## 4 Discussion
Only a few studies have compared Hyrax and Haas appliances, but none used deciduous molars as anchorage. Nevertheless, a similar study was performed regarding nasal changes induced by RPE comparing Hyrax and Hass appliances anchored to primary teeth [15]. In that study, it was reported that no statistically significant changes in skeletal expansion were found comparing the Hyrax-type and Haas-type expanders; even if the results of skeletal expansion are similar to those of the present study, it is impossible to calculate the ratio between dental and skeletal expansion caused by the lack of the amount of dental expansion obtained on anchorage teeth. Comparing the total expansion of dental or skeletal measurement is not a reliable method as it depends on the amount of screw activations needed by each patient. The same trial analyzed the reduction in the permanent molar tipping since no direct forces were exerted on them, also reported by other authors and in accordance with the findings of the present study [7]. Another study reported a rate between skeletal expansion--the distance between nasal floor points--and dental expansion at the level of the upper first permanent molars of about 55% when RPE was performed by Hyrax expander anchored to deciduous teeth [30]; this data were comparable with that obtained in our study. However, as already mentioned, different methods from other trials mean a comparison with our result is not possible. The expanders analyzed in the present study are certainly the most recognized in the literature but their mechanotherapy is not totally well known. The Hyrax expander has easier hygiene, greater comfort, and prevention of decubitus lesions to the palatal vault; on the other hand, the Haas expander, thanks to the acrylic pads, may guarantee a more significant expansion of the maxillary base because of the distribution of the expanding forces between the posterior teeth and the palate. Independently of the design, RPE should be performed applying forces in the range of 7 to 15 kg to maximize the skeletal separation of the midpalatal suture before any dental movement can occur [31]; for this reason, the skeletal parameters are the most investigated and clinically relevant due to smaller dentoalveolar side-effects. It has been stated that only the Haas expander optimizes the orthopedic effects of RME, avoiding all dentoalveolar effects, but this statement seems to be controversial [9]. It is probable that the different effects produced by the two analyzed devices may reflect the different force delivery systems; as a matter of fact, it has been reported that the sutural expansion designs that use an acrylic interface with teeth were far less stiff than those constructed solely of stainless-steel wires [32]. Increasing the rigidity of the appliance with acrylic results in a superior migration of the center of resistance to the frontonasal suture, resulting in a more linear separation of maxillary halves and decreasing buccal tipping of the anchorage teeth [33]; less tilting of the alveolar process and a reduced degree of dental tipping are probably the effects of the use of the Haas expander as observed in the present study. In fact, if the entire amount of dental expansion on the anchorage t teeth--deciduous molars--that is the amount of screw activation considered to be 100% of the transversal expansion achievable, then it has been shown that the Haas expander produces a greater skeletal expansion (44%) compared to the Hyrax expander (27%) as the maxillary permanent molars expanded spontaneously more in the Haas group (67%) than the Hyrax group (53%). Starting with these different dentoalveolar effects, a subsequent decompensation in the inclination of the permanent molars was observed.