I Have Had Knee Arthoscopy On Both My Knees. I

I have had knee arthoscopy on both my knees. I know the doc says I have arthuritis in them both but absolutely have no pain thus far. I do however have a strange symptom of almost numbness or locking when I have sat for at least 90 minutes then get up to walk and I can't without standing still and hanging onto something for a bit (minute or so). Then it is like the fluid comes back in and I can move it. It takes approx. 40 steps or so and then it is normal. No pain just the sensation of giving out or locking. Doc wants to give cortisone again and again and it helps sometimes if he gets it in the correct placement. I really feel it is the fluid that helps but like I said.....no pain. Not the least bit. What do you suggest?