Body mass index as a prognostic factor in development of infantile Blount disease.

BACKGROUND: Obesity has been associated with infantile Blount disease, yet no specific relationship has been established. The purpose of this study was to determine the relationship between body mass index (BMI) and the development of infantile Blount disease.

METHODS: A retrospective study was performed reviewing charts and radiographs of 69 consecutive children between 2 and 4 years old who presented during a 5-year period with the diagnosis of idiopathic genu varum.

RESULTS: Forty-nine of these children were noted to have physiological bowing that resolved. Twenty children were diagnosed with infantile Blount disease that required treatment. Logistic regression analysis compared the 2 groups and showed no statistical difference between their age at presentation and age of walking. An independent group Student t test showed a significant statistical difference for body weight, BMI percentile, and weight for height percentile between children with physiological bowlegs and Blount disease. A highly significant difference between the 2 groups was shown in the patient's BMI, proximal tibial metaphyseal-diaphyseal angle, and tibial femoral angle. Based on these data, criteria were established for predicting Blount disease: a tibial metaphyseal-diaphyseal angle greater than or equal to 10 degrees and a BMI greater than or equal to 22. Using these criteria, this prediction method has a sensitivity of 95%, specificity of 100%, true-positive predictive value of 100%, and true-negative predictive value of 98%.

CONCLUSIONS: The establishment of a statistically significant relationship between BMI and infantile Blount disease will be helpful to the orthopaedic surgeon in deciding which children would benefit from early treatment of bowlegs. In addition, nutritional counseling can be emphasized for those at risk.