Fatigue design of a mechanically biocompatible lattice for a proof-of-concept femoral stem

Abstract

A methodology is proposed to design a spatially periodic microarchitectured material for a twodimensional femoral implant under walking gait conditions. The material is composed of a graded lattice with controlled property distribution that minimizes concurrently bone resorption and interface failure. The periodic microstructure of the material is designed for fatigue fracture caused by cyclic loadings on the hip joint as a result of walking. The bulk material of the lattice is Ti6AL4V and its microstructure is assumed free of defects. The Soderberg diagram is used for the fatigue design under multiaxial loadings. Two cell topologies, square and Kagome, are chosen to obtain optimized property gradients for a two-dimensional implant. Asymptotic homogenization (AH) theory is used to address the multiscale mechanics of the implant as well as to capture the stress and strain distribution at both the macro and the microscale. The microstress distribution found with AH is also compared with that obtained from a detailed finite element analysis. For the maximum value of the von Mises stress, we observe a deviation of 18.6 % in unit cells close to the implant boundary, where the AH assumption of spatial periodicity of the fluctuating fields ceases to hold. In the second part of the paper, the metrics of bone resorption and interface shear stress are used to benchmark the graded cellular implant with existing prostheses made of fully dense titanium implant.The results show that the amount of initial postoperative bone loss for square and kagome lattice implants decreases, respectively, by 53.8% and 58%. In addition, the maximum shear interface failure at the distal end is significantly reduced by about 79%.A set of proof-of-concepts of planar implants have been fabricated via Electron Beam Melting (EBM) to demonstrate the manufacturability of Ti6AL4V into graded lattices with alternative cell size. Optical microscopy has been used to measure the morphological parameters of the cellular microstructure, including cell wall thickness and pore size, and compared them with the nominal values. No sign of fracture or incomplete cell walls was observed, an assessment that shows the satisfactory metallurgical bond of cell walls and the structural integrity of the implants.

Fully porous 3D printed titanium femoral stem to reduce stress-shielding following total hip arthroplasty

“…The design domain of the prosthesis is assumed to possess a 3D lattice micro‐architecture, obtained through an aperiodic tessellation of a tetrahedron based unit cell, which has been shown appropriate for both load bearing orthopaedic applications and bone ingrowth . Mechanical properties, in particular the homogenized stiffness tensor [ E H ] and the multiaxial yield surface { σ y }, are calculated via Asymptotic Homogenization (AH) theory . We have shown that AH theory can capture stress distribution within the micro‐structure with a considerably higher accuracy compared to other homogenization approaches .…”

“…Some existing works on porous materials focus on their use as surface coating on the implant to allow bone in‐growth to achieve biologic fixation . Other works attempt to use porous materials for bone replacement, but they are mainly limited to computational modeling, manufacturing and testing of small samples, morphological characterization, and proof‐of‐concept implants with uniform porosity . So far, no work has successfully tackled the challenge of using a fully porous material for femoral stems.…”

Specific Yielding of Selective Laser-Melted Ti6Al4V Open-Porous Scaffolds as a Function of Unit Cell Design and Dimensions

“…This phenomenon known as stiffness mismatch can result in stress shielding [5,6], which has been identified as a cause for the aseptic loosening of orthopaedic implants, the main cause of their failure [7,8]. In order to reduce the risk of stress shielding, metal alloy implant components can be fabricated for instance with open-porous structures that provide an elastic modulus low enough for this specification [9][10][11][12][13][14]. Open porous structures were generated and tested as irregular [15][16][17][18] and regular or non-stochastic scaffolds [9,10,[19][20][21][22][23][24][25][26].…”

Microstructure Interpolation for Macroscopic Optimisation

“…In addition to the idealised domain, the objective function is missing a component dealing with the strength of the prosthesis. Strength is an important requirement and it should be ensured that a bone prosthetic be able to withstand the rigours of day-to-day use over a long time period (Arabnejad Khanoki and Pasini, 2013). More suitable loading conditions, geometry and non-linear analysis can be seen in the works of Arabnejad Khanoki and Pasini (2013); Chanda et al (2015b); and Arabnejad Khanoki et al (2016a) for example, which consider multiple load cases on a more complicated geometry generated from CT data.…”

“…Aseptic loosening leads to revision surgeries which are costly and cause patient discomfort. Mathematically, the chance of implant failure is typically quantied as a function of the stress on the interface between the prosthetic and the existing bone (Kuiper and Huiskes, 1997;Arabnejad Khanoki and Pasini, 2012;Chanda et al, 2015b), however fatigue has been considered (Arabnejad Khanoki and Pasini, 2013). A second major consideration in implant design is how much existing bone material will be removed by the body in response to changed loading conditions, termed bone resorption (Engh et al, 1992).…”