Rapid determination of internal strains in soft tissues using an experimentally calibrated finite element model derived from magnetic resonance imaging

Abstract

Background: Finite element models (FEMs) of medical images can provide information about the underlying tissue that cannot be obtained from the original images. Preforming an accurate simulation requires the careful experimental calibration of boundary conditions. Here we describe a method for deriving a geometric mesh for soft biological materials using a magnetic resonance imaging (MRI) system, and an experimental workflow for calibrating the boundary conditions and optimizing the mesh density in these simulations. Methods: A three-dimensional image stack of a ballistic sphere gel, a bovine caudal intervertebral disc (IVD), and a human lumbar IVD were generated using a positional MRI system. These images were then segmented using a semi-automated process, converted to a tetrahedral mesh, and then modeled as a linear elastic solid. The mesh density was optimized based on simulation time and convergence with the experimental results. The modulus of the ballistic gel was determined experimentally, while the material properties for the nucleus pulposus (NP) and the annulus fibrosus (AF) within the bovine and human IVDs were assigned from literature. The simulation for the spherical gel and the bovine IVD matched the reaction forces determined experimentally in compression. We then simulated a 0.3 MPa compressive load on the human lumbar IVD at the optimal mesh density and material properties determined from the bovine model and then examined the resultant internal strains. Results: The scaled mesh density demonstrated excellent correspondence with the experimental results, confirming that accuracy was not compromised. Both the ballistic gel and the IVD samples exhibited a wide range of internal strains. The NP of the IVD underwent greater deformation than the AF under loading. Conclusions: This study validated a strategy for mesh optimization and FEM of soft biological materials from data generated from MRI scans. This calibrated approach allows for the rapid examination of internal strain distributions medical images that can be performed on the order of minutes.

The Radial Bulging and Axial Strains of Intervertebral Discs during Creep Obtained with the 3D-DIC System

“…Based on this, a novel structural model for the elastic fibers in the NP were proposed. With the rapid development of computational simulations, finite element modeling (FEM) with rational simplifications [ 59 ] can provide information on the underlying tissue that cannot be assessed from in vitro experiments [ 15 , 60 ] or original images [ 61 ]. Komeili et al [ 62 ] and Castro et al [ 63 ] systemically developed spinal models with linear elastic, hyper-elastic, and biphasic material constitutive models and applied various physiological conditions.…”