Prediction of backside micromotion in total knee replacements by finite element simulation

The micromotion at the interface between the polyethylene tibial insert and metal tibial tray [corrected] in modular total knee replacements [corrected] has been shown to contribute to wear particle-induced osteolysis and may [corrected] cause implant failure. Therefore, studying the design parameters that are involved in the backside wear process is an important task that may lead to improvement in new total knee replacements. In the present study, a finite element model was developed to predict the backside micromotion along the entire modular interface. Both the linear elastic constitutive model and non-linear J2-plasticity constitutive model were considered in the finite element model for polyethylene and were corroborated against published results obtained from displacement controlled knee simulator wear tests. The finite element simulation with the non-linear J2-plasticity constitutive model was able to predict backside micromotion [corrected] more accurately than the simulation with the linear elastic constitutive model. [corrected] The developed finite element model (including the non-linear J2-plasticity constitutive model) was then applied to assess the effects of the tibial tray locking mechanism design (dovetails versus fullperipheral [corrected] design) and different levels of interference fit on insert micromotion. The developed finite element model, implementing the non-linear J2-plasticity constitutive model, was shown to successfully predict clinical amounts of backside micromotion and could be used for the design and development of total knee replacements for the reduction of backside micromotion and polyethylene [corrected] wear.     

Custom fabrication of composite tibial hemi-knee joint combining CAD/CAE/CAM techniques

A custom fabrication approach combining computer-aided design (CAD), computer-aided engineering (CAE), and computer-aided manufacturing (CAM) techniques for constructing a novel composite tibial hemi-knee joint is presented. Anatomical modelling was used to provide the computer model with specific geometry for individuals and the finite element method (FEM) was adopted to understand the loading distribution on each component of the composite substitute. Rapid prototyping (RP) was employed to build the negative patterns, based on which the titanium alloy tibial tray and the porous artificial bone were custom fabricated through quick casting and powder sintering techniques. The results show that the titanium alloy component bears most of the loading while the artificial bone shares little, which could prevent it from fracturing in vivo. The final porous artificial bone has controllable microchannels (600 microm) and random micropores (100-200 microm), which ensures full interconnectivity and is expected to address the biological consideration. Clinical application demonstrates that the composite tibial hemi-knee joint has enough mechanical strength and can fit with the upper hemi-knee joint. This novel approach provides a new way to repair large bone defects in the loading sites.     

Load transfer in the proximal tibia following implantation with a unicompartmental knee replacement: a static snapshot

Unicompartmental knee replacement (UKR) is an appealing alternative to total knee replacement when the patient has isolated medial compartment osteoarthritis. A common observation post-operatively is radiolucency between the tibial tray wall and the bone. In addition, some patients complain of persistent pain over the proximal tibia antero-medially; this may be related to elevated bone strains in the tibia. Currently, there is no intentionally made mechanical bond between the vertical wall of an Oxford UKR and the adjacent bone; whether one exists or not will influence the load transmission in the proximal tibia and may affect the elevated tibia strain. The aim of this study was to investigate how introducing a mechanical tie between the tibial tray wall and the adjacent bone might alter the load carried into the tibia for both cemented and cementless UKRs. Strain energy density in the region of bone adjacent to the tray wall was considerably increased when a mechanical tie was introduced; this has the potential of reducing the likelihood of a radiolucency occurring in that region. Moreover, a mechanical tie had the effect of reducing proximal tibia strain, which may decrease the incidence of pain following implantation with a UKR.     

