Abstract
The market share of zirconia (ZrO2) dental implants is steadily increasing. This material comprises a polymorphous character with three temperature-dependent crystalline structures, namely monoclinic (m), tetragonal (t) and cubic (c) phases. Special attention is given to the tetragonal phase when maintained in a metastable state at room temperature. Metastable tetragonal grains allow for the beneficial phenomenon of Phase Transformation Toughening (PTT), resulting in a high fracture resistance, but may lead to an undesired surface transformation to the monoclinic phase in a humid environment (low-temperature degradation, LTD, often referred to as ‘ageing’). Today, the clinical safety of zirconia dental implants by means of long-term stability is being addressed by two international ISO standards. These standards impose different experimental setups concerning the dynamic fatigue resistance of the final product (ISO 14801) or the ageing behavior of a standardized sample (ISO 13356) separately. However, when evaluating zirconia dental implants pre-clinically, oral environmental conditions should be simulated to the extent possible by combining a hydrothermal treatment and dynamic fatigue. For failure analysis, phase transformation might be quantified by non-destructive techniques, such as X-Ray Diffraction (XRD) or Raman spectroscopy, whereas Scanning Electron Microscopy (SEM) of cross-sections or Focused Ion Beam (FIB) sections might be used for visualization of the monoclinic layer growth in depth. Finally, a minimum load should be defined for static loading to fracture. The purpose of this communication is to contribute to the current discussion on how to optimize the aforementioned standards in order to guarantee clinical safety for the patients.
Highlights
Zirconia was introduced for medical use several decades ago [1]
The lattice structure of pure zirconia depends on the temperature, resulting in phase transformations at 1170 ◦ C (< monoclinic, > tetragonal) and 2370 ◦ C (< tetragonal, > cubic), concomitant with changes in density [2]
Roughened or porous implant surfaces presented an up to 60% higher transformation rate compared to specimen with polished surfaces [13]
Summary
Zirconia was introduced for medical use several decades ago [1]. The lattice structure of pure zirconia depends on the temperature, resulting in phase transformations at 1170 ◦ C (< monoclinic, > tetragonal) and 2370 ◦ C (< tetragonal, > cubic), concomitant with changes in density [2]. These phase configurations/modalities are reversible to a certain degree.
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