Silly Internet Game
" And if a large piece wedges itself between the bones, the knee may suddenly lock. "
The game instructions: Grab the nearest book, open it to page 23, find the 5th sentence, and post its text along with these instructions. I would add to the instructions: point back to where you got the idea so that we can follow the threads.
( And yes, the quoted sentence applies to me. I am not looking forward to visiting the doctor. )