The biomechanics of plate repair of periprosthetic femur fractures near the tip of a total hip implant: the effect of cable-screw position

Optimal surgical positioning of cable-screw pairs in repairing periprosthetic femur fractures near the tip of a total hip implant still remains unclear. No studies in the literature to date have developed a fully three-dimensional finite element (FE) model that has been validated experimentally to assess these injury patterns. The aim of the present study was to evaluate the biomechanical performance of three different implant-bone constructs for the fixation of periprosthetic femoral shaft fractures following total hip arthroplasty. Experimentally, three bone-plate repair configurations were applied to the periprosthetic synthetic femur fractured with a 5 mm gap near the tip of a total hip implant. Constructs A, B, and C, respectively, had successively larger distances between the most proximal and the most distal cable-screw pairs used to affix the plate. Specimens were oriented in 15 degrees adduction, subjected to 1000 N of axial force to simulate the single-legged stance phase of walking, and instrumented with strain gauges. Computationally, a linearly elastic and isotropic three-dimensional FE model was developed to mimic the experimental setup. Results showed excellent agreement between experimental versus FE analysis strains, yielding a Pearson linearity coefficient, R2, of 0.90 and a slope for the line of best data fit of 0.96. FE axial stiffnesses were 601 N/mm (Construct A), 849 N/mm (Construct B), and 1359 N/mm (Construct C). FE surface stress maps for cortical bone showed maximum von Mises values of 74 MPa (Construct A), 102 MPa (Construct B), and 57 MPa (Construct C). FE stress maps for the metallic components showed minimum von Mises values for Construct C, namely screw (716MPa), cable (445MPa), plate (548MPa), and hip implant (154MPa). In the case of good bone stock, as modelled by the present synthetic femur model, optimal fixation can be achieved with Construct C.     

Finite element analysis of bone stress and strain around a distal osseointegrated implant for prosthetic limb attachment

Direct skeletal attachment techniques have recently been identified as an alternative method for percutaneous attachment of prosthetic limbs. Osseointegrated implants for prosthetic attachments are subjected to a complex load condition. This finite element study investigates the effect of varying geometries of the implant on the stress and strain distribution in the area of the bone/implant interface. Simplified three-dimensional axisymmetric models of the femur and seven implants with different core diameters, external diameters, implant lengths and thread pitches were derived. The resulting stress and strain distributions were compared. The significance of each implant geometry was identified for improving implant design in the light of benefit to the bone/implant osseointegration and the bone remodelling of the femur.     

The biomechanics of a validated finite element model of stress shielding in a novel hybrid total knee replacement

This study proposes a novel hybrid total knee replacement (TKR) design to improve stress transfer to bone in the distal femur and, thereby, reduce stress shielding and consequent bone loss. Three-dimensional finite element (FE) models were developed for a standard and a hybrid TKR and validated experimentally. The Duracon knee system (Stryker Canada) was the standard TKR used for the FE models and for the experimental tests. The FE hybrid device was identical to the standard TKR, except that it had an interposing layer of carbon fibre-reinforced polyamide 12 lining the back of the metallic femoral component. A series of experimental surface strain measurements were then taken to validate the FE model of the standard TKR at 3000 N of axial compression and at 0 degreeof knee flexion. Comparison of surface strain values from FE analysis with experiments demonstrated good agreement, yielding a high Pearson correlation coefficient of R(2)= 0.94. Under a 3000N axial load and knee flexion angles simulating full stance (0O degree, heel strike (200 degrees, and toe off (600 degrees during normal walking gait, the FE model showed considerable changes in maximum Von Mises stress in the region most susceptible to stress shielding (i.e. the anterior region, just behind the flange of the femoral implant). Specifically, going from a standard to a hybrid TKR caused an increase in maximum stress of 87.4 per cent (O0 degree from 0.15 to 0.28 MPa), 68.3 per cent (200 degrees from 1.02 to 1.71 MPa), and 12.6 per cent (600 degrees from 2.96 to 3.33 MPa). This can potentially decrease stress shielding and subsequent bone loss and knee implant loosening. This is the first report to propose and biomechanically to assess a novel hybrid TKR design that uses a layer of carbon fibrereinforced polyamide 12 to reduce stress shielding.     

