Photonics looks set to provide dentists with a convenient new way of analysing the mineral content of tooth enamel, an important indicator of the onset of dental decay.

Reporting in the online journal Optics Express, scientists from Australia and Taiwan describe how laser-generated ultrasound can be used to measure the elasticity of a human tooth, which relates directly to the level of mineralization in its enamel (Opt. Express 17, 15592–15607; 2009). Although so far the scheme has only been tested on extracted teeth in an experimental set-up, the expectation is that in the future it could be developed into a hand-held probe and applied to patients in vivo.

Credit: © 2009 OSA

The strength of enamel, the hard outer surface of a tooth that protects the soft dentin underneath, is dependent on having a sufficiently high mineral content. Poor dental hygiene can allow the build-up of acids that dissolve the minerals, leading to weakened enamel and ultimately tooth decay. Having knowledge of the level of mineralization is therefore important for assessing the health of teeth.

Hsiao-Chuan Wang and co-workers from the University of Sydney, Australia, and National Cheng Kung University, Taiwan, illuminated a tooth with a line of ultrashort (∼5 ns duration) pulses of UV (266 nm) light emitted by a Q-switched frequency-quadrupled Nd:YAG laser.

The laser pulses excite an ultrasonic surface-acoustic-wave that travels across the tooth and is detected by highly sensitive optical interferometry through the tip of an optical fibre suspended just above the surface of the tooth. The velocity of the ultrasonic waves depends on the elasticity of the tooth; thus, by comparing travel-time and dispersion characteristics for different ultrasonic frequencies, the tooth's elasticity — and thus mineralization — can be determined.

An attraction of the technique is that it is non-destructive, in contrast with conventional physical nanoindentation impact tests, which can perform tooth elasticity measurements but damage the tooth in the process.

Interestingly, this is not the first time that photonics has been used as a diagnostic tool for the early detection of tooth decay. Researchers have also considered the use of terahertz imaging, optical coherence tomography, second-harmonic microscopy, near-infrared transillumination and Raman spectroscopy among others. In all likelihood, you'll be seeing more lasers at your dental surgery in the future.