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By applying ‘pure’ microseparation conditions, the finite element method was used to assess the affects of contact pressure and stress. This therefore extended previous modelling and studies of hip implant device edge loading. Laxity of the joint occurred when there was no contact between the femoral head and acetabular cup and the centre point of the acetabular cup did not coincide with the centre point of the femoral head. The centre point of the femoral head was moved to a specified distance relative to the acetabular cup in the inferior-medial direction for the left hip or in the inferior-lateral direction for the right hip. Following laxity of the joint and relocation by a vertical load to re-engage the hip joint using an explicit finite element solver, initial contact occurred on the rim of the acetabular cup. The magnitude of the contact pressure was larger than under normal loading conditions; however the value was less than that observed when lateral displacement control edge loading was applied as the boundary condition. This methodology led to lower values of contact pressure and a symmetric pressure distribution about the centre of contact. As the cup inclination angle increased the contact pressure also increased, which agreed with the trend observed from in vivo and in vitro retrievals. Along with this finding, the importance of acetabular cup rim radius was observed from the contact pressure results under ‘pure’ microseparation.
5.2. Model B: Segmented Hip Device Models
As expected the maximum contact pressure and stress values reduced over those observed from model A due to the elasticity of the bone backed model material properties and reduced model rigidity. Initial results showed the maximum contact pressure to be located off the centre of the contact, which differed to the more predictable contact pressure profile observed for the results in model A. The maximum contact pressure under vertical loading conditions applied to model B-1 occurred towards the posterior end of the cup. The contact pressure distribution under lateral displacement edge loading also differed to that observed in model A. The maximum contact pressure of 142 MPa did not occur at the centre of contact (i.e. contact
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radius equal to zero). In all contact cases, the initial non-zero value of contact pressure occurred at t = 0. This was due to displacement control being used to establish equilibrium before the application of load control.
As with model A, under maximum vertical loading conditions, no edge loading or rim contact was observed. When edge loading was simulated by a lateral displacement, the occurrence of the off centre maximum contact pressure was caused by a combination of peak load and lateral displacement. In the case where ISO gait loading was considered, the amount of contact between the head and cup was significantly less than in the case where constant peak loading was modelled along with normal and edge loading. The contact area between the bearing components with microseparation at peak loading and ISO loading conditions were denoted by Mp and MI respectively. In addition to this, the contact area between the bearing components
without microseparation at peak loading and ISO loading conditions were denoted by Np and
NI respectively. The contact area profiles and magnitudes are clear to see from Figure 4.14,
where a varying contact area profile was observed when an ISO loading profile was applied. However, a steady state increase in contact area was observed as the contact load was increased during the application of peak loading conditions. The contact area for both types of loading conditions was reduced when microseparation was applied.
As discussed in chapter 2, previous literature suggests that the magnitude of vertical loading has negligible effect on the contact pressures between the hip bearing components, however the results from model B have shown that increasing vertical load magnitude by 30%, i.e. from FI to Fy, increased the maximum contact pressure by 22%, which leads to a proportional
increase in wear according to the wear theories referred to and used in this project.
Non-symmetric contact pressures and magnitudes were due to the influences of bone geometry and as expected the contact pressure increased when a larger lateral displacement was applied (from 200 µm to 500 µm). Modelling development following the methodology proposed in
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Figure 3.14 was repeated to ensure that this was not caused by any errors with the modelling (referred to as model B-2), however the same results were obtained.
For model B-1, when a lateral displacement was applied in combination with a peak vertical load the maximum contact pressure increased by a factor of 7.9 over the maximum contact pressure observed under vertical loading. This increase in contact pressure above the rim of the acetabular cup was caused by a 907 N lateral reaction force as a result of applying the lateral displacement boundary condition of 250 µm. Although the reaction force at the rim of the cup was lower than that observed for model A, this is still larger than the lateral force applied in experimental in vitro simulator studies.
Following the calculation and study of edge loading factors, further analysis considered the flexion-extension profile in line with ISO standards and experimental simulator testing methodologies. When assessing the effect of varying the anteversion angle under ISO gait loading conditions and flexion-extension hip rotations, no effect on the contact pressure was observed. The cup diametral clearance was also increased from 80 µm to 150 µm, however, this had very little effect on the contact pressure magnitudes and the contact profiles were almost matching. The variation of shear stress occurred in line with variation of ISO loading, where the maximum shear stress of 8.5 MPa was present on the surface of the acetabular cup, at the time of peak vertical loading. Varying the surface coefficient of friction had a negligible effect on the surface shear stresses under ISO loading and component sliding contact simulations. The importance of checking the energy results of any implicit or explicit analysis was discussed and justified. The energy results and assessments provided an additional level of confidence in the results obtained during this project using the finite element method, especially with regards to the complexity of modelling sliding contact.
The profile of the maximum contact pressure throughout the time step was representative of the variation in vertical load throughout the single gait cycle. Reducing the cup rim radius
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had no effect on reducing the maximum contact pressure throughout the gait cycle as no rim contact was observed without the inclusion of microseparation. The fixation area between the cup and pelvis also did not have a considerable effect on the results. Less than a 1% difference of the average maximum contact pressure was observed when the contact fixation area between the superior surface of the acetabular cup and inferior surface of the pelvis acetabulum was increased by 15%. The total edge load magnitude was applied in the first step of the analysis and therefore the maximum edge load was present at the beginning of the subsequent step, which in this case was the flexion-extension cycle. This type of kinematic and kinetic condition meant that the maximum contact pressure distribution did not follow the gait cycle loading profile applied in the analysis. As the edge load was removed the femoral head was free to relocate under the application of the ISO gait vertical loading. The combined effect of a maximum edge load and the application of the gait cycle meant that the maximum contact pressure occurred between the cycle time 1 s to 1.6 s which was within the region of the loading cycle where more than 10% of the vertical loading occurs and the peak loads occurred at 10% and 50% of the cycle time.
From the contact pressures of model A and model B under normal vertical loading with the application of an edge load, the average edge load factor was determined to be 5.6. This means that an edge load can increase the maximum contact pressure to around 5.6 times greater than the maximum pressure observed under normal/vertical loading conditions. This includes the analysis where the edge loading was modelled with a lateral displacement and an analysis where the edge loading was observed through a pure separation of the joint, i.e. laxity of the joint leading to relocation by the application of a vertical load causing acetabular cup rim contact.
Model B-2 was also used to assess the performance of using the computer’s central processing unit (CPU) with hyperthreading technology when using the Abaqus postprocessor. Such