Abstract: After the fracture of the human long bone， the intramedullary nail needs to be inserted into the medullary cavity to fix the fracture area. Then the screw is required to be placed in the distal hole of the nail. The difficulty of this operation is that the distal hole’s pose will change with the shape of the medullary cavity. Therefore， it is difficult to accurately locate the distal hole’s pose by the traditional manual method. To solve this problem， a three-dimensional spatial location method using single intraoperative X-ray image for distal hole is proposed. Firstly， the initial pose of the distal hole is determined by comparing the contour similarity between the virtual and real nails. Then， the nail’s contour is aligned using a correction algorithm to drive the iterative optimization of the virtual nail pose. Finally， the accurate pose of distal hole is determined. In addition， in order to extract the contour features of X-ray film accurately， a two-stage extraction method from coarse to fine intramedullary nail contour is proposed combined with the target detection algorithm. The experiments are carried out in simulated and clinical environments. Comparison is drawn between the calculated distal hole’s axis and the real hole’s axis. The distance and angle errors in the simulated environment are 0.42 mm， 0.46°， and the errors in the clinical environment are 0.75 mm， 0.81°. The proposed method can meet the actual surgical needs and improves the efficiency and planning in the distal locking nail surgery.

Key words: surgical navigation, inter-locking hole location, distal deformation, deep regression, spatial registration