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

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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.