Modeling the Deformable Body

August 2007 saw a surge of new open-source software for simulating musculoskeletal movement. In addition to OpenSim 1.0 (described in the Fall 2007 issue of this magazine), FEBio arrived on the scene. While OpenSim uses rigid body mechanics—simulating the body moving essentially as a series of segments attached at joints—FEBio (Finite Elements for Biomechanics) addresses the other part of the problem. It can simulate how movement deforms and places stresses upon solid parts of the body such as muscles, tendons, ligaments, cartilage and bone.


In this FEBio model of a shoulder capsule—the soft-tissue envelope that surrounds the shoulder joint—the upper arm bone (the humerus) is moved upward and then rotated around its axis. The left image shows the initial undeformed mesh, the middle image shows an intermediate state, and the rightmost image shows the stresses on the capsule in the final deformed state (blue means low stress, red means high stress). During a shoulder examination, clinicians typically move the shoulder in various ways in an attempt to determine the source of a problem. Models of this kind could eventually help clinicians better understand the results of such tests. Courtesy of Jeff Weiss and Steve Maas.Created by Jeff Weiss, PhD, associate professor of bioengineering at the University of Utah, and his colleagues, FEBio already has 200 to 250 users. “Initially we developed FEBio for our use in-house,” says Weiss, “but we saw the potential for it to be a really popular tool in the research community and decided to make it available to everyone.”


Before now, biomechanics researchers studying the solid mechanics of soft tissue have relied upon costly general-purpose finite-element programs such as Abaqus or LS-DYNA. But because these programs are proprietary, it’s hard to add new features to the code. “We saw that as a major shortcoming in our field,” says Weiss. So he and his colleagues tailored FEBio to address the kinds of problems that come up in biomechanics.


In addition to FEBio itself, Weiss and his colleagues also released programs that allow users to prepare their models in advance of using FEBio (PreView) and to analyze and visualize the results of an FEBio simulation (PostView). “That’s one of the advantages of FEBio,” says Steve Maas, a software developer who works with Weiss. “You can do your model creation and post-processing on your own computer and use a high performance computer only for the FEBio step.”


FEBio’s users come from many different disciplines including orthopedics, ophthalmology and cardiovascular mechanics. Weiss himself has used FEBio for a variety of research projects including a study of hip stresses in people with displasia and a study of the shoulder capsule. He and his colleagues are also continuing to add new features to FEBio.


Weiss and one of the OpenSim creators Scott Delp, PhD, a professor of bioengineering at Stanford University, have begun a collaboration to link the two programs to address problems that can’t be handled by either program alone. Although Delp’s group has combined dynamics with finite element approaches in previous work (for example in a study of knee pain), “development of advanced methods in biomechanics would be accelerated if one could use two open-source programs connected in a straightforward way. I’m looking forward to that day,” Delp says.


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