Statement of problem

Tools will gradually deteriorate with repetitive milling. However, tool lifetime can vary depending on the type of milling machine, the hardness of the ceramic material, and the size of the restoration. Studies evaluating the effect of tool deterioration on the trueness of milled restorations are lacking.

Purpose

The purpose of this study was to evaluate the effect of tool deterioration on the trueness of milled restorations.

Material and methods

A patient requiring a ceramic crown was recruited. Repetitive milling of zirconia crowns (inCoris TZI; Dentsply Sirona) with tungsten carbide rotary instruments and glass-ceramic crowns (VITABLOCS mark II; Vita Zahnfabrik) with diamond rotary instruments was performed by using a 5-axis milling machine (inLab MC X5; Dentsply Sirona) until the machine software program indicated replacing the largest tool. The percentage of deterioration for the largest tool was recorded before each milling. The trueness for different crown areas was evaluated by using a 3-dimensional evaluation software program (Geomagic Control X; 3D systems), and the crowns were clinically evaluated. The crowns were grouped into 4 quadrants as per the deterioration percentage. The Pearson correlation (r) between the deterioration percentage and trueness and linear contrast analysis (partial eta squared [η]) among the groups were performed.

Results

Glass-ceramic crown areas had low correlation except for mesial (high, r=0.63, high, η=0.37) and distal (medium, r=0.42, high, η= 0.34) areas, whereas zirconia crowns had low correlation for inner and internal areas and medium to high correlation for mesial (r= -0.31, η=0.33), distal (r= -0.53, η=0.36), occlusal (r= -0.32, η=0.164), external (r= 0.69, η=0.52), and marginal (r= -0.44, η=0.19) areas. Zirconia crowns had excellent marginal fit and occlusal contact and light to excellent proximal contact, whereas glass-ceramic crowns had excellent marginal fit, minor to major occlusal correction, and heavy proximal contact.

Conclusions

The effect of tool deterioration on trueness depended on material type and crown area. Only proximal areas were affected in the glass-ceramic crowns, whereas external, proximal, occlusal, and marginal areas were affected in the zirconia crowns. Trueness did not necessarily reflect the clinical quality of the crown, as both crown types were clinically satisfactory. Although zirconia crowns were overmilled in comparison with the reference design and glass-ceramic crowns, they had better clinical quality.

中文翻译：

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CAD-CAM工具劣化对陶瓷修复体真实性的影响

问题陈述

工具会随着重复铣削而逐渐退化。然而，刀具寿命可能因铣床类型、陶瓷材料硬度和修复体尺寸而异。缺乏评估工具劣化对铣削修复体真实性影响的研究。

目的

本研究的目的是评估工具劣化对铣削修复体真实性的影响。

材料与方法

招募了一名需要陶瓷牙冠的患者。使用 5 轴铣床 (inLab MC X5; Dentsply) 重复铣削氧化锆牙冠 (inCoris TZI; Dentsply Sirona) Sirona），直到机器软件程序指示更换最大的刀具。在每次铣削之前记录最大刀具的劣化百分比。使用3维评估软件程序（Geomagic Control X；3D系统）对不同冠区域的真实性进行评估，并对冠进行临床评估。根据劣化百分比将牙冠分为 4 个象限。

结果

除近中（高，r=0.63，高，η=0.37）和远中（中，r=0.42，高，η=0.34）区域外，玻璃陶瓷冠区域的相关性较低，而氧化锆冠与内部和内侧（r= -0.31，η=0.33），远中（r= -0.53，η=0.36），咬合（r= -0.32，η=0.164），外部（r= 0.69， η = 0.52）和边缘（r = -0.44，η = 0.19）区域。氧化锆牙冠具有出色的边缘贴合和咬合接触以及轻到极好的近端接触，而玻璃陶瓷冠具有出色的边缘贴合、轻微到主要的咬合矫正和重度近端接触。

结论

刀具劣化对正确度的影响取决于材料类型和冠部面积。玻璃陶瓷冠中只有近端区域受到影响，而氧化锆冠中的外部、近端、咬合和边缘区域受到影响。真实性不一定反映牙冠的临床质量，因为两种牙冠类型在临床上都令人满意。尽管与参考设计和玻璃陶瓷冠相比，氧化锆冠被过度研磨，但它们具有更好的临床质量。