This paper presents a prototype system for computer assisted surgery, the purpose of which is to assist orthopaedic surgeons when performing distal locking of intramedullary nails. This system comprises three components, namely: an Intelligent Image Intensifier, a Trajectory Tactician and an Intelligent Trajectory Guide. The Intelligent Image Intensifier is an X-ray vision system that provides accurate X-ray images. Such images enable the Trajectory Tactician software to analyse the operation site and calculate the trajectory required for a screw to lock an intramedullary nail. This involves the capture of two X-ray images from which are extracted the projections of the nail's edge boundaries and its distal locking holes. Using an analytical mathematical model of the nail, the position and orientation of the nail is determined. The trajectory is then implemented by the surgeon using the Intelligent Trajectory Guide. Evaluation in the laboratory suggests that the system is capable of reliably inserting a locking screw into an intramedullary nail. The rapidity with which this computer assisted method achieves locking should benefit both patient and surgeon by reducing radiation dosage and the length of time required to lock a nail.