UCSF

Dental caries is one of the most prevalent chronic diseases affecting populations worldwide. With the prevalence of tooth-matched restorative options, the occurrences of recurrent and secondary caries have dramatically increased due to leakage in the interfaces between restorative materials and tooth structure, allowing fluid and bacterial acid infiltration. It is not sufficient to simply detect carious lesions; methods are needed to assess the activity of the lesion and determine if chemical or surgical intervention is necessary. Previous studies have demonstrated the potential of non-ionizing optical imaging technologies for the early detection and diagnosis of lesion severity and depth. The purpose of my project is to quantify the structural differences between active carious and arrested lesions using optical imaging methods. The goal is to provide imaging methods to better assess lesion activity to avoid unnecessary surgical intervention and to allow safe routine monitoring of dental caries without radiation exposure. All carious lesions have a surface layer due to the occurrence of some redeposition of mineral during lesion formation. For arrested lesions, the surface layer has a much higher mineral content due to additional mineral deposition during remineralization, which leads to decreased permeability of fluids including water and plaque generated acids. The higher mineral content can approach that of sound tissues and can produce a zone that appears more transparent to visible, near-IR and SWIR light, called a transparent surface zone. Previous in vitro and in vivo studies on enamel and root surfaces have shown that the thickness of this transparent surface zone can be measured nondestructively using optical coherence tomography (OCT). Other studies also have demonstrated that short-wave infrared (SWIR) reflectance and thermal imaging coupled with dehydration can be used to measure changes in the permeability of lesions in enamel and dentin. However, it has not been established how thick the surface zone should be to inhibit the penetration of these fluids. In these studies, SWIR reflectance and thermal imaging during dehydration with forced air were used to measure changes in the permeability of lesion structure. SWIR imaging at 1400-2300 nm wavelengths coincides with higher water absorption and exploits the increased contrast caused by loss of water from porous lesion areas. Thermal imaging at 6-10 μm wavelengths is sensitive to changes in temperature on tooth surfaces produced by water evaporation from porous lesion areas during dehydration. SWIR reflectance and thermal imaging were used to image simulated enamel and dentin lesions, primary coronal lesions on extracted teeth, and secondary lesions on extracted teeth. The lesions were also assessed by optical coherence tomography (OCT) and correlated with dehydration rates to determine lesion activity. Micro-computed tomography (MicroCT) was used to further confirm lesion severity and structure. The results from these studies suggest that the relationship between surface layer thickness and lesion permeability is highly non-linear, but significantly negatively correlated. A small increase in the surface layer thickness may lead to a significant decrease in permeability. Increasing transparent surface layer thickness led to decreased permeability of lesions, potentially indicating an arrest in activity at thicknesses exceeding 70 μm. Permeability measurements performed best on smooth surface lesions. Based on these results, custom handheld probes employing SWIR, thermal, and OCT imaging were fabricated for small feasibility studies for in vivo assessment of lesion activity in patients. Infrared imaging may significantly improve caries diagnosis and management. These technologies are suitable to produce portable handheld instruments to detect and monitor decay without ionizing-radiation and to increase dental outreach globally.