A mathematical model was created to predict the location and orientation of vertebrae relative to artificial markers. Because the stereovision system detects the position and orientation of artificial markers, the model was developed to transform the marker motion into vertebra motion. Geometry, vector analysis, and stereo-radiographs were used to relate die artificial markers to the vertebrae. Experiments were done using stereo-radiographs and an electromagnetic digitizer to determine die orientation and position of vertebrae in a simulated operating room (OR) environment. Results demonstrated Uiat the mathematical model performed as expected when supplied accurate data from an electromagnetic digitizer. Systematic errors in stereo-radiographs however resulted in 3D reconstruction inaccuracies of 6-7mm