Novel Zirconia Materials in Dentistry

Abstract

Zirconias, the strongest of the dental ceramics, are increasingly being fabricated in monolithic form for a range of clinical applications. Y-TZP (yttria-stabilized tetragonal zirconia polycrystal) is the most widely used variant. However, current Y-TZP ceramics on the market lack the aesthetics of competitive glass-ceramics and are therefore somewhat restricted in the anterior region. This article reviews the progressive development of currently available and next-generation zirconias, representing a concerted drive toward greater translucency while preserving adequate strength and toughness. Limitations of efforts directed toward this end are examined, such as reducing the content of light-scattering alumina sintering aid or incorporating a component of optically isotropic cubic phase into the tetragonal structure. The latest fabrication routes based on refined starting powders and dopants, with innovative sintering protocols and associated surface treatments, are described. The need to understand the several, often complex, mechanisms of long-term failure in relation to routine laboratory test data is presented as a vital step in bridging the gaps among material scientist, dental manufacturer, and clinical provider.

Keywords: ceramics; dental prostheses; durability; graded structure; nanostructure; translucency.

Figure 1.

Failure modes in ceramic prostheses from tensile stresses at the occlusal surface and cementation interface. (a) Images of failure in laboratory tests on crowns, splitting (left) and chipping (right), and (b) corresponding schematic diagram depicting various individual fracture modes: full cone cracks outside (O) and within (I) the contact zone; median cracks (M) beneath contact; partial cone cracks (P) from sliding; chipping cracks (C) adjacent to the side wall; flexure radial cracks (R) at the cementation surface, either below the contact or at the margins; delamination cracks (D) at the cementation/substrate interface. (c, d) Corresponding image and schematic diagram for fixed dental prosthesis, indicating dominant flexure cracks (F) at connector sites. Arrows indicate occlusal loading. Modified from Zhang et al. (2013).

Figure 2.

Factors affecting durability of zirconia ceramics. (a) Schematic diagram showing flaws in ceramic structure: microcracks at internal grain boundaries and surface damage zone (shaded) from machining/sandblasting; pores and inclusions from sintering. (b) Schematic diagram of degradation from damage at surface from occlusal contact at load P (arrow) and number of cycles n, showing generation of quasiplastic zone with initiation of macroscopic cone-like crack (C) from microcrack coalescence. (c) Flexural stress versus number of cycles (S-n curve) for an ultrastrong 3Y-TZP in as-polished and heavily sandblasted states. Data from Zhang et al. (2004). (d) Static strength for a 3Y-TZP after repeat surface contact with a metal sphere (radius, 3.18 mm). Box at left axis designates laboratory strengths (unindented specimens, SD bounds). Data from Jung et al. (2000). Pronounced fatigue is apparent in both data sets. 3Y-TZP, 3 mol% yttria stabilized tetragonal zirconia polycrystal.

Figure 3.

Graded zirconia. (a) Cross section of 3Y-TZP (3 mol% yttria stabilized tetragonal zirconia polycrystal) after surface infiltration with glass. Dark region at left is residual glass layer. Note change in microstructure near the surface to a net depth of ~150 µm (skin depth: 30 µm + penetration: depth 120 µm). (b) Profile of elastic modulus. Vertical dashed lines delineate glass-layer, graded, and core regions. (c) Fabricated anatomically correct crowns (left to right—ungraded white, graded white, graded tinted) and (d) fixed dental prosthesis framework (graded tinted). After Ren et al. (2011).

Figure 4.

Nanoscale 3Y-TZP (3 mol% yttria stabilized tetragonal zirconia polycrystal), showing (a) small-grain microstructure, (b) improved transmittance of nanostructure slab (left) relative to second-generation Zpex (right), and (c) measured transmittance in visible spectrum for slabs of 1-mm thickness.