--you may well be experiencing a compressive neuropathy--bone or ligament has shifted in position and impacted the nerves in that area.

The knee is a well-known site for entrapment neuropathies; the branch of the sciatic nerve that eventually forms the peroneal nerve that goes down the calf bends around the knee at a very oblique angle in many people, and it doesn't take much to impact it or trap it.

Take a look at:

http://emedicine.medscape.com/article/1234809-overview

http://emedicine.medscape.com/article/1141734-overview