A computer assisted surgery system is described for assisting a surgeon in aligning a drill with the interlocking holes of an implanted intramedullary (IM) rod used for fixation of long bone fractures. With the IM rod inserted, a localizing device measures the pose of a tracked adapter attached to the rods exposed end. Approximate AP and lateral fluoroscopic x-ray images are acquired of the end of the rod with the interlocking holes. Image processing algorithms determine the actual position of the rod and calculate an adjustment to the pose of the tracked adapter and IM rod. Using the adjusted pose information, the system displays, in three roughly orthogonal views, a representation of the drill trajectory relative to the images of the IM rod and relative to a graphic representation of the IM rod.
