462 DISCUSSION

tally superimposed on the real patient allows surgery to become less invasive. In orthognathic surgery and in osteotomies of the facial skeleton, augmented reality concepts have, for the first time, allowed transparent transfer of preoperative planning to intraoperative vi- sualization.

Whenever bone segments are transferred, virtual structures serve as guidelines intraoperatively and, in addition show every motion of the osteotomized bone in relation to imaging data and cephalometric drawing (Figs 8, 9). For instance, in correction of posttraumatic enophthalmos, augmented reality can be used for intra- operative visualization of a symmetrical position of the globe.

System accuracy and consistancy are over-riding concerns. Because the surgeon relies on other cues and skills during surgery, he or she can always check the accuracy of the system using fiducial markers and noting what is encountered compared with what the virtual images show. The system showed good, but still insufficient, accuracy for extremely accurate pro- cedures (ie, less than 1 mm) in the operating room environment, because the system’s coregistration error cannot be expected to decrease the tracking system’s error. For instance, for determination of occlusal rela- tions, a splint is still better. In this sense, it can be stated that it is not always possible to be highly accu- rate; therefore, it is of paramount importance that the surgeon can always quantify system inaccuracy by direct visualization. Despite little shortcomings, how- ever, our system has already proved to be a valuable addition to our surgical armamentarium and is there- fore used in selected cases.

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Discussion Virtual Reality in Orthognathic Surgery: The

Augmented Reality Environment Concept

Herman F. Sailer, MD, DDS and Ulrich Longerich, MD, DDS University Hospital Zurich, Zurich, Switzerland

The Virtual Patient System (VPS) described in this article and the further development, the Interventional Videotomog- raphy (IVT), offer the possibilities of so-called composite or augmented reality. VPS and IVT are based on the same software, with the difference being that IVT transmits the image data added to a special conference system via Internet, ISDN, or TCP/IP.

SAILER AND LONGERICH 463

The augmented reality of the VPS is technically a drawing overlayed on an image data set. It is possible to plan an operative approach preoperatively on the monitor, plan the location of the osteotomies, or mark the tumor borderlines from a computed tomography (CT) image data set. The sur- geon draws the tumor borders manually as an overlay with VPS/IVT software tools into the CT data set and adjusts the overlay in all three dimensions. After this, the overlay can be transmitted into any other data set, ie, into the video image data set of the operative field, and then can be projected onto a head-up display (HUD) worn by the surgeon. In this way the surgeon can now see the CT-border lines in his or her operative field. Then a new overlay can be created and cali- brated to an instrument, ie, a scalpel linked to a sensor. The scalpel can be seen in relation to the tumor border lines. An advantage of the VPS/IVT System is that the surgeon with the HUD is not limited in his or her movements during the operation. Also, the position of the patient during surgery can be changed at any time without any new adjustments. The only requirement is that every movable object is linked to a sensor. Because of these technical freedoms, the system is very user friendly.

Wagner et al applied the VPMVT system to osteotomies in orthognathic surgery to reduce the risk of damage of the palatal vessels intraoperatively during the Le Fort I osteot- omy. We would not use a navigation system for that reason.

It also was not explained how the exact position of the palatal vessels was determined. Using the correct technique for the Le Fort I osteotomy, the risk of damaging of the palatal vessels is not great. This example, however, should represent the splendid possibility of navigation to manage complex osteotomies for craniofacial deformities.

The authors also describe the use of the VPSAVT system to compare the preoperative prediction tracing of the soft tissue profile with the intraoperative result after positioning the upper jaw. This is an innovative way of controlling the soft tissue profile intraoperatively, but the intraoperative swelling has to be taken into account. Unfortunately, the authors do not show the preoperative and postoperative con- ditions of the patient.

We disagree with the authors’ use of the VPS/IVT for positioning the maxilla via a virtual interincisal point. This procedure refers only to the incisor position, ie, the sagittal plane, but not to the horizontal plane, which must be defined in all asymmetric cases with facial rotation.

It is not possible at the moment to program the VPS/IVT with the model operation data that would provide further accuracy within a necessary range of 20.5 mm.

We agree with the authors that the VPSIIVT provides good intraoperative guidelines for osteotomies in complex cases. In our opinion computer-assisted surgery will be a very important tool in all kinds of operations of hard and soft tissues in the near future.