After the diagnostic tests showed nothing, the doctor questions her: "There must be something you're doing that you haven't told me. Can you think of anything that might be doing this to your knees?"
"Well." She said a little sheepishly. "My husband. and I have sex doggy-style on the floor every night."
"That's got to be it." Said the doctor. "There are plenty of other positions and ways to have sex, you know."
Spoiler:
