Based on the findings of the current study, a history of periodontal disease is the only identified parameter significantly associated with sinus mucosal thickening. This finding indicates that clinicians should expect some degree of mucosal thickening when performing sinus augmentation procedures in a previously periodontally involved site. This finding is consistent with several previously published studies [27–29]. Phothikhun et al. [28] reported that sinuses with severe periodontal bone loss were three times more likely to have mucosal thickening. In a more recent study, Ren et al. [29] reported an odds ratio of 4.62 for patients with severe periodontal bone loss with mucosal thickening. A possible explanation for this phenomenon is that increased inflammatory cytokines resulting from periodontal diseases might also reach the maxillary sinus, and thereby trigger an increased membrane thickening. With regards to implant treatment outcomes, while our study found that there is no association between mucosal thickening and future implant survival, a higher chance of sinus membrane perforation during sinus lift procedures has been reported when a thicker membrane is present [3, 30].

Our study did not find a significant association between endodontically treated teeth and mucosal thickening. Though this finding is consistent with some previously published studies [27, 28], other studies [21, 31] did report an association. These discrepant findings could be the result of different inclusion criteria in the study design. Since our study did not include any patients with radiographic signs of pulpal pathoses, these data suggest that successful root canal treatment without signs of apical radiolucency should not be considered as a risk indicator of future mucosal thickening. On the other hand, it has been reported [32, 33] that the presence of apical periodontitis is related to sinus mucosal thickening, which should alert clinicians when planning future implant-related procedures. Because periapical infections are considered a multifactorial entity, they should be carefully evaluated and treated to ensure a favorable implant treatment outcome [34, 35]. In addition, the influence of a periapical scar of dense collagen tissue, formed after conventional root canal treatment, on implant treatment outcomes has not yet been fully explored. Therefore, additional future investigations are needed to examine these unresolved issues.