Varus alignment in total knee replacement (TKR) results in a larger portion of the joint load carried by the medial compartment.[1] Increased burden on the medial compartment could negatively impact the implant fixation, especially for cementless TKR that requires bone ingrowth. Our aim was to quantify the effect varus alignment on the bone-implant interaction of cementless tibial baseplates. To this end, we evaluated the bone-implant micromotion and the amount of bone at risk of failure.[2,3] Finite element models (Fig.1) were developed from pre-operative CT scans of the tibiae of 11 female patients with osteoarthritis (age: 58–77 years). We sought to compare two loading conditions from Smith et al.;[1] these corresponded to a mechanically aligned knee and a knee with 4° of varus. Consequently, we virtually implanted each model with a two-peg cementless baseplate following two tibial alignment strategies: mechanical alignment (i.e., perpendicular to the tibial mechanical axis) and 2° tibial varus alignment (the femoral resection accounts for additional 2° varus). The baseplate was modeled as solid titanium (E=114.3 GPa; v=0.33). The pegs and a 1.2 mm layer on the bone-contact surface were modeled as 3D-printed porous titanium (E=1.1 GPa; v=0.3). Bone material properties were non-homogeneous, determined from the CT scans using relationships specific to the proximal tibia.[2,4] The bone-implant interface was modelled as frictional with friction coefficients for solid and porous titanium of 0.6 and 1.1, respectively. The tibia was fixed 77 mm distal to the resection. For mechanical alignment, instrumented TKR loads previously measured Introduction
Methods
The interaction between the mobile components of total elbow replacements (TER) provides additional constraint to the elbow motion. Semi-constrained TER depend on a mechanical linkage to avoid dislocation and have greater constraint than unconstrained TER that rely primarily in soft tissue for joint stability. Greater constraint increases the load transfer to the implant interfaces and the stresses in the polyethylene components. Both of these phenomena are detrimental to the longevity of TER, as they may result in implant loosening and increased damage to the polyethylene components, respectively[1]. The objective of this work was to compare the constraint profile in varus-valgus and internal-external rotation and the polyethylene stresses under loads from a common daily activity between two semi-constrained TER, Coonrad/Morrey (Zimmer-Biomet) and Discovery® (DJO), and an unconstrained TER, TEMA (LimaCorporate). We developed finite element (FE) models of the three TER mechanisms. To reduce computational cost, we did not include the humeral and ulnar stems. Materials were linear-elastic for the metallic components (ETi6Al4V=114.3 GPa, ECoCr=210 GPa, v=0.33) and linear elastic-plastic for the polyethylene components (E=618 MPa, v=0.46; SY=22 MPa; SU=230.6 MPa; εU=1.5 mm/mm). The models were meshed with linear tetrahedral elements of sizes 0.4–0.6 mm. We assumed a friction coefficient of 0.02 between metal and polyethylene. In all simulations, the ulnar component was fixed and the humeral component loaded. We computed the constraint profiles in full extension by simulating each mechanism from 8° varus to 8° valgus and from 8° internal to 8° external rotation. All other degrees-of-freedom except for flexion extension were unconstrained. Then, we identified the instant during feeding that generated the highest moments at the elbow[2], and we applied the joint forces and moments to each TER to evaluate the stresses in the polyethylene. To validate the FE results, we experimentally evaluated the constraint of the design with highest polyethylene stresses in pure internal-external rotation and compared the results against those from a FE model that reproduced the experimental setup (Fig.1-a).Introduction
Methods
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. 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 ( 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 Introduction
Methods
Finite element (FE) models are commonly used to analyse the mechanical behaviour of the bone under different conditions. They provide detail information but they can be numerically expensive and this limits their use in cases where large or numerous simulations are required. On the other hand, 2D models show less computational cost but the precision of results depends on the approach used for the simplification. Three 2D approaches are commonly used: models without side-plate (WOSP)[1]; models with variable thickness side-plate and constant cortical thickness (SPCT)[2]; models with side-plate and variable cortical thickness (SPVT)[3]. The aim of this study is to determine which 2D approach reproduces best the FE results obtained with a 3D model involving hip stems. The 2D models were generated by the intersection of the 3D model with the stem symmetry plane. Three approaches were considered to assure 3D-2D correspondence: 1) consider variable thickness for the cortical elements so that their transversal area moment of inertia equals the cross-sectional area moment of inertia from the 3D model (model WOSP); 2) include an additional side-plate with variable thickness to match the area moment of inertia from the 3D model, and consider constant thickness for the cortical bone elements (model SPCT); 3) include the side-plate but consider variable thickness for the cortical bone elements, derived from the 3D model (model SPVT). In all cases, the cancellous bone and stem elements had variable thickness computed so that their transversal area moment of inertia was equal to the cross-sectional area moment of inertia measured in the 3D model. This was done at different levels (Fig.1), providing a thickness distribution for the 2D elements. FE analyses were carried out for the static loading condition simulating stair climbing[4]. All materials were defined as linear isotropic and homogeneous. The post-operative situation where bone ingrowth is achieved was considered, resulting in bonded contact between the bone and the implant. The comparison between the 2D and 3D models was done based on three physical quantities: the Von Mises stresses (σVM); the strain energy density (Introduction
