In Brief: OCD lesions are uncommon and poorly understood joint surface defects that are found most frequently in the knees of children and young adults. While some OCD lesions heal on their own, many require surgical treatment. Unhealed or unsuccessfully treated OCD defects compromise the "ball-bearing" function of the knee's gliding surfaces (which are normally quite smooth and almost frictionless), leading to premature joint arthritis. The goal of surgical management is to preserve or restore a normally contoured, smooth, firm joint surface that will function properly in load-bearing throughout life. This goal is by no means easy to achieve.

Osteochondritis dissecans is an unusual affliction of human joints that is not rare but also cannot be considered common. The knee is the most frequently affected joint in the body. Curiously, one particular location on the medial femoral condyle (inner aspect of the lower end of the thigh-bone) is where the majority of knee OCD lesions are found (see FIGURE 1).

FIGURE 1 - Schematic illustration of femoral condyle anatomy, demonstrating the most common areas afflicted with OCD.

The next most common site is the posterior aspect (rear portion) of the lateral (outer) femoral condyle (see FIGURE 1), followed by rarer forms in the patella (kneecap) and upper tibia ("shinbone"). OCD begins in childhood and is therefore most commonly seen in teenagers and young adults. Severe (large) OCD lesions that remain unhealed can ultimately wreak havoc on a knee joint, with long-term arthritic consequences that may require joint replacement surgery.

Osteochondritis dissecans is a truly mysterious joint disease process that was studied by 19th century pathologists and given its name because it was considered an inflammatory (the "itis" in osteochondritis refers to inflammation) condition of articular (joint surface) cartilage and the underlying (subchondral) bone. (Note: "osteo" refers to bone and "chondro" refers to cartilage). The disease causes a section of joint surface cartilage and the bone beneath it to loosen and separate (by way of a gradual dissection process, hence the "dissecans" in OCD's name) from the main or "parent" bone structure such as a femoral condyle (see FIGURE 2).

FIGURE 2 - This MRI scan image shows a side-view section of a 13-year-old patient's knee. The upper bone is the femur, and the back of the knee is toward the right side of the picture. The arrows are pointing to a large OCD fragment of articular surface cartilage and underlying bone that has dissected itself loose and has almost completely separated from the parent femoral condyle.

The actual cause of OCD is unknown. There are three theories regarding the origin of OCD, the first being that it is the result of traumatic impact injuries delivered to the adolescent joint surface, either acutely (suddenly) or chronically (repetitively) over time. This view considers OCD to represent a fracture of sorts. The second theory is that the separating osteochondral (bone and cartilage) fragment starts out as a small, anomalous (aberrant or extra), independent zone of ossification (bone formation) during early adolescent skeletal growth that simply fails to fuse (merge) with the main ossification center as the bone matures. This ultimately leaves the bone tissue in that extra ossification center (the "ossific nucleus" or subchondral ossicle of OCD) isolated and without a blood supply, depriving it of oxygen and nutrients. The bone ossicle may then shrink and atrophy, thereby undermining the overlying joint surface and making the involved segment of articular cartilage (with attached ossicle) subject to gradual loosening and separation from the parent bone. The third theory is that normal bone underlying a region of articular joint cartilage somehow loses its circulation suddenly and therefore dies, similar to the way a localized area of heart muscle dies when a blood clot cuts off its circulation in the case of a myocardial infarct (heart attack). While my personal experience with many cases of OCD over the years has led me to believe that the anomalous ossification center theory is the most likely of the three to be correct, no one knows for sure.