FSI Analysis to Understand Carpal Tunnel Syndrome
One of the major advantages of ADINA in modeling fluid structure interaction problems is that both the structure and the fluid can be represented in their most general form without compromise in accuracy. The structure may include any source of nonlinearities (geometric, material and contact) and the fluid can be a general (incompressible or compressible) Navier-Stokes fluid. The deformations of the structure interacting with the fluid can be arbitrarily large. ADINA uses an effective ALE formulation and adaptive meshing for the fluid region to accommodate such large deformations. All of these capabilities are tightly integrated into a single software environment, which results in cost-saving and greater efficiency for the end user.
In this News, we present a study of an interesting biomechanics problem that further demonstrates some of these capabilities. The subject of the study is the mechanics of carpal tunnel syndrome. The carpal tunnel is a narrow passageway located on the palm side of the wrist. This tunnel protects a main nerve to the hand and nine tendons that bend the fingers (see Figure 1). Pressure placed on the nerve causes numbness and pain that characterize carpal tunnel syndrome. A better understanding of the mechanics involved in this syndrome can be instrumental in designing potential remedies.
The 3D finite element model of the carpal tunnel is depicted in Figure 2. The geometry is reconstructed using MRI images. The finite element model consists of two phases: a solid phase, consisting of the median nerve, the tendons and the carpal tunnel wall, and a fluid phase representing the fluid contained in the carpal tunnel within which the median nerve and tendons are immersed (see Figure 2) .
In the model, the solid is represented as an incompressible viscoelastic material while the fluid is modeled as a Newtonian fluid.
Contact conditions are invoked between the nerve and the tendons, between the individual tendons, between the nerve and the tunnel wall, and between the tendons and the tunnel wall. The contact offset feature is used to avoid fluid mesh overclosure in the narrow gap between solid elements, i.e., to ensure fluid mesh preservation between contacting solids .
The magnitude and direction of displacements for the median nerve and tendons are prescribed by considering an actual pattern of tendon excursions obtained from MRI for different types of maneuvers (power grip, wrist flexion, etc.).
A transient dynamic analysis was performed for a duration of 0.5 s. During the simulation, the tendons undergo large deformations. The fluid region is re-meshed a number of times to accommodate the large motions of the solid phase.
Both 2D and 3D finite element models of the carpal tunnel were studied [1,2]. Results of the 2D analysis, where prescribed movements of tendons are related to the power grip maneuver, are presented in Figures 3 and 4 . The animations at the top of the page depict the velocity vector field in the fluid region and the effective stress and the contact tractions in the solid phase during the wrist flexion maneuver .
Figure 3 shows plots of the effective stress in the solid phase and the pressure in the fluid phase. To ensure a high quality of the contact tractions, the solid phase was meshed with quadrilateral elements. The analysis shows that the median nerve undergoes finite deformation of the cross section due to solid-solid contact with the tendons and the carpal tunnel wall and experiences high levels of stresses due to its entrapment between the tunnel wall and the tendons. Figure 4 shows snapshots of the velocity vector field in the fluid during the simulation.
This study shows some of the advanced capabilities of ADINA FSI where multiple flexible bodies interact with each other due to contact, and undergo finite deformations while all are immersed in a fluidic environment.
For more information on ADINA FSI, refer to our fluid-structure interaction page where we have provided an overview of the features, many case studies, and more than 130 publications where researchers have used ADINA FSI for solving a wide range of challenging fluid-structure interaction problems.
Courtesy of C. Ko, J. Goetz and T.D. Brown, Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City