Study on bubble fluid dynamics of ultrasound-activated irrigation in apical region of root canal

The mechanism of fluid dynamics is essential for the rational clinical application of the root canal irrigation technique. This study aimed to explore the dynamic characteristics of bubble fluid in the apical canal during ultrasound-activated irrigation. Ultrasound-activated irrigation was performed in canal models with different sizes as group #30/0.04 and #30/0.06. High-speed flow visualization technology was used to capture real-time images at 4000 frames per second. The air bubble behavior in the apical region was observed and analyzed via ImageJ and MATLAB software. The bubble movement distance and the bubble lock formation time and length were recorded. The dimensionless cavitation area was calculated. Fluid activated by ultrasound in the apical canal presented initiation, growth and stability stages. In stage initiation, numerous small cavitation bubbles were generated within 1 ms at the file tip. Concurrent with small bubble generation, certain bubbles underwent coalescence, forming individual larger bubbles approximately 100 μm in diameter within 1 s. The presence of multiscale-size bubbles from micrometres to millimetres indicated the stage growth. Fate of most bubbles ended up with collapse, while aggregation was captured. When the large bubble diameter reached the root canal diameter, new small bubbles could not break and pass through the bubble layer. This was denoted as the bubble lock formation and stage stability starting at 9.80 ± 2.74 s in group #30/0.06 and 5.00 ± 2.26 s in group #30/0.04 (P < 0.05). Taper size of the root canal could influence the bubble fluid dynamics during ultrasound-activated irrigation. Larger canal taper with an apical diameter of 0.30 mm significantly prolonged bubble lock formation time and enhanced apical bubble movement distance during ultrasound-activated irrigation.