Bitewing Radiography for Caries Diagnosis in Children: When and Why?

Diagnostic yield of bitewings

Kidd and Pitts's 1990 literature review concluded that bitewing radiography is essential to ensure proximal caries is not missed in the primary or permanent dentition.⁸ Most studies included in the review found that 50% more lesions were detected compared to those identified clinically, and that in some cases 250% more lesions could be detected from bitewings. A more recent systematic literature review also confirmed that, for proximal surfaces, the radiographic prevalence of carious lesions was considerably higher than clinical prevalence.⁹ Further, Newman et al found that 48% more proximal carious lesions were diagnosed with bitewing radiography than without. Bitewing radiography is considered particularly important in diagnosing early proximal lesions, allowing the possibility for preventive intervention.^10,11Figures 1a and 1b show an apparently caries-free lower arch. However, radiographs reveal distal dentine caries in the lower first primary molars and enamel caries in the mesial surface of the lower right second primary molar. There is a slight shadow visible through the marginal ridge of the lower left first primary molar. This is a result of the camera flash and was not seen clinically.

As well as providing information regarding proximal surfaces, bitewing radiography will also demonstrate occlusal caries once it has reached dentine. Weerheijm et al in two separate studies found that, in the permanent dentition in children, 15-37.5% more occlusal lesions were detected where bitewing radiography was employed.^12,13 Similar findings were found by Newman et al, where 12% more occlusal lesions were detected with the use of bitewing radiography.¹⁰ However, several other studies have shown that bitewing radiography adds little in the detection of occlusal lesions.^14,15 In a review by Braga et al it was suggested that, if a thorough clinical examination was carried out on cleaned dry teeth, then occlusal lesions will not be missed.¹⁶ That being said, whenever a bitewing radiograph has been obtained it should always be examined for occlusal caries in dentine.¹⁷

A study involving 126 children in the primary dentition looked at the effect on treatment planning of the additional information provided by bitewing radiograph by comparing treatment plans based on clinical assessment alone, with a treatment plan on the same patient after assessment was supplemented with bitewing radiography.¹⁸ The examiners used a meticulous caries diagnostic system (ICDAS). After use of bitewing radiography, the number of surfaces that changed from no treatment, to non-operative management, and to operative management increased. While the overall percentage increase was small, this has to be taken in context, as the authors considered all surfaces, including occlusal ones, when caries in the primary dentition is centred on the approximal surfaces, particularly the distal surface of the first primary molar and the mesial surface of the second primary molar. Therefore, the percentage increase for clinically important (approximal) surfaces may well have been greater. This is reflected in the fact that a greater effect was seen for proximal surfaces. Fifty-two (3.2%) surfaces believed sound moved into requiring non-operative treatment and 46 (2.8%), moved to requiring operative care, as did 50 (6.2%) of surfaces originally thought amenable to prevention. Therefore, the additional diagnostic information available following bitewing radiographic examination altered a significant number of treatment plans. Specificity of bitewing radiography has been found to be high, at over 90%, therefore the rate of false positives and over treatment would be low.¹⁰

Caries risk

A patient's caries risk should be determined following thorough history-taking (including medical, social and dental) and examination, thus requiring accurate caries diagnosis. In children, caries experience is the single best predictor for future caries development, but the findings of a recent systematic review and a review of longitudinal studies have shown that other factors may be useful, including at a sociodemographic/socioeconomic level, dietary habits, oral hygiene, fluoride use, presence of lactobacilli/Streptococci mutans, salivary flow rate and the post-eruptive age.^19,20,21 Using these risk factors, patients can be categorized into very high, high, and low caries risk, with preventive treatment tailored appropriately.²² The additional diagnostic yield from bitewing radiography is higher in the high-risk groups and lower in the low-risk groups.²³

Bitewing interval guidelines

The Faculty of General Dental Practice UK (FGDP) have recommended appropriate time intervals between bitewing investigations.²³ Summarized in Table 1, they are informed by risk assessment but reassessment of caries risk should be undertaken at each recall appointment. It is recognized that, for the low risk group, longer radiographic recall intervals may be more appropriate.

The European Academy of Paediatric Dentistry (EAPD) has also produced guidelines regarding the timing and frequency of bitewings in children.²⁴ Patients are categorized as either ‘high risk’ or ‘low risk’. Baseline bitewings are suggested at age 5, but this may be too late to diagnose lesions that may potentially be reversed before cavitation if detected earlier, particularly considering the proportion of 5-year-olds in the UK with caries experience.³

