Widening of the femoral proximal diaphysis--metaphysis angle in fetuses with achondroplasia

Abstract

Objectives: It has recently been reported that fetuses with achondroplasia have a wider than expected femoral proximal diaphysis-metaphysis angle (femoral angle). The aim of this case-control study was to investigate this finding.

Methods: Cases with confirmed achondroplasia (n = 6), small-for-gestational-age fetuses (n = 70) and a group of normal fetuses (n = 377) were included in this study. The ultrasound image of the femur was examined by two independent experienced observers blinded to the diagnosis, who measured the femoral angle. These values were converted into multiples of the expected median (MoM), after adjustment for gestational age and femur length. Prevalence of various prenatal ultrasound signs of achondroplasia was determined in affected fetuses. Intra- and interobserver agreement of measurement of femoral angle was assessed using 95% limits of agreement and kappa statistics.

Results: The femoral angle can be measured accurately by ultrasound, and increases with both increasing gestational age and increasing femur length. The femoral angle-MoM was significantly higher in fetuses with achondroplasia than in the control group (1.36 vs 1.00 MoM, P < 0.001) and in the SGA group (1.36 vs 1.04 MoM, P < 0.001). It measured more than 130° in five of the six cases with achondroplasia (83.3%), which was the most consistent finding other than shortening of the long bones.

Conclusions: The femoral angle is wider in fetuses with achondroplasia. This new ultrasound sign appears promising as an additional discriminatory marker when clinicians are faced with a case of short long bones in the third trimester.

Keywords: achondroplasia; femoral proximal diaphyseal-metaphyseal angle; prenatal diagnosis; screening; ultrasound.