It is an odd thing, and it doesn't really make sense, but it's true that weak hips can cause knee pain. "How is that?" you may wonder.
Look at this video taken about a week ago. This patient had a a new hip put in, so she is weak on her right (left side of the video). Watch her step down, and you will see her right hip shifts to the outside, and her right thigh and knee roll in. Because her right thigh rolls in her kneecap is tugged to the outside of its normal groove causing knee pain.
Check back tomorrow to see if her hip gets stronger and her knee pain goes away!