A Narrative Review on Polycrystalline Ceramics for Dental Applications and Proposed Update of a Classification System

Abstract

Dental zirconias have been broadly utilized in dentistry due to their high mechanical properties and biocompatibility. Although initially introduced in dentistry as an infrastructure material, the high rate of technical complications related to veneered porcelain has led to significant efforts to improve the optical properties of dental zirconias, allowing for its monolithic indication. Modifications in the composition, processing methods/parameters, and the increase in the yttrium content and cubic phase have been presented as viable options to improve zirconias' translucency. However, concerns regarding the hydrothermal stability of partially stabilized zirconia and the trade-off observed between optical and mechanical properties resulting from the increased cubic content remain issues of concern. While the significant developments in polycrystalline ceramics have led to a wide diversity of zirconia materials with different compositions, properties, and clinical indications, the implementation of strong, esthetic, and sufficiently stable materials for long-span fixed dental prostheses has not been completely achieved. Alternatives, including advanced polycrystalline composites, functionally graded structures, and nanosized zirconia, have been proposed as promising pathways to obtain high-strength, hydrothermally stable biomaterials. Considering the evolution of zirconia ceramics in dentistry, this manuscript aims to present a critical perspective as well as an update to previous classifications of dental restorative ceramics, focusing on polycrystalline ceramics, their properties, indications, and performance.

Keywords: alumina; fixed dental prostheses; prosthodontics; restorative ceramics; zirconia.

Figure 1

Schematic diagram of the updated classification of dental ceramics and ceramic-like materials, adapted from Gracis et al. (2015) [14] (the updated sections are presented in dark green). As glass-infiltrated ceramics produced by slip-casting and pure alumina ceramics have been withdrawn from the market, these groups were not considered in this update. Ceramic-like materials, such as resin-matrix ceramics represented by resin composite blocks for CAD/CAM, currently have more compositions than those presented in this paper. Due to the prolific launching in the market of new such products, including 3D-printed materials, they are not explored in this review. Modified with permission from CopyRightClearanceCenter.

Figure 2

Scanning electron microscope (SEM) image of the 3Y-TZP framework. A dense microstructure is depicted with regular and homogeneous spherical zirconia grains and few microstructural defects (white arrows), which may be related to ceramic processing.

Figure 3

SEM micrograph of monolithic 3Y-TZPs, where a dense surface with slightly larger grains and fewer structural defects are observed in comparison to 3Y-TZPs of the first generation, being used as a framework material.

Figure 4

SEM micrographs of (A) 4Y-PSZ and (B) 5Y-PSZ depicting a dense microstructure with a predominance of cubic grains that are significantly larger than those of the 3Y-TZP versions used as framework and monolithic, as presented in Figure 2 and Figure 3, respectively. The high content of the non-birefringent cubic phase allows for a higher light transmittance and, therefore, higher translucency.

Figure 5

SEM micrograph of a multilayered system composed of 3Y-TZP and 5Y-PSZ, depicting the transition layer composed of interpenetrated areas of tetragonal (white asterisks and unfilled arrow) and cubic grains (white circles and solid white arrows) in low (A) and high (B) magnifications.

Figure 6

X-ray diffraction (XRD) pattern of (A) framework 3Y-TZP; (B) monolithic 3Y-TZP; and (C) 5Y-PSZ before (orange, bottom) and after (green, top) artificial aging in a hydrothermal reactor for 20 h at 134 °C under 2.2 bars of pression. A significant increase in monoclinic peaks (m) is depicted for both 3Y-TZPs, with higher peaks recorded for the monolithic, second-generation 3Y-TZP. Otherwise, 5Y-PSZ presents characteristic tetragonal (t) and cubic (c) peaks and no significant alterations after aging.

Figure 7

SEM micrograph of: (A) zirconia-toughened alumina (80% alumina and 20% zirconia) and (B) alumina-toughened zirconia (80% zirconia and 20% alumina) composites. Both polycrystalline composites present a dense microstructure with secondary phases that are homogeneously distributed in the alumina and zirconia matrices, respectively. Moreover, the white arrows indicate microstructural defects originating from ceramic processing.

Figure 8

XRD patterns of (A) ZTA (70/30) and (B) ATZ (80/20) before (green, top) and after (orange, bottom) autoclave aging for 20 h at 134 °C under 2.2 bars of pression. While the high hydrothermal stability of ZTA is observed with almost no modifications in the diffractogram after aging, a limited amount of phase transformation is depicted in ATZ after aging. Letters represent (m) monoclinic peaks, (t) tetragonal peaks, (c) cubic peaks, and (α) alpha alumina peaks.

Figure 9

Clinical cases rehabilitated with different kinds of zirconias. (A) 3Y-TZP infrastructures fully veneered with feldespathic ceramic in a tooth-supported upper arch rehabilitation including single-crown and three-unit fixed dental prostheses (Clinical case conducted by Benalcázar-Jalkh EB, and Pegoraro LF, and Dental Technician M. Portaluppi). (B) Partially veneered monolithic zirconia used for full-arch implant-supported prosthesis. The buccal aspect of the teeth and gingiva were veneered to achieve esthetic results, while occlusal and palatal aspects were designed in monolithic zirconia to reduce the risk of porcelain chipping (Clinical case conducted by Laura Firmo de Carvalho and Dental Technician Marcos Celestrino). (C) 5Y-PSZ, or “ultra-translucent” zirconia, was used to manufacture an implant-supported single crown over a Ti-base abutment (Clinical case conducted by Raphaelle SM de Sousa and Dental Technician/DDS Ricardo Tanaka). (D) Full-arch tooth-supported prosthesis manufactured in a monolithic multilayered zirconia (3Y-TZP/5Y-PSZ) and stained to achieve esthetic results (Clinical case conducted by Benalcázar-Jalkh EB, Bonfante EA, and Dental Technician M. Celestrino).