AN FRANCISCO, US: Accurate assessment of carious lesion activity, depth and severity is important for clinicians to decide whether intervention is necessary. In a recent study, a research group from the University of California San Francisco (UCSF) evaluated whether short-wavelength infrared (SWIR) and thermal imaging could be used to accurately diagnose the activity of secondary carious lesions. While previous studies using these techniques showed promising results for assessing lesion activity, this was the first study to investigate the use of thermal imaging for secondary carious lesions.

In recent years, the frequent use of shade-matched and radiopaque dental restorative materials to replace carious tooth structure after cavity preparation has led to a significant increase in secondary carious lesions. Poor adaptation of bonding materials to the tooth structure allows fluids and bacterial acid to escape, resulting in demineralisation of the tooth structure under the restoration and the formation of secondary caries.

“Dentists now spend more time replacing failed restorations than placing new ones due to the maladaptation of bonding materials to tooth structure,” commented study author Dr Nai-Yuan N. Chang from the Department of Preventive and Restorative Dental Sciences at UCSF in a press release.

He explained: “The traditional methods relying on tactile sensation via a dental explorer and visual inspection based on texture and colour are highly subjective and unreliable. However, there is currently no established dental imaging technology that can provide diagnostic information with high specificity and sensitivity when assessing dental decay activity.”

Both imaging techniques were used to determine the rate of fluid loss from teeth, effected via air-drying. This helps determine activity of the lesions because the outer layers of active lesions are more permeable compared with arrested caries and healthy teeth. For SWIR, active lesions can be detected by observing changes in the SWIR reflectivity as the tooth dries out. Thermal imaging relies on temperature changes reflecting the amount of water diffusing from the surface of active lesions during evaporation compared with arrested caries and healthy teeth.


When combined with air-drying, short-wavelength infrared imaging can be used to detect active caries. This is possible because of the porous outer layers of active cavities, producing different light scattering patterns to those of sound tooth structure and arrested caries as the tooth dries. (Image: Chang et al.)

In assessing the reliability and accuracy of SWIR and thermal imaging in detecting active lesions, the results of these methods were compared with measurements of lesion structure, depth and severity by optical coherence tomography (OCT), a technique that uses near-infrared light to produce high-resolution 3D images, and of micro-CT.

The study results showed that SWIR imaging outperformed thermal imaging overall in assessing secondary carious lesions on coronal surfaces. Thermal imaging performed well in identifying crevices, that is, areas of microleakage, between composite material and tooth structure, but produced incorrect results at times for complex occlusal anatomy and crevices of interfaces that tend to retain water, since this interfered with thermal dehydration. SWIR imaging was less prone to such interference, as it can distinguish composite materials, sound tooth structure and lesions with high contrast. Despite the complex geometry and topography in a few lesions, SWIR imaging assessed permeability with almost the same accuracy as OCT.

The authors concluded that the results “further demonstrate the potential of SWIR reflectivity and OCT imaging methods for the clinical monitoring of the activity of secondary caries lesions”. “Our work provides further developmental milestones towards meeting the need for better diagnostic and easily operable clinical devices,” emphasised Dr Chang.

The study, titled “Assessment of the activity of secondary caries lesions with short-wavelength infrared, thermal, and optical coherence tomographic imaging”, was published in the September 2023 issue of the Journal of Biomedical Optics.