The degradation of UHMWPE and polymers in general is a process that alters the chemical structure of the material that diminishes the molecular weight of the material. Degradation diminishes the polyethylene mechanical characteristics and therefore, causes precocious wear.
Degradation of a polymer comes with the breakage of the chemical bonds and the creation of new free radicals, when for example the material is exposed to a quantity of energy superior to that of the chemical links themselves that make up the molecular chain of the polymer. In the presence of oxygen this process is called oxidative degeneration or oxidation.
Once the oxidation process begins, it is irreversible. The formation of new free radicals that react with oxygen then trigger the breakage of other chemical ties which then generate other free radicals and so on, creating a chain reaction that quickly self generates. The oxidation velocity depends on the number of free radicals present and from the amount of oxygen that could be present in the atmosphere during the manufacturing and sterilization processes of the polyethylene, which can also penetrate during the storage of the product or once implanted in the patient.
Polyethylene used in joint replacement systems typically undergoes elevated doses of energy in the form of gamma or beta ray irradiation to sterilize or increase the polyethylene level of crosslinking. Such irradiation produces the formation of active free radicals inside the material. The major part of the free radicals recombine forming new traversal links during the amorphous stage of the polyethylene – the crosslinking stage. The free radicals instead, that are formed during the polyethylene crystalline stage long remain trapped without recombining into new links. In the presence of oxygen these radicals are responsible for the progressive breakage of the polymeric links through oxidative chain reactions and consequently, are responsible for the reduction of the polyethylene mechanical properties.
Oxidation determines the degradation of the polyethylene molecular structure and therefore, in time causes a progressive decline of the mechanical and tribological properties.