Effect of bone material properties on the initial stability of a cementless hip stem: a finite element study

In previous finite element studies of cementless hip stems reported in the literature, the effect of bone quality on the initial micromotion and interface bone strain has been rarely reported. In this study, the effect of varying cortical and cancellous bone modulus on initial stem micromotion and interface bone strain was examined and the potential consequence of these changes on bone ingrowth and implant migration was reported. A finite element (FE) model of a total hip replacement (THR) was created and the Young's moduli of cortical and cancellous bone were systematically varied to study the relative effect of the quality of both types of bone on the initial stability of a cementless THR. It was found that the initial micromotion and interface bone strain in a THR was significantly affected by the overall stiffness of the femur. In other words, both the reduction of the modulus of cortical and cancellous bone caused an increase in the initial micromotion and interface bone strain. This suggests that for FE studies to be truly predictive, a range of bone quality must be examined to study the performance envelope of a particular stem and to allow comparison with clinical results.     

Primary and long-term stability of a short-stem hip implant

The new generation short-stem hip implants are designed to encourage physiological-like loading, to minimize stress-strain shielding and therefore implant loosening in the long term. As yet there are no long-term clinical studies available to prove the benefits of these short-stem implants. Owing to this lack of clinical data, numerical simulation may be used as a predictor of longer term behaviour. This finite element study predicted both the primary stability and long-term stability of a short-stem implant. The primary implant stability was evaluated in terms of interface micromotion. This study found primary stability to fall within the critical threshold for osseointegration to occur. Longer term stability was evaluated using a strain-adaptive bone remodelling algorithm to predict the long-term behaviour of the bone in terms of bone mineral density (BMD) changes. No BMD loss was observed in the classical Gruen zones 1 and 7 and bone remodelling patterns were comparable with hip resurfacing results in the literature.     

Deformation of the cement mantle of tibial components following total knee arthroplasty: a laboratory study.

An in vitro model has been developed to measure in-plane strains of the cement mantle, sandwiched between the tibial component and the underlying cancellous bone following total knee arthroplasty. Maximal in-plane strains occurred in the cement mantle below the contact points between the femoral and tibial components. These strains were significantly reduced by increasing the thickness of the polyethylene and even more impressively by metal backing. Eccentric loading, by as little as 5 degrees, increased the strains in the loaded compartment by 26 per cent and decreased those in the unloaded compartment by 62 per cent. The addition of torsion to axial loading did not significantly alter the principal direct strains or the principal shear strains. Although surface-covering tibial components have been advocated, continuous support of the cortical rim did not appear to be important in reducing cement mantle strains. While other studies have emphasized the critical stresses that may occur in the polyethylene tibial components of total knee implants, this study highlights the potential for localized cement fatigue with improperly sized components or with eccentric loading.     

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.     

Analysis on stress and micromotion on various peg fixation at glenoid implant

Six commercially available implant designs were simulated via finite element analysis. The objectives of this study are (1) to determine the effect of peg numbers and orientations to the stress distribution at the implant and cement (2) to compare the micromotion between fully cemented and partially cemented implant. An applied load of 750 N was subjected to the implant at inferior location in which pegs were distributed. The result showed, by increasing the peg numbers, implant stress decreased and increased maximum stress at cement. While, fully cemented implant had higher micromotion compared to partially cemented implant. There are no significant differences between four-fins and six-fins implants under both load conditions. In conclusion, increase in peg numbers results in decreased stress at implant and partially cemented implant can provide better stability in terms of micromotion than fully cemented as it allows bone in-growth between fins. While, adding more fins in partially cemented implant does not influence the micromotion of the implant at all.     

An investigation into the fixation of hip replacements.

This paper describes an experimental study of hip prosthesis stems fixed with bone cement in model femora and subjected to cyclic loading. Loosening of the stems did not occur due to fatigue of the bone cement, even when reduction of medial support from the calcar had been simulated. Measurement of stresses in the cement layer and comparison with published properties of bone cement indicated that fatigue failure was not likely under the experimental conditions. Fatigue failure of cement could not be solely regarded as initiating loosening. It is concluded that the results uphold theories that biological factors must also contribute to the initiation of loosening, rather than being a response to mechanical failure.     

