A comparative evaluation of tibial metaphyseal-diaphyseal angle changes between physiologic bowing and Blount disease

The purpose of this study was to estimate the rate of spontaneous improvement in tibial metaphyseal-diaphyseal angle (TMDA) in physiologic bowing in comparison to that in Blount disease and to provide reference values of TMDA for monitoring patients with highly suspected to have Blount disease. We retrospectively reviewed patients with physiologic bowing meeting the following criteria: (1) TMDA greater than 9° before 36 months of age at initial evaluation; (2) two or more standing long bone radiographs available; and (3) follow-up conducted up to resolution of deformity. Patients with Blount disease had (1) more than 2 standing long bone radiographs obtained before 36 months of age and (2) underwent no treatment during the period in which these images were obtained. TMDA measurements were obtained from 174 patients with physiologic bowing and 32 patients with Blount disease. Rates of TMDA improvement were adjusted by multiple factors using a linear mixed model, with sex and laterality as fixed effects and age and individual patients as the random effects. In the physiologic bowing group, TMDA improved significantly, by 3° per 6 months and by 6° per year. Changes in TMDA were not significant in the Blount disease group. Knowing the rate of TMDA change can be helpful for physicians seeking to monitor infants with suspected as having Blount disease with a high TMDA and to avoid unnecessary repeat radiographic evaluations.