Vibration measurements for diagnosis of structural defects on human teeth

In this paper, the authors propose and analyse a non-invasive measurement procedure for the diagnosis of structural defects on human teeth based on non-contact excitation and measurement of vibration by laser techniques. The tooth is excited in vibration by Nd:YAG laser pulses, below the ablation threshold, which induces local thermal expansion with consequent propagation of bending waves. Vibrations are then measured by Laser Doppler Vibrometry (LDV), which permits non-invasive and accurate measurements. The presence of structural defects, caused, for example, by caries, can be detected by variation in the dynamic behaviour of the tooth. The technique and the results of in-vitro tests on artificially defected human teeth are presented. The metrological problems connected with measurement repeatibility in different operating conditions are addressed. The effect of pulse energy level has also been measured, with the aim of determining the minimum energy level sufficient to achieve satisfactory signal-to-noise ratio in vibration measurement. In fact, if high-energy pulses are utilised, overheating or ablation on the tooth structure may be caused with serious consequences for the patient.