More mysteries of the knee.

? About eight months ago I was slowly sitting down into a low chair when I felt a sharp pain under the lower part of my knee. I'm 6' 2", 200 lbs. I had been running 10 to 15 miles a week. I now have crunching--and a little pain--in my knee when I walk. I can actually hear what I presume to be the fibular head slipping out and popping back into the socket when my left heel touches the ground. If I walk a lot and then sit, the knee gets stiff and sore. What can I do to get through this and back to running?

Fred Upton, Rochester Hills, MI

Instability of the fibular head is possible, but is usually caused by significant trauma, for example, a car accident; and there are usually other ligament injuries as well. A simple exam by an orthopedic surgeon can determine whether the head is subluxing (slipping). One treatment for this consists of fusing the head of the fibula to the adjacent tibia. This is a relatively short and predictable operation and should allow you to return to running. That said, I don't quite have a good explanation for why this has occurred in the first place, so upon examination you may find yourself with an altogether different diagnosis.

G. Klaud Miller, MD, Evanston, IL

If the fibular head is slipping, there must be an unstable tibio-fibular joint. An MRI will assess this. Also, have your doctor check for a tight biceps femoris muscle, which is associated with a lumbo-pelvic dysfunction.

Mitch Goldflies, MD, Chicago, IL