## 4 Discussion  
The results of the present study rejected the null hypothesis (H\({}_{0}\)) that states that there would be no difference between the operator's experience regarding the placement accuracy of the micro-screws and the intraoperative complications. Micro-screws are frequently used to achieve absolute anchorage during tooth movement, and one of the most frequent intraoperative complications is screw loss and pulpal and periodontal ligament damage resulting from root contact [16]. Root contact is described as a potential risk factor for clinical failure of micro-screw anchorage [17]. Because the dental root process becomes narrower and interradicular bone space increases to the apex, an oblique insertion angle is recommended to prevent root perforations. Unlike the vertical insertion angle, the oblique insertion angle also increases the bone-micro-screw contact surface and micro-screw stability [18]. However, [PERSON] et al. reported that an oblique insertion angle might influence the accuracy of orthodontic micro-screws' placement and hence could increase the risk of intraoperative complications [19]. Furthermore, the oblique insertion angle could lead to the exposition of the orthodontic micro-screw head over the mucous, which could increase dental plaque accumulation. In addition, [PERSON] et al. reported that oblique insertion angles of orthodontic micro-screws might require a higher cortical bone penetration, which might increase the tension at the bone-micro-screw interphase, increasing the tooth movement and promoting release of cytokines, macrophages and inflammatory mediators, which could affect the stability and hence the survival of orthodontic micro-screws [20]. Nevertheless, [PERSON] et al. recommended placing the micro-screw at an angle of 90\({}^{\circ}\) to prevent dental root perforations, increasing orthodontic micro-screws primary stability [21], but [PERSON] et al. reported that an angle between 30-40\({}^{\circ}\) reduces the risk of root contact and allows the placement of longer orthodontic micro-screws, which influences their stability [22]. [PERSON] et al. reported that the insertion angle also influences the mechanical resistance of orthodontic micro-screws and concluded that orthodontic micro-screws of 1.8 mm diameter and 6.0 mm length placed at an insertion angle of 90\({}^{\circ}\) with respect to the longitudinal axis of the teeth showed higher resistance values than orthodontic micro-screws placed at 45\({}^{\circ}\) with respect to the longitudinal axis of the teeth [23]. [PERSON] et al. agreed with the above results and concluded that the optimal insertion angle for Jeil orthodontic micro-screws was within 90\({}^{\circ}\), according to its characteristics [15]. This is why the Jeil orthodontic micro-screws in our study were placed at an insertion angle of 90\({}^{\circ}\) with respect to the longitudinal axis of the teeth. Suitable locations for orthodontic micro-screws placement should be analyzed preoperatively through radiographic techniques to avoid intraoperative complications and ensure orthodontic micro-screws' stability and retention. [PERSON] et al. analyzed the inter-radicular insertion locations through orthopantomography scans and reported that orthodontic micro-screws of 1.2-2 mm diameter could be placed safely at 3-4 mm inter-radicular spaces, leaving 1 mm of bone tissue around the orthodontic micro-screw and avoiding dental root damage [24]. [PERSON] et al. described the interradicular width of maxillary and mandible teeth to analyze the potential risk of dental root damage during orthodontic micro-screws placement and reported that interradicular spaces of 3 mm width were usually located in the upper anterior teeth (mainly between central incisors and lateral incisors and canines) and in the lower posterior teeth (mainly between first and second molars and between the first molars and the second premolars) [25]. In addition, [PERSON] et al. reported that the interradicular width was usually wider and considered safer at the apical root third [22].