Caries Diagnosis Using Infrared Imaging

Despite advances in the biocompatibility and esthetics of modern restorative materials, direct composites do not always bond well to the surrounding tooth structure. In these cases, microleakage of fluids and bacterial acids can lead to secondary caries around previously restored lesions. According to Nai-Yuan N. Chang, DDS, PhD, from the Fried Group in the Department of Preventive and Restorative Dental Sciences at the University of California, San Francisco School of Dentistry, “Dentists now spend more time replacing failed restorations than placing new ones due to the maladaptation of bonding materials to tooth structure.”

Directed by Daniel Fried, PhD, the Fried Group develops and evaluates new techniques for caries diagnosis. In a study published in the Journal of Biomedical Optics, a team that included Chang, Fried and colleagues investigated emerging imaging modalities as a means of detecting active lesions. “Traditional methods relying on tactile sensation via a dental explorer and visual inspection based on texture and color are unreliable and highly subjective,” Chang reports. “Clinically, there are no established dental imaging technologies that can provide diagnostic information with high specificity and sensitivity when assessing lesion activity.”

To address this issue, the researchers examined whether shortwave infrared (SWIR) and thermal imaging could be combined with air drying to accurately diagnose secondary lesion activity. The idea underlying both methods is that active lesions are more porous than healthy tooth structure, and these pores hold water. In the SWIR-based approach, clinicians can detect active lesions by observing changes in SWIR reflectivity as the tooth dries out. In comparison, thermal imaging relies on temperature changes caused by water evaporating from the pores during air drying.

In their work, the team acquired 63 human tooth samples and analyzed 109 suspected secondary lesions with both SWIR and thermal imaging. In addition, the researchers used optical coherence tomography (OCT) to assess the samples, which is an advanced technique that emits near-infrared light to create high resolution three-dimensional images. These findings were compared with the SWIR and thermal imaging results to see if these simpler modalities could reliably detect active lesions.

Reporting in the paper, “Assessment of the Activity of Secondary Caries Lesions With Short-Wavelength Infrared, Thermal, and Optical Coherence Tomographic Imaging,” the authors note that SWIR proved superior to thermal imaging overall. In addition, they found that SWIR permeability measurements closely correlated with the thickness of the transparent surface layer (TSL) of lesions, as measured with OCT. These results indicate the highly mineralized TSL is thickest when a lesion has been fully arrested — at which point no further intervention is needed. Based on the OCT findings, a TSL thicker than 70 µm is a potential indicator a lesion is no longer active.

Noting this study could help pave the way to new approaches in caries management, Chang characterizes their research as a “developmental milestone” in the quest for simpler and more reliable diagnostic technologies.