Zirconia structures used for dental purposes are fabricated using CAD-CAM (computer-aided design and computer-aided manufacturing) technology in two possible ways.
One method mills the fully sintered block of zirconia with no shrinkage to the final structure. The disadvantages are the great wear of the grinding tools and the population of flaws produced during the machining that may lower the mechanical reliability of the structure.
In the other method, the zirconia structure is milled from a pre-sintered block, reaching its final mechanical properties after sintered, which produces structural shrinkage that can be partly compensated at the designing stage, and the fit of the zirconia restoration will be warranted.
Both CAD-CAM processes have three main steps: acquisition of digital data, computer processing and designing, and fabrication of the zirconia structure.
Most importantly, the CAD-CAM technique has the ability to produce zirconia restorations with sufficient precision for dental use.
Traditionally, zirconia is dull white in color and its opacity can mask the underneath structure. Most dental zirconia systems indicate structural coloring to enhance the esthetic. Currently, full-contoured (anatomical-shaped) monolithic zirconia dental restorations are offered, which could abbreviate or extinguish the dental laboratory work on zirconia-based restorations.
Several studies reported, however, that Y-TZP would lose its stability in wet environment, leading to strength degradation mostly because of the crystallographic transformation from metastable tetragonal phase to monoclinic phase (T-M transformation) and inherent cyclic fatigue from chewing and para-functional habits (e.g., bruxism and clenching). Nevertheless, the influence of low temperature degradation (LTD) on dental zirconia is still in need of further investigation.
Even so, the most popular zirconia-based restorations have a zirconia infrastructure that is porcelain veneered to adequate anatomic contour and esthetic. There are two main ways of veneering zirconia infrastructures: the traditional layering technique and the hot pressing method.