Fig. 10.3 Cross section of mesial root of mandibular molar demonstrating remaining tissue following root canal preparation utilizing NiTi rotary Þles [73]

placement of calcium hydroxide greatly reduces the number of viable bacteria in the canal [29, 30, 31, 38, 39, 46, 53, 121, 125, 132, 138, 142, 144, 145]. However, it does not eliminate all of them. Removal of bioÞlm is also limited, especially in the isthmuses and canal ramiÞcations [28].

The ßow characteristics of various size and shaped needles (open-ended versus side-vented) have also been described in the endodontic literature. Shen et al. [140] described how the velocity of ßuid ßow is affected by the needle design. Open-ended or beveled needles deliver ßuid at a quicker pace than side-vented needles. This increase in velocity may aid in removing debris from the canal walls. The side-ported needles have their maximum velocity at the site of the port and the energy dissipates apically. Openended/beveled needles deliver irrigation solution about 1Ð2 mm past the end of the needle. This is a beneÞt if the needle cannot reach the apex of the root canal but a drawback if it does (potential irrigation accident). The side-ported needles allow solution to only reach about 1 mm past the end of the tip and still have similar risks and beneÞts of the open-ended needles. Shen et al. [140] also calculated the ßuid pressure of the irrigant. This pressure may be of beneÞt in cleaning the canal walls by dislodging material such as bioÞlm. Of course the three dynamic parameters (ßuid ßow pattern, velocity, ßuid wall pressure) are all affected by the ßuid ßow rate placed into the needle. Unfortunately, research has continuously shown that traditional needle irrigation fails to clean isthmuses, lateral canals, and cul- de-sacs to any extent ([15, 22, 69, 73, 74, 82, 94, 112, 151, 170]). Activation of the endodontic irrigant appears to be a necessary step in the more

complete cleaning of the root canal system. A survey conducted by Dutner et al. [65] on the use of irrigants and adjunctive devices to aid irrigation found that almost 50 % of respondents use some type of irrigation aid. Of that group, 48 % used ultrasonics and 34 % utilized some form of sonic (subsonic) activation.

Ultrasonic Activation

Richman [128] Þrst 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 [48Ð50, 104Ð109] reported on the use of ultrasound as a primary method of canal preparation and debridement in root canal therapy. The studies evaluated the efÞcacy 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 Þnal canal shaping. The ultrasonically energized Þle was reported to rapidly instrument the canal wall more efÞciently with less operator fatigue. The Òultrasonically activatedÓ 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 Òsynergistic system.Ó The irrigating solution achieves its active biological-chemical effects when it undergoes ultrasonation. The authors deÞned 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 pressurevacuum effect which cleans irregularities in canals and kills microorganisms. The oscillatory effect of the ultrasonic instrument, which vigorously agitates the irrigating solution, is deÞned 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 highpowered shockwaves that dissipate repeatedly at a rate of 25,000 ~ 30,000 times per second (25Ð 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 ßuid 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/Þle placed in a ßuid. 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 Þle can also be associated with small gas bubbles set into oscillation by the ßuctuating pressure Þeld generated by the ultrasonic Þle. The group looked at transient cavitation using a photometric-sensitive image intensiÞcation system. This detection system monitored the light produced by the violent collapse of cavitation bubbles. A rectangular container Þlled with methylene blue dye and a dispersed Þlm 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 Þles. However, cavitation was produced when a scaler tip was inserted into the unit. The endosonic Þles produced acoustic streaming. When the amount of remaining debris was evaluated, there was no statistically signiÞcant 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 sufÞcient acoustic streaming to effectively clean the canal. Dampening of the Þles 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 deÞned acoustic streaming as the generation of time-independent, steady unidirectional circulation of ßuid 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 Þles were studied at different power settings. The power generated by the Þles was estimated by measuring the transverse displacement amplitudes that were produced. Transverse displacement amplitude was deÞned as half of the total distance moved by the pinpoint of light that appeared as a thin transverse line when a Þle oscillated. Twenty extracted maxillary anterior teeth were divided into two groups and instrumented with the second group using a modiÞed technique in which a #15 endosonic Þle was allowed to freely vibrate at working length for 5 min. The results showed that each Þle generated an acoustic streaming Þeld comprised of a primary Þeld consisting of rapidly moving eddies in which the ßuid element oscillated about a mean position, and a superimposed secondary Þeld consisting of patterns of relatively slow, time-independent ßow (Fig. 10.4). Approximately four clusters of eddies were generated by the #15 and 20 endosonic Þles. In the primary Þeld, the direction of rotation of the ßuid elements in each eddy was opposite to that of its immediate neighbor. The secondary Þeld showed symmetrical longitudinal ßows on both sides of the Þle (Fig. 10.5). Fluid was generally transported from the apical to coronal end of the Þle. The streaming velocity was greatest at the apical and least at the coronal end of the Þle. Smaller Þles generated relatively greater acoustic streaming, the velocity of which increased with increased power. These results were later conÞrmed by Jiang et al. [83]. Canals instrumented with the modiÞed method were found to exhibit cleaner surfaces. The authors concluded that the

Fig. 10.4 Photo of acoustic streaming around a size 15 endosonic Þle [3]

freely vibrating Þle 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 Þle, 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

Fig. 10.5 Depiction of the waves generated around the vibrating ultrasonic Þle (ACTEON North America/ Clinical Research Dental)

activity of ultrasound. This conclusion was based on his postulation that although the displacement amplitudes of the vibrating Þle were adequate to produce cavitation, the streamlined shape of the endosonic Þle was not conducive to generating a sound pressure Þeld large enough to produce cavitation. Walmsley [167] also concluded that because of the transverse nature of the vibration pattern of the activated Þle, the effectiveness of ultrasonic instrumentation is limited by the dampening of the Þle 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 Þle is not allowed to vibrate freely. Recently how-

ever, 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 Þle, but that canal size (in relation to the Þle size) did impact the amount of cavitation produced.

Ahmad et al. [5] also reported that ultrasonic Þles can generate acoustic streaming both in the free Þeld and in a small channel. Higher-velocity streaming was observed when smaller size Þles were employed and when the Þle was precurved (for curved canals). Light Þle-wall contact did not totally inhibit streaming, while severe Þlewall contact inhibited movement of the Þle and, as a result, no streaming was observed. The positions and length scales of the streaming vortices appeared to be inßuenced by the presence of boundaries. In the free Þeld, two rows of vortices were situated along the sides of the Þle (Fig. 10.6a), while in the small channel, the vortices were positioned above the surface of the Þle (Fig. 10.6b). These results indicated that it is possible for acoustic streaming to occur in a conÞned space, as in a root canal, provided that severe Þle-wall contact is avoided. They recommended that allowing the Þle 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 Þles. 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 Þle, provided the wall loading did not inhibit Þle motion. In all cases studied, steady acoustic streaming and stable cavitation were observed to varying degrees, depending on the amount of Þle to wall contact. Although the imposition of Þle-wall contact served to inhibit the production of transient cavitation, this action had relatively