I biked about 250 miles last month on the trainer. It was all good until that last ride, and now my left knee hurts. It seems to hurt in exactly the same spot it hurt prior to the accident after I did an energetic ride, so I can only assume it's not related to my broken hip at all, but a regular old case of strain. It's not too bad at all, although I took six days off bicycling, and when I go back to it tomorrow, I'll be ratcheting down the speed from 20 mph to something more like 16. Nice and easy with a bit of Advil and ice.
Just so I have a documentation to compare against next time, left leg, outside of the knee the nobby bit is somewhat tender, and then the knee itself sort of locks a little with an ache, particularly after using the stairs, but not always. If I sit crosslegged with my right leg resting on the left, the pressure just above the knee translates into a bit of an ache on the top, side and bottom (not under the kneecap proper). It's not a minor bit of pain when it kicks in, it's more of a quick ramp up that's rather surprisingly in intensity. The closest comparison I have is a kidney stone without that level of pain.
Interestingly, I had an IT band issue before the accident (that the accident eliminated) and one of the symptoms of a recurring IT band issue seems to be pain that translates to the knee. I hope it's not coming back. I remember last time this happened, it was just before spring, and transitioning off the trainer to outside, where the bicycling is a little more varied in pace and on/off the bike happens more frequently, seemed to resolve the problem. No chance of that for another month, so I'll have to work at keeping it healthy until then.