Search

Finite element (FE) analysis can predict proximal human femoral strength. Automated meshing and identifying subregions with high relevance for strength prediction could reduce the laborious modeling process. Mesh morphing based on free-form registration provides a high level of automation and inherently creates isotopological meshes. The goals of this study were to investigate if FE models based on free-form transformed meshes predict experimental femoral strength as well as manually created FE models and to identify regions and parameters with highest correlation to femoral strength. Subject-specific meshes and FE models were created from a set of quantitative CT images (QCT) using a B-Spline registration-based algorithm. Correlation of FE-predicted bone strength and local parameters with experimental bone strength were investigated. FE models based on transformed meshes closely resembled manually created counterparts, with equally strong correlations with experimental bone strength (R2=0.81 vs. R2=0.80). The regional analysis showed strong correlations (0.6

Anatomic models have an important role in the medical domain. However, soft tissue mechanical properties’ representation is limited in mass-produced and 3D-printed models. In this study, a multi-material 3D printer was used to print a human liver model featuring tuned mechanical and radiological properties, with the goal of comparing the printed model with its printing material and real liver tissue. The main target was mechanical realism, while radiological similarity was a secondary objective. Materials and internal structure were selected such that the printed model would resemble liver tissue in terms of tensile properties. The model was printed at 33% scaling and 40% gyroid infill with a soft silicone rubber, and silicone oil as a filler fluid. After printing, the liver model underwent CT scanning. Since the shape of the liver is incompatible with tensile testing, tensile testing specimens were also printed. Three replicates were printed with the same internal structure as the liver model and three more out of silicone rubber with 100% rectilinear infill to allow a comparison. All specimens were tested in a four-step cyclic loading test protocol to compare elastic moduli and dissipated energy ratios. The fluid-filled and full-silicone specimens had initial elastic moduli of 0.26 MPa and 0.37 MPa, respectively, and featured dissipated energy ratios of 0.140, 0.167, 0.183, and 0.118, 0.093, 0.081, respectively, in the second, third, and fourth loading cycles. The liver model showed 225 ± 30 Hounsfield units (HU) in CT, which is closer to real human liver (70 ± 30 HU) than the printing silicone (340 ± 50 HU). Results suggest that the liver model became more realistic in terms of mechanical and radiological properties with the proposed printing approach as opposed to printing only with silicone rubber. Thus, it has been demonstrated that this printing method enables new customization opportunities in the field of anatomic models.

Background Experimental validation is the gold standard for the development of FE predictive models of bone. Employing multiple loading directions could improve this process. To capture the correct directional response of a sample, the effect of all influential parameters should be systematically considered. This study aims to determine the impact of common experimental parameters on the proximal femur’s apparent stiffness. Methods To that end, a parametric approach was taken to study the effects of: repetition, pre-loading, re-adjustment, re-fixation, storage, and μCT scanning as random sources of uncertainties, and loading direction as the controlled source of variation in both stand and side-fall configurations. Ten fresh-frozen proximal femoral specimens were prepared and tested with a novel setup in three consecutive sets of experiments. The neutral state and 15-degree abduction and adduction angles in both stance and fall configurations were tested for all samples and parameters. The apparent stiffness of the samples was measured using load-displacement data from the testing machine and validated against marker displacement data tracked by DIC cameras. Results Among the sources of uncertainties, only the storage cycle affected the proximal femoral apparent stiffness significantly. The random effects of setup manipulation and intermittent μCT scanning were negligible. The 15∘ deviation in loading direction had a significant effect comparable in size to that of switching the loading configuration from neutral stance to neutral side-fall. Conclusion According to these results, comparisons between the stiffness of the samples under various loading scenarios can be made if there are no storage intervals between the different load cases on the same samples. These outcomes could be used as guidance in defining a highly repeatable and multi-directional experimental validation study protocol.

Digital volume correlation (DVC) enables to evaluate the ability of μFE models in predicting experimental results on the mesoscale. In this study predicted displacement fields of three different linear and materially nonlinear μFE simulation methods were compared to DVC measured displacement fields at specific load steps in the elastic regime (Step

Purpose of Review Image-based finite element analysis (FEA) to predict and understand the biomechanical response has become an essential methodology in musculoskeletal research. An important part of such simulation models is the constitutive material model of which recent advances are summarized in this review. Recent Findings The review shows that existing models from other fields were introduced, such as cohesion zone (cortical bone) or phase-field models (trabecular bone). Some progress has been made in describing cortical bone involving physical mechanisms such as microcracks. Problems with validations at different length scales remain a problem. Summary The improvement of recent constitutive models is partially obscured by uncertainties that affect overall predictions, such as image quality and calibration or boundary conditions. Nevertheless, in vivo CT-based FEA simulations based on a sophisticated constitutive behavior are a very valuable tool for clinical-related osteoporosis research.

