Introduction: various irrigation devices and chemical solutions are being developed and improved nowadays for better penetration deep into the complex root canal system. Activation of irrigating solutions enhances the solution effectiveness. Evidence in the literature suggests that ultrasonic irrigation is much more effective in this respect than syringe irrigation alone. Passive ultrasonic irrigation (PUI) and/or ultrasonically activated irrigation (UAI): nowadays, the two terms, PUI and UAI, are interchangeable in the literature and represent the same technique. Literature data have shown that combining ultrasonically activated NaOCl with EDTA results in significantly greater removal of the smear layer. Conclusion: the use of ultrasonic activation of the irrigating solution after root canal preparation significantly improves the removal of debris from hard-to-reach areas such as isthmuses, lateral canals and the apical delta.

Purpose: The aim of this in vitro study was to examine the distinct types of isthmuses using a light microscope and cone-beam computed tomography (CBCT). Material/Methods: Fifty-five extracted human molars were used for this study. CBCT was used to conduct the study. After the CBCT analysis, all teeth were inspected under a light microscope, and the results were compared with those of the CBCT examination. Results: The results from the CBCT at the coronary third of the root canals were as follows: Types I, II, III, IV, and V isthmuses were found in 50.9%, 16.4%, 1.8%, 7.3%, and 23.6% of all samples, respectively. For the middle third, we found Types I, II, III, IV, and V isthmuses in 40.0%, 23.6%, 1.8%, 16.4%, and 18.2% of the samples, respectively. For the apical third, we identified Types I, II, III, IV, and V isthmuses in 60.0%, 29.1%, 5.5%, 1.8%, and 3.6% of all samples, respectively. The results obtained from the microscopic examination were similar to those from the CBCТ study. There is no confirmed statistically significant difference between the examination methods (p = 0.07). Conclusion: Three-dimensional examinations have a significant role among the methods for diagnosing and detecting an isthmus in the complex root canal system, especially in the apical part.