Finite element model of the proximal femur under consideration of the hip centralizing forces of the iliotibial tract
Introduction
Preliminary findings of the biomechanics of the human hip joint were examined within the scope of the finite element method (FEM) especially bony structures. FEM-supported femur model was developed with and without hip prosthesis by many authors (Besdo and Händel, 1994, Couteau et al., 1998, Cristofolini et al., 1994, Huiskes, 1982, Jacobs et al., 1997, Lengsfeld et al., 1996, Rohlmann et al., 1983, Taylor et al., 1996). Huiskes (1982) wrote already about one main problem of all femur FEM — the inhomogeneous material properties of human bone with the difficulty to integrate them accordingly to the Hook law in the FEM model.
The influence of the abductor muscles on the surface tension in the investigations and FEM was intensively discussed in the past (Cheal et al., 1992, Crowninshield et al., 1980, Huiskes, 1987, Huiskes et al., 1987, Huiskes et al., 1988, Huiskes et al., 1989a, Huiskes et al., 1989b). Rohlmann et al. (1982) confirmed already that the sole view on the gluteal muscles is not sufficient for describing the hip joint.
Wirtz et al. (1998) developed an orthotropic FEM of the proximal femur. In this model the direction of the material properties was defined for each finite element by using the vectorial preference direction appropriate to the trabecular orientation of the spongiosa and the three-dimensional direction of the osteons of the cortical bone.
The clinical aspect of our investigations is the important predictability of the changing forces on the femoral head after displacement osteotomy which can be necessary for the surgical treatment of the hip dysplasia with a varus or valgus centrum-collum-diaphysis-angles (CCD-angle) of the femoral neck. The displacement osteotomy leads to an alteration of the forces of the iliotibial tract (IT).
Our constructed FEM model of the hip joint was applied to the two-leg standing under consideration of the IT with physiological (125°), varus (115°) and valgus (155°) CCD-angles. The aim of our investigations was the calculation of the hip centralizing forces of the IT along the femoral neck and the thereby resulting force on the femoral head namely the acetabulum.
Section snippets
Miscellaneous to the finite element model
With the FEM a complex component geometry will be separated in many geometrical simple ground elements like cuboids and tetrahedrons. By this means the developing separate element equations will be summarized to a total equation and solved by means of the numerical mathematics. Therefore the calculations of the equations of states were formulated at the particular element nodal points. By summarizing the existing tension and strain relationships we obtain the strain distribution about the total
FEM of the hip joint and the proximal femur at a 128° CCD-angle
The anatomical position of the femur has been accounted for in the FEM. The sagittal positioning of the IT has been modelled considering the natural course, using an angle of 23° in the frontal plane and 6° in the sagittal plane. The compensating muscle force, acting in the opposite direction, was 1460 N and was applied at the upper edge of the greater trochanter (force direction 24° in the frontal plane, 15° in the sagittal plane) (Birnbaum et al., 2004).
The preload of the IT has been set to
Discussion
The investigations simulated the influence of the IT and the gluteal muscles for the hip joint apart from each other. The main reason is the fact that in the literature no conciliation about the including muscles and ligaments could be found (Stolk et al., 2001).
Ling et al., 1996, Taylor et al., 1996 saw in their investigations a high influence of the IT regarding the reduction of the bending forces in the course of the femoral diaphysis. They demanded a consideration of the IT in numeric
Conclusion
Under consideration of the anatomical and the biomechanical properties of the IT we constructed a FEM of the proximal femur. We determined the hip centralizing forces of the IT with various CCD-angles. With these results the surgeon has the opportunity to predict the expected hip centralizing forces of the IT and the force on the femoral head after displacement osteotomy.
Within the scope of a developing rigid body model with simulation of dynamic movements of the hip joint we may examine which
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