International Endodontic JournalVolume 42, Issue 5 p. 391-398 Ultrasonic Debridement of Root Canals: Acoustic Cavitation and Its Relevance* M. Ahmad BDS, MDSc, M. Ahmad BDS, MDScSearch for more papers by this authorT. R. Pitt Ford BDS, PhD, FDSRCPS, T. R. Pitt Ford BDS, PhD, FDSRCPSSearch for more papers by this authorL. A. Crum BS, MS, PhD, L. A. Crum BS, MS, PhDSearch for more papers by this authorA. J. Walton BA, MSc, PhD, A. J. Walton BA, MSc, PhDSearch for more papers by this author M. Ahmad BDS, MDSc, M. Ahmad BDS, MDScSearch for more papers by this authorT. R. Pitt Ford BDS, PhD, FDSRCPS, T. R. Pitt Ford BDS, PhD, FDSRCPSSearch for more papers by this authorL. A. Crum BS, MS, PhD, L. A. Crum BS, MS, PhDSearch for more papers by this authorA. J. Walton BA, MSc, PhD, A. J. Walton BA, MSc, PhDSearch for more papers by this author First published: 08 April 2009 https://doi.org/10.1111/j.1365-2591.2009.01560.xCitations: 9 * Reprinted from the Journal of Endodontics, 14/10, M. Ahmad, T.R. Pitt Ford, L.A. Crum and A.J. Walton, Ultrasonic debridement of root canals: acoustic cavitation and its relevance, pp.486-493, 1988, with permission from The American Association of Endodontists. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Citing Literature Volume42, Issue5May 2009Pages 391-398 RelatedInformation

The incidence of breakage of Piezon-Master ultrasonic K files were evaluated. Three groups of unused files were subjected to three treatments, namely; free vibration in air without irrigation, free vibration in root canal while minimizing contact with the wall of canal in the presence of irrigation and light filing in root canal with free flow of irrigation. Cavitation produced by files in contact and free of contact with a glass surface was examined in order to observe the relationship between cavitation defects and breakage. In addition, the fractured and unfractured files were examined under a scanning electron microscope for the presence of cavitation pits. The results indicated that more files broke in air. In water, a higher incidence of breakage occurred when files were allowed to freely vibrate while no breakage occurred when the files were used in filing. All files generated cavitation which resulted in pitting of their surfaces. However, it was considered unlikely that the pits contributed to fracture. Fatigue cracks which could be the result of the manufacturing process were observed at some of the corners of the cross sections of the fractured files and could be the main contributory factor to fracture.

The present study was undertaken to see if there was any variability in the power output of Piezon-Master 400 ultrasonic files when driven using different generators, tranducers and file holders. The displacement amplitude of the oscillating tip of the file in air was used as a measure of the power output. The results showed that there was considerable variability in the power output of Piezon-Master 400 ultrasonic files of similar size and length when driven using different generators, transducers and file holders. In consideration of this, it is recommended that a calibration device be incorporated in the ultrasonic unit so that the operator will have some knowledge of when the unit is working at its maximum efficiency.

The pattern of oscillation of a Piezon-Master 400 ultrasonic file driven by a piezoelectric transducer was studied in air and on water. In addition, the displacement amplitudes of the files were measured. The findings were compared with those observed with the Cavi-Endo unit reported in another study (Ahmad 1969). It was observed that the file vibrated such that a standing wave was formed on the file and it exhibited points of maximum deflection (antinode) and points of minimum deflection (node) with the largest deflection occurring at the apical end. This pattern of oscillation was similar to that exhibited by the Cavi-Endo file which employed a magnetostrictive transducer. However, the displacement amplitudes were very much higher than those exhibited by the Cavi-Endo. It is considered that the 120 degrees angle of the file holder inherent in the Piezon-Master 400 unit and the more effective power transmission with the piezoelectric transducer may have contributed to the large amplitudes.

The steady acoustic streaming generated around straight and precurved oscillating ultrasonic files driven by the Piezon-Master 400 unit was examined in the free field and in small channels using a stereomicroscope. In addition, the effect of file-wall contact on streaming production was also investigated. The results indicated that the ultrasonic files can generate acoustic streaming both in the free field and in the small channel. Higher velocity streaming was observed when smaller size files were employed and when the file was precurved. 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 while in the small channel, the vortices were positioned above the surface of the file. 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. It is therefore recommended that light filing or allowing the file to freely vibrate during some stage of treatment should be carried out in order to generate streaming in the root canal.