Development and numerical validation of a finite element model of the muscle standardized femur

The human femur is one of the parts of the musculo-skeletal system most frequently analysed by means of the finite element (FE) method. Most FE studies of the human femur are based on computed tomography data sets of a particular femur. Since the geometry of the chosen sample anatomy influences the computed results, direct comparison across various models is often difficult or impossible. The aim of the present work was to develop and validate a novel three-dimensional FE model of the human femur based on the muscle standardized femur (MuscleSF) geometry. In the new MuscleSF FE model, the femoral attachment of each muscle was meshed separately on the external bone surface. The model was tested under simple load configurations and the results showed good agreement with the converged solution of a former study. In the future, using the validated MuscleSF FE model for numerical studies of the human femur will provide the following benefits: (a) the numerical accuracy of the model is known; (b) muscle attachment areas are incorporated in the model, therefore physiological loading conditions can be easily defined; (c) analyses of the femur under physiological load cases will be replicable; (d) results based on different load configurations could be compared across various studies.     

Finite element analysis of a subtrochanteric fractured femur with dynamic hip screw, dynamic condylar screw, and proximal femur nail implants--a comparative study

Selection of the correct type of implant for fracture fixation has become a very interesting problem in the orthopaedic community. The present work studies the biomechanical behaviour of the femur with three different implant configurations for simple transverse subtrochanteric fracture and the intact femur using finite element analysis. The implants considered in this study are as follows: dynamic hip screw (DHS), dynamic condylar screw (DCS), and proximal femur nail (PFN). The modelling software Unigraphics and finite element simulation software ANSYS are used for the present analysis. The three implants are compared for deflection, stress, and strains. The simulation also includes modelling of the cortical defect near the fracture. An estimation of the critical depth of the cortical defect based on the von Mises stress is obtained using this study on the DHS implant. The displacement and principal stress on the proximal femur have been compared for all the implant models. The stresses on the cortical screws for DCS and DHS implants have also been compared. The result shows that the DHS and DCS implants behave in a similar way to the intact femur compared with the PFN implant.     

The biomechanics of plate fixation of periprosthetic femoral fractures near the tip of a total hip implant: cables, screws, or both?

Femoral shaft fractures after total hip arthroplasty (THA) remain a serious problem, since there is no optimal surgical repair method. Virtually all studies that examined surgical repair methods have done so clinically or experimentally. The present study assessed injury patterns computationally by developing three-dimensional (3D) finite element (FE) models that were validated experimentally. The investigation evaluated three different constructs for the fixation of Vancouver B1 periprosthetic femoral shaft fractures following THA. Experimentally, three bone plate repair methods were applied to a synthetic femur with a 5 mm fracture gap near the tip of a total hip implant. Repair methods were identical distal to the fracture gap, but used cables only (construct A), screws only (construct B), or cables plus screws (construct C) proximal to the fracture gap. Specimens were oriented in 15 degrees adduction to simulate the single-legged stance phase of walking, subjected to 1000 N of axial force, and instrumented with strain gauges. Computationally, a linearly elastic and isotropic 3D FE model was developed to mimic experiments. Results showed excellent agreement between experimental and FE strains, yielding a Pearson linearity coefficient, R2, of 0.92 and a slope for the line of best data fit of 1.06. FE-computed axial stiffnesses were 768 N/mm (construct A), 1023 N/mm (construct B), and 1102 N/mm (construct C). FE surfaces stress maps for cortical bone showed Von Mises stresses, excluding peaks, of 0-8 MPa (construct A), 0-15 MPa (construct B), and 0-20 MPa (construct C). Cables absorbed the majority of load, followed by the plates and then the screws. Construct A yielded peak stress at one of the empty holes in the plate. Constructs B and C had similar bone stress patterns, and can achieve optimal fixation.     

Finite element analysis of intramedullary devices: the effect of the gap between the implant and the bone

This paper examines the interaction interface between the implant and the bone for an intramedullary femoral nailing system using a finite element (FE) model and specifically considers the hypothesis that the local geometry at the interface is significant to the resulting localized contact stress between the medial and lateral aspect of nail and endosteum. Contact mechanics algorithms are used in the FE modelling technique that can be developed to deal with any form of intramedullary device for which contact at the bone-implant interface is important. Global stiffness data from the FE model are compared with available data from an experiment carried out on a construct of the bone and the device that uses intramedullary femoral nails. Acceptable agreement is obtained. The results demonstrate that the mechanical interface between the implant and the bone is significantly affected by the gap geometry and magnitude. In particular, larger gaps lead to greater concentrations of stress on the medial side, while the distribution of stress is more uniform at the lateral contacts. Furthermore, the results show that the gap can have a marked effect on the stresses that occur on the fracture plane.     

