Richman [128] first reported the application of ultrasonics in endodontics. He used a Cavitron® ultrasonic dental unit and concluded that since these cases were treated without untoward postoperative sequelae, the use of ultrasonics in root canal therapy held great promise.

In a series of articles published in the endodontic literature from 1976 to 1985, Martin and Cunningham [48–50, 104–109] reported on the use of ultrasound as a primary method of canal preparation and debridement in root canal therapy. The studies evaluated the efficacy of the endosonic method, its ability to eliminate bacteria from the canal, and its effect on extrusion of debris. Martin and Cunningham [106] concluded that endosonic root canal preparation was superior to hand preparation in mechanical and chemical debridement, disinfection, and final canal shaping. The ultrasonically energized file was reported to rapidly instrument the canal wall more efficiently with less operator fatigue. The “ultrasonically activated” irrigant facilitated cleansing and disinfecting actions within the root canal system.

Other studies of ultrasound as a primary method of instrumentation did not support the claims of Martin and Cunningham [107] that ultrasound removes more tissue from the canal than hand instrumentation. These studies [52, 67, 92, 154] found no difference in tissue removal between ultrasound and hand instrumentation. Also, when antibacterial effects were evaluated, no difference was found between the two instrumentation techniques [16, 60]. The overall performance of ultrasound as a primary method of instrumentation was not found to be superior to hand instrumentation. There was also a reported increase risk in straightening and perforating canals.

Martin and Cunningham [106] attributed the success of ultrasonic instrumentation to the interaction of the ultrasonic energy and the irrigating solution. They called this interaction the “synergistic system.” The irrigating solution achieves its active biological-chemical effects when it undergoes ultrasonation. The authors defined the primary effects of ultrasound as being cavitation and acoustic streaming. Transient cavitation was said to occur when the ultrasonic energy creates a bubble which grows to a certain point and then collapses. This collapse creates a pressure-vacuum effect which cleans irregularities in canals and kills microorganisms. The oscillatory effect of the ultrasonic instrument, which vigorously agitates the irrigating solution, is defined as resonant or stable cavitation. Combined with these effects of cavitation is a dispersal of physical energy which leads to physical acoustic (sound wave) streaming. This acoustic streaming purportedly enhances cleansing and disinfection.

When an ultrasonic wave is projected in liquid, negative pressure is created and causes the liquid to fracture, a process known as cavitation. Cavitation creates bubbles that oscillate in the projected ultrasonic waves. As the ultrasonic waves continue, these bubbles grow larger and become very unstable, eventually collapsing in a violent implosion. The implosions radiate high-powered shockwaves that dissipate repeatedly at a rate of 25,000 ~ 30,000 times per second (25–30 kHz). Additionally, the implosion of cavitation bubbles creates temperatures that exceed 5,000 ° C and pressures that exceed 500 atmospheres. The shock waves that are generated by the implosion travel at speeds over 500 mph within the fluid and this current is called acoustic streaming (www.​tmasc.​com, http://​bluewaveinc.​com) [21, 157]. Acoustic streaming can also be derived from the ultra-high-frequency oscillation of the ultrasonic tip/file placed in a fluid. Cleaning an object requires dissolving a contaminant (removing substance/object from a wall and putting it into solution) and then displacing the saturated layer of the contaminant so that fresh cleaning solution can come in contact with the unsaturated surface of the contaminant. The ultrasonic cavitation implosion effect is incredibly effective in doing this. The cavitation implosion effect is especially effective on unsmooth and out of reach surfaces that are normally inaccessible through conventional means such as irrigation alone.

