Validation of Subject Specific Computed Tomography-based Finite Element Models of the Human Proximal Tibia using Full-field Experimental Displacement Measurements from Digital Volume Correlation

Quantitative computed tomography-based finite element (QCT-FE) modeling is a computational tool for predicting bone’s response to applied load, and is used by musculoskeletal researchers to better understand bone mechanics and their role in joint health. Decisions made at the modeling stage, such as the method for assigning material properties, can dictate model accuracy. Predictions of surface strains/stiffness from QCT-FE models of the proximal tibia have been validated against experiment, yet it is unclear whether these models accurately predict internal bone mechanics (displacement). Digital volume correlation (DVC) can measure internal bone displacements and has been used to validate FE models of bone; though, its use has been limited to small specimens. The objectives of this study were to 1) establish a methodology for high-resolution peripheral QCT (HR-pQCT) scan acquisition and image processing resulting in low DVC displacement measurement error in long human bones, and 2) apply different density-modulus relationships and material models from the literature to QCT-FE models of the proximal tibia and identify those approaches which best predicted experimentally measured internal bone displacements and related external reaction forces, with highest explained variance and least error. Using a modified protocol for HR-pQCT, DVC displacement errors for large scan volumes were less than 19μm (0.5 voxels). Specific trabecular and cortical models from the literature were identified which resulted in the most accurate QCT-FE predictions of internal displacements (RMSE%=3.9%, R2>0.98) and reaction forces (RMSE%=12.2%, R2=0.78). This study is the first study to quantify experimental displacements inside a long human bone using DVC. It is also the first study to assess the accuracy of QCT-FE predicted internal displacements in the tibia. Our results indicate that QCT-FE models of the tibia offer reasonably accurate predictions of internal bone displacements and reaction forces for use in studying bone mechanics and joint health

Validation of an in vivo model for monitoring trabecular bone quality changes using finite element analysis.

A combination of three techniques – high resolution micro computed tomography (micro CT) scanning, Archimedes-based volume fraction measurement and serial sectioning or milling – were used to determine the volume fraction, trabecular thickness, trabecular separation, trabecular number and micro finite element analysis combined with mechanical testing was used to determine the apparent stiffness and tissue modulus to quantify bone quality in rabbit distal femur trabecular bone. The objectives of this dissertation were two-fold. First, to develop the capabilities of micro CT scanning and micro CT image segmentation based on a slice-by-slice global thresholding technique to investigate trabecular microstructural changes in vivo and in vitro; and second, to develop the capability of translating micro CT scans into three dimensional finite element models based on direct voxel conversion technique. These results were validated within the in vivo and in vitro scans at the same time, and validated with the Archimedes-based volume fraction measurements and serial sectioning or milling experiments. The micro FE models were executed as linear analyses and the same bone cubes of the models were mechanically tested (compressive testing) to determine the correct tissue modulus of the bone specimens. The apparent stiffness of these micro FE models was recalculated using the average tissue modulus. A total of six six-month-old New Zealand white rabbits were utilized in this study. Three rabbits were scanned twice in vivo seven days apart (T1 and T7) and three rabbits were only scanned once in vivo. All of the femurs were scanned in vitro. All micro CT images were obtained at 28 um (in vivo) or 14 um (in vitro) nominal resolutions. Specimens from six left and right rabbit distal femurs (medial and lateral) were measured based on Archimedes\u27 principle and serial milling. The volume fraction for lateral condyles between two in vivo scans T1 (0.401+0.015) and T7 (0.397+0.021), between in vitro micro CT (0.352+0.035) and Archimedes (0.365+0.031) and between in vitro micro (0.352+0.035) and serial milling (0.369+0.031) were not significantly different. The medial condyles were also not significantly different: T1 (0.513+0.010), T7 (0.515+0.011), in vitro micro CT (0.454+0.049), Archimedes (0.460+0.060) and serial milling (0.467+0.505). Specimens from another six left and right distal femurs (medial and lateral) were mechanically tested along the anterior-posterior directions. The tissue modulus of each specimen was determined by making the calculated apparent stiffness values from FEA to be equal to mechanical compressive testing (MTS). Based on a new constant tissue modulus, the recalculated FEA apparent stiffness (1.77E9+6.45E8) and MTS apparent stiffness (1.76E9+7.37E8) were linearly correlated (r-value = 0.8721). These findings suggest that the capabilities of slice-by-slice global thresholding and direct voxel conversion are sensitive, reliable and consistent for the study of trabecular bone microstructural changes in vivo utilizing high resolution (\u3c 28 um) micro CT scanning and micro FEA

