Specific Finite Element Models Research Articles

Seismic Analytical Model for Retrofitted Old Reinforced Concrete Structures

Abstract In seismic areas, a majority of such old buildings have only been designed for gravity loads or according to outdated seismic codes. Before1970's, smooth rebars were also extensively used in RC structures. As some of these buildings are still functioning, they need to be reassessed against seismic demands. Load bearing behaviour of concrete structures reinforced with smooth rebars is considerably affected by the slip deformation of the plain rebars. In this study a specific finite element model has been proposed for evaluating the seismic performance of these structures. The slipping characteristics of the smooth rebar have been incorporated in this model. To this aim, a tailored stress-strain property has been assigned to the steel fibers in tension. The model has then been calibrated/verified against several sets of experimental results, from other. In general satisfactory correlations have been noticed between the experimental results and the predictions from the proposed fiber element model. The model has also been used for a full seismic assessment of an existing RC building, having smooth rebars. In addition, the structure has been retrofitted with steel bracing, viscose dampers and base isolators schemes and their nonlinear seismic performance have been evaluated using the proposed model.

Generic Rules of Mechano-Regulation Combined with Subject Specific Loading Conditions Can Explain Bone Adaptation after THA

Bone adaptation after total hip arthroplasty is associated with the change in internal load environment, and can result in compromised bone stock, which presents a considerable challenge should a revision procedure be required. Under the assumption of a generic mechano-regulatory algorithm for governing bone adaptation, the aim of this study was to understand the contribution of subject specific loading conditions towards explaining the local periprosthetic remodelling variations in patients.CT scans of 3 consecutive THA patients were obtained and used for the construction of subject specific finite element models using verified musculoskeletal loading and physiological boundary conditions. Using either strain energy density or equivalent strain as mechano-transduction signals, predictions of bone adaptation were compared to DEXA derived BMD changes from 7 days to 12 months post-implantation. Individual changes in BMD of up to 33.6% were observed within the 12 month follow-up period, together with considerable inter-patient variability of up to 26%. Estimates of bone adaptation using equivalent strain and balanced loading conditions led to the best agreement with in vivo measured BMD, with RMS errors of only 3.9%, 7.3% and 7.3% for the individual subjects, compared to errors of over 10% when the loading conditions were simplified.This study provides evidence that subject specific loading conditions and physiological boundary constraints are essential for explaining inter-patient variations in bone adaptation patterns. This improved knowledge of the rules governing the adaptation of bone following THA helps towards understanding the interplay between mechanics and biology for better identifying patients at risk of excessive or problematic periprosthetic bone atrophy.

IMPROVED FEMORAL NECK FRACTURE PREDICTIONS USING ANISOTROPIC FAILURE CRITERIA MODELS

Finite element models are widely used to assess long bone strength, implant stability and other clinical problems. In most of the models presented so far in the literature, the bone is taken to be isotropic, and the occurrence of failure is predicted by defining a threshold von Mises stress. However, human bone is found to show orthotropic behavior. Studies so far have focused only on the use of anisotropic criteria in orthotropic models designed to predict the occurrence of human femur failure. The aim of this study was therefore to investigate how specific finite element models for human femora combined with composite failure theories could be used to improve failure predictions in vitro. For this purpose, nine human proximal femora were tested mechanically up to failure under the loading conditions present during the one-leg stance phase in walking. Specific finite element models using various materials to represent the bone were generated for each femur. First, the bone material was modeled in the form of an isotropic brittle material, and the von Mises criterion was used to predict the occurrence of fracture. Second, the bone was modeled as a transversely isotropic brittle material with asymmetric strength characteristics, and the occurrence of fracture was predicted using the Hill and the Tsai–Wu criteria. The results obtained here show that the transversely isotropic model combined with Tsai–Wu and Hill criteria accurately predicted the fracture load (values of R2 = 0.94 and SEE = 10.3% were obtained with the Tsai–Wu criteria and R2 = 0.82 and SEE = 22.9% were obtained with the Hill criteria), while the isotropic model combined with the von Mises criterion overestimated the fracture load, although a good correlation was generally observed with the experimental results (R2 = 0.77, SEE = 30.6%).

