There are so many factors that come into play, both with deep knee bends and knee problems in general. First of all, you stress your knee less if you are on the ball of your foot with your heel lifted and your chest over your knee over your toes. If you do a deep knee bend flat-footed without pushing your chest forward, it's a lot more strain on the knee. Also, in the long run, knee problems for skaters commonly develop from an imbalance in the muscles surrounding the knee, not just deep knee bends. That's why cross-training is important. The repetitive movements in figure skating tend to build outer quads more than inner quads and hamstrings, creating an imbalance. I know there is at least one physical therapist on this board who can explain further, but this is what I have learned so far from mine.