213 research outputs found
Cell Method and Related Meshless Methods
No full textCOMPUTER MODELING IN ENGINEERING & SCIENCES CMES Vol. 94, No. 4, 2013
ISSN: 1526-1492 (print)
ISSN: 1526-1506 (on-line)
Special Issue of Cell Method
organized by Professor Elena Ferretti
CONTENTS
The Cell Method: Quadratic Interpolation with Tetrahedra for 3D Scalar Fields
Martino Pani, Fulvia Taddei . . . . . . . . . . . . . . . . . . . . . . . . . 279
Soft Core Plane State Structures Under Static Loads Using GDQFEM and Cell
Method
E. Viola, F. Tornabene E. Ferretti and N. Fantuzzi . . . . . . . . . . . . . 301
GDQFEM Numerical Simulations of Continuous Media with Cracks and Discontinuities
E. Viola, F. Tornabene E. Ferretti and N. Fantuzzi . . . . . . . . . . . . . 33
A new meshless approach for subject-specific strain predictionin long bones: Evaluation of accuracy
No full textThe Finite Element Method is at present the method of choice for strain prediction in bones from Computed Tomography
data. However, accurate methods rely on the correct topological representation of the bone surface, which requires a massive operator
effort, thus restricting their applicability to clinical practice. Meshless methods, which do not rely on a pre-defined topological
discretisation of the domain, might greatly improve the numerical process automation, but currently their application to biomechanics
is negligible.
A meshless implementation of an innovative numerical approach based on a direct discrete formulation of physical laws,
the Cell Method, was developed to predict strains in a cadaver femur from Computed Tomography data. The model accuracy was estimated
by comparing the predicted strains with those experimentally measured on the same specimen in a previous study. As a reference,
the results were compared to those obtained with a state-of-the-art finite element model.
Findings. The Cell Method meshless model predicted strains highly correlated with the experimental measurements (R2 = 0.85) with a
good global accuracy (RMSE = 15.6%). The model performed slightly worse than the finite element one, but this was probably due to
the need to sub-sample the original data, and the lower order of the interpolation used (linear vs parabolic).
Although there is surely room for improvement, the accuracy already obtained with this meshless implementation of
the Cell Method makes it a good candidate for some clinical applications, especially considering the full automation of the method,
which does not require any data pre-processing.
2008 Elsevier Ltd. All rights reserved
Finite element modelling of human bones from ct data: a sensitivity study on the influence of geometry and material uncertainties
No full textThe aim of the present work was to investigate the
influence that the uncertainties related to the
segmentation process and to the material assignment
procedures may have onto the results of a finite element
analysis of a human femur generated from Computer
Tomography (CT) data. Starting from a CT dataset of a
normal femur, the finite element model was generated
using an already validated procedure. The errors
associated to the segmentation algorithm adopted, to the
CT dataset calibration and to the material assignment
procedure were estimated. Simplified loading conditions
were applied to the femur to generate a compressive
bending and torque loading. The distribution of the
mostly used biomechanical indicators was analyzed. The
results showed that the level of influence of the input
variable is dependent on the load case and on the
considered output variable. The coefficients of variation
were, however, always smaller than 10%, for all variables
THE EFFECT of COMPUTED TOMOGRAPHY CURRENT REDUCTION on PROXIMAL FEMUR SUBJECT-SPECIFIC FINITE ELEMENT MODELS
No full textMany studies have addressed the modulation of computed tomography (CT) parameters, and particularly of tube current, to obtain a good compromise between the X-ray dose to the patient and the image quality for diagnostic applications. This study aimed at evaluating the influence of dose reduction by means of tube current reduction on the CT-based subject-specific finite element (FE) modeling. To this aim, CT scans at stepwise reduced values of tube current from 180mAs to 80mAs were performed on: (i) a densitometric phantom, to quantify the changes in the calibration equation; (ii) a fresh-frozen, water submersed, human cadaver femur, to quantify changes in geometry reconstruction and material mapping from CT, as well as strain prediction accuracy, based on the in vitro strain measurements available; (iii) a fresh-frozen human cadaver thigh with soft tissues attached, to quantify FE results changes in conditions similar to those found in vivo. The results showed that the tube current reduction does not affect the 3D modeling and the femur FE analysis. Our pilot study highlights the possibility of performing CT scans with reduced dose to generate biomechanical models, although a confirmation by performing larger studies with clinical CT data is needed
Effect of lower-limb joint models on subject-specific musculoskeletal models and simulations of daily motor activities
No full textUnderstanding the validity of using musculoskeletal models is critical, making important to assess how model parameters affect predictions. In particular, assumptions on joint models can affect predictions from simulations of movement, and the identification of image-based joints is unavoidably affected by uncertainty that can decrease the benefits of increasing model complexity. We evaluated the effect of different lower-limb joint models on muscle and joint contact forces during four motor tasks, and assessed the sensitivity to the uncertainties in the identification of anatomical four-bar-linkage joints. Three MRI-based musculoskeletal models having different knee and ankle joint models were created and used for the purpose. Model predictions were compared against a baseline model including simpler and widely-adopted joints. In addition, a probabilistic analysis was performed by perturbing four-bar-linkage joint parameters according to their uncertainty. The differences between models depended on the motor task analyzed, and there could be marked differences at peak loading (up to 2.40 BW at the knee and 1.54 BW at the ankle), although they were rather small over the motor task cycles (up to 0.59 BW at the knee and 0.31 BW at the ankle). The model including more degrees of freedom showed more discrepancies in predicted muscle activations compared to measured muscle activity. Further, including image-based four-bar-linkages was robust to simulate walking, chair rise and stair ascent, but not stair descent (peak standard deviation of 2.66 BW), suggesting that joint model complexity should be set according to the imaging dataset available and the intended application, performing sensitivity analyses
