Pitfalls in femur length measurements.

The following study was performed to determine the precise anatomic correlate for the end points of sonographic measurement of the fetal femur. A cadaveric fetal thigh was dissected in layers and correlated with sonographic images. The margins of the osseous portion of the femur were determined on sonograms. Potential for underestimating femur length (oblique images) and overestimating femur length (including nonosseous portions of the femur in measurement) can be eliminated by requiring a view of the femur which includes the epiphyseal cartilages. In this way, the margins of the osseous portion may be accurately estimated, and there is assurance that the entire shaft has been visualized. Images of the dissected specimen indicate that the "distal femur point," which may cause confusion in measurement, is not a part of the osseous femur, but probably represents a specular reflection from the lateral surface of the distal epiphysis.