Inverse bone (re)modeling (IBR) can infer physiological loading conditions from the bone microstructure. IBR scales unit loads, imposed on finite element (FE) models of a bone, such that the trabecular microstructure is homogeneously loaded and the difference to a target stimulus is minimized. Micro-FE (µFE) analyses are typically used to model the microstructure, but computationally more efficient, homogenized FE (hFE) models, where the microstructure is replaced by an equivalent continuum, could be used instead. However, also the target stimulus has to be translated from the tissue to the continuum level. In this study, a new continuum-level target stimulus relating relative bone density and strain energy density is proposed. It was applied using different types of hFE models to predict the physiological loading of 21 distal radii sections, which was subsequently compared to µFE-based IBR. The hFE models were able to correctly identify the dominant load direction and showed a high correlation of the predicted forces, but mean magnitude errors ranged from − 14.7 to 26.6% even for the best models. While µFE-based IBR can still be regarded as a gold standard, hFE-based IBR enables faster predictions, the usage of more sophisticated boundary conditions, and the usage of clinical images.

The apparent stiffness tensor is relevant for characterizing trabecular bone quality. Previous studies have used morphology-stiffness relationships for estimating the apparent stiffness tensor. In this paper, we propose to train

Strains on the surface of engineering structures or biological tissues are non-homogeneous. These strain fields can be captured by means of Digital Image Correlation (DIC). However, DIC strain field measurements are prone to noise and filtering of these fields influences measured strain gradients. This study aims to design a novel tensile test specimen showing two linear gradients, to measure full-field linear strain measurements on the surface of test specimens, and to investigate the accuracy of DIC strain measurements globally (full-field) and locally (strain gauges’ positions), with and without filtering of the DIC strain fields. Three materials were employed for this study: aluminium, polymer, and bovine bone. Normalized strain gradients were introduced that are load independent and evaluated at two local positions showing 3.6 and 6.9% strain change per mm. Such levels are typically found in human bones. At these two positions, two strain gauges were applied to check the experimental strain magnitudes. A third strain gauge was applied to measure the strain in a neutral position showing no gradient. The accuracy of the DIC field measurement was evaluated at two deformation stages (at $$\approx $$ ≈ 500 and 1750 μstrain) using the root mean square error (RMSE). The RMSE over the two linear strain fields was less than 500 μstrain for both deformation stages and all materials. Gaussian low-pass filter (LPF) reduced the DIC noise between 25% and 64% on average. As well, filtering improved the accuracy of the local normalized strain gradients measurements with relative difference less than 20% and 12% for the high- and low-gradient, respectively. In summary, a novel specimen shape and methodological approach are presented which are useful for evaluating and improving the accuracy of the DIC measurement where non-homogeneous strain fields are expected such as on bone tissue due to their hierarchical structure.

An efficient solver for large-scale linear $$\mu \hbox {FE}$$μFE simulations was extended for nonlinear material behavior. The material model included damage-based tissue degradation and fracture. The new framework was applied to 20 trabecular biopsies with a mesh resolution of $${36}\,{{\upmu }\hbox {m}}$$36μm. Suitable material parameters were identified based on two biopsies by comparison with axial tension and compression experiments. The good parallel performance and low memory footprint of the solver were preserved. Excellent correlation of the maximum apparent stress was found between simulations and experiments ($$R^2 > 0.97$$R2>0.97). The development of local damage regions was observable due to the nonlinear nature of the simulations. A novel elasticity limit was proposed based on the local damage information. The elasticity limit was found to be lower than the 0.2% yield point. Systematic differences in the yield behavior of biopsies under apparent compression and tension loading were observed. This indicates that damage distributions could lead to more insight into the failure mechanisms of trabecular bone.

Predicting pathologic fractures in femora with metastatic lesions remains a clinical challenge. Currently used guidelines are inaccurate, especially to predict non-impeding fractures. This study evaluated the ability of a nonlinear quantitative computed tomography (QCT)-based homogenized voxel finite element (hvFE) model to predict patient-specific pathologic fractures. The hvFE model was generated highly automated from QCT images of human femora. The femora were previously loaded in a one-legged stance setup in order to assess stiffness, failure load, and fracture location. One femur of each pair was tested in its intact state, while the contralateral femur included a simulated lesion on either the superolateral- or the inferomedial femoral neck. The hvFE model predictions of the stiffness (0.47 2 2

Finite element analysis is a common tool that has been used for the past few decades to predict the mechanical behavior of bone. However, to our knowledge, there are no round-robin finite element analyses of long human bones with more than two participating biomechanics laboratories published yet, where the results of the experimental tests were not known in advance. We prepared a fresh-frozen human femur for a compression test in a universal testing machine measuring the strains at 10 bone locations as well as the deformation of the bone in terms of the displacement of the loading point at a load of 2 kN. The computed tomography data of the bone with a calibration phantom as well as the orientation of the bone in the testing machine with the according boundary conditions were delivered to seven participating laboratories. These were asked to perform a finite element analysis simulating the experimental setup and deliver their results to the coordinator without knowing the experimental results. Resultantly, four laboratories had deviations from the experimentally measured strains of less than 40%, and three laboratories had deviations of their numerically determined values compared to the experimental data of more than 120%. These deviations are thought to be based on different material laws and material data, as well as different material mapping methods. Investigations will be conducted to clarify and assess the reasons for the large deviations in the numerical data. It was shown that the precision of finite element models of the human femur is not yet as developed as desired by the biomechanics community.

