[Establishment and characterization of an artificial caries-affected dentin model with demineralization and discoloration].

Objective: To investigate the establishment, structural, and bonding interface characteristics of an artificial caries-affected dentin model with demineralization and discoloration as a basis of research on caries-affected dentin bonding repair. Methods: One hundred intact molars without caries were collected (acquired from Department of Oral and Maxillofacial Surgery, School & Hospital of Stomatology, Wuhan University from March to May 2023) and prepared as 5 mm thick dentin specimens. Then, they were screened and divided into 3 parts. One part of dentin specimens was subjected to bacterial biofilms to prepare artificial carious dentin (ACD). They were further ground by 600-grit SiC paper for 0, 12, 24, 36, and 48 s, respectively to obtain 5 groups with different layers of ACD: ACD-0, 12, 24, 36, and 48 s. Sound dentin was used as the control group. To determine the preparation parameter for artificial caries-affected dentin (ACAD), the first part of specimens was used for bacterial visualization observation (n=3) and demineralization analysis experiments (micro-CT, Raman spectroscopy, and surface micro-hardness analyses, n=3). Another part of dentin specimens was allocated to 3 groups: control group (sound dentin), artificial caries-infected dentin group (ACD-0 s) and ACAD group (prepared according to the parameter determined by the experiments above). They were used for color tests (n=10), Raman spectroscopy analysis (n=6) and scanning electron microscope (SEM) observation (n=1), thus comparing color, chemical composition and structure, and micro-morphology of 3 groups. The rest of dentin specimens were divided into 2 groups: sound dentin and ACAD (n=6), which were bonded to composite resin with Single Bond Universal in a self-etch mode. Then, the bonding interface was measured using an electron probe micro-analyzer (EPMA). Results: The depth of bacterial invasion for ACD-0 s was (142.4±25.8) μm. And obvious bacteria were observed in the dentin tubules for the ACD-12 s group. For micro-CT, the demineralization depth was (283.9±25.6) μm for ACD-0 s and (139.2±27.9) μm for ACD-36 s. The grey values in some regions of the dentin surface for ACD-48 s resembled those of sound dentin. For Raman spectroscopy, the peak ratio of phosphate to amide Ⅰ was significantly lower for ACD-24 s [4.2 (3.2,6.7)] than ACD-36 s [6.7 (6.0,7.7)] (P<0.05). Additionally, there was no significant difference in surface micro-hardness between ACD-24 s [8.3 (7.0,10.2) HV] and ACD-36 s [10.2 (9.1,11.4) HV] (P>0.05). The preparation parameter of ACAD was determined to be grinding for 36 s based on the experimental results above. The brightness (L^* value) and the yellow-blue chromaticity (b^* value) of ACAD (76.69±2.54, 33.15±1.89) were significantly lower than those of the control group (85.23±1.68, 35.87±1.55) (P<0.05). The red-green chromaticity (a^* value) of ACAD (5.38±1.20) was significantly higher than that of the control group (0.71±0.86) (P<0.05). Moreover, the collagen structure parameter in Raman spectroscopy (the peak ratio of amide Ⅲ to CH₂) of ACAD (1.089 7±0.038 5) was significantly higher than that of the control group (0.985 2±0.020 1) (P<0.05). As shown in EPMA, the hybrid layer of ACAD [(4.72±1.03) μm] was significantly thicker than that of sound dentin [(3.02±0.66) μm] (F=21.09, P<0.001) in a self-etch mode. Conclusions: ACAD is established through bacterial biofilm challenges followed by grinding for 36 s. It is partly demineralized and discolored with collagen structure changes, making it suitable for research on caries-affected dentin bonding.