or rather their porphyrins, can have a characteristic red fluorescence [45]. Fluorescence is thus an indirect measure of the degree of tooth demineralisation [46–48]. It is applicable on occlusal surfaces, smooth surfaces, and some devices also measure carious lesions in approximal surfaces [49].
There are pen-type devices that measure the red fluorescence locally in small areas and camera-based systems that can visualise the fluorescence for one or more tooth surfaces at once. One of the most popular pen-type fluorescence devices operates on a wavelength of about 650 nm and has a numeric output between 0 and 99 (DIAGNOdent pen, KaVo, Biberach, Germany). This device measures the fluorescence and allows for monitoring over time. In numerous studies a greater reliability was found for the pen-type fluorescence device compared to visual caries detection [49–51]. Among the camera-based systems, there are devices that visualise fluorescence qualitatively (e.g., Sopro, Acteon, La Ciotat, France), and other devices that have a quantitative output (e.g., Vistacam iX, Dürr, Bietigheim-Bissingen, Germany). There is a heterogeneous body of evidence derived from clinical studies [52]. While certain authors found that use of fluorescence devices increased the diagnostic accuracy [53–58], others found that it did not add to clinical decision making compared to visual inspection and radiography [59, 60], at least not in primary teeth [61].
Like radiography, fluorescence-based methods cannot detect surface cavitation, but they may add to the information on lesion depth. This might be helpful especially when radiography cannot be applied (repeatedly in children or pregnant women, anxious patients etc.) and visually dubious or conspicuous sites are present. One problem with fluorescence is the sensitivity to false positive results because of the inherent autofluorescence of biofilm, calculus, some prophylaxis pastes [62] or some filling materials [63, 64]. Fluorescence-based methods therefore require professional tooth cleaning before application, and – within the limits outlined above – they are most notably suitable for adjunct primary caries detection. Although thresholds have been published indicating the presence of enamel caries or dentine caries, the use of fluorescence-based methods is usually advocated as a “second opinion” device [46, 47], and treatment decisions should not be based on fluorescence measurements alone.
Transillumination
Especially for anterior teeth, fibre optic transillumination (FOTI) is an easy, inexpensive, and fast method to improve the visualisation of the depth of approximal decay. A bright light source is necessary to transilluminate the teeth. Light is applied to the side of the tooth and its transmission observed from either the opposing side or occlusally in the case of molars and premolars. Because light is scattered more in carious dental tissue than in sound dental tissue, caries appears darker against the healthy surrounding tissues. Thus, FOTI can assist the judgement as to whether the lesion is confined to enamel or if it has already extended to dentine because it enhances the contrast between sound and carious tissue. However, it is not possible to see cavitation or lesion activity with transillumination. Temporary tooth separation with a spatula or gentle flossing can add valuable information on surface continuity [25]. For premolars and molars, the digitised FOTI (DIFOTI), where a charge-coupled device sensor replaces the human eye, has produced heterogenous results with limited evidence [52]. Compared to radiographs, for detecting enamel carious lesions, a better agreement with the reference standard was reported for DIFOTI, while the detection of dentinal carious lesions was similar [65]. However, differences between reported studies might be attributable to inadequate light source or insufficient calibration [66].
Recently, a device with advanced near-infrared light transillumination (NILT) technology has been marketed and validated in clinical studies (DIAGNOcam, KaVo). This device consists of an intraoral camera with a wide-angle objective. The light source operates on a near-infrared wavelength of about 760 nm. Two branches illuminate the apical region from the oral and buccal aspect; the light enters the tooth via the roots, and the illuminated tooth is viewed from the occlusal aspect. This method is primarily suitable for approximal caries detection. Sound enamel looks shiny and transparent because light is transmitted in an unhampered way, while approximal enamel lesions are depicted as grey shadows. Because lesion depth in dentine is visible only in very deep dentine lesions for diagnostic purposes, the geometry of the approximal enamel lesion as viewed from the occlusal aspect is linked to lesion depth [67]. It has been found that an enamel shadow in broader contact with the enamel-dentine junction is indicative of a dentinal carious lesion, and in a clinical trial with 127 lesions in 85 patients the agreement between NILT and bitewing radiography for dentine carious lesion detection was reported to be almost 100% [68]. However, approximal lesions restricted to enamel were less likely to be reliably detected with interexaminer reliability of 0.51 (weighted kappa) [69].
Unfortunately, with NILT it is neither possible to detect surface cavitation nor to visualise approximal cervical secondary lesions, because the restauration hampers light transmission. For occlusal surfaces, no validated lesion detection criteria have been described so far.
Lesion Activity Assessment
Active enamel carious lesions are more likely to turn into cavitated lesions than inactive enamel lesions [6, 7]. Using visual-tactile criteria, active enamel carious lesions look whitish, matte, and opaque, they feel rough on gently running a probe across the surface and are often covered by “tacky” plaque before professional tooth cleaning [6]. By contrast, inactive enamel carious lesions look shiny and they feel smooth on running a probe over the surface. In a 2-year survey, it was found that these criteria are more predictive for occlusal sites than for smooth surfaces [70]. This is probably due to the fact that, as with carious lesion detection, lesion activity assessment in approximal surfaces is hampered by the neighbouring teeth and by the gingival papilla and/or gingival bleeding. Therefore, and because lesion activity assessment despite calibration can result in less than perfect agreement [71], objective methods using reflection intensity [72] or chemiluminescence [73] have been described. However, these have only