the cast models for each patient. Transferring the preoperative planning to the operating theatre The usefulness of the preoperative planning, no matter how accurate, depends on the accuracy of the reproduction of the simulated osteotomy in the surgical field. The transfer of the planning was mainly based on the surgeon's visual skills. Different guiding headframes were further developed to mechanically guide bone fragment repositioning. Such a headframe is attached to the patient's head, during CT or MRI, and surgery. There are certain difficulties in using this device. First, exact reproducibility of the headframe position on the patient's head is needed, both during CT or MRI registration, and during surgery. The headframe is relatively uncomfortable to wear, and very difficult or even impossible to use on small children, who can be uncooperative during medical procedures. For this reason headframes have been abandoned in favor of frameless stereotaxy of the mobilized segments with respect to the skull base. Intraoperative registration of the patient's anatomy with the computer model is done such that pre-CT placement of fiducial points is not necessary. Surgical Segment Navigator Initial bone fragment positioning efforts using an electro-magnetic system were abandoned due to the need for an environment without ferrous metals. In 1991 Taylor at IBM working in collaboration with the craniofacial surgery team at New York University developed a bone fragment tracking system based on an infrared (IR) camera and IR transmitters attached to the skull. This system was patented by IBM in 1994. At least three IR transmitters are attached in the neurocranium area to compensate the movements of the patient's head. There are three or more IR transmitters are attached to the bones where the osteotomy and bone repositioning is about to be performed onto. The 3D position of each transmitter is measured by the IR camera, using the same principle as in satellite navigation. A computer workstation is constantly visualizing the actual position of the bone fragments, compared with the predetermined position, and also makes real-time spatial determinations of the free-moving bony segments resulting from the osteotomy. Thus, fragments can be very accurately positioned into the target position, predetermined by surgical simulation. More recently a similar system, the Surgical Segment Navigator (SSN), was developed in 1997 at the University of Regensburg, Germany, with the support of the Carl Zeiss Company. Clinical use of bone segment navigation The first clinical report of the use of this