## 4 Discussion  
The area of the IZC is considered a safe zone for mini-screw placement, as this region is devoid of nerves, major blood vessels, or teeth [9]. Moreover, adequate bone density provides good primary stability, making IZC a valuable source of anchorage. Nevertheless, the close proximity of the maxillary sinus and the risk of Schneiderian membrane perforation, whose integration is vital for undisturbed sinus function [14], involves the need to accomplish the implantation procedure in a precise manner. As stated in the literature, when an implant invades the maxillary sinus less than 2 mm, the Schneiderian membrane is elevated and new bone is formed in this region. In the case of deeper insertion, the maxillary membrane becomes perforated, which may promote the development of sinusitis [15]. Theoretically, to obtain sufficient stability of the IZC mini-screw a minimum bone thickness of 6 mm is required [7]. [PERSON] et al. report that the mean bone depth measured in Taiwan patients at the level of the mesial root of the first maxillary molar, 14 mm above the occlusal plane, at the insertion angle of 70\({}^{\circ}\) was 8 mm. According to our study, mean bone thickness at the same point and angle of insertion was 2.54 \(\pm\) 2.42 mm, which implies much poorer conditions for implantation. [PERSON] et al. conducted a study on Brazilian patients to identify the optimal areas for extra-alveolar mini-screws of patients with different facial patterns [16]. There was no significant difference in the case of IZC bone thickness among groups, but the mean bone depth at the level of the distal root of the first maxillary molar 13 mm above the occlusal plane and with the insertion angle of 70\({}^{\circ}\) ranged from 7.11 \(\pm\) 1.95 mm in dichlorofacial patients to 7.51 \(\pm\) 2.16 mm in brachyfacial patients, which gives much higher results when compared to analogical measurements performed in our study (3.71 \(\pm\) 2.76 mm at the height of 12 mm and 3.11 \(\pm\) 2.35 mm at the height of 14 mm). Values obtained at the level of interpersonal contact of the first and second maxillary molars in another Brazilian study (7.3 \(\pm\) 3.0 mm) also surpass our results (6.03 \(\pm\) 2.64 mm) [17]. Nevertheless, to confirm that the mean bone thickness of the IZC in Caucasian patients is lower when compared to the non-Caucasian population, multicenter studies in a much larger group of patients are needed. The most favorable conditions for IZC mini-screws placement were found at the level of interdental space between the first and the second molar, where bone depth ranged from 3.46 \(\pm\) 1.93 mm at the point I3 to 6.03 \(\pm\) 2.64 mm at I1. The thinnest bone was identified at the area of the mesial root of the first molar (from 2.42 mm to 2.54 mm), which indicates that bone thickness increases from front to back. A study conducted by [PERSON] et al. [18] on Arabian patients, according to which mean bone depth at the level of the mesial root of the first molar with an insertion angle of 70\({}^{\circ}\) was 3.90 mm, confirmed our conclusion that the available bone in this area is insufficient and the risk of sinus injury could be high. According to [PERSON] et al., mean bone depth tends to decrease with an increase in the distance from the occlusal plane (an increase of 2 mm in height was connected with a 2 mm decrease in the bone thickness) [16]. Although the results of our study seem to be in line