There is no nail in my knee.
Here is the X-ray:
See the little fine wire in the lower right of each image (click on the image to make it larger)? that is a piece of wire of unknown origin lodged in my calf. This doesn't eliminate the alien tracking device, but I think polonium pellet is ruled out.
The knee pain is just some random injury I must have incurred recently. Nothing serious. It will probably eventually go away.