Abstract: The invention concerns an osteosynthesis device, particularly for the spine by means of internal implantation. Comprising firstly one or more bars (2) used to support or move the spine, and secondly at least one implant (3) connecting the bars and the vertebrae (V), said implant comprising firstly bone anchorage means attached to the body of said implant and secondly fixation means for said bars, said fixation being carried out by clamping means (31) clamping said bar against the internal walls (320) of a channel (32) formed in said body of the implant (3), characterised in that said bars comprise a transversal bearing surface (21) on at least part of their length, said transversal bearing surface being produced by a cross-section of said bars comprising at least one flat part or part of lower or opposed convexity in relation to the rest of said cross-section.