A new approach for the detection of cracks in a tooth is presented using a microwave coaxial sensor. In the test, the probe tip is in close contact with the tooth surface, and the fringe electromagnetic fields out of the probe aperture interact with the tooth material. The presence of any crack affects the effective permittivity, causing changes in the signal energy received. Cracks in two representative teeth are evaluated, and they cannot be readily identified in X-ray images. Low signal power was applied during the scan, and no heating effect was produced. From the scanning of the positions right on and away from the cracks, significant differences are seen in the magnitude and phase of the reflection coefficient. By comparison, it is found that good performance can be achieved using the phase at a high frequency. The probe is also used for permittivity determination. From the measurement of standard liquids, high accuracy is achieved. The permittivity of the crown is computed using the error correction technique, and the data are used in further analytical modelling.In the analysis, by varying the thickness of the dielectric layer, the phase greatly changes when the thickness is below the aperture diameter. Thus, the characteristic of the limited signal penetration is revealed, enabling local inspection and accurate identification. With a simplified geometric model, it is shown that the air gap between the probe tip and the tooth surface does not contribute much to the whole sensing volume. In addition, from the numerical simulation, there is close correlation between the extent of the crack and the phase value, suggesting the potential for quantitative characterisation.The methodology proposed here could provide an alternative solution to efficient non-destructive detection of cracked teeth.

Tooth diseases including dental caries, periodontitis and cracks have been public health problems globally. How to detect them at the early stage and perform thorough diagnosis are critical for the treatment. The diseases can be viewed as defects from the perspective of non-destructive testing. Such a defect can affect the material properties (e.g., optical, chemical, mechanical, acoustic, density and dielectric properties). A non-destructive testing method is commonly developed to sense the change of one particular property. Microwave testing is one that is focused on the dielectric properties. In recent years, this technique has received increased attention in dentistry. Here, the dielectric properties of human teeth are presented first, and the measurement methods are addressed. Then, the research progress on the detection of teeth over the last decade is reviewed, identifying achievements and challenges. Finally, the research trends are outlined, including electromagnetic simulation, radio frequency identification and heating-based techniques.