These distinctions are important with regard to management. Lesions at levels D1 and D2 are generally managed using preventative measures, whereas lesions at the D3 or D4 level are likely to require restorative treatment. Detection of lesions of caries and being able to assess the level of disease are therefore crucial in determining clinical treatment.

Diagnosis and detection of caries

Diagnosis has been defined as identifying a disease from its signs and symptoms. In caries diagnosis that would involve both detecting the lesion and determining the activity of the process. Dental radiography is one method used to detect the lesion but it gives no information on the activity of the process. Various detection methods are available for different sites.

Radiographic detection of lesions of caries

Bitewing radiographic techniques, using both film packets and digital sensors (solid-state and phosphor plates) as the image receptor, were described in Chapter 10. For caries detection, film packets and phosphor plates are preferred as the imaging area of the equivalent-sized solid-state sensors is smaller. It has been reported that on average three fewer interproximal tooth surfaces are shown per image.

Approximal lesions of caries are detectable radiographically only when there has been 30–40% demineralization, so allowing the lesion to be differentiated from normal enamel and dentine. The importance of optimum viewing conditions for both film and digital images, as described in Chapter 19, cannot be overemphasized when looking for these early subtle changes. Magnification is of particular importance, as shown in Fig. 20.1.

Fig. 20.1 The effect of magnification. A Bitewing radiograph showing almost invisible very early approximal lesions in the molar and premolar teeth. B Magnified central portion of the same bitewing showing the approximal lesions (arrowed) more clearly.

Radiographic assessment of caries activity

A single bitewing radiograph may illustrate one or more caries lesions but it gives no information on caries activity. As caries is a slowly progressing disease, this progression, and therefore caries activity, can be assessed over time by periodic radiographic investigations.

In the UK, the 2013 Faculty of General Dental Practice (UK)’s booklet Selection Criteria for Dental Radiography recommends that the frequency of these follow-up radiographs be linked to the caries risk of the patient. There are three risk categories – high, moderate or low risk – for both children and adults. The Selection Criteria booklet contains a number of evidence-based recommendations, the main ones of which are summarised in Table 20.1. As can be seen for high risk children and adults 6-monthly time intervals are recommended, for moderate risk patients 12-monthly intervals and for low risk children 12–18 month intervals and for low risk adults 2-yearly intervals.

Table 20.1

Summary of the main recommendations for using radiographs for the diagnosis of dental caries (linked to caries risk), based on the Faculty of General Dental Practice (UK)’s 2013 Selection Criteria for Dental Radiography (3rd Ed)

Recommendation	Evidence-based Grading*
All *children* designated as *high caries risk* should have six-monthly posterior bitewings taken until no new or active lesions are apparent and the individual has entered another risk category (Bitewings should not be taken more frequently and it is imperative to reassess caries risk in order to justify using this interval again.)	B
All *children* designated as *moderate caries risk* should have annual posterior bitewings taken until no new or active lesions are apparent and the individual has entered another risk category.	B
All *children* designated as *low caries risk* should have posterior bitewings taken at approximately 12–18 month intervals in the primary dentition and at approximately two-yearly intervals in the permanent dentition. More extended radiographic recall intervals may be employed if there is explicit evidence of continuing low caries risk.	B
All *adults* designated as *high caries risk* should have six-monthly posterior bitewings taken until no new or active lesions are apparent and the individual has entered another risk category (Bitewings should not be taken more frequently and it is imperative to reassess caries risk in order to justify using this interval again. It is also important to remember that rates of caries progress in enamel and dentine will differ and that rates in adults may well be slower than in children)	C
All *adults* designated as *moderate caries risk* should have annual posterior bitewings taken until no new or active lesions are apparent and the individual has entered another risk category	C
All *adults* designated as *low caries risk* should have posterior bitewings taken at approximately two-yearly intervals. More extended radiographic recall intervals may be employed if there is explicit evidence of continuing low caries risk.	C
The taking of ‘routine’ radiographs based solely on time elapsed since the last examinations is not supportable.	C
Intervals between subsequent radiographic examinations must be reassessed for each new period as patients can move in and out of caries risk categories over time.	C
CBCT should not be used as a routine method of caries diagnosis	B

^*Evidence-based grading B = based on evidence from well conducted clinical studies but with no specific in vitro validation studies; C = based on evidence from expert committee reports or opinions and/or clinical experience of respected authorities and indicates an absence of directly applicable studies of good quality.

Radiographs used to monitor the progression of caries lesions need to be geometrically identical – hence the need for image receptor holders and beam-aiming devices (see Ch. 10). They should also be similarly exposed to give comparable contrast and density.

Geometrically reproducible digital images can be superimposed and the information in one image subtracted from the other to create the subtraction image which shows the changes that have taken place between the two investigations (see Ch. 5).

