PHYSICAL-MECHANISMS GOVERNING THE HYDRODYNAMIC RESPONSE OF AN OSCILLATING ULTRASONIC FILE

Ultrasonically driven vibrating files are known to enhance the efficie ncy of root canal debridement. This paper presents a phenomenological view of the hydrodynamic response of an oscillating ultrasonic file an d the relationship between the file response and various physical fact ors such as file size and curvature, file surface properties, file vel ocity amplitude, root canal geometry, and the type of irrigant. Releva nt hydrodynamic properties include the propensity of a file to produce stable and transient cavitation, steady streaming, and cavitation mic rostreaming. These relationships were explored by experiment. Sonolumi ncscence was employed as an indicator of transient cavitation activity and photographic analysis was utilized as a means for detecting stead y streaming, microstreaming, and stable cavitation. Measurements faile d to indicate any strong correlation between registered driving power and the propensity to produce transient cavitation. Files that were pi tted or possessed salient edges were very effective at generating tran sient cavitation. When observed, transient cavitation activity general ly occurred near the tip of the straight file, provided the wall-loadi ng 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 enh anced by the addition of moderate amounts of dissolved gas into the ir rigant. Although the imposition of file-wall contact served to inhibit the production of transient cavitation, this action had relatively li ttle effect on the ability of a file to produce a nominal level of str eaming, microstreaming, and stable cavitation. The relationship betwee n these hydrodynamic properties and the process of root canal debridem ent is addressed. Observations suggest that it is not prudent to ascri be enhanced cleaning effects to any one phenomenon, for it is likely t hat several factors are involved to varying degrees depending on the l ocal conditions of application.