The present invention relates to an osteosynthesis device, particularly for spinal support or correction, enabling easier and compact implantation, that can be particularly used in the case of implantation via the anterior approach, and a preassembly method for such a device. Said device comprises implants screwed into two vertebrae and one plate characterized in that the plate is joined to at least one implant by attachment structure able to hold said plate without preventing the implant from rotating on its screwing axis, or without preventing a specified clearance of the plate with respect to the implant. In this way, said attachment structure make it possible to continue screwing the implant, or adjust the position of the plate after the plate has been on the implant.