Sintering Distortion in Multi-Composition-Layered Zirconia Disks for Dental Prostheses: An Experimental Analysis

Zirconia is a high-strength ceramic and has increasing importance, particularly in the field of aesthetic dentistry for crown restorations. Multi-composition-layered-type (MCL) zirconia is attracting attention as a new material that integrates high light transmittance with mechanical strength. However, there are no reports on the deformation induced by sintering in MCL zirconia. Therefore, we aimed to investigate the sintering distortion of MCL zirconia. An experimental fixed dental prosthesis (FDP) was designed based on a 4-unit monolithic zirconia FDP. A MCL with no color gradation and an MCL with color gradation were selected. Particularly, three milling areas-the top end of the disk (area I) (n = 7), vertical center (area II) (n = 7), and bottom end of the disk (area III) (n = 7)-were investigated. Moreover, sintering distortions generated by experimental FDPs were measured. Sintering distortion was detected in all areas. The direction of distortion varied by area-positive in area I, negative in area II, and approximately zero in area III-with a significant difference between areas I and II (p = 0.001). The largest absolute distortion was observed in c-MCL-A (area I); the corresponding marginal gaps were ~89.4 μm (second molar) and ~56.9 μm (first premolar), both below the clinical threshold of 120 μm.