After my knee locked up, my doctor did some tests and found that part of my knee cartilage had broken off. She called it osteochondritis dessicans. What is this problem?

Osteochondritis dessicans (OCD) of the knee is when a piece of bone just under the joint cartilage is injured and dies. The piece of bone and the cartilage above it may actually separate from the joint. When this happens, the fragment can float around inside the joint. Because it floats free, the fragment can get stuck in the joint, causing the knee to pop, click, or even lock up.