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An
artificial knee joint is a more challenging problem. The components move
with respect to each other and the implant life should be as long as possible.
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Corrosion
of the metal structural components and fatigue in a corrosive ambient,
wear of the polymeric material and fretting with the release of small particles,
and the bioactivity of these components must all be considered.
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Fixing
the implant to the bones involves a stem that is placed in a hollowed out
region of the bone. Bone cement is used to make a mechanical bond with
the bone. In more modern devices, the surface of the stem may be porous
and coated with hydroxyapatite (the mineral component of bone) to encourage
bone growth into the implant for better bonding.
· The
wear of the ultrahigh molecular weight polyethylene used for the surface
of the components can produce sub-micron debris that is shed into the surrounding
tissue. The immune system attempts to remove these particles, but the enzymes
released attack the bone and can destroy the prosthetic junction. |
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