Physical mechanisms governing the hydrodynamic response of an oscillating ultrasonic file

Ultrasonically driven vibrating files are known to enhance the efficiency of root canal debridement. This paper presents a phenomenological view of the hydrodynamic response of an oscillating ultrasonic file and the relationship between the file response and various physical factors such as file size and curvature, file surface properties, file velocity amplitude, root canal geometry, and the type of irrigant. Relevant hydrodynamic properties include the propensity of a file to produce stable and transient cavitation, steady streaming, and cavitation microstreaming. These relationships were explored by experiment. Sonoluminescence was employed as an indicator of transient cavitation activity and photographic analysis was utilized as a means for detecting steady streaming, microstreaming, and stable cavitation. Measurements failed to indicate any strong correlation between registered driving power and the propensity to produce transient cavitation. Files that were pitted or possessed salient edges were very effective at generating transient cavitation. When observed, transient cavitation activity generally occurred near the tip of the straight file, provided the wall-loading did not inhibit file motion. In all cases studied, steady streaming and stable cavitation were observed to varying degrees, depending on the amount of file to wall contact. Stable cavitation was probably enhanced by the addition of moderate amounts of dissolved gas into the irrigant. Although the imposition of file-wall contact served to inhibit the production of transient cavitation, this action had relatively little effect on the ability of a file to produce a nominal level of streaming, microstreaming, and stable cavitation. The relationship between these hydrodynamic properties and the process of root canal debridement is addressed. Observations suggest that it is not prudent to ascribe enhanced cleaning effects to any one phenomenon, for it is likely that several factors are involved to varying degrees depending on the local conditions of application.