You say that it is a totally different kind of pain, so I'd trust my gut feeling, and have the doctors rule out anything else besides the RSD. There's more to a knee than you think, it's a very complicated joint, and there could be all sorts of things wrong with it.

My cartilage pain is a deep achy feeling, sometimes sharp, that lessens when I take it really easy. I can't do stairs, for instance (I lack strength for a full flight of stairs) or the knees will start acting up to such a degree that I could basically scream in pain, it enhances my RSD too (it will cause a flare-up in the knees like you wouldn't believe). It will then take more than 2 weeks for the knee pain to lessen to a tolerable level (and no my regular meds don't cut it at all). It's mostly my left (unoperated knee) that gives me the most problems and that has the most intense flare-up. I have to be careful with the knees, because if it comes to the day where I will need to have the knees replaced, that's an operation I won't be able to undergo due to RSD.