In Situ Fatigue Crack Initiation Monitoring in NiTi Rotary Endodontic Instruments

During clinical operation in root canals, NiTi endodontic rotary instruments are subjected to alternating tension and compression and are prone to accidental fracture without prior warning. Once broken, extracting the fractured part from the canal is difficult and is annoying to both the patient and the dentist. Warning of an imminent fracture during clinical use will be a great help to avoid medical and legal complications. To this end, a monitoring technique that involves the picking up and analysis of the stress wave signals due to the cutting and rubbing at the canal wall-instrument blade interface was investigated. Both the stress wave conducted through solid and the sound wave conducted in air were monitored. The former was picked up by a Fiber Bragg Grating (FBG) and the latter with a microphone. When cracking developed in a rotary instrument, we expect the natural vibration frequency of the instrument as well as the characteristics of cutting/rubbing with the canal wall change. From the raw signal intensity history, it is possible to differentiate whether the instrument is operating but there is no clue when a fatigue crack has initiated. By employing Fast Fourier Transform (FFT) on the signal, we can reveal the energy associated with different frequencies. With characteristic frequency we refer to the frequency that is associated with the highest signal intensity. It was found that the energy intensity of the characteristic frequency varied in a well-defined pattern during the process of fatigue testing of the rotary instrument. Towards the end of fatigue life, a large intensity spike of the characteristic frequency was observed and this was shown to be associated with the occurrence of a fatigue crack. It is hoped that with this information, the fatigue failure of rotary instruments can be closely monitored to avoid/alleviate the occurrence of unexpected fracture during clinical use.