## 4 Discussion  
A few authors have studied the effects of maxillary expansion with appliances anchored to deciduous teeth and, to the best of our knowledge, none of them have compared Hyrax and Haas with CBCT technology. The present study compared three-dimensional dental and skeletal effects of rapid palatal expansion using deciduous teeth as anchorage but comparing a tissue-borne and a tooth tissue-borne maxillary expander. Based on the obtained results, skeletal maxillary expansion is lightly affected by the appliance design, and it can be stated that, statistically and clinically, no differences between these two appliances exist; the null hypothesis was therefore confirmed. In the literature, there are several studies that used 2D radiographs and dental casts to evaluate and measure the structural changes produced by RME [2; 3; 6; 7; 18; 19; 20]. Some authors studied these effects using 3D technologies when the first permanent molars were used as anchorage [17; 21; 22], also comparing tooth-borne and tooth tissue-borne appliances [2; 9; 13; 14]. Furthermore, a few authors have studied the three-dimensional effects of maxillary expansion with appliances anchored to deciduous teeth [19; 23] but, to the best of our knowledge, none of them compared the Hyrax and Haas expanders in mixed dentition using CBCT technology. As a matter of fact, CBCT technology allowed clinicians to measure the distance between anatomic landmarks, eliminating the drawbacks of traditional auxiliary examinations such as 2D radiographs and dentals casts, thus ensuring more reliable and accurate measurements, which may have been biased in previous studies [16]. Based on these premises, a stable 3D reference system was used to analyze any dental or skeletal changes after RPE; the result of intraoperator error confirmed the reliability and sensitivity of this method. RPE on deciduous teeth is, nowadays, a common strategy to correct several transversal and sagittal discrepancies. The high forces exerted during RPE can affect the periodontal and endodontic status; thus, the anchorage onto deciduous teeth could be performed avoiding root resorption, bone loss, gingival recessions, and carious lesions on permanent dentition [18]. All of these side-effects could be avoided by using deciduous teeth as anchorage but without any statistically significant difference concerning the skeletal  
Figure 2: Representation of the percentage amount of expansion at different levels (NF, FURCA, and 6 + 6) compared to the total amount of dental expansion on the anchorage unit (E + E) between Hyrax and Haas group after RPE. expansion compared to the RPE with anchorage onto permanent molars [6, 7, 24, 25]. There was an advantage in not using permanent molars as anchorage: they were free from buccal tipping, as has been described in several studies in which the appliance was bonded on them [9, 17, 21, 22]. Due to this, special attention must be paid regarding the spontaneous movement of the upper first molars when the expander in bonded onto deciduous teeth, independently of Hyrax or Hass type expanders. As a matter of fact, expansion onto deciduous teeth does not direct forces to the permanent molars and they can achieve a more physiological inclination and rotation to counteract bone bending and dentoalveolar compensation of posterior cross-bites, obtaining a less buccal angulation, as previously reported [26]; in the present study, first permanent molars spontaneously expanded by approximately 48% and 33% less than deciduous molars, in favor of the Haas expander compared with the Hyrax expander. This is due to a simultaneous expansion and decompression that reduces the ability to overexpand but achieves a more physiological molar inclination. Furthermore, a spontaneous distorotation of maxillary molars implies a significant increase in the upper arch length, a possible improvement in the Class II molar relationship, and less invasive orthodontic treatments [25, 27, 28, 29]. RME was effective in increasing maxillary transverse dimensions.