To gain an insight into the mechanisms involved in ultrasonic instrumentation, Ahmad et al. [2] investigated the phenomena of cavitation and acoustic streaming as seen within the root canal space. In this initial study, the authors combined the phenomenon of resonant or stable cavitation, as described by Martin and Cunningham [106], with the phenomenon of acoustic streaming. These terms were combined because the rapid vortex-like motion associated with the vibrating file can also be associated with small gas bubbles set into oscillation by the fluctuating pressure field generated by the ultrasonic file. The group looked at transient cavitation using a photometric-sensitive image intensification system. This detection system monitored the light produced by the violent collapse of cavitation bubbles. A rectangular container filled with methylene blue dye and a dispersed film of polystyrene spheres was used to detect acoustic streaming. These spheres were illuminated so that patterns of acoustic streaming could be detected. Forty extracted maxillary anterior teeth were divided into four groups and instrumented either by hand or ultrasonically (Cavi-Endo®), using either water or 2.5 % sodium hypochlorite as an irrigating solution. The teeth were split longitudinally and evaluated for presence of a smear layer using a scanning electron microscope (SEM). It was determined that transient cavitation did not occur with the Cavi-Endo® unit and endosonic files. However, cavitation was produced when a scaler tip was inserted into the unit. The endosonic files produced acoustic streaming. When the amount of remaining debris was evaluated, there was no statistically significant difference between ultrasonic and hand instrumentation when either water or sodium hypochlorite was used as an irrigating solution. The authors concluded that acoustic streaming was more important to cleaning than cavitation. It was also concluded that the recommended technique of ultrasonic instrumentation did not produce sufficient acoustic streaming to effectively clean the canal. Dampening of the files may have caused the limitation in the production of acoustic streaming in the constricted canal system.

Ahmad et al. [3] continued the investigation of ultrasonic debridement by examining acoustic streaming. The authors defined acoustic streaming as the generation of time-independent, steady unidirectional circulation of fluid in the vicinity of a small vibrating object. Using the same method to detect acoustic streaming as described in the previous study [2], different size files were studied at different power settings. The power generated by the files was estimated by measuring the transverse displacement amplitudes that were produced. Transverse displacement amplitude was defined as half of the total distance moved by the pinpoint of light that appeared as a thin transverse line when a file oscillated. Twenty extracted maxillary anterior teeth were divided into two groups and instrumented with the second group using a modified technique in which a #15 endosonic file was allowed to freely vibrate at working length for 5 min. The results showed that each file generated an acoustic streaming field comprised of a primary field consisting of rapidly moving eddies in which the fluid element oscillated about a mean position, and a superimposed secondary field consisting of patterns of relatively slow, time-independent flow (Fig. 10.4). Approximately four clusters of eddies were generated by the #15 and 20 endosonic files. In the primary field, the direction of rotation of the fluid elements in each eddy was opposite to that of its immediate neighbor. The secondary field showed symmetrical longitudinal flows on both sides of the file (Fig. 10.5). Fluid was generally transported from the apical to coronal end of the file. The streaming velocity was greatest at the apical and least at the coronal end of the file. Smaller files generated relatively greater acoustic streaming, the velocity of which increased with increased power. These results were later confirmed by Jiang et al. [83]. Canals instrumented with the modified method were found to exhibit cleaner surfaces. The authors concluded that the freely vibrating file produced hydrodynamic shear stresses large enough to remove debris and the smear layer from the walls of the root canal, resulting in enhanced cleansing action. This hydrodynamic shear stress was proportional to streaming velocity. Therefore, the authors deduced that since streaming velocity was highest at the apical tip of the file, a concentration of stresses in the vicinity of the tip facilitated debridement.

In another investigation into the mechanisms of ultrasound, Ahmad et al. [4] examined the effects of acoustic cavitation in debridement of root canals. The authors concluded that cavitation should not be regarded as an important mechanism in root canal debridement. Walmsley [166] also investigated the mechanisms of ultrasound in root canal treatment. His results agreed with Ahmad et al. [2], as he concluded that cavitation had little if any bearing on the debridement activity of ultrasound. This conclusion was based on his postulation that although the displacement amplitudes of the vibrating file were adequate to produce cavitation, the streamlined shape of the endosonic file was not conducive to generating a sound pressure field large enough to produce cavitation. Walmsley [167] also concluded that because of the transverse nature of the vibration pattern of the activated file, the effectiveness of ultrasonic instrumentation is limited by the dampening of the file against the root canal wall. Acoustic streaming is an effective mechanism in disrupting debris within the canals but is reduced when loading occurs against canal walls. Also, the synergistic activity of ultrasound and the irrigating solution does not take place when the file is not allowed to vibrate freely. Recently however, Jiang et al. [83] and Macedo et al. [97] showed that, within a simulated root canal system, cavitation did occur around the tip of an ultrasonically activated file, but that canal size (in relation to the file size) did impact the amount of cavitation produced.

