I have an artificial hip and am not supposed to break a 90 degree angle. Can I use a Bowflex Treadclimber?

Without seeing it (the treadclimber) — and not being your doctor or physical therapist — I would say no, since you will probably need to bear your own weight on the hip, as well as deal with the resistance of the machine.

I would run the idea by your doctor and physical therapist before trying this activity.