Plastic Spacer Exchange
The long term concern with knee joint replacements is wear of the plastic spacer. If it becomes badly worn (which it will do beyond 12-15 years) the plastic wear particles set up cellular reactions about the implants, resulting in loss of bone support and implant loosening.
The implant then needs to be re-done ('revised') before too much bone is lost.
A revision procedure carries substantially increased risk of complications. It is technically straightforward to change the plastic spacer before it starts to show advanced wear. I recommend this should be done beyond the 10-12 year point.
This is a similar principle to changing the camshaft belts in a car engine when they reach the end of their service life. Should the belts fail, major damage can be done to the engine. Similarly, we need to change the plastic spacer in a knee joint replacement before it wears and causes loosening. If the implant is well fixed and otherwise undamaged, it should theoretically last another 10-12 years (until the next plastic exchange!).