1,721,038 research outputs found
Effect of varus/valgus malalignment on bone strains in the proximal tibia after TKR: an explicit finite element study
No full textMalalignment is the main cause of tibial component loosening. Implants that migrate
rapidly in the first two post-operative years are likely to present aseptic loosening. It has
been suggested that cancellous bone stresses can be correlated with tibial component
migration. A recent study has shown that patient-specific finite element (FE) models have
the power to predict the short-term behavior of tibial trays. The stresses generated within
the implanted tibia are dependent on the kinematics of the joint; however, previous
studies have ignored the kinematics and only applied static loads. Using explicit FE, it is
possible to simultaneously predict the kinematics and stresses during a gait cycle. The
aim of this study was to examine the cancellous bone strains during the stance phase of
the gait cycle, for varying degrees of varus/valgus eccentric loading using explicit FE. A
patient-specific model of a proximal tibia was created from CT scan images, including
heterogeneous bone properties. The proximal tibia was implanted with a commercial
total knee replacement (TKR) model. The stance phase of gait was simulated and the
applied loads and boundary conditions were based on those used for the Stanmore knee
simulator. Eccentric loading was simulated. As well as examining the tibial bone strains
(minimum and maximum principal strain), the kinematics of the bone-implant construct
are also reported. The maximum anterior–posterior displacements and internal–external
rotations were produced by the model with 20 mm offset. The peak minimum and maximum
principal strain values increased as the load was shifted laterally, reaching a
maximum magnitude for ?20 mm offset. This suggests that when in varus, the load
transferred to the bone is shifted medially, and as the bone supporting this load is stiffer,
the resulting peak bone strains are lower than when the load is shifted laterally (valgus).
For this particular patient, the TKR design analyzed produced the highest cancellous
bone strains when in valgus. This study has provided an insight in the variations produced
in bone strain distribution when the axial load is applied eccentrically. To the
authors’ knowledge, this is the first time that the bone strain distribution of a proximal
implanted tibia has been examined, also accounting for the kinematics of the tibio–
femoral joint as part of the simulation. This approach gives greater insight into the
overall performance of TKR
A combined RSA and FE study of the implanted proximal tibia: correlation of the post-operative mechanical environment with implant migration
No full textInter-patient evaluation of stresses in proximal implanted tibiae
No full textIn biomechanics finite element analysis (FEA) is still only a comparative tool. To the authors' knowledge, no study has examined multiple tibiae or included patient specific data. Only by constructing finite element models taking into account these parameters in combination with prospective clinical studies can the predictive power of FEA be assessed. The purpose of this study was to evaluate the differences in the predicted stresses and risk ratios observed on the resected surface of models of proximal implanted tibiae created from patient specific data.
Finite element models of four proximal implanted tibiae were analysed. The models were created from quantitative computed tomography (QCT) data. The immediate post-operative situation was modelled by assuming frictionless contact between the tibia and the tibial plateau. Post-operative alignment of the implants were considered in the study. The loads used in the models were equivalent to three times the weight of each patient. A bi-condylar load case was used, in which 60% of the total force was applied on the medial side and 40% on the lateral side. The forces were applied directly on the tibial plateau.
A program called Bonemat, was used to assign the material properties on an element-by-element basis, based on the correlation between QCT data and the material properties of the bone (i.e. apparent density and stiffness). Meshes of linear tetrahedral elements were created in I-DEAS for both bone and implant. Risk ratio values (defined as the Von Mises stress divided by the ultimate compressive strength) on the resected surface of each tibia were examined and compared between all four models. All the analyses were carried out using MARC K7.3.2.
In all four models, a similar overall risk ratio distribution on the surface of interest was observed, with peak values of 319%, 315%, 322% and 327% for patients 1,2,3 and 4, respectively. In all the models, the peak values were found in the portion of cancellous bone supporting the posterior side of the tibial plateau.
