## 1 Introduction  
Maxillary transverse deficiency is one of the most common skeletal problems in the craniofacial region. However, it was demonstrated that the transverse dimension of the maxillary complex is the most malleable [1]. Traditionally, rapid palatal expansion (RPE) was the preferred standard treatment, especially for younger patients. Although the main purpose of RPE is to expand the maxilla by splitting the midpatalatature, the surrounding circum-maximillary sutures are also affected [2]. In addition, dental tipping and alveolar bone bending are commonly observed [3; 4; 5; 6]. RPE treatment for maxillary skeletal expansion is most effective when performed prior to the pubertal growth spurt, to minimize dentoalveolar side effects [7]. As patients mature and the midpatalat suture interdigitates, maxillary skeletal transverse expansion becomes more difficult, and the lateral forces from the tooth-borne RPE appliance are translated into more significant alveolar bone bending and dental tipping. This is true even in young patients, though to a lesser extent. These dentoalveolar changes can include fenestration, bone dehiscence, and a decrease in alveolar bone height [8].  
In order to avoid these undesired dentoalveolar side effects, a variety of bone-supported or hybrid micro-implant-assisted rapid palatal expanders (MARPE) were introduced [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. These MARPE appliances have grown in popularity in recent years as a potentially effective way to minimize dentoalveolar side effects in mature patients due to their ability to anchor directly onto bone. However, the results of different designs of MARPE appliances are varied, and significant dentoalveolar changes were still reported in many MARPE studies despite bone anchorage [11; 14; 15; 16; 17; 18; 19; 20]. One study reported negligible molar tipping [21], while others claimed that side effects such as alveolar bone bending and dental tipping are not preventable but can be reduced compared to RPE [9; 10].  
The midfacial skeletal expander (MSE) is one particular design of MARPE appliance that has been described in the literature since 2014 [12; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. There have been many studies on the use and successful application of the MSE in mature patients in recent years [12; 21; 28]. [PERSON] et al. described a downward and outward movement of the zygomaticomaxillary complex in the coronal plane, with the fulcrum slightly localized above the frontozygomatic suture when performing expansion with the MSE [21]. However, the amount of skeletal, alveolar, and dental expansion achieved with RPE and MARPE were mostly measured using linear measurements rather than angular measurements and were based on arbitrary points [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 30].  
Previous studies showed that MARPE expansions produce stable outcomes 1 year after expansion [31]. However, it was shown that MSE expansion is rotational in nature [21]. Two previous studies by [PERSON] et al. introduced a novel angular measurement system that takes into account the true fulcrum, providing a method to more accurately assess the rotational expansion created by the MSE [32; 33]. These studies illustrated about 95% skeletal rotations and negligible dentoalveolar changes with the MSE immediately after expansion. Although the immediate result was groundbreaking, the stability of skeletal expansion can be questioned. The purpose of this study is to assess the long-term skeletal, alveolar, and dental expansion effects of the midfacial skeletal expander by comparing the changes immediately after expansion and at the completion of orthodontic treatment.