Maxillary sinus is an anatomic structure located in a vital location with close proximity to nasal cavity and the roots maxillary posterior teeth.  Maxillary sinuses are often associated with anatomic variations particularly the osteo-meatal complex which predisposes them to disease conditions.13 AMO is one among the variations of the osteo-meatal complex. It is important to note that sinus disease does not necessarily mean the presence of osteo-meatal variation.14 The term AMO was first coined in the year 1993 by Rice and Scheaffer, as a terminology for all the openings on the lateral nasal wall, other than a single primary ostium.  It is not very clear whether AMO is congenital or acquired. Some researchers believe that AMOs usually occurs after an episode of acute maxillary sinusitis.  Some recent studies have highlighted the "recirculating mucus ring" phenomenon in which mucus circulation takes place between the normal ostium and the maxillary sinus AMO.Apart from routine imaging procedures nasal endoscopy has been used for the detection of AMO. [18, 19]
In our study, 35.5% of the maxillary sinuses in the population were found to have AMOs. The prevalence of AMOs was examined in hospital / clinic settings using computerized tomography (CT) and endoscopy, while in anatomical research, cadavers were assessed. AMOs were present in 29.5 percent of paranasal CT scans of the research cohort in one published study on the Jordanian population.7] Recent CT based studies on AMOs reported a prevalence 19.1–46.3% of the sinuses in Turkish population and 18% in Indian population.[6, 20, 21] In another CT and endoscopy-based research on the Indian population conducted in 2018, AMOs were found in 23 percent of the cohort. Similar percentages were reported by purely endoscopic studies in the Indian population.  AMOs were reported in 13.8–26% of the cadavers as per few recently study published anatomical studies. [15,22−25] Studies carried out on Chinese, Indian and Turkish population revealed prevalence rates of AMOs to be 47.2 percent, 23.7 percent and 38.8 percent, respectively. 9, 26, 27 In general, the prevalence of AMOs ranges from 20 percent to 50 percent in studies using CT, CBCT scans and endoscopy. The likely explanations for this difference may be ethnic difference and the sensitivity of investigative imaging system. However, cadaveric studies have shown a lower prevalence of AMOs relative to studies using live subjects using CT, CBCT, or endoscopy. Post mortem anatomical distortion may be the likely cause for this lower prevalence.  Therefore prevalence studies using imaging are more reliable.
Intra- and inter observer agreement is a vital issue in medical imaging interpretation and this must be assessed with the most suitable test for an accurate outcome of any imaging study.  In our study the inter observer reliability was 0.83 and intra-observer reliability for detecting the presence of AMO was 0.85. The observers' reliability values in our study were consistent with the values found in the study by Hung et al.  Nevertheless, few other researchers did not evaluate the components of inter and intra observer variability in their studies. 
In our study there was no statistically significant difference in the occurrence of AMOs between male and female subjects, although the occurrence was numerically higher in male subjects. Similar observations were found in the studies by Bani-Ata et al and Ghosh et al. [1, 7] However, Hung K et al found AMOs to be more commonly present in, CBCT scans of female subjects.9 It is important to note that there was no statistical difference in terms of gender in any of the studies reiterating the fact that, gender had no significant influence on the occurrence of AMO.1,7 We found no statistically significant difference in the occurrence of AMOs when age was considered. The mean age of the research subjects was higher than that of the Hussein et al and Sahin et al studies. [30, 31] In most studies, the frequency of AMOs has not been significantly dependent on the age group. [1, 6, 7, 18] However, there was a higher prevalence of AMOs in older age groups in one study by Dedeoğlu N and Altun O. The authors attributed the greater incidence of AMOs in the elderly to be due to the age-related phenomenon and the resorption phenomenon that accompanied age-related edentulism. 
