Suggest Treatment For Burning Sensation In The Knees

My mother stretched one night and heard a loud pop in July of 2010. Since that day she has had chronic pain in her left leg and right side of lower back. She also has a burning sensation in her knees and her feet are numb. She has seen about 5 different dr's and they do not know what it is. Not nerve issues or bones. Please help, she is 57 years old.