The problem can be traced back to the companies' sterilization practices.

Howmedica, as well as several other companies, used a technique known as gamma irradiation in air to sterilize the devices (hip and knee prostheses) made of high molecular weight polyethylene, which is like a pliable plastic. The sterilization process caused the polyethylene part of the devices, once implanted in the body, to break up into small pieces that lodge into the patient's hip or knee joint. In the body's fight to rid itself of those pieces, the immune system also turns on the patient's healthy bone, causing it to decay, a condition known as osteolysis . These patients inevitably need a second surgery to replace the device long before expected.

Oxidation, which is a time-dependent byproduct of gamma sterilization in air , has an adverse effect on the material properties and wear resistance of polyethylene. Oxidation occurs when oxygen combines with free radicals in the polyethylene that are generated by radiation. The oxidation can occur during irradiation, or it can occur over time as oxygen diffuses into the polyethylene and combines with residual free radicals.

Recognizing the adverse effects of oxidation, some manufacturers have abandoned gamma sterilization and began using ethylene oxide (EtO) or gas plasma to sterilize the components, thereby avoiding immediate and long-term oxidation. However, particularly in the absence of oxygen , the free radicals generated by irradiation can combine to each other, forming "crosslinks" between two carbon atoms on adjacent polyethylene molecules.

When packaging/sterilizing in a low-oxygen environment (e.g., partial vacuum, inert gas, or with an oxygen scavenger), the degree of oxidation that will occur is reduced. In order to gain the benefit of the crosslinking, some manufacturers continue to sterilize their polyethylene components with gamma radiation.

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