Recent advances in dental zirconia: 15 years of material and processing evolution

Abstract

Objectives: The objective was to discuss the research on zirconia published in the past 15 years to help the dental materials community understand the key properties of the types of zirconia and their clinical applications.

Methods: A literature search was performed in May/2023 using Web of Science Core Collection with the term "dental zirconia". The search returned 5102 articles, which were categorized into 31 groups according to the research topic.

Results: The current approach to improving the translucency of zirconia is to decrease the alumina content while increasing the yttria content. The resulting materials (4Y-, 5Y-, and above 5 mol% PSZs) may contain more than 50% of cubic phase, with a decrease in mechanical properties. The market trend for zirconia is the production of CAD/CAM disks containing more fracture resistant 3Y-TZP at the bottom layers and more translucent 5Y-PSZ at the top. Although flaws located between layers in multilayered blocks might represent a problem, newer generations of zirconia layered blocks appear to have solved this problem with novel powder compaction technology. Significant advancements in zirconia processing technologies have been made, but there is still plenty of room for improvement, especially in the fields of high-speed sintering and additive manufacturing.

Significance: The wide range of zirconia materials currently available in the market may cause confusion in materials selection. It is therefore imperative for dental clinicians and laboratory technicians to get the needed knowledge on zirconia material science, to follow manufacturers' instructions, and to optimize the design of the prosthetic restoration with a good understanding where to reinforce the structure with a tough and strong zirconia.

Keywords: Composition-gradient materials; Consolidation methods; Dental zirconia; Microstructure; Monolith; Powder technology; Review; Sintering; Techniques; Translucency.

[Figure 1] –

Distribution by topics of zirconia papers published between 2008 and 2023.

[Figure 2]:

Scanning electron microscopy (SEM) views of a bulk defect in the fracture surface of the distal connector of a three-unit monolithic zirconia bridge (Katana STML, Kuraray Noritake). (a) and (b) Lower magnification overviews of the bulk defect, and (c) a higher magnification image of the defect, showing free-air sintered zirconia grains inside the open crack. Note that this bulk defect was not related to the fracture origin.

[Figure 3]:

Same Katana STML three-unit bridge distal connector fracture surface at a different site. (a) A very small bulk defect, and (b-c) detailed SEM analysis revealed free-air sintered zirconia grains inside the defect. Again, this defect is not related to the fracture origin.

[Figure 4]:

Examples of powder compact related defects resulting in (a) poorly densified areas, and (b) flaws in the microstructure of sintered zirconia, which ultimately acted as fracture origins in fatigue fractured specimens [34].

[Figure 5]:

SEM micrographs of the occlusal surface of a monolithic zirconia molar (Katana STML) bonded to an implant-supported titanium base after 10 months of intraoral chewing. (a) shows the worn surface with the glaze microcracking away exposing the zirconia grains (b).

[Figure 6]:

SEM images at 10000 x of commercial polychromic zirconia with uniform composition from Kuraray Noritake. (a): Katana ML (3.7 mol% yttria); (b) Katana STML (4.8 mol% yttria); and (c) Katana UTML (5.4 mol% yttria). The exact mol% were obtained from a previous study [89].

[Figure 7]:

SEM images of the microstructure at 10000 x of a composition-gradient 3-5Y zirconia (IPS e.max ZirCAD Prime, Ivoclar) block. The bottom part illustrates the 3Y (a) and the top part shows the 5Y zirconia with large translucent zirconia grains (b).