Qualitative analysis of a microtomographic apparatus and measurement of the bone tissue density with reference to microgravity conditions

Computed Thomography is a relatively new field in the area of non destructive imaging.It allows to reconstruct the internal structure of opaque objects without destroy them. This is a great advantage compared to conventional microscopy techniques, any optical or electronic microscope, in fact, provides information on the internal structure of samples only if samples are properly processed and sectioned. Information about three-dimensional structure could be obtained by the image of a surface or a combination of several thin slices, but in both cases information cannot be certain since methods of cutting and preparation can dramatically change the structure of the sample. Microcomputed tomography, commonly referred to as µCT, like conventional computed tomography is based on the collection of projections of X rays through a specimen and the application of tomographic principles to reconstruct the 3-D structure of the specimen. Itis based on the interaction of X-rays with matter. The attenuation ofX-rays, passingthrough an object, is dependent on thedensity and atomic number of the object under investigation. This radiation is converted in a radiographic image of the object. Images obtained from different angles are analyzed by analgorithm called Filter back projection in order to reconstruct a virtual slice through the object. When different consecutive slices are reconstructed, a 3D visualizationcan be obtained. The term "micro" denotes a scanning system much higher in resolution than conventional clinical scanners. Clinical tomographic scanners may have resolutions on the order of a millimeter or less. However, high-resolution µCT scanners may have resolutions below five microns. The high resolution of this system makes it useful in the analysis of small objects such as trabecular bone samples. Trabecularbone consists of a complicated three-dimensional network of plates and rods, arranged ina lattice-like network.The architectural parameters of trabecular bone could be strongly influenced by aging or bone diseases such as osteoarthritis or osteoporosis. Until recently, information about thesestructural parameters of trabecular bone were only available by histomorphometry, adestructive procedure limited to two-dimensional analysis. Nowadays Micro-CT, because of its capability to allow three-dimensional and non destructive analysis, found largeapplications in pre-clinical bone research.The increasing incidence and prevalence of bone pathologies on the population, increases the interest of improve an accurate bone characterization by Micro-CT. Micro-CT system, object of this study is the Skyscan 1072, located at the Technology and Health Department of the Italian National Institute of Health.One of the goal of this research is set at optimizing the system for the analysis of bone samples. The first part is dedicated on determining the resolution of the system. The performance of an imaging system is usually described by the measurement of its Modulation Transfer Function or MTF whichgives a description of how much contrast at a specific spatial frequency is maintained by the imaging process.The second part of this study is focused on the process of images reconstruction, fundamental in a Micro-CT analysis. Micro-CT images are affected by several artifacts which will be widely discussed in the following chapters. One of the most difficult artifact is beam hardening. It depends on the polychromatic X-ray tube used in these systems. The X-rays beam investing the sample is composed of X-rays with a spectrum of different energies. The attenuation of an X-ray depends on its energy, the lowestX-ray energies are preferentially absorbed. Assuming that the grey level of CT images corresponds to the linear coefficientof attenuation, which is constants depending on the material, because of the beam hardening, the attenuationof a given material is not strictly proportional to its thickness. This implies visual distortions on the images and the consequent origin of quantitative problems. In order to better understand the effect of beam hardening on Micro-CT images, the filtered back projection algorithm will be implemented in LabVIEW (Version 8.2). The Skyscan 1072 allows to correct the effect of beam hardening during the process of images reconstruction by the definition of a proper parameter. In order to define the correct value of this parameter for a bone sample analysis, a comparison between the results of both the algorithm implemented and the Skyscan reconstruction software will be evaluated. After the optimization of the system for bone analysis, nineteen trabecular bone samples, extracted from femoral heads of eight patients subject to a hip arthroplasty surgery, will be analyzed. The main problem of bone analysis by micro-CT is the processing of the reconstructed cross-sections images for the sample morphometric analysis. The post-processing of the images for the morphometric characterization usually requires a process named binarization of the images which consists on the definition of a threshold value of grey-level, necessary to distinguish bone from background. The choice of this value is a crucial task since a standard method doesn’t exist. Moreover, the inhomogeneity of bone causes another problem during the binarization process. Binarization associates each pixel of the image to bone or air, not considering that each pixel can be composed by both of them. This effect is called Partial Volume Effect and it affects especially pixels at the edges of the analyzed sample. In order to avoidproblems related to the binarization, the main goal of this study is the evaluation of a new method for the histomorphometric analysis of bone sample from the direct processing of the greylevel histogram of the images. Finally, the last part of this research will be dedicated on the remodeling process of bone. The remodeling of bone is an important research topic because of its importance in the study of bone pathologies such as osteoporosis. Osteoporosis is a bone disorder characterized by an inadequate amount and faulty structure of bone, resulting in fractures from relatively minor trauma. It leads to a bone mineral density (BMD) reduction, a bone microarchitecture deterioration and an alteration of the amount and variety of proteins in bone. Aging is the main factor of osteoporosis incidence but in the last years, another factor related to long-duration spaceflight, has been considered. Because of the difficult in reproducing in-vivo space conditions, the development of numerical models is a good alternative for the remodeling process study

