Unfortunately I don't. I sold it a year or so after the second round of knee surgery under the impression that I'd been told not to do things like that. It wasn't until recently (I just started back bike riding in April) that it dawned on me that bike riding is a low-impact activity and should be fine. I can't stand up to pedal because my knee will instantly go out from under me and I'll be on the ground, but as long as I'm sitting down I can get up some impressively steep hills (27 gears help).

Part of the reason I'm struggling with this is that I do a lot of geocaching, and end up pedaling down some pretty rough roads/trails which would destroy a road bike in short order, I think. I need to decide if I'll be riding paved roads enough to justify a road bike, or just go ahead and use the mountain bike for everything.