## 1 Introduction  
The infrazygomatic crest (IZC) is a thick bony ridge located between the first and the second maxillary molar. It has been used as a site for orthodontic miniplate placement when absolute anchorage or intrusive or distally directed forces were required during orthodontic treatment [1, 2]. [PERSON], the author of the method, recommends this location due to its solid bone structure and safe distance from the roots of the upper molars [1]. The Zygomach Anchor System, designed and popularized by [PERSON], consists of a titanium miniplate adjusted to the shape of the infrazygomatic buttress and three miniscrews (5 or 7 mm long), that fixes the miniplate to the bone. Although miniplates provide effective orthodontic anchorage, the process of their placement requires advanced surgical skills. The procedure consists of preparation of the mucoperiosteal flap, bending of the miniplate to the proper shape, fixation with miniscrews, repositioning of the mucoperiosteum, and placing sutures. After completion of the orthodontic treatment, miniplates have to be removed. To simplify the clinical procedure and minimize the financial cost of orthodontic treatment, extra-alveolar IZC miniscrews have been introduced. They are placed on the buccal surface of the alveolar process at the base of the zygomatic crest eminence as an alternative to miniplates. [PERSON], who popularized IZC, recommends its use for the retraction of posterior teeth and rotation of the whole dental arch in order to manage even challenging malocclusions without extractions or orthonghatic surgery [3].  
Despite their undeniable advantages, extra-alveolar miniscrews inserted in the IZC area can perforate the maxillary sinus and initiate sinus infection or mini-implant loss [4]. Although according to some authors [5], sinus penetration depth within 1 mm is advocatedfor mini-implant anchorage, to avoid this complication, adequate bone depth should be available at the site of mini-screw insertion. Typically, IZC mini-screws are longer than intra-radicular mini-implants and manufactured in two dimensions: 2 mm diameter and 12 mm or 14 mm length (a 14 mm screw is recommended in the case of thick soft tissue in the buccal vestibule) [6]. As stated by the literature, a minimum of 6 mm of the bone is necessary to provide sufficient stability of the IZC during orthodontic treatment [7; 8]. According to our clinical observations of Polish patients, in the vast majority of cases, the depth of bone ridge in this area is thinner, making IZC mini-screw placement challenging or even impossible. In the face of a lack of studies assessing the maxillary bone thickness for optimum IZC mini-screw placement in the Polish population, the aim of the present study was to evaluate the thickness of the IZC area.  
It was found that the IZC is located between the maxillary second premolar and the first molar in youngs and at the level of the maxillary first molar in adults [9]. [PERSON] developed a method for IZC screw placement adjacent to buccal surfaces of the maxillary first molar [7], while [PERSON] recommends a more distal site that is buccal to the maxillary second molars [10; 11]. Nevertheless, the region between the first and the second molar is also recommended for IZC screw placement [12]. To localize the most favorable site for IZC mini-screw implantation, we compared bone thickness at different levels considering sagittal plane and vertical distance from the occlusal plane. In view of the fact that both young and adult patients are seeking orthodontic treatment, another objective was to assess dependency between bone availability, sex, and age.