Investigations on the effects of friction modeling in finite element simulation of machining

Accurately predicting the physical cutting process variables, e.g. temperature, velocity, strain and stress fields, plays a pivotal role for predictive process engineering for machining processes. These predicted field variables, however, are highly influenced by workpiece constitutive material model (i.e. flow stress), thermo-mechanical properties and contact friction law at the tool-chip-workpiece interfaces. This paper aims to investigate effects of friction modeling at the tool-chip-workpiece interfaces on chip formation process in predicting forces, temperatures and other field variables such as normal stress and shear stress on the tool by using advanced finite element (FE) simulation techniques.For this purpose, two distinct FE models with Arbitrary Lagrangian Eulerian (ALE) fully coupled thermal-stress analyses are employed to study not only the effects of FE modeling with different ALE techniques but also to investigate the influence of limiting shear stress at the tool-chip contact on frictional conditions, which was never done before. A detailed friction modeling at the tool-chip and tool-work interfaces is also carried by coupling sticking and sliding frictions. Experiments and simulations have been performed for machining of AISI 4340 steel using tungsten carbide tooling and the simulation results under increasing limit shear stress have been compared to experiments. The influence of limiting shear stress on the tool-chip contact friction was explored and validity of friction modeling approaches was examined. The results presented in this work not only provide a clear understanding of friction in FEM modeling of machining but also advance the process knowledge in machining.     

Numerical investigations of stress shielding in total hip prostheses

Aseptic loosening of the prosthesis is still a problem in artificial joint implants. The loosening can be caused by, among other factors, resorption of the bone surrounding the prosthesis owing to stress shielding. In order to find out the influence of the prosthesis type on post-operative stress shielding, a static finite element analysis of a femur provided with the conventional uncemented stem BICONTACT and of one with the femoral neck prosthesis SPIRON was carried out. Strain energy densities and maximal principal strain distributions were calculated and compared with the physiological situation. Here, stress shielding was demonstrated in both periprosthetic femora. To determine the areas of the stress shielding, the bone in each FE model was subdivided into three regions of interest (ROI): proximal, diaphyseal, and distal. The numerical computations show stress shielding in the proximal ROI of both periprosthetic femora. Diaphyseally, the femoral neck prosthesis SPIRON, in contrast to the conventional uncemented long-stem prosthesis BICONTACT, causes no decrease in the strain distribution and thus no stress shielding. Distally, no change in the load distribution of either periprosthetic femur could be found, compared with the physiological situation.     

Effect of constraint on fracture behavior of welded 17mn4 and aisi304 steels

In this study, 17Mn4 (P295GH) pressure vessels steel and AISI304 stainless steel were welded with ER309L austenitic consumable. In experimental part of the study, tensile tests were conducted on welded plates and variation of hardness values along specimen was measured. J-integral fracture toughness values were investigated for different crack locations. In order to determine the regions where plastic deformation did not take place due to constraint, uni-axial tensile test was performed on welded tensile specimen after attaching strain gauges. In numerical part of the study, finite element (FE) analyses were conducted by fixing 2-D models precracked on different locations by using ANSYS software. In these models, stress triaxiality and plastic deformation characteristics around crack tip were determined for each crack locations after stress — strain analyses. The limitation on the extension of plastic deformation at diffusion line causes extra increase in stress triaxiality at crack tip.     

圆柱壳开孔补强的弹性和弹塑性有限元分析

考虑补强圈与圆柱壳作为一个整体子模型和接触子模型，对开孔补强结构的圆柱壳进行了弹性和弹塑性有限元应力分析。比较两个子模型和弹性／弹塑性有限元分析的结果，发现接触子模型和整体子模型的圆柱壳开孔应力分布相似，但接触子模型和整体子模型在壳体和补强圈之间的接触面上的应力分布存在差异。对不同直径的接管和不同倾角的接管进行了有限元分析，分析表明，应力分布及最大应力受倾角的影响较大，受直径变化的影响不大。     

A proximal femoral implant preserves physiological bone deformation: a biomechanical investigation in cadaveric bones

The aim of this study was to compare the perturbances in bone deformation patterns of the proximal femur due to a conventional cemented femoral stem and a novel uncemented implant designed on the principles of osseointegration. Five matched pairs of fresh frozen human femora were mechanically tested. Bone deformation patterns, measured with a video digitizing system under 1.5 kN joint force, showed that the cemented Spectron femoral implant caused significant alterations to the proximal femoral deformation pattern, whereas the Gothenburg osseointegrated titanium femoral implant did not significantly alter the bone behaviour (p < 0.05). Vertical micromotions measured under 1 kN after 1000 cycles were within the threshold of movement tolerable for bone ingrowth (21 microm for the Gothenburg system and 26 microm for the cemented implant).     

