Abstract: Objective: To analyze the variation of refractive index (RI) of caries-affected dentin by using OCT. Methods: Nineteen extracted third molars were selected, and four of which were caries free and other fifteen had occlusal dentin caries. The dentin slices with thickness ranging from 300 to 500μm were prepared perpendicular to long axis of the tooth crown for all teeth. Ten slices from caries free teeth were chosen as the control group. Fifteen slices were chosen as the study group. All dentin slices were scanned by micro-CT and a self-built OCT system (Dental 1.0, demo), and reconstructed to be three-dimensional digital slices. The RI of the caries-free and caries-affected dentin slices were calculated by optical path length-matching method basing on an improved canny edge detection algorithm. The mineral density (MD) of the caries-affected dentin slices was evaluated by comparing the grayscale difference between the slice and the calibration phantom using K₂HPO₄ liquids. Results: ith an increase of the distance from the reference center to enamel-dentin junction (EDJ), the RI of caries-free dentin increased firstly and then decrease, while that of caries-affected dentin is less affected. The mean value of RI of the caries-free dentin was 1.578, which was significantly higher than that of the caries-affected dentin (P<0.001). A good linear correlation was found between the RI and MD (Pearson's r=0.78) in the caries-affected dentin (P<0.001). Conclusion: Caries would lead to variation of refractive index of dentin. The mineral density was an important factor causing the decrease of dentin refractive index.

Key words: Optical coherence tomography, Dentin remaining thickness, Refractive index, Caries-affected dentin