The ability of soft collagenous tissue (SCT) to withstand propagation of a defect in the presence of a macroscopic crack is termed the 'fracture toughness parameter'. In soft tissues not undergoing significant plastic deformation, it is purported that a considerable amount of additional energy is dissipated during failure processes, due to viscoelasticity. Hence the total work, measured experimentally during failure, is the sum of fracture and viscoelastic energies. Previous authors have aimed to apply constitutive modeling to describe viscoelastic hysteresis for fracture toughness determination with a tendency of models to either over or underestimate the viscous energy. In this study, the fracture toughness of porcine muscle tissue is determined using two strategies. Firstly, it was determined experimentally by calculation of the difference in dissipated energy of notched and unnotched tissue specimens undergoing cyclic 'triangular wave' excitation with increasing strain levels in uniaxial tension. The second strategy involved the extension and use of the adaptive quasi-linear viscoelastic model (AQLV) to model cyclic loading (model parameters were obtained from a previous study) and sequentially the dissipated energy was calculated. The mean value of the dissipated energy based on the AQLV approach was then subtracted from the total dissipated energy of notched porcine muscle tissue samples to determine the fracture toughness. The mean experimental viscous dissipated energy ratio was 0.24 ± 0.04 in the experimental approach, compared to 0.28 ± 0.03 for the AQLV model. Fracture toughness determined experimentally yielded 0.84 ± 0.80 kJ/m

Anatomic models are important in medical education and pre-operative planning as they help students or doctors prepare for real scenarios in a risk-free way. Several experimental anatomic models were made with additive manufacturing techniques to improve geometric, radiological, or mechanical realism. However, reproducing the mechanical behavior of soft tissues remains a challenge. To solve this problem, multi-material structuring of soft and hard materials was proposed in this study, and a three-dimensional (3D) printer was built to make such structuring possible. The printer relies on extrusion to deposit certain thermoplastic and silicone rubber materials. Various objects were successfully printed for testing the feasibility of geometric features such as thin walls, infill structuring, overhangs, and multi-material interfaces. Finally, a small medical image-based ribcage model was printed as a proof of concept for anatomic model printing. The features enabled by this printer offer a promising outlook on mimicking the mechanical properties of various soft tissues.

Anatomical models for research and education are often made of artificial materials that attempt to mimic biological tissues in terms of their mechanical properties. Recent developments in additive manufacturing allow tuning mechanical properties with microstructural designs. We propose a strategy for designing material microstructures to mimic soft tissue viscoelastic behaviour, based on a micromechanical Mori–Tanaka model. The model was applied to predict homogenised viscoelastic properties of materials, exhibiting a matrix-inclusion microstructure with varying inclusion volume fractions. The input properties were thereby obtained from compression relaxation tests on silicone elastomers. Validation of the model was done with experimental results for composite samples. Finally, different combinations of silicones were compared to mechanical properties of soft tissues (hepatic, myocardial, adipose, cervical, and prostate tissue), found in literature, in order to design microstructures for replicating these tissues in terms of viscoelasticity. The viscoelastic Mori–Tanaka model showed good agreement with the corresponding experimental results for low inclusion volume fractions, while high fractions lead to underestimation of the complex modulus by the model. Predictions for the loss tangent were reasonably accurate, even for higher inclusion volume fractions. Based on the model, designs for 3D printed microstructures can be extracted in order to replicate the viscoelastic properties of soft tissues.

