Dental surveillance surveys and large epidemiological studies have traditionally depended on the use of visual dental examination techniques as the caries detection method. This is because for dental public health purposes, visual determination is simple, requires low technology and is easy to administer. This method of caries detection is however not appropriate in comparative studies where examiners collecting caries information need to be “blind” to various attributes of the different populations; for example the residential status of participants in the evaluation of water fluoridation schemes or to the allocation of participants in randomized control trials. Other caries detection methods such as radiographs, or more innovative techniques/technologies such as Quantitative Light-induced Fluorescence (QLF), fibre-optic trans-illumination (FOTI) and electronic caries monitoring (ECM) [8] will not be suitable to use in such studies for a variety of reasons.

The use of radiographs to ensure examiner “blinding” is an unviable proposition as it is fraught with ethical dilemmas in terms of justifiable risks from exposure to ionizing radiation and problems with validity in detecting occlusal caries [9]. Although dental caries and enamel fluorosis present as different lesions, it may be difficult to use the QLF techniques that are commercially available to make a distinction [10]. QLF is more suited to laboratory based research and clinical work involving precise measurement of changes in mineralization of tooth-tissue. ECM is a very sensitive caries detection method which can be affected by factors such as the presence of water, ambient light, and tooth temperature [11].

As digital imaging fibre-optic trans-illumination (DiFOTI) produces images that can be stored, it could be considered for ensuring examiner “blinding”. It is however very cumbersome to handle and time consuming to use. It also requires considerable amount of training to achieve the level of competence needed for it to be used as a caries detection method [8]. As such, none of the so called “novel” methods for caries detection appear appropriate for use in epidemiological studies where blinding is required.

However, a simple and economical method of “blinding” may be for examiners to inspect photographs of participants’ teeth rather than examine the subjects visually.

Photographic images have been used in dentistry in a variety of ways [12-14] and intra- and extra- oral cameras have evolved rapidly over recent years with a commensurate decrease in cost and complexity of use. With the advancement in technology, there are various intra-oral cameras now in use in clinical dental practice. There are however very few studies in the literature that have investigated the use of intra-oral images and caries diagnosis. In a study by Elfrink et al [15], intra-oral photographs were used to score caries and hypo-mineralization on primary molars in a clinical setting and the results suggest that intra-oral photographs may be used in clinical practice and large epidemiological studies with some degree of confidence. The main method of caries determination in the UK National Health Service epidemiological surveys is via a visual examination method developed and described by the British Association for the study of Community Dentistry (BASCD) [16-18]. Before intra-oral photographs can be recommended for use in epidemiological studies their performance must be assessed against the established BASCD visual examination method and the reference standard for caries diagnosis of histological section.

Fifty permanent extracted teeth, 32 molars and 18 premolars, varying from sound to grossly carious teeth were used for this study. The teeth were examined visually for caries (without probing) using the method developed and described by BASCD [16]. The BASCD codes used for scoring, classified teeth as being sound (caries-free), having arrested caries, having caries into dentine or having caries extending into the pulp (Figure ​(Figure1).1). The teeth were also photographed using an intra-oral camera and the obtained images assessed for caries using the same BASCD codes as were used for the visual examination. The teeth were then sectioned for a histological assessment to detect the presence of dental caries.

The level of agreement between the first and second photographic assessment for the individual examiners (to compare standardized with varying physical conditions for viewing photographs) ranged from moderate agreement to almost perfect agreement with weighted kappa from 0.59 to 0.92 with a median = 0.74. [20]

The measure of inter-examiner reliability for the visual examinations was a multi-rater kappa of 0.66. This showed there was substantial agreement within the group for the visual scores. The measure of inter-examiner reliability within the group for the photographic assessment was a multi-rater kappa of 0.60. Again this showed a substantial agreement within the group.

McNemar test (p<0.05) calculated to compare the performance of the visual and photographic methods showed no significant difference between the methods.

Sensitivity and specificity as a measure of the diagnostic performance of the visual examination and the photographic assessment methods as compared to the reference standard of histological section assessments are presented in Table ​Table33.

The difficulty of convening examiners from wide geographical area made a longer washout period for the visual examinations problematic in this study period. It had to be expected that a decision (one of 50) could not be recalled by the examiners after the washout time allowed in this study. In this study caries was only diagnosed when it was determined by the examiners to have reached dentine. This is because the established visual examination method developed by BASCD that was used for the comparisons determines caries at the caries into dentine level as part of its protocol. Although early caries is not accounted for, this method has provided the main epidemiological data on the state of the dental health of the child population in the UK for almost 30 years [21-23]. Any other caries detection method to be used in dental public health studies in the UK should at least be comparable to the BASCD examination in ease of application, reliability and validity.

The detection of caries was restricted to only the occlusal surfaces of the teeth. The purpose of this approach was to minimize the number of photographs per tooth that had to be assessed by the examiners. This may be seen as limiting the external validity of the findings of this study. However majority of carious lesions occur in pits and fissures on occlusal surfaces [24] and moreover as lesions on occlusal surfaces are the most difficult to reliably diagnose [25] the findings of this study will contribute to the available literature. Future research could include caries detection on other tooth surfaces in vivo. Also weighted kappa rather than non-weighted kappa values were computed and used for the reliability comparisons because the caries diagnosis was not just recorded as dichotomous scores (of caries present or absent) but rather as category of scores.

The physical conditions by which photographs are viewed and how they are presented could potentially influence assessment, reducing intra examiner reliability. However we found good agreement between the standardized and non-standardized presentation of the photographs, suggesting that the mode of presentation has little impact on diagnostic decision making from photographs.

Decisions about the introduction of caries prevention strategies such as water fluoridation and the evaluation of such interventions will depend on detecting the presence of the disease at the appropriate level. The requirements of a caries detection tool needed for such a task will not be the same as that required in, for example, general practice to aid management of the early stages of the disease. Examiners collecting data should however do so at a consistent level; the intra examiner reliability for both the visual and photographic methods was high.

Histology/microscopy is used as the reference standard in many comparison and validity studies [26-29]. Some researchers believe that there is tissue loss during sectioning of teeth and thus histology/microscopy cannot be said to be the ultimate reference standard. This may be important in studies concerning very early enamel lesions but not detecting caries into dentine used for dental public health purposes. The two methods in this study were compared to histology as the reference standard to determine their validity. The photographic assessments method had a higher sensitivity (median value 81.3%) for caries detection than the visual examination (median value 65.6%). The two methods however had the same specificity (82.4%) which was high. The photographic assessment method in this study has caries detection capability that is at least comparable to that of the BASCD visual examination method.

As well as the possible advantages concerning blinding in comparative studies, another benefit that the photographic method could provide for dental public health epidemiological research is the archiving of images. This is a concept which is already widely used in “Store and forward” telemedicine [30,31]. The use of intra-oral photographs would allow remote dental examination and screening as piloted by a previous study [32]. In theory, trained dental nurses or other dental care professionals could take the intra-oral photographs for a single examiner to assess them all. The use of this approach in epidemiological studies will eliminate potential errors that could be introduced by using multiple assessors if potential logistical issues of using the system in the field can be mitigated.

Also archived photographic records would allow true comparative public health studies on the same cohort of population or different cohorts based on archived images in future research. This could include future water fluoridation evaluation studies. These could be conducted as prospective cohort studies where examiners assess the photographs of participants in both the test and control groups “blind” to participants’ exposure to water fluoridation thereby reducing the risk of bias.