Introduction

Relative motion at the modular head-neck junction of hip prostheses can lead to severe surface damage through mechanically-assisted corrosion. One factor affecting the mechanical performance of modular junctions is the frictional resistance of the mating surfaces to relative motion. Low friction increasing forces normal to the head-neck interface, leading to a lower threshold for slipping during weight-bearing. Conversely, a high friction coefficient is expected to limit interface stresses but may also allow uncoupling of the interface in service. This study was performed to examine this trade-off using finite element models of the modular head-neck junction

Current implant designs and materials provide a high grade of quality and safety, but aseptic implant loosening is still the main reason for total hip revision. Highly cross-linked polyethylene (HX-PE) is used successfully in total hip replacements (THR) since several years. The good wear properties lead to a reduction of wear debris and may contribute to a longer survival time of the THRs. Furthermore, thin HX-PE liner allows the use of larger femoral heads associated with a decreased risk of dislocation and an improved range of motion. However, the cross-linking process is associated with a loss of mechanical properties of the polyethylene material which compromise the use of thin HX-PE liner in terms of high stress situations.

The aim of the present study was the experimental wear analysis of HX-PE liner under steep acetabular cup position. Furthermore, a finite element analysis (FEA) was performed in order to calculate the stress within the HX-PE material in case of steep cup position under physiological loading.

Experimental wear testing was performed for 5 Mio load cycles, using highly cross-linked polyethylene (HX-PE) acetabular liner combined with 44 mm ceramic femoral heads at a standard position of the acetabular cup (30° inclination) according to ISO 14242 as well as at 60° cup inclination. The wall thickness of the HX-PE liner was 3.8 mm. A hip wear simulator, according to ISO 14242 (EndoLab GmbH, Rosenheim, Germany), was used and wear was determined gravimetrically. Moreover, finite element models of the THR system at standard and steep cup position was created by Abaqus/CAE (Dessault Systemes Providence, USA). Using the finite element software Abaqus (Dessault Systemes Providence, USA) the total hip implants were physiologically loaded with maximum force of the gait cycle (3.0 kN). Thereby, the stresses within the HX-PE material were analysed.

The average gravimetrical wear rates of the HX-PE liners at standard implant position (30°) and 60° cup inclination showed small wear amounts of 3.15 ± 0.32 mg and 1.92 ± 1.00 mg per million cycles, respectively. The FEA revealed a clear increase of stresses at the HX-PE liner with respect to steep cup position (von Mises stress of 8.78 MPa) compared to ISO standard implant position (von Mises stress of 5.70 MPa).

The wear simulator tests could not demonstrate significant differences of gravimetrical wear amount of HX-PE liners under steep hip cup position compared to standard implant position. The small contact surface between the femoral head and the SX-PE liner during the wear testing may lead to the low wear rate of the misaligned acetabluar cup. Moreover, the FEA showed that the effect of a misaligned acetabular cup on the stresses within the polyethylene liner can be critical. Although an increase of wear could not be detected a steeper acetabular cup position using thin HX-PE liners should be avoided due to higher stresses preventing implant failure in clinical application.

Background. Surgical reconstruction of the anterior cruciate ligament is a common practice to treat the disability or chronic instability of the knee. Several factors associated with success or failure of the ACL reconstruction, including surgical technique and graft material and graft tension. We aimed to show how we can optimize the graft properties and achieve better post surgical outcomes during ACL reconstruction using 3-dimensional computational finite element simulation. Methods. In this paper, 3-dimensional model of the knee was constructed to investigate the effect of graft tensioning on the knee joint biomechanics. Four different grafts were compared: 1) bone-patellar tendon-bone graft (BPTB) 2) Hamstring tendon 3) BPTB and a band of gracilis 4) Hamstring and a band of gracilis. The initial graft tension was set as “0, 20, 40, or 60N”. The anterior loading was set to 134 N. Findings. Our study shows that the use of the discarded gracilis tendon, which usually excised after graft fixation, could be associated with a host of merits. Our results show that preserving this excess part of gracilis would decrease the required pretention load and, subsequently, could optimize biomechanical properties of the knee. Conclusion. Required pretension during surgery will have decreased significantly by adding a band of gracilis to the proper graft. Therefore, in addition to achieving normal stability of the knee, we can have lower risk of degradation

