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I am a doctoral physical therapy student considering conducting a study on Legg-Calvé-Perthes disease for my research. I started looking at X-rays of children at our regional pediatric center. But I noticed not all radiologists use the same classification method for this condition. Is there one system that is best that I should look for when choosing which patients to include (or exclude) from the study?
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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.
As you have pointed out, it's best to have one classification system to study all patients. Treatment depends on a careful classification to determine severity. The natural history and prognosis for final outcomes depend on an accurate classification.
Right now, there isn't one standard classification method for Perthes used by everyone. Instead, there are a half dozen classification methods described for Perthes disease 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).
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.
In a recent review of Perthes classification methods, the author pointed out there is 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. You may want to take this information with you and ask your group of radiologists for their ideas and opinions as well when planning the research design for your study. Good luck!
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