ABSTRACT
<p><b>BACKGROUND</b>Treating developmental dysplasia of the hip is often challenging. The difficulties include not only the hip surgery itself but also the treatment of the associated lower-limb valgus deformity. However, there have been very few studies on such deformity in patients with developmental hip dysplasia. In this study, we investigated the prevalence and severity of lower-limb valgus deformity, along with the relationship between the severity of valgus deformity and mechanical alterations of the hip or the ipsilateral knee.</p><p><b>METHODS</b>Two hundred and six affected lower limbs of 116 adult patients with untreated developmental dysplasia of the hip were included in the study, grouped according to the severity of hip dysplasia. Each study participant's radiographs were measured to quantitatively evaluate the mechanical axis deviation of the lower limb, and further to evaluate the prevalence and severity of the lower-limb valgus deformity. Some mechanical alterations of the hip and the ipsilateral knee were also measured on the radiographs.</p><p><b>RESULTS</b>Of the affected lower limbs, 14.1% had valgus deformities. Study participants with Crowe type III hip dysplasia had the most severe deformity and the highest prevalence of deformity. Severity of valgus deformity had a strong positive correlation with the lateral migration of the femoral head but not with the superior migration. A decreased lateral distal femoral angle contributed to the lower-limb valgus deformity, and the lateral distal femoral angle had a strong negative correlation with the severity of valgus deformity.</p><p><b>CONCLUSIONS</b>Hip dysplasia is commonly associated with lower-limb valgus deformity, and the severity of the lower-limb valgus deformity is mostly affected by lateral migration but not superior migration of the femoral head. The valgus deformity may originate mainly in the distal femur, in addition to the hip joint itself. These findings can be taken into account when planning to treat the patients with hip dysplasia.</p>