Osteoporosis is the most common bone disease and is conventionally classified as a decrease of total bone mass. Current diagnosis of osteoporosis is based on clinical risk factors and dual energy X‐ray absorptiometry (DEXA) scans, but changes in bone quantity (bone mass) and quality (trabecular structure, material properties, and tissue composition) are not distinguished. Yet, osteoporosis is known to cause a deterioration of the trabecular network, which might be related to changes at the tissue scale—the material properties. The goal of the current study was to use a previously established test method to perform a thorough characterization of the material properties of individual human trabeculae from femoral heads in cyclic tensile tests in a close to physiologic, wet environment. A previously developed rheological model was used to extract elastic, viscous, and plastic aspects of material behavior. Bone morphometry and tissue mineralization were determined with a density calibrated micro‐computed tomography (μCT) set‐up. Osteoporotic trabeculae neither showed a significantly changed material or mechanical behavior nor changes in tissue mineralization, compared with age‐matched healthy controls. However, donors with osteopenia indicated significantly reduced apparent yield strain and elastic work with respect to osteoporosis, suggesting possible initial differences at disease onset. Bone morphometry indicated a lower bone volume to total volume for osteoporotic donors, caused by a smaller trabecular number and a larger trabecular separation. A correlation of age with tissue properties and bone morphometry revealed a similar behavior as in osteoporotic bone. In the range studied, age does affect morphometry but not material properties, except for moderately increased tissue strength in healthy donors and moderately increased hardening exponent in osteoporotic donors. Taken together, the distinct changes of trabecular bone quality in the femoral head caused by osteoporosis and aging could not be linked to suspected relevant changes in material properties or tissue mineralization. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research., Individual trabeculae of human femoral heads were tested cyclically in tensile mode. Tensile modulus was determined in each loading and unloading cycle and significantly larger in the unloading phase. Determination of material properties with a rheological model revealed no significant difference between control and osteoporotic trabeculae. In contrast, bone morphometry was significantly deteriorated in osteoporosis and with increased ageing.

Ultimate force of the proximal human femur can be predicted using Finite Element Analysis (FEA), but the models rely on 3D computed tomography images. Landmark-based statistical appearance models (SAM) and B-Spline transformation-based statistical deformation models (SDM) have been used to estimate 3D images from 2D projections, which facilitates model generation and reduces the radiation dose. However, there is no literature on the accuracy of SDM-based FEA models of bones with respect to experimental results. In this study, a methodology for an enhanced SDM with textural information is presented. The statistical deformation and texture models (SDTMs) are based on a set of 37 quantitative CT (QCT) images. They were used to estimate 3D images from two or one projections of the set in a leave-one-out setup. These estimations where then used to create FEA models. The ultimate force predicted by FEA models estimated from two or one projection using the SDTMs were compared to the experimental ultimate force from a previous study on the same femora and to the results of standard QCT-based FEA models. High correlations between predictions and experimental measurements were found for FEA models reconstructed from 2D projections with R 2 = 0.835 when based on two projections and R 2 = 0.724 when using one projection. The correlations were comparable to those reached with standard QCT-based FE-models with the experimental results ( R 2 = 0.795 ). This study shows the high potential of SDTM-based 3D image reconstruction and FEA modelling from 2D projections to predict femoral ultimate force.

Haeusler et al. (1) suggest that our analysis (2) of the distribution of relative bone volume across the articular surface (figure 5) does not justify different taxonomic allocations or locomotor classifications. We agree with their first suggestion, and we did not use these data to make direct arguments for the taxonomic attribution of either specimen in our paper. With regard to locomotor classifications, we note that our results “further [support] the inferred differences in loading between these two specimens as evidenced by their internal BV/TV distribution” (2). The strongest evidence for locomotor differences between StW 522 and StW 311 comes from the internal distribution of trabecular bone, which is not captured by the geometric … [↵][1]1To whom correspondence may be addressed. Email: m.skinner{at}kent.ac.uk. [1]: #xref-corresp-1-1

Digital image correlation (DIC) systems have been used in many engineering fields to obtain surface full-field strain distribution. However, noise affects the accuracy and precision of the measurements due to many factors. The aim of this study was to find out how different filtering options, namely, simple mean filtering, Gaussian mean filtering and Gaussian low-pass filtering (LPF), reduce noise while maintaining the full-field information based on constant, linear and quadratic strain fields. Investigations are done in two steps. First, linear and quadratic strain fields with and without noise are simulated and projected to discrete measurement points which build up strain window sizes consisting of 6&times, 5, 12&times, 11, and 26&times, 17 points. Optimal filter sizes are computed for each filter strategy, strain field type, and strain windows size, with minimal impairment of the signal information. Second, these filter sizes are used to filter full-field strain distributions of steel samples under tensile tests by using an ARAMIS DIC system to show their practical applicability. Results for the first part show that for a typical 12&times, 11 strain window, simple mean filtering achieves an error reduction of 66&ndash, 69%, Gaussian mean filtering of 72&ndash, 75%, and Gaussian LPF of 66&ndash, 69%. If optimized filters are used for DIC measurements on steel samples, the total strain error can be reduced from initial 240&minus, 300 &mu, strain to 100&ndash, 150 &mu, strain. In conclusion, the noise-floor of DIC signals is considerable and the preferable filters were a simple mean with s*&macr, = 2, a Gaussian mean with &sigma, *&macr, = 1.7, and a Gaussian LPF with D0*&macr, = 2.5 in the examined cases.

