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Every year more than one million hip joint replacements are performed worldwide, 135,000 thereof in Germany. In the majority of clinical centers worldwide, it is still state-of-the-art to do preoperative planning on the basis of X-ray images of the patient’s hip joint. In consequence of this approach, the surgeon can make his choice for the best fitting size of an endoprosthesis only on 2D X-ray data, using transparent template sheets with the outlines of the implants. The drawbacks and limitations of such a two-dimensional approach are obvious: Rotational misalignment of the implant is not controlled and the position of the implant is only revisable in the coronar plane.

New approaches of preoperative planning were pursued in the last years by the use of 3D information from patient specific CT-data to overcome these shortcomings. With a 3D virtual planning system surgeons are able to visualize and intuitively understand the patient specific bone geometry. Such systems provide three-dimensional control to the position of the implant in the bone and even support the design of custom-made implants for a specific bone contour. But still the planning and decision support that is provided by a digital 3D model in the preoperative decision phase is limited to pure geometrical information.

The objective of this research project is the development of an interactive simulation platform that provides the indispensable knowledge about patient specific biomechanical properties of the affected bone in order to select the optimal implant design, size and position according to the prediction of individual load transfer from the implant to the bone. Beside a sophisticated visualization platform that allows the intuitive exploration of the bone geometry and particularly the mechanical response to various load situations of the physiological state and the post-operative state of an implant-bone situation in terms of stresses and strains, the development, verification and validation of the newly developed Finite Cell Method as simulation core of the software platform has another focus of this project [1,2]. The Finite Cell Method turns out to be highly suited for an accurate, numerically efficient and reliable simulation of voxel-based data of complex and patient specific bone geometry. The validation of the numerical simulation of the bone mechanical behavior includes experiments with bone donors that provide a data basis for comparison as well as bone specific parameters to enrich the simulation model. For the mechanical testing a standardized procedure is developed to ensure reliable and comprehensible results. In addition a setup for the test of bone samples of the critical trabecular region of the femur’s head is developed to provide test data as a validation basis for the constitutive model of the simulation [3].

 

[1] C. Dick, J. Georgii, R. Burgkart, R. Westermann. Stress Tensor Field Visualization for Implant Planning in Orthopedics. IEEE Transactions on Visualization and Computer Graphics 15(6), pg 1399-1406, 2009

[2] A. Düster, J. Parvizian, Z. Yang, E. Rank. The Finite Cell Method for three-dimensional problems of solid mechanics. Computer methods in applied mechanics and engineering 197, pg 3768-3782, 2008

[3] E. Grande Garcia, T. Obst, M. Ruess, R. Gradinger, R. Burgkart. Biomechanische Modellbildung der anisotropen Spongiosa auf Basis von CT-Daten. Internationale Biomechanik und Biomaterial Tage München, 7/2009

Simulation Results

femur/implant model - principle stress lines
source: Chair for Computation in Engineering

femur model - von Mises stresses
source: Chair for Computer Graphics & Visualization

[click on images to enlarge]

 

The left image shows the von Mises stresses of the proximal femur from an analysis with the Finite Cell Method. The femur was loaded on top with a predescribed displacement and clamped on the distal face. The image on the right-hand side shows the principal stress lines after a simulated implant surgery, using a classical FEM analysis.

 

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Friday, 09. June 2017
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