Abstract: An implantable osteosynthesis device is made up of two portions, a so-called fixed portion and a movable portion, which can move relative to each other and are provided with coupling elements capable of blocking the translation of the portions so as to obtain a clamping effect on the bone, wherein the fixed portion has an elongated element (1) and the movable portion has a body (5) forming a sleeve (6) capable of sliding along the elongated element. The coupling of the invention comprise one-way combined locking arrangements (2, 10-13) for allowing the relative displacement of the fixed and movable portions in a single direction adapted for providing the clamping effect, and for locking the portions so as to prevent the translation thereof in the other direction.