Optimal design of femoral hip prosthesis using topology optimisation to reduce stress shielding

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Date
2005
Authors
Ridzwan, Mohamad Ikhwan Zaini
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Abstract
Introducing an implant into a femur might reduce the natural stress distribution of the femur. The reduction could cause its density and volume shrinkage. The implant starts to loose and causes patients hardly to move, thus needed a revision surgery. The phenomenon of reduction in load was identified as stress shielding. This study was conducted to find the factors that will contribute to the stress shielding and to apply topology optimisation method to minimise the problem. A simple composite beam theory was used and it proved that implant material and its crosssectional area would mostly affect the load distribution in femur (Ft/F). The results showed that the implant materials with higher modulus of elasticity compared with femur such as Titanium (Ti) and Cobalt-chromium (Co-Cr) would reduce Ft!F to 43.04% and 58.23%. Hollow implant would increase the Ft!F to 15.19% (iso-elastic), 73.33% (Ti) and 109.10% (Co-Cr). Topology optimisation method was employed in the analysis of mod~l of implant, cement and femur in 3-dimension by using ANSYS 7.1. The objective of the optimisation was to minimise implant compliance subjected to percentage of reduction in its initial volume (V0 ) ranges from 30% V0 up to 70% V0 . The analysis was performed with the assumption that the models responded linearly, isotropic and also homogeneous. Titanium was chosen as an implant material. The load and reaction from muscle of greater trochanter occurred during walking were used in the analysis. Model was constrained at the distal end of femur along x, y and z-axes. Results showed that implant with 50% V0 or 60% V0 would produce closed boundary and hence were acceptable in shape. Both implants were compared in stress distribution with conventional implant and intact femur (without implant). All values were obtained from locations along medial and lateral sides of femur, cement and implant. Results showed that, stresses produce in both optimum models were very close to each other. Load transfer has increased in femur with the optimised implants almost 4% compared to before optimise in medial and lateral side. Although the differences were not too far, but, it has been proved that optimised implants have tried to bring the stress as closed as in intact femur especially along the length of implants. Hence, it showed that the new optimised implants were better than the conventional implant in order to reduce stress shielding problem.
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Keywords
Topology optimisation , Stress shielding
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