The degradation of UHMWPE and polymers in general is that process such that there is an alteration of the chemical structure of the material with a decrease in molecular weight. Degradation decreases the mechanical properties of polyethylene and, therefore, causes premature wear.

The degradation of a polymer occurs with the breaking of certain chemical bonds and the creation of new free radicals when, for example, the material is exposed to more energy than the chemical bonds themselves of which the molecular chains of the polymer are made. This process in the presence of oxygen is called oxidative degeneration or more simply oxidation.

Once triggered, the oxidative process is irreversible. As new radicals are formed and react with oxygen, they trigger breaks in other chemical bonds that generate other radicals and so on, creating a chain reaction that is self-perpetuating at increasing speed. The rate of oxidation depends on the number of radicals present and the amount of oxygen that may be present in the atmosphere during polyethylene processing and sterilization processes into which it may penetrate by diffusion during product storage or once implanted in the patient.

Polyethylenes used in arthroplasty are typically subjected to high doses of energy in the form of gamma or beta irradiation to sterilize or to increase the degree of cross-linking of the polyethylene. Such irradiation produces the formation of active free radicals within the material. Most free radicals recombine to form new cross-links in the amorphous phase of polyethylene, cross-linking precisely. Instead, free radicals that have formed within the crystalline phase of polyethylene remain trapped for a long time without recombining into new bonds. Such radicals in the presence of oxygen are responsible for the progressive breakdown of polymer chains through oxidation cascade reactions and consequently, are responsible for the reduction of the mechanical properties of polyethylene.

Oxidation results in the degradation of the molecular structure of polyethylene and thus over time causes its mechanical and tribological properties to progressively decay.