In this paper, we introduce the concept of functional anteversion of the femur and its positional change. It emerged from studies using B-mode and real-time ultrasound to determine femoral anteversion in two knee positions. Twenty healthy children were scanned at the hips whilst lying supine with knees extended and then with the knees flexed to about 90°. The anteversion angle between the head-trochanter line and the horizontal measured with knees extended and feet together is determined by three factors: 1) anatomical femoral anteversion, 2) femoro-tibial rotation at the knee, and 3) tibial torsion. This is one type of functional anteversion of the femur. The best reproducibility for functional femoral anteversion was obtained by scanning with real-time ultrasound and the knees extended (95% confidence limits within ± 2.8°); the use of B-mode ultrasound in the knees-extended position is significantly less reproducible (95% confidence limits within ± 6.1°). In the flexed-knee position, the reproducibility is similar with both B-mode and real-time ultrasound (± 2.6-3.5°). Both positions have limitations in attempts to measure anatomical femoral anteversion by ultrasound. The functional femoral anteversion angle measured by B-mode and real-time ultrasound is significantly larger (by 9-10°) with the knees flexed than with the knees extended. This positional change of measured femoral anteversion involving axial rotation at the hip is attributed mainly to 1) lateral rotation (unlocking) of the femur at the knee during flexion from the fully extended position and 2) any lateral tibial torsion which rotates the femur nedially in the knees-extended position with the feet vertical. The considerable individual variation and asymmetry of this axial rotational change may have relevance to the etiology of certain clinical disorders of the spine, hip, and knee joints
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