Zirconia Ceramics as Biomaterials - Radioactivity Issues

Background

Zirconium compounds are refined from naturally occurring ores, notably zircon, and usually contain trace amounts of other elements depending upon the source of the original ore.  In particular, zirconia typically contains trace amounts of radionuclides of the actinide series such as ²²⁶Ra and ²²⁸Th.  As a consequence there have been some concerns about the use of zirconia ceramics as implant materials.

The Radioactivity of Zirconia Biomaterials

However, the radioactivity of zirconia is negligible.  For example, the radiation emitted by a 3 mol% Y₂O₃-ZrO₂ powder was the same order of magnitude as alumina powder, both of which were several orders of magnitude less than that typically measured for water, milk, vegetables and meat (in Europe).  The radioactivity of Y-TZP zirconia femoral heads were shown to be similar to that of alumina and cobalt-chrome alloy femoral heads accepted for human implantation.  The radiation dose of each material was well below European radiation limits specified for general external exposure of the human body and also for local internal exposure of organs and tissue, and was no more than the ambient radiation of the surroundings.

The Radioactivity of Commercially Available Zirconia Powders

Similarly, the radiation levels of several commercially-available zirconia powders (Nilcra Ceramics, Melbourne, Australia) measured by the Australian Radiation Laboratory were found to be minimal and well below acceptable limits.  The Radiation Health Committee of the National Health and Medical Research Council of Australia consider that magnesia partially stabilised zirconia femoral heads made from these powders would pose very little hazard to surrounding tissue.

Radiation Absorption

In a dense ceramic, almost all the radiation will be absorbed within the bulk of the material.  Any external radiation could come only from radionuclides at the surface in the case of α-emission, or from radionuclides in the first few millimetres in the case of β-emission.  Furthermore, in the case of artificial hip joints, any radiation from the femoral head will be absorbed mostly by the ultra high molecular weight polyethylene acetabular cup or the metal femoral stem.  The radiation risk of zirconia ceramics is negligible and certainly no more than that of alumina.

Primary author: Dr. Owen Standard.

Source: Abstracted from Ph.D Thesis “Application of Transformation-Toughened Zirconia Ceramics as Bioceramics”, University of New South Wales, Australia, 1995.

For more information on this source please visit Owen Standard.