Vibration Modes Generated in Magnetostrictive Ultrasonic Dental Scaler Inserts

Introduction: Ultrasonic dental scalers are commonly used for the removal of plaque and calculus from tooth surfaces. Most studies to assess ultrasonic scalers concentrate on the working tip of the instrument. This study investigated the vibration modes of both the magnetostrictive stacks which drive dental scaler inserts and the tips themselves using single-point and scanning laser vibrometry and mechanical Finite Element Modelling (FEM). Methods: Single-point laser vibrometry was used to assess a Slimline (Dentsply, York, PA, USA) ultrasonic scaler. Vibration measurements of the stack showed that the scaler had a resonance at 32 kHz. This originates from the dynamic magnetostriction of the insert's laminated nickel stack. This resonance, in turn, excited scaler insert tip vibrations that are then used to remove the dental calculus. 2D and 3D Femlab™ mechanical FE models were created to explore the vibration modes associated with a whole insert at the excitation frequency of ~32 kHz. From 2D solutions, flexural modes in the stack could be identified in the region of the measured resonance. 3D FEM produced tip vibration mode-shape models at ~30kHz similar to the vibrations of dental scalers physically evaluated using scanning laser vibrometry. Results: The 3D FEM models suggested that longitudinal strain produced by magnetostrictive action stimulates a flexural mode in the insert stack that results in the observed tip vibration. It has been noted that magnetostrictive dental scalers have a tendency to fail after prolonged use with the stack becoming distorted. The shape of such a distorted stack resembles the theoretical flexural mode-shape and so it is suggested that stacks can degrade as a result of flexural-mode stimulated stress cycling. Conclusion: The nature of magnetostrictive dental scaler vibrations has been identified through a comparison of measured data and FE models.

Supported by EPSRC grants GR/T22568/01 and GR/T22551/01