Researchers from two German institutions have used near-infrared Raman spectroscopy to identify tooth decay in a variety of stages, and in tooth-colored fillings. Traditionally, dentists have examined suspicious teeth with a dental probe, a time-consuming and subjective process that, on occasion, may leave the remains of cavities undetected. Wieland Hill of the Institut für Spektrochemie und Angewandte Spektroskopie and Verena Petrou of Universität Erlangen-Nürnberg investigated this new method in the wake of developments that use NIR lasers to ablate infected tooth tissue instead of drilling. It holds the promise of a device that can both detect and treat tooth decay automatically.Near-infrared spectroscopy of decayed teeth may allow dentists to detect early forms of tooth decay and to ensure that all infected dentin is removed before a cavity is filled. Courtesy of Applied Spectroscopy (1997); 51:9:1266.The group uses the different Raman spectra of natural tooth substances and composite resin fillings as well as broadband luminescence of the decay itself. The benefit of using a near-infrared laser to detect decay is that the low level of quantum energy ensures that luminescence intrinsic to the teeth will not overpower the detection apparatus. The method also is chemical-free, painless and nondestructive and, unlike x-rays, uses no ionizing radiation. Five decay typesUsing an Nd:YAG laser from I E Optomech Ltd. operating at 1064 nm and sold by Perkin-Elmer as a part of its Fourier transform Raman spectrometer package, the group investigated teeth that have five types of decay: Initial caries, white spots where acid has demineralized the tooth's enamel. Ordinary caries, mildly progressing decay of the dentin. Chronic caries, slow but significant decay. Rampant caries, rapid destruction of dentin. Fissure caries, characterized by deep, infected pits. The group's results -- published in the Journal of Applied Spectroscopy -- indicate that decay with significant bacterial infection also shows high luminescence. The researchers think this may come from the by-products of bacterial activity or from other sources related to infection. The luminescence spectra was strikingly similar to the spectra of a humic substance found in peat. According to Hill, the more advanced stages of tooth decay are easily distinguished from healthy tooth enamel, the hard outer surface of a tooth. However, infected dentin, tooth tissue just under the enamel, may look almost identical to healthy dentin."The instrument may be especially helpful," Hill said, "for detecting initial forms of caries and small amounts of infected material left after a dentist has removed the caries." Benefits for dentistsAlthough there is virtually no clinical difference between spots of initial caries, the researchers found that some spots were luminescent while others were not. Hill said that his group thinks that the difference may be caused by bacterial infection. If this proves to be correct, the system could be enormously beneficial to dentists, because eliminating infection is important to treating tooth decay. Because of the inaccuracies from irregular tooth surfaces and a hand-held fiber optic probe, Hill said that a clinical system should not examine absolute luminescence intensities. The system should detect tooth decay by using ratios of luminescence and Raman signals. A clinical system would use an inexpensive NIR diode laser and a dispersive spectrometer, and would examine intensities at only a few spectral points.