterior segment of the dental arch, exerting minimal influence on the molar region [29; 30]. This appliance integrates a jackscrew within an acrylic-bonded expander framework. The hinge point of this screw is precisely located tangent to the distal surfaces of the upper first permanent molars, thereby selectively expanding the inter-canine region of the maxilla [29; 30]. The Expander with Differential Opening (EDO) is specifically designed to facilitate differential expansion between the anterior and posterior region. Originally, the EDO was developed to achieve enhanced anterior expansion of the maxillary dental arch in patients with complete bilateral cleft lip and palate (BCLP) [79]. This appliance incorporates two parallel-opening screws located strategically within the palate--one positioned anteriorly and the other posteriorly. The differential activation of these anterior and posterior screws results in a trapezoid-shaped opening of the appliance, with a wider divergence toward the front [31]. Compared to the fan-type expander, the EDO produces greater transverse skeletal expansion, while the effects in the vertical and anteroposterior directions are similar [32]. Figure 1: Designs of HYRAX and Hass Expanders. (**a**) The HYRAX expander with two premolar bands and two molar bands.