The observation that a small number of patients with first-generation MOM prostheses exhibited good clinical and radiographical results after 20 years in vivo led to the development of second-generation MOM hip prostheses, and in 1988 the Metasul prosthesis was introduced into clinical practice. This comprised of a cobalt chrome alloy femoral head articulating on a cobalt chrome alloy acetabular cup. Over 200,000 Metasul combinations have been implanted to date. Short-term clinical performance has been encouraging; with low wear rates, and few prostheses requiring revision. However, long-term clinical performance is as yet unknown.