I am not a doctor or physical trainer.
 I recommend seeing a/your doctor.
 That having been said, if you want cardio, you want something relatively low-impact for your joints.
 I suspect bicycling (real bikes or stationary at a gym) would be easier on your knee & ankle and joints in general than the treadmill or ellipical machines, for example. If your joints can handle if there are rowing machines.
 And perhaps better for joints, as well as good for heart/lungs and overall body work: swimming!
"Habe nun, ach! Philosophie, Juristerei und Medizin, Und leider auch Theologie Durchaus studiert ..." (Goethe, "Faust")
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