Japanese Dental Science Review 56 (2020) 1–23 Contents lists available at ScienceDirect Japanese Dental Science Review journal homepage: www.elsevier.com/locate/jdsr Review Article Strength and aging resistance of monolithic zirconia: an update to current knowledge Eleana Kontonasaki a,∗ , Panagiotis Giasimakopoulos b , Athanasios E. Rigos b a Department of Prosthodontics, Faculty of Dentistry, School of Health Sciences, Aristotle University of Thessaloniki, Greece b Dentist, Thessaloniki, Greece a r t i c l e i n f o Article history: Received 20 May 2019 Received in revised form 2 August 2019 Accepted 17 September 2019 Keywords: Monolithic zirconia ceramics Strength Aging a b s t r a c t New zirconia compositions with optimized esthetic properties have emerged due to the fast-growing technology in zirconia manufacturing. However, the large variety of commercial products and synthesis routes, make impossible to include all of them under the general term of “monolithic zirconia ceramics”. Ultra- or high translucent monolithic formulations contain 3–8 mol% yttria, which results in materials with completely different structure, optical and mechanical properties. The purpose of this study was to provide an update to the current knowledge concerning monolithic zirconia and to review factors related to strength and aging resistance. Factors such as composition, coloring procedures, sintering method and temperature, may affect both strength and aging resistance to a more or less extend. A significant reduction of mechanical properties has been correlated to high translucent zirconia formualtions while regarding aging resistance, the findings are contradictory, necessitating more and thorough investigation. Despite the obvious advantages of contemporary monolithic zirconia ceramics, further scientific evidence is required that will eventually lead to the appropriate laboratory and clinical guidelines for their use. Until then, a safe suggestion should be to utilize high-strength partially-stabilized zirconia for posterior or long span restorations and fully-stabilized ultra-translucent zirconia for anterior single crowns and short span fixed partial dentures. © 2019 The Authors. Published by Elsevier Ltd on behalf of The Japanese Association for Dental Science. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). 1. Introduction Zirconia was suggested as the first candidate for full contour monolithic restorations due to its significant advantages, such as excellent mechanical properties, superior to those of other ceramic systems, esthetic performance comparable to that of metal-ceramic restorations, radiopacity, low corrosion potential, good chemi- cal properties, volumetric stability and elastic modulus values comparable to steel [1–6]. According to in-vitro studies, zirco- nia restorations exhibit flexural or bending strength values of 900–1200 MPa and resistance to fracture of 9–10 MPa [7]. However, zirconia ceramics do suffer from low-temperature degradation (LTD), also known as aging [8]. In particular, the spontaneous and progressive transformation of the tetragonal phase to monoclinic degrades the mechanical properties of the Y-TZP and, in particular, ∗ Corresponding author at: Laboratory of Prosthodontics, Department of Dentistry, Faculty of Health Sciences, Aristotle University of Thessaloniki, Thessaloniki GR- 54124, Greece. E-mail address: kont@dent.auth.gr (E. Kontonasaki). its strength. Specific processing conditions, environmental humid- ity, stress and temperatures between 200–300 ◦ C may accelerate it [5,8–10]. Many experimental studies have shown that water molecules can penetrate the zirconia structure when exposed to a hygroscopic environment. The triatomic character of yttrium con- tributes to the presence of several oxygen vacancies in the zirconia lattice, which facilitates the diffusion of water molecules into its mass [8]. The diffusion of water initially causes a lattice contraction that will result in tensile stresses’ concentration on the surface of the zirconia grains resulting in the transformation of the tetragonal phase to monoclinic phase. The increase in volume, that accompa- nies the transformation of a tetragonal grain to monoclinic, causes surface uplifts (Fig. 1) and grain pull out. This causes microcracks, which facilitate further the access of the water molecules within the internal grains and therefore the surface initial tetragonal to monoclinic transformation progresses deeper and deeper into the bulk of the material. As the microcracks grow and this process con- tinues, the material fractures. Microstructural factors such as grain size, percentage of stabilizer, residual stresses and manufacturing imperfections may affect this phenomenon [10–12]. https://doi.org/10.1016/j.jdsr.2019.09.002 1882-7616/© 2019 The Authors. Published by Elsevier Ltd on behalf of The Japanese Association for Dental Science. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).