COBALT-CHROMIUM MOLYBDENUM METAL COMBINATION FOR MODULAR HIP PROSTHESES

The development of a metal combination for modular hip systems was mot ivated by the following observations: (1) wear particles from polyethy lene acetabular components can lead to a foreign body reaction and lat e aseptic loosening and (2) well designed all metal hip prostheses had very low wear rates, usually causing no osteolytic problems. The foll owing 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, m odular hip systems with metal combinations while maintaining compatibi lity with existing components. The realization of a metal combination consisted of the stable anchoring of a standard metal lining in a poly ethylene insert that, combined, is intended to provide adequate load t ransfer and fit to either the bone cement bed or the titanium shell. T he metal lining is manufactured from a carbide containing cobalt chrom ium molybdenum wrought alloy (Protasul-21WF). From 1988 to 1995, appro ximately 40,000 metal combinations (Metasul) were implanted. From thes e, 44 single components, with a maximum time in situ of 5.5 years, wer e retrieved and examined. The total linear wear rate averaged 2 to 5 m u m per year per component after the initial conditioning phase. Under these conditions, particle induced late aseptic loosening is not to b e expected.