One widely employed bioceramic is alumina, which is considered bioinert. The search for an ideal bioceramic has included alumina, hydroxyapatite, calcium phosphate, and other ceramics. The first use of aluminas for implants in orthopedics and dentistry was in the 1960's. They were later employed in hip prostheses as early as 1970. Since those early days the quality and performance of aluminas have improved. High-purity, high-density, fine-grained aluminas are currently used for a wide range of medical applications, e.g. dental implants, middle ear implants, and hip or knee prostheses.
Although the aluminas currently available perform satisfactorily, a further improvement in strength and toughness would increase the safety factor and may extend usage to higher stressed components. A proposed candidate to add to this list is stabilized-zirconia, because of its potential advantages over alumina of a lower Young's modulus, higher strength, and higher fracture toughness. Another advantage of stabilized-zirconia is low-wear residue and low coefficient of friction. Because, zirconia undergoes a destructive phase change at between 1000° and 1100° C., changing from monoclinic to tetragonal, phase stabilization admixtures of calcia, magnesia, ceria, yttria, or the like are required.
Tetragonal zirconia polycrystalline ceramic, commonly known as TZP, which typically contains 3 mole percent yttria, coupled with the small size of the particles, results in the metastable tetragonal state at room temperature. Under the action of a stress field in the vicinity of a crack, the metastable particles transform, with a 3% to 4% volume increase, by a shear-type reaction, to the monoclinic phase. Crack propagation is retarded by the transforming particles at the crack tip and by the compressive back stress on the crack walls behind the tip, due to volume expansion associated with transformation to the monoclinic phase.
The well-known transformation toughening mechanism is operative in zirconia ceramics whose composition and production are optimized such that most of the grains have the tetragonal crystal structure. These TZP ceramics, most notably their mechanical properties in air at room temperature, are superior to those of zirconia-toughened aluminas and to other classes of zirconias. While the biocompatibility of TZP ceramic has not been fully assessed, it has been preliminarily investigated.
For example, in one study by Thompson and Rawlings [see I. Thompson and R. D. Rawlings, “Mechanical Behavior of Zirconia and Zirconia-Toughened Alumina in a Simulated Body Environment,” Biomaterials, 11 [7] 505–08 (1990)]. The result was that TZP demonstrated a significant strength decrement when aged for long periods in Ringer's solution and was therefore unsuitable as implant material.
Drummond [see J. L. Drummond, J. Amer. Ceram. Soc., 72 [4] 675–76 (1989)] reported that yttria-stabilized zirconia demonstrated low-temperature degradation at 37° C. with a significant decrement in strength in as short a period as 140 to 302 days in deionized water, saline, or Ringer's solution. He also reports on similar observation by others, where yttria-doped zirconia demonstrated a strength decrement in water vapor, room temperature water, Ringer's solution, hot water, boiling water, and post-in vivo aging.
TZP components suffer a decrement in strength properties after exposure for only a few days to humid environments. This degradation of mechanical properties occurs when moisture is present in any form, for example, as humidity or as a soaking solution for the TZP component. TZP components have been observed to spontaneously fall apart after times as short as a few weeks in room temperature water. This is of particular importance in living-tissue implanted devices that contain components made of this class of material. Long-term implantation of devices that contain yttria-stabilized (or partially-stabilized) zirconia components is not feasible with available materials.
One approach to preventing the low-temperature degradation of zirconia that was doped with 3 mole percent yttria is presented by Chung, et al. [see T. Chung, H. Song, G. Kim, and D. Kim, “Microstructure and Phase Stability of Yttria-Doped Tetragonal Zirconia Polycrystals Heat Treated in Nitrogen Atmosphere,” J. Am. Ceram. Soc., 80 [10] 2607–12 (1997)]. The TZP sintered material was held for 2 hours at 1600° or 1700° C. in flowing nitrogen gas.
Analysis showed that the resulting surface consisted of cubic grains with tetragonal precipitates, while the interior was only slightly affected by the nitrogen exposure. Chung reported that low-temperature degradation was prevented because degradation of TZP started at the surface, which is protected from degradation by the stable cubic phase.
An alternate material and an easy to apply method of producing stable material to prevent the detrimental low-temperature phase change are needed.