Hybrid visualization for maxillofacial surgery planning and simulation Carlos Buchart CEIT and Tecnun (University of Navarra) cbuchart@ceit.es Gaizka San Vicente CEIT gsanvicente@ceit.es Aiert Amundarain CEIT aamundarain@ceit.es Diego Borro CEIT and Tecnun (University of Navarra) dborro@ceit.es Abstract Currently, many visualization methods are used in com- puter assisted medicine. It is commonly considered that a unique visualization scheme makes difficult the interac- tion and limits the quality and quantity of the information shown. In this paper we study the specific requirements of a maxillofacial surgery simulation tool for facial appearance prediction. The different stages of the application lead to present medical information in different ways. We propose to adapt visualization techniques to give a more suitable answer to these needs: a hybrid volumetric and polygonal visualization for the planning stage, as well as a scheme for surgery definition in 2D. Finally, we propose the use of mesh visualization for the simulated model, which previously re- quires the 3D reconstruction of the surface in order to be visualized. 1. Introduction In the last decades, computer science has given a new dimension to medical information, enabling the pre- diction and simulation of surgical treatments. Indeed, computer-aided surgical simulation (CASS) is widely used in medicine since it allows saving much time in the surgery planning process. One of the many developed applications that uses this techniques is the maxillofacial surgery plan- ning since it appears to be at least as good as traditional methods [20], [19]. Maxillofacial surgery treats abnormalities, injuries, and diseases in the head by bone fragment repositioning, restoration of bone defects, and implant insertion. In this field there is a huge demand from surgeons to be able to predict, before performing the surgery, the new facial out- look of the patient. Actually, there is a growing interest in CASS since it helps during surgery planning, and improves the communication between dentists, orthodontists and sur- geons. For some application examples in this area we refer the reader to the following research works [15], [3], [21]. The medical information acquired in the diagnostic and planning phase can be very varied; nonetheless, one of the most common techniques is the computed tomography (CT). This source of medical images is usually stored in DI- COM (Digital Imaging and Communication in Medicine) format, which is a widely standard used by the medical community. These images encode the volumetric informa- tion of the head using pixels that are given in sets of 2D slices. The color of the pixels is expressed in gray levels and they allow identifying different tissues because of their different densities. This procedure is called segmentation. After the segmentation, a 3D reconstruction of the different issues is accomplished. The essential steps of a computer tool capable of plan- ning and simulating maxillofacial surgeries are the follow- ing (see Figure 1): read the DICOM files, make the segmen- tation, reconstruct the 3D model, make the surgery plan- ning, and simulate the surgery. The specific requirements pointed out by the medical personnel/staff for this tool were: • Show user-defined specific tissues like soft tissue and hard tissue. • Show region of interest (ROI) and allow navigation through the whole data. • Easy and interactive surgery planning module. • Fast and accurate simulation module. According to the requirements of the simulation module we developed a simulation module based on mass-spring models (MSM). In a word, MSM assumes that mass is dis- cretely concentrated in mass-points which are connected by springs to each other. Those springs represent the elastic