[Mechanical characteristics of ultrasonic device for root canal treatment]

Recently, many studies have demonstrated the superiority of the ultrasonic devices in endodontic treatment, while other reports did not recognize the advantage of the ultrasonic instrumentation over hand instrumentation. This disagreement in opinion might be due to the fact that the mechanical characteristics of ultrasonic devices have not been clarified. Therefore, some factors which might contribute to the mechanical characteristics of ultrasonic device for root canal treatment were examined. These factors were: (a) applied force on file; (b) angle of connector; (c) position of substrate material being cut; (d) filing speed; and (e) resonance of file. These factors were examined by measuring the cutting depth on acrylic wafer using the cutting efficiency testing apparatus. This apparatus with a push-pull stroke approximately 4 mm in length, was propelled the root canal instruments in linear motion. The filing speed was adjusted to 100 strokes per minute. The results were as follows: 1. The greater load was given to the file, the higher was the cutting ability of ultrasonic device. But cutting efficiency of #15, #25 and #35 files were inhibited over the load of 50, 60 and 70g each. It was considered that the adequate load for filing utilized ultrasonic device was #15:50g, #25:60g and #35:70g. 2. The file attached to the angled connector had a cantilevered vibration. And the cutting ability was effected by connector angle. The 120 degree-type connector was more effective than straight type (180 degree-type connector). 3. Substrate materials were made deeper cut when it was applied against the vibrating direction of the file than were applied at right angle to vibrating direction of the file. 4. Cutting efficacy of ultrasonic device was evaluated under the conditions as filing speed of 25, 50 and 100 strokes/min by comparing each ratio of cutting depth per total number of filing strokes after 20 minutes. The ultrasonic vibration showed tighter cutting ability when this ultrasonic device was operated 25 strokes/min. 5. Cutting efficiency of two methods was examined under the same amplitude. One was to set up the file length slightly shifted from the resonance point and to give the amplitude to file using resonance phenomenon. The other was to set up the file length of nonresonance condition and to give the amplitude by power supplying from the device. The latter showed higher cutting ability without the risk of the file fracture.(ABSTRACT TRUNCATED AT 400 WORDS)