## 4 Discussion  
One of our study objectives was to determine the pattern of expansion and positional changes of the maxilla after TPD. We found insignificant change of the SNA, which is consistent with other studies, showing that correction of the maxillary constriction with a transpalatal distractor does not influence the anteroposterior position of the A point [1; 13]. On the other hand, [PERSON] et al. reported a post-TPD increase of the SNA angle by 1.6\({}^{\circ}\)\(\pm\) 2.57\({}^{\circ}\) (\(p\) = 0.0001); however, such standard deviation value proves the high variability of the outcomes. Moreover, 20-month follow-up and an orthodontic closure of the diastema may also affect the TPD results, because remodeling of the alveolar bone between the maxillary central incisors is likely to influence position of the A point, and thus the SNA angle [14]. Neither position of the mandible (SNB angle) nor inclination of the maxilla to the cranial base (SN/PNS-ANS angle) changed after our surgical protocol. Nonetheless, although this is in accordance with recently published results [13; 14], it must be emphasized that studies on TPD effects give contradictory results. [PERSON] et al. described counter clockwise mandibular rotation, while other authors observed the opposite [14; 15; 16]. [PERSON] et al. suggest that an increase in the inclination of the maxilla to the cranial base induced by TPD can be only expected when the S-N/PNS-ANS angle is pre-operatively low [14]. Our study seems to support this hypothesis, as the discussed angle was low at T1, and changed insignificantly at T2 and T3. Except for increasing the S-N/PNS-ANS angle, the whole maxilla moved downward: the N-ANS and the S-PNS measured at T3 significantly increased by 1.27 mm and 0.54 mm, respectively, compared with T1. This is in accordance with the majority of researchers' observations [1; 13; 14]. To us, two factors seem to be of key importance for vertical control: 1. position of the distractor in relation to the center of resistance of the mobilized maxilla and 2. whether the pterygomaxillary disjunction is performed or not. The TPD (or the bone-borne SARME) generates a force vector located close to the center of resistance of the maxilla, which ensures more parallel movement of the separated segments. During expansion, one observes buccal movement of the maxillary halves, which results in downward rotation of the structures located close to the cranial midline, including A, ANS, and PNS points. This rotation increases together with elongation of the force moment arm, which is supported by the study of [PERSON] et al., who--comparing results of the tooth-borne Hyrax and TPD application--observed a greater downward movement of the maxilla resulting from the previous appliance [1]. Greater mobility of the separated segments at the anterior part of the maxilla and--in turn--more pronounced downward movement in the canine region may also be explained by the lack of pterygomaxillary disjunction in our study. This way our TPD protocol results resemble the Hyrax screw-induced pattern. This is in accordance with the report of [PERSON] et al., who--using finite element analysis--showed that pterygoid disjunction provokes an even downward movement of both the front and the lateral aspects of maxilla, measured at the level of the midpalatal suture, whilst alteration the surgical protocol by omitting the disjunction and applying merely the Hyrax appliance produces downward displacement that is greater anteriorly. These results seem to confirm the fact that the extent of the surgical osteotomy affects the pattern of maxillary displacement [17]. We achieved transverse skeletal changes that were greater at the anterior part of the maxilla: the ratio of anterior to posterior skeletal expansion measured at the level of the canine (ARC) and the first molar roots (ARM) was 1.33 at T3. The pattern of skeletal expansion was V-shaped, with the base at the area of the anterior nasal spine, and the apex at the distal part of the palatal suture.