Tribological optimisation of hip endoprostheses by developing an artificial synovial fluid and a novel hip tribometer

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In Germany, around 200,000 people opt for an artificial hip each year and have their hip joint replaced by an endoprosthesis. The operation is one of the standard procedures in orthopedic surgery. The hip prosthesis is exposed to heavy loads during movement, especially the sliding pairing between the femoral head and the acetabulum and is therefore also subject to heavy wear generation. The mechanical work of the two friction partners creates considerable abrasion of material, which can lead to inflammation of the surrounding tissue and subsequent loosening of the endoprosthesis which than requires a replacement. However, a revision surgery of the hip prosthesis is more difficult than the first implantation, because the fixation of the endoprosthesis in the bone is more difficult compared to the first operation. The cause of this is the inflammation caused by the metal and plastic wear from the artificial joint, which leads to a decrease in the bone substance. The lubricating fluid of a healthy hip joint is the synovial fluid produced by the synovial membrane. Implantation of an endoprosthesis and joint inflammation change the composition of this synovial fluid, which leads to a reduced lubricity of the fluid. In the present project, the properties and chemical composition of the synovial fluid before and after prosthesis implantation will be examined in detail. With the help of the results obtained in this project, an artificial synovial fluid will be produced that meets the requirements for optimal lubrication of the implant. To investigate the tribological properties, a new test stand will be developed, which is designed as a femoral ball / socket test stand and can measure the friction directly on the test pairing, taking into account a more realistic joint-like amount of lubricant (approx. 3-10 ml). This makes it possible to identify an optimal lubricating fluid for each material pairing, to check the similarity between artificial and healthy synovial fluid, and the positive influencing factors on the lubricating properties. The aim is to optimize a synthetic synovial fluid for the use in the patient with hip implants in the future.