Determining the orientation of the knee joint from stereo radiographs and 900 bi-plane radiographs

In this work is investigated the measurement of the spatial direction of the axis of rotation of the knee joint from stereo radiographs and from 900 bi-plane radiographs. The methods to be described are based on geometrical properties of the femur condyles and their contours in radiographs. Therefore, a use of metal markers in the patient is not necessary. Taking into account the properties of contours in radiographs, the double tangency to the contours can be interpreted as the picture of a tangent plane to the femur condyles. Measurements were carried out on a phantom. They have been done in order to find out, how good the hypothesis of a cylindrical envelope to the condyles is. Radiographs have been made for different orientations of the knee with respect to the X-ray apparatus. The results have been transformed into a unique coordinate system, which was fixed to the femur. The accuracy of the reconstructed direction of the knee axis is +-1 degree for the component in the cranio-caudal direction and +-2 to 3 degrees for the component in the dorso-ventral direction. (orig./MG)