Method of determining the position of an object using projections of markers or struts

A 3-D model of a body part is created by using an object comprising markers, fixation members, and struts. The method involves first and second roentgenograms of the body part and the object disposed between an x-ray source and an x-ray imager. It then determines first and second sets of distances between projections of the markers or the struts, and determines first and second 3-D positions of the x-ray source and of the object with respect to the x-ray imager using predetermined distances between the markers or struts and the first and second sets of distances between their projections. The method then aligns the first and second 3-D object projections in a 3-D reference frame using the 3-D positions of the markers or struts with respect to the x-ray imager in the two orientations. The method creates the 3-D model of the object based on the 3-D object projections.