A plastic implant device for a mammal that contains a rare earth metal compound tracer and a method for detecting degradation such as wear of the implanted device are disclosed. The tracer can also be present with a separate antioxidant or the tracer compound can be can be the salt of a C.sub.6-C.sub.22 unsaturated carboxylic acid. The rare earth metal compound tracer is released when the prosthetic is worn down or otherwise degraded in the mammalian body in which it was implanted. The presence and amount of released tracer present in a body fluid or tissue sample measured and is proportional to the degree of degradation of the implant.