An instrument (10, 100) for locating an axis of a blocking screw is disclosed. The instrument (10, 100) is applicable for a retrograde installation of a femoral intramedullary device or an antegrade installation of a tibial intramedullary device. The instrument (10, 100) includes a drill jig (11, 105) with a radiolucent frame portion (14, 120) and a mounting portion (12, 110). The mounting portion (12, 110) is adapted to connect to an intramedullary device (204), and the frame portion (14, 120) has at least one aperture (18, 22, 24, 25, 28, 29, 126, 137, 337) for locating the axis of the blocking screw (210). The drill jig (11, 105) is adjustable to locate the aperture (18, 22, 24, 25, 28, 29, 126, 137, 337) in a longitudinal or rotational direction relative to the intramedullary device (204).
