A CPM (continuous passive motion) machine is an apparatus which is attached to the leg, and which takes the leg passively through the range of motion set by the physiotherapist.
This has the effect of minimising stiffness from internal swelling. It is of particular value after knee replacement, cruciate ligament surgery and microfracture, but may be applied routinely post knee surgery, generally only for one or two sessions a day.
The machines are generally set to allow a limited range of motion, so as to not place the swollen knee under undue stress. The speed is also variable.
CPM machines do not come cheap. For a home machine, expect to pay about $4,000!