The efficacy of two ultrasonic units in shaping curved canals in teeth were compared. Twenty teeth were instrumented using the Cavi-Endo unit at a power setting 1 using the technique recommended by the manufacturer. Another group of 20 teeth received similar treatment but were instrumented with the Enac unit. The time taken to instrument each canal was recorded. The pre- and post-instrumented radiographs of the teeth of x 10 magnification were taken using a microfocal technique. The radiographs were subjected to a subtraction technique to result in composite images of the pre- and post-instrumented shapes. The canal shape and the incidence of elbows were evaluated using various measurements taken from the radiographs. The manner the dentine was removed was similar in both groups. All canals exhibited unequal removal along the canal with more dentine being removed at the coronal end. The Enac group exhibited a higher incidence of elbows which occurred further apically than those in the Cavi-Endo group. There was no significant difference between groups in the following: time of instrumentation, amount of apical and coronal canal enlargement, apical deviation and change in width at the elbow. These findings were no different from those of another study using simulated canals.

The physical mechanisms of ultrasound, namely cavitation and acoustic streaming, generated by the Enac-Osada ultrasonic unit were investigated for effectiveness in disrupting Streptococcus mitis. In addition, the bactericidal effect of ultrasound in the presence of 2.5% sodium hypochlorite was examined. Bacterial suspensions were irradiated directly with ultrasound in simulated root canals, and the viability of bacteria was examined after growth on a blood agar medium under anaerobic conditions at 37° C for 5 days. The results indicated that ultrasound per se failed to disrupt bacteria but resulted in increases in the viable counts; the former was considered to be because of the lack of cavitation and the latter because of the dispersal effects of acoustic streaming. The 2.5% sodium hypochlorite solution demonstrated powerful bactericidal activity.

The temperature rise of the irrigant in the root canal during free vibration of the ultrasonic file was studied in vitro in 10 human teeth. The mean temperature rise was found to be 0.6 degrees C. The minimal temperature increase may not significantly contribute to the effectiveness of ultrasonic root canal instrumentation.

The resonant characteristics of ultrasonic files driven by the Cavi-Endo unit were examined. The investigation was carried out by evaluating the pattern of oscillation and the power emitted by the ultrasonic files. It was observed that the files oscillated in a sinusoidal fashion, exhibiting a standing wave pattern along the file. The power emitted was found to be directly proportional to the flexibility of the file. Such behaviour is typical of a system that exhibits resonance.

Abstract This study investigated the feasibility of using K‐Flex files with the Cavi‐Endo ultrasonic unit. The effectiveness of the K‐Flex files in shaping canals was compared with the ultrasonic Cavi‐Endo K‐files. Simulated curved canals in clear resin blocks were instrumented ultrasonically using the 2 file types. Pre‐ and postinstrumentation photographs were taken and the following were evaluated: apical and coronal areas, the incidence of the narrowing of the canals coronal to the apical transport (elbows). Displacement amplitude measurements were carried out in order to see any relationship between the incidence of elbows and displacement amplitudes of the files. The results indicated that the K‐Flex files were more efficient than the ultrasonic K‐files and resulted in less elbow formation. No direct relationship was observed between displacement amplitudes and the incidence of elbow formation. It is suggested that the manufacturers of ultrasonic units consider different file designs.

Two physical mechanisms of ultrasound, cavitation and acoustic streaming, were investigated in an ultrasonic endodontic unit. In addition, the potential of ultrasonic instrumentation for disruption of Bacteroides intermedius was examined at various time intervals. The ultrasonic file could not generate cavitation within the recommended power settings indicated for endodontic purposes. However, there was evidence of acoustic streaming. The latter phenomenon resulted in destruction of 21.6%, 30.4% and 92.9% of test bacteria after 1, 5 and 15 min sonication respectively. It appears that ultrasonic instrumentation of root canals has little bactericidal effect.

The incidence of fracture of Cavi-Endo ultrasonic K-files during ultrasonic root canal preparation was evaluated. In addition, the appearance and the sites of fractures were examined in relation to the nodal points on the file. The displacement amplitudes and the hardness of the files were quantified to determine their relationship to fracture. It was observed that the files exhibited a low incidence of fracture, which was of the brittle type and occurred at the apical ends. The displacement amplitude and the hardness of the files did not appear to influence the incidence of fracture.

The effects of ultrasonic instrumentation on the preparation shape of curved simulated canals were investigated. Measurements of the displacement amplitudes of the files were made to examine the possible relationship between displacement amplitude and the cutting ability of the file. Twenty-five simulated canals in clear resin blocks were instrumented using a Cavi-Endo unit at a power setting of 1 with differing times of instrumentation and with water as irrigant. Another group of 25 canals received similar treatment but were instrumented with an Enac unit at a power setting of 1. The canal shape and the incidence of elbow formation were evaluated using various measurements taken from photographic prints of the canals. All canals exhibited unequal removal along the canal with more removal occurring coronally. The Enac group exhibited a higher incidence of elbows which occurred further apically than those in the Cavi-Endo group. There was no significant difference between groups in the amount of apical canal enlargement. Coronally, the Enac appeared to cut significantly better. The Enac also caused significantly greater apical deviation and change in width. In both units more elbows were formed following instrumentation, with less flexible files having smaller displacement amplitudes.

