Combining dissimilar metals in orthopaedic implants: revisited

The use of metals as implant materials has become common practice in the field of orthopaedics. A wide variety of conditions are treated with metallic implants, and designers have used an assortment of materials to meet the unique mechanical demands of each application. The majority of implants used today, whether pins, plates, screws, or total joints, are made of cobalt-chrome alloy, stainless steel, or titanium. Common metallurgic wisdom cautions against bonding dissimilar metals in a biologically active environment. Surgeons have therefore shied away from combining dissimilar metal implants because of the fear of inciting corrosion that could potentially compromise the implants and lead to aseptic loosening, implant failure, or adverse biological reaction in host tissue. As surgical reconstruction and arthroplasty options expand with the advent of newer implants and expanded operative techniques, the orthopaedic surgeon will increasingly be faced with weighing the risks and benefits of combining implants made of dissimilar metals in a patient. Here, the authors examine the origins of the concern over using mixed metals, discuss mechanisms of corrosion as they relate to surgical implants, and review both in vitro and in vivo studies concerning the most common combinations of dissimilar metals in order to guide the surgeon in choosing implants.