Our son is going to a large children’s hospital for evaluation of a hip problem that has been diagnosed as Perthes disease. I’m on-line trying to understand the X-ray report we received from the first doctor that puts it as a Group A disease. The second radiologist called it Class I. What’s the difference?

Perthes disease affects children between the ages of four and eight most often. 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. 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. But there isn’t one standard classification method used by everyone for Perthes. Instead, there are numerous ways to classify this condition. You are probably seeing different descriptions based on two classification methods. These methods rely on X-ray evaluation of the hip.

Group A most likely refers to a method called the Salter-Thompson classification. This approach divides X-rays of Perthes hips as Group A (less than half of the femoral head involved) and Group B (more than half the femoral head is affected). This is a fairly simple approach based on how much of the bone is changed.

A more detailed description is provided by looking at the shape of the femoral head. There are five classes from mild (class I) to severe (Class V). In Class I, the normally round shape of the femoral head remains round and symmetric. Patients with Class I Perthes have not developed any signs of early arthritis.

So using both methods for your son gives the orthopedic physician an idea of how much of the hip is affected and provides a bit of a prognostic outlook. Treatment decisions can be made using these systems.