State of the art of zirconia for dental applications
Introduction
Zirconia holds a unique place amongst oxide ceramics due to its excellent mechanical properties. This situation ensues from the considerable amount of research work that has been carried out since the discovery of the transformation toughening capabilities of zirconia in the mid-1970s [1].
At ambient pressure, unalloyed zirconia can assume three crystallographic forms depending on the temperature. At room temperature and upon heating up to 1170 °C, the symmetry is monoclinic (P21/c). The structure is tetragonal (P42/nmc) between 1170 and 2370 °C and cubic () above 2370 °C and up to the melting point [2], [3]. The transformation from the tetragonal (t) phase to the monoclinic (m) phase upon cooling is accompanied by a substantial increase in volume (∼4.5%), sufficient to lead to catastrophic failure. This transformation is reversible and begins at ∼950 °C on cooling. Alloying pure zirconia with stabilizing oxides such as CaO, MgO, Y2O3 or CeO2 allows the retention of the tetragonal structure at room temperature and therefore the control of the stress-induced t → m transformation, efficiently arresting crack propagation and leading to high toughness [1], [4], [5].
The recent introduction of zirconia-based ceramics as restorative dental materials has generated considerable interest in the dental community. The mechanical properties of zirconia are the highest ever reported for any dental ceramic. This may allow the realization of posterior fixed partial dentures and permit a substantial reduction in core thickness. These capabilities are highly attractive in prosthetic dentistry, where strength and esthetics are paramount. However, due to the metastability of tetragonal zirconia, stress-generating surface treatments such as grinding or sandblasting are liable to trigger the t → m transformation with the associated volume increase leading to the formation of surface compressive stresses, thereby increasing the flexural strength but also altering the phase integrity of the material and increasing the susceptibility to aging [6]. The low temperature degradation (LTD) of zirconia is a well-documented phenomenon, exacerbated notably by the presence of water [7], [8], [9], [10], [11], [12]. The consequences of this aging process are multiple and include surface degradation with grain pullout and microcracking as well as strength degradation. Although LTD has been shown to be indirectly associated with a series of femoral head prostheses failures in 2001 and despite a well established definition of the conditions for which LTD is susceptible to occur, there seem to be no clear relationship between LTD and failure predictability when zirconia is used as a bioceramic [13]. Greater insight into LTD is proposed in a companion review.
The various types of zirconia commercially available in dentistry are summarized in the first part of this article. The adequacy of surface treatments and their possible consequences on the reliability of zirconia for dental restorations is also examined. An update on the status of clinical trials occurring worldwide is provided.
Section snippets
Different types of zirconia ceramics available for dental applications
Although many types of zirconia-containing ceramic systems are currently available [14], [15], only three are used to date in dentistry. These are yttrium cation-doped tetragonal zirconia polycrystals (3Y-TZP), magnesium cation-doped partially stabilized zirconia (Mg-PSZ) and zirconia-toughened alumina (ZTA).
Soft machining of pre-sintered blanks
Since its development in 2001 [19], direct ceramic machining of pre-sintered 3Y-TZP has become increasingly popular in dentistry and is now offered by a growing number of manufacturers. Briefly, the die or a wax pattern is scanned, an enlarged restoration is designed by computer software (CAD) and a pre-sintered ceramic blank is milled by computer aided machining. The restoration is then sintered at high temperature. Several variations of this process exist depending on how the scanning is
Hard machining of 3Y-TZP and Mg-PSZ
At least two systems, Denzir® (Cadesthetics AB) and DC-Zirkon® (DCS Dental AG) are available for hard machining of zirconia dental restorations. Y-TZP blocks are prepared by pre-sintering at temperatures below 1500 °C to reach a density of at least 95% of the theoretical density. The blocks are then processed by hot isostatic pressing at temperatures between 1400 and 1500 °C under high pressure in an inert gas atmosphere [33], [55]. This latter treatment leads to a very high density in excess of
Clinical studies of ZrO2 fixed prostheses
There are approximately fifteen major studies of zirconia prostheses underway at this time; characteristics of which are annotated in Table 1. It seems notable that these studies mostly involve multi-unit and posterior prostheses. It is apparent that sponsoring manufacturers have some confidence in the structural potential of zirconia frameworks. This also signals some comfort regarding the performance of this core ceramic for the restoration of single anterior teeth; reflecting the clinical
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