| 1. |
- Grassi, Lorenzo, et al.
(author)
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Experimental Validation Of Finite Element Model For Proximal Composite Femur Using Optical Measurements
- 2013
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In: Journal of the Mechanical Behavior of Biomedical Materials. - : Elsevier BV. - 1751-6161. ; 21, s. 86-94
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Journal article (peer-reviewed)abstract
- Patient-specific finite element models have been used to predict femur strength and fracture risk in individuals. Validation of the adopted finite element modelling procedure against mechanical testing data is a crucial step when aiming for clinical applications. The majority of the works available in literature used data from strain gages to validate the model, thus having up to 15 experimental measurements. Optical techniques, such as Digital Image Correlation, can help to improve the models by providing a continuous field of deformation data over a femoral surface. The main objective of this study was to validate finite element models of six composite femora against strain data from digital image correlation, obtained during fracture tests performed in quasi-axial loading configuration. The finite element models were obtained from CT scans, by means of a semi-automatic segmentation. The principal strains both during the elastic phase and close to the fracture were compared, and showed a correlation coefficient close to 0.9. In the linear region, the slope and intercept were close to zero and unity, while for the case when fracture load was simulated, the slope decreased somewhat. The accuracy of the obtained results is comparable with the state-of-the-art literature, with the significant improvement of having around 50000 data points for each femur. This large number of measurements allows a more comprehensive validation of the predictions by the finite element models, since thousand of points are tracked along the femoral neck and trochanter region, i.e., the sites that are most critical for femur fracture. Moreover, strain measurement biases due to the strain gage reinforcement effect, were avoided. The combined experimental-numerical approach proved to be ready for application to in-vitro tests of human cadaver femurs, thus helping to develop a suitable mechanistic fracture risk criterion.
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| 2. |
- Grassi, Lorenzo, et al.
(author)
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Full-field Strain Measurement During Mechanical Testing of the Human Femur at Physiologically Relevant Strain Rates
- 2014
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In: Journal of Biomechanical Engineering. - : ASME International. - 0148-0731 .- 1528-8951. ; 136:11
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Journal article (peer-reviewed)abstract
- Understanding the mechanical properties of human femora is of great importance for the development of a reliable fracture criterion aimed at assessing fracture risk. Earlier ex vivo studies have been conducted by measuring strains on a limited set of locations using strain gauges. Digital Image Correlation (DIC) could instead be used to reconstruct the full-field strain pattern over the surface of the femur. The objective of this study was to measure the full-field strain response of cadaver femora tested at a physiological strain rate up to fracture in a configuration resembling single stance. The three cadaver femora were cleaned from soft tissues, and a white background paint was applied with a random black speckle pattern over the anterior surface. The mechanical tests were conducted up to fracture at a constant displacement rate of 15 mm/s, and two cameras recorded the event at 3000 frames per second. DIC was performed to retrieve the full-field displacement map, from which strains were derived. A low-pass filter was applied over the measured displacements before the crack opened in order to reduce the noise level. The noise levels were assessed using a dedicated control plate. Conversely, no filtering was applied at the frames close to fracture to get the maximum resolution. The specimens showed a linear behavior of the principal strains with respect to the applied force up to fracture. The strain rate was comparable to the values available in literature from in-vivo measurements during daily activities. The cracks opened and fully propagated in less than 1 ms, and small regions with high values of the major principal strains could be spotted just a few frames before the crack opened. This corroborates the hypothesis of a strain-driven fracture mechanism in human bone. The data represents a comprehensive collection of full-field strains, both at physiological load levels and up to fracture. About 10000 measurements were collected for each bone, providing superior spatial resolution compared to ~15 measurements typically collected using strain gauges. These experimental data collection can be further used for validation of numerical models, and for experimental verification of bone constitutive laws and fracture criteria.
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| 3. |
- Vaananen, Sami P., et al.
(author)
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Estimation of 3D rotation of femur in 2D hip radiographs
- 2012
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In: Journal of Biomechanics. - : Elsevier BV. - 1873-2380 .- 0021-9290. ; 45:13, s. 2279-2283
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Journal article (peer-reviewed)abstract
- Femoral radiographs are affected by the degree of rotation of the femur with respect to the plane of projection. We aimed to determine the 3D rotation of the proximal femur in 2D radiographs. A 3D Statistical Appearance Model (SAM), which was built from CT images of cadaver proximal femurs (n = 33) was randomly sampled to form a training set of 500 bones. Nineteen clinical CT images were collected for testing. All CT images were rotated to +/- 20 degrees in 2 degrees division around the shaft axis, +/- 10 degrees around medial-lateral axis, and by simultaneous rotation of both axes (+/- 16 degrees and +/- 8 degrees around shaft and medial-lateral axes). In each orientation, a 2D projection was recorded for generating a 2D SAM. The outcome parameters of the 2D SAM were used as input for a linear regression model and an artificial neural network to predict the rotation. The artificial neural network estimated the rotation more accurately than the linear regression. For artificial neural networks the mean errors were 4.0 degrees and 2.0 degrees around the shaft and medial-lateral axes, respectively. For an individual radiograph, the confidence interval of estimation was still relatively large. However, this method has high potential to differentiate the amount of rotations in two image sets. (C) 2012 Elsevier Ltd. All rights reserved.
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| 4. |
- Vaananen, Sami P., et al.
(author)
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Repeatability of digital image correlation for measurement of surface strains in composite long bones
- 2013
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In: Journal of Biomechanics. - : Elsevier BV. - 1873-2380 .- 0021-9290. ; 46:11, s. 1928-1932
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Journal article (peer-reviewed)abstract
- Digital image correlation (DIC) can measure full-field surface strains during mechanical testing of hard and soft tissues. When compared to traditional methods, such as strain gauges, DIC offers larger validation data (similar to 50,000 points) for, e.g., finite element models. Our main aim was to evaluate the repeatability of surface strain measurements with DIC during compressive testing of composite femurs mimicking human bones. We also studied the similarity of the composite femur samples using CT. Composite femurs were chosen as test material to minimize the uncertainties associated with the use of cadaveric tissues and to understand the variability of the DIC measurement itself. Six medium-sized fourth generation composite human proximal femora (Sawbones) were CT imaged and mechanically tested in stance configuration. The force-displacement curves were recorded and the 3D surface strains were measured with DIC on the anterior surface of the femurs. Five femurs fractured at the neck-trochanter junction and one at the site below the minor trochanter. CT image of this bone showed an air cavity at the initial fracture site. All femurs fractured through a sudden brittle crack. The fracture force for the composite bones was 5751 +/- 650 N (mean +/- SD). The maximum von Mises strain during the fractures was 2.4 +/- 0.8%. Noise in one experiment was 5-30 mu epsilon. When applied loads were equalized the variation in strains between the bones was 20-25%, and when the maximum strains were equalized, variation in the other regions was 5-10%. DIC showed that the ability of nominally identical composite bones to bear high strains and loads before fracturing may vary between the samples. (C) 2013 Elsevier Ltd. All rights reserved.
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