There is evidence to suggest that, despite the publication of evidence-based guidelines, many dentists do not carry out caries-risk assessments for their patients, with an underuse of bitewings in the diagnosis of dental caries.^25,26 A study found that less than half of GDPs regularly carried out risk assessments in all children. The same study looked at use of bitewing radiography in children, and 24% of GDPs considered bitewing radiography for caries diagnosis in primary teeth to be of little value for preventive care, with only 15% considering it to be very valuable.¹¹ Regarding restoration of proximal dentinal lesions, 19% thought bitewings were of little value, with only 12% saying that they were very valuable, and only 9% thought that bitewings were very valuable for assessing caries progression; 21% assigning them of little value.¹¹ Factors considered to be very important in influencing the decision to take bitewing radiographs were child co-operation and past caries, with these being very important to 42% and 52% of respondents, respectively.¹¹ Past caries experience is indeed an important indicator for radiographic examination, by informing risk status and therefore interval between examinations. Co-operation is a relevant factor, research at a UK dental school found that 75% of children aged 5-10 found radiographic examination easy or very easy, with only 10% finding it hard or very hard²⁷ (Table 2). Furthermore, only 7% would be unhappy or very unhappy to have a radiograph taken again. Therefore, co-operation would not seem to be the limiting factor that explains the gap between clinical practice and national standard.

Radiographic diagnosis of caries using bitewings

Ideally, bitewings require an image receptor holder and beam-aiming device but associated discomfort can make this difficult in young children. Image receptor holders result in less beam angulation error in comparison to traditional bitewing tabs, which is important as false proximal lesion progression can be suggested due to small changes in horizontal beam angulation.^28,29,30

Examples of bitewings taken using holders and tabs are shown in Figure 2. Both sets of images are Grade 1 quality.

For conventional film radiography, a size 0 (22 mm x 35 mm) film packet should generally be used, but when the permanent second molars have erupted, a size 2 film packet (30.5 mm x 40.5 mm) ensures adequate coverage of the teeth. It is important that the bitewing radiographs are of a consistently high quality in order to maximize diagnostic outcomes and reduce radiation dose. Summarized below are the European Commission quality standards for bitewing radiography.³¹

These standards refer to conventional radiography but similar standards would also be applied to radiographs acquired using digital systems.

Radiation dose and risk

The effective dose from 2 bitewing radiographs is low and ranges from 0.6-43 µSv.²³ However, bitewing radiography risk in young children is 2-3 times that of an adult, making it vital that radiographic exposures are fully optimized.³¹ Modern x-ray equipment should be used, with correct exposure factors and rectangular collimation used routinely. Alone, the latter may reduce the effective dose 5-fold.³² F-speed film should be used as it requires significantly lower radiation exposure, with no differences in diagnostic accuracy in terms of caries detection compared to slower film speeds.^33,34

Digital radiography

Conventional radiography is being steadily replaced with digital systems in the UK, with more digital systems in use than conventional.³⁵ Most of these units used solid state detectors (SSDs), with the remainder using photostimulable phosphor plates (PSPs). Digital radiography has many advantages, including the elimination of chemical processing, image enhancement, and dose reduction. Interestingly, however, this dose reduction has not been realized in practice. This has been attributed to a lack of knowledge of appropriate exposure settings, and because it may not be possible to reduce the exposure times sufficiently on older x-ray sets.³⁵

Generally, there is a higher retake rate using solid state detectors in comparison to film.³⁶ Bitewing SSDs are more difficult to position than PSPs, with first premolar and canine teeth often missed.³⁷ SSD bulk can make them particularly uncomfortable for the patient, but this can also be so for PSPs as some have sharp edges on their outer envelopes.³⁷

The image enhancement available on digital systems may help improve caries detection.^38,39 The main adjustments clinicians make to digital images are to the brightness and contrast. High brightness, low contrast images are associated with higher number of true negative cases and a decrease in caries.⁴⁰ In addition, high contrast images may overestimate the presence and extent of caries lesions.⁴⁰ The perception of ‘Mach bands’ which can mimic caries are also influenced by image adjustments of the image. Examples of image enhancements are shown in Figure 3.

Most studies have shown that the diagnostic accuracy of digital systems is similar to conventional radiography.^{34,41,42,43,44,45} Examples of PSP and SSD detectors set up for bitewing radiography are shown in Figure 4.

Image automated analysis

An early in vitro study on digitized conventional films showed automated image analysis, where software extracts data from digital radiographs, for example to aid diagnosis, was accurate and reproducible.⁴⁶ While Wenzel's early testing of the only commercially available system (Logicon caries detector) showed inconsistency, more recent studies show that software improvements make it more reliable.^47,48 More research is required before these systems can be used routinely in dental practice. Subtraction radiology involves the superimposition of two images taken at different times. The software compares the images and shows areas of lesion progression or regression visually as dark or light, respectively.⁴⁷ Currently, no commercial systems are available, but this technique offers great potential for monitoring lesions.

Conclusion

Caries diagnosis is important as it can allow prevention and treatment, potentially resulting in disease reduction and treatment, with beneficial outcomes for children, families, practitioners and the health service. Bitewing radiographs reveal superior diagnostic yield of caries to clinical assessment alone, and are recommended in national and international guidelines for children. Bitewing radiography is best practice for diagnosis of proximal caries in children, and therefore must be carried out in primary care to provide the best standard of care.