Not to long ago I posted how I hurt my knee on a geared bike. I did not go to a doctor. Basically as I recuperated I got back on my fixed gear off and on. After a month I was back commuting an a ragular basis with no pain.

I think it should also be said that no matter if you are on a freewheel or fixed the gearing you choose is more importing becuse that will determine the stresses you will put on your joints.