Osteochondritis dissecans or OCD is seen most often in children and young adults. In OCD a piece of joint cartilage pulls away or tears along with the first layer of bone under the cartilage. Sometimes the fragment separates partly from the joint surface. In some cases it is completely separated.

Reports estimate about 20 cases in every 100,000 people so it's not too common. Children with OCD range in age from seven to 19 or 20 with an average age of onset of 13 years. The condition was first described over 100 years ago.

The cause remains unknown. It could be a combination of defects in how the bone is made, genetic tendencies, trauma, or even loss of blood supply to the area. The shape of the meniscus may have something to do with it, too. According to a recent study in Japan a narrow crescent-shape meniscus instead of a mature crescent-shape is linked with increased numbers of OCD.

More study is needed to find the cause and ways to prevent this condition.