Experimental knee simulators for component evaluation or in vitro testing provide valuable insight into the mechanics of the implanted joint. The Kansas knee simulator (KKS) is an electro-hydraulic whole joint knee simulator, with five actuators at the hip, ankle and quadriceps muscle used to simulate a variety of dynamic activities in cadaveric specimens. However, the number and type of experimental tests which can feasibly be performed is limited by the need to make physical component parts, obtain cadaveric specimens and the substantial time required to carry out each test. Computational simulations provide a complementary toolset to experimental testing; experimental data can be used to validate the computational model which can subsequently be used for early evaluation and ranking of component designs. The objective of this study was to explore potential improvements to loading and boundary conditions in current computational/experimental models, specifically the KKS, in order to develop representations of several activities of daily living (ADLs) which reproduce in vivo knee joint loading measurements.

An existing finite element model of the KKS was modified to extend the capability, and improve the fidelity, of the computational model beyond the experimental setup. An actuator to allow anterior-posterior (A-P) motion of the hip was included and used to prescribe relative hip-ankle A-P kinematics during the simulations. The quadriceps muscle, which in the experimental simulator consisted of a single quadriceps bundle with a point-to-point line of action, was divided into four heads of the quadriceps with physiological muscle paths. The hamstrings muscle, which was not present in the experiment, was represented by point-to-point actuators in four bundles. A flexible control system was developed which allowed control of the quadriceps and hamstrings actuators to match a knee flexion profile, similar to actuation of the experimental KKS, but also allowed control of the compressive tibiofemoral (TF) joint force, medial-lateral (M-L) load distribution, internal-external (I-E) torque and A-P load at the joint. A series of sensors, measuring all six load components on the medial and lateral compartments of the tibial insert, as well as knee flexion angle, were incorporated into the simulation. Instantaneous measurements from the sensors were fed to a control system, implemented within an Abaqus/Explicit user subroutine (Figure 1). The controller was used to drive actuators in the FE model to match target in vivo joint loading profiles, measured from telemetric patient data. The control system was applied to recreate in vivo loading conditions at the knee joint during three ADLs for three different subjects (Figure 2), with excellent agreement between simulation joint loading conditions and the target profiles; RMS differences were less than 1°, 80N, 2.5%, and 0.8Nm for knee flexion angle, compressive joint load, M-L load split and I-E torque, respectively, throughout the cycle for all three activities (Figure 3). The flexible nature of the control system ensures that it can be used to evaluate an expansive variety of ‘effect of’ studies, as well as to determine advanced loading profiles for the experimental simulator.

The wear particles released from the polyethylene (PE) tibial insert of modular total knee replacements (TKRs) have been shown to cause wear particle induced osteolysis, which may necessitate revision surgery [1]. Wear occurs at the backside surface of the PE insert of modular TKRs, resulting from the relative movement between the PE insert and the tibial tray [2]. Wear particles generated from the backside surface of the PE insert have been shown to be smaller in size than those originating from the articular surface [1], and may therefore have increased biological activity and osteolytic potential [3-4]. The ability to predict backside micromotion and contact pressure by finite element simulation has previously been demonstrated by O'Brien et al. [6-7]. Although the effect of insert thickness on articular surface contact pressure has been investigated [5], the effects of insert thickness on backside contact pressures, backside micromotion, and wear has not received adequate attention. Brandt et al. [2] has suggested that increased insert thickness was associated with increased backside damage (Fig. 1). In the present study, finite element simulations were conducted using the Sigma - Press Fit Condylar TKR (Sigma-PFC®, DePuy Orthopedics Inc., Warsaw, IN) with inserts of different insert thickness ranging between 5, 10, 15, 20 and 25 mm. The TKRs were simulated under ISO 14343-2 [8]. A non-linear PE material model was implemented by means of the J2-plasticity theory [6] and the effects of insert thickness on backside micromotion and contact pressure were analyzed. At the peak loading of the simulated gait cycle (time=13%), the 5 mm thick PE insert showed a greater backside peak contact pressure than the 25 mm thickness PE insert. Increasing insert thickness from 5 mm to 25 mm lead to approximately 15% greater peak micromotion at the modular interface (Fig. 2). This effect may be attributed to the ability of the PE material to distribute the load more evenly through deformation at the modular interface and reduce micromotion for thinner inserts. It is suggested that increased insert thickness results in increased moments at the modular interface that could lead to increased backside wear in silico. Although an increase in PE insert thickness was only associated with a moderate increase in backside micromotion in the present study, it was deemed likely that backside micromotion could be accelerated for thicker inserts in vivo as the PE locking mechanism has been shown to degrade after extended implantation periods

