In vitro evaluation of hip prostheses

Sammanfattning: Aseptic loosening and periprosthetic osteolysis are the most common complications (5-10% in 10 years follow up) after total hip arthroplasty. Polyethylene debris appears to play a major role in this process. To avoid polyethylene wear induced osteolysis, second generation of metal on metal bearing hip prosthesis made of cobalt alloy has been introduced. The aim of this study was to assess tribological and bio compatibility characteristics of metal on metal hip prosthesis systems in vitro prior to the clinical trial. A hip joint simulator was used to asses the tribological. characteristic of the hip prosthesis system. The hip joint simulator was validated against retrieval reports. The wear rate from metal on metal bearing prosthesis system was compared with the wear rate from metal on polyethylene prosthesis system and the wear phases in metal on metal prosthesis system were characterised. The effect of pseudosynovial fluid from patients with loosened hip prostheses on osteobalst proliferation was examined. Finally, the influence of cobalt on human osteoblast proliferation and function was studied. By reproducing the head size effect reported by Charnley the joint simulator showed to be a reliable tool to evaluate wear rate. The use of bovine serum as synovial fluid substitute in the simulator is clearly superior compared to water. The wear rate of metal on metal hip prostheses is 100 times less compared to metal on polyethylene prosthesis systems. Characterisation of wear phases in metal on metal prosthesis systems show a biphasic wear pattern with an initial rapid followed by a slower steady state phase. This indicates that patients with metal on metal hip prostheses will be exposed to a higher volume of metallic wear in the first year after implantation compared to the following years. The pseudosynovial fluid from patients with loosened prosthesis systems inhibited the proliferation of human osteoblast cells. The inhibition could be a result of accumulation of cytokines and /or dissolved metallic wear in synovial fluid. Penetration of pseudosynovial. fluid in bone implant interface may inhibit osteoblasts and cause prosthesis loosening. Human osteoblast's function and proliferation were reduced in the presence of cobalt. The osteoblast activities, such as DNA synthesis, osteocalcin and collagen type I production have been reduced and the amount of interleukine 6 was increased with increasing cobalt concentration. In vitro evaluation of metal on metal prosthesis systems where tribological properties are assessed provide us data about expected wear rate. The wear from prosthesis system can be dissolved in body fluid and influence the homeostasis of the cells both in periprosthetic tissue and distanced organs. Our biocompatibility studies showed that psuedosynovial fluid influenced osteoblast cell proliferation and cobalt inhibited the ostoeblast function and proliferation. This thesis advocates combined tribological and biocompatibility assessments of hip prosthesis prior to the clinical trial. In vitro evaluation of prosthesis systems and material may prevent disastrous clinical outcomes after total hip arthroplasties.