The application of digital volume correlation (DVC) to evaluate strain predictions generated by finite element models of the osteoarthritic humeral head

Continuum-level finite element models (FEMs) of the humerus offer the ability to evaluate joint replacement designs preclinically; however, experimental validation of these models is critical to ensure accuracy. The objective of the current study was to quantify experimental full-field strain magnitudes within osteoarthritic (OA) humeral heads by combining mechanical loading with volumetric microCT imaging and digital volume correlation (DVC). The experimental data was used to evaluate the accuracy of corresponding FEMs. Six OA humeral head osteotomies were harvested from patients being treated with total shoulder arthroplasty and mechanical testing was performed within a microCT scanner. MicroCT images (33.5 µm isotropic voxels) were obtained in a pre- and post-loaded state and BoneDVC was used to quantify full-field experimental strains (≈ 1 mm nodal spacing, accuracy = 351 µstrain, precision = 518 µstrain). Continuum-level FEMs with two types of boundary conditions (BCs) were simulated: DVC-driven and force-driven. Accuracy of the FEMs was found to be sensitive to the BC simulated with better agreement found with the use of DVC-driven BCs (slope = 0.83, r2 = 0.80) compared to force-driven BCs (slope = 0.22, r2 = 0.12). This study quantified mechanical strain distributions within OA trabecular bone and demonstrated the importance of BCs to ensure the accuracy of predictions generated by corresponding FEMs

Quantitative assessment and mechanical consequences of bone density and microstructure in hip osteoarthritis

Osteoarthritis (OA) is a chronic, painful, and currently incurable disease characterized by structural deterioration and loss of function of synovial joints. OA is known to involve profound changes in bone density and microstructure near to, and even distal to, the joint. The prevailing view is that these changes in density and microstructure serve to stiffen the subchondral region thereby altering the mechanical environment (stresses and strains) within the epiphyseal and metaphyseal bone, and that these alterations trigger the aberrant cellular signaling and tissue damage characteristic of the progression of OA. Critically, however, these alterations in mechanical environment have never been well documented in a quantitative fashion in hip OA. Separately, although OA is generally thought to be inversely associated with fragility fracture, recent data challenge this idea and suggest that OA may actually modulate which regions of the proximal femur are at risk of fracture. Therefore, the goal of this work was to provide a spatial assessment of bone density and microstructure in hip OA and then examine the mechanical consequences of these OA-related abnormalities throughout the proximal femur. First, micro-computed tomography and data-driven computational anatomy were used to examine 3-D maps of the distribution of bone density and microstructure in human femoral neck samples with increasing severity of radiographic OA, providing evidence of the heterogeneous and multi-faceted changes in hip OA and discussion of the implications for OA progression and fracture risk. Second, the feasibility of proton density-weighted MRI in image-based finite element (FE) modeling, to examine stress, strain, and risk of failure in the proximal femur under sideways fall, was assessed by comparison to the current standard of CT-based FE modeling. Third, phantom-less calibration for CT-based FE modeling was used with clinically available pre-operative patient scans to assess bone strength and failure risk of the proximal femur in hip OA. Overall, the results of this work provide a rich, quantitative definition of the ways in which the bone mechanical environment under traumatic loading differ in association with hip OA, and then highlight the potential for clinical image-based FE methods to be used opportunistically to assess bone strength and failure risk at the hip. This work is significant because it directly tests the long-standing premise that OA is associated with changes in the mechanical environment of the bone tissue in ways that are impactful for OA progression; further, this work examines how these changes may influence risk of hip fracture. The results can be used to identify mechanistic predictors of OA progression, to inform development of bone-targeting treatments for OA, and to more broadly understand bone damage and fracture in this population

