Abstract
Introduction
New challenges arise in total hip arthroplasty (THA) as patients are younger and perform higher levels of activity. Implants need to stand increased loads, last longer and improve bone stock conservation[1] for future revision. Additive manufacturing allows optimizing the implant shape and material properties imposing few restrictions. The mechanical properties of porous meta-materials can be adjusted by tailoring their meso-structure, allowing for a functional gradation of the material properties (i.e. elastic modulus) throughout the stem.
The objective of this paper is to use finite element analysis for optimizing the shape and the functional gradation of material properties distribution of hip stems in order to reduce the bone loss and to obtain lower and more homogeneous interfacial stresses.
Methods
The 2D stem geometry (initially Profemur®TL) was parameterized with 8 variables. Limits were established to keep tapered stem shape, avoid intersecting the cortexes and assure proper cortical contact. A functional gradation of the stem's material properties was generated by prescribing the values of the elastic modulus (E) on a 53 points grid. Values for E were between 2 GPa (highly porous meta-material made of Ti6Al4V) and 110 GPa (solid Ti6Al4V). The stem neck and a 1.5 mm layer around the stem were kept solid.
Two contradictory objective functions were considered: 1) a function of the total bone loss, accounting for the bone losses due to the resection for the implant insertion and due to stress shielding; 2) a function of the interfacial shear stresses, accounting for their uniformity and value. This multi-objective optimization problem was solved using genetic algorithms for stair climbing load case[2], with 30090 stem design evaluations for a total of 50 generations (iterations).
Two representative optimized stem designs were selected to undergo a second step of tailoring their porous meta-material for obtaining the desired material properties distribution. Simple-cubic unit cell was considered at the mesoscale of the porous meta-material, with a fixed unit-cell length of 1.5 mm. The strut diameter at each point of the grid was optimized to match the prescribed E using a previously developed model of porous meta-materials that includes the manufacturing irregularities[3].
Results
Fig.1 shows FBLFIS functions for the optimized stem designs at the last generation. For optimized stems FBL=21.5–26.6% and FIS=0.27–3.64; for the original stem design FBL=35.4% and FIS=10.20. Optimized stems are shorter, thinner and less stiff than the original stem design; and they show smaller extent of bone resorption and smaller and more homogeneous interfacial shear stresses (Fig.2). The E distribution is adequately reproduced by the porous material (Fig.2).
Conclusion
Combining shape and material properties distribution optimization of hip stems can improve their mechanical compatibility with the bone, reducing the bone loss and interfacial shear stresses. The material properties distribution can be adequately obtained by tailoring additively manufactured porous meta-materials. This is expected to improve the THA long-term outcome and the conditions for future revision.
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