From a posterior approach long Schanz screws are inserted through the pedicles into the bodies of the two vertebrae just adjacent to the lesion and connected by the threaded Fixateur-interne-rods. By tightening the nuts the Schanz screws are fixed in all directions. The advantages of the F.I.-system are: excellent reposition by the long lever-arm of the Schanz screws, immobilization of only two segments avoiding the iatrogenic loss of mobility and lumbar lordosis, stability against flexion forces better than with Harrington distraction rods, additional rotational stability.