Craniofacial biomechanics: in vivo to in silico

Craniofacial biomechanics: in vivo to in silico

A numerical investigation of mid-femoral injury tolerance in axial compression and bending loading

Bone fractures occur frequently at mid-shaft femoral site of the front seat vehicle occupants during frontal and offset automotive crashes. A numerical investigation of femoral shaft tolerance under axial and bending loading corresponding to traffic accidents is presented in the current study. A subject specific finite element (FE) model of a femur is developed and the parameters of two material models of cortical bone (isotropic elastic–plastic and elastic transversally isotropic) are identified based on three-point bending test data using optimisation techniques. A Monte Carlo analysis is performed on a surface approximation of the optimised models over a domain of +/− 20% of the optimised parameter values and showed that the elastic moduli of femur are the most influential parameters on the bone stiffness curve prior to bone fracture. The mid-shaft femoral tolerance curves demonstrate sensitivity with respect to the impact direction of transversal load due to the initial curvature of the femur, but insignificant dependence on the material model, or the failure criteria used for femoral cortical bone. The results highlight the predominant role of bending in a combined axial-bending loading of the femur, which may be used in redefining the current injury criteria of femur used in anthropometric test devices.

The Effect of Under-Reaming on the Cup/Bone Interface of a Press Fit Hip Replacement

This paper explores the effect of under-reaming on micromotion at the cup/bone interface of a press-fit acetabular cup. A cadaver experiment was performed on 11 acetabuli implanted with a cementless acetabular cup. The loading profile simulated hip impingement at the extremes of motion and subluxation relocation of the hip joint. Micromotion of each cup was measured in a custom made jig with linear variable differential transducers. A CAT scan and DEXA scan of the acetabulum and femoral head respectively were used to construct a three-dimensional patient specific finite element model of the hemi-pelvis. The model predicted cup micromotion under loading conditions and stresses in the acetabulum as a result of cup insertion. Micromotion was then calculated as a function of variable bone density and variable degree of underreaming. Simulated cup insertion with under-reaming of 2 mm or more approached or exceeded the yield strength of bone in acetabula with reduced bone mass density.

Dor procedure for dyskinetic anteroapical myocardial infarction fails to improve contractility in the border zone

Endoventricular patch plasty (Dor) is used to reduce left ventricular volume after myocardial infarction and subsequent left ventricular remodeling. End-diastolic and end-systolic pressure-volume and Starling relationships were measured, and magnetic resonance images with noninvasive tags were used to calculate 3-dimensional myocardial strain in 6 sheep 2 weeks before and 2 and 6 weeks after the Dor procedure. These experimental results were previously reported. The imaging data from 1 sheep were incomplete. Animal specific finite element models were created from the remaining 5 animals using magnetic resonance images and left ventricular pressure obtained at early diastolic filling. Finite element models were optimized with 3-dimensional strain and used to determine systolic material properties, T(max,skinned-fiber), and diastolic and systolic stress in remote myocardium and border zone. Six weeks after the Dor procedure, end-diastolic and end-systolic stress in the border zone were substantially reduced. However, although there was a slight increase in T(max,skinned-fiber) in the border zone near the myocardial infarction at 6 weeks, the change was not significant. The Dor procedure decreases end-diastolic and end-systolic stress but fails to improve contractility in the infarct border zone. Future work should focus on measures that will enhance border zone function alone or in combination with surgical remodeling.

Predicting the effects of knee focal articular surface injury with a patient-specific finite element model

Successful focal articular surface injury (FAI) repair depends on appropriate matching of the geometrical/material properties of the repaired site, and on the overall dynamic response of the knee to in-vivo loading. There is evidence linking the pathogenesis of lesion progression (e.g. osteoarthritis) to weightbearing site and defect size. The paper investigates further this link by studying the effects of osteochondral defect size on the load distribution at the human knee. Experimental data from cadaver knees ( n = 8) loaded at 30° of flexion was used as input to a validated finite element (FE) model. Contact pressure was assessed for the intact knees and over a range of circular osteochondral defects (5 mm to 20 mm) at 30° of flexion with 700 N axial load. Patient specific FE models and the specific boundary conditions of the experimental set-up were used to analyze the osteochondral defects. Stress concentration around the rims of defects 8 mm and smaller was not significant and pressure distribution was dominated by the menisci. Experimental data was confirmed by the model. For defects 10 mm and greater, distribution of peak pressures followed the rim of the defect with a mean distance from the rim of 2.64 mm on the medial condyle and 2.90 mm on the lateral condyle (model predictions were 2.63 and 2.87 mm respectively). Statistical significance was reported when comparing defects that differed by 4 mm or greater (except for the 5 mm case). Peak rim pressure did not significantly increase as defects were enlarged from 10 mm to 20 mm. Peak values were always significantly higher over the medial femoral condyle. Although the decision to treat osteochondral lesions is multifactorial, the results of this finite element analysis indicate that a size threshold of 10 mm, may be a useful early adjunct to guide clinical decision-making. This modified FE method can be employed for in-vivo studies.

