Human blood has a salt concentration of about 0.9% (seawater is 3.5%), which makes it corrosive to metals. Hip implants go back to the late 1970's, so they've been around a long time, and have markedly improved over the years. The greatest wear item is the very dense plastic cup that is part of the acetabular replacement (the socket portion of the joint, part of the hip bone). Early ones would wear out in 5-7 years, now they can go 12-15 years. The metal portions are made mostly of cobalt steel, very strong and generally benign in the body, but some people seem to be more sensitive than others to foreign bodies, which any joint prosthesis actually is. There was for a while a hip replacement that uses a metal-on-metal joint, designed to reduce that wear, but there were problems with deterioration and deposition of metal in the tissue. Two disparate metals in a saline environment are essentially a battery, and the current flow will cause one of the "poles" to corrode, requiring replacement. When I first started working in the O.R., in 1984, there were two companies that made total hip replacements, now there are close to a dozen different types, so a patient can get a "custom fit" for their size and activity level. The patient population has gotten younger over time; originally, only older patients who were not very physical got them, now athletes get them (think of Bo Jackson), and people who get them are living long enough that they may need a revision at some point.
I cannot count the number of hip and knee replacement surgeries I have assisted on or circulated on during my 37 year career in the O.R. as a Registered Nurse. This includes revisions to replace the plastic cup, as well as replacements for corroded prosthetic parts, and of course, the primary joint replacement itself.
The guy I do landscaping with is 68, and has had both his hips replaced, one in 2012 and one in 2014. It amazes me what he is able to do, there's a lot of heavy lifting, bending over, sitting in odd positions, and he is about 230 pounds, so they get a workout. he says all the time that he would be unable to do most of what we do if he'd not had his hips replaced, from degenerative joint disease. he will probably outlive his joint replacements and need a revision in another 5 years