A intramedullary fixation device for use in fixating, after an osteotomy or fracture, segments of a long, tubular bone that has been prepared for the use with the device by cutting into the bone a longitudinal-axis-aligned slot that extends from the free surface to the medullary cavity of the bone, includes: (a) an intramedullary beam whose beam cross-sectional shape varies along the beams longitudinal axis, (b) a fin whose fin cross-sectional shape varies along the fins longitudinal axis and a portion of which is attached to the beam surface, (c) wherein the area of any beam cross-sectional section is set so that it can be located in the bones medullary cavity, and (e) wherein the area of any fin cross section is set so that it can be accommodated in the
