Abstract: A method of designing a pair of fixation rods for implantation on a patient's spine includes: modelling the spine: determining a corrected vertebrae position; modelling the corrected spine by: realigning the vertebrae in the frontal plane, rotating the sacral plateau in the sagittal plane, modifying angulation in the sagittal plane between the adjacent plates of the discs, changing angulation between one or two vertebrae in the sagittal plane, and modifying the height of each disc; calculating a point cloud passing through the centers of the vertebral bodies; deducing two point clouds on either side of the first, spaced in the frontal plane by the width of the vertebra multiplied by X, and located in the sagittal plane at a distance equivalent to the depth of the vertebra in the sagittal plane multiplied by Y, X and Y being a function of the instrumentation connecting the rod to the spine.