In the present study, we evaluated the amount of anterior extension of maxillary sinus beyond the long axis of the maxillary canine and the relationship between the root apex of the permanent maxillary canine and the maxillary sinus floor in 300 CBCT images.
The proximity of the sinus floor and root apices of the maxillary teeth is critical in several clinical procedures such as endodontic surgery, orthodontic treatment, and implant placement. CBCT scans, with dedicated 3D software, allow image reconstruction in three orthogonal planes and precise morphometric measurements, which provide accurate information for case selection, treatment planning, and avoiding collateral damage during surgery [10,11,12].
The result of the present study shows that most of the maxillary sinuses (68.83%) extended to the canine area, which is almost equal to Zhang et al. finding (68.9%) [4]. In contrast, Kim et al. [21] and Kopecka et al. [22] reported much lower percentages of canine area pneumatization by maxillary sinus, 33%, and 2.4%, respectively. Regarding the pneumatization of the incisor region, Zhang et al. [4] reported a frequency of 15.5% (12.1% in lateral and 3.4% in central incisor) in their study population. In the present study, however, the anterior border of the maxillary sinus reached the incisor area in only 2.5% of sinuses. The difference in the percentages reported by the studies may be due to ethnicity and the differences in methodology, including the approach used for localizing the maxillary sinus and the number of subjects.
In this study, most of the maxillary sinuses (81.33%) had bilateral symmetry in the location of the anterior border in the jaw. This finding was in accordance with previous studies [2, 23, 24]. Comparably, Shahbazian et al. [25] reported the symmetric morphology of maxillary sinus in 83% of their cases, and according to Hamdy et al. [26], the average linear craniocaudal, anteroposterior, and mediolateral measurements were almost bilaterally matched in all cases. Based on our results, bilateral symmetry was even more common in those maxillary sinuses which extended into the canine area. Further studies are required to confirm this result.
The frequency of maxillary sinus extended to the anterior region was similar in both sexes in the present study. No study evaluated this relationship previously.
The frequency of maxillary sinus extended to the anterior region, and the mean AEMS beyond the canine long axis was significantly lower in older cases (40–50 years) compared with the other age groups. Additionally, in the vertical dimension, the MS-NF was significantly higher in the older age group. These findings confirm those of Belgin et al. [23], Takahashi et al. [27] and Velasco-Torres et al. [28] who found decreasing maxillary sinus volume with increasing age. Jun et al. [1] also showed that the maxillary sinus increases in size until the completion of skeletal development in both sexes. Then, an age-related decrease occurs in its volume. Similarly, Ariji et al. [2] reported increasing maxillary sinus volume up to 20 years of age and a declining process subsequently. Contrary to all these findings, Sahlstrand-Johnson et al. [29] reported that maxillary sinus volume was not related to age in their study sample.
Oishi et al. [20] found a significant negative correlation between age and the distance of the maxillary sinus floor to the root apices of all teeth (posterior teeth and canine) in CBCT images. The same result was found by AL Qasab et al. [30] regarding the distance between the canine apex and maxillary sinus floor in periapical radiographs. In the present study, the vertical distance between the maxillary sinus floor and canine apex was not statistically related to age. Further research with a larger sample size is needed to clarify these conflicting results.
As part of this study, it is found that when the maxillary sinus extended to the canine area, the apices were most commonly (84.99%) located below the sinus floor and at more than 2 mm distance (type I). It was followed by types II and III relationship with much less percentage (8.96% and 6.05% respectively). This order was in accordance with a recent study by Oishi et al. [20]. They defined type 0 for those cases in which the sinus floor did not appear above the root apices. Type I, II, and III are reserved for separate, in-contact, and interlock relationships between the maxillary sinus floor and canine apices. They reported that whenever maxillary sinus appeared above the canine apices, type I was the most, and type III was the least frequent relationship, similar to our findings.
Oishi et al. [20] assessed the proximity of the posterior teeth and the canine tooth to the maxillary sinus floor in standard sagittal and coronal planes and did not consider the anatomic curvature of the jaw, which is especially important in the canine area. In contrast, in the present study, the classification and measurements were done based on bucco-lingual cross-sections perpendicular to the dental arch of each side of the jaw. This corrected plane seems to be more reliable for evaluating the relationship and distance of the root apices and the sinus floor. In the study of Oishi et al. [20], the subjects was not evenly distributed in according to age and most of their sample (184 subjects) were between 18–29 years. Moreover, their sample size weighted toward the female group (225), which was three times more than the male group (76). To minimize the possible biases and to have more accurate results, we tried to have an even subject distribution in different age groups (each age group contained 100 subjects/200 maxillary sinuses) as well as having similar numbers of male (154) and female (146) subjects. However, based on the limited number of older individuals meeting the study's inclusion criteria, we set the upper age at 50 years old. We encourage future studies with a broader age span to overcome this limitation.
Another limitation of the present study is that the effect of facial biotypes has not been considered in the design of the present study. Okşayan et al. [31] reported lower values in maxillary sinus length and width in high-angle subjects. Costea et al. [32] found that the relationship between the maxillary second molar root apices and the maxillary sinus floor is affected by the facial biotype. Future studies can address whether the different facial biotypes influence the canine tooth and maxillary sinus relationship.