I think that CPMs are greatly over-rated. What gets your knee going best is muscles with good tone and strength. A CPM doesn't do that - it's a continuous passive motion machine, meaning it the muscle contribute nothing to the activity. Use one by all means but do not expect it to achieve everything. It cannot and should not, be used instead of tone/strength improving exercises. For that you need walking, straight leg raises, heel slides and a stationary bike.