INTRODUCTION
Maxillofacial surgery is concerned with the treatment of skull deformations resulting from heavy injuries or malformations during adolescence. In the operative procedure the surgeon resects several skull fragments and rearranges them. The esthetic appearance of the patient has to be taken into account as well as the function of the treated organ structures. For example, a good dental occlusion has to be achieved. Every surgical step must therefore be thoroughly planned in order to accurately predict the postoperative shape of the skull and soft tissue. The aim is to supplement conventional methods with virtual reality techniques, thus significantly contributing to clinical understanding and improving treatment planning as well as surgical intervention.

CTThe primary goal of our research has been to implement a completely computer-based maxillofacial surgery planning system. An important step toward this goal is to make virtual tools available to the surgeon in order to carry out a 3D cephalometrical analysis and to interactively define bone segments from skull and jaw bones. An easy-to-handle user interface employs visual and force-feedback devices to define subvolumes of a patient's volume dataset. The defined subvolumes together with their spatial arrangements based on the cephalometrical results lead to an operation plan. We have evaluated modern low-cost, force-feedback devices with regard to their ability to emulate the surgeon's working procedure. Once the planning of the procedure is complete, the planning results are transferred to the operating room. In our intra-operative concept the visualization of planning data is speech controlled by the surgeon and tracked with the patient's position by an electromagnetic 3D sensor system.