Zirconia biocompatibility

Fluoride-Modified Zirconia as a Biocompatible Stationary Phase... 

Some ceramics exhibit biocompatibility in the human body. Alumina and zirconia are employed as the ball for hip replacements. Hydroxyapatite (Caio(P04)6(OH)2) is used as bone replacements, as ocular implants, and as a coating for metallic implants. Ceramics also find application in dentistry for restorative work. 

Rieger, W. (1993) Biocompatibility Studies on Zirconia and Alumina in Orthopaedic Joint Applications, Ascona, Switzerland. 

Coated substrates used in implantology can be ceramics such as alumina, zirconia or titania, metals such as magnesium and titanium and their alloys, and austenitic medical stainless steels, as well as several biocompatible polymers. In the following text, some recent research will be reviewed. 

As a metal, zirconium is used in bone and muscle implant materials. The combination of mechanical properties and excellent biocompatibility makes tetragonal zirconia polycrystal (TZP) ceramics one of the best biomaterials for prosthetic joints. (Covacci et al. 1999). 

Biomedical-grade zirconia was introduced 20 years ago to solve the problem of alumina brittleness, and the consequent potential failure of implants. The reason for this is that biomedical-grade zirconia exhibits the best mechanical properties of oxide ceramics as a consequence of transformation toughening, which increases its resistance to crack propagation. Likewise, partially stabilized zirconia shows excellent biocompatibility, and it has therefore been applied to orthopedic uses such as hip and knee joints [255]. 

This concept of biocompatibility, which equates the quality to inertness and biological indifference, has resulted in the selection of a portfolio of acceptable or standard biomaterials which have widespread usage. These range from the passivatable alloys such as stainless steel and titanium alloys, the noble metals gold and platinum, to some oxide ceramics such as alumina and zirconia, various forms of carbon and a range of putatively stable polymeric materials including silicone elastomers (poly-siloxanes), polyolefins, fluorocarbon polymers and some polyacrylates. Of course, if this was all there was to biocompatibility, there would be few problems other than optimizing inertness and there would be little to write about. 

Biocompatible high-strength ceramics have recently been obtained from hot-pressed mixtures of hydroxyapatite and zirconia. Sintered composites are reported to have excellent fatigue resistance [43-45]. 

Schafer et al. used several spectroscopic techniques to characterize the surface species on phosphate-modified zirconia particles. Their results show that phosphate merely adsorbs on the surface of zirconia under the mildest phosphate concentration, i.e., neutral pH, room temperature, and short contact times. However, at acidic pH and higher temperarnres, esterification of the phosphate with surface hydroxyls takes place as the kinetic barriers are overcome. The solid NMR studies clearly show the presence of covalently bound phosphate. This phosphate modification effectively blocks the sites responsible for the strong interaction of certain Lewis bases with the zirconia surface, resulting in a more biocompatible stationary phase. Unlike fluoride-modified zirconia, phosphate-modified zirconia behaves as a classic cation exchanger and not as a mixed-mode medium analogous to hydroxyapatite, despite spectroscopic evidence of zirconium phosphate formation on the surface. This limits the applicability of the supports, as most proteins and enzymes are anionic at neutral pH. Nevertheless, its ability to separate proteins with high p/ values still deserves much attention. The preparative-scale separation of murine IgGs from a fermentation broth demonstrates the utiUty of the supports for solutes that are retained. 

It is likely that cordierite, titanate and zirconate ceramics will record the most rapid rates of growth, due to their uses in environmental systems, medical products, electronic components, and household appUances. A continued expansion in electronic component shipments will provide opportunities for titanate per-ovskites and other ceramics. Nonetheless, cordierite, titanates and other ceramics will undoubtedly benefit from a continued, environmentally driven trend to reduce the amounts of particulates, nitrogen oxides (NO,), and sulfur oxides that are released into the atmosphere. Technological advances in the medical product market will also provide many opportunities, notably for monohthic ceramics such as alumina and zirconia used for femoral balls in hip endoprostheses, as well as biocompatible hydroxyapatite and tricalcium phosphate coatings for the metal stems of hip implants (see also Chapter 10). Likewise, dental ceramics wiU continue to experience high growth rates through 2010. 

Bioceramic Applications The performance requirements of yttria-stabilized tetragonal zirconia polycrystal (TZP) to form biocompatible, strong components for use as hip, knee, and dental prostheses, and which demonstrate long-term resistance against aggressive body fluids and mechanical wear and tear, during a predicted lifetime of 15-20 years in the human body, include ... 

Biocompatibility of Zirconia Today, substantial controversy persists in relation to... 

Another noteworthy effort at nanocomposite fabrication applied ceramic nanoparticles to a ceramic material to enhance osteoconductivity and mechanical performance. Nawa et al. [49] developed a ceria-stabiHzed tetragonal zirconia polycrystal (Ce-TZP) ceramic and incorporated alumina (AI2O3) nanocrystals into it via wet chemistry methods for load-bearing bone applications. Further studies of this material investigated its ability to induce apatite formation [50], in vivo biocompatibility, and resistance to wear [10] with favorable results. 

High density zirconia oxide showed excellent compatibility with autogenous rhesus monkey bone and was completely nonreactive to the body environment for the duration of the 350 day study [Hentrich et al, 1971]. Zirconia has shown excellent biocompatibility and good wear and friction when combined with ultra-high molecular weight polyethylene [Kumar et al, 1989 Murakami and Ohtsuki, 1989]. 

Diffusion bonding eliminates any foreign material as needed in brazing so it would be preferred for implantable medical device applications. Alumina can be diffusion bonded to a few biocompatible metals including tungsten, platinum, molybdenum, stainless steel, and niobium [58,61]. Zirconia has been successfully diffusion... 

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