Sensitivity of the primary stability of a cementless hip stem to its position and orientation
No full textAbstract: Using computed tomography (CT)-based preoperative
planning software, we can define with good
accuracy the position of a cementless hip stem inside the
host bone, but previous studies suggest that the pose the
surgeon achieves during freehand surgery may differ from
the planned one even by some millimeters. Advances in
simulation now make it possible to predict the primary
stability of the stem in a given position during the preoperative
planning, but is the stability predicted for the
planned pose indicative of that we can expect for the
achieved pose? The aim of the present study was to verify
how this prediction is affected by the differences observed
between the planned and the achieved poses. Two finite
element models of an implanted femur were generated,
one with the stem in the planned pose, and one with the
stem in the achieved pose, as defined from postoperative
CT scans. When compared to experimental measurements,
the model with the achieved position was clearly
more accurate (0.6 vs. 12% error over measured peak
micromotion); however, the predictions of induced micromotions
were different between the two models for less
than 13%. It is thus concluded that while the implant
position does have an effect on primary stability, the
estimate of micromotion we can get from the planned
position remains a clinically relevant indicator
Finite-element modeling of bones from CT data: sensitivity to geometry and material uncertainties
No full textThe aim of the present work was to investigate the
influence that the uncertainties related to the
segmentation process and to the material assignment
procedures may have onto the results of a finite element
analysis of a human femur generated from Computer
Tomography (CT) data. Starting from a CT dataset of a
normal femur, the finite element model was generated
using an already validated procedure. The errors
associated to the segmentation algorithm adopted, to the
CT dataset calibration and to the material assignment
procedure were estimated. Simplified loading conditions
were applied to the femur to generate a compressive
bending and torque loading. The distribution of the
mostly used biomechanical indicators was analyzed. The
results showed that the level of influence of the input
variable is dependent on the load case and on the
considered output variable. The coefficients of variation
were, however, always smaller than 10%, for all variables
Individual Trajectories of Bone Mineral Density Reveal Persistent Bone Loss in Bone Sarcoma Patients: A Retrospective Study
No full textMultiagent chemotherapy offers an undoubted therapeutic benefit to cancer patients, but is also associated with chronic complications in survivors. Osteoporosis affects the quality of life of oncologic patients, especially at the paediatric age. However, very few studies have described the extent of loss of bone mineral density (BMD) in bone sarcoma patients. We analysed a retrospective series of children and adolescents with primary malignant bone tumours (52 osteosarcoma and 31 Ewing sarcoma) and retrieved their BMD at diagnosis and follow-up as Hounsfield units (HU). We studied their individual BMD trajectories before and after chemotherapy up to 5 years, using routine chest CT scan and attenuation thresholds on T12 vertebrae ROI. At one year, bone sarcoma patients showed significant bone loss compared to diagnosis: 17.6% and 17.1% less for OS and EW, respectively. Furthermore, a bone loss of more than 49.2 HU at one-year follow-up was predictive of the persistence of a reduced bone mass over the following 4 years, especially in patients with EW. At 4 years, only 26% and 12.5% of OS and EW, respectively, had recovered or improved their BMD with respect to the onset, suggesting a risk of developing morbidities related to a low BMD in those subjects
Tetrahedral micro-FE models of human trabecular bone: a computationally efficient alternative to voxel models?
No full textMedial and lateral knee contact forces during walking, stair ascent and stair descent are more affected by contact locations than tibiofemoral alignment in knee osteoarthritis patients with varus malalignment
No full textIntroduction: Knee OA progression is related to medial knee contact forces, which can be altered by anatomical parameters of tibiofemoral alignment and contact point locations. There is limited and controversial literature on medial-lateral force distribution and the effect of anatomical parameters, especially in motor activities different from walking. We analyzed the effect of tibiofemoral alignment and contact point locations on knee contact forces, and the medial-lateral force distribution in knee OA subjects with varus malalignment during walking, stair ascending and stair descending.Methods: Fifty-one knee OA subjects with varus malalignment underwent weight-bearing radiographs and motion capture during walking, stair ascending and stair descending. We created a set of four musculoskeletal models per subject with increasing level of personalization, and calculated medial and lateral knee contact forces. To analyze the effect of the anatomical parameters, statistically-significant differences in knee contact forces among models were evaluated. Then, to analyze the force distribution, the medial-to-total contact force ratios were calculated from the fully-informed models. In addition, a multiple regression analysis was performed to evaluate correlations between forces and anatomical parameters.Results: The anatomical parameters significantly affected the knee contact forces. However, the contact points decreased medial forces and increased lateral forces and led to more marked variations compared to tibiofemoral alignment, which produced an opposite effect. The forces were less medially-distributed during stair negotiation, with medial-to-total ratios below 50% at force peaks. The anatomical parameters explained 30%-67% of the variability in the knee forces, where the medial contact points were the best predictors of medial contact forces.Discussion: Including personalized locations of contact points is crucial when analyzing knee contact forces in subjects with varus malalignment, and especially the medial contact points have a major effect on the forces rather than tibiofemoral alignment. Remarkably, the medial-lateral force distribution depends on the motor activity, where stair ascending and descending show increased lateral forces that lead to less medially-distributed loads compared to walking
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