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New Data on Predicting Blount Disease

Obesity measured by body mass index (BMI) has been linked with Blount disease. But how much obesity and why BMI is a risk factor remain unknown. Blount disease is a condition of extreme bowlegs. The bones of the lower leg start to form an angle and rotate inwardly.

In this study from Shriners Hospital in Texas, research shows that both the child's BMI and the angle of the bones are important factors. Using the data, they were able to form a prediction rule for Blount disease.

A prediction rule is a way to look at risk factors and tell if a child will develop this condition. Such a rule would help orthopedic surgeons decide which children need nutritional counseling and early treatment. Some children will outgrow bowlegs. They don't need medical treatment.

The prediction rule is: if the tibial metaphyseal-diaphyseal angle (TMDA) is 10 degrees or more and the BMI is 22 or more, then treatment is advised.

The metaphysis and diaphysis are the areas where growth occurs in the child's bone. In the early stages of Blount's disease, excess weight and load causes the medial side (inside edge) of the metaphysis to break down. Growth then stops. The lower leg starts to curve outwardly. The child does not outgrow the problem.

The authors report that this prediction rule is very sensistive and specific. That means when the test is positive, there's no chance that the result is a mistake.

In a very small number of cases, a negative test may be in error. In other words, the test results suggest the child has bowlegs but not Blount disease when the child really does have the disease.

References: Allison C. Scott, MD, et al. Body Mass Index as a Prognostic Factor in Development of Infantile Blount Disease. In Journal of Pediatric Orthopaedics. December 2007. Volume 27, Number 8. pp. 921-925

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