## 1 Introduction  
When an orthodontist diagnoses a skeletal constricted maxillary arch, the orthopedic expansion involving the separation of midpalatal suture is the treatment of choice to normalize the transversal dimension of the upper jaw. Rapid palatal expansion (RPE) is recommended, especially in growing patients before the complete calcification of the craniofacial sutures, to prevent any development of skeletal asymmetries and to relieve anterior arch crowding [1]. RPE technique was first described by [PERSON] in 1860; since then, several banded loops or helices, jackscrew devices, and spring-loaded devices have been incorporated in the group of fixed tooth-borne maxillary expanders [2]. Several criteria should influence the choice of an appliance, e.g., the age of patients, transversal discrepancy amount, rate of screw activation, and the patient's capability for correct care of oral hygiene. Usually, the appliance is anchored to the first maxillary permanent molars, even if the expansion can often cause a deleterious buccal tipping of these teeth [3] that can partially relapse during the postretention phase [4]. Furthermore, anchoring teeth may develop gingival recessions, root resorption, and pulp diseases affecting their survival and functioning [5].  
The uniqueness of this procedure lies in the appliance anchorage. As a matter of fact, to avoid these undesirable effects on permanent teeth and attain a more physiological tissue reaction, RPE in the transitional dentition can be achieved by using primary teeth to anchor the appliance [6; 7; 8]. Two types of palatal expanders have been mostly recognized in the literature and the main difference between them is reported to be only the presence in the Haas type, or the absence in the Hyrax type, of acrylic pads close to the palate, in contact with mucosa. The first type was assumed to distribute the expanding force between the posterior teeth and the palatal vault [9]. The Hyrax expander was believed to deliver the force mostly to the maxilla through the appliance-supporting teeth, jointly, so as to be easy to clean [10].  
Despite this, cephalometric analysis has not demonstrated any differences between these two appliances, and there is no consensus regarding their method of action [11]. The skeletal and dental effects of RPE have usually been investigated on dental casts and radiographic evaluations such as latero-lateral and posteroanterior cephalograms, which have their limitations and do not allow for an accurate assessment of the structure involved without structure overlapping [12]. Nowadays, it is possible to use cone beam computed tomography (CBCT) for more precise reproduction of maxillary structures in all three spatial planes. This technology offers a reproducible three-dimensional (3D) image without the limiting factors of magnification and distortion, which affect conventional two-dimensional (2D) radiography and its subsequent accuracy.  
To date, only a few 3D studies are available to know if there is any difference between the Hyrax and Haas appliances when using permanent teeth as a support for the appliances, without any evidence about which is the most efficient device [9; 13; 14]. Moreover, only one other study has compared these two appliances when using primary second molars as anchorage and analyzing the data by CBCT images indirectly obtained from a 3D airway analysis [15]. Indeed, primary teeth have weaker roots when compared to permanent teeth and the different design and rigidity of the appliances, with or without the acrylic pad, might make a difference when compared to permanent teeth, with longer supporting roots, used as the anchor unit.  
Thus, the present randomized clinical trial aimed to three-dimensionally compare the skeletal and dental effects of Haas-type and Hyrax-type expanders using the primary teeth of growing patients as anchorage units, and to test the null hypothesis that there are no differences between tissue-borne and tooth tissue-borne expanders in growing patients treated by RPE.