Methods
Stress shielding is one of the major concerns of load bearing implants (e.g. hip prostheses). Stiff implants cause stress shielding, which is thought to contribute to bone resorption1. On the contrary, low-stiffness implants generate high interfacial stresses that have been related to pain and interfacial micro-movements². Different attempts have been made to reduce these problems by optimizing either the stem design3 or using functionally graded implants (FGI) where the stem's mechanical properties are optimized4. In this way, new additive manufacturing technologies allow fabricating porous materials with well-controlled mesostructure, which allows tailoring their mechanical properties. In this work, Finite Element (FE) simulations are used to develop an optimization methodology for the shape and material properties of a FGI hip stem. The resorbed bone mass fraction and the stem head displacement are used as objective functions. The 2D-geometry of a femur model (Sawbones®) with an implanted Profemur-TL stem (Wright Medical Technology Inc.) was used for FE simulations. The stem geometry was parameterized using a set of 8 variables (Figure 1-a). To optimize the stem's material properties, a grid was generated with equally spaced points for a total of 96 points (Figure 1-b). Purely elastic materials were used for the stem and the bone. Two bone qualities were considered: good (Ecortical=20 GPa, Etrabecular=1.5 GPa) and medium (Ecortical=15 GPa, Etrabecular=1 GPa). Poisson ratio was fixed to v=0.3. Loading corresponded to stair climbing. Hip contact force along with abductors, vastus lateralis and vastus medialis muscles were considered5 for a bodyweight of 847 N. The resorbed bone mass fraction was evaluated from the differences in strain energy densities between the intact bone and the implanted bone2. The displacement of the load point on the femoral head was computed. The optimization problem was formulated as the minimization of the resorbed bone mass fraction and the head displacement. It was solved using a genetic algorithm.Introduction
Methodology
After arthroplasty, stress shielding and high shear stresses at the bone-implant interface are common problems of load bearing implants (e.g. hip prostheses). Stiff implants cause stress shielding, which is thought to contribute to bone resorption1. High shear stresses, originated by low-stiffness implants, have been related to pain and interfacial micro-movements², prohibiting adequate implant initial fixation. A non-homogeneous distribution of mechanical properties within the implant could reduce the stress shielding and interfacial shear stresses3. Such an implant is called “functionally graded implant” (FGI). FGI require porous materials with well-controlled micro-architecture, which can now be obtained with new additive manufacturing technologies (e.g. Electron Beam Melting). Finite element (FE) simulations in ANSYS-v14.5 are used to develop an optimization methodology to design a hip FGI. A coronal cut was performed on a femur model (Sawbones®) with an implanted Profemur®TL (Wright Medical Inc.) stem to obtain the 2D-geometry for FE simulations. The central part of the FGI stem was made porous, the neck and inferior tip were solid. Ti6Al4V elastic material was assumed (E=120 GPa, v=0.3). Three bone qualities were considered for the optimization: poor (E=6GPa; v=0.3); good (E=12GPa; v=0.3); excellent (E=30GPa; v=0.3). The structure of bone evolves to maintain a reasonable level of the strains. Similarly in the proposed algorithm, the strut sections of the porous material evolve to keep stresses (proportional to strains) at a reasonable level. Starting with a very small strut section, resulting in an almost zero-rigidity stem, strut sections are increased or decreased as a function of the stresses they support. This is done incrementally, until force values corresponding to normal walking of an 80 kg person (1867 N)4 are reached. Force direction was vertical and no action of the abductors was considered, to analyze the worst case scenario. The optimized FGI microstructure is defined by the strut diameter distributions. Since the distance between struts remain constant, variations in strut diameters result in variations in density. Optimized FGI porous structure was compared for the three bone qualities considered and with a solid stem in terms of bone stresses.Introduction
Methodology
Porous metallic materials, due to their capability of tailoring their mechanical properties to those of bone, have been suggested to be utilized in prosthesis to avoid the stress shielding phenomenon1, believed to increase the risk of implant loosening2. The aim of this work is to obtain the most simplified model possible to simulate the mechanical behavior of a Ti6Al4V porous structure. For this purpose, a beam element model was analyzed and the results were then compared to a 3D-solid model. Two computational models of the porous structure were developed: a 3D solid model, considered as the reference for comparison, and a beam model as a simplified and computationally inexpensive approximation (Fig. 1). CATIA V5R20 (3D modelling) and ANSYS V13 (simulations) were used. Isotropic elastic material model was used. Strut diameter (ϕb) was set to 450 μm, pore diameter (ϕp) was varied between 600 and 5000 μm, and pore number (np) between 2 and 9. Structures sizes varied from 2.1 × 2.1 × 2.1 mm3 to 49.05 × 49.05 × 49.05 mm3. Apparent elastic modulus (Eap) and its difference between both models (error) were analyzed for the different values of ϕp and slenderness ratio (SR). In addition, the influence of loading direction was analyzed with the beam model for cubic and diamond cell geometries. Eap variations were compared.INTRODUCTION
EXPERIMENTAL METHODS
