When I was doing research about knee replacements, because I will probably need one in the near future, I read of “polyethylene wear”. I assume that has something to do with the implant wearing down, but can you explain what happens?

Part of a knee replacement implant, a total knee arthroscopy implant, is made of polyethylene, a type of plastic polymer, usually on the tibial (shin bone) and patellar (kneecap) surfaces. After a while, the wear and tear that the knee undergoes can cause the polyethylene parts to become damaged. If this wears away completely, the prosthesis will likely need to be replaced.

One thing to also be aware of is if the polyethylene is wearing away, particles may break loose and irritate the tissue. This can cause pain and swelling.