Hi. Sorry to hear of your problem :-( I modify out a lot of the high impact from Cathe's tapes because of knee problems. I would say that all of her tapes are pretty high impact, so you'll have to modify rather than try to find one that doesn't have the impact in the first place. You've got to be creative to keep your heart rate up, since Cathe relies on those power moves to make the workout intense. It's difficult to find a move to repalce jumping jacks that is as intense, but if you use very big arm movements and get down low with all the other moves (use your legs!), it can be done.
That said, I have begun incorporating a lot of less-intense cardio into my program, and have been completely satisfied with my results. You do not have to do power moves to get a good cardio workout--in fact, I find that just one intense one a week, with longer, less intense workouts (Christi Taylor is my favorite)is great for me.
Of all the Cathe cardio tapes I have, I would say that Rhythmic Step may be the easiest to modify down, because, it isn't quite as power-move based as most of Cathe's other stuff. CTX is the hardest to modify, since it has tons of power 7's and 15's Cardio Kicks is pretty easy on the knees, if you take out the jumps from the drill section, and keep the other stuff to low impact. (I actually get more out of the upper body moves in the drill section when I take out the jumps.) Circuit Max--next to impossible to modify! Step Works can be done, but there are a lot of over the tops and hop turn type moves that you may have to entirely delete, which makes the owrkout less fun than in it's original form...
Hope this helps!
Wendy