Japanese researchers may have an answer to the problem of osteochondritis dissecans (OCD). This painful knee condition affects teens and young adults who are usually still growing. That means the growth plates around the joints have not closed completely yet. Damage to the joint cartilage and first layer of bone (called subchondral bone) occurs causing knee pain with activity. Until now, it's been unclear just what happens to cause this condition.

Rest from activity and walking with crutches (nonweight-bearing status) is the first-line of treatment. Surgery may be needed to fix or hold any pieces or fragments of cartilage/subchondral bone that may have broken off. It was during 12 of these operations that the surgeons who conducted this study removed a plug of cartilage and bone for close examination. The plug was taken right from the center of the osteochondritis dissecans (OCD) lesion and then examined under a microscope.

All 12 plugs were taken from the same anatomic location (medial femoral condyle) of the knee. This area is located along the bottom of the femur (thighbone) on the side closest to the other knee. When they took a closer look at the plugs of bone, they found a cleft (division) in the specimens.

The cleft divided the plug into two parts (upper fragment and lower base piece). The surface of the base portion was covered with a fibrous cartilage tissue. Underneath that was active bone cells trying to repair the damage. The bottom of the upper fragment was also covered in the same kind of dense fibrocartilage. The top of the fragment had normal articular cartilage like you would see covering the surface of any joint. But underneath was dead or dying bone cells. Some specimens didn't have any bone tissue left (alive or dead) -- just cartilage cells.

Examining cells from bone and cartilage under a microscope is called a histologic study. Histologic findings can help explain how and why osteochondritis dissecans develops. The goal, of course, is to find better ways to treat this problem.

The results of this histologic study suggest a series of steps in the breakdown of cartilage and bone. First, repetitive stress from activitiy appears to cause a fracture of the subchondral bone. The subchondral bone has a limited supply of blood normally. This feature combined with trauma to the subchondral areas (from continued movement) results in osteonecrosis (death of bone). The necrosis affects the trabeculae, a scaffold or framework of bone cells in the subchondral layer.

The body's natural response to any cell death is to remove or resorb those dead cells. That's when the fibrous tissue is formed in an attempt to repair the damage. Deeper areas remodel to form cartilage or bone. Bone formation is more likely when there's no separation or only a partial separation of the subchondral bone. A full division seems to cut off the body's ability to mend the area with the necessary bone.

Even with this new histologic information, researchers still don't know why osteonecrosis (bone death) occurs. Is it because the osteochondritis dissecans (OCD) fragment detaches and loses all blood supply? Is it an automatic response after a subchondral fracture? Is the poor blood supply to this area a cause or effect of the fracture? More study is needed to look into the mechanisms of this condition.

What this study shows is why certain treatment technques do or don't work. For example, patients with cartilage and trabecular bone can recover with nonoperative care. The presence of trabecular bone makes it possible to restore the natural layer of cartilage with the underlying subchondral bone. Without this trabecular bridge, fragments that are made up of just cartilage may not reunite with bone even after surgery. Those type of injuries seem to only be able to make more fibrous cartilage, which isn't enough to repair the damage and restore the cartilage to subchondral interface.

If researchers can find a way to examine the tissue types that make up the osteochondritis dissecans (OCD) fragments without doing surgery to remove a plug, then it might be possible to predict which treatment plan would work best for each patient. Special MRI studies using contrast dye might give this information. Three-dimensional (3-D) CT scans are another diagnostic possibility.

For now, we know more about the pathologic process of osteochondritis dissecans (OCD). Future studies may give more information about the cause of this problem so that instead of looking for optimal treatment techniques, it may be possible to prevent OCD instead.

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.