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

Abstract

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.

Figure 1

Representation of the tibial metaphyseal-diaphyseal angle (TMDA). A line drawn perpendicular to the lateral border of the tibial cortex and another is drawn through the 2 beaks of the metaphysis to determine the transverse axis of the tibial metaphysis. The TMDA is the angle bisected by those 2 lines.

Figure 2

A 15-month-old boy presented with bilateral bow-leg deformities. (A) At the initial visit, radiographs revealed tibial metaphyseal-diaphyseal angles (TMDAs) of 13° on the right and 12° on the left. (B) There was slight improvement after 4 months, with TMDAs of 9° on the right and 10° on the left. (C) Radiograph showing definite improvement at the age of 31 months. The TMDA measured 0.5° on the right and 0.5° on the left.

Figure 3

The reference values for tibial metaphyseal-diaphyseal angles (TMDAs) are shown in each group. The red lines represent estimates of improvement in the TMDA according to a linear mixed model. (A) Physiologic bowing group, (B) Blount disease group.