1. A method for tracing rib posteriors comprising the steps of: providing an incomplete rib tracing comprising a digitized 3-dimensional representation of a rib and a digital image from which said rib tracing was extracted, said image comprising a plurality of intensities corresponding to a domain of points on a 3-dimensional grid; initializing a tracing direction for a remainder of said rib; detecting a plurality of ridge points in a sub-volume of said digital image about said initial tracing direction; and deforming a closed curve in a plane perpendicular to said tracing direction until a rib-edge contour is obtained, using said ridge points as constraints.