Hi Patricia,
Sorry for the late response. There are many things you can and should do to protect your knees while doing Dragon & Tiger, or any qigong. Bruce has an excellent section devoted to learning to protect your knees on pages 59 – 72 of Volume 1 of his Dragon & Tiger Medical Qigong book. Bruce covers five components that are essential to the health of your knees, and suggests ways to practice them until you can integrate them fully into the movements.
In addition, make sure that you only move or turn as far as you can go comfortably, even if this means that you are only doing a very small movement. Over time your comfortable range of motion will increase. If you cannot turn at all without pain or discomfort at first, simply do the movement without turning.
I hope this is helpful.
Dan Winter