The effect of primary stability on load transfer and bone remodelling within the uncemented resurfaced femur

One of the major causes of aseptic loosening in an uncemented implant is the lack of any attachment between the implant and the bone. The implant's stability depends on a combination of primary stability (mechanical stability) and secondary stability (biological stability). The primary stability may affect the implant-bone interface condition and thus influence the load transfer and mechanical stimuli for bone remodelling in the resurfaced femur. This paper reports the results of a study into the affect of primary stability on load transfer and bone adaptation for an uncemented resurfaced femur. Three-dimensional finite element models were used to simulate the intact and resurfaced femurs and the bone remodelling. As a first step towards assessing the immediate post-operative condition, a debonded interfacial contact condition with varying levels of the friction coefficient (0.4, 0.5, and 0.6) was simulated at the implant-bone interface. Then, using a threshold value of micromotion of 50 microm, the implant-bone interfacial condition was varied along the implant-bone boundary to mechanically represent non-osseointegrated or osseointegrated regions of the interface. The considered applied loading conditions included normal walking and stair climbing. Resurfacing leads to strain shielding in the femoral head (20-75 per cent strain reductions). In immediate post-operative conditions, there was no occurrence of elevated strains in the cancellous bone around the proximal femoral neck-component junction resulting in a lower risk of neck fracture. Predominantly, the micromotions were observed to remain below 50 microm at the implant-bone interface, which represents 97-99 per cent of the interfacial surface area. The predicted micromotions at the implant-bone interface strongly suggest the likelihood of bone ingrowth onto the coated surface of the implant, thereby enhancing implant fixation. For the osseointegrated implant-bone interface, the effect of strain shielding was observed in a considerably greater bone volume in the femoral head as compared to the initial debonded interfacial condition. A 50-80 per cent peri-prosthetic bone density reduction was predicted as compared to the value of the intact femur, indicating bone resorption within the superior resurfaced head. Although primary fixation of the resurfacing component may be achieved, the presence of high strain shielding and peri-prosthetic bone resorption are a major concern.     

A dynamic model of simulating stress distribution in the distal femur after total knee replacement

The aim of this study has been to develop a dynamic model of the knee joint after total knee replacement (TKR) to analyse the stress distribution in the distal femur during daily activities. Using MSC/ADAMS and MSC/MARC software, a dynamic model of an implanted knee joint has been developed. This model consists of the components of the knee prosthesis as well as the bones and ligaments of the knee. The femur, tibia, fibula, and patella have been modelled as mixed cortico-cancellous bone. The distal part of femur has been modelled as a flexible body with springs used to simulate the ligaments positioned at their anatomical insertion points. With this dynamic model a gait cycle was simulated. Stress shielding was identified in the distal femur after TKR, which is consistent with other investigators' results. Interestingly, higher stresses were found in the bone adjacent to the femoral component peg. This dynamic model can now be used to analyse the stress distribution in the distal femur with different load conditions. This will help to improve implant designs and will allow comparison of prostheses from different manufacturers.     

Experimental measurement of strain and stress state at the contacting helical gear pairs

Contact stress evaluation in gears has been a complex area of research, due to its non-linear and non-uniform nature of stress distribution. The high contact stress on gears results in pitting and scuffing, which leads to tooth failure. Furthermore the effects of friction on gear contacts make the problem more complicated. Hence, in this paper, attempt has been made to study contact stress in gears. The experimental testing and analysis of the helical gear was carried out using Gear Dynamic Stress Test Rig (GDSTR). GDSTR is a newly designed test rig to compute the contact stresses on the gear pair contact, under real gear conditions. GDSTR uses the strain gauge and carbon slip rings to measure the surface contact stresses at the contacting points of a meshed gears. The experimental analysis showed promising results which have been verified by the finite element frictional contact analysis. The experimental testing was carried out on 5° and 25° helical gear pairs. Helical gear models with the same specifications and for different frictional coefficient conditions were also generated using FE modelling. The frictional contact stress analysis using FEM has been used for comparison with the experimental results.     