Forty teeth with severely curved canals were divided into two groups and the root canals were prepared either ultrasonically or with hand instruments. Canal shapes were compared qualitatively and quantitatively using subtraction macroradiography; this allowed the pre- and postinstrumented canal shapes to be viewed on the same print. The times to carry out instrumentation and the incidence of elbows were recorded. In addition, the following measurements were taken from the radiographs using a digitizer: apical and coronal areas, distance of elbow from canal curvature, change in width at elbow and 0.5 mm apical to it, and apical transport width. Subjective evaluation revealed that no difference was apparent between the methods as to the places where the dentin was removed; both instruments exhibited unequal dentin removal along the canal length with more removal occurring coronally. The quantitative results indicated an absence of differences in the following: the time of instrumentation, the incidence of elbows, the distance of elbow from the point of curvature, the change in width at the region of the elbow, and the apical transport width. These findings indicated that ultrasonic files behaved similarly to hand instruments when used in a filing action.

The validity of using simulated root canals in resin blocks for evaluating the effects of ultrasonic root canal instrumentation was examined. Curved canals in resin blocks and in natural teeth were ultrasonically instrumented using the Cavi-Endo unit at power setting 1. Canal shapes were compared qualitatively and quantitatively from photographs and subtraction macroradiographs of simulated canals and teeth, respectively. The following measurements were taken: apical and coronal areas, changes in width at the elbow and at 0.5 mm apical to the elbow, and apical transport width. Qualitatively, there did not appear to be any differences in the manner of removal of material along the length of the canal in both test models. Quantitatively, there was a similarity in the following: areas of canal walls removed, the incidence of elbows, the change in width at the region of the elbow, and the apical transport width. The lack of differences showed that the ultrasonic file cut in a similar fashion in both test models and indicated that simulated canals are valid models for evaluating the effects of ultrasonic instrumentation.

The phenomena of cavitation and acoustic streaming were investigated in an ultrasonic endodontic unit. A comparison between the cleaning efficiency of ultrasonic and hand instruments using either tap water or 2.5% sodium hypochlorite was made by scanning electron microscopy. With the use of a sensitive image intensification system, no light emission, indicative of transient cavitation, was observed. However, examination of the surface of water containing polystyrene spheres near the vibrating file indicated intense acoustic streaming. Under scanning electron microscopy no difference in the surface debris was observed between the two techniques, although less smear was apparent in the ultrasonic groups. Canals instrumented with sodium hypochlorite exhibited less debris regardless of the techniques used. It is concluded that transient cavitation does not play a role in canal cleaning; however, acoustic streaming does appear to be the main mechanism involved. The findings with sodium hypochlorite reemphasized its usefulness as an irrigant. The phenomena of cavitation and acoustic streaming were investigated in an ultrasonic endodontic unit. A comparison between the cleaning efficiency of ultrasonic and hand instruments using either tap water or 2.5% sodium hypochlorite was made by scanning electron microscopy. With the use of a sensitive image intensification system, no light emission, indicative of transient cavitation, was observed. However, examination of the surface of water containing polystyrene spheres near the vibrating file indicated intense acoustic streaming. Under scanning electron microscopy no difference in the surface debris was observed between the two techniques, although less smear was apparent in the ultrasonic groups. Canals instrumented with sodium hypochlorite exhibited less debris regardless of the techniques used. It is concluded that transient cavitation does not play a role in canal cleaning; however, acoustic streaming does appear to be the main mechanism involved. The findings with sodium hypochlorite reemphasized its usefulness as an irrigant.

Light microscopic comparisons were made of acoustic streaming generated by Cavi-Endo endosonic files of varying sizes at different power settings. To estimate power generated by the files, measurements of the transverse displacement amplitude were carried out. The optimum parameters established from these findings were used in a subsequent study which compared the effectiveness of ultrasonic debridement of two techniques, the one advocated by the manufacturer and a modified technique developed by us. The results indicated that smaller files generated relatively greater acoustic streaming, the velocity of which tended to increase with increases in power. Canals instrumented using the modified technique exhibited cleaner surfaces. It is proposed that in the modified technique, acoustic streaming was freely generated and played an important role in debridement.