Abstract
Stiff total hip arthroplasty implants can lead to strain shielding, bone loss and complex revision surgery. The aim of this study was to develop topology optimisation techniques for more compliant hip implant design. The Solid Isotropic Material with Penalisation (SIMP) method was adapted, and two hip stems were designed and additive manufactured: (1) a stem based on a stochastic porous structure, and (2) a selectively hollowed approach. Finite element analyses and experimental measurements were conducted to measure stem stiffness and predict the reduction in stress shielding. The selectively hollowed implant increased peri-implanted femur surface strains by up to 25 percentage points compared to a solid implant without compromising predicted strength. Despite the stark differences in design, the experimentally measured stiffness results were near identical for the two optimised stems, with 39% and 40% reductions in the equivalent stiffness for the porous and selectively hollowed implants, respectively, compared to the solid implant. The selectively hollowed implant’s internal structure had a striking resemblance to the trabecular bone structures found in the femur, hinting at intrinsic congruency between nature’s design process and topology optimisation. The developed topology optimisation process enables compliant hip implant design for more natural load transfer, reduced strain shielding and improved implant survivorship.
Highlights
Total hip arthroplasty (THA) is one of the most successful surgical procedures in modern medicine, providing a treatment option for debilitating diseases such as end-stage osteoarthritis and osteonecrosis of the femoral head
As topology optimisation does not presume a priori material distribution, the algorithm has the liberty to leave specific locations void of material, whereas the corresponding size optimisation algorithm would be unable to remove unit elements from the lattice, merely reducing their sizes to a specified minimum. Such an approach has been demonstrated to reduced strain shielding in fracture plates [23], and has shown success in modelling the growth of internal bone structures when implemented with geometric constraints [24]
The technique is useful, as imposing a volume fraction of less than one will always result in a concomitant increase in the minimum compliance when compared to a volume fraction equal to one
Summary
Total hip arthroplasty (THA) is one of the most successful surgical procedures in modern medicine, providing a treatment option for debilitating diseases such as end-stage osteoarthritis and osteonecrosis of the femoral head. As topology optimisation does not presume a priori material distribution, the algorithm has the liberty to leave specific locations void of material, whereas the corresponding size optimisation algorithm would be unable to remove unit elements from the lattice, merely reducing their sizes to a specified minimum Such an approach has been demonstrated to reduced strain shielding in fracture plates [23], and has shown success in modelling the growth of internal bone structures when implemented with geometric constraints [24]. The technique is useful, as imposing a volume fraction of less than one will always result in a concomitant increase in the minimum compliance when compared to a volume fraction equal to one This approach will inherently produce more compliant stems despite maximizing the stiffness and strength at the set volume fraction. Two proof-ofconcept designs were manufactured and tested: (1) a porous implant, similar to those previously described but utilising a stochastic trabecular-like structure rather than cellular lattices, and (2) a novel selectively hollowed implant that maintains the outer shape of the implant such that only the stiffness of the stem is reduced, with no other design changes
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