In our study majority of the AMOs (90.83%) were less than 4 mm in size. This was consistent with the findings of Hung K et al.  Based on the maximal length, we divided the AMOs into three classes. We also identified the location of the AMO based on its distance from the floor and the anterior wall of the maxillary sinus. The location of the AMO was determined by some of the studies and case reports based on the landmarks on the lateral nasal wall, such as the anterior and posterior fontanelle [1, 9, 32, 33, 34] Other studies have identified the location of the ostium based on their distance from the landmarks such as floor of the sinus and anterior wall of the sinus.[35, 36, 37] Due to the variability of landmarks on the lateral nasal wall, we chose the latter approach based on measurements.[38, 39] In our study the height of the AMO from the floor of the sinus was 19.93 ± 1.68 mm. Radiographical studies have shown that the normal maxillary sinus height varies from 28 mm to 34 mm.[40, 41] Following the above measurements, AMOs are most likely to be found at a point between half and three-fourths of a line connecting the floor of the sinus to the roof. The fragility of the lateral nasal wall in this area might be the explanation for the incidence of AMOs at this site.  During inflammation, the maxillary sinus is almost half filled with inflammatory fluid in gel consistency. The cohesive forces prohibit normal ciliary transport of the fluid into the ostium, which is situated at a higher level. Therefore, the fluid content finds a point of structural fragility on the lateral nasal wall to escape the sinus. 
MT is associated with collection of inflammatory fluid within the maxillary sinus.[43, 44, 45] Many recent studies have highlighted the relationship between periapical and periodontal health of maxillary dentition, sinus floor mucosa and maxillary sinusitis.  There are many approaches used by researchers to classify mucosal thickening in the maxillary sinus using CT and CBCT. [47, 48, 49] Magnetic resonance imaging (MRI) was also used in one of the earlier studies for the classification of MT in the maxillary sinus. [50, 51] MT has been graded by researchers either on the basis of thickening (mild/ moderate / severe, polyps, pseudocyst retention) or on the basis of numerical measurement ranges. [10, 52] We used Sheiki et al's classification, which is based on measurements. In the present study, there was a statistically significant difference in the occurrence of AMOs when the thickening size exceeded 3 mm. It is important to note that, there is no consensus on the threshold values above which MT is considered to be pathological.  In our study, sinuses with radiographic evidence of MT, showed significantly higher occurrence of AMOs than those without such features. Similar results were obtained in a CT based study by Gusrov M et al, who suggested that AMO may be an accelerating factor in the transformation of sinus mucosal pathologies like retention cyst to antrochoanal polyps. This association was also reflected in endoscopy-based and CBCT studies which revealed higher frequency of occurrence of AMOs in rhinosinusitis patients.[18, 31, 54] A CT-based analysis by Yenigun A et al concluded that a probability of finding MT in the same sinus was correlated with the existence of AMOs. Additional observations from our study also suggest that the probability of finding an AMO during the radiographic evaluation of the maxillary sinus is higher if there is co-existing mucosal thickening of more than 3 mm in the sinus floor. Such results suggest the Acquired Development Hypothesis model for the existence of AMO rather than the theory of congenital development. The length of the ostium plays a major role in the mucous circulation and thus will influence the mucosal thickening of the maxillary sinus. If the size of the AMO is up to 4 mm, the mucous secretions with a normal viscidness tend to circumvent the AMO in the maxillary sinus. In this situation the secretions do not pass through the AMO. However, the same phenomenon does differ for situations where the size of the AMO exceeds 4 mm in diameter, whereby the mucous secretion portion of the mucous carpet flowing over the centre of the AMO flows into the centre of the meatus. The part of the mucous secretions in the margin of the AMO continues to pass along its borders of the AMO to finally reach the main natural ostium. The mucous secretions that have moved out of the maxillary sinus, through the main natural ostium, return to the same maxillary sinus when it makes a downward journey due to gravity and encounters AMO on its path. The secretions are laden with pathogenic micro-organisms from the nasal cavity layer during the re-entry process into the maxillary sinus. This malicious recirculation of secretions regularly aggravates the sinus condition, causing the sinus mucosa to pathologically thicken. [2, 16]
Our study revealed that the MT did not have any statistically significant correlation with the location of the AMO. However, when we evaluated the maximal length the of AMOs with the type of MT, it was observed that larger AMOs were associated with higher degree of MT. This was similar to the observation in the study by Hung K et al.  They proposed that AMO decreases the clearance of mucus secretions in the corresponding maxillary sinus, which could make the sinus susceptible to pathologies. [6, 9]