磁悬浮式纳米级微动工作台的理论分析与建模研究

磁悬浮式微动工作台由于运动平台和驱动机构采用非接触式的磁悬浮驱动技术而易于实现大范围纳米级精度的微运动。本文构建了一种新型的磁悬浮微动工作台，从理论上对磁悬浮式微动工作台的运动机理进行了详尽分析，建立了磁悬浮式微运动的电磁驱动模型，用Matlab对磁悬浮式微动工作台的运动控制进行了仿真研究，结果表明，设计的磁悬浮式微动工作台能达到纳米级的微运动。本文的研究成果为磁悬浮式微动工作台的设计及其控制提供了理论基础。     

Effect of joint conformity on glenoid component fixation in total shoulder arthroplasty

Results of shoulder replacements are inferior and must be improved. Two of the major problems of total shoulder replacements are loosening of cemented glenoid components and wear of polyethylene inlays of uncemented, metal-backed glenoid components. The aim of this study is to investigate the influence of joint conformity on glenoid-component fixation. Keeled glenoid components, with radii of curvature of 24, 25, or 29 mm, were cemented in bone substitutes, placed in a force-controlled test set-up, articulating against a 24 mm humeral head. They were loaded by a constant joint compression force (725 +/- 10 N) and a superior subluxation force (shear force), cyclically varying between 0 and 350 +/- 1 N. After 200,000 load cycles, the upper and lower glenoid component rim-displacements were measured by custom-made displacement sensors. Additionally, the shear-out strength has been measured to investigate the residual strength. The glenoid component structures with radii of curvature of 24, 25, and 29 mm showed maximum superior rim-displacements of 0.163 (SD = 0.01), 0.299 (SD = 0.0306), and 0.350 (SD = 0.0197) mm respectively, which is a significant difference (p < 0.05). The maximum shear-out strength of glenoid components with radii of curvature of 24, 25, and 29 mm was 2707 (SD = 452), 2648 (SD = 299), and 2631 (SD = 312) N respectively, which is not a significant difference (p < 0.05). However, the results indicate that a conform articulation shows smaller glenoid rim-displacements, which might be beneficial for long-term component fixation.     

Mechanics of the passive knee joint. Part 1: The role of the tibial articular surfaces in guiding the passive motion

The motion of the unloaded knee is associated with tibial internal rotation and femoral posterior translation. Although it is known that the passive motion is the result of the interaction between the articular surfaces and the ligaments, the mechanism through which the particular pattern of motion is guided is not completely understood. The goal of this study was to focus on the tibial geometry and to identify the roles that its geometric features have in guiding the passive knee motion. The method used in this study simplified the geometry of the tibial plateaux and the menisci into basic features that could be eliminated individually. The generated tibial geometry was implemented in a computer model to simulate the passive motion. Different parts of the geometry were eliminated individually and the comparison between the simulation results was used to identify the role that each part of the geometry had in guiding the passive motion. The medial meniscus was found as the feature that promoted the tibial internal rotation and restrained the femoral posterior translation. The lateral meniscus and the medial aspect of the tibial eminence, on the other hand, were found as the elements that confined the tibial internal rotation.     

Coded Control of a Multilayer Piezo Actuator Producing Nano- and Micromotion

The mechanical and control characteristics of a multilayer piezo actuator producing nano- and micromotion are derived, in the case of coded control. The static and dynamic characteristics of such actuators with longitudinal, transverse, and shear piezoelectric effects are determined.     

Prediction of scratch resistance of cobalt chromium alloy bearing surface, articulating against ultra-high molecular weight polyethylene, due to third-body wear particles

