An apparatus for treating ankle fractures includes an external fixer formed by a tibial clamp and a heel clamp connected to a central member extendable by means of respective universal joints. The heel clamp is articulated such that its rotation axis is arrangeable to be approximately centered on the tibial-heel joint axis. The apparatus also includes a positioner device of the true rotation axis of the tibial-heel joint, wherein such device includes a support removably associated with a screw freely insertable in the astragalus, a Kirschner string adapted to identify said true rotation axis, a locking member of the Kirschner string selectively orientable with respect to the support until the axis of the string is substantially stationary during the rotation of the tibial-heel joint.
