Today Vitallium is used for surgical implants. It is a casting alloy which, with advances in casting technology, is also used commercially for making instruments of fairly complex shape. Because of its expense, however, it is not widely used in Japan. Instead, a series of 18-8 Mo alloys are used in Japan even though of insufficient strength. Used over a long period of time in the body, especially for the purpose of preserving structual functions as part of the human skeleton, it often corrodes,

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... resulting in either abnormalities in tissue cells or, because of its insufficient strength, danger of bending and breaking with aging. In spite of a marked advance in fracture treatment, we have hardly any suitable materials for making instruments appropriate to the internal fixation of fractures in Japan. We, therefore, conducted various experiments to develop an alloy with sufficient corrosive resistance and strength that could be formed into a complex shape to take the place of Vitallium alloy, finally succeeding in developing an alloy we call "COP". The characteristic properties of COP may be summarized as follows: 1. The main components are 20% Cr, 20% Ni, 20% Co and 4% Mo aside from 0.2% P. 2. As it contains "P", it shows a marked age-hardening. In its molten state its machinability is excellent, and later it can readily be hardened by heat-treatment. 3. It has not only a marked yield point and tensile strength but also has toughness in elongation and reduction of area, showing a strength which surpasses Vitallium. 4. Its corrosive resistance is great. 5. Its cost is far cheaper than Vitallium.