Q: I am a 13 year old girl with knee pain that only goes away when I sit and do nothing. The doctor says I have osteochondritis dissecans. I looked this up on-line and found out it's from an injury or repetitive sports activity. I'm not a sports freak, ballerina, gymnast, or athlete of any kind. So why do I have this problem?

A: Osteochondritis dissecans (OCD) is a problem that affects the knee, mostly at the end of the big bone of the thigh (the femur). The problem occurs where the cartilage of the knee attaches to the bone underneath. The area of bone just under the cartilage surface is injured, leading to damage to the blood vessels of the bone. Without blood flow, the area of damaged bone actually dies. This area of dead bone can be seen on an X-ray and is sometimes referred to as the osteochondritis lesion.

Doctors aren't sure what causes OCD. Many doctors think that OCD in children is caused by repeated stress to the bone. Most young people with juvenile OCD (JOCD) have been involved in competitive sports since they were very young. A heavy schedule of training and competing can stress the femur in a way that leads to JOCD.

But many people who develop OCD don't have any particular risk factors. In some cases, other muscle or bone problems can cause extra stress and contribute to JOCD. For people like you, symptoms often develop gradually over time. OCD with no known cause can be just as severe as for those individuals with a history of repeated sports events.

There are other causes of knee pain in teens, so it's important to make sure the diagnosis is correct. The diagnosis of JOCD is usually made by asking many questions about your medical history and current symptoms. The doctor will examine both knees for a comparison.

X-rays of the knee usually show OCD lesions. If not, a bone scan may be needed. A bone scan involves injecting a special type of dye into the blood stream and then taking pictures of the bones with a special camera. This camera is similar to a Geiger counter and can pick up very small amounts of radiation. The dye that is injected is a very weak radioactive chemical. It attaches itself to areas of bone that are undergoing rapid changes, such as a healing fracture. A bone scan is the best way to see the lesions in the very early stages.

Other imaging tests, such as magnetic resonance imaging (MRI) are able to create pictures that look like slices of the knee. The images clearly show the anatomy and any undetected injuries. These tests may help determine the extent of damage from JOCD. They also help rule out other problems.

If you have any questions about your diagnosis, talk with your doctor. Find out if any further tests are needed to confirm the diagnosis. Sometimes a conservative program of rest and inactivity is all that's needed to clear up this problem. If that doesn't help, then the physician may order the next round of tests. Unless you are a candidate for surgery, expensive imaging tests may not be necessary at this time.

Hiroaki Uozumi, MD, et al. Histologic Findings and Possible Causes of Osteochondritis Dissecans of the Knee. In The American Journal of Sports Medicine. October 2009. Vol. 37. No. 10. Pp. 2003-2008.