It has long been recognized that one of the most difficult steps of intramedullary nailing of femoral shaft fractures is distal locking – the insertion of distal transverse interlocking screws, for which it is necessary to know the positions and orientations of the distal locking holes of the intramedullary nail. This paper presents a constrained optimization approach for solving this problem using single calibrated fluoroscopic image. The problem is formulated as a sequential two-stage model-based optimal fitting process. The first stage, nail axis determination, automatically estimates the axis of the distal part of the IMN through a constrained optimization. The second stage, pose recovery of DLHs, resolves the translation and rotation of the distal locking holes around the estimated axis by iteratively fitting the geometrical models of the DLHs to the image. We report the results of our in-vitro experiments, which demonstrate promising accuracy of the present approach.