The entrapment of abrasive particles within the articulation between a cobalt chromium alloy (CoCrMo) femoral component and an ultra-high molecular weight polyethylene (UHMWPE) cup of artificial hip joints or tibial inserts of artificial knee joints usually scratches the metallic bearing surface and consequently increases the surface roughness. This has been recognized as one of the main causes of excessive polyethylene wear, leading to osteolysis and loosening of the prosthetic components. The purpose of this study was to use the finite element method to investigate the resistance of the cobalt chromium alloy bearing surface to plastic deformation, as a first approximation to causing scratches, due to various entrapped debris such as bone, CoCrMo and ZrO2 (contained in radiopaque polymethyl methacrylate cement). A simple axisymmetric micro contact mechanics model was developed, where a spherical third-body wear particle was indented between the two bearing surfaces, modelled as two solid cylinders of a given diameter, under the contact pressure determined from macro-models representing either hip or knee implants. The deformation of both the wear particle and the bearing surfaces was modelled and was treated as elastic-plastic. The indented peak-to-valley height on the CoCrMo bearing surface from the finite element model was found to be in good agreement with that reported in a previous study when the third-body wear particle was assumed to be rigid. Under the physiological contact pressure experienced in both hip and knee implants, ZrO2 wear particles were found to be fully embedded within the UHMWPE bearing surface, and the maximum von Mises stresses within the CoCrMo bearing surface reached the corresponding yield strength. Consequently, the CoCrMo bearing surface was deformed plastically and the corresponding peak-to-valley height (surface roughness) was found to increase with both the hardness and the size of the wear particle. Even in the case of CoCrMo wear particles, with similar mechanical properties to those of the CoCrMo bearing surface, a significant plastic deformation of the bearing surface was also noted; this highlighted the importance of considering the deformation of the wear particles. These findings support the hypotheses made by clinical studies on the contribution of entrapped debris to increased surface roughness of CoCrMo femoral bearing surfaces.     

An experimental technique for the investigation of three-dimensional stress in bone cement underlying a tibial plateau

The technique of experimental model testing was applied to the analysis of stress at selected sites in bone cement underlying a tibial plateau. The investigation utilized a large model knee fabricated from materials which had mechanical properties similar to the actual tibial plateau and acrylic cement but which did not duplicate adequately the complexity of bone. A porous interface was created in the model between the materials representing the bone and cement. Three-dimensional strain rosettes were embedded into the cement and the model was loaded in a varus or valgus mode. Overloading resulted in breakdown of the modelled anterior and part of the posterior cement-bone interfaces, producing non-linear and in some cases erratic strains in the anterior section but repeatable linear results in the posterior section. The investigation highlighted the necessity for three-dimensional strain gauge investigations as opposed to two-dimensional studies. It is suggested that the approach could provide comparative information about different products and form the basis for a valuable design tool.     

Experimental versus computational analysis of micromotions at the implant-bone interface

In total hip arthroplasty, micromotions at the implant-bone interface influence the long-term survival of the prosthesis. These micromotions are often measured using sensors that are fixed to the implant and bone at points that are remote from the interface. Given that the implant-bone system is not rigid, errors may be introduced. It is not possible to assess the magnitude of these errors with the currently available experimental methods. However, this problem can be investigated using the finite element method (FEM). The hypothesis that the actual interface micromotions differ from those measured in the experimental manner was tested using a case-specific FE model, validated against deflection experiments. The FE model was used to simulate an 'experimental' method to measure micromotions. This 'experimental' method was performed by mimicking the distance between the measurement points; the implant point was selected at the interface while the bony point was at the outer surface of bone. No correlation was found between the micromotions computed at the interface and when using remote reference points. Moreover, the magnitudes of micromotions computed with the latter method were considerably greater. By reducing the distance between the reference points the error decreased, but the correlation stayed unchanged. Care needs to be taken when interpreting the results of micromotion measurement systems that use bony reference points at a distance from the actual interface.     

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.     

