ReviewLow temperature degradation -aging- of zirconia: A critical review of the relevant aspects in dentistry
Introduction
Typing the words “zirconia OR ZrO2” on any scientific search engine provides close to 25,000 publications from 1969 to 2008. Clearly also review papers centered on this material are numerous and, hence, it is necessary to justify the supposed need for a new one. We believe that the need stems from two concomitant aspects: (i) the world of dentistry has begun to deal with zirconium oxide only in the recent years and some aspects of the long-term behavior of this materials are not sufficiently known among dentists and researchers in the field [1], [2], [3], and (ii) more specifically, in spite of the above mentioned body of literature, there are actually few contributions reporting experimental data, and not extrapolations, on the long-term degradation (aging) of zirconia at temperatures low enough to be of interest for the dentists. For example, the comprehensive review of Lawson [4] on the degradation of zirconia due to the environment reports literally 3 data points below 100 °C. On the other hand, the case of the unusually large premature failures of ceramic heads in hip joint prostheses is widely known: between 2000 and 2002 a consistent number of ceramic balls made of yttria-stabilized zirconia, produced by Saint Gobain Desmarquest and marketed with the name Prozyr®, failed prematurely because of a change in the processing procedure which resulted in increased monoclinic content (the best clinical description of the episode can be found in [5] whereas a sound scientific interpretation of the problem has been proposed only very recently, in 2009 [6]).
Chevalier, a worldwide expert in the field of aging of zirconia, recently presented an informed opinion on this problem [7]; in the concluding remarks, he adds that “The use of zirconia for dental implants is quite young and in development phase. The issue of aging is still not discussed for these applications.” Another review on the use of zirconia in dentistry concludes also very conservatively, warning that long-term studies are badly needed in the field [8]. A recent review focused on the bacteriological aspects of zirconia for dental applications concludes that “A need for references concerning resistance to failure in long-term clinical trials is of paramount importance for such systems.” [3]. The cited work of Denry and Kelly [2] finishes stating that “It seems wise to keep in mind that some forms of zirconia are susceptible to aging and that processing conditions can play a critical role in the low temperature degradation of Zirconia”. In yet another very recent review [6], in the few lines dedicated to zirconia in dental applications, the authors conclude that aging “is expected to be no less of an issue for their manufacturers” [6]. This should induce the dental community to be very cautious and in wont of a clear knowledge, based on experimental data, about the performances of zirconia at low temperatures and long terms.
Thus, the present contribution will focus on the experimental evidence gathered in the temperature range of interest for implanted dental materials, that is between 0 and 100 °C on the problem of long-term degradation, or aging, of zirconia. The paper is organised as follows: firstly, a background on zirconia will be presented, then a paragraph will be dedicated to each of the three main factor affecting LTD: (i) stabilizer type and content, (ii) stress and (iii) grain size. Then the experimental methods used to study the LTD will be discussed followed by a section dedicated to a critical evaluation of the lifetime predictions of zirconia implants. Finally, based on a comprehensive evaluation of the experimental evidence, some engineering guidelines for the use of zirconia in restorative and prosthetic dentistry are proposed in the last section.
Section snippets
Terminology and background
Zirconium oxide, ZrO2, is chemically an oxide and technologically a ceramic material. It is basically insoluble in water. It can be dissolved in H2SO4 and in HF. In nature it is relatively abundant (about 0.02% of the earth crust) [9]. Large deposits are present in Brazil as baddeleyte (monoclinic zirconia) and in Australia and India as zircon (ZrSiO4) sands. Pure zirconia presents the phenomenon of allotropy, that is same chemical composition but different atomic arrangement, among the
LTD and stabilizers
Though, in principle, all stabilizers could be used to obtain both PSZ and TZP, practically alkaline earth oxides are used for commercially available PSZ and Y2O3 and CeO2 for TZP. As all other stabilizers, they are characterized by a large solubility in zirconia and by the ability to form fluorite-type phases over large temperature and compositional ranges. By definition PSZ is a two-phase material, with the tetragonal phase precipitating in the cubic matrix [34]: the precipitation is a rather
LTD and stress
As mentioned in paragraph II, stress can affect the t–m transformation and the behavior of zirconia with respect to stress can be easily rationalized in terms of Le Chatelier principle: “when perturbed, a system in equilibrium tends to react in order to minimize the perturbation”. When t-zirconia is put in tension, it minimizes this perturbation by expanding its dimensions and transforming to m-zirconia, because this expansion decreases the initially applied tensile stress (the same principle
LTD and grain size
Numerous researchers have reported that reducing the average GS in zirconia-based ceramics has a beneficial effect on the stability of the tetragonal phase [42], [48], [77], [78], [79], and therefore on LTD. Some confusion seems to arise in the literature on this subject, much of it generated by the ambiguous definition of the critical GS. Some researchers, who have focused on a purely thermodynamic approach to the problem, identify it as the maximum GS for which the tetragonal phase is
Experimental methods
In this section we briefly present the experimental methods which can provide evidence for the occurrence of the aging phenomenon. A more extensive review can be found in Ref. [18], with the exception of the stress measurements, which are not treated at all in that work.
Low temperature aging of zirconia is usually conducted in autoclave or steam chambers, where the pressure of water vapor, the temperature and the elapsed time are the controlled experimental variables. In some cases the ceramic
Limits of lifetime predictions at low temperature derived from accelerated aging tests
Degradation rates at room or body temperature of Y-TZP ceramics are currently not available, and accelerated tests at intermediate temperature (100–300 °C) are the only basis for extrapolating an estimate of the transformation rate and, hence, of the product lifetime. This approach relies on the assumption that the transformation rate follows the same Arrhenius-like trend down to room/body temperature. Unfortunately, such extrapolation could lead to a significant error in estimating room/body
Conclusions and engineering guidelines for the use of zirconia as dental materials
After presenting the most relevant experimental data, we proceed now to formulate some possible guidelines for a practical use of zirconia as dental materials. First of all the issue must be addressed whether the aging of t-zirconia to the m-polymorph should be avoided or can be lived with, also in light of the fact that there is some evidence that a limited and superficial t–m transformation can causes an increase of the strength of the material [102]. The reason is that the t–m transformation
Acknowledgement
FONDO TRIESTE is acknowledged for partial funding.
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