It's a scenario familiar to many pet owners - your dog is running across the yard chasing a ball or Frisbee, when in mid-stride he yelps in pain and yanks a hind leg off the ground. Moments later he's off again, but sporting a limp which goes away after 10 to 20 minutes.
What your dog likely experienced here is a luxated patella, or in layman's terms, a dislocated kneecap. In a normal knee, the patella fits into a groove at the end of the femur (thigh bone), and slides up and down as the knee joint bends back and forth. It also acts as a protective cover for the knee joint. The joint's movement follows a limited track, guided by the grooves in the femur. [...]