A new technique has been developed for the measurement of 3D motion from 2D perspective projections of an object's silhouette. The technique uses measured perspective projections, a simplified model of perspective geometry, and an accurate description of the object surface geometry to implement a contour matching based position/orientation estimation scheme. Computer modelling studies and in vitro calibration experiments indicate that the technique can achieve measurement accuracies of ±0.2mm (±1 standard deviation) for object translations in the image plane, ±5mm for translations normal to the image plane, and ±0.75 degrees for all three object rotations. The current implementation is used for measuring the motions of Total Knee Replacements (TKR) in x-ray images The technique was used to measure the knee motions for four subjects with TKR's as they performed a constrained stair-rise maneuver. The human studies' results are consistent with previously reported results for similar studies of TKR subjects. It is proposed that this technique can provide an unique and valuable new tool for quantifying the performance of existing total knee designs and for developing improved knee replacement devices.