ObjectiveThis study aimed to evaluate the thickening of sinus membrane, which is also named Schneiderian membrane (SM), in patients and its relationship with periapical lesions (PAL) in the posterior maxillary region to provide reference for the prevention and treatment of odontogenic maxillary sinusitis.MethodsA retrospective analysis was conducted on 554 cone beam computed tomography (CBCT) imaging data of maxillary sinuses from 301 patients who met the inclusion criteria to determine the correlation between PAL and SM thickening in the posterior maxillary region. Cases of pathological SM were recorded and classified on the basis of the degree and type of SM thickening. The correlation between SM thickening and the diameter of PAL, the relationship between the upper edge of PAL and the maxillary sinus floor, and its relationship with whether affected teeth with PAL undergo root canal treatment were evaluated.ResultsThe detection rate of SM thickening in patients with PAL was significantly higher than in those without PAL, so PAL was correlated with SM thickening. Analysis on the correlation between PAL detection indicators and SM thickening degree showed that SM thickening degree was positively correlated with PAL diameter (cone beam computed tomography-periapical index) and not correlated with the three spread effects between the upper edge of PAL and the maxillary sinus floor, as well as whether the teeth with PAL undergo root canal treatment. The correlation analysis between PAL detection indicators and SM thickening types showed that whether the teeth with PAL undergo root canal treatment was not correlated with SM thickening types, and the diameter of PAL, the three spread effects between the upper edge of PAL, and the maxillary sinus floor were not correlated with SM thickening types.ConclusionThe PAL of posterior maxillary teeth is closely related to SM thickening, and the diameter of PAL is positively correlated with the degree of SM thickening. Patients with PAL who have undergone root canal treatment often exhibit SM polyp thickening. In addition, the relationship between the upper edge of PAL and the maxillary sinus floor does not affect the possibility of SM development.