In the years following your first knee replacement surgery, the metal portion of your implant slowly wears down this plastic portion. In fact, your regular daily activities may produce as many as two million cycles per year on these parts.
Depending on the amount of wear to this plastic piece, your surgeon may be able to replace only the plastic piece alone. In other cases, though, you may have worn through it and damaged the metal components as well. In this case the entire implant will be replaced.