Introduction. Metallic resurfacing systems have been widely used until pseudotumors and ALTR have been clinically found and related to excessive wear of these metal-on-metal hip systems. Hence, surgeons widely abandoned the use of resurfacing systems. Meanwhile, there is a ceramic on ceramic (CoC) resurfacing system (Embody, London, UK) made of zirconia toughened alumina (BIOLOX. ®. delta, CeramTec, Plochingen, Germany) in a clinical safety study. Even though conventional CoC hip systems are known for their excellent wear behavior, it has to be ensured that intraoperative and in-vivo deformations of the ceramic acetabular cup do not infringe the proper functionality of the system. The method of determining the minimum clearance of such a system will be presented here. Materials and Methods. Combined experimental and numerical results were used to determine the deformation of the ceramic shell. In a cadaver lab, the resulting deformations after impaction of generic metal shells have been measured, see e.g. [1] for the method of measurement. The maximum deformation has been chosen for further calculation. Additionally, the stiffness of both generic metal and ceramic shells has been measured using ISO 7206–12. The deformation of the ceramic shells were then calculated by the equation. where u. c. and u. m. are the deformations of the ceramic and the metal shell, respectively, and K. m. and K. c. are the respective stiffnesses. Additionally, in a finite element simulation, the resulting deformation of the ceramic shell under in-vivo conditions was calculated and superposed with u. c. The resulting deformation was used as the minimum value of the clearance for the ceramic resurfacing system. Results. The average value of the maximum deformation of the 8 generic metal shells was 177 µm (StD. 68 µm). Using the stiffness values for the ceramic and the metal shells, a maximum deformation for the ceramic shells (with the smallest and the largest outer diameter) were calculated to 56 µm and 74 µm, respectively. The superposition with the results from the FE studies led to deformation values of 69 µm (smallest shell) and 87 µm (largest shell), respectively. These values were chosen as the minimum values for the realization of the minimum clearance. Discussion. The above described minimum clearance results from a worst-case scenario for the long-term deformation of the ceramic shells. The values from the experimental measurements were taken ten minutes after impaction in the cadaveric hips, when first relaxation already took place. Any other bone remodeling in the long-term, leading to further relaxation of the ceramic shell, has not been taken into account. The maximum deformations resulting from the numerical investigations have been superposed to the experimental values, assuming that both maximum deformations are acting in the same direction. In reality, this is most likely not the case because the line-of-action of the in-vivo forces acting on the hip are not collinear with the direction where the maximum deformation during intra-operative impaction takes place. Additionally, the experimentally chosen underreaming (1 mm) can also be considered as a worst-case. Hence, the calculated minimum clearances are representing the maximal deformation that in the long-term may take place in-vivo

