PSI - Issue 13

Milena Babić et al. / Procedia Structural Integrity 13 (2018) 438 – 443 E. D. Pasiou, S. K. Kourkoulis , M. G. Tsousi, Ch. F. Markides/ Structural Integrity Procedia 00 (2018) 000 – 000

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3.1. Boundary conditions

In the first case considered, the applied boundary conditions are such that displacements of the whole femoral shaft surface were fixed, up to the collar of the femoral component, as shown in Fig. 3a. This boundary condition prevents displacements of every node on this surface. The described boundary condition was applied because in this case the femoral shaft is completely fixed in femur or bone cement and there is no debonded area. In the second, third and fourth studied cases, with partly loosened femoral shaft, only one part of the femoral shaft surface was fixed, as shown in Figs. 3b, c and d.

(a) (d) Fig. 3. Boundary conditions. (a) The first case; (b) the second case; (c) the third case; (d) the fourth case. (b) (c)

3.2. Loads The load considered on the studied femoral component of total hip prosthesis is the force which appears between femoral head and acetabulum during human walking. In hip joint stress modelling and analysis, the load on the femoral component during human walking is often simplified and estimated based on the static resultant force on the hip joint, Ruszkowski et al. (1985). In case of normal walking the resultant force on the femoral head is closing an angle of approximately 16° with the line of gravity, Ruszkowski et al. (1985) and Byrne and Baker (2010). The resultant force R closes an angle of approximately 21° with longitudinal axis of the femoral neck (Fig. 4a,b). The force applied in studied cases was 238% of body weight (Bergmann et al. (2001)) of a man with body mass of 90 kg, R = 2.38 ∙ T = 2.38∙90∙9.81 = 1765.8 N . In the FEM model the concentrated force was applied at a point located above the head of the femoral component of the total hip prosthesis, as shown in Figs. 4a and b. The point lays on an axis that passes through the center of the femoral head. Concentrated force was transferred to the whole surface of the femoral head by coupling, as depicted in Fig. 4c, in order to avoid the stress concentration.

(a) (c) Fig. 4. (a) Applied concentrated force; (b) components of the concentrated force; (c) coupling. (b)