Face-lift simulation to predict soft tissue behaviour after surgical bone rearrangement

Berlin 23 June 2000In the Konrad-Zuse-Zentrum für Informationstechnik Berlin (ZIB), researchers at the Department of Scientific Visualisation have started a project on Computer Assisted Surgery (CAS) in the last quarter of 1999. The objective of the CAS project is to develop an integrated 3D planning and simulation environment for cranio-maxillofacial surgery, by means of the Amira visualisation system and the Kaskade toolkit, which equally have been designed by the scientific visualisation team.

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During the one-year project time, the team will primarily focus on the simulation of natural jaw movements and muscle-based mimics in order to predict the behaviour of facial tissues. The idea is to provide a qualitative impression of the patient's post-operative appearance after a surgical bone rearrangement. This requires the generation of a virtual patient model, that is able to simulate the complex elasto-mechanical behaviour, performed by the soft facial tissues.

In practice, the researchers are developing a consistent finite element model (FEM) of anisotropic, heterogeneous materials, in which different parameters easily can be introduced. This will allow to simulate the way in which tissues are being deformed under different loading conditions. The Kaskade toolkit provides a wide range of fast, adaptive, multi-level finite element methods to generate a numerical simulation. The ZIB researchers have no intention to determine the true mechanical properties of soft tissues however, neither to accurately describe the natural visco-elastic behaviour, as observed in the tissues.

The primary objective is to develop an integrated framework for pre-operative surgical treatment planning, allowing a surgeon to visualise in 3D how bone movement and muscle contraction may cause soft tissue deformation. In this way, the surgeon is able to judge more confidently on alternative strategies. To this purpose the research team will combine specifically designed cut and transformation tools with 2D and 3D input devices. The planning system will also integrate force feedback and stereo projection for the surgeon to perform the simulation in a fully immersive and intuitive manner.

The CAS project comprises a variety of sophisticated visualisation techniques which will be built into the Amira system. As such, the system requires the importation of medical image data in the standardised ACR-NEMA/DICOM format. The image data needs to be segmented either automatically or semi-automatically with a subvoxel accuracy. Next, the 3D surface models have to be reconstructed according to their correct topology, whereas tetrahedral grids of arbitrary resolution need to be generated. The individual structures of the resulting 3D model are being separated and transformed. Next, finite element modelling is utilised to numerically solve the Lamé-Navier equations. Then, the model is ready to be viewed in 2D or 3D in the capacity of computed tomography (CT) slices, surfaces, vector fields, and so on.

Eventually, CAS will become a novel paradigm of health care since this 3D planning and simulation environment will permit to simulate interactively enforced bone and muscle movements. Cranio-maxillofacial surgeons will be able to functionally analyse mobile skeletal structures and perform computer assisted osteotomy planning, as well as simulations of bone rearrangement. The ZIB team is actively working together with surgeons from the Klinik und Poliklinik für Mund-Kiefer-Gesichtschirurgie in Munich, and with the Technical University of Munich. This co-operation has been set up in order to guarantee the clinical assessment of the CAS environment. More details and illustrative movies are available at the Web site of the Konrad-Zuse-Zentrum für Informationstechnik Berlin.


Leslie Versweyveld

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