Effect of alignment.

The effect of socket alignment upon the calculated interface pressure distribution was investigated by repeating the model of the subject’s issued limb, again using the measured mechanical properties, but assuming that the neutral axis of the limb was vertical during loading. Only a z-wise component of displacement to the socket was used, therefore, to model the half-bodyweight loading.

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(a) (b)

Figure 8.9 • Effect of alignment upon interface pressure distribution. (a) with bench alignment.

(b) with neutral alignment.

In figures 8.9 (a) and (b) the pressure distribution under these conditions is compared with that which incorporated the bench alignment

Chapter 8 - Models based upon measured data 170

8.5 Discussion of results.

Effect of rectification

The correlation between the rectification grids and pressure grids of figures 8 . 6 and

8.7 is intuitively reasonable. Areas which are heavily rectified have high normal interface pressures. In addition, the influence of the distribution of mechanical properties is seen with higher pressures in the stiffer regions.

The peak pressure encountered in the model of the issued socket was 196 kPa in magnitude and was located at the patellar bar. In this model, other peaks were identified in the popliteal depression and in the anteromedial and anterolateral tibial regions. The maximum pressures at these locations were 115,71 and 105 kPa respectively.

When heavy rectification was applied to the limb shape, the peak pressure at the patellar tendon rose to 291 kPa; the rectification here had been increased by approximately 50%. In the popliteal, anteromedial and anterolateral regions peak pressures increased to 152, 105 and 287 kPa respectively. At each of these locations

pressures could be predicted, within 2 0%, by linearly scaling those values found in the

previous model by the proportional increase in rectification. The results from this model might be expected to show a redistribution of pressure with higher localised pressures in those areas where rectification has been increased and reduced pressures elsewhere. Nevertheless, the analysis does not show this, and normal pressures all over the interface have been increased. The influence of shear forces at the interface in these 'totally rough' models must not be forgotten, however, and it may be that in the heavily rectified socket shear forces contribute less to the support of the limb.

With no rectification, a maximum pressure of 44 kPa was found at the base of the patella and a large region of normal pressure existed to the lateral side of the tibia, although pressures here were all below 42 kPa. The wall of this 'socket' will have only small inclination to the vertical over most of its surface and the action of shear forces may contribute efficiently to the support of the limb. This may account for the low pressures calculated.

Chapter 8 - Models based upon measured data 171

Shear force distributions have not been produced since no measured experimental values have been found with which comparisons may be made and because it is not possible to define a consistent set of axes on the irregular socket surface from which components of these forces may be calculated. With no knowledge of the directions in which shear forces are acting it is difficult to determine what contribution they make to the support of the limb. It might, however, be assumed that an increase in the relative movement between the bone and socket will increase the shearing action at the interface. Relative displacements in the heavily rectified socket, issued socket and unrectified socket were 1.9, 3.3 and 4.1 mm respectively and therefore the influence of shear forces may be expected to increase where less rectification is used. An overall reduction in normal pressures would be expected to accompany increased shear and this is consistent with the results of the models.

Effect of material properties

Where a uniform modulus was used, a redistribution of pressure is observed. In this model, lower pressures were found in the patellar tendon and anterolateral regions, which were made more compliant with respect to the measured moduli. In the popliteal and anteromedial regions, which had been stiffened, pressures rose, although less significantly. This is to be expected since the deviations from the measured moduli here are smaller than those in the stiffer regions. The maximum pressure in this model was located at the patellar bar and was 138 kPa.

With the range of moduli exaggerated, the reverse trend was observed, as expected. Pressures in the stiffer regions rose with respect to those modelled using the measured properties, while pressures fell in the more compliant regions. Again, the changes in calculated pressures were more significant in the stiffer regions. The maximum pressure in this model was 272 kPa. This pressure is at the high end of the range of published pressured measured during walking and it would seems unlikely that such high pressure will exist in a comfortable socket in standing. This might indicate that the exaggerated range of moduli is unlikely to exist in a real limb.

Variations in the effects of shear in these models are expected to be small. Relative movements in models using the uniform or the wide range of moduli were 3.2 and 3.5 mm respectively.

Chapter 8 - Models based upon measured data 172

Effect of alignment

With bench alignment, flexion and adduction of the knee are encouraged and pressures on the anterior and lateral aspects of the limb are expected to be higher than with the socket in neutral alignment. This is found in the FE models. The model using neutral alignment shows increased pressures on the posterior and medial aspect of the limb with respect to those found using the bench alignment; reduced pressures are calculated on the anterior and lateral aspects. Alterations to the pressure distribution are not great; a maximum difference in local peak pressures of 13% is found in the anterolateral region.

The vertical displacement with neutral alignment was 3.2 mm and therefore a similar shearing action at the interface may be expected to that encountered with the bench alignment.