Advaced Microtomographic Techniques for in vitro and in vivo evaluation of Biomaterials in Orthopedic Research

Micro-CT is a very useful non-destructive technique for the 3D study of bone, biomaterials and their interactions because it is able to supply structural and densitometric information by obtaining images of the internal structure of a small object with a high spatial resolution. Besides, Micro-CT is an important tool to study the interactions between biomaterials and bone tissue from a 3D point of view. Typical uses in biomedical fields include the study of in vitro bone samples and biomaterials such as hydroxyapatite (HA), bioactive glasses, pharmaceutical granules, metals and composites (polymers + HA or calcium phosphate). Usually, the most evaluated material parameter is porosity because it plays a dominant role in the biomechanical characteristics, the initial cell attachment and thus the subsequent tissue regeneration. Due to the linear attenuation coefficient specific for every material, is important to set X-ray source voltage and image reconstruction corrections depending on involved materials. More specifically, bone is a connective tissue composed by an average of about 2/3 of inorganic substances and 1/3 of organic substances. The main constituents of the inorganic matrix of bone is a mixture of HA and TCP while biomaterials in orthopedics can be enclosed within two categories:- biomaterials used as replacement of bone to stimulate tissue regeneration and that can be completely absorbed and degraded (mainly ceramic and polymers); - biomaterials for prosthetic implants that have mechanical properties that ensure stabilization of fractures or damaged joints (mainly metals, ceramics or high density polymers). Moreover, micro-CT, deriving from clinical CT, allow studying the interactions between biomaterials and bone tissue also in a longitudinal way. The statistical power of longitudinal studies compared to cross-sectional studies has a great meaning principally because the effect of an implanted biomaterial can be evaluated over time in the same sample. In fact, an in vivo micro-CT analysis allows an assessment and quantification of the development over time in healing processes due to the application of engineered medical devices. The ability to study the time evolution of anatomical changes occurring in the course of the experiment could exclude the use of different experimental groups and, thus, has a significant ethical meaning that should not be underestimated. Limitations of the in vivo micro-CT image acquisition are: the complexity of displaying small structures that are moving dynamically due principally to breathing and heart beating; the limitation in administered X-ray dose distribution per animal due to the destructive effects on living organisms and cells; and the linear attenuation coefficient specific for every material including animal soft and hard tissues. The evaluation of the mineral content is another important and peculiar task of micro-CT. The variations in mineral content are determinable due to the principle that the gray levels of every micro-CT section give a map of the distribution of the absorption coefficients of the X-rays related to the analyzed sample. Such coefficients depend on the material density, from the atomic number of the elements that constitute it and from the incidental energy used. In orthopedic preclinical studies, usually two different densitometric parameters are evaluated: Bone Mineral Density (BMD) and Tissue Mineral Density (TMD). Mineralization is an important aspect in several bone disease evolution such as, for example, osteoporosis or osteoarthritis.The first year of the research was dedicated to the micro-CT analysis of different kinds of biomaterials in the pre-implantation phase studying new procedures to widen the acquisition possibilities and the kinds of quantitative analytical methods. In particular the project followed 3 different research lines: a) the study of thixotropic carboxymethylcellulose (CMC) hydrogels added with iron magnetic-nanoparticles (CMC-NPs) examining the differences in magnetic particles distribution and using a micro-CT freezing chamber to overcome the limitation of movements during the acquisition; b) the study of the 3D cell (MG63) distribution seeded onto a polymeric scaffold using osmium tetroxide as contrast agent and developing a new micro-CT segmentation protocol; c) the study of dimensional metrology establishing an approach for the quantification of wear in ZrO2 head prosthesis components using micro-CT and to validate the method comparing it with the gold standard, i.e. the gravimetric analysis. The second year was dedicated to the study of the interactions between different types of biomaterials implanted in bone tissue. The micro-CT analyses were performed as a result of in vivo preclinical studies and clinical retrieved studies. During this year, the project was divided in the following research lines: a) the evaluation of the in vivo behaviour of ceramic custom made prosthesis in a suitable animal model (adult sheep) at 6 and 12 months from surgical cranioplasty; b) the evaluation of the characteristics of bone quality and its microarchitecture in retrieved metal-on-metal Metal-on-metal HR; c) the analysis of granules characteristics using a new injectable multiphasic bone substitutes based on gel-coated OsproLife® HA/TTCP. During the third year, the non “functional”, i.e. non quantitative, information obtainable from a micro-CT analysis was deepened, testing the most important computer algorithms for 3D visualization and modelling: maximum intensity projection (MIP), shaded surface display (SSD) and volume rendering (VR). Moreover, the Micro-CT analyses performed were divided in the following research lines: a) the evaluation of the in vivo osteoinductive behaviour of three-dimensional interconnected porous scaffolds of gelatin with or without contents of nanocrystalline HA over time; b) the evaluation of bone quality in terms of mineral content in a study of osteoarthritis treatment in a large animal model with engineered hyaluronic acid scaffolds. The main objective of this PhD research was to develop innovative techniques and procedures of 3D image analysis for the characterization of polymeric, ceramic and metal biomaterials used in various fields of bone tissue preclinical research. In detail the aims can be summarized in: – assessing micro-CT procedures applicable to pre-implanted biomaterials through both metrological studies and in vitro studies of 3D cell scaffold colonization, with the definition of effective segmentation techniques; – developing micro-CT techniques to evaluate different kinds of implanted biomaterials both ex vivo and in vivo establishing standard test protocols and identifying the mechanisms of material resorption and degradation in physiological environment and the mechanisms of bone remodeling; – exploring the densitometric analysis in relation to the contribution of mineralization in healing process at peri-implant site. These program objectives have been achieved through the development of reliable experimental procedures for the morphological and mechanical evaluation of implantation biomaterials (scaffold and prostheses); the investigation of a large number of possible applications of biomaterials in orthopedic preclinical studies through in vitro, ex-vivo and in vivo micro-CT analysis; the elaboration of new interpretative models of the bone regeneration mechanisms through 3D morphometric parameters; and the extension of the knowledge and the expertise in 3D biomaterial evaluations used in orthopedic research

