Finite Element Analysis Interfacing
Applications in orthopedic and trauma biomechanics often require information on the internal forces in the body parts. Therefore, frequently Finite Element Analysis (FEA) is utilized to compute stresses and strains in the tissues.
Essential to get reliable, meaningful results out of this are realistic boundary conditions (BC). Force-wise these BCs are composed from external loadings, dynamic forces and the corresponding muscle activations for the given situation. As the AnyBody™ Modeling System (AMS) is capable of providing these forces (an image visualizing this is shown to the below) it seems to be straight forward to use this data as input to a FEA.
A typical workflow of such FEA with loads estimated by AnyBody™ is summarized in the figure below. Typically, a surface and a volume model is created based on medical images (CT or MRI scans) using image processing and segmentation methods available in various software packages. Based on the volume model, a FE model can be generated. In this tutorial, we shall assume that you are familiar with methods for segmentation and FE mesh-generation and we shall focus on how to transfer computed forces from the AMS to the FE code.
Based on the segmented bone surface, also used to generate the FE mesh, one can derive parameters used to scale the generic AnyBody™ model for a suitable fit the actual scanned subject. It is also beyond the scope of this tutorial to consider these details, but let us just notice that this can be done at many different levels of accuracy depending on the resources and data available. An STL file from the segmentation of the bone can be useful in this process and scaling can be done, for instance, based on bony landmarks of the bone. The STL file can also be imported into AnyBody™ model for visualization, but this alone will not change the musculoskeletal system if you dont change the model definition). Please refer to the tutorial on Scaling for details on this matter.
A registration of the coordinate system used in the FE mesh, i.e., the scanned configuration, to the coordinate system of the bone in AnyBody™ is naturally needed for transferring the load data properly. We shall just touch briefly upon this issue, showing how you can deal with different coordinate systems. However, the examples, you find in the lessons of this tutorial, are typically using FE meshes based on geometries from the generic AnyBody™ models in AMMR; thus, scaling and registration is not a relevant issue.
The boundary conditions consist of the applied loads (exported from AnyBody™) and fixation conditions. The loads exported from AnyBody™ should include all loads on the bones, including reactions to applied external and muscle loads, i.e. the joint loads and this should in principle be sufficient for loading the FE model. The exported loads do provide a situation of equilibrium between all forces in the AnyBody™ model; however, this includes all inertia forces as well and these is not easily to apply to a FE model of a bone. The bone is just a part of a segment in the rigid-body concept in AnyBody™ models, where the segment inertia also includes the soft tissues surrounding the bone. Neglecting parts of the inertia in FE bone model and maybe all accelerations, we will not have a situation of equilibrium. Therefore, we need to apply proper fixation BCs to accommodate for this. This will also remove issues of singular stiffness matrix, in standard static analyses.
In the following tutorial, it is shown how AnyBody™
can be used to analyze the stress state of bones. In the
first lesson, the built-in facilities of AnyBody™ to
generate output for a FEA are explained in general. A
simple example is included, which shows all the steps from
the AnyBody™ application to the nice colored Finite
Element stress plots. We will briefly go through the
Generate a force output file from AnyBody™
Export the bone surface from AnyBody™
Mesh the bone
Apply material description and boundary conditions
Solve the Finite Element Model
In the first showcase freely available software is used to generate the mesh and solve the problem. Therefore, also users without access to a commercial FEA software package can step through it. But obviously there are plenty of possibilities on the market to do FEA and this example should just illustrate the flow of the task, without claiming to do high-end FEA.
The following lessons consider interfaces to selected commercial FE packages, Ansys and Abaqus. In these cases, small interface tools are applied to automatically convert the standard load output file from AnyBody™ to formats directly readable by the FE packages.
Now it's time for Lesson 1: Export of data for FEA.