Hello and welcome to Mayo Connect.
This was not a metal allergy - there was a generation of hip implants, used in the 1990's and early 2000's where the femur head (ball) and the acetabulum (socket) were composed of chromium and cobalt, with no "cushion" of polymer between. The theory was that it would last longer in younger, more active people, so need for a revision would be far into the future (at the time about 15 years was the estimated time to revision.)

Unfortunately, unless perfectly implanted and aligned, the two surfaces wear against one another, releasing excessive quantities of chromium and cobalt ions into the body, which causes a condition known as metalosis or heavy metal poisoning. This can cause a wide variety of health issues - tissue breakdown, and affect heart, brain, thyroid, liver... The cure is to remove the offending implants, clean up the tissue damage, and use traditional implants in their place.

Over the next 6-12 months, the body rids itself of the excess metal ions and life goes on.

May I ask what brought you to Mayo Connect? If you have concerns about hip implants, there are many helpful people here who can share their experiences.
Sue