Radiographic appearance of caries lesions

As lesions of caries progress and enlarge, they appear as differently shaped areas of radiolucency in the crowns or necks of the teeth. These shapes are fairly characteristic and vary according to the site and size of the lesion. They are illustrated diagrammatically in Fig. 20.2 and examples are shown in Fig. 20.3.

Fig. 20.2 Diagrams illustrating the radiographic appearances and shapes of various lesions of caries.

Fig. 20.3 Bitewing radiographs showing examples of typical caries lesions (arrowed). A Small approximal lesions . B Large approximal lesions with extensive dentine involvement and a small lesion . C Approximal lesion extending into dentine and recurrent caries . D Small and extensive approximal lesions . E Small occlusal lesion and extensive occlusal lesion , apart from the small approximal enamel lesion, the enamel cap appears intact. F Root caries and recurrent caries .

Other important radiographic appearances

Radiographic detection of caries lesions is not always straightforward. It can be complicated by:

• Residual caries

• Radiodensity of adhesive restorations

• Cervical burn-out or cervical translucency

• Dentinal changes beneath amalgam restorations.

Residual caries

The rationale for removal of caries has changed in recent years. The primary aim nowadays when excavating dentine caries is to remove only the highly infected, irreversibly demineralized dentine in order to allow effective restoration of the cavity and surface anatomy of the tooth and to prevent disease progression. In other words, demineralized tissue (residual caries) is left behind in the base of the cavity and the tooth restored. Radiographically, this inactive caries may show a zone of radiolucency beneath the restoration. This appearance is identical to that of an active caries lesion under a restoration (see Fig. 20.4).

Radiodensity of adhesive restorations

The development of successful adhesive restorative materials in recent years, including dentine-bonding agents and glass ionomer cements, has resulted in many teeth being restored with non-metallic fillings. These various adhesive materials vary considerably in their radiodensity and do not appear as white on radiographic images as traditional amalgam, as shown in Fig. 20.5. Identifying subtle radiolucent lesions of caries adjacent to these materials can be very difficult, if not impossible.

Fig. 20.5 A RIGHT and B LEFT bitewings showing numerous posterior teeth that have been restored with adhesive restorations. Note: the restorations look less radiopaque than amalgam, and as a result the reduced contrast difference between dentine and restoration makes identification of adjacent radiolucent caries difficult. (Kindly provided by Prof A. Banerjee.)

Cervical burn-out or cervical translucency

This radiolucent shadow is often evident at the neck of the teeth, as illustrated in Fig. 20.6. It is an artefactual phenomenon created by the anatomy of the teeth and the variable penetration of the X-ray beam.

Fig. 20.6 A Diagram illustrating the radiographic appearance of cervical *burn-out*. B Bitewing radiograph showing extensive cervical *burn-out* of the premolars (arrowed). Compare this with the appearance of cervical caries on distal aspect .

Cervical burn-out can be explained by considering all the different parts of the tooth and supporting bone tissues that the same X-ray beam has to penetrate:

• In the crown – the dense enamel cap and dentine

• In the neck – only dentine

• In the root – dentine and the buccal and lingual plates of alveolar bone (see Fig. 20.7).

Fig. 20.7 A Diagrammatic representation of from the side showing the three-dimensional structures involved in the formation of the radiographic image. Note that in the cervical region there is less tissue present. B Plan view at the level of the necks of the teeth. Through the centre of the teeth there is a large mass of dentine to absorb the X-ray beam, while at the edges there is only a small amount. The edges of the necks of the teeth are therefore not dense enough to stop the X-ray beam, so their normally opaque shadows do not appear on the final radiograph.

Thus, at the edges of the teeth in the cervical region, there is less tissue for the X-ray beam to pass through. Less attenuation therefore takes place and virtually no opaque shadow is cast of this area on the radiograph. It therefore appears radiolucent, as if some cervical tooth tissue does not exist or that it has been apparently burnt-out.

Cervical burn-out is of diagnostic importance because of its similarity to the radiolucent shadows of cervical and recurrent caries. However, burn-out can usually be distinguished by the following characteristic features:

• It is located at the neck of the teeth, demarcated above by the enamel cap or restoration and below by the alveolar bone level

• It is triangular in shape, gradually becoming less apparent towards the centre of the tooth

• Usually all the teeth on the radiograph are affected, especially the smaller premolars.

In contrast, root and recurrent caries lesions, although they also often affect the cervical region, have no apparent upper and lower demarcating borders. These lesions are saucer-shaped and tend to be localized, as shown in Fig. 20.2. If in doubt, the diagnosis should be confirmed clinically by direct vision and gentle probing having cleaned and dried the area.