Validation of an anatomy specific finite element model of Colles’ fracture

Osteoporotic fractures are harmful injuries and their number is on the rise. Distal radius fractures are precursors of other osteoporotic fractures. The wrist's bony geometry and trabecular architecture can be assessed in vivo using the recently introduced HR-pQCT. The goal of this study was the validation of a newly developed HR-pQCT based anatomy specific FE technique including separation of cortical and trabecular bone regions using an experimental model for producing Colles’ fractures. Mechanical compression tests of 21 embalmed human radii were conducted. Continuum level FE models were built using HR-pQCT images of the bones and nonlinear analyses were performed using boundary conditions highly similar to the mechanical tests. Density and fabric based material properties were taken from previous tests on biopsies and no adjustments were made. Numerical results provided good prediction of the experimental stiffness ( R 2 = 0.793 ) and even better for strength ( R 2 = 0.874 ). High damage zones of the FE models coincided with the actual failure patterns of the specimens. These encouraging results allow to conclude that the developed method represents an attractive and efficient tool for simulation of Colles’ fracture.

Vertebral trabecular main direction can be determined from clinical CT datasets using the gradient structure tensor and not the inertia tensor—A case study

Osteoporosis is a wide spread disease with one-third of all women beyond their menopause and a fifth of men above the age of 50 years suffering from it. Patient specific finite element models would be a great improvement for the diagnosis of vertebral fracture risk. Different material models have been proposed, which incorporate information about the anisotropy of trabecular bone in addition to bone mineral density (BMD) using a second rank structure tensor.Two alternative structure measurement methods, gradient structure tensor (GST) and inertia tensor (IT), were investigated. Structure was determined from in situ scans. This was compared to structure computed with the mean intercept length (MIL) tensor from μCT scans at the same locations.GST delivered information comparable to MIL regarding the structural main direction even at normal dose standard clinical settings (median of the scalar products of up to ≈0.98). IT was not comparable to MIL (median ≈0.4). Neither of the alternatives could determine eigenvalues comparable to these determined from MIL (p>0.5).In conclusion, this study could show that the measurement of the structural main direction is possible for in situ scans in a clinical setting. It was shown that the method of choice to determine trabecular main direction in situ is GST. Knowing the main direction a transverse isotropic fabric tensor can be constructed.

Subject specific finite element analysis of implant stability for a cementless femoral stem

Background The primary stability of a cementless implant is crucial to ensure long term stability through osseointegration. In the present study we have examined how subject specific finite element models can be used to evaluate the stability of a cementless femoral stem. Methods Micromotion on the bone–implant interface of a cementless stem was measured experimentally in six human cadaver femurs. Subject specific finite element models were built from computed tomography of the same femurs, and used to simulate the same load scenario used experimentally. Findings Both experimental measurements and numerical analyses showed a tendency of increased rotational stability for bigger implants. Good correlation was found between measurements and calculated values of axial rotation ( R 2 = 0.74, P < 0.001). The finite element models produced interface micromotion of the same magnitude as measured experimentally, with micromotion generally below 40 μm. Bigger femoral stems were found to decrease the micromotion in the experimental measurements. This tendency could not be recognised in the interface micromotion from the finite element models. Interpretation The finite element models showed limited success in predicting interfacial micromotion, but reproduced a similar pattern of rotational stability for the implants as seen experimentally. Since rotation in retroversion is often the main concern when studying implant stability, subject specific finite element models could be employed for pre-clinical evaluation of implants.

