Department of Biomaterials Science, College of Dentistry, Dankook University, 119 Dandaero, Cheonan, Chungnam 31116, Republic of Korea.
Division of Fixed Prosthodontics and Biomaterials, University Clinics of Dental Medicine, University of Geneva, Geneva, Switzerland.
Dent Mater. 2024 Apr;40(4):653-663. doi: 10.1016/j.dental.2024.02.003. Epub 2024 Feb 19.
This study aimed to compare the four-point flexural strength of CAM-milled and sintered (as-sintered, AS) specimens with those of high-polished (HP) specimens using chairside polishing systems to simulate clinical surface conditions.
Seven full-contour zirconia CAM/CAM blanks with various yttria contents (3, 4, 5 mol%) including three high-translucent groups (5Y) were selected to prepare flexural specimens. The bend bar specimens (2.0 × 4.0 × 25.0 mm) were fabricated by using STL file and dental CAM machine with the respective zirconia blanks (98 mm ϕ and 10-14 mm in height). Twelve bar specimens were machined from one zirconia puck and a total of 24 specimens were prepared from each group. The pre-sintered bar specimens were sintered by using a dental zirconia furnace at 1530-1550 °C for 2 h according to the instructions. All sintered specimens were divided into two groups: as-sintered (AS) group and high-polished (HP) groups (n = 12). HP groups were subjected to polishing one surface of specimens using a three-step polishing system and finally finished with diamond polishing. After cleaning and drying, the flexural strength of all specimens was determined by a fully articulating four-point flexure fixture consisting of a 1/4-point test configuration with an inner/outer span of 10/20 mm. Statistical differences between AS and HP groups were conducted with Weibull analysis. The fractured surfaces of zirconia specimens were observed using a field emission SEM and EDS to detect failure origins.
The mean AS flexural strength values were significantly lower than those of HP counterparts. However, Weibull moduli expressing the reliability of HP groups were generally decreased although not significantly in comparison to their AS. The fracture of the AS specimens mostly originated from extrinsic CAM-milling defects, while the HP specimens were fractured from intrinsic subsurface or volume defects including pores, large grain clusters, inclusions, and corner-located critical flaws. Two high-translucent (5Y) zirconia groups were not affected in their strength and reliability after polishing, whereas one 5Y zirconia significantly increased its strength but significantly lowered its reliability.
The extrinsic and intrinsic strength-limiting defects should be considered in evaluating the flexural strength and reliability of dental CAD/CAM zirconia ceramics for full-contour restorations. For the materials tested in this study, more optimized processing of blanks and milling protocols of pre-sintered zirconia blanks should be developed including post-sintering surface finishing to reduce the flaw population regulating strength and reliability which will affect the survivability of dental zirconia prostheses.
本研究旨在通过椅旁抛光系统模拟临床表面条件，比较 CAM 铣削和烧结（即烧结，AS）的四点弯曲强度与高抛光（HP）的试件。
选择了七种不同氧化钇含量（3、4、5mol%）的全轮廓氧化锆 CAM/CAM 毛坯，包括三个高透光组（5Y），以制备弯曲试件。使用 STL 文件和牙科 CAM 机以及各自的氧化锆毛坯（98mmφ和 10-14mm 高）制造了 2.0×4.0×25.0mm 的弯曲棒试件。从一个氧化锆压块加工了 12 个棒试件，每组共制备了 24 个试件。根据说明，使用牙科氧化锆炉在 1530-1550°C 下烧结预烧结棒试件 2 小时。所有烧结试件分为两组：烧结后（AS）组和高抛光（HP）组（n=12）。HP 组使用三步抛光系统对一个表面进行抛光，最后用金刚石抛光完成。清洁干燥后，用全关节四点弯曲夹具测定所有试件的弯曲强度，夹具由内/外跨距为 10/20mm 的 1/4 点测试配置组成。采用威布尔分析比较 AS 和 HP 组之间的统计学差异。使用场发射 SEM 和 EDS 观察氧化锆试件的断裂面，以检测失效起源。
AS 弯曲强度值明显低于 HP 组。然而，尽管威布尔模数表示 HP 组的可靠性普遍降低，但与 AS 相比，其降低并不显著。AS 试件的断裂主要源于外在的 CAM 铣削缺陷，而 HP 试件的断裂则源于内在的表面下或体积缺陷，包括孔隙、大晶粒簇、夹杂物和位于角部的临界缺陷。两个高透光（5Y）氧化锆组在抛光后强度和可靠性不受影响，而一个 5Y 氧化锆组的强度显著提高，但可靠性显著降低。
在评估全轮廓修复用牙科 CAD/CAM 氧化锆陶瓷的弯曲强度和可靠性时，应考虑外在和内在的强度限制缺陷。对于本研究中测试的材料，应开发更优化的毛坯加工和预烧结氧化锆毛坯铣削方案，包括烧结后表面处理，以减少影响牙科氧化锆修复体存活率的缺陷种群调节强度和可靠性。