For all four tibiae there were areas of bone, particularly around the posterior cruciate cut-out, where the localised risk of failure was high. However, in general, the risk of failure was below 100% over the majority of the resected surface. RSA studies of tibial plateaus have shown that during the first six months after surgery, there is a period of rapid migration. This is widely thought to be due to the implant "bedding in". The findings of this study support this view, as some regions presented considerably high risk ratios. In these regions, the cancellous bone will be crushed and the load redistributed on the resected surface until an equilibrium position is reached. This study has shown that patient specific FE model do demonstrate subtle differences in the predicted stress and risk ratio distributions. This emphasises the importance of moving away from the traditional generic modelling approach to patient specific modelling
Assessment of the effect of mesh density on the material property discretisation within QCT based FE models: a practical example using the implanted proximal tibia
No full textthree-dimensional, quantitative computed tomography based finite element model of a proximal implanted tibia was analysed in order to assess the effect of mesh density on material property discretisation and the resulting influence on the predicted stress distribution. The mesh was refined on the contact surfaces (matched meshes) with element sizes of 3, 2, 1.4, 1 and 0.8 mm. The same loading conditions were used in all models (bi-condylar load: 60% medial, 40% lateral).Significant variations were observed in the modulus distributions between the coarsest and finest mesh densities. Poor discretisation of the material properties also resulted in poor correlations of the stresses and risk ratios between the coarsest and finest meshes. Little difference in Young's modulus, von Mises stress and risk ratio distributions were observed between the three finest models; hence, it was concluded that for this particular case an element size of 1.4 mm on the contact surfaces was enough to properly describe the stiffness, stress and risk ratio distributions within the bone. Poor convergence of the material property distribution occurred when the element size was significantly larger than the pixel size of the source CT data. It was concluded that unless there is convergence in the Young's modulus distribution, convergence of the stress field or of other parameters of interest will not occur either.<br/
The importance of tibial alignment
No full textThe influence of the tibial plateau orientation on cancellous bone stress was examined by finite element analysis for a cemented device. The objectives of the study were i) to examine the effect of the plateau-ankle angle on the cancellous bone stress, ii) to analyze the significance of the anteroposterior angles of the tibial component on these stresses, and iii) to compare the finite element predictions with clinical data. In general, positioning the tibial plateau in valgus resulted in lower cancellous bone stresses. These results support previous clinical studies, which suggest that overall alignment in valgus results in lower migration rates and lower incidence of loosening
A combined RSA and FE study of the implanted proximal tibia: correlation of the post-operative mechanical environment with implant migration
No full textThere is strong evidence to suggest that inducible displacements, migration and implant loosening are closely related to the initial mechanical environment of the implanted tibia. If this is true, then it should be possible to predict the likelihood of implant migration using patient-specific finite element models. Finite element models of the proximal implanted tibiae were analysed based on pre-operative quantitative computed tomography data of four patients entered into a radiographic migration study. These four patients were also part of an radiostereometric analysis (RSA) study. A variety of load cases were analysed and the risk of bone failure determined for a 2 mm layer of bone immediately beneath the tibial tray. The results were compared with the RSA data measured 1 year post-operatively for each patient. For each patient, an appropriate load case was selected based on patient weight and on the varus–valgus migrations observed in the migration study. The two patients with press-fit implants were predicted to have the highest risk of failure and were found to migrate the most. The two patients with bonded implants (one HA coated and one cemented) were found to have a low risk of failure and these implants migrated the least.This study suggests that the degree of implant migration is dependent on the initial mechanical environment and can be determined using patient-specific finite element analysis
Effect of Strain-Rate and Temperature on Mechanical Response of Pure Tungsten
No full textThis paper presents the results obtained from the investigation of the mechanical behaviour of two different batches of pure tungsten specimens. The interest in pure tungsten is due to its special properties, which has led to it finding applications in several fields, including nuclear physics. At this moment, it is used as a core material for fixed particle producing targets in several accelerator facilities around the world and it is a candidate for future ones. In these facilities, tungsten directly interacts with high energy proton beams and, consequently, is subjected to considerable deformation at high strain-rates and temperatures. From these considerations, there comes the need to properly investigate the material response under these extreme conditions. For this purpose, an ad-hoc testing campaign was performed on small dog-bone specimens in tension. The results will be applicable to the ongoing design of CERN’s AD-target as well as to other future tungsten targets operating at high power and dynamically loaded in multiple accelerator facilities. Due to tungsten’s high Ductile-to-Brittle Transition temperature, it was not possible to test it at temperatures less than 250 °C. Tests were performed at two different strain-rates (nominal value of 1 s−1 and 103 s−1) reaching a maximum temperature of 1000 °C. The dynamic tests were performed by using a Hopkinson Bar setup in the direct impact configuration. Both at low and high strain-rates, heating of the specimen was achieved using an induction coil system. A numerical inverse procedure was applied to analyse the experimental data with the aim to obtain the equivalent stress versus effective plastic strain at the different loading conditions to be used for calibration of the strength model and for the evaluation of strain-rate and thermal softening sensitivities of the material
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