Comparative Evaluation of the Radiographic Parameters for Screening Early Blount Disease

Radiographic findings in young children with physiological bowing sometimes difficult to distinguish from early Blount disease. However, early diagnosis of the disease is critical because of the poor treatment outcomes for Blount disease. In this study, we aim to evaluate the accuracy of the metaphyseal-diaphyseal angle (MDA) compared with the medial metaphyseal beak (MMB) angle for differentiating between physiological bowing and early Blount disease and to determine which parameter to adequately screen for the subsequent development of Blount disease.

A retrospective study was conducted on children aged 1 to 3 years old who were brought to our outpatient clinic with bowed leg between 2000 and 2017. Data on the patients' age, sex, and affected sides were collected. Radiographic measurements of the femorotibial angle (FTA), MDA, and MMB angle were evaluated from the initial radiographs. An observer repeated the measurements on all the radiographs 2 weeks after they were first done.

In total, 158 legs were considered from 79 children (48 males/31 females), whose average age was 26.0±6.1 months old. Eighty-seven legs were diagnosed with Blount disease and 71 legs had physiological bowing. Using single cutoff values of 16 degrees for the MDA showed low sensitivity (50.6%), very high specificity (100.0%), and a very high positive predictive value (PPV); while using MMB angle cutoff values ≥122 degrees showed very high sensitivity (92.0%), high specificity (80.3%), and a high PPV. Considering the MDA and MMB angle simultaneously showed very high sensitivity (93.1%), high specificity (80.3%), and a high PPV. The area under the receiver operating characteristic curve of the MDA and MMB showed excellent (0.89) and outstanding (0.93) discriminative ability, respectively. When combining the MDA and MMB angles, it was also considered outstanding performance (area under the receiver operating characteristic curve=0.95).

The MMB angle represents a potential radiographic screening parameter for predicting early Blount disease in children 1 to 3 years old, offering high sensitivity and specificity. The MDA showed excellent specificity as a confirmation parameter for Blount disease patients. Applying both the MDA and MMB angles is another option to increase early recognition and confirm the diagnosis in early Blount disease patients.

Level II.