基于PRO/E与LS-DYNA的齿轮副动态接触分析

利用PRO/E软件构建精确啮合齿轮副模型,采用动力学有限元软件ANSYS/LS-DYNA对齿轮动态接触问题进行分析,计算结果清晰地反映了齿轮在不同啮合位置时齿面的接触应力、齿根应力及应变变化等。该方法克服了二维接触与静态接触分析的缺点与不足,为三维齿轮动力学接触分析提供了新的方法。     

Finite element analysis of spherical indentation to study pile-up/sink-in phenomena in steels and experimental validation

The ball indentation technique based on deforming a material with a spherical indenter is an useful non-destructive tool for evaluating mechanical properties from a very small volume of material. In this work, the indentation test carried out using a 1.0 mm diameter tungsten carbide ball to penetration depths of around 100–200 μm is modeled using finite element (FE) method and analyzed for three steels having different yield stress and strain hardening exponent. The FE generated load–depth curve is compared and verified with the experimental load–depth data for the three materials. The role of the contact friction at the indenter–specimen interface on both the load—depth plot and indentation profile are examined. The development of pile-up/sink-in during indentation and its dependence on strain hardening characteristics of the material, contact friction and indentation depth are analyzed using the FE model. The indentation profiles obtained from simulation are compared with experimental profiles and the implication of pile-up phenomenon on accurate evaluation of stress–strain values from the experimental indentation load–depth data is discussed.     

准双曲面齿轮冷摆辗加工新方法及其数值模拟

针对目前准双曲面齿轮冷摆辗技术存在模具结构复杂、摆辗力相对较大等问题,提出了一种新型的冷摆辗方法。该方法简化了模具结构,采用局部线接触连续塑性成形。基于刚塑性有限元法基本理论,借助通用三维有限元软件DEFORM-3D对准双曲齿轮摆辗成形全过程进行了三维动态分析,研究了速度场量、接触区以及模具受力的分布和变化规律。研究结果揭示了准双曲面齿轮冷摆辗的变形规律。     

Effect of temperature on the frictional behaviour of an aluminium alloy sliding against steel during ball-on-disc tests

Ball-on-disc tests were carried out at elevated temperatures to determine the friction behaviour of the 7475 aluminium alloy against steel, resembling the material mating during hot extrusion. Friction coefficient was found to increase markedly with rising temperature, suggesting that the commonly accepted assumption of an unvaried friction coefficient in the FE simulation of the aluminium extrusion process involving temperature evolution is incorrect. A theoretical analysis showed that the high friction values measured during high-temperature ball-on-disc tests could be partly attributed to the increased plowing friction caused by the severe deformation of the aluminium alloy. The effect of temperature on the stress and strain distributions at the ball/disc interface was revealed by means of three-dimensional finite-element (3D FE) simulation. Increased deformation in the aluminium disc around the contact point with rising temperature was confirmed. The alterations of the strain state and stress state around the contact point were considered responsible for the generation of wear debris, the oscillations of friction coefficient along the wear track and the increase of friction coefficient with rising temperature.     

Studies in dynamic design of drilling machine using updated finite element models

The aim of the present work is to develop updated FE models of a drilling machine using analytical and experimental results. These updated FE models have been used to predict the effect of structural dynamic modifications on vibration characteristics of the drilling machine. Two studies have been carried out on the machine. In the first study, modal tests have been carried out on a drilling machine using instrumented impact hammer. Modal identification has been done using global method of modal identification. For analytical FE modeling of the machine, a computer program has been developed. The results obtained using FEM, have been correlated with the experimental ones using mode shape comparison and MAC values. Analytical FE model has been updated, with the help of a program, which has been developed using direct methods of model updating. In the second study, modal testing has been carried out using random noise generator and modal exciter. Global method has been used for modal identification. Analytical FE modeling has been done using I-DEAS software. Correlation of FE results with the experimental ones has been carried out using FEMtools software. Updating of the analytical FE model has also been done using the above software, based on an indirect technique viz. sensitivity based parameter estimation technique. The updated FE models, obtained from both the studies have been used for structural dynamic modifications (SDM), for the purpose of dynamic design and the results of SDM predictions are seen to be reasonably satisfactory.