Quantitative Analysis On Refractive Index And Mineral Density Of Dentin Around Pulp By Using Optical Coherence Tomography: An In Vitro Study

Dentin is an important barrier against external stimulation.The external environment can easily stimulate the pulp tissue and produce severe pain when remaining dentin thickness?RDT?is too low.For a deep caries tooth that penetrate into deep dentin layer,an excavation should be taken to remove decay dentin while risking low RDT and exposure of the pulp cavity due to difficulty in controlling the RDT around pulp.In addition,accurate quantification of the RDT is also an objective indicator for doctors in judging the state of the pulp before and after treatment in the course of tooth defect repair and preparation of abutment teeth.Due to the defect of radiation,high cost,anatomical structure superposition,low resolution of the images produced by X-ray radiography,cone-beam CT and other common clinical technological approaches,the spectral-domain optical coherence tomography?SD-OCT?was used in quantitatively evaluating the RDT in this article.The key to accurately measure the RDT by using optical imaging technology is to get the precise refractive index of dentin.However,few research have been reported on the refractive index of dentin with different physiological states.Therefore,this article analyzed the refractive index of dentin with different pathological properties in different regions and introduced two main factors affecting the refractive index of dentin:?1?the main composition of dentin;?2?light-guide effect of dentinal tubules.Consider to the strong correlation between mineral density and refractive index in dentin,the potential role of OCT in quantitatively evaluating mineral density of decay dentin was also explored.After the collection of freshly isolated teeth of human by dental clinician,followed by cutting and polishing,17 orthodontic adult molars were finally selected as the source of the normal cross-cut and long-cut?those sections containing the pulp cavity?slices in this experiment.8 adult molars of natural occlusal surfaces dentin caries after excavation were selected as a source of cross-cut slices of caries marked decay area.For the method of calculating refractive index of biological tissue by OCT technology,the traditional optical-path length matching method based on extracting characteristic peak has some errors in judging the location of scattered signals at the weak bottom edge due to the strong scattering effect of dentin,which can cause the errors in calculating refractive index.From the above considerations,two-dimensional refractive index images of dentin slices were obtained by using an improved Canny edge extraction technology to obtain more precise location of bottom boundary points for cross-cut slices in this paper.ICCs and Bland-Altman analysis was applied to determine the repeatability of experiment.In order to study the change of the refractive index of normal dentin relative to the anatomical position,a special method of ROI selection was designed to get the trend of the refractive index in the depth direction and horizontal direction.Besides,the refractive index of decay dentin was also calculated to compare the refractive index of normal dentin.In addition,in order to analyze the potential correlation between local refractive index and mineral density in decay area,an X-ray computed tomography microscope was applied to scan the caries slices.The beam-harden correction and seven K₂HPO₄ solution with different concentration was used to calculate the mineral density of the caries slices at each point.Finally,the average local refractive index and mineral density of each decay area were obtained.In the repeatability test,ICCs were basically greater than 0.8 and the scattered points on the Bland-Altman plots were basically evenly distributed on the both sides of zero-line in this article,which indicated the refractive index obtained from experiments has a good consistency.In the term of trend of refractive index,for the normal cross-cut slices,the refractive index was increased firstly and then decreased from center to enamel-dentinal junction?horizontal direction?and the regional maximum value,minimum value,average value was 1.657±0.065,1.498±0.025,1.578±0.034,respectively.There was a similar trend in decay dentin whose mean value was 1.507±0.054,however,refractive index of decay dentin was significantly smaller than that of normal dentin?P<0.001?.Besides,no significant change of refractive index from pulp to enamel-dentinal junction?depth direction?was observed for the long-cut slices whose mean value was 1.551±0.025.In the term of mineral density,a good linear relationship was found between local refractive index and mineral density?y=0.132x+1.334,Pearson's r=0.78?.