In the present study, we investigated the prevalence, position, and diameter of the AAA canal in 280 Thai patients using CBCT. The AAA canal was detected in 94.6% of the patients, which was higher than in previous studies. Previous studies using CBCT detected the AAA in 60.3–92.75% [14, 17, 19,20,21,22,23,24,25] of the patients while the studies using multiple detector computed tomography (MDCT) detected it in 47–64.5% [10, 11, 13, 26,27,28] of the patients. Therefore, our finding and the literature clearly demonstrate CBCT is superior to MDCT in detecting the AAA canal. Image resolution is a key factor affecting the detectability of the canal. The small isotropic voxel size of CBCT helps with the visuality of small canals. This is difficult with MDCT which has anisotropic voxel, larger voxel size, and the partial volume averaging effect.
We also found a higher prevalence of the detected AAA canal detected than in all previous similar studies using CBCT [14, 17, 19,20,21,22,23,24,25]. This may have been due to our selection of a fixed FOV of 6 cm × 6 cm (voxel size 0.125 mm) in combination with a high-resolution monitor (2560 × 1600 pixels). The small FOV guaranteed a high image resolution, and the 0.125 mm voxel size allowed visualization of canals with diameters as small as 0.125 mm. Visualization was also improved by using a medical grade monitor. There is a possibility that the AAA exists in every patient, which is suggested by studies in cadavers [29,30,31]. However, CBCT still will not be able to detect all canals because of limitation in detecting canals with diameters smaller than its voxel size. We suggested that the appropriate selection of FOV and voxel size should be concerned before taking CBCT for implant placement planning in the posterior maxilla area.
We found the most common mediolateral positions of the AAA were the intrasinus position (72.5%), followed by the intraosseous position (24.7%), and the superficial position (2.8%). These frequencies were similar to those found by Jung et al.  and Bischof et al. . Conversely, some studies found the most common mediolateral position of the canal was the intraosseous position [13, 14, 19]. In the present study, our selected small voxel size may have caused more detections of the intrasinus position of the AAA canal, which is usually small and may partially involve the maxillary sinus wall. Knowing the mediolateral positions of the AAA canal may help lessen the possibility of injuring the artery . While performing the sinus lift procedure, a piezoelectric instrument can be used to detach the AAA in the intrasinus position, in which the artery is partially located within soft tissue . However, it is impossible not to breakdown the artery with the intraosseous position. Therefore, if the intraosseous position was seen in CBCT images, the patient should be informed of the increased risk of bleeding and membrane perforation before the surgery or the double window technique should be considered .
To the best of our knowledge, we are the first to study the route of the mediolateral position of the AAA. Our classification was the secondary outcome of the mediolateral position classification. We assumed that the relations between the AAA canals in each tooth locations may simulate the route of the AAA and explained how the AAA passes through the maxillary sinus wall. We found more than 70% of the routes were the all-in type, the intrasinus position from the premolar area to second molar area (Fig. 9), followed by the out-to-in type, the superficial position at the premolar area to intraosseous position at the second molar area (Fig. 10). This suggests the mediolateral position of the AAA may vary in different tooth locations. Hence, it may be possible that the AAA canal is invisible or too small to be seen in a designated implant area (Fig. 10-1M). Our classification can be used to explain that even the AAA cannot be detectable in CBCT images on that area, it may be inside the maxillary sinus or outside the sinus wall. To define its location, it is advisable to check for the existence of the AAA canal in the adjacent location. The mediolateral position of the AAA could be estimate by considering the mediolateral routes from our classification and their prevalence (Fig. 11). Moreover, the pathway of the canal in our study tends to be parallel to the maxillary sinus floor, so the distance from the maxillary sinus floor to the AAA canal can be estimated from the distances to the canal at the adjacent locations.
The mean perpendicular distance from the AAA canal to the maxillary sinus floor (distance A) in the present study was comparable to that of previous studies [10, 19, 33]. The mean perpendicular distance from the AAA canal to the edentulous alveolar crest (distance B) was different from the previous studies in the first premolar and second premolar and was similar in the first and second molar [10, 17, 23, 33, 34].
According to the alveolar ridge height, edentulous areas with alveolar ridge height ≤ 6 mm had a significantly longer mean perpendicular distance from the sinus floor to the AAA canal (distance A) compared with those with alveolar ridge height > 6 mm. This result represents the pneumatization of the maxillary sinus in an edentulous area, which increased the distance from the sinus floor to the AAA canal. Although the mean distance was higher in edentulous areas with alveolar ridge height ≤ 6 mm, the risk that the surgery might involve the AAA is still higher in this group because these areas certainly need sinus lift procedure to increase the alveolar ridge height.
In the alveolar ridge height ≤ 6 mm, the shortest mean distances around 14–16 mm from the AAA canal to the edentulous alveolar crest (distance B) were founded in the first and the second molar locations (Table 4). These distances are crucial when performing the sinus lift procedure. The inferior cut may need to be 2–3 mm from the maxillary sinus floor and the size of the lateral window may need around 6 mm for easy instrument access. The safety margin to prevent the bleeding may need to be more than 2 mm from vital structures. Thus, the superior cut may range from 10–17 mm from the alveolar crest, depending on the residual ridge height. Apparently, there is a high possibility that the AAA canal may be involved in the surgery because a study of sinus lift in cadavers which showed that the AAA canal was involved in 83% of first and second molar locations when antrostomy was performed . Therefore, these two locations need to be considered carefully when performing the sinus lift procedure.
In the present study, the mean ± SD diameter of the AAA canal was 1.05 ± 0.34 mm, which was similar to the previous studies [14, 19, 20, 25]. The diameter was not significantly different by age or dentate status. However, the mean ± SD diameter of the AAA canal was significantly larger in males than females (1.13 ± 0.33 vs. 0.97 ± 0.28 mm, respectively). Canals with diameters of less than 1 mm may not cause any complication during surgery while those with diameters larger than 1 mm may cause severe bleeding . Just under half of the canals in the present study showed a diameter larger than 1 mm.
The limitation of the present study was the small numbers of the AAA canals found in the first premolar area, which precluded analysis by difference of means tests that would allow generalization to the Thai population. It may have been because the anterior wall of the maxillary sinus in our cohort often began after the first premolar location. From the literature, the size and extension of the maxillary sinus may vary depending on the gender  and ethnicity [37, 38]. Even though there have been no study about the maxillary sinus size in Thai population, some study in other populations have shown that maxillary sinus seems larger in a larger facial skeleton, such as is found in the Caucasoid compared with Mongoloid individuals [36,37,38].