Fast Soft Tissue Deformation with Tetrahedral Mass Spring Model for Maxillofacial Surgery Planning Systems Wouter Mollemans, Filip Schutyser, Johan Van Cleynenbreugel, and Paul Suetens Medical Image Computing (Radiology - ESAT/PSI), Faculties of Medicine and Engineering, University Hospital GasthuisBerg, Herestraat 49, B-3000 Leuven, Belgium Abstract. Maxillofacial surgery simulation and planning is an ex- tremely challenging area of research combining medical imagery, com- puter graphics and mathematical modelling. In maxillofacial surgery ab- normalities of the skeleton of the head are treat by skull remodelling. Since the human face plays a key role in interpersonal relationships, peo- ple are very sensitive to changes to their outlook. Therefore planning of the operation and reliable prediction of the facial changes are very im- portant. Recently, the use of 3D image-based surgery planning systems is more and more accepted in this field. Although the bone-related planning concepts and methods are maturing, prediction of soft tissue deforma- tion needs further fundamental research. In this paper we present a soft tissue simulator that uses a fast tetrahedral mass spring system to cal- culate soft tissue deformation due to bone displacement in a short time interval. Results of soft tissue simulation for patients who had a max- illofacial surgery are shown. Finally we truly validated the simulation results and compared our method with others. 1 Introduction Simulation of the deformation of the facial soft tissues due to bone movement, demands a mathematical model that is able to imitate the behavior of the facial tissues. Various models have been proposed for this simulation. The Finite Element Method (FEM) [1] [2] is a common and accurate way to compute complex deformations of soft tissue, but conventional FEM has a high computational cost and large memory usage. This makes FEM models inap- propriate for realtime simulation. Hybrid models based on global parameterized deformations and local deformations based on FEM, have been introduced to solve this problem[3]. Most of these methods, however, are only applicable to linear deformations and valid for small displacements. Furthermore they rely on pre-computing the complete matrix system and are therefore unable to cope with topological changes when these occur during simulation. Mass Spring systems (MSS) [4] [5] are widely used to model deformable objects. They are applied to a variety of problems, such as cloth modelling, C. Barillot, D.R. Haynor, and P. Hellier (Eds.): MICCAI 2004, LNCS 3217, pp. 371–379, 2004. c  Springer-Verlag Berlin Heidelberg 2004