Abstract: A system for spinal osteosynthesis is provided comprising a pedicle screw having a pedicle thread for introduction into the pedicle of a vertebra and a saddle thread adapted to receive a saddle member, a rod, at least one saddle member with a hole of a diameter substantially corresponding to the diameter of the saddle thread and a throat portion the axis of which is substantially perpendicular to the axis of the head and which includes a passage for the rod. A device secures the saddle member on the screw adapted to render the pedicle screw integral with the saddle member at the head thereof prior to application of mechanical effort for effecting spinal reduction. A part of the saddle thread is thereby left free. The rod is secured in the throat portion and is integral with the throat portion of the saddle member after application of mechanical effort for effecting spinal reduction.