I rode all last summer with no issues, my first year on the road. Then, after I stopped for the winter, my left knee got puffy. I ignored it for a while, but it started to hurt more and more. So, a few weeks ago I went in and got it drained. They took xrays, did an mri, and did blood work. Nothing. So, they went in with a scope and instantly found cracked cartilage. Cleaned it up and a week later, back on the bike.
So, first thing to know is that all the tests they can run might not show anything. Second, if they go in and scope it, it's not a big deal at all. If they don't do anything when they are in there, there is no real down time. Even if they do work while in there, the down time might be minimal. I still have to be aware of my knee while it gets better, but I can ride all I want.
On a side note, I talked to my doctor ahead of time, and he told me I could watch if I wanted. When the anesthesiologist came in, he said he was going to use general on me. I said no way, I was told I could have a spinal. He didn't like that, as he didn't get to do the spinal, they called in a different guy. But, I got to watch, and it was very cool to see inside my knee. He talked to me as he worked, explained what I was seeing and what he was doing. So, it's worth asking about if you end up being scoped and want to watch.