An intramedullar osteosynthetic device includes a body with an elongated shape and a flat cross-section having, in succession, from one of its ends, a first fixation zone cooperating with one of the bone parts to be immobilized, a median zone suitable for withstanding the shear and bending stresses, and a second fixation zone in the other bone part to be immobilized. Each of the fixation zones is made from a material suitable for enabling their deformation by thermal action (tepid memory) or mechanical action (superelasticity), to permit an introduction into the bone parts without pulpar approach, followed by a fixation in the bone parts, while avoiding any rotational movement, withstanding the tensile stresses, and maintaining a compressive force. At least one of the fixation zones has two tabs or fins and which are separable under the effect of the deformation.