Metallic materials used for implants have a passive layer which protects the surface from the leaching out of metallic ions. Co, Cr and Ti ions have been found to show cytotoxic effects when the levels reach significant concentrations. Ion levels in the tissue result in cell death by necrosis and apoptosis, the rate of which is influence by ion concentration. The presence of dying cells can eventually stimulate inflamation and rejection of orthopedic implants.
Since wear particles are shown to result in a bioactive response when in the micron sized range, it is thought that nano-sized particles would be relatively inert.
However the effect of their increased surface area to volume ratio cannot be ignored. As a result of their larger surface area particles are more likely to release ions. The release of ions, if toxic can result in genotoxic and cytotoxic reactions. The release of ions has been found to not be limited to metallic debris but has also been reported for ceramic wear debris.
Although different wear particles (polymer, elastomeric, ceramic and metal) do influence the cell response in the body differently the most important parameters are particle size and concentration. Larger particles in the micron range are more bioactive, and have a greater effect on initiating the cell responses that lead to osteolysis and implant loosening.
Wear particle studies during biomechanical testing are important because it is the debris produced in vivo which influences whether an implant will be sucessful or not.
|
|