Lacunar-canalicular network in femoral cortical bone is reduced in aged women and is predominantly due to a loss of canalicular porosity

The lacunar-canalicular network (LCN) of bone contains osteocytes and their dendritic extensions, which allow for intercellular communication, and are believed to serve as the mechanosensors that coordinate the processes of bone modeling and remodeling. Imbalances in remodeling, for example, are linked to bone disease, including fragility associated with aging. We have reported that there is a reduction in scale for one component of the LCN, osteocyte lacunar volume, across the human lifespan in females. In the present study, we explore the hypothesis that canalicular porosity also declines with age. To visualize the LCN and to determine how its components are altered with aging, we examined samples from young (age: 20-23 y; n = 5) and aged (age: 70-86 y; n = 6) healthy women donors utilizing a fluorescent labelling technique in combination with confocal laser scanning microscopy. A large cross-sectional area of cortical bone spanning the endosteal to periosteal surfaces from the anterior proximal femoral shaft was examined in order to account for potential trans-cortical variation in the LCN. Overall, we found that LCN areal fraction was reduced by 40.6% in the samples from aged women. This reduction was due, in part, to a reduction in lacunar density (21.4% decline in lacunae number per given area of bone), but much more so due to a 44.6% decline in canalicular areal fraction. While the areal fraction of larger vascular canals was higher in endosteal vs. periosteal regions for both age groups, no regional differences were observed in the areal fractions of the LCN and its components for either age group. Our data indicate that the LCN is diminished in aged women, and is largely due to a decline in the canalicular areal fraction, and that, unlike vascular canal porosity, this diminished LCN is uniform across the cortex

