**5.2.2 Finite element models**

For fatigue testing, we performed three finite element models in which, in each case, it was changed the material properties of the implant. The models accounted for stem, the test stand and also a piece to apply the load. Full 3D model were considered, with solid tetrahedral and hexahedral elements.


Table 6. Mechanical properties of implant materials considered in the simulation

Because the stem consists of two parts and a fastener, frictional contact was modelled at the interface. The remaining interactions were assumed as tied. A total of 106,195 elements and 26,192 nodes were used in the analyses. The meshed finite element model is shown in Figure 1.

For implant attached to bone finite elements models were realized, in which the hip implant and the femur were represented. Four nodes solid elements were used in the models, to realize 4 finite elements models. Three of them were developed with the implant and an additional model was analyzed without the implant. This was considered as a control solution for evaluation of stress shielding.

One of the meshed finite element model is shown in Figure 2.

For those models with implant, the implant was completely fastened to the bone through an interaction in which "slave" nodes are tied to the master surface of the bone. So the degrees of freedom in the exterior side of the implant associates to the degrees of freedom of the bone surface in contact to it.

Titanium as a Biomaterial for Implants 159

In all cases for fatigue testing, the applied load was the load specified in the standard, of

For implant attached to bone the applied load was an arbitrary load of 1000 N in the acetabular component of the prosthesis. Besides the pre-tensioning bolt load was simulated.

Due to the assembly load between the stem components, high levels of compressive contact stress were obtained. Therefore, and to wean the study of the influence of these stresses, we considered in the analysis the stress components S33, which would be responsible for a possible material fatigue. The simulation results obtained for the three models using




Vertical displacement in the head of the implant (mm)

Figure 3 shows the S33 stress distribution in the implant made of beta titanium alloy.

stress S33 (MPa)

(\*) The indicated compression component was mainly due to the contact stresses.

Whereas the applied load varies between zero and 2300 N, the minimum stresses are related to those produced by the preload of the bolt connecting both parts of the implant, and the

2300 N. The load was applied on the upper surface of the piece that applys the load.

**5.2.3 Loads** 

**5.2.4 Results a. Fatigue testing:** 

different materials are given in Table 7.

Material Implant maximum flexural

316L +379

Ti6Al4V +397

Ti35Nb7Zr5Ta +455

Table 7. Simulation results for the three models

Fig. 3. S33 stress distribution (MPa)

maximum stresses are due to the load of 2300 N.

Fig. 1. Fatigue testing. The 3D finite element model developed in the analysis

Fig. 2. Implant attached to bone. Finite element model developed in the analysis

All the models were created, analyzed and afterwards the results were processed using Abaqus 6.4.5.
