Property of sintering zirconia ceramics that is important for metal-free bridge restoration

由于无金属修复体在牙科中的应用，氧化锆(Zirconia：ZrO2)有望成为一种新型的陶瓷材料。然而，众所周知，氧化锆在高温下会随着烧结过程而收缩。此外，氧化锆的硬度太高，无法通过刀具形成复杂的图案。本研究的目的是调查CAD/CAM制作的氧化锆支架的边缘适合性和内部适合性。此外，还研究了在20000次热应力作用下的力学性能。使用商用CAD/CAM系统(Katana[0！R]，Noritake Dental Supply Co.，Noitake Dental Supply Co.Ltd.，Aichi，Katana[0！R]，Noritake Dental Supply Co.Ltd.，日本爱知县Noritake Dental Supply Co.，Katana[0！R]，日本爱知县Noritake Dental Supply Co.Ltd.)，使用原始氧化锆坯料(Katana[0！R]，Noritake Dental Supply Co.Ltd.，日本爱知)，制作了单冠覆盖件(A：基台)、三单元桥架(APA，P：Pontic)、四单元桥架(APPA)和五单元桥架(A1P1A2P2A3)。将冠覆盖和框架粘结到各自的主基牙模型上，并在特定测量点测量粘固层的厚度。APA和APPA的边缘适合度和内部适合度均在临床可接受范围内。APA和APPA桥桥侧轴面的边缘间隙和粘固层厚度均略高于非桥桥侧。A1P1A2P2A3支架的边缘间隙优于APA和APPA支架，这是因为中心基牙支撑支架防止了变形。因此，框架的基准部(A2)的轴线保持垂直，从而使A2具有良好的边缘适合性。因此，本研究的结果清楚地表明，骨架设计对于防止长骨架在加工后烧结过程中的变形是重要的。另一方面，氧化锆棒经过热循环后的三点弯曲强度低于存放在大气中的棒。在这些有限的数据中，人们怀疑氧化锆可能受到储存环境的影响。

Zirconium oxide (Zirconia: ZrO_2) has been expected as a new ceramics material because metal-free restoration is available in dentistry. However, it is well-known that zirconia contracted with sintering process at the high temperature. Additionally, zirconia was too hard to form the complex design by cutting tool. The purpose of this study was to investigate the marginal and internal fit of CAD/CAM-fabricated zirconia frameworks. Furthermore, mechanical property was also investigated under the storage in the thermal stress with 20, 000 cycle. Single crown copings (A: abutment), three-unit bridge frameworks (APA, P: pontic), four-unit bridge frameworks (APPA), and five-unit bridge frameworks (A_1P_1A_2P_2A_3) were fabricated with raw-stage zirconia blanks using a commercial CAD/CAM system(KATANA[○!R], Noritake Dental Supply Co. Ltd., Aichi, Japan). Crown copings and frameworks were cemented to their respective master abutment models, and thickness of the cement layer was measured at specific measuring points. Marginal and internal fit of both APA and APPA were within clinical acceptance. However, the marginal gap and thickness of the cement layer on the axial surface of the pontic side of APA and APPA were slightly higher than those of the non-pontic side. As for the marginal gap of A_1P_1A_2P_2A_3 framework, it was superior to those of APA and APPA because the center abutment supported the framework to prevent distortion. Consequently, the axis of the abutment portion (A2) of the framework was kept vertical, thereby resulting in an excellent marginal fit for A2. Thus, findings of this study clearly showed that framework design was important to preventing distortion of long frameworks during post-machining sintering. On the other hand, the three point bending strength of zirconia bar after thermal cycling was less than that of bars stored in the atmosphere. Within this limited data, it was suspected that zirconia might be influenced by the storing environment.