Introduction. In total hip arthroplasty, press-fit anchorage is one of the most common fixation methods for acetabular cups and mostly ensures sufficient primary stability. Nevertheless, implants may fail due to aseptic loosening over time, especially when the surrounding bone is affected by stress-shielding. The use of acetabular cups made of isoelastic materials might help to avoid stress-shielding and osteolysis. The aim of the present numerical study was to determine whether a modular acetabular cup with a shell made of polyetheretherketone (PEEK) may be an alternative to conventional titanium shells (Ti6Al4V). For this purpose, a 3D finite element analysis was performed, in which the implantation of modular acetabular cups into an artificial bone stock using shells made of either PEEK or Ti6Al4V, was simulated with respect to stresses and deformations within the implants. Methods. The implantation of a modular cup, consisting of a shell made of PEEK or Ti6Al4V and an insert made of either ceramic or polyethylene (PE), into a bone cavity made of polyurethane foam (20 pcf), was analysed by 3D finite element simulation. A two-point clamping cavity was chosen to represent a worst-case situation in terms of shell deformation. Five materials were considered; with Ti6Al4V and ceramic being defined as linear elastic and PE and PEEK as plastic materials. The artificial bone stock was simulated as a crushable foam. Contacts were generated between the cavity and shell (μ = 0.5) and between the shell and insert (μ = 0.16). In total, the FE models consisted of 45,282 linear hexahedron elements and the implantation process was simulated in four steps: 1. Displacement driven insertion of the cup; 2. Relief of the cup; 3. Displacement driven placement of the insert; 4. Load driven insertion of the insert (maximum push-in force of 500 N). The FE model was evaluated with respect to the radial deformations of the shell and insert as well as the principal stresses in case of the ceramic inserts. The model was experimentally validated via comparison of nominal strains of the titanium shells. Results. The maximum radial deformation of the shell made of PEEK was 581 μm (insertion) and 470 μm (relief) and therefore multiple times higher compared to the Ti6Al4V shell (42 μm and 21 μm). As a result, larger deformations occurred at the PE and ceramic inserts in combination with the PEEK shell. Partially, the deformations were above an usual clearance of 100 μm. When the ceramic insert was combined with the shell made of PEEK, maximum principal stresses in the ceramic insert amounted to 30 MPa and were clearly lower than approved bending strength of the ceramic material (948 MPa). Conclusion. The examined acetabular shell made of PEEK was intensively deformed during insertion compared to the geometrically identical Ti6Al4V shell and is therefore not suitable for modular acetabular cups. In future studies it should be clarified to what extent acetabular cups with shells made of carbon fiber reinforced PEEK materials with higher stiffness lead to reduced deformations during the insertion procedure

Introduction. As population grows older, and patients receive primary joint replacements at younger age, more and more patients receive a total hip prosthesis nowadays. Ten-year failure rates of revision hip replacements are estimated at 25.6%. The acetabular component is involved in over 58% of those failures. From the second revision on, the pelvic bone stock is significantly reduced and any standard device proves inadequate in the long term [Villanueva et al. 2008]. To deal with these challenges, a custom approach could prove valuable [Deboer et al. 2007]. Materials and methods. A new and innovative CT-based methodology allows creating a biomechanically justified and defect-filling personalized implant for acetabular revision surgery [Figure 1]. Bone defects are filled with patient-specific porous structures, while thin porous layers at the implant-bone interface facilitate long-term fixation. Pre-operative planning of screw positions and lengths according to patient-specific bone quality allow for optimal fixation and accurate transfer to surgery using jigs. Implant cup orientation is anatomically analyzed for required inclination and anteversion angles. The implant is patient-specifically analyzed for mechanical integrity and interaction with the bone based upon fully individualized muscle modeling and finite element simulation. Results. Ten clinical severe pelvic bone defects, classified Parosky IIIb, have benefited from this methodology so far. Implant outline, thickness, fixation and cup orientation was adapted to the anatomical situation. Stress shielding of the bone was eliminated by taking into account personalized muscle anatomy, bone quality and patient weight while evaluating the design performance. All implants were applied smoothly intra-operatively because of personalized case documentation provided and jig technology for accurate pre-drilling of screw holes. Postoperative imaging showed excellent results. Short term follow-up indicates good outcome for the patients treated, restoring mobility and functionality, and showing solid anchorage to the bone. Conclusion. Complex acetabular bone defects can be treated adequately and efficiently, using a personalized solution which addresses both patient's and surgeon's needs

