The use of Zirconia ceramics for orthopedic implants derives from the exceptional mechanical properties of this family of ceramics: high strength and high toughness. These properties are a consequence of the phase transformation toughening mechanism that is particular to Zirconia ceramics.
Introduction to zirconia ceramics for orthopedic implants
Ceramic materials are made of an inorganic non-metallic oxide. Usually ceramics are divided into two groups:
- Silicon ceramics
- Aluminous ceramics
Depending on inner molecular organization, ceramics are also divided into:
Depending on their in vivo behavior, ceramics are classified as:
Zirconia, the metal dioxide of zirconium (ZrO2), is a bioinert ceramic; its low reactivity together with its good mechanical features (low wear and high stability) led to use it in many biomedical restorative devices. Its most popular application is in arthroprosthetic joints where it has proven to be very effective. Also dental use of Zirconia is widely accepted to achieve aesthetic and reliability of dental restorations.
Mechanical chemical features of zirconia
Zirconia’s evolution from one metastable, tetragonal polymorph to the stable monoclinic one explains both the phase transformation toughening mechanism responsible for the high mechanical properties of Zirconia and its sensitivity to low-temperature degradation. This degradation goes under the name of “aging”.
From high temperature to low temperature, pure Zirconia exists under 3 stable phases:
- Cubic (between the melting point, 2.680ºC and 2.370ºC)
- Tetragonal (down to 1.170ºC)
- Monoclinic (room temperature stable phase)
The processing of Zirconia ceramics by sintering involves temperatures up to 1.500ºC, temperatures at which the tetragonal phase is stable. Upon cooling to room temperature, pure Zirconia transforms to its monoclinic stable phase. The ~5% volume increases accompanying the tetragonal to monoclinic transformation inevitably lead to extensive cracking and ruin of the pure zirconia pieces. Thus, pure Zirconia can retain neither tetragonal nor cubic phase at room temperature and is never used alone for structural applications. Doping Zirconia with a few percent of other oxides allows the retention of an intact body after sintering, at room temperature.
Impact of aging on the durability of implants
Low Temperature Degradation (LTD) was first described by Kobayashi in 1980, in a paper that showed that tetragonal-monoclinic transformation could take place on the surface of Zirconia pieces at 250ºC, and results in a decrease of the mechanical properties. Since the first reports dealt with temperatures significantly higher than body temperature, LTD was first considered as negligible at 37ºC. However, it was soon to be discovered that aging is a thermally activated phenomenon, that may be involved in at least 2 sequences of the implant lifetime: during sterilization and during in vivo residence.
In 1997, the Food and Drugs Administration (FDA) issued a warning cautioning surgeons against the use of steam sterilization on Zirconia implants, as this procedure was proven to roughen the surface and provoke increased wear of polyethylene cups. This increased wear rate due to LTD resulted in increased risks of aseptic loosening , thus shorter lifetime.
Aging may give rise to other problems, mainly related to fracture of the ceramic. Indeed, LTD results in microcracking of the aged surface and the microcracked area may act as potentially critical defects. As soon as microcracking appears, one can expect a decrease of the strength and of the lifetime.
An important development in ceramics for orthopedics is the combination of ceramic heads with ceramic inserts to eliminate Polyethylene (PE) wear. Whereas the preferred combination of ceramic hard-hard couplings still seems to be the one pairing Alumina against Alumina, recent developments have shown that Zirconia against Alumina appears to be another option.
Many manufacturers of orthopedic ceramic implants have discontinued the manufacture of pure Zirconia components, due to the problem of the material’s long-term stability. However, other manufacturer continue to produce them, relying on the improvement in material technology, processing technique, machining capability, and quality control.