Natural history of radiological coronal alignment of lower limbs in children and adolescents with achondroplasia.

Abstract

Achondroplasia (ACH) is the most common skeletal dysplasia worldwide, caused by mutations in the fibroblast growth factor receptor 3 gene and characterized by a disproportionately short stature, and many orthopaedic complications. Genu varum affects between 80% and 90% of children with ACH, but the longitudinal natural history of lower limb alignment remains unclear. The aim of this study was to describe age-related changes in lower limb alignment in children with ACH by analyzing longitudinal radiological parameters from infancy to skeletal maturity. This retrospective cohort study included 42 children with ACH, who had at least two radiological assessments at our institution between January 2003 and December 2024. Whole-leg supine anteroposterior radiographs were obtained before the age of two years and in the standing position thereafter. There were 22 females and 20 males, and a total of 254 limbs were analyzed. The parameters which were analyzed included the femorotibial angle (FTA), mechanical axis angle (MAA), mechanical lateral proximal femoral angle (mLPFA), mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle (MPTA), and lateral distal tibial angle (LDTA). Radiographs were grouped into nine age groups between six months and 18 years. Longitudinal changes were analyzed using one-way analysis of variance with Bonferroni post hoc testing. The FTA and MAA demonstrated significant age-related changes (p < 0.05). Genu varum, which was present in all age groups, improved until the age of nine years, then deteriorated slightly until skeletal maturity, yielding approximately 10° of varus alignment at the age of 18 years. Other joint-orientation angles (mLPFA, mLDFA, MPTA, and LDTA) also showed significant age-related changes (p < 0.05). This study provides the first longitudinal evidence of changes in lower limb alignment in children with ACH. Genu varum improved until the age of nine years and subsequently worsened slightly, with persistent varus at skeletal maturity. These baseline data will inform decisions about the timing of surgical correction and the evaluation of emerging forms of pharmacological treatment, as well as the design of future trials dealing with the management of children with ACH.