Metal on metal hip prostheses are critically reviewed as examples of a tribologic system. Because the inputs (load, velocity, fluid) are given, the designer has relatively few options, which can be exploited only if operative lubrication and wear mechanisms are known. Nothing can be done about the fluid, but film thickness can be influenced by macrogeometry (diameter and clearance) and by provision of film enhancing features. Microgeometry (surface topography, including asperity geometry and spacing) is a powerful contributor, but has not been sufficiently characterized or controlled. Compared with metal/polymer bearings, the total knowledge is small, and extensive research is called for. The high cost and long duration of hip joint simulation tests makes them more suitable to evaluate devices and to validate findings from bench tests. A particularly important task is to establish which experimental fluid adequately simulates the fluid formed around an all metal prosthesis. Much basic work can be conducted in bench tests, particularly in a form of oscillating twist compression test with cyclic loading, which induces squeeze film and mixed film lubrication. There is need for friction and wear studies with different metals and coatings, with controlled surface topography, and with lubricants specially formulated to clarify the roles played by various constituents of the fluid formed in metal on metal joints.

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