I know they're expensive, but they've really helped me. There are other stirrups out there that bend and stuff for a lot less. I can't comment on those because I don't have them.
I do have knee problems when I'm not riding. I can't run for long because my knees will hurt (so bad that I start limping) I also can't have my legs straight for a long time (like laying down or standing) as it gets really painful. So when I'm standing I have to keep shifting weight, and when I'm laying down I either need a pillow under my knees, or I need to keep them bent.
My grandfather had bad knees and I was told I got his knees. I've dealt with it for so long, I don't really pay it any attention anymore. I just know what I can and cannot do anymore. I never went to a doctor to look at my knees.. figure he would say there were bad and would want me to either get knee surgery, stop riding
, or have them in a brace.. none of which I wanted to do.
I have found when I ride bareback they don't hurt at all.
Here's a pair for $80 that's said to alleviate knee pain Intec UMS 6-Way Stirrup Irons - Dover Saddlery.
This one has a double joint so it flexes. This one's $45 Horse-S Jointed Irons - Dover Saddlery...