Validation of swept-source optical coherence tomography (SS-OCT) for the diagnosis of occlusal caries
Introduction
Dental caries is an infectious microbial disease of the teeth that results in localized dissolution and destruction of dental calcified tissues. Despite the dramatic decline of caries incidence, the disease is far from eradicated, particularly in children and young adults. The occlusal fissures of the first permanent molars are generally the first sites in the permanent dentition to develop caries.1
Traditional diagnostic methods, such as visual inspection, appear to have low sensitivity in diagnosing occlusal caries.2, 3 The clinician relies on visual observation of discoloration, and clinical judgment based upon experience and tactile sense by probing with an explorer. While the probing with a sharp explorer can increase the specificity for detection of occlusal caries,4 it may also cause damage to newly erupted teeth or create a cavity at the site of a superficial carious lesion,5 and therefore its use has been questioned by several authors.6, 7 Since the cavitation due to the probing has been recognized histologically,5 the necessity of early treatment after the iatrogenic cavitation due to the probing has also been pointed out from the distant past.5
Occlusal caries has usually been diagnosed along with the WHO standard, in which lesions are recorded at the cavitation level.8 In conformity with WHO standard, caries requires operative treatment when exposed dentine is visible or undermined enamel exists where softened margins can be felt. However, several studies showed that such lesions can extend well into dentine, and then rapidly progress.9, 10, 11
On the other hand, there is evidence that the regular use of fluorides reinforces and remineralizes the superficial enamel, which may obscure underlying lesions in dentine; the so-called “hidden caries”.10 Since detection of caries lesions on occlusal surfaces is diagnostically challenging, dentists need an imaging technology that can non-invasively and reliably quantify the caries progression. However, there is no method that can independently detect the caries process with high sensitivity and specificity. Dental radiographs do not have the sensitivity for early lesions, particularly occlusal lesions, and by the time the lesions are radiolucent they have often progressed well into dentine.11
Optical coherence tomography (OCT) is an emerging diagnostic method for creating cross-sectional imaging of internal biological structures.12, 13 OCT is a promising imaging technology, because it performs “optical biopsy”; the real time and in situ visualization of tissue microstructure, without the need to remove and process specimens nor radiation dose.12, 13 Earlier OCT systems were based on time domain detection where echo time delays of light were detected by measuring the interference signal as a function of time, while scanning the optical path length of the reference arm.13 Conventional OCT combines light from a low coherence light source with a Michelson interferometer to produce cross-sectional images of tissue structures generated as a result of interaction between a partially coherent beam of optical radiation and tissue components.14 OCT images differentiate the tissue optical properties, which includes the effects of both optical absorption and scattering.13
Swept-source optical coherence tomography (SS-OCT) is a variant of OCT,14, 15 of which a light source is a tunable laser that sweeps the wavelength over a certain range. SS-OCT time-encodes the wavenumber by rapidly turning the narrowband and source through a broad optical bandwidth. Fringe response versus frequency is detected with a balanced detector and the signal is Fourier transformed to obtain a depth-reflectivity profile from which a cross-sectional image is reconstructed.16
In this article, we examined SS-OCT as a diagnostic methodology for occlusal caries in the laboratory. The results of SS-OCT were compared with those of visual inspection and direct observation of sectioned samples under confocal laser scanning microscope.
Section snippets
Materials and methods
Sixty-two extracted human molar teeth with stained occlusal fissures and/or small open caries lesions with diameter within 1 mm were used in this study (36 non-cavitated teeth and 26 cavitated teeth). The teeth, of which usage was approved by the Institutional Review Board of Tokyo Medical and Dental University, were stored at 4 °C in saline containing a few crystals of thymol. The examiners who took part in this study were three experienced dentists (YS1, JT and YS2), with over 20 years
Results
The results and values of sensitivity, specificity and area under ROC curve (Az) for detection of caries lesion (score 0 versus score 1–3) and dentine caries (score 0–1 versus score 3) are given in Table 1, Table 2. The sensitivity for visual inspection was 0.80 for caries lesion and 0.36 for dentine caries. The sensitivity values for the SS-OCT were over those of the visual inspection; 0.98 for caries lesion and 0.60 for dentine caries. Specificity for visual inspection was 0.69 for caries
Discussion
In this study, CLSM observation was carried out as a diagnostic validation to examine the presence and extent of caries lesions on the investigation sites. Transversal microradiography (TMR) is regarded as the gold standard to determine mineral loss and depth of enamel caries lesions but requires preparation of thin sections.19, 20, 21 CLSM direct observation is advantageous over the conventional histological examination, since specimens examined by CLSM do not require any special preparation
Conclusions
Within the limitation of this laboratory study, it is suggested that the SS-OCT is a promising diagnostic modality for the detection of early occlusal caries. Since imaging depth range of the SS-OCT was approximately 2 mm in this study, further technological development, as well as the association with other diagnostic methodology, seems necessary for the examination of advanced lesion depth.
Acknowledgements
This work was in part supported by the Grant-in-Aid for Scientific Research (No. 21592415) from the Japan Society for the Promotion of Science, in part by the Research Grant for Longevity Sciences (21A-8) from Ministry of Health, Labor and Welfare, and in part from the Japanese Ministry of Education, Global Center of Excellence (GCOE) Program, International Research Center for Molecular Science in Tooth and Bone Diseases.
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