Subject specific finite element analysis of stress shielding around a cementless femoral stem

Background Stress shielding around a femoral stem is usually assessed experimentally using composite or human cadaver femurs. In the present study we have explored the feasibility of using subject specific finite element models to determine stress shielding in operated femurs. Methods Cortical strain was measured experimentally on seven human cadaver femurs, intact and implanted with a straight cementless prosthesis. Two load configurations were considered: single leg stance and stair climbing. Subject specific finite element models derived from computed tomography of the same femurs were analysed intact and with an implant. Principal cortical strain was used to validate the finite element models. Stress shielding was defined as the change in equivalent (von Mises) strain between pre- and post-operative femurs. Findings Cortical strain predicted by the finite element analyses showed to be close to unity with the experimental observations for both intact ( R 2 = 0.94, slope = 0.99), operated femurs ( R 2 = 0.86, slope = 0.86) and stress shielding ( R 2 = 0.70, slope = 0.90). In the proximal calcar area, the region most prone to periprosthetic remodelling, the finite element models were found to successfully reproduce the stress shielding observed experimentally. Interpretation The study shows that subject specific finite element models manage to describe the stress shielding pattern measured in vitro in the different femurs. Finite element models based on actual human femurs (cadaver and/or patient) could thus be a useful tool in the pre-clinical evaluation of new implants.

IN VIVO CONTACT STRESSES AT THE RADIOCARPAL JOINT USING A FINITE ELEMENT METHOD OF THE COMPLETE WRIST JOINT

A small number of cadaveric studies have been carried out looking at the force transmission through the radiocarpal joint. In this study subject specific finite element models were created of the whole wrist joint using measured biomechanical data to capture the forces acting on the wrist with the hand generating a maximum gripping force.

Évaluation tridimensionnelle et optimisation du traitement orthopédique de la scoliose idiopathique adolescente

Adolescent idiopathic scoliosis involves complex tridimensional deformities of the spine, rib cage and pelvis. Moderate curves generally are treated using an orthosis. This paper presents different studies performed over the last fifteen years related to the biomechanical evaluation and optimization of the orthopedic treatment of scoliotic deformities. Patient specific 3D models of the spine, pelvis and rib cage are computed from calibrated radiographs, and are used to calculate 2D and 3D clinical indices. The torso shape is acquired using surface topography. With such internal and external 3D models, the efficacy of the most frequently used orthoses can be analyzed and new treatments can be developed. Pressures generated by a brace on the patient's trunk were measured using a flexible matrix of pressure sensors and displayed over the patient's internal geometry in order to analyze the brace efficacy. Patient specific finite element models have been developed, including the osseo-ligamentous structures as well as the muscles, the neuro-control, trunk growth and its adaptation to the stress. These models were used to analyze the effects of the Boston brace. The electro-myographic activity also was measured to analyze the << active >> correction mechanisms. Adjustment techniques and software are used to help the orthotists with real time feedback when the brace is being fabricated and adjusted to the patient. Residual growth potential is also being added to the computer model to simulate the long term effect of a brace. The improvement of the orthotic treatments of scoliotic deformities is very encouraging. The exploitation of such tools is expected to allow reaching optimal treatment personalized to each patient. double dagger.

Limitation of scaling methods in child head finite element modelling

During growth, a child's head undergoes different modifications in morphology and structure. This paper presents an anthropometric study in terms of dimension compared to the scaling method developed by Mertz which consists of reducing the adult head model with a scaling coefficient to obtain a child head. A detailed sizes and shape analysis of brain contour in sagittal and frontal plans is then proposed, for child head versus a scaled adult head. The superimposition of those contours allowed pointing to main differences. Numerical simulations performed with the detailed three year old child head model developed in this present study, and a scaled adult head finite element model, showed that reducing an adult finite element model to obtain a child head by scaling method does not seem to be realistic. Then, the creation of specific finite element models of child head seems necessary to understand paediatric injuries.