Biomechanical importance of proximal human femur morphology and mechanics in orthopaedic purposes

Bone morphology is essential in orthopedic surgery to perform precise preoperative planning and surgery as well as to appropriately design optimal medical implants. In this study we provided a database of surgically important morphological parameters of proximal human femur for orthopedic and biomedical research purposes (study 1), indicated accuracy of the 3D reconstructed images in comparison with the optical 3D scan of real human femur (study 2), and reported the accuracy and reliability of the developed image-based finite element model in comparison with the experimental results (study 3)

Micro-CT Image-Based Mesh Generation and Finite Element Analysis of Bone

This study investigated the reduced platen compression (RPC) test of the distal femur metaphysis (DFM) of adult male rats. The objectives were to develop a finite element (FE) mesh generation algorithm that would take existing µCT images and create a model for use in commercial finite element analysis (FEA) software, and to investigate the RPC test using different boundary conditions (BC) for comparison with modern bone FEA studies. The MATLAB Image Processing Toolbox was used to manipulate the µCT images from their exported format into a usable volume in the form of a 3D binary image. The voxels in this image were directly converted to hexahedral elements in a FE mesh using custom developed MATLAB scripts. The meshes were imported to ABAQUS for FE processing. Post-processing was done in both ABAQUS and MATLAB. ABAQUS was used to visualize the results in contour plots. The stress and strain values for each element were exported for distribution analysis in MATLAB. To validate the FEA techniques developed in this study, two animals were chosen based on their RPC mechanical testing results; one with significantly above average and one with significantly below average maximum load. Both whole specimen compression (WSC) and RPC tests were simulated using rough and smooth BCs. The reaction force results of the simulations mirrored past mechanical testing, but the outlier effect was muted due to greater consistency in platen sizing in the simulations than in RPC testing. The contour plots and strain distributions indicated that the RPC test has little influence from the cortical tissue, supporting the assumption that the RPC test effectively measures cancellous tissue properties. The computational requirements of the image processing and mesh generation techniques, along with the FE processing in ABAQUS are analyzed for future study feasibility. Computation time is not a pressing issue, but memory becomes the limiting factor for both simulation processing and result storage. Overall, the mesh generation technique developed in this study is powerful due to its generality; any binary image can be converted in a FE mesh. Future possibilities include extending the analysis to nonlinear FEA and incorporating tissue failure models

Mapping Trabecular Bone Fabric Tensor by in Vivo Magnetic Resonance Imaging

The mechanical competence of bone depends upon its quantity, structural arrangement, and chemical composition. Assessment of these factors is important for the evaluation of bone integrity, particularly as the skeleton remodels according to external (e.g. mechanical loading) and internal (e.g. hormonal changes) stimuli. Micro magnetic resonance imaging (µMRI) has emerged as a non-invasive and non-ionizing method well-suited for the repeated measurements necessary for monitoring changes in bone integrity. However, in vivo image-based directional dependence of trabecular bone (TB) has not been linked to mechanical competence or fracture risk despite the existence of convincing ex vivo evidence. The objective of this dissertation research was to develop a means of capturing the directional dependence of TB by assessing a fabric tensor on the basis of in vivo µMRI. To accomplish this objective, a novel approach for calculating the TB fabric tensor based on the spatial autocorrelation function was developed and evaluated in the presence of common limitations to in vivo µMRI. Comparisons were made to the standard technique of mean-intercept-length (MIL). Relative to MIL, ACF was identified as computationally faster by over an order of magnitude and more robust within the range of the resolutions and SNRs achievable in vivo. The potential for improved sensitivity afforded by isotropic resolution was also investigated in an improved µMR imaging protocol at 3T. Measures of reproducibility and reliability indicate the potential of images with isotropic resolution to provide enhanced sensitivity to orientation-dependent measures of TB, however overall reproducibility suffered from the sacrifice in SNR. Finally, the image-derived TB fabric tensor was validated through its relationship with TB mechanical competence in specimen and in vivo µMR images. The inclusion of trabecular bone fabric measures significantly improved the bone volume fraction-based prediction of elastic constants calculated by micro-finite element analysis. This research established a method for detecting TB fabric tensor in vivo and identified the directional dependence of TB as an important determinant of TB mechanical competence