OPTICAL BASED TECHNOLOGIES FOR DETECTION OF DENTAL CARIES

Background: A conservative, non-invasive or minimally invasive approach to clinical management of dental caries requires diagnostic techniques capable of detecting and quantifying lesions at an early stage, when progression can be arrested or reversed. Objective evidence of initiation of the disease can be detected in the form of distinct changes in the optical properties of the affected tooth structure. Caries detection methods based on changes in a specific optical property are collectively referred to as optically based methods. The present thesis evaluated the feasibility of three such technologies for quantitative or semiquantitative assessment of caries lesions. Two of the techniques are well-established: quantitative light-induced fluorescence (QLF), which is used primarily in caries research and laser-induced fluorescence (LF), a commercially available method used in clinical dental practice. The third technique, based on near-infrared transillumination (TI), is in the developmental stages. Aims: The general aims of this thesis were twofold: firstly, to evaluate the LF and QLF methods, with special reference to validity and reliability in detecting lesions; and secondly, to implement a procedure to characterise an imaging technique based on TI of dental enamel with near-infrared light (NIR). Material and Methods: In Study I (in vivo) the LF method was validated for occlusal dentinal caries detection by comparing LF readings with the actual lesion depth, determined by opening the fissures. LF readings obtained on smooth surface white spot lesions were compared with corresponding measurements by QLF, which served as a reference method. Operator agreement was also tested. In Study II (in vivo) QLF was applied to monitor a preventive intervention: caries active adolescents were instructed to brush weekly with amine fluoride gel to test whether this treatment could enhance the remineralisation of early caries lesions on smooth surfaces. In Study III (in vitro) LF readings and colour and surface texture of root caries lesions were investigated in relation to histopathological lesion depths. The influence of discolouration and surface texture on LF readings, as well as the operator agreement, was investigated. In Study IV (in vitro) a procedure to characterise a TI imaging system, using wavelengths in the NIR spectrum, was developed for detection of early caries lesions. The performance of the system with reference to high resolution and low noise of captured images was characterised and quantified by the modulation transfer function. The potential for this imaging technique to detect and indicate the relative position of a lesion on the approximal surface was also explored. Results: LF readings of occlusal dentinal caries (Study I) showed very poor correlation with clinically assessed lesion depth (r < 0.15). With respect to root caries lesions (Study III), LF readings showed extremely low correlation with histopathological lesion depth (r < 0.01). The agreement between LF readings and corresponding measurements by QLF (Study I) was satisfactory. For root caries, discolouration and surface texture denoted as hard were associated with higher LF values (Study III): the study disclosed moderate correlation between colour and histological depth, as well as between surface texture and histological depth. Operator agreements (Study I, III) were good to excellent. Monitoring by QLF disclosed no enhancement of remineralisation of white spot lesions by additional weekly brushing with amine fluoride gel (Study II). Study IV demonstrated that the TI system was able to detect features as small as 250 μm with 30 % modulation in captured images. The location of a caries lesion on the approximal surface was demonstrated in thin tooth sections. Conclusions: In their present form, neither of the two established optical methods evaluated in this thesis can be regarded as ideal for clinical application. LF readings showed very poor correlation with lesion depth, both on occlusal and root surfaces. The QLF method was appropriate for monitoring small changes in white spot lesions. When illuminated through dental enamel, the novel TI imaging system disclosed extremely small features. Reduction of modulation indicated the relative position of a simulated approximal caries lesion in thin tooth sections. These promising results indicate that the TI method warrants further investigation and development as an aid to traditional caries detection and assessment methods.