Theoretically, visual inspection can be performed on approximal surfaces when the patient receives orthodontic separating rubber bands for 2 to 3 days to create space between the teeth [examples of this are shown in Fig. 3 in Schwendicke, Lamont and Innes, this vol., p. 36 and Fig. 3 in Fontana and Innes, this vol., p. 107]. This method works well for single approximal spaces and has been used as a gold standard in several clinical trials [25, 26]. For multiple approximal spaces, however, the effect would be less pronounced. This method also has the disadvantages of discomfort for the patient and requiring several appointments. Whilst in other medical disciplines several appointments for several tests to derive a complex diagnosis are usually accepted by patients, in dentistry there seems to be a need for immediate findings and treatment decisions [2].
Cleaning of teeth is necessary to inspect the tooth surfaces [27, 28]. Having defined approximal surfaces and occlusal fissures and grooves as high-risk caries sites, these sites are often covered by plaque or even food remnants that render a meticulous visual inspection impossible without prior cleaning. It is essential that plaque present be identified by means of a probe, because the position of the plaque may be an indicator for the presence of “active” enamel carious lesion(s) [6]. Clinically, the patient can be asked to brush the teeth in the usual way before inspection. This facilitates the identification of chronically neglected sites easier. Depending on the cleaning habits of the patients, chronically swollen and bleeding papillae hamper visual inspection of the interdental areas, and an initial periodontal therapy might be necessary before assessing the surface for the presence of a carious lesion.
Regarding occlusal caries diagnostics, some authors recommend the use of air abrasion because the small particles are able to clean the fissures better than pumice and brush or rubber cup [29]. Dental practitioners should be aware of the powder they use, because alumina air abrasion changes the surface morphology, especially of brittle enamel in carious fissures [30]. Sodium bicarbonate powder will cause bleeding when targeting the gums [31], so for cleaning prior to visual inspection air-abrasion with atraumatic glycine or erythritol powders should always be used [32].
Radiography
In order to enhance the detection of approximal carious lesions on bitewing, radiography is usually recommended. This technique allows the visualisation of the demineralisation depth both in enamel and in dentine. For dental practitioners, bitewing radiography is the primary means of supplementing findings obtained by visual inspection. Bitewing radiography shows mineral loss by change of radiolucency. It should be kept in mind that the change of radiolucency is a mere estimate for the degree of demineralisation. It does not show if active biofilm is present and thus if the lesion is progressing or is in a stagnating phase. Only the comparison with previous or future radiographs allows for lesion activity judgement. It is not possible to see on a bitewing radiograph whether the surface is cavitated or not [33]. There is too much overlap of dental hard tissue both in occlusal and approximal surfaces that does not allow the determination of whether cavitation has already occurred in the outermost surface. In a recent meta-analysis, sensitivity and specificity for carious lesion detection in approximal surfaces have been reported to be 0.36 and 0.94, respectively, at the dentine level [34]. When enamel carious lesions are included, sensitivities and specificities changed to 0.24 and 0.97 [34].
Because dental radiography consists of ionising radiation, its use should be justified [35]. Practically all dental scientific societies have published regulations on the frequency for bitewing radiographs, depending on the individual caries risk and on the age of the patient [35–38].
Depending on the detection threshold, bitewing radiography can be both helpful and misleading. It has been shown that if enamel carious lesions are included in the detection threshold level, more enamel lesions can be detected visually than radiographically [19, 20]. This might be because the crowns should be free from overlapping structures on the radiograph for superficial enamel carious lesion detection. Crowding of teeth, height of the palate, compliance of the patient and positioning of the intraoral film can reduce the quality of the radiographs, resulting in less diagnostic information for the dentist. When the caries prevalence is low, the possible risk of false-positive diagnoses outweighs the risk of overlooked carious lesions [3].
Several clinical studies have been carried out to assess the state of the surface in relation to depth of radiolucency in bitewing radiographs (for a review see Wenzel [33]). If dentinal carious lesions are visible in approximal surfaces, the surface has been found to be broken in earlier studies in more than 70% of cases [33].
If occlusal dentinal carious lesions are visible radiographically on otherwise clinically inconspicuous occlusal surfaces, the clinician should be aware of ongoing demineralisation, and a minimally invasive restoration should be applied [22]. However, it is not possible to see occlusal carious lesions restricted to enamel in bitewing radiographs.
As a more recent development, cone beam computerised tomography (CBCT) has been advocated by some authors who reported that a higher sensitivity with unchanged specificity might be achieved in vitro [39–41]. Under optimum circumstances, with high resolution, crowns without restorations can be viewed with CBCT almost as well as with microcomputed tomography, and it might be possible to even detect a cavitation in the transverse plane. However, since most devices have a limited field of view, one CBCT scan would be necessary for each side. Taking this into account and the fact that CBCT images are compromised by radiation artefacts from existing fillings or even from thick enamel [42], the much higher radiation dose, costs, and, last but not least, the necessity to interpret all radiologic findings for legal reasons [43], CBCT cannot be regarded as a recommended means for caries lesion detection. It is safe to stipulate that for caries detection, CBCTs absolutely cannot be justified.
Fluorescence-Based Technologies
The interaction of light with matter may lead to several specific phenomena: light can be reflected, or scattered (back-scattering or diffuse transmission), there can be transmission of the light, absorption with heat production or absorption with fluorescence [44]. Under the impact of suitable wavelengths,