The fixation of press-fit orthopaedic devices depends on the mechanical properties of the bone that is in contact with the implants. During the press-fit implantation, bone is compacted and permanently deformed, finally resulting in the mechanical interlock between implant and bone. For the development and design of new devices, it is imperative to understand these non-linear interactions. One way to investigate primary fixation is by using computational models based on Finite Element (FE) analysis. However, for a successful simulation, a proper material model is necessary that accurately captures the non-linear response of the bone. In the current study, we combined experimental testing with FE modeling to establish a Crushable Foam model (CFM) to represent the non-linear bone biomechanics that influences implant fixation. Mechanical testing of human tibial trabecular bone was done under uniaxial and confined compression configurations. We examined 62 human trabecular bone samples taken from 8 different cadaveric tibiae to obtain all the required parameters defining the CFM, dependent on local bone mineral density (BMD). The derived constitutive rule was subsequently applied using an in-house subroutine to the FE models of the bone specimens, to compare the model predictions against the experimental results.Introduction
Methods
Fretting corrosion at the taper interface of modular connections can be studied using Finite Element (FE) analyses. However, the loading conditions in FE studies are often simplified, or based on generic activity patterns. Using musculoskeletal modeling, subject-specific muscle and joint forces can be calculated, which can then be applied to a FE model for wear predictions. The objective of the current study was to investigate the effect of incorporating more detailed activity patterns on fretting simulations of modular connections. Using a six-camera motion capture system, synchronized force plates, and 45 optical markers placed on 6 different subjects, data was recorded for three different activities: walking at a comfortable speed, chair rise, and stair climbing. Musculoskeletal models, using the Twente Lower Extremity Model 2.0 implemented in the AnyBody modeling System™ (AnyBody Technology A/S, Aalborg, Denmark; figure1), were used to determine the hip joint forces. Hip forces for the subject with the lowest and highest peak force, as well as averaged hip forces were then applied to an FE model of a modular taper connection (Biomet Type-1 taper with a Ti6Al4V Magnum +9 mm adaptor; Figure 2). During the FE simulations, the taper geometry was updated iteratively to account for material removal due to wear. The wear depth was calculated based on Archard's Law, using contact pressures, micromotions, and a wear factor, which was determined from accelerated fretting experiments.Introduction
Methods
Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) studies. Manufacturing tolerances can result in a mismatch between the femoral head and stem, which can influence the taper mechanics leading to possibly more wear. Using FE models the effect of these manufacturing tolerances on the amount of volumetric wear can be studied. The removal of material in the FE model was validated against experiments simulating the clinical fretting wear process, subsequently the mismatch and assembly force were varied to study the effect on the volumetric wear. An FE model was developed in which the geometry can be updated to account for material removal due to wear. In this model the geometry was updated based on Archard's Law, using contact pressures, micromotions and a wear factor, which was determined based on accelerated fretting experiments. The linear wear was calculated using H=k*p*S. Where H is the linear wear depth in mm, k is a wear factor (mm3/Nmm), p is the contact pressure (MPa) and S is the sliding distance (mm). 10 million cycles were simulated using 50 virtual steps. Using this scaling and the measured volumetric wear from the experiments a wear factor of 2.7*10−5 was applied. Based on general manufacturing tolerances the resulting mismatch in taper angles were determined to be ± 1.26°. Using this mismatch a tip fit (figure 1a) and base fit (Figure 1b) model were created. In combination with a perfect fit, meaning no mismatch, and two different assembly forces of 4 kN and 15 kN, 6 different situations were studied.Introduction
Methods
Fretting at modular junctions is thought to be a ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to increased contact stresses and micromotions at the taper interface. We adopted a finite element approach to model the head-taper junction, to analyse the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants, to demonstrate the importance of a good assembly during surgery. Models of the Bimetric taper and adaptor were created, with elastic-plastic material properties based on material tests with the actual implant alloy. FE contact conditions were validated against push-on and pull-off experiments. The models were loaded according to ISO 7206-4 and −6, after being assembled at 2-4-15kN, both axially and at a 30° angle. Average micromotions and contact pressures were analysed, and a wear score was calculated based on the contact pressures and micromotions.Background
Methods
Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) investigations. In FE analyses, different parameters can be varied to study micromotions and contact pressures at the taper interface. However, to truly study taper wear, the simulation of micromotions and contact pressures in non-adaptive FE models is insufficient, as over time these can change due to interfacial changes caused by the wear process. In this study we developed an FE approach in which material removal during the wear process was simulated by adaptations to the taper geometry. The removal of material was validated against experiments simulating the clinical fretting wear process. Experimental test: An accelerated fretting screening test was developed that consistently reproduced fretting wear features observed in retrievals. Biomet Type-1 (4°) tapers and +9 mm offset adaptors were assembled with a 4 kN force (N=3). A custom head fixture was used to create an increased offset and torque. The stems were potted in accordance with ISO 7206–6:2013. The set-up was submerged in a 37°C PBS solution with a pH adjusted to 3 using HCL and NaCl concentration of 90gl−1. The components were cyclically loaded between 0.4 – 4 kN for 10 million cycles. After completion, the volumetric and linear wear was measured using a Talyrond-585 roundness measurement machine. FE model: This was created to match the experimental set up (Figure 1). Taper geometry and experimental material data were obtained from the manufacturer (Zimmer Biomet). The coefficient of friction of the studied combination of components was based on previous experiments (Introduction