Ahmad et al. [5] also reported that ultrasonic files can generate acoustic streaming both in the free field and in a small channel. Higher-velocity streaming was observed when smaller size files were employed and when the file was precurved (for curved canals). Light file-wall contact did not totally inhibit streaming, while severe file-wall contact inhibited movement of the file and, as a result, no streaming was observed. The positions and length scales of the streaming vortices appeared to be influenced by the presence of boundaries. In the free field, two rows of vortices were situated along the sides of the file (Fig. 10.6a), while in the small channel, the vortices were positioned above the surface of the file (Fig. 10.6b). These results indicated that it is possible for acoustic streaming to occur in a confined space, as in a root canal, provided that severe file-wall contact is avoided. They recommended that allowing the file to freely vibrate during some stage of treatment should be carried out in order to generate streaming in the root canal.

Roy et al. [133] used sonoluminescence as an indicator of transient cavitation activity and photographic analysis was utilized as a means for detecting steady streaming, microstreaming, and stable cavitation with ultrasonic files. 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 acoustic streaming and stable cavitation were observed to varying degrees, depending on the amount of file to wall contact. 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. They concluded that it was 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. Boutsioukis et al. [26] confirmed that an ultrasonically activated file contacts the root canal wall at least 20 % of time during activation. They reported that the depth of penetration of the file, the power utilized to activate the file, and the size of the root canal preparation all affected the amount of contact. However, they did report that cavitation in the fluid was detected even though there was file-wall contact.

The terminology for the activation of irrigating fluids in root canals can be a bit confusing. Weller et al. [168] compared the efficacy of ultrasonics as a primary method of instrumentation and as an adjunct to hand instrumentation versus hand instrumentation alone. The authors concluded that ultrasonic instrumentation is not an alternative to hand cleaning but acts as an aid to increase debridement efficacy after hand instrumentation. In this study, the ultrasonic instrument was still used as an adjunct in canal preparation. Later research [12, 36, 69, 74, 82, 94, 112] looked at the use of ultrasonic instrumentation in a more passive manner, that is, it was utilized after hand instrumentation and without the intent to enlarge, instrument, or impact the walls of the root canal. Thus, the term passive ultrasonic irrigation (PUI) came to be. The “passive” portion indicated no active or intentional removal of dentin. Unfortunately, even though no intent is made to contact or alter the root canal walls, contact of the oscillating ultrasonic instrument on the wall occurs (see above). Due to this, the phrase ultrasonically activated irrigation (UAI) was recently suggested by Boutsioukis et al. [26]. Unfortunately the reader must be aware that these terms can and will be used interchangeably in the dental literature and that they represent the same general technique.

Research into PUI/UAI has looked at the ability of the technique to remove tissue and debris, bacteria, biofilm, calcium hydroxide and other medicaments, and smear layer. Research has also looked at the impact of using PUI/UAI in curved canals, the use of a smooth instrument versus an endodontic file, and the effects the size of the instrument and canal preparation size have on cleaning/debridement results. In general, PUI/UAI consists of the use of a size 15 or 20 endodontic-type file or wire attached to an ultrasonic handpiece from which ultrasonic energy is supplied. The depth of the file within the canal and the manner in which irrigating solution is supplied during the process has also been evaluated.

Available products that a clinician can utilize to provide PUI/UAI include file-holder tips (Brasseler). These tips allow for the insertion of a hand file (k-type file, r-type file, spreader, etc.) or a specially designed hand file-type inserts (diamond coated, fluted, smooth-sided, etc.) and secured for use in the canal (Fig. 10.7). Also available is the Irrisafe™ ultrasonic tip (Fig. 10.8) produced by Satelec Acteon which comes in different lengths and diameters and includes a port for the delivery of irrigating fluid, and the Sonofile tips (Fig. 10.9) by Satelec which are similar to the Irrisafe™ files but without the irrigation port.