Abstract
Joint replacement implants, especially in their modular forms, are subjected to wear and corrosion at various sites in their articulation, such as the bearing surfaces, the undersurface of the insert, the femoral head-neck junction and the implant or polymethylmethacrylate-bone interface. Movement of the bearing surfaces is not the only cause, as faulty implant positioning can initiate wear through impingement between two parts of the articulation.
These wear products of polyethelene or metal, in particulate form, are influential to the ultimate fate of the prostheses through the initiation of local and systemic immune reactions.
These debris are phagocytised by macrophages and phagocytised proteins are partly degraded in intracellular vesicles, where they become associated with the major histocompatability complex molecule HLA-DR. This molecule when transported to the cell membrane, interacts with CD4+ lymphocytes to activate an immune response and initiate the production of interleukin1b, interleukin 6 and tumor necrosing factor a. These cytokines mediate the inflammatory response and activate osteoclasts causing periprosthetic osteolysis.
Polyethelene and metal wear particles, in addition to their local effects, can be disseminated beyond the periprosthetic tissues and reach distant organs and regional lymphnodes. The concentrations of certain elements of metallic implants, such as iron, cobalt, chromium or titanium have been detected in lymphnodes, the liver and the spleen in levels a lot higher than normal, especially in patients with loose prostheses and, less so, in patients with stable prostheses.
The reported values of metal ions in published series vary. Thus certain investigators (Brodner et al) have reported continuous systemic cobalt release during a five year follow-up period and in levels slightly above detection values, while others (Clarke et al, Lohtka et al) have reported consistently high levels of cobalt and chromium ions in metal on metal articulations. The diameter of the femoral head appears to be a significant factor. In surface hip replacements with large diameter heads the amount of detected metal ions was significantly higher compared with total hip replacements with use of 28mm diameter femoral heads. In that type of replacement the levels of cobalt was 50 times higher than normal and of chromium 100 times higher.
Polyethelene particles, similarily have been detected in paraaortic lymphnodes in percentages similar to metal ions. However the detection of PE particles in the liver or the spleen was less, compared to metal ions, possibly due to the difficulty of modern methods to detect PE particles of submicrometre size. The relevance of the dissemination of metal ions and of PE wear debris in organs distal to the operated joint need to be carefully evaluated since certain of these elements are known carcinogens. Two studies have reported slight increase of haemopoeitic cancers in patients with cobalt alloy implants and in patients with metal on metal devices, while others have documented the development of malignant tumours in the vicinity of total hip replacements.
Since prostheses with metal on metal bearing surfaces are used more and more frequently in younger patients, these patients require careful monitoring for longer periods.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
