That is true, bavarian - grass/dirt terrain is usually uneven and you can easily mess up your ankles, knees and back if you step in the wrong place. I didn't know your condition is that bad, though.
Most highschool tracks are made of soft dirt/gravel mix but I suppose they may be no better than the paved trails you are currently running on. Maybe you should give the treadmill a try if you can accept the indoors conditions and the physics of the treadmill.
I'm sorry about my outlandish behavior - there's too much blood in my alcohol system.