High‐performance parallel simulation of structure degradation using non‐local damage models

AbstractSimulating damage and failure of metallic or composite structures often fails when using standard finite element discretizations and a Newton–Raphson solving procedure. The difficulties arise from an uncontrolled mesh dependency caused by damage localization, to structural instabilities and to an increase of computational costs. The first problem can be overcome by using non‐local damage constitutive equations together with a specific finite element model. The second problem can then be solved with an arc‐length method which manages instabilities and snap‐backs. They are caused by the loss of strength‐carrying capacity of the damaged material. Finally, the whole computational scheme is parallelized. The main contribution of this paper consists in bringing together the three numerical techniques: non‐local material model; piloting and parallel computations. The resulting numerical scheme allows to go beyond the overly simplistic cases often encountered in the literature. It allows robust industrial simulations with a high number of degrees of freedom, both in two and three dimensions, and deepened studies involving a large number of simulations. Copyright © 2006 John Wiley &amp; Sons, Ltd.

CASMIL: a comprehensive software/toolkit for image-guided neurosurgeries

CASMIL aims to develop a cost-effective and efficient approach to monitor and predict deformation during surgery, allowing accurate, and real-time intra-operative information to be provided reliably to the surgeon. CASMIL is a comprehensive Image-guided Neurosurgery System with extensive novel features. It is an integration of various modules including rigid and non-rigid body co-registration (image-image, image-atlas, and image-patient), automated 3D segmentation, brain shift predictor, knowledge based query tools, intelligent planning, and augmented reality. One of the vital and unique modules is the Intelligent Planning module, which displays the best surgical corridor on the computer screen based on tumor location, captured surgeon knowledge, and predicted brain shift using patient specific Finite Element Model. Also, it has multi-level parallel computing to provide near real-time interaction with iMRI (Intra-operative MRI). In addition, it has been securely web-enabled and optimized for remote web and PDA access. A version of this system is being used and tested using real patient data and is expected to be in use in the operating room at the Detroit Medical Center in the first half of 2006. CASMIL is currently under development and is targeted for minimally invasive surgeries. With minimal changes to the design, it can be easily extended and made available for other surgical procedures.

Accurate prediction of strain values from subject specific finite element models of long bones

Accurate prediction of strain values from subject specific finite element models of long bones

Finite element simulation for the prediction of mechanical failure in the lumbar spine surgery

Finite Element spine modeling can be useful for computer assisted surgery planning providing help in assessing the risk of mechanical failure. The aim of this study is to provide a patient specific finite element model of the lumbar spine where surgical gesture was considered and to evaluate its relevance as a predictive tool for lumbar spine surgery. A three-dimensional FEM of the L1-Sacrum lumbar spine with geometrical personalization was used. Mechanical properties were based on published data. Then, main characteristics of degenerative pathologies and surgical gestures were considered, and posterior implants were modeled. The sacrum was fixed and Flexion compression loads were applied on the L1 vertebra. Then, results of simulations were post-processed to obtain stresses in the discs and along implants. Clinical cases were collected to evaluate the models relevance. 66 cases instrumented with rigid screw rod systems were considered. 13 of the collected cases were “failed cases” in which patients had screw breakage or screw loosening. Blind simulations were performed for all these patients and the results were qualitatively compared with the clinical outcome. Numerical results highlighted the specific behavior of 12 models for which loads on screws and rods were markedly higher than those of all others. The analysis of the clinical outcome emphasized that 9 of these 13 models corresponded to patients with a mechanical failure. For the first time a patient specific model was proposed for lumbar spine surgery planning. By increasing the number of modeled clinical cases, it could be possible to improve this model and to provide an help for surgery planning.

Numerical and experimental analysis of second-order effects and loss mechanisms in axisymmetrical cavities

This paper deals with the analysis of finite amplitude acoustic waves in three-dimensional resonant cavities. A specific finite element model is proposed which includes: (i) the pressure field nonlinearity using a perturbative method; (ii) the loss mechanisms using an experimentally determined effective bulk attenuation or modeling viscous and thermal losses at the walls and acoustic radiation through the aperture with complex impedances. An axisymmetrical cavity with transversal dimension larger than the wavelength has been experimentally studied. The acoustic field is generated by a high-power flexurally vibrating transducer which generates high pressures. Measurements are performed for the fundamental and second-order pressure components for several cavity resonance modes. An important standard deviation is observed. Experimental data are compared to predicted pressure field distributions. Numerical models describe the pressure distribution correctly but overestimate the amplitude.