A surgical device implantable in the subtalar tunnel for treating the flat foot condition, composed of: a central cylindrical body having an outer cylindrical surface provided with a first screw thread; an expansion cone having a longitudinally extending internal bore provided with a second screw thread mateable with the first screw thread, the cone further having an external surface of conical form; and an external cylinder having an internal conical surface engageable with the cone and constructed to be radially expandable in response to movement of the cone in a longitudinal direction relative to the cylinder, the cone being longitudinally moveable by rotation relative to the cylindrical body while the first and second threads are in threaded engagement with one another.