Hello Doctor. I have been having strong/painful stiffness behind my left knee for the last month. The pain generated in the center of the back of the knee. Then one evening while stepping down a flight of stairs .....all of a sudden I heard a loud pop and an incredible amount of raw pain in the back of the knee. What was in the center of the back of the knee shifted to the right corner of the back of the knee. Today I cannot climb or decent stairs, it will not support my weight and it generates too much pain. If I stand still for over 5 minutes the calf muscles turn rock solid with stiffness. I get radiating pain that runs from the back of the knee down the leg to my left foot. I feel I have some kind of kink or clamp behind my left leg.