Titanium is an attractive metal to use in orthopedic implants. It has a high strength-to-weight ratio, a low modulus of elasticity, and good resistance to corrosion. Titanium can be categorized as either commercially pure titanium (cp-Ti) or a titanium alloy. Colloquially, both cp-Ti and titanium alloys are often referred to simply as titanium, but the distinction is important when it comes to medical implants and devices.
Commercially pure titanium is more than 99% pure titanium, but up to 1% of its volume can be comprised of impurities. In titanium alloys, the alloy elements are intentionally added to create a material with optimal properties. The 2 most common types of titanium that are used for orthopedic implants are cp-Ti and Ti6Al4V, a titanium alloy containing approximately 90% titanium, 6% aluminum, and 4% vanadium. Similar to cp-Ti, titanium alloys also can contain impurities such as aluminum, beryllium, cobalt, chromium, iron, nickel, and palladium, among many others.
Although these impurities often are considered negligible from a metallurgy perspective, as they do not change the properties of the material, these trace elements may be present in large enough quantities to cause hypersensitivity reactions.Several weeks after implantation of a titanium alloy metal plate in the left foot, a widespread eczematous eruption developed in our patient who had no prior skin disease. The eruption was steroid responsive but did not clear until the plate was removed. Detailed metallurgy analysis confirmed that vanadium was present and was not homogeneously distributed in the plate.
The plate also was different in composition from the screws. Additional studies showed that galvanic corrosion between the plate and the chemically different screws might have contributed to the release of vanadium in the tissue.There is still a great deal of uncertainty around metal implant allergy. Allergy to metal implants can be difficult to diagnose for several reasons. Some metals are not conducive to patch testing because of their low bioavailability.
Additionally, we lack validated and standardized patch test formulas for metals that can be diagnosed by patch testing. Furthermore, there is uncertainty about what to do after allergy to a metal implant is diagnosed; in some cases (eg, with more extensive procedures such as total joint replacements), removal or replacement of the implant may be associated with increased risk of further complications