Introduction: Cortical bone thinning and a rarefaction of the trabecular architecture represent possible causes of increased femoral neck (FN) fracture risk. Due to X-ray exposure limits, the bone microstructure is rarely measurable in the FN of subjects but can be assessed at the tibia. Here, we studied whether changes of the tibial cortical microstructure, which were previously reported to be associated with femur strength, are also associated with structural deteriorations of the femoral neck.Methods: The cortical and trabecular architectures in the FN of 19 humans were analyzed ex vivo on 3D microcomputed tomography images with 30.3 mu m voxel size. Cortical thickness (Ct.Th-tibia), porosity (Ct.Po-tibia) and pore size distribution in the tibiae of the same subjects were measured using scanning acoustic microscopy (12 mu m pixel size). Femur strength during sideways falls was simulated with homogenized voxel finite element models.Results: Femur strength was associated with the total (vBMD(tot); R-2 = 0.23, p 100 mu m in tibial cortical bone (relCt.Po100 mu m-tibia) indicated higher Tb.Sp(FN) (R-2 = 0.36, p Conclusion: Bone resorption and structural decline of the femoral neck may be identified in vivo by measuring cortical bone thickness and large pores in the tibia.

The Pistoia criterion (PC) is widely used to estimate the failure load of distal radius segments based on linear micro Finite Element (μFE) analyses. The advantage of the PC is that a simple strain-threshold and a tissue volume fraction can be used to predict failure properties. In this study, the PC is compared to materially nonlinear μFE analyses, where the bone tissue is modelled as an elastic, damageable material. The goal was to investigate for which outcomes the PC is sufficient and when a nonlinear (NL) simulation is required. Three types of simulation results were compared: (1) prediction of the failure load, (2) load sharing of cortical and trabecular regions, and (3) distribution of local damaged/overstrained tissue at the maximum sustainable load. The failure load obtained experimentally could be predicted well with both the PC and the NL simulations using linear regression. Although the PC strongly overestimated the failure load, it was sufficient to predict adequately normalized apparent results. An optimised PC (oPC) was proposed which uses experimental data to calibrate the individual volume of overstrained tissue. The main areas of local over-straining predicted by the oPC were the same as estimated by the NL simulation, although the oPC predicted more diffuse regions. However, the oPC relied on an individual calibration requiring the experimental failure load while the NL simulation required no a priori knowledge of the experimental failure load.

Background Using fewer distal screws in volar plate fixation of distal radius fractures could reduce treatment costs and complications. However, there is currently no consensus on the ideal screw configuration, likely due to experimental limitations and its subject-specific nature. In this study, finite element analysis was used to investigate (1) if reducing the number of screws is biomechanically feasible and (2) if an optimal screw configuration is subject-specific. Methods Validated subject-specific finite element models of 16 human radii with extra articular distal radius fractures and volar plate fixation with six distal screws were used as a baseline. 41 additional configurations with three to six distal screws were simulated for each subject. Axial stiffness and peri-implant strains around the distal screws (a correlate for number of load cycles to failure) were evaluated. Subject-specific optimum configurations were determined using a lower bound for the axial stiffness and minimizing peri-implant strains. Findings Even using just three distal screws led to only minor deterioration of the biomechanical properties in the best configuration (axial stiffness: −11.2%, peri-implant strains: −35.0%), but a considerable deterioration in the worst configuration (axial stiffness: −46.2%, peri-implant strains: +112.4%). The optimization showed that the ideal screw configuration is subject-specific and on average 1.9 screws could be saved based on the herein used optimization criterion. Interpretation This study highlights that not only how many, but which screws are used in volar plate fixation of distal radius fractures is critical. Using a patient-specific selection of distal screws bears potential to save costs and reduce complications.

In the human femoral neck, the contribution of the cortical and trabecular architecture to mechanical strength is known to depend on the load direction.In this work, we investigate if QCT-derived homogenized voxel finite element (hvFE) simulations of varying hip loading conditions can be used to study the architecture of the femoral neck. The strength of 19 pairs of human femora was measured ex vivo using nonlinear hvFE models derived from high-resolution peripheral QCT scans (voxel size: 30.3 mu m). Standing and side-backwards falling loads were modeled. Quasi-static mechanical tests were performed on 20 bones for comparison. Associations of femur strength with volumetric bone mineral density (vBMD) or microstructural parameters of the femoral neck obtained from high-resolution QCT were compared between mechanical tests and simulations and between standing and falling loads.Proximal femur strength predictions by hvFE models were positively associated with the vBMD of the femoral neck (R-2 > 0.61, p 0.27, p 0.38, p

The analysis of internal trabecular and cortical bone has been an informative tool for drawing inferences about behaviour in extant and fossil primate taxa. Within the hand, metacarpal bone architecture has been shown to correlate well with primate locomotion; however, the extent of morphological differences across taxa is unexpectedly small given the variability in hand use. One explanation for this observation is that the activity-related differences in the joint loads acting on the bone are simply smaller than estimated based on commonly used proxies (i.e. external loading and joint posture), which neglect the influence of muscle forces. In this study, experimental data and a musculoskeletal finger model are used to test this hypothesis by comparing differences between climbing and knuckle-walking locomotion of captive bonobos ( Pan paniscus ) based on (i) joint load magnitude and direction predicted by the models and (ii) proxy estimations. The results showed that the activity-related differences in predicted joint loads are indeed much smaller than the proxies would suggest, with joint load magnitudes being almost identical between the two locomotor modes. Differences in joint load directions were smaller but still evident, indicating that joint load directions might be a more robust indicator of variation in hand use than joint load magnitudes. Overall, this study emphasizes the importance of including muscular forces in the interpretation of skeletal remains and promotes the use of musculoskeletal models for correct functional interpretations.

