Cobalt chromium molybdenum metal combination for modular hip prostheses.

The development of a metal combination for modular hip systems was motivated by the following observations: (1) wear particles from polyethylene acetabular components can lead to a foreign body reaction and late aseptic loosening and (2) well designed all metal hip prostheses had very low wear rates, usually causing no osteolytic problems. The following challenges had to be met: (1) metal alloy with the maximum wear resistance; (2) the optimal clearance (difference in diameter) between 28-mm ball head and acetabular component; and (3) equipping modern, modular hip systems with metal combinations while maintaining compatibility with existing components. The realization of a metal combination consisted of the stable anchoring of a standard metal lining in a polyethylene insert that, combined, is intended to provide adequate load transfer and fit to either the bone cement bed or the titanium shell. The metal lining is manufactured from a carbide containing cobalt chromium molybdenum wrought alloy (Protasul-21WF). From 1988 to 1995, approximately 40,000 metal combinations (Metasul) were implanted. From these, 44 single components, with a maximum time in situ of 5.5 years, were retrieved and examined. The total linear wear rate averaged 2 to 5 micrometers per year per component after the initial conditioning phase. Under these conditions, particle induced late aseptic loosening is not to be expected.