Elsevier

Journal of Biomechanics

Volume 36, Issue 2, February 2003, Pages 289-293
Journal of Biomechanics

Short communication
Concept and development of an orthotropic FE model of the proximal femur

https://doi.org/10.1016/S0021-9290(02)00309-3Get rights and content

Abstract

Purpose: In contrast to many isotropic finite-element (FE) models of the femur in literature, it was the object of our study to develop an orthotropic FE “model femur” to realistically simulate three-dimensional bone remodelling.

Methods: The three-dimensional geometry of the proximal femur was reconstructed by CT scans of a pair of cadaveric femurs at equal distances of 2 mm. These three-dimensional CT models were implemented into an FE simulation tool. Well-known “density-determined” bony material properties (Young's modulus; Poisson's ratio; ultimate strength in pressure, tension and torsion; shear modulus) were assigned to each FE of the same “CT-density-characterized” volumetric group.

In order to fix the principal directions of stiffness in FE areas with the same “density characterization”, the cadaveric femurs were cut in 2 mm slices in frontal (left femur) and sagittal plane (right femur). Each femoral slice was scanned into a computer-based image processing system. On these images, the principal directions of stiffness of cancellous and cortical bone were determined manually using the orientation of the trabecular structures and the Haversian system. Finally, these geometric data were matched with the “CT-density characterized” three-dimensional femur model. In addition, the time and density-dependent adaptive behaviour of bone remodelling was taken into account by implementation of Carter's criterion.

Results: In the constructed “model femur”, each FE is characterized by the principal directions of the stiffness and the “CT-density-determined” material properties of cortical and cancellous bone. Thus, on the basis of anatomic data a three-dimensional FE simulation reference model of the proximal femur was realized considering orthotropic conditions of bone behaviour.

Conclusions: With the orthotropic “model femur”, the fundamental basis has been formed to realize realistic simulations of the dynamical processes of bone remodelling under different loading conditions or operative procedures (osteotomies, total hip replacements, etc).

Introduction

In literature, many isotropic simulation models of the proximal femur have been described to calculate the functional adaptation of cortical and cancellous structures, (Couteau et al., 1998; Jacobs et al., 1997; Lengsfeld et al., 1996; Weinans et al., 1992). Since bony materials are not isotropic but rather anisotropic and orthotropic, respectively (Carter et al., 1989; Savvidis and Stabrey, 1996; Wirtz et al., 2000; Yang et al., 1999), it was the object of our study to develop an orthotropic finite-element (FE) “femur reference model” to simulate bone remodelling more reasonably adapted to the physiological situations in vivo.

Section snippets

Materials and methods

For orthotropic modelling of bony structures, four main developing steps had to be performed: (step 1) the three-dimensional reconstruction of femoral geometry and the generation of an FE mesh for the bone model, (step 2) the allocation of the material properties of the bone according to their density for each FE, (step 3) the exact definition of the principal stiffness directions for each FE, (step 4) the implementation of a numerical algorithm for time- and load-dependent remodelling of bone.

Discussion

Although since several years, some studies have been performed to generate anisotropic FE modelling of the proximal femur (Bagge, 2000; Doblaré and Garcia (2001), Garcia et al. (2001); Jacobs et al., 1997), no real solution exists to the problem of finding the orthotropic material orientation to an optimal structure within a three-dimensional bone remodelling tool.

The presented new simulation tool is a more comprehensive approach to this “three-dimensional problem” of orthotropy. Each FE has

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