The Influence of Passive Ultrasonic Irrigation (PUI) on the Healing of Maxillary Sinusitis of Endodontic Origin (MSEO) After Non-Surgical Endodontic Treatment

<b>Background/Objectives</b>: Half of diagnosed unilateral maxillary sinusitis may have odontogenic origin, and root canal treatment (RCT) can be beneficial as a single-mode treatment for full resolution of maxillary sinusitis of endodontic origin (MSEO) symptoms. The aim of the study was to investigate the influence of passive ultrasonic irrigation (PUI) on the healing of MSEO after non-surgical endodontic treatment. <b>Methods</b>: A single-center, retrospective study was conducted on CBCT data and medical records of 240 patients, who underwent non-surgical endodontic treatment, performed by the same operator between 2016 and 2025. One hundred and thirty-six teeth entered the study due to inclusion/exclusion criteria. <b>Results</b>: Complete healing was observed in 75.74% of the cases (<i>n</i> = 103). The tooth most frequently associated with MSEO was the first upper molar (52.21%, <i>n</i> = 71). PUI was applied in 66.91% of the treatments (<i>n</i> = 91). PUI was an independent predictor of MSEO healing (<i>p</i> = 0.001; 95% CI [1.768; 9.136]). When PUI was introduced in RCT, complete healing of MSEO was observed in 84.62% of the cases (<i>n</i> = 77). PUI was independently associated with higher odds of radiological resolution of MSEO in univariate logistic regression (OR = 4.019) and multiple logistic regression (OR = 12.388) models. <b>Conclusions</b>: PUI application in the irrigation protocol was associated with higher rates of MSEO healing after non-surgical endodontic treatment (<i>p</i> = 0.001; 95% CI [1.768; 9.136]). The rate of MSEO healing after non-surgical RCT is high (75.74%); therefore it should be considered as a single-mode treatment option in cases of unilateral maxillary sinusitis.