Abstract: A femoral nail which can be implanted in a bone has a transverse hole, and an arrangement is provided for generating a magnetic field which has a maximum strength along an axis of the transverse hole and which decreases in strength in directions radially away from the axis. A drill guide for axially aligning a drill with the transverse hole has an axial drill opening through it, and first and second sensor arrangements are provided at axially spaced locations on the drill guide and each have a plurality of sensors which are at angularly spaced locations and are spaced radially from the opening through the drill guide. An arrangement responsive to the first and second sensor arrangements detects a deviation of the drill opening in the drill guide from coaxial alignment with the axis of the hole in the member, and provides an operator perceptible indication of a radial direction from the drill opening to the axis.