This study aimed to address the predictive value of a micro-computed tomography (μCT)-based finite element (μFE) model of a human cadaveric edentulous posterior mandible, rehabilitated by short dental implants. Hereby, three different prosthetic/implant configurations of fixed partial dentures ("Sp"-3 splinted crowns on 3 implants, "Br" - Bridge: 3 splinted crowns on 2 implants, and "Si"- 3 single crowns) were analysed by comparing the computational predictions of the global stiffness with experimental data.Experimental displacement of the bone/implant/prosthesis system was measured under axial and oblique loads of 100 N using an optical deformation system (GOM Aramis) and the overall movement of the testing machine (Zwick Z030). Together with the measured machine force, an "Aramis" (optical markers) and "Zwick" (test machine) stiffness were calculated. FE models were created based on μCT-scans of the cadaveric mandible sample (n = 1) before and after implantation and using stl-files of the crowns. The same load tests and boundary conditions were simulated on the models and the μFE-results were compared to experimental data using linear regression analysis.The regression line through a plot of pooled stiffness values (N/mm) for the optical displacement recording (true local displacement) and the test machine (machine compliance included) had a slope of 0.57 and a correlation coefficient RThe current study showed good qualitative agreement between the experimental and predicted global stiffness of different short implant configurations. It could be deduced that 1:1 splinting of the short implants by the crowns is most favorable for the stiffness of the implant/prosthesis system. However, in the clinical context, the absolute in silico readings must be interpreted cautiously, as the FEA showed a considerable overestimation of the values.

Mechanical characterisation of soft viscous materials is essential for many applications including aerospace industries, material models for surgical simulation, and tissue mimicking materials for anatomical models. Constitutive material models are, therefore, necessary to describe soft biological tissues in physiologically relevant strain ranges. Hereby, the adaptive quasi-linear viscoelastic (AQLV) model enables accurate modelling of the strain-dependent non-linear viscoelastic behaviour of soft tissues with a high flexibility. However, the higher flexibility produces a large number of model parameters. In this study, porcine muscle and liver tissue samples were modelled in the framework of the originally published AQLV (3-layers of Maxwell elements) model using four incremental ramp-hold experiments in uniaxial tension. AQLV model parameters were reduced by decreasing model layers (M) as well as the number of experimental ramp-hold steps (N). Leave One out cross validation tests show that the original AQLV model (3M4N) with 19 parameters, accurately describes porcine muscle tissue with an average R

Osteoporosis is defined as a decrease of bone mass and strength, as well as an increase in fracture risk. It is conventionally treated with antiresorptive drugs, such as bisphosphonates (BPs) and selective estrogen receptor modulators (SERMs). Although both drug types successfully decrease the risk of bone fractures, their effect on bone mass and strength is different. For instance, BP treatment causes an increase of bone mass, stiffness and strength of whole bones, whereas SERM treatment causes only small (4%) increases of bone mass, but increased bone toughness. Such improved mechanical behavior of whole bones can be potentially related to the bone mass, bone structure or material changes. While bone mass and architecture have already been investigated previously, little is known about the mechanical behavior at the tissue/material level, especially of trabecular bone. As such, the goal of the work presented here was to fill this gap by performing cyclic tensile tests in a wet, close to physiologic environment of individual trabeculae retrieved from the vertebrae of beagle dogs treated with alendronate (a BP), raloxifene (a SERM) or without treatments. Identification of material properties was performed with a previously developed rheological model and of mechanical properties via fitting of envelope curves. Additionally, tissue mineral density (TMD) and microdamage formation were analyzed. Alendronate treatment resulted in a higher trabecular tissue stiffness and strength, associated with higher levels of TMD. In contrast, raloxifene treatment caused a higher trabecular toughness, pre-dominantly in the post-yield region. Microdamage formation during testing was not affected by either anti-resorptive treatment regimens. These findings highlight that the improved mechanical behavior of whole bones after anti-resorptive treatment is at least partly caused by improved material properties, with different mechanisms for alendronate and raloxifene. This study further shows the power of performing a mechanical characterization of trabecular bone at the level of individual trabeculae for better understanding of clinically relevant mechanical behavior of bone.

This research was supported by The Max Planck Society and the European Research Council Starting Grant #336301.

