Although the relationship between the MS volume and shape and upper posterior teeth have been investigated before [16, 17], scarce information is available regarding the association between the maxillary canine displacements and MS dimension and volume. Hence, the present investigation was undertaken to determine the association of the PDC and BDC with the MS dimension and volume in unilaterally displaced maxillary canines. Only, subjects with unilateral canine displacement were included to avoid any confounding factors.
The MS usually present at birth and with age and increases in size afterwards. The most extensive period of growth occurs during the first 8 years and, by the end of the 16th year, the maximal values of all diameters and volume usually are reached . The age of included subjects in this study was at least 16 years to allow for the MSs to reach its adult size and the maxillary canines to fully erupt into its final position.
The vast majority of previous studies demonstrated that the dimensions of the maxillary sinuses were larger in males than in females  which was explained by male’s higher functional need (due to bigger body size and larger craniofacial skeleton). However, in the current study, males and females showed insignificant differences in MS dimensions and volume. This was in partial agreement with Urooge and Patil  who reported a statistically insignificant gender difference with respect to the maxillary sinus length, height, area and volume. It also is possible that the small number of males in the current study may have masked any gender differences.
In the current study, CBCT images were used to measure the maxillary sinus dimensions and volume. These images were taken for patients to detect anomalies associated with displaced canines which was in line with the recommendations of the American Academy of Oral and Maxillofacial Radiology . Bjerklin and Ericsson  reported that treatment plans of 43.7% of their cases were altered on basis of the additional information gained from the CBCT investigation. Maxillary sinus height, width, length and volume and the 3D measurements of maxillary canine crowns and roots are not feasible with periapical or panoramic radiographs.
In the present study, MS anterior pneumatization into the canine area was more pronounced in BDC subjects (48% of the displaced side and 34% on non-displaced side) whereas in PDC subjects, it was found in 23% of the displaced side and in 36% of the non-displaced side. This finding is comparable to that reported by Kopecka et al.  but lower than that (69%) reported by others [14, 23]. Additionally, MS pneumatization in the incisor region was found in 10% and 13% of buccal and palatal canine displacement sides, and in 10% and 6% of canine non-displacement sides, respectively. This was lower than 15.5% reported by Zhang et al.  However, this was in agreement with Kopecka et al.  who suggested that the more MS pneumatization to the canine area occurred in a type I vertical relationship between MS and canine apex (canine apices located at more than 2 mm distance below the sinus floor) followed by type II (less than 2 mm distance) and type III relationship (interlock). MS pneumatization is related to dental position and the variation between displaced and nondisplaced sides and between PDC and BDC sides may be explained by the position of the displaced canine which may have prevented or allowed anterior sinus pneumatization. This was in agreement with Oz et al.  who reported an increase in MS dimensions after orthodontic traction if the impacted canines were closer with respect to the MS.
When the displaced and non-displaced sides of BDC and PDC subjects were compared, the position of maxillary canine in relation to MS wall showed significant differences which is expected based on the position of the displaced canine. The variation in the vertical, lateral and A-P position of the canines between right and left sides resulted in significant differences between the displaced and non-displaced canines’ sides.
In BDC subjects, MS volume was larger on the displaced side compared to the non-displaced side. This was accompanied by similar MS dimensions (length, height and width) between the displaced and non-displaced sides. The lack of correlation between MS dimensions and MS volume may be attributed to the method used for their measurement. In the current study, the MS length, width and height were measured as a line between the most prominent points on MS walls at a specific location which does not reflect the actual dimension throughout the MS wall.
In the current study, although the BDC crown tip was located more laterally on the displaced side, the MS width was similar to the non-displaced side. This may be explained by the closer position of the BDC’s root tip to the MS. This was in agreement with previous reports that the shape and size of the maxillary sinuses were affected by the proximity of the roots .
In PDC subjects, the MS volume was reduced and the maxillary displaced canine crown and root tip were closer to the MS laterally as compared to the non-displaced side. Similar to that found in BDC subjects, MS dimensions between displaced and non-displaced sides did not differ significantly. The lack of correlation between MS dimensions and MS volume may be explained by the method used to measure MS dimensions as previously explained. In PDC subjects, the canines are positioned in the palate and are close to the MS, therefore, their presence in the palatal area may have reduced MS volume in these subjects.
In both BDC and PDC displaced sides, the maxillary canine root tips were closer to the anterior MS wall than in the non-displaced sides. Because the MS length in this study was similar between the displaced and the non-displaced sides, this finding may be explained by the position of the displaced canine root and not to an increased MS length in the displaced sides.
It has been reported that the maxillary ectopic canines cause displacement of the midline toward the non-displaced side . This midline displacement may affect the MS dimensions in the non-displaced side making it smaller. In the current study, MS width was similar between the displaced and non-displaced sides. However, the effect of the PDC displacement on the width of MS may have been masked by the great variability in the position of displaced canines in relation to MS wall and the possible increased transverse arch width in PDC side .
In the current study, significant differences between BDC and PDC were detected. In the displaced sides, BDCs were more laterally and more inferiorly located and the MS volume was increased. In the non-displaced side, PDCs were more laterally placed compared to BDCs which may suggest a larger maxillary width in PDC subjects. This was in agreement with Al-Nimri and Gharaibeh  who investigated arch width of Jordanian subjects with PDCs and reported that the transverse arch dimension was significantly wider in the PDC group. However, the increased lateral distance in the non-displaced side of the PDCs as measured from the maxillary canine crown tip to the MS wall was not associated with a wider MS or larger MS volume. This was in disagreement with AlHazmi  who reported a strong correlation between maxillary arch width and MS volume. Anyway, this distance (crown tip to MS) does not reflect the true maxillary arch width due to the large MS width variability in these subjects.
Regression analysis revealed that PDCs are associated with smaller MS volume, increased MS height and Class III skeletal pattern. This finding is in partial agreement with Oz et al.  who reported that impacted canines have smaller MS volume if the impacted canines were positioned high and closer to MS and that MS dimensional changes were associated with orthodontic traction of the impacted canines. In addition, regression analysis revealed that the OR of having PDC is reduced in a class II skeletal pattern. This finding was in agreement with Al Balbeesi et al.  who reported that the lowest frequency of canine impaction was found in patients with a Class II skeletal discrepancy and Basdra et al.  who observed impacted canines in 9% of Class III subjects compared to 1.3% in Class II subjects. On the other hand, this finding was in contrary to others who concluded that skeletal Class III subjects did not show a different prevalence of canine impaction  and that PDCs are not associated with altered skeletal features . Furthermore, in the current study, maxillary canine displacement was not associated with the vertical skeletal relationship. This was in disagreement with the three times higher canine impaction reported in hypodivergent patients compared to normal face subjects .
Limitations of this study include a high female/male ratio and the number of subjects with a reduced vertical pattern was small. It has been suggested that subjects with a reduced vertical relationship have an increased maxillary sinus width and height compared to subjects with increased vertical skeletal pattern .
The findings of this study spot the lights into the importance of the assessment of MS during orthodontic treatment planning for patients with maxillary canine displacement. Early diagnosis and treatment of PDC could improve the MS volume and dimensions. In addition, clinicians would be aware of the challenges during orthodontic traction of maxillary displaced canines in presence of anterior MS pneumatization and improve orthodontic treatment outcome.