Simulation and analysis of the wear of metal on metal articulations in artificial hip joints
Metal on metal articulations used in hip joint replacement were demonstratedth rough the 1960's and 70's to perform adequately and comparably to metal on plastic articulations but their use then fell into decline as orthopaedic surgeons preferred metal on polyethylene articulations. However, one of the main causes of failure of prosthetic hip joints was then discovered to be aseptic loosening due to particle induced osteolysis, such particles arising from wear of the prosthetic components, and in particular from polyethylene bearing surfaces. Through the late 1980's and the 1990's the concept of metal on metal bearings has re-emerged as one potential solution to the clinical problems associated with polyethylene particle induced osteolysis. In this thesis both theoretical analysis and practical testing using a 12 station MMED hip joint simulator were employed to investigate how metal on metal articulations may be optimized so as to reducet he amount of wear debris generatedth rough their use. Hence, such articulations should function effectively within the environment of a human hip joint when they form part of a total hip replacement design. To this end the effect of diametral clearance, head size, and material composition were both tested and analyzed. Finally a modular clinical design of hip joint replacemenwt as tested to assess its performance. Wear was found to be strongly related to bearing clearance in practical tests as predicted by the theory. A lower limit of clearance existed due to deviations from perfect form which were unavoidable with current manufacturing technology. A band of clearance was defined for 22, 28, and 35mm diameter bearings within which reduced wear was exhibited. Steady state wear rates, following the initial bedding in period, were generally equal regardless of total wear volume. High carbon against high carbon content cobalt chromium articulations did not produce the lowest wear contrary to previous studies in the literature. The mean hardness of, and hardness difference between, bearing surfaces influenced the wear performance of metal on metal articulations. It was possible to design a clinical metal on metal bearing having modular femoral (head and stem) and acetabular (insert and shell) components with optimized metal on metal articulation. This metal on metal device has been introduced into clinical use throughout the world.