Pathologic fractures of femora in patients with metastatic cancer are associated with high morbidity and mortality. Prediction of impending fractures is based on unspecific clinical criteria or past clinician's experience, which leads to underestimation or overtreatment. The aim of this study was to investigate the effect of the site of metastatic lesions on biomechanical behavior of the proximal femur. Sixteen pairs of human femora were scanned with quantitative computed tomography (QCT) to asses bone mineral density. One femur of each pair remained intact while a defined lesion was reamed out in either the superolateral or inferomedial portion of the femoral neck of the contralateral femur. All femora were loaded in a mechanical test set-up mimicking one-legged stance and stiffness, failure load, and fracture location were determined. In the biomechanical experiments the superolateral lesion and the inferomedial lesion caused a stiffness reduction of 19% and 66%, respectively. The average failure load was 40% and 75% lower for specimens with the superolateral (4.53 ± 1.56 kN) and inferomedial (1.89 ± 1.73 kN) lesions, respectively, compared to intact specimens (7.66 ± 3.34 kN). Lesions in the femoral neck led to reduction in both stiffness and failure load of the proximal femur. Further, the site of the lesion had a large effect on the magnitude of the reduction in biomechanical properties. The presented data emphasize the importance of differentiating between locations of the lesion in pathologic fracture prediction of the metastatic femur and underline the insufficient accuracy of current predictive guidelines. This article is protected by copyright. All rights reserved

Fractures of the distal section of the radius (Colles’ fractures) occur earlier in life than other osteoporotic fractures. Therefore, they can be interpreted as a warning signal for later, more deleterious fractures of vertebral bodies or the femoral neck. In the past decade, the advent of HR-pQCT allowed a detailed architectural analysis of the distal radius and an automated but time-consuming estimation of its strength with linear micro-finite element (μFE) analysis. Recently, a second generation of HR-pQCT scanner (XtremeCT II, SCANCO Medical, Switzerland) with a resolution beyond 61 μm became available for even more refined biomechanical investigations in vivo. This raises the question how biomechanical outcome variables compare between the original (LR) and the new (HR) scanner resolution. Accordingly, the aim of this work was to validate experimentally a patient-specific homogenized finite element (hFE) analysis of the distal section of the human radius for the fast prediction of Colles’ fracture load based on the last generation HR-pQCT. Fourteen pairs of fresh frozen forearms (mean age = 77.5±9) were scanned intact using the high (61 μm) and the low (82 μm) resolution protocols that correspond to the new and original HR-pQCT systems. From each forearm, the 20 mm most distal section of the radius were dissected out, scanned with μCT at 16.4 μm and tested experimentally under compression up to failure for assessment of stiffness and ultimate load. Linear and nonlinear hFE models together with linear micro finite element (μFE) models were then generated based on the μCT and HR-pQCT reconstructions to predict the aforementioned mechanical properties of 24 sections. Precision errors of the short term reproducibility of the FE analyses were measured based on the repeated scans of 12 sections. The calculated failure loads correlated strongly with those measured in the experiments: accounting for donor as a random factor, the nonlinear hFE provided a marginal coefficient of determination (Rm 2) of 0.957 for the high resolution (HR) and 0.948 for the low resolution (LR) protocols, the linear hFE with Rm 2 of 0.957 for the HR and 0.947 for the LR protocols. Linear μFE predictions of the ultimate load were similar with an Rm 2 of 0.950 for the HR and 0.954 for the LR protocols, respectively. Nonlinear hFE strength computation led to precision errors of 2.2 and 2.3% which were higher than the ones calculated based on the linear hFE (1.6 and 1.9%) and linear μFE (1.2 and 1.6%) for the HR and LR protocols respectively. Computation of the fracture load with nonlinear hFE demanded in average 6 h of CPU time which was 3 times faster than with linear μFE, while computation with linear hFE took only a few minutes. This study delivers an extensive experimental and numerical validation for the application of an accurate and fast hFE diagnostic tool to help in identifying individuals who may be at risk of an osteoporotic wrist fracture and to follow up pharmacological and other treatments in such patients.

In order to produce anatomical models that feel realistic to the touch, artificial materials need to be found that mimic tactile properties of biological tissues. The aim of this study was to provide a guideline for identifying materials that feel similar to biological tissues, based on a quantifiable and reproducible measure. For this, a testing procedure was developed to identify mechanical properties that contribute to tactility. Bovine and porcine liver tissues were compared to different silicone elastomers and a soft 3D printed polymer. Macroindentation was chosen to simulate the palpation of material cubes with loading occurring during actual finger and material interaction. Elastic behaviour was considered by conducting quasistatic loading and unloading for extracting contact stiffness S and equivalent spring stiffness k. Viscoelasticity was quantified by means of force relaxation for calculating loss tangent tanδ based on a Prony series approach. Furthermore, Shore 00 hardness H was measured with a hand-held durometer. For assessing how well materials mimicked liver in terms of tactile properties, a mean error of all measured properties was introduced, referred to as tactile similarity error Q. The 3D printed polymer exhibited the highest error (Q=100-150%), while the material with the lowest error - thus representing liver best - was a super-soft silicone elastomer (nominal hardness of 30 Shore Units) with Q~50%. In conclusion, a suitable material was found that best represented liver. However, the relatively high tactile similarity error, even for the best material tested, indicates that there is still room for improvement concerning material choice.

