## 4 Discussion
This is probably due to our inclusion of canine teeth. The roots of the maxillary premolars, molars, and in some cases the canine teeth have a close relationship with the MS, and may even protrude into it [2; 46; 47]. In our study the teeth most frequently associated with mucosal thickening (class >2) were the maxillary first molars, followed by maxillary second molars and the third molars. The canines were rarely associated with the maxillary sinus. Other differences could stem from the methods of measuring MS membrane thickness used in the various studies. [PERSON] (2016) described six categories of the MS membrane, including a number of abnormalities, such as sinus polyp, antral pseudocyst, nonspecific opacification, periositis, and antrolith [45]. Cases with such sinus abnormalities were excluded from our study. Although many authors in the literature have accepted a mucosal thickness >2 mm as a sign of sinus patohsis [7; 22; 41; 48; 49; 50], we employed the categories of [PERSON] et al. [12], who concluded that mucosal thickening of up to 3 mm is common and lacks clinical significance [12]. Periapical status has also been evaluated by a variety of scales. [PERSON] et al. described a periapical lesion as an at least 0.5 mm thick area of apical radiolucency associated with the apical portion of the root or ligament space [51]. [PERSON] et al. [52] divided the periapical status of teeth in CBCT scans into three categories, while [PERSON] et al. and others evaluated the periapical status of teeth from CBCT scans using the periapical index scoring created by [PERSON] (1986) and based on periapical two dimensional films [7; 24]. In his study, [PERSON] used a new classification system, called the periapical and endodontic status scale (PESS). This is based on the periapical index (COPI), which was proposed for the classification of periapical lesions in teeth with apical periodontitis, and the endodontically treated tooth index (ETTI) for the assessment of the quality of endodontic treatment by CBCT [23; 53]. It should be noted that evaluation of the quality of root canal treatment by means of CBCT scans can be problematic due to artifacts (1,22). The extrusion of endodontic materials inside the sinus may result in inflammatory reactions of the sinus mucosa (1). While the root canal sealers are considered biocompatible, proper root canal instrumentation and sealing is important in order to prevent sinus pathologies of endodontic origin (22). In this study we evaluated the periapical status of teeth with the six category CBCT PAI score described by [PERSON] et al. [35]. We consider that the multiple categories are important in grading the severity of the periapical lesion, in contrast to the dichotomy categorization used in [PERSON] study [51]. One of the findings in our study was the correlation between the size of the periapical lesion and the degree of the thickening of the mucosal membrane. Our results revealed a high frequency of periapical alterations in endodontically treated teeth (54%). The presence of periapical radiolucency in CBCT may present the ongoing healing of a previously larger lesion; other studies that reported similar results attributed the high rate of periapical lesions to failure to follow recommended technical criteria [52; 54; 55].