In recent years 3D preoperative planning has become increasingly popular with orthopaedic surgeons. One technique that has shown to be successful in transferring this preoperative plan to the operating room is based on surgical templates that guide various surgical instruments. Such a patient-specific template is designed using both the 3D reconstructed anatomy and the preoperative plan and is then typically produced via additive manufacturing technology. The combination of a preoperative plan and a surgical template has the potential to result in a more accurate procedure than an unguided one, when the following three criteria are met: the template needs to achieve a stable fit on the surgical field, it needs to fit in a unique position, and the surgeon needs to be able to determine the correct, planned position during the surgery. When the template fails one of these conditions, it can be used incorrectly. Consequently the process could result in an inaccurate outcome. This research focuses on modelling the stability of a surgical template on bone. The relationship between the contact surface of the template and the resulting stability is investigated with a focus on methods to quantify the template stability. The model calculates a quality score on the designed contact surface, which reflects the likelihood of positioning the template on the bone in a stable position. The model used in this study has been experimentally validated to verify its ability to provide a reliable indication of the template stability. This was analysed using finite element analysis where multiple templates and support models with different contact surface shapes were created. The application of forces and moments in varying directions was simulated. Stability is then defined as the ability of a template to resist an applied force or moment. The displacements of the templates were computed and analysed. The results show a minimal displacement of less than 0.01 mm and a maximal displacement larger than 10 mm. The former is considered to be a very stable template design; the latter to be very unstable and hence, would result in an insecure contact. The geometry of the contact surface had a clear influence on the template stability. Overall, the coverage of curvature variations improved the stability of the template. The displacements of the different finite element simulations were used as criterion for ranking the tested template designs according to their stability on their corresponding model surface. This ranking is then compared to that resulting from the quality score of the stability model. Both rankings showed a similar trend. This evaluation phase thus indicates that the developed stability model can be used to predict the stability of a surgical template during the preoperative design process

In biomechanical finite element (FE) simulations, the mechanical nonlinear behaviors must be considered frequently and depend on several properties, such as structural, material, and contact situation. The hexahedral meshes were widely applied to the modeling with the mechanical nonlinearities and can decrease the computer resources and improve the accuracy of the simulations. However, it is quite difficult to construct the three-dimensional hexahedral meshes of complicated shapes such as human joints. This study proposes the development of the semi-automatic meshing technique which consists of only hexahedral elements, thereby reducing the number of elements without spoiling the shape fidelities. In order to create the three-dimensional models of the tibial plateau and femoral condyle, the simply-shaped ‘seed’ models consisting of only hexahedral elements were prepared. The seed meshes were located into the surface of the target bone and expanded until they fitted the target surface. When the seed meshes expanded and intersected with the target surface, the contact condition was detected and the seed surface slide on the target one. These procedures are repeated until the seed meshes filled up inside the complicated target surface. Figure 1 shows the transformed and filled seed meshes inside the surface. The boundary between the cortical and cancellous bone was kept clearly. In the finite element meshes, there was no concentration of elements, and each hexahedral element had the good aspect ratio. Figure 2 shows the impact FE simulation of the TKR joint model, which was constructed by hexahedral elements using this technique. The impact stress propagated cleary through the TKR joint. The number of elements were reduced by a sixth, compared with that of the tetrahedral ones. Because the number of nodes and elements of the model can be defined beforehand, it is easy to predict the scale of the final model. Therefore, this technique is very effective in creating the huge skeltal models which build the complicated biomaterial shapes by the hexahedral elements

Tibial component loosening is an important failure mode in unicompartmental knee arthroplasty (UKA) which may be due to the 6–8 mm of bone resection required or the limited surface area. To address component loosening and fixation, a new Early Intervention (EI) design is proposed which reverses the traditional material scheme between femoral and tibial components. That is, the EI design consists of a plastic inlay component for the distal femur and a thin metal plate for the proximal tibia. With this reversed materials scheme, the EI design requires minimal tibial bone resection compared to traditional UKA to preserve the dense and stiff bone in the proximal tibia. This study investigated, by means of finite element (FE) simulations, the potential advantages of a thin metal tibial component compared with traditional UKA tibial components, such as an all-plastic inlay or a metal-backed onlay. We hypothesized that an EI component would produce comparable stress, strain, and strain energy density characteristics to an intact knee and more favorable values than UKA components. Indeed, the finite element results showed that an EI design reduced stresses, strains and strain energy density in the underlying support bone compared to an all-plastic UKA component. Analyzed parameters were similar for an EI and a metal-backed onlay, but the EI component had the advantage of minimal resection of the stiffest bone

