Structural modifications induced by ion implantation in metals and polymers used for orthopaedic prostheses

Abstract

Ti–6Al–4V titanium alloy, AISI 316L stainless steel, and UHMW (ultrahigh molecular weight) polyethylene are commonly used as friction materials in orthopaedic joint prostheses. The most interesting property of the titanium alloy is its high corrosion fatigue resistance. However, its friction and wear behaviour is unsatisfactory, even when rubbing against a soft material such as polyethylene; it cannot be used without a surface treatment. The 316L stainless steel has superior friction and wear characteristics, but its mechanical properties are inferior and crevice corrosion is often associated with wear. The UHMW polyethylene wears and flows. Wear is related to the surface properties, whereas creep is linked to the bulk mechanical properties. Ion implantation is a surface treatment mainly used in industry for increasing the wear resistance of metals, such as in cutting tools, and for changing the electrical or optical properties of ceramics and polymers. In the present work, the technique is applied to orthopaedic materials, to improve their surface properties and to increase the lifetime of the prostheses. The treatment does not deteriorate the bulk characteristics of the materials. As far as metals are concerned, structural modifications are produced in a thin layer of <1 μm thickness. They depend on the implantation parameters, namely, nature of the ions, energy, dose, and temperature, and are analysed using glow discharge spectrometry and grazing incidence X-ray diffraction. For polymers, ion implantation leads to two different effects: a modification of the first atomic layers, which induces an increase in wettability, and a variation of the structure at greater depths, which influences the mechanical properties. These effects are analysed using specific methods such as electron spectroscopy for chemical analysis, electron spin resonance, and attenuated total reflectance infrared spectroscopy.MST/1689