Method
Fretting corrosion at the taper interface has been implicated as a possible cause of implant failure. Using Experimental test set-up: An accelerated wear test was developed that consistently reproduced fretting wear features observed in retrievals. Biomet stems with smooth 4° Type-1 tapers were combined with Ti6Al4V Magnum +9 mm adaptors using a 2 or 15 kN assembly force. The head was replaced with a custom head fixture to increase the offset and apply a torque at the taper interface. The stems were potted according to ISO 7206-6:2013. The set-up was submerged in a test medium containing PBS and 90gl-1 NaCl. The solution was pH adjusted to 3 using HCl and maintained at 37°C throughout the tests. For each assembly case, n=3 tests were cyclically loaded between 0.4–4 kN for 10 Million cycles. Volumetric wear measurements were performed using a Talyrond-365 roundness measurement machine. The FE model was created to replicate the experimental set up. Geometries and experimental material data were obtained from the manufacturer (Biomet). The same assembly forces of 2 and 15 kN were applied, and the same head fixture was used for similar offset and loading conditions. The 4 kN load was applied at the same angles in accordance with ISO 7206-6:2013. Micromotions and contact pressures were calculated, and based on these a wear score was determined by summation over all contact points.Introduction
Methods
Recent reports implicate fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. Fretting observed at modular junctions is thought to be a type of ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to an increase in contact stresses and micromotions at the taper interface. These may include: intra-operative taper assembly, taper contamination by debris or body fluids, patient weight and ‘toggling’ of the head or increased frictional torque in a poorly functioning bearing. We adopted a finite element approach to model the head-taper junction, to analyze the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants. Models of the Biomet Type-1 taper, a 60 mm head and a taper adaptor were created. These models were meshed with a mesh size based on a mesh density convergence study. Internal mesh coarsening was applied to reduce computational cost. Elastic-plastic material properties based on tensile tests were assigned to all titanium components. The contact conditions used in the FE analyses were validated against push-on and pull-off experiments, resulting in a coefficient of friction of 0.5. To analyze micromotions at the taper-adaptor connection, the models were loaded with 2300N (ISO 7206-4) and 5340N (ISO 7206-6), after being assembled with 2-4-15 kN, axially and under a 30º angle. This ISO standard is commonly used to determine endurance properties of stemmed femoral components. Micromotions and contact pressures were analyzed by scoring them to an average micromotion and average contact pressure for the surface area in contact.Introduction
Materials and methods
Recent reports have implicated fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. While it has been suggested that larger MOM heads, may induce greater frictional torques at the taper connection, the exact mechanisms underlying fretting corrosion remain poorly understood. It is likely that the onset of the corrosion process is caused by mechanical factors, such as contact stresses and micromotions occurring at the interface. These stresses and micromotions depend on the fixation of the head onto the stem and may be affected by blood, fat, bone debris or other contaminations. The fixation of the head is achieved intraoperatively through impaction. To further study this phenomenon, we adopted a finite element approach in which we modeled the head-taper junction fixation mechanics. In this model, we analyzed the effect of impaction force on the micromotions occurring at the head-stem interface. We created a model of a BIOMET Type-1 taper and an adapter that is typically used for larger heads. Titanium alloy material properties were assigned to both components, and frictional contact (μ = 0.5) was simulated between the adapter and the taper. To ensure that the model accurately represented the contact mechanics, we first simulated experiments in which the head was assembled on the taper in a load-controlled manner, at different load (4 and 15 kN), after which it was disassembled axially. The disassembly loads predicted by the FEA simulations were then compared to the experimental values. After ensuring a correct prediction of the disassembly loads, we used various impaction loads (2, 4, and 15 kN) to assemble the taper, after which a 2.3 kN load (ISO 7206-4) was applied to the adapter/taper assembly. This loading regime is commonly used to determine endurance properties of stemmed femoral components. Under these loading conditions, we then analyzed the contact stresses and micromotions, and the effect of impaction load on these quantities.Introduction
Materials and methods
1. Mucopolysaccharides were analysed in the urine of thirteen patients with Morquio's syndrome aged between three and fifty-nine years and of fourteen controls of comparable ages. 2. There were no significant qualitative or quantitative differences between patients and controls. 3. The clinical and radiological findings suggested that these patients did not have the "Morquio-Ullrich" form of the disease, which appears on retrospective assessment of case reports to be more uniform and less diffuse than the Morquio-Brailsford form which may include a number of possibly unrelated diseases. 4. Keratosulphate has so far been demonstrated in the urine only of patients with the "Morquio-Ullrich" form of the disease, although the mucopolysaccharide excretion has been investigated in only a few patients with the Morquio-Brailsford form. The normal mucopolysaccharide excretion of the present series of patients suggests that a normal mucopolysaccharide excretion distinguishes the Morquio-Brailsford from the Morquio-Ullrich form, the latter having a number of features overlapping with Hurler's disease, where large amounts of mucopolysaccharide other than keratosulphate are excreted. 5. Both qualitative and quantitative analysis of urinary mucopolysaccharide are thus necessary to distinguish between Hurler's disease, the Morquio-Ullrich form and the Morquio-Brailsford's form of Morquio's syndrome.