The interface condition between the prosthesis and the bone tissue must play important roles during dynamic loading transfer through the knee joint. In this study, the three- dimensional impact finite element (FE) simulations were performed to investigate the impact stress propagation. The FE models of a totally replaced knee joint were constructed with the high shape fidelity. The models included the cortical and cancellous bone, articular cartilage, bone marrow, and the artificial femoral and tibial components. The artificial components were set to the femoral and tibial contact area. The FE meshes had 7251 nodal points and 5547 hexahedral elements (Figure 1). The interfacial condition between the artificial component had two kind of contact situations, bonding situation and no-bonding ones. In the bonding situation, the interface between the artificial components and the cancellous bone had fully fixations. The no-bonding allowed the tie-breaking of each other although the interface had the high coefficient of friction. The three kind of the impact loading (1, 5, and 10kgW) were applied from the proximal femur to the distal side of tibia. In the FE simulations, the impact stress propagated to the tibia through the TKR joint components during several milliseconds. On the interfacial surface at the cancellous side of the proximal tibia, the difference in the stress distribution was observed according to the contact situation of the TKR component (Figure 2). The fully fixation (tied to each other) model showed the high compressive stress on the interface. On the other hand, in the no-bonding model, the compressive stress distributed discontinuously and the high compressive stress was observed only in the hole area and edge of the tibial component during the impact loading. In previous research, the cancellous bone had important roles for the load transmission inside the joint especially under the impact loading condition. However, this study indicated that the stress shielding was caused by the imperfect bonding at the interface. More consideration of the interface situation between the bone and component is required to keep stability for impact loading

Introduction. Pre-clinical testing of orthopaedic devices could be improved by comparing performance with established implants with known clinical histories. Corail and Summit (DePuy Synthes, Warsaw) are femoral stems with proven survivorship of 95.1% and 98.1% at 10 years [1], which makes them good candidates as benchmarks when evaluating new stem designs. Hence, the aim of this study was to establish benchmark data relating to the primary stability of Corail and Summit stems. Methods. Finite Element (FE) simulations were run for 34 femurs (from the Melbourne femur collection) for a diverse patient cohort of joint replacement age (50 – 80 yrs). To account for the diversity in shape, the cohort included femurs with the maxima, minima and medians for 26 geometric parameters. Subject-specific FE models were generated from CT scans. An in-house developed algorithm positioned idealized versions of Corail and Summit (Figure 1) into each of the femur models so that the stem and femur shaft axes were aligned, and the vertical offset between the trunnion centre and the femoral head centre was minimised. For such a position, the algorithm selected the size that achieved maximum fill of the medullary canal without breaching the cortical bone boundaries. Joint contact and muscle forces were calculated for level gait and stair climbing[2] and scaled to the body mass of each subject. Femurs were rigidly constrained at the condyles. Risk of failure was assessed based on (i) stem micromotion, (ii) equivalent strains (iii) percentage of the bone-prosthesis contact area experiencing micromotions < 50 μm, micromotions > 150 μm and strains > 7000 μstrains [3]. Results. Stair climb loads resulted in higher micromotion and interface strains, compared to level gait loads. For level gait, on average, Corail had 89% and Summit had 91% of the contact area experiencing less than 50 μm and less than 1% of the contact area with micromotion greater than 150 μm. For stair climbing, the average area experiencing <50 μm was about 75% for both stems. On average, Corail and Summit had less than 1% of the contact area with micromotion greater than 150 μm during stair climbing. The average percentage of the contact are with strains greater than 7000 μstrains was about 2% for both stems during level gait, and 8% (Corail), 10% (Summit) during stair climbing (Figure 2). Discussion and Conclusion. It is desirable for the micromotion at the entire contact area to be below 50 μm. Despite the reported good survivorship of Corail and Summit [1], results of the FE simulations do not show such a distribution. Instead, results suggest that primary stability may be achieved with up to 25% of the contact area with micromotion greater than 50 μm. Hence, the 75th percentile may be a suitable metric for benchmarking femoral stems

