Material Wear

The wear of the friction surfaces of the prosthesis causes the appearance of particles of worn material in the joint, with which the body reacts to try to eliminate them, producing a chronic inflammation that leads to the destruction of the bone around the implants (osteolysis) until they are loosened from the bone causing pain to the patient. The wearer of a loosened prosthesis begins to notice discomfort in certain movements, such as rotation, until it becomes pain in load and even rest after activity. The difficulty of surgery to replace a prosthesis lies not only in tackling the joint for previously damaged tissues and scar tissue, but also in achieving a good integration of the implants in lower quality bone due to osteolysis and the damage caused by the mobility of the components on the bone. In many cases there are bone defects that have to be recomposed, either with prosthetic material or with bone grafting from the patient, from a bench or bone substitutes.

The failure of total hip and knee arthroplasties due to the wear and tear of the interposition polyethylene, whether or not associated with periprosthetic osteolysis, is one of the problems that has aroused the greatest interest in the world orthopaedic community at the turn of the century. The great benefit that patients and society obtain from these arthroplasties is limited, in the medium and long term, by the wear that forces the prosthetic replacement after the failure of the implant. Intense research has been carried out on materials over the last decade, especially polyethylene, so it is important for any orthopaedic surgeon to know the current situation and trends in this field, as well as proposals for the future. We have had surgeries where even the material can completely fracture. Further research is needed to understand why these materials fail?

During this time continuous improvements have been made in this material trying to extend its useful life in the prosthesis. In spite of its good performance, polyethylene presents aseptic faults that trigger the subsequent revision of the prosthesis with the consequent negative effects on the patient and socioeconomic losses. One of the failures is associated with the presence of submicroscopic particles originating in wear, which cause a chain of complex reactions that ultimately generate bone loss around the prosthesis, a phenomenon known as osteolysis, with the corresponding loosening of the prosthesis.

To combat this problem, polyethylene is irradiated with gamma radiation or electron beams. Several techniques have been used for this purpose, such as eliminating radicals by heating the material after irradiating it, thus increasing its resistance to oxidative processes inside the body, but it also entails a loss of mechanical properties such as fatigue resistance and toughness, important aspects in the tibial components of knee prostheses. It has also been experimented with the incorporation of vitamin E into polyethylene, a natural antioxidant that traps radicals and avoids subjecting the material to thermal processes.