» The Value of MRIs for Children with Osteochondritis Dissecans
The Value of MRIs for Children with Osteochondritis Dissecans

The forceful and repeated actions of sports can strain the immature surface of the knee joint in children and teens. The bone under the joint surface weakens and becomes injured, which damages the blood vessels going to the bone. Without blood flow, the small section of bone dies. The injured bone cracks. It may actually break off. When this condition occurs in this age group, it is called juvenile osteochondritis dissecans (JOCD).

JOCD can also occur in children and adolescents with no known cause. The condition can be stable and without symptoms but more often there is knee pain and/or tenderness around the joint. Mild swelling may be present but more often than not, there is no swelling. With stable lesions, motion is normal. Loss of knee motion is more common with unstable lesions.

An OCD lesion is unstable when a piece of cartilage or cartilage and bone breaks loose and is free to float around inside the joint. How does this happen? The articular (joint surface) cartilage in children is newly formed. It can't handle the type of forces placed on it with repetitive activity.

The subchondral bone (under the articular cartilage) takes the brunt of the stress. A portion of the bone may eventually weaken, and possibly even crack. When the bone is damaged, the tiny blood supply going to the area is somehow blocked. Without blood supply, the small area of bone dies. This type of cell death is called avascular necrosis. (Avascular means without blood, and necrosis means death).

The crack may begin to separate. Eventually, the small piece of dead bone may break loose. This produces a separation between the articular cartilage and the subchondral bone, which is the condition called OCD. If the dead piece of bone comes completely detached, it becomes a loose body that is free to float around inside the joint. And that is how an OCD lesion becomes unstable.

In the absence of a specific injury, the child may at first feel bothersome knee discomfort only while playing sports or during physical activity. The soreness generally goes away quickly when the leg is rested. Over time, however, the joint pain worsens, is hard to pinpoint, and may linger after using the leg. The knee may feel stiff, and it may not completely straighten out.

In advanced (unstable) cases of OCD, the patient may notice that the joint grinds (called crepitus). The knee may catch, or even lock up occasionally. These sensations may mean that a loose body is floating around inside the joint. The joint may also feel warm and swollen, and the muscles around the knee may appear to have shrunk (atrophied).

Studies show that the best way to get a good look at the condition of the joint surface and cartilage is to do an MRI. X-rays may look normal early in the condition when there is, in fact, a real problem. As the condition worsens, the X-ray image shows changes in the bone and joint.

The normal shape of the bony knob at the end of the femur (thigh bone) called the femoral condyle may appear irregular. In bad cases of knee juvenile OCD, the condyle might even look like it has flattened out, suggesting that the bone has collapsed. The X-ray could show a crack in the bone or even a loose body.

A magnetic resonance imaging (MRI) scan may show more detail. The MRI can give an idea of the size of the affected area. It can show bone irregularities and also help detect swelling. Doctors may repeat the MRI test at various times to see if the area is healing. MRIs are also very helpful when considering and/or planning surgery. That's where this study comes in.

The authors examined 132 juvenile OCD lesions using MRIs and then compared the results to their actual findings on arthroscopic examination. In this way, they could judge the accuracy of MRIs when diagnosing and evaluating this condition. This information is important because it provides surgeons with the predictive value of MRI regarding the stability of JOCD lesions.

What they found is that MRIs are "reliably sensitive" to articular (joint) surface lesions associated with juvenile osteochondritis dissecans (JOCD). This is especially true for low-grade, stable lesions. More severe unstable lesions are less likely to show up on MRIs, especially if they are located in unusual areas of the joint (e.g., the nonweight-bearing surface of the lateral femoral condyle). The lateral condyle is the round bottom of the femur that forms the top portion of the knee joint. Lateral means it occurs on the side away from the other leg. The medial femoral condyle is on the side closest to the other knee.

For surgeons who need the statistical specifics on sensitivity, specificity, and predictive values, here is the breakdown:

  • Agreement between MRI and arthroscopic evaluation was 62.1 per cent.
  • MRI sensitivity was 92 per cent. Sensitivity shows the ability of a test to determine a true positive for the condition (i.e., the patient really has a lesion).
  • MRI specificity was 55 per cent. Specificity reflects the ability of a test to show a true negative for the problem (the patient does not have the problem).
  • Negative predictive value of MRI was 97 per cent. Negative predictive values indicate the certainty that a negative test result truly indicates the absence of a condition.
  • The chances of finding an unstable lesion on the lateral side of the femoral condyle was 15.7 times greater than the medial side. It was also more likely that unstable lesions would occur on the lateral side compared with the medial side for both weight-bearing and nonweight-bearing surfaces of the joint.

Signals transmitted by the MRI show the surgeon low-grade (stable) versus high-grade (unstable) lesions. Thickening of the articular cartilage and cracks or fissures in the cartilage are visible. Loose fragments of tissue and the hole or "defect" where the tissue came from are also visible. The challenge in reading MRIs is in recognizing signal patterns that suggest healing in progress or edema in the fracture line. The authors say that such findings are more important than the location of the lesion.

The results of this study confirm findings from other similar studies. The value of these findings is that children who have unusual (atypical) lesions can get treatment early with the help of MRIs to identify changes that might go undetected otherwise. Untreated, OCD lesions can gradually get worse and lead to early joint arthritis.

In summary, MRIs have prognostic value in determining the severity of JOCD lesions. MRI may be the best method available to check the condition of joint cartilage and the subchondral bone (first layer of bone under the cartilage).

Reference: Walter P. Samora, MD, et al. Juvenile Osteochondritis Dissecans of the Knee: Predictors of Lesion Stability. In Journal of Pediatric Orthopaedics. January/February 2012. Vol. 32. No. 1. Pp. 1-4.