Patellofemoral arthroplasty (PFA) is a partial replacement of the knee, involving the kneecap portion of the joint. This may be an option for a knee that has only worn out in one of the three areas of the knee.
If the cartilage is lost from underneath the kneecap (patella), then climbing stairs and getting up and down out of a chair can be very difficult. A metal and plastic replacement of just that area may be an option, but fewer people qualify for this than a total knee replacement. It is best to consult your surgeon for more details.