CHARACTERISTICS OF METALS USED IN IMPLANTS

The performance of any material in the human body is controlled by two sets of characteristics: biofunctionality and biocompatibility, With the wide range of materials available in the mid-1990s, it is relative ly easy to satisfy the requirements for mechanical and physical functi onality of implantable devices, Therefore, the selection of materials for medical applications is usually based on considerations of biocomp atibility, When metals and alloys are considered, the susceptibility o f the material to corrosion and the effect the corrosion has on the ti ssue are the central aspects of biocompatibility, Corrosion resistance of the currently used 316L stainless steel, cobalt-chromium, and tita nium-based implant alloys relies on their passivation by a thin surfac e layer of oxide, Stainless steel is the least corrosion resistant, an d it is used for temporary implants only, The titanium and Co-Cr alloy s do not corrode in the body; however, metal ions slowly diffuse throu gh the oxide layer and accumulate in the tissue, When a metal implant is placed in the human body, it becomes surrounded by a layer of fibro us tissue of a thickness that is proportional to the amount and toxici ty of the dissolution products and to the amount of motion between the implant and the adjacent tissues, Pure titanium may elicit a minimal fibrous encapsulation under some conditions, whereas the proliferation of a fibrous layer as much as 2 mm thick is encountered with the use of stainless steel implants, Superior fracture and fatigue resistance have made metals the materials of choice for traditional load-bearing applications, In this review, the functionality of currently used meta ls and alloys is discussed with respect to stenting applications, In a ddition, the ''shape memory'' and ''pseudo-elasticity'' properties of Nitinol--an alloy that is being considered for the manufacturing of ur ologic stents--are described.