Early caries detection is an ever-changing field where better techniques and more effective methods which offer greater sensitivity and specificity are constantly being developed. There have been several novel methods of caries detection (with the potential to be applied in diagnosis) which have been proposed in recent years, most of which are still in the testing phase in laboratories and are generally many years away from routine clinical application. In this section, a few of the latest technologies will be discussed, offering us a glimpse of what the future of caries detection and diagnosis might look like. These include Infrared Thermography, Ultrasound technology, Optical Coherence Tomography (OCT), and Polarised Raman Spectroscopy (PRS).
This method makes use of the physical property that thermal radiation energy travels in the form of waves, and changes in thermal energy when fluid evaporates from a lesion may be measured. Sound tooth structure emits a different amount of thermal energy compared to carious tooth structure, and this amount of thermal energy is detected by thermal sensors.
A technique described by Kaneko et al.(1999) uses indium/antimony thermal sensors, which can detect changes in temperature in the order of 0.025°C. With a constant air flow over the tooth surface, the change in temperature of the lesion is compared with that of the surrounding sound tooth structure. The sensor is placed 20 cm away from the source, and the time taken to capture the data from a lesion is roughly 2 minutes. In a study conducted by Matsuyama et al.(1998), a reasonable correlation was found between temperature changes and mineral loss and lesion depth.
There are limitations to this method, however. Variations to the temperature of the mouth as a result of respiration or fluid evaporation from other surfaces in the mouth may lead to inaccurate measurements. The 20 cm distance between the source and the sensor makes it unsuitable for posterior teeth. While remaining as a potential concept of caries/lesion detection, it will probably not translate into a viable clinical tool of detection and diagnosis.
Sound waves are longitudinal waves capable of passing through solids, liquids, or gases, and the boundaries between them. Ultrasound waves are basically sound waves with a frequency of more than 20,000 Hz, and possess the usual properties of waves, such as reflection, scattering, refraction, and absorption. When ultrasound passes through different mediums, its physical properties (eg. velocity, acoustic impedance) changes. This forms the basic principle of using ultrasound as a means of detecting variations in teeth.
A group of researchers (Bab et al. ) proposed using ultrasound waves which travel along the surface of the tooth along the interface between enamel and air to detect surface discontinuity from cavitated proximal lesions. Using a flexible probe tip which fit into wedge shaped interproximal contours, the method was able to show stronger ultrasound reflections from cavitated lesions compared to non-cavitated lesions. A later study by the same group of researchers in 1998 involving 253 caries sites showed that their Ultrasonic Caries Detector (UCD) could distinguish beteween dentin interproximal caries from an intact site, using bitewing radiography as a standard.
While findings have been limited to only a few studies, Ultrasound shows great potential as a caries detection and diagnosis tool. It is quick and reliable for detecting caries near the enamel. In the study conducted in 1998, the UCD showed better diagnostic results compared to bitewing radiography. Moreover, the use of sound waves as means of measurement is harmless, compared to the possibility of some degree of radiation exposure from bitewing radiography. Certainly, with more research and testing, this technique could be a very feasible diagnostic tool of the future.
Optical Coherence Tomography (OCT)
Optical Coherence Tomography (OCT) is a form of imaging technology which utilises light waves to provide high resolution (10-30 µm) morphological depth images. This method is simliar to ultrasound, except that it uses light waves instead of sound waves to obtain the image, and is more suited for imaging near-surface structures, compared to deep tissues structures commonly imaged using ultrasound.
This method employs the physical principle of coherent back scattered light. A brief description of the OCT method would involve using a Michelson interferometer to split a laser light beam into two paths which bounce off two mirrors and recombine to produce an interference pattern. The intereference pattern generates a depth profile at a single point, known as an A-scan. As the laser moves across the surface of the tooth, adjacent A-scans are put together to create a two-dimensional image, known as a B-scan. The back-scatter of OCT light is influenced by the refractive index of the medium which it encounters, hence different images are produced for an A-scan of different tissue types or structures (e.g. enamel vs dentin, healthy vs carious regions). An example of a B-scan of a tooth containing sound and carious regions is shown below.