Introduction. 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. Methodology. 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. Results. For the Profemur-TL design, bone resorption was around 36% and 56% for good and medium bone qualities, respectively (Fig. 2). The corresponding head displacements were 11.75 mm and 21.19 mm. Optimized solutions showed bone resorption from 15% to 26% and from 44% to 65% for good and medium bone qualities, respectively. Corresponding head displacements ranged from 11.85 mm to 12.25 mm and from 16.9 mm to 22.6 mm. Conclusion. The obtained set of solutions constitutes an improvement of the implant performance for this functionally graded implant (FGI) compared to the original implant for both bone qualities. From these simulations, the final solution for the FGI could be chosen based on manufacturing restrictions or another performance indicator

Introduction. The influence of the bone mineral density (BMD) on the mechanical behavior of bones can be examined using computer tomography (CT) data and finite element (FE) simulations, because the BMD correlates with the Hounsfield scale (HU) of the CT data. Therefor the material mapping strategy, which is required to assign the HU values to the FE mesh, is of crucial importance. In this study a nodal mapping strategy was analyzed concerning its sensitivity towards FE mesh parameters and an averaging of HU values from the area around the respective nodes. Method. The FE simulation is based on CT data of a human proximal femur. Once the bone shape was reconstructed, the resulting model was meshed with quadratic tetrahedral elements in ABAQUS/CAE and all nodes were assigned an HU value from the CT data by using the respective node coordinates. In this process, the mesh density, the threshold, which could be used to exclude connective tissue and fat from the material mapping process, the considered volume around the nodes and the method of averaging were varied. The material assignment was realized by an HU value dependent, linear elastic material definition. The femur model was clamped at the level of the isthmus and a displacement of 0.5 mm was applied at the femoral head. The evaluation was based on the resulting reaction forces. Results. The sensitivity analysis demonstrated, that threshold and mesh density mainly influenced the reaction force [Fig. 1]. If a threshold was applied, the reaction force increased by about 20 % in average. A threefold increase of the mesh density led to an average gain of the results of about 24 %. For a specific mesh density the curve progressions of the respective results intersected, i.e. an alteration of the considered volume or the method of averaging barely affected the reaction force [Fig. 2, Fig. 3]. Apart from this intersection, the comparison of the small and the large average volume led to a deviation of up to 11 %. On the other hand, the examination of different methods of averaging revealed only a maximum deviation of 4 % between “mean” and “median”. Discussion. The present study indicates, that the material mapping strategy is an influential part of the modeling process, which should be validated to avoid misjudgments of the load situation. Accordingly, the use of a threshold to exclude non-bone tissue could be a helpful tool. But with the exclusion of lower HU values, the load-bearing structure gains stiffness and the reaction force in the femur rises. A finer mesh leads to a higher resolution of the bone structure and, therefore, to a higher accuracy of the results. The “equilibrium” between the different models at the intersection is caused by a more homogeneous distribution of the material property which is increased by a larger considered volume and the method “mean”

Introduction. 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 resorption. 1. 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 stresses. 3. 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. Methodology. 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. Results. Different bone qualities result in slightly different strut diameter distribution (Fig.1). An excellent bone quality (E=30 GPa) results in a less dense porous structure, where some dense zones are substituted by a thick strut surrounded by a low density area. As can be expected, a poor bone quality (E=6 GPa) results in a denser porous structure. Compared with the solid stem, in general the FGI stem produced higher bone stresses. Locally, the stresses augmented proximally, while diminished distally (Fig.2). This is expected to result in a smaller influence of stress shielding, and better load transfer. Conclusion. The presented algorithm succeeded obtaining an optimal strut diameter distribution from low rigidity struts, using a strategy similar to bone remodelling (i.e. maintaining certain stress level within the struts). Optimized diameter distribution was obtained with little computational cost