My 15-year old son has been diagnosed with OCD of the knee. He's in a cast and on crutches. The nurse warned us about "cast disease," and suggested a short course of physical therapy to avoid this. What is it?

OCD or osteochondritis dissecans is a condition in which a loose piece of bone and cartilage separates from the end of the bone. It usually affects the knee or elbow.

Standard nonsurgical treatment for this problem is reduced activity and non-weight bearing. The patient is often put in a cast to ensure limited use of the joint.

Cast disease refers to changes that occur in and around a joint that's been immobilized for long periods of time. The joint becomes stiff. It doesn't exactly fuse but it starts to act like it's going to fuse to protect itself. The muscles start to atrophy or waste away from lack of use. The bone can even lose mass.

The best way to prevent this from happening is through movement and activity that doesn't stress the joint or bone. Range of motion and isometric exercises work well for this problem. Follow your doctor's advice and see a physical therapist. He or she can give your son a home program to do daily while waiting for healing to take place.