Hydroxyapatite particle migration
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HA-coatings have a complex structure due to modification of the crystallographic structure of the HA and the appearance of other calcium phosphate phases during spraying. These modifications are responsible for enhancing of the coating degradation rate by cells and extracellular fluids. Calcium phosphate debris of different sizes and characteristics is released during this degradation and phagocytosed by macrophages or integrated within the newly formed bone. The phagocytosed debris are dissolved in the low pH compartment of the cells and do not trigger osteoclast activation. Some concerns have been raised about the hypothetical particle migration to the surface of the polyethylene cup increasing the release of polymeric debris by a third body wear process. However, the presence of calcium phosphate particles is observed at the surface of polyethylene cups even when implanted with non-coated devices. Thus, the fate of calcium phosphate debris differs to that of metal and polymer due to the characteristics and degradability within the cells of this former.
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