title: Concept and development of an orthotropic FE model of the proximal femur.
abstract: OBJECTIVE
  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 2mm. 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 2mm 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).
authors.affiliation: Department of Orthopaedic Surgery, Technical University of Aachen, Paulwelsstr 30, 52074, Aachen, Germany. dcwirtz@ukaachen.de
authors.forename: Dieter Christian
authors.initials: DC
authors.lastname: Wirtz
authors.forename: Thomas
authors.initials: T
authors.lastname: Pandorf
authors.forename: Frank
authors.initials: F
authors.lastname: Portheine
authors.forename: Klaus
authors.initials: K
authors.lastname: Radermacher
authors.forename: Norbert
authors.initials: N
authors.lastname: Schiffers
authors.forename: Andreas
authors.initials: A
authors.lastname: Prescher
authors.forename: Dieter
authors.initials: D
authors.lastname: Weichert
authors.forename: Fritz Uwe
authors.initials: FU
authors.lastname: Niethard
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods; D016880:Anisotropy; D015519:Bone Density / Q000502:physiology*; D016723:Bone Remodeling / Q000502:physiology*; D002102:Cadaver; D004548:Elasticity; D005269:Femur / Q000000981:diagnostic imaging / Q000502:physiology*; D020342:Finite Element Analysis; D006801:Humans; D066298:In Vitro Techniques; D008954:Models, Biological; D012015:Reference Standards; D013314:Stress, Mechanical; D013382:Subtraction Technique; D014057:Tomography, X-Ray Computed / Q000379:methods; D014783:Viscosity