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Understanding the Severity of Perthes Disease

Posted on: 09/28/2011
There is a condition that affects some children called Legg-Calvé-Perthes or Perthes disease. It involves a loss of blood supply and the death of bone at the top of the femur (thigh bone). The condition can range from mild-to-severe. The round head of the femur may collapse and shift and become more oval shape. Later in life, these children can develop early and sometimes severe hip arthritis.

Treatment depends on a careful classification to determine severity. But there isn't one standard classification method used by everyone. Instead, there are numerous ways to classify this condition. The authors of this article review the various classification methods and make recommendations for the most useful one to use based on research studies.

Classification of any condition like Perthes is useful because it helps give a picture of what is happening for each individual patient. Using a classication system allows us to look at groups of patients with Perthes to predict results with and without treatment.

It's best to have one classification system to study all patients. Right now, there are a half dozen methods described in the literature. There's the Catterall, Salter-Thompson, lateral pillar (Herring), Mose, and Stulberg classifications. These methods all rely on X-ray evaluation of the hip.

The Catterall classification takes a look at the location and amount of bone collapse. Patients are placed in one of four groups based on the specific effects of Perthes on the epiphysis (growth center). The presence of collapse, degeneration, absorption, and regeneration are described with this system.

The Salter-Thompson classification model is much simpler. There are only two groups based on how much of the femoral head is involved (less than half or more than half). The Stulberg classification places children in one of three categories based on the shape of the femoral head. The Stulberg classification method has been tested, modified, and eventually expanded to include five groups instead of the original three.

Herring also took Stulberg's original three-group classification method and revised it and published several studies. Focus has shifted now to using the Herring (lateral pillar approach) because it has good interobserver reliability. In other words, different radiologists using X-rays to classify the disease using this method came up with the same results.

There is another type of classification based on bone scintigraphy. Bone scintigraphy looks at the distribution of blood flow and active bone. It helps show blood flow to and through the bone and shows places throughout the skeletal system where the bone is actively metabolizing.

The advantage of bone scintigraphy is that changes in bone metabolism show up on the bone scan before structural changes would appear on an X-ray. Conditions such as fractures, infections, tumors, and Perthes can be recognized with a bone scan long before they can be seen with plain radiographs.

Scintigraphy is not used routinely because it is an invasive test. It requires injecting radioactive tracers in the child's blood. It is also much more expensive than plain radiographs (X-rays).

In summary, taking a look at all the classification methods and comparing final results is a good way to see which approach is the most accurate. Although the Catterall classification has been used most often in the past, there's a shift now toward using the Herring (lateral pillar) method instead. The Herring method has been shown to have good interrater reliability as well as the ability to predict final outcomes.

The author of this review says there is still room for improvement in adopting a standard way to classify Perthes disease based on X-rays. For example, it's very easy to have slight differences in the child's hip position when the X-ray is taken. What the radiologist sees of the hip can be very influenced by a subtle difference in hip rotation. The child could also be standing or lying down and that makes a difference.

References:
Ken N. Kuo, MD, et al. Classification of Legg-Calvé-Perthes Disease. In Journal of Pediatric Orthopaedics. September 2011. Vol. 31. No. 2. Pp. 168-173.

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