Statement of problem

Zirconia is a widely used restorative material, yet its white color does not meet esthetic requirements and coloring is needed. However, the effects of different durations of coloring have not been thoroughly investigated.

Purpose

The purpose of this in vitro study was to evaluate the effects of coloring liquid immersion of different durations on the flexural strength, Vickers hardness, and color of zirconia.

Material and methods

Fifty bar-shaped and 60 disk-shaped zirconia specimens were milled from nonshaded (Copran Zri) and preshaded (Copran Zri Precoloured Medium) disks by using a computer-aided design and computer-aided manufacturing (CAD-CAM) system. Preshaded specimens were assigned to the group PS and received no further immersion. Nonshaded specimens were divided into 4 subgroups as per immersion time in coloring liquid (Copran Color A2 Shade): control (C; no immersion), G45 (45 seconds of immersion), G90 (90 seconds of immersion), and G135 (135 seconds of immersion). Bar-shaped specimens were subjected to a 3-point flexural strength test by using a universal testing machine (Lloyd LRX), following the International Organization for Standardization (ISO) 6872:2015 specification. Color coordinates of disk-shaped specimens were measured with a spectrophotometer (VITA Easyshade Advance 4.0) over a gray background. Color differences were calculated by using both CIE76 and CIEDE2000 formulas, followed by a comparison of obtained values with perceptibility (CIE76: 3.7, CIEDE2000: 0.8) and acceptability (CIE76: 6.8, CIEDE2000: 1.8) thresholds of each formula. Subsequently, Vickers hardness measurements were conducted. The data were statistically analyzed by using the Shapiro-Wilk test, 1-way ANOVA, and the Tukey HSD test (α=.05).

Results

Immersing zirconia specimens in coloring liquid decreased the flexural strength and hardness values, as group C presented the highest values for both parameters. In addition, prolonged duration had a negative effect on mechanical properties. The flexural strength of groups PS and G45 (P=.993) and groups G90 and G135 (P=.999) was statistically similar. For Vickers hardness, group G135 presented the lowest values with the difference between groups G90 and G135 being statistically similar (P=.061). Color differences among groups varied from 2.49 to 27.33 for ΔE∗, whereas ΔE₀₀ values ranged from 0.58 to 8.26. Compared with group C, ΔE∗ values of all shaded specimens exceeded the acceptability threshold of 6.8. The color difference between each group was higher than the acceptability threshold values of both ΔE∗ and ΔE₀₀ except for groups G45 and PS.

Conclusions

Colored zirconia specimens had lower flexural strength and Vickers hardness values than nonshaded specimens, regardless of the immersion time or coloring technique.

中文翻译：

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着色液浸渍对氧化锆弯曲强度、维氏硬度和颜色的影响

问题陈述

氧化锆是一种广泛使用的修复材料，但其白色不符合美学要求，需要着色。然而，尚未彻底研究不同着色持续时间的影响。

目的

这项体外研究的目的是评估不同时间的着色液浸渍对氧化锆的弯曲强度、维氏硬度和颜色的影响。

材料与方法

通过使用计算机辅助设计和计算机辅助制造 (CAD-CAM) 系统，从未着色 (Copran Zri) 和预着色 (Copran Zri Precoloured Medium) 圆盘上铣削出 50 个棒状和 60 个圆盘状氧化锆试样。预先着色的标本被分配到 PS 组，不再接受进一步的浸泡。未着色的样品根据在着色液（Copran Color A2 Shade）中的浸泡时间分为 4 个亚组：对照（C；未浸泡）、G45（浸泡 45 秒）、G90（浸泡 90 秒）和 G135（135 秒）浸入）。根据国际标准化组织 (ISO) 6872:2015 规范，使用万能试验机 (Lloyd LRX) 对棒状试样进行 3 点弯曲强度测试。用分光光度计（VITA Easyshade Advance 4.0）在灰色背景上测量圆盘状样品的颜色坐标。通过使用 CIE76 和 CIEDE2000 公式计算色差，然后将获得的值与每个公式的可感知性 (CIE76: 3.7, CIEDE2000: 0.8) 和可接受性 (CIE76: 6.8, CIEDE2000: 1.8) 阈值进行比较。随后，进行维氏硬度测量。使用Shapiro-Wilk检验、单向方差分析和Tukey HSD检验对数据进行统计分析（α=.05）。随后，进行维氏硬度测量。使用Shapiro-Wilk检验、单向方差分析和Tukey HSD检验对数据进行统计分析（α=.05）。随后，进行维氏硬度测量。使用Shapiro-Wilk检验、单向方差分析和Tukey HSD检验对数据进行统计分析（α=.05）。

结果

将氧化锆试样浸入着色液中会降低弯曲强度和硬度值，因为 C 组的两个参数值都最高。此外，延长的持续时间对机械性能有负面影响。PS 和 G45 组 ( P =.993) 和 G90 和 G135 组 ( P =.999)的弯曲强度在统计学上相似。对于维氏硬度，G135 组呈现最低值，G90 和 G135 组之间的差异在统计上相似（P = .061）。对于 ΔE*，各组之间的颜色差异从 2.49 到 27.33 不等，而 ΔE ₀₀值范围从 0.58 到 8.26。与 C 组相比，所有阴影样本的 ΔE* 值都超过了 6.8 的可接受阈值。除 G45 和 PS 组外，每组之间的色差均高于 ΔE* 和 ΔE ₀₀的可接受阈值。

结论

无论浸泡时间或着色技术如何，彩色氧化锆试样的弯曲强度和维氏硬度值都低于非着色试样。