It is known that severe cases of intervertebral disc (IVD) disease may lead to the loss of natural intervertebral height, which can cause radiating pain throughout the lower back and legs. To this point, surgeons perform lumbar fusion using interbody cages, posterior instrumentation and bone graft to fuse adjacent vertebrae together, thus restoring the intervertebral height and alleviating the pain. However, this surgical procedure greatly decreases the range of motion (ROM) of the treated segment, mainly caused by high cage stiffness. Additive manufacturing can be an interesting tool to reduce the cage's elastic modulus (E), by adding porosity (P) in its design. A porous cage may lead to an improved osteointegration since there is more volume in which bone can grow. This work aims to develop a finite element model (FEM) of the L4-L5 functional spinal unit (FSU) and investigate the loss of ROM induced by solid and porous cages. The Intact-FEM of L4-L5 was created, which considered the vertebrae, IVD and ligaments with their respective material properties1. The model was validated by comparing its ROM with that of other studies. Moments of 10 Nm were applied on top of L4 while the bottom of L5 was fixed to simulate flexion, extension, lateral bending and axial rotation2. The lumbar cages, posterior instrumentation and bone graft were then modelled to create the Cage-FEMs. Titanium was chosen for the instrumentation and cages. Cages with different stiffness were considered to represent porous structures. The solid cage had the highest modulus (E0=110 GPa, P0=0%) whereas the porous cages were simulated by lowering the modulus (E1=32.8 GPa, P1=55%; E2=13.9 GPa, P2=76%; E3=5.52 GPa, P3=89%; E4=0.604 GPa, P4=98%), following the literature3. The IVD was removed in Cage-FEMs to allow the implant's insertion [Fig. 1] and the previous loading scenarios were simulated to assess the effects of cage porosity on ROM.Background
Materials and 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
Cementless stems are fixed to the surrounding bone by means of mechanical press-fit. Short-, mid-, and long term outcomes are good for this type of fixation despite that only a part of the stem surface is in contact with the surrounding bone. Several studies show that the contact ratio achieved after surgery between the stem and the surrounding bone ranged between 15% and 60%. Then, only a part of the stem-bone interface presents a press-fit. The rest of the stem-bone interface is only in contact or presents an interfacial gap inherent to the surgical technique. Therefore, this study aimed to investigate the difference in the primary stability of a cementless stem between a press-fit combined with contact and a press-fit combined with gap achieved after the surgery. A finite element study was carried out on a composite bone implanted with a femoral stem and subjected to physiological loading simulating stair climbing [1]. All materials were defined as isotropic homogeneous. The stem-bone interface was divided into 4 areas: the superior plasma spray, the inferior plasma spray, the polished surface of the stem in contact with the cancellous bone, and the plasma spray surface of the stem in contact with the cortical bone. Each contact area can be either in contact with a press-fit, either in contact without press-fit or can present a gap. This result in a total of 28 cases: 14 where there is a press-fit combined with contact and 14 cases where there is a press-fit combined with gap.Introduction
Materials & Methods
Modeling the press-fit that occurs in Total Hip Arthroplasty (THA) cementless implants is crucial for the prediction of micromotion using finite element analysis (FEA). Some studies investigated the effect of the press-fit magnitude and found a direct influence on the micromotion [1,2]. They assumed in their model that press-fit occurs throughout the prosthesis. However [3] found using computed tomography measurement that only 43% of the stem-bone interfaces is really in contact. The aim of this study is to investigate the press-fit effect at the stem-bone interface on the implant micromotion. Finite element analysis (FEA) was performed on a Profemur® TL implanted into a Sawbones®. The implant orientation was validated in a previous study [4]. All materials were defined as linear isotropic homogeneous. FEA was carried out for the static loading conditions defined by [5] simulating walking fastly. Frictional contact between the bone and the prosthesis was assumed all along the prosthesis with a coefficient μ set to 0, 63 for the plasma spray (Fig. 1a) and 0,39 for the polished surface (Fig. 1b) [6]. Firstly, FEA was performed without press-fit (Fig. 2a) and then press-fit was simulated with an interference of 0,05 mm [2] between stem and bone in specific areas: superior (Fig. 2b), intermediate (Fig. 2c), inferior (Fig. 2d), and cortical alone (Fig. 2e) and finally over the entire surface in contact with the bone. The press-fit effect at the stem-bone interface on the micromotion was investigated. Measurement of the micromotion was realised on different points located on the plasma spray surface by calculating the difference between the final displacement of the prosthesis and the final displacement of the bone.Introduction
Methods
