Root canal cleaning: through cavitation and microstreaming

Abstract

We have investigated the flow from a needle using Computational Fluid Dynamics simulations and high-speed imaging experiments on sub-millimeter fluidic channels. These have shown that the flow is not effective in delivering the bleach near the bacteria, due to the complex geometry of the root canal. Laser-Activated Irrigation (LAI) and Passive Ultrasonic Irrigation (PUI) are techniques for improving the irrigant distribution, of which we have investigated the cleaning mechanism, being acoustic streaming, cavitation bubbles, a chemical effect, or a combination of these. PUI makes use of a miniature file, vibrating at ultrasonic frequencies around 30 kHz. The oscillation characteristics of the miniature file have been investigated with vibrometry experiments and computer simulations, showing a pattern of nodes and antinodes along the file. In vitro measurements with twenty dentists have shown that, due to the small dimensions of the root canal, contact between the file and the root canal walls nearly always takes place. This affects the file oscillation but not the cleaning mechanisms. The induced acoustic streaming was measured using high-speed photography, revealing a complex flow pattern. The flow consists of both an oscillatory flow and a steady flow, which together help in exerting a force on the bacteria and in distributing the bleach. By evaluating the amount of debris removed from holes in the walls of extracted teeth we have gained insight into the cleaning characteristics and efficacy of PUI. Cavitation bubbles are also observed in the high-speed movies and with sonochemical techniques. While experiments indicated that such bubbles are very efficient in removing biofilms and other materials from nearby surfaces, during a root canal treatment their contribution to cleaning is limited, because these bubbles rarely collapse onto the wall of the root canal. Additional visualizations showed that the steady part of the flow does most of the removal of a biofilm-mimicking hydrogel, but the narrow root canal limits this effect. Interestingly, stable bubbles were observed to be able to remove the hydrogel even more efficiently.