This is a joint project with the department of Numerical Analysis and Modelling.
Due to the expanding application of computer technology in medicine new methods are evolving for medical diagnosis, education and training, as well as of surgical treatment planning, assistance and assessment. The planning of complex surgical procedures necessitates highly reliable computerized models of the human anatomy. The ultimate goal is to accurately simulate surgical interventions on virtual patient models in view of better preparation, improved surgical outcome, and shorter operation time. Therefore Computer Assisted Surgery (CAS) is likely to become a new paradigm of health care.In cranio-maxillofacial surgery, physicians are often faced with skeletal malformations that require complex bone relocations. Especially in severe cases of congenital dysgnathia (misalignment of upper and lower jaw) or hemifacial microsomia (asymmetric bone and tissue development of the head), where multiple bone segments are to be mobilized and relocated simultaneously and in relation to each other, careful preoperative planning is mandatory. At present in clinical routine not all possible strategies can be planned and assessed with regard to functional rehabilitation. Moreover, the aesthetic outcome, i.e. the postoperative facial appearance, can only be estimated by a surgeon's experience and hardly communicated to the patient. On this account, a preoperative planning of complex osteotomies with bone relocations on a computerized model of a patient's head, including a reliable three-dimensional prediction and visualization of the post-surgical facial appearance is a highly appreciated possibility cranio-maxillofacial surgeons are longing for.
The CAS CMFS project, being performed at Zuse Institute Berlin (ZIB), addresses such a computer based 3D surgery planning. A processing pipeline has been established and a simulation environment has been developed on basis of the software Amira, enabling a surgeon (or a planning assistant) to perform bone cuts and bone rearrangements interactively and in an intuitive manner on virtual patient models. In addition, a prediction of the patients' postoperative appearance according to the relocated bone can be simulated and visualized realistically. For a meaningful planning of surgical procedures, anatomically correct patient models providing all relevant details are reconstructed from tomographic data with high fidelity. These patient models reliably represent bony structures as well as the facial soft tissue. Unstructured volumetric grids of the soft tissue are generated for a fast and efficient numerical solution of partial differential equations, describing tissue deformation on the foundation of the theory of 3D elastomechanics.
The current state of our development is as follows:
For medical diagnosis, visualization, and model-based therapy planning three-dimensional geometric reconstructions of individual anatomical structures are indispensable. Computer-assisted, model-based planning procedures typically cover specific modifications of virtual anatomy as well as numeric simulations of associated phenomena, like e.g. mechanical loads, fluid dynamics, or diffusion processes, in order to evaluate a potential therapeutic outcome. Since internal anatomical structures cannot be measured optically or mechanically in vivo, three-dimensional reconstruction of tomographic image data remains the method of choice. We have implemented a process chain of individual anatomy reconstruction which consists of import and segmentation of medical image data in DICOM format (Fig. 1), geometrical reconstruction of all relevant tissue interfaces, up to the generation of geometric approximations (boundary surfaces and volumetric meshes) of three-dimensional anatomy being suited for finite element analysis (cf. Fig. 2).
Computer-assisted cephalometric analysis, based on tomographic data in combination with 3D surface models of anatomical structures (Fig. 3), allows for a very precise quantification of asymmetries or facial deformities, i.e. deviations from normal anthropometric measures. Landmarks can be defined either within the image data (e.g. sella point) or on top of the surface reconstruction of bone and soft tissue. Distances and ratios can be easily measured between landmark sets, and planes can be easily constructed from or fitted to such landmarks. 3D cephalometry is a valuable tool for planning combined surgical and orthodontic treatment as well as for assessing and monitoring craniofacial morphology and growth.
The planning of osteotomies (bone cuts) for the mobilization and relocation of bone segments is performed in accordance to the planning on basis of life size replicas of a patient's skull, i.e. stereolitographic models. Osteotomy lines can be drawn on top of the polygonal planning models using suitable input devices (cf. Fig. 4). After evaluation of the consequence of a planned cut with regard to vulnerable inner structures (nerves, vessels, teeth, etc.) the model is separated accordingly.
A relocation of bone segments can be performed unrestrictedly in 3D or restricted to a translation or rotation within arbitrarily chosen planes under consideration of cephalometric guidelines. Bone and tooth collisions can be evaluated for functional analysis or orthodontic treatment planning with possible integration of digitized dental plaster casts. As a result of the preoperative planning, a single transformation matrix, encoding translation and rotation, or a sequence of such matrices are provided for each bone segment. Both the osteotomy paths and the transformation parameters can finally be used for intra-operative navigation.
In the course of the planning, the relocated positions of bone segments serve as an input for the simulation of the resulting soft tissue deformation. Since bone and surrounding soft tissue share common boundaries that are either fixed or translocated, the resulting configuration of the entire tissue volume can be computed from the given boundary displacements by numerical minimization of the internal strain energy on basis of a biomechanical model, using a finite-element approach. A primary concern is on the development of an adequate model of the complex elastomechanical behaviour of soft tissues. We do not focus on the determination of the true mechanical properties of soft tissues, nor do we claim to realistically describe any tissue's natural viscoelastic behaviour. Rather our goal is to develop a consistent and easy to parametrize finite element model of anisotropic, inhomogeneous materials. With such a model the deformation of tissues under different loading conditions can be simulated. Our numerical simulation is based on fast, adaptive multilevel finite element methods, as available with the KASKADE toolkit. Simulating the behaviour of facial tissues will give a qualitative impression of the patient's postoperative appearance. This will help a surgeon to judge on alternative strategies considered in surgical treatment planning, and allows an improved patient information, quality assurance, and documentation.
In addition to osteotomy planning with soft tissue prediction we are working on automatic segmentation and surgical reconstruction of facial bone structures (neurocranium, mandible, bony orbit), implantology, and statistical 3D shape modeling for computer-assisted cranio-maxillofacial surgery. A first study for simulating facial expressions based on muscle actions was also performed in our group.
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| Example 2007 (15 MB) | Overview 2002 (26 MB) | �berblick 2002 (26 MB) |
In collaboration with various surgeons and hospitals more than 40 clinical cases have been accompanied by preoperative planning so far, ranging from dysgnathia, mandibular and midfacial hypoplasia to complex craniofacial microsomia. Simulation results were validated on the basis of photographs, 3D photogrammetry as well as of postoperative CT data, showing a good correlation between simulation and postoperative outcome.
