"Functional retroversion" of the femoral head in Legg-Calvé-Perthes disease and epiphyseal dysplasia: analysis of head-neck deformity and its effect on limb position using three-dimensional computed tomography.

We studied femoral head and neck deformity in Legg-Calvé-Perthes disease (LCPD) and epiphyseal dysplasia using three-dimensional computed tomography (3DCT) in 17 patients (22 hips). The deformed femoral head can be divided into two portions: (i) the portion that protrudes anterolaterally and inferiorly (false head), often blocking internal rotation (lower margin is radiographic "sagging rope" sign) and (ii) the posteromedial superior portion (true head), representing the original articulating femoral head. This remodeled segment is retroverted in relation to the anterolateral segment resulting in "functional retroversion," which causes an externally rotated gait. This position also produces "functional coxa vara" measured by the relationship with the greater trochanter. These studies contradict previous reports of increased anteversion in LCPD and explain the paradox of supposed increased anteversion on radiograph in a child who walks with the limb externally rotated.