Figure 1: OCT B-Scan of a tooth sample presenting images of sound enamel (B) and incipient lesion (C)
(L.P Choo-Smith et al., 2008)
OCT is a technique that has been employed in medical imaging for many years, especially in opthalmologic circles for diagnostic imaging of retina tissue. It is only in recent times where this technique has been modified for use in studies on teeth. An example of OCT imaging used on a permanent maxillary second premolar is shown in Figure 1 above, and the actual tooth is shown in the left hand side. Clear morphological features may be identified on the OCT image, and in the colour scheme used, blues and greens represent low-intensity light back-scattering, while yellows and reds represent high-intensity back-scattering. For the OCT scan of sound enamel (B), high-intensity signals (yellow and red) are only present at the air-enamel or air-root surface, and the signal decreases rapidly with depth into the tooth. THe OCT scan of the carious lesion (C), high intensity signals are present deeper into the tooth, suggesting the presence of a carious lesion. This is because demineralisation increases tooth porosity and opens up more space within the tooth matrix, allowing scattered light to penetrate deeper into the enamel.
Figure 2: A light photomicrograph showing the histologic section of the carious lesion on the same tooth as figure 1
(L.P Choo-Smith et al., 2008)
The accuracy of the sample in figure 1 may be compared with a light photomicrograph of a histological section of the same tooth (figure 2, above), and the depth of the carious lesion measured. A comparison of the depths on both figures shows that the lesion is approximately 507 µm using OCT imaging, and approximately 427 µm on the histological section. The measurement taken from the OCT image is quite similar to the histological section (the most accurate standard), highlighting the potential of the non-destructive OCT imaging technique as a method of detecting and diagnosing caries.
One of the drawbacks of this method, however, is that regions of hypocalcification in a tooth may occasionally show increased light back-scattering, and this may be misread as a sign of early caries, resulting in a false positive diagnosis. In order to rule out such a diagnosis, and to increase the specificity of this method, OCT can be used together with Polarised Raman Spectroscopy (PRS), which will be discussed next.
Polarised Raman Spectroscopy (PRS)
Like OCT, Polarised Raman Spectroscopy (PRS) also makes use of light scattering. It uses a physical principle known as the Raman effect, which describes the phenomenon when scattered light impinges upon a molecule and interacts with the electron cloud and its bonds. A photon is used to excite the molecule from the ground state to the virtual energy state. When the molecule emits a photon and reverts to its ground state, it returns to a different vibrational state. The difference in energy state may be detected and quantified in a spectrum. (The detailed description of the actual mechanism is very technical, and is beyond the scope of this project. In a nutshell, PRS uses a polarised laser excitation and a polarisation anaylser to detect the difference in energy before and after excitation, and presents it on a spectrum. The spectrum may be studied and analysed to provide information on the molecular content of the sample.
PRS is applicable to analysis of the tooth because the properties of scattered light would differ depending on the type of medium (sound or carious) being explored. A study done using the PRS method on teeth showed that PRS is able to detect changes in the biochemical signature of a tooth based on the state of demineralisation and remineralisation. This information may be made useful when used with a quantitative parameter which will interpret the intensity of the peaks of the specrum obtained.
When used hand in hand with OCT, PRS proves to be an excellent and accurate diagnostic tool. While OCT imaging is able to show the presence, location, depth, and characteristics of an incipient lesion, PRS can be used to confirm that the suspicious carious sites are indeed sites of demineralisation. This technique, if made applicable in a clinical setting, would certainly be an excellent diagnostic tool in the diagnosis of early caries.
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Electrical Conductance | Laser Fluorescence And QLF | Towards The Future: Other New And Emerging Technologies | A Summary
1) Hall, A., & Girkin, J. M. (2004). A review of potential new diagnostic modalities for caries lesions. J Dent Res, 83 Spec No C, C89-94.
2) Choo-Smith, L. P., Dong, C. C., Cleghorn, B., & Hewko, M. (2008). Shedding new light on early caries detection. J Can Dent Assoc, 74(10), 913-918.