I don't have a medical condition so severe to cause me to consider not riding...yet...but I do have a knee that is bad enough it has caused me to alter my bike...
The reason I removed the Boarzilla from my SEUC and installed the Jackpot 2-1-2 pipe is to allow me to hold my right leg much closer to my bike when stopped. My right knee was severely damaged in a skiing accident about 12 years ago. Today, it has very minimal side to side stability and the only real solution is to replace it. My doctor has encouraged me to delay replacement as long as possible, as the technology continues to improve, and if I replaced it now (at my current age), it is likely I would have to replace it again in later years.
The upper loop of the rear cylinder pipe of the Boarzilla is high enough and sticks out far enough that I have to hold my right leg further away from the bike, putting to much strain on my knee. Twice last year, while at a complete stop, my knee "buckled", once causing me to have to lay down my bike. (It's happened about 5 times in the previous 4 or 5 years.) When this happens, my knee almost immediately fills with fluid, which takes days to re-absorb. Of course, the worst instance last year occurred when we were about 1500 miles from home, riding two up and the bike loaded to the gills... And, of course, with both instances, my wife was sitting behind me, which has caused her a bit of concern. There's no issue at all with riding, just stopping or sitting still. The 2-1-2 pipe is tucked in closer to the bike and is lower, allowing me to plant my foot and keep my knee much "straighter". I'm sure I've lost a few ponies with the change, but I figure my 120 has a few to spare... The EMS seems to have adapted well to the pipe change...