Always get your orthopedic surgeons opinion as far as the time frame for flying is concerned. Most patients I see will fly however after 4-6 weeks from the date of surgery. The same holds true in regards to hip replacements if you must.
I would prefer the patient wait until at least six weeks out after the hip replacement if possible just to avoid any possibility of dislocation.
With your knee replacement you may want to wear a compression stocking if possible to keep the swelling down , of course it is recommended you sit possibly by the emergency exit or somewhere where you can get up easily to stretch if needed. This is easier said then done due to the fact multiple people are requesting the same locations! First come first served.
The planes are packed anyway and room is at a premium. No matter what you do you will experience some increased swelling in the affected knee unless you are taking a very short flight less then an hour.
I flew to India in 2009 spent 16 hours on the plane. My knee was replaced in 1999. I did experience swelling in my right foot due to the cramped quaters of the plane. The knee or rest of the leg did not swell just the foot. Like all of us should do on those long flights, you should be geting up and moving around a bit to keep the circualtion going in your legs and the rest of your body.
Just look to keep yourself as comfortable as possible. Some increase in swelling will happen. When you get to your desination you work on getting the swelling down with elevation and some rest.
No harm is done to the knee itself.
If you are prone to blood clots or have a had them in the past please discuss this with the doctor before leaving.
Otherwise enjoy the flight.
Richard Haynes PTA
Total Joint Fitness LLC
Orthopedic Fitness and Rehabilitation Products and Services