I have had meniscectomies in both knees and now have osteoarthritis. I still ride fixed (48/19) but use a brake. All my geared bikes use compact chain sets as my knees really don't need or like to grind at 60rpm. Ride sensible low gears, keep your knees warm and you should be ok.
Riding around on track ratios pulling whipskidz brklz is a mugs game if your body is worn out.