Mechanics of the passive knee joint. Part 2: interaction between the ligaments and the articular surfaces in guiding the joint motion

The aim of this study was to examine how the interaction between ligament tensions and contact forces guides the knee joint through its specific pattern of passive motion. A computer model was built based on cadaver data. The passive motion and the ligament lengthening and force patterns predicted by the model were verified with data from the literature. The contribution of each ligament and contact force was measured in terms of the rotational moment that it produced about the tibial medial plateau and the anterior-posterior (AP) force that it exerted on the tibia. The high tension of the anterior cruciate ligament (ACL) and the geometric constraints of the anterior horns of the menisci were found to be key features that stabilized the knee at full extension. The mutual effect of the cruciates was found as the reason for the screw-home mechanism at early flexion. Past 300, the AP component of contact force on the convex geometry of the lateral tibial plateau and tension of the lateral collateral ligament (LCL) were identified as elements that control the joint motion. From 60 degrees to 90 degrees, reduction in the tension of the ACL was determined as a reason for continuation of the tibial anterior translation. From 90 degrees to 120 degrees, increase in the tension of the posterior cruciate ligament and the AP component of the contact force on the convex geometry of the lateral tibial plateau pushed the tibia more anteriorly. This anterior translation was limited by the constraining effects of the ACL tension and the AP component of the contact force on the medial meniscus. The important guiding role observed for the LCL suggests that it should not be overlooked in knee models.     

Pore-scale analysis of Newtonian flow in the explicit geometry of vertebral trabecular bones using lattice Boltzmann simulation

The geometric and transport properties of trabecular bone are of particular interest for medical engineers active in orthopaedic applications and more specifically in hard tissue implantations. This article resorts to computational methods to provide some understanding of the geometric and transport properties of vertebral trabecular bone. A fuzzy distance transform algorithm was used for geometric analysis on the pore scale, and a lattice Boltzmann method (LBM) for the simulation of flow on the same scale. The transport properties of bone including the pressure drop, elongation, and shear component of dissipated energy, and the tortuosity of the bone geometry were extracted from the results of the LBM flow simulations. Whenever suitable, dimensionless numbers were used for the analysis of the data. The average pore size and distribution of the bone were found to be 746 microm and between 75 and 2940 microm, respectively. The permeability of the flow in the cavities of the specific bone sample was found to be 5.05 x 10(-8) m2 for the superior-inferior direction which was by a factor of 1.5-1.7 higher than the permeability in the other two anatomical directions (anterior-posterior). These findings are consistent with experimental results found 3 years prior independently. Tortuosity values approached 1.05 for the superior-inferior direction, and 1.13 and 1.11 for the other two perpendicular directions. The low tortuosities result mainly from the large bone porosity of 0.92. The flow on the pore scale seems to be shear dominated but 30 per cent of the energy dissipation was because of elongational effects. The converging and diverging geometry of the bone explains the significant elongation and deformation of the fluid elements. The transition from creeping flow (the Darcy regime), which is of interest to vertebral augmentation and this study, to the laminar region with significant inertia effects took place at a Reynolds number of about 1-10, as usual for porous media. Finally, the authors wish to advise the readers on the significant computational requirements to be allocated to such a virtual test bench.     

Effect of interface conditions between ultrahigh molecular weight polyethylene and polymethyl methacrylate bone cement on the mechanical behaviour of total shoulder arthroplasty

The aim of this study was to investigate the effect of the interface condition between polymethyl methacrylate (PMMA) bone cement and the ultrahigh molecular weight (UHMWPE) glenoid component on cement stresses and glenoid component tilting in a finite element (FE) model. The background of this research is that most FE models assume bonding between the PMMA bone cement and the UHMWPE component, although it is very doubtful that this bonding is present. An FE model of a cemented glenoid component was developed and a joint compression force and subluxation force of 725 and 350 N respectively were applied. The maximal principal stresses in the cement layer ranged between 21.30 and 32.18 MPa. Glenoid component tilting ranged between 0.943 degrees and 0.513 degrees. It was found that the interface condition has a large effect on the maximal principal stresses and glenoid component tilting. Whether adhesion between the UHMWPE component and PMMA bone cement occurs is unknown beforehand and, as a result, design validation using the FE technique should be carried out both by using contact elements in combination with a coefficient of friction as well as by a full bonding at this interface.