To investigate the effects of grain size and immersion dyeing time on the dye penetration depth and color parameters of zirconia, and analyze the dyeing behavior in relation to the material's pore characteristics to provide experimental evidence for developing personalized immersion staining protocols for clinical applications targeting zirconia with different microstructures.

A total of 135 pre-sintered zirconia specimens with three different grain sizes were fabricated using computer aided design/manufacturing technology and divided into three groups (S1, S2, S3; n = 45). Randomly select 6 specimens from each group, among which 3 zirconia pre sintered specimens are soaked in 10 mL A2 staining solution for 60 s, dried and sintered, and the other 3 are not treated. Observe the microstructure of the outer surface and bonding surface of the specimen under SEM, and use Image J software to count and measure the grain size. From each group, three randomly selected specimens were subjected to mercury intrusion porosimetry to analyze porosity and pore size distribution. For the remaining specimens, the bottom surface (10 × 10 mm) was designated as the staining surface and immersed in 10 mL of A2 staining liquid. The remaining specimens were stained at six time gradients (10, 20, 30, 60, 90, 120 s). After staining, half of the specimens were sectioned perpendicular to the stained surface and fully sintered to measure the staining penetration depth. The other half were directly fully sintered. A spectrophotometer was used to measure the color parameters (L*, a*, b*) of the stained surface, and the color difference (ΔE₀₀) relative to a standard A2 shade tab was calculated.

The average grain sizes (G) for the three groups were G_S1=(221.64±62.16) nm, G_S2=(197.80±46.72) nm, and G_S3=(150.21±42.11) nm; the average porosities (P) were P_S1=37.82%, P_S2=46.02%, and P_S3=47.36%. Both grain size and staining time significantly affected the staining outcome of zirconia (P < 0.001), with a significant interaction effect (P < 0.001). Staining penetration depth increased with prolonged staining time and decreased grain size (P < 0.001). All fully sintered specimens exhibited a perceptible color difference (ΔE₀₀>0.8) compared with the standard A2 shade tab (VITA, USA). Specifically, specimens from group S1 stained for 20 s, group S2 stained for 60 s, and group S3 stained for 10 s exhibited color differences below the clinically acceptable threshold (ΔE₀₀<1.8), meeting clinical thickness requirements. Specimens stained for other time intervals exceeded the clinically acceptable color difference threshold (ΔE₀₀>1.8).

The porosity of pre-sintered zirconia with different grain sizes varied. Higher porosity correlated with deeper staining penetration. The staining time required to achieve the standard A2 shade differed among zirconia materials with different grain sizes. In clinical applications, the immersion dyeing protocol for zirconia should be optimized based on the grain size and pore characteristics of zirconia with different grain sizes, so as to avoid color deviations caused by improper dyeing time.

To investigate the changes of dentin surface and the effects of different surface treatments on the rebonding effect following non-destructive restoration removal by an Er:YAG laser and to provide reference for oral clinical operation.

This study was approved by the ethics review committee of the unit. Using computer-aided design and computer-aided manufacturing (CAD/CAM) technology, 102 zirconia specimens (4 mm × 4 mm × 1.5 mm) were fabricated. In total, 110 impacted third molar teeth were extracted, and 102 dentine blocks (4 mm × 4 mm × 2 mm) were prepared. The zirconia specimen and dentin blocks were bonded with resin cement before removal with an Er: YAG laser. Three disassembled dentin blocks were randomly selected, and the components of dentin surface elements were analyzed by energy dispersive X-ray spectroscopy (EDX). The removed dentin blocks were randomly divided into three groups (n = 33) based on the different surface treatments: control group (no treatment), sandblasting group (50 μm, Al₂O₃ sandblasting), and laser irradiation group (Er: YAG laser irradiation, parameters were set to 10 Hz, 60 mJ, 0.6 W). Three dentin blocks were randomly selected in each group for scanning electron microscopy (SEM) observation, and the residual resin on dentin surface of remaining 30 dentin blocks in each group were observed under an optical microscope at 20 times magnification. Scores were obtained using the adhesive remnant index (ARI) method. Three groups of dentin blocks (n = 30) that underwent different surface treatments were rebonded with resin cement according to standard procedures and then divided into two subgroups for aging (n = 15). One subgroup was subjected to a 37 ℃ water bath for 24 h, and the other subgroup was subjected to 5000 thermal cycles after a 37 ℃ water bath for 24 h, and the microshear bonding strength of each group was measured. The microshear bonding strength of each group was measured, and fracture modes were analyzed. The differences of dentine surface ARI between the three groups, as well as the inter-group differences in fracture mode, and bonding strength, and the intra-group differences before and after aging were compared between the three groups.

When zirconia was removed by Er: YAG laser, there was no obvious damage on the dentin surface, but C and Si elements in dentin increased significantly. After different surface treatments, the ARI scores of the sandblasting and laser irradiation groups were lower than those of the control group (P＜0.05), while ARI was not significantly different between the sandblasting and laser irradiation groups (P＞0.05). The dentin surface morphology was also different. There was a large amount of residual resin on the dentin surface of the control group. In the sandblasting group, the residual resin was lower, the dentin surface was rough, and the dentin tubules were visible. A large amount of residual resin was observed on the dentin surface of the laser irradiation group. After 24 h of water bath at 37 ℃, the bonding strengths of the control group, sandblasting group, and laser irradiation group were (6.13 ± 2.40) MPa, (9.39 ± 2.00) MPa, and (5.85 ± 1.44) MPa, respectively, and the bonding strength of the sandblasting group was significantly higher than that of the other two groups (P＜0.05). After being subjected to 24 h of water bath at 37 ℃ and 5000 thermal cycles, the bonding strengths of the control group, sandblasting group, and laser irradiation group were (5.39 ± 0.83) MPa, (8.45 ± 1.20) MPa and (4.84 ± 1.43) MPa, respectively. The bonding strength of the sandblasting group was significantly higher than that of the other two groups (P＜0.05). There was no significant difference between the control group, sandblasting group, and laser irradiation group before and after 5000 thermal cycles following 24 h of water bath at 37 ℃ (P＞0.05). In the control group, sandblasting group, and laser irradiation group, cohesive fracture was not observed. The fracture mode was mainly adhesive fracture. Before and after 5000 thermal cycles, the frequency of mixed fracture in the sandblasting group was significantly higher than that in the other two groups (P＜0.05).

Er: YAG laser removal of zirconia does not damage dentin, but a large amount of resin remains on the dentin surface after removal. The sandblasting process can effectively remove these residual resins, thereby improving the dentine rebonding effect.