Introduction Knowledge of internal finger loading during human and non-human primate activities such as tool use or knuckle-walking has become increasingly important to reconstruct the behaviour of fossil hominins based on bone morphology. Musculoskeletal models have proven useful for predicting these internal loads during human activities, but load predictions for non-human primate activities are missing due to a lack of suitable finger models. The main goal of this study was to implement both a human and a representative non-human primate finger model to facilitate comparative studies on metacarpal bone loading. To ensure that the model predictions are sufficiently accurate, the specific goals were: (1) to identify species-specific model parameters based on in vitro measured fingertip forces resulting from single tendon loading and (2) to evaluate the model accuracy of predicted fingertip forces and net metacarpal bone loading in a different loading scenario. Materials & Methods Three human and one bonobo (Pan paniscus) fingers were tested in vitro using a previously developed experimental setup. The cadaveric fingers were positioned in four static postures and load was applied by attaching weights to the tendons of the finger muscles. For parameter identification, fingertip forces were measured by loading each tendon individually in each posture. For the evaluation of model accuracy, the extrinsic flexor muscles were loaded simultaneously and both the fingertip force and net metacarpal bone force were measured. The finger models were implemented using custom Python scripts. Initial parameters were taken from literature for the human model and own dissection data for the bonobo model. Optimized model parameters were identified by minimizing the error between predicted and experimentally measured fingertip forces. Fingertip forces and net metacarpal bone loading in the combined loading scenario were predicted using the optimized models and the remaining error with respect to the experimental data was evaluated. Results The parameter identification procedure led to minor model adjustments but considerably reduced the error in the predicted fingertip forces (root mean square error reduced from 0.53/0.69 N to 0.11/0.20 N for the human/bonobo model). Both models remained physiologically plausible after the parameter identification. In the combined loading scenario, fingertip and net metacarpal forces were predicted with average directional errors below 6° and magnitude errors below 12%. Conclusions This study presents the first attempt to implement both a human and non-human primate finger model for comparative palaeoanthropological studies. The good agreement between predicted and experimental forces involving the action of extrinsic flexors—which are most relevant for forceful grasping—shows that the models are likely sufficiently accurate for comparisons of internal loads occurring during human and non-human primate manual activities.

The ability to measure bone tissue material properties plays a major role in diagnosis of diseases and material modeling. Bone’s response to loading is complex and shows a viscous contribution to stiffness, yield and failure. It is also ductile and damaging and exhibits plastic hardening until failure. When performing mechanical tests on bone tissue, these constitutive effects are difficult to quantify, as only their combination is visible in resulting stress–strain data. In this study, a methodology for the identification of stiffness, damping, yield stress and hardening coefficients of bone from a single cyclic tensile test is proposed. The method is based on a two-layer elasto-visco-plastic rheological model that is capable of reproducing the specimens’ pre- and postyield response. The model’s structure enables for capturing the viscously induced increase in stiffness, yield, and ultimate stress and for a direct computation of the loss tangent. Material parameters are obtained in an inverse approach by optimizing the model response to fit the experimental data. The proposed approach is demonstrated by identifying material properties of individual bone trabeculae that were tested under wet conditions. The mechanical tests were conducted according to an already published methodology for tensile experiments on single trabeculae. As a result, long-term and instantaneous Young’s moduli were obtained, which were on average 3.64 GPa and 5.61 GPa, respectively. The found yield stress of 16.89 MPa was lower than previous studies suggest, while the loss tangent of 0.04 is in good agreement. In general, the two-layer model was able to reproduce the cyclic mechanical test data of single trabeculae with an root-mean-square error of 2.91 ± 1.77 MPa. The results show that inverse rheological modeling can be of great advantage when multiple constitutive contributions shall be quantified based on a single mechanical measurement.

Background In medical training and research fresh human tissue is often replaced by preserved human or fresh animal tissue, due to availability and ethical reasons. Newer preservation approaches, such as the Thiel method, promise more realistic mechanical properties than conventional formaldehyde fixation. Concerning animal substitute material, porcine and bovine tissue is often chosen, as it is easily obtainable and certain similarity to human tissue is assumed. However, it has not been thoroughly investigated how Thiel preservation changes non-linear and viscoelastic behaviour of soft organ tissues. Furthermore, differences in these properties between animal tissue and human tissue have not been previously corroborated. Methods We conducted ramp and relaxation tensile tests on fresh human and Thiel preserved hepatic tissue, extracting strain-specific elastic moduli, and viscoelastic properties. The results for fresh human liver were then compared to corresponding results for Thiel preserved liver, as well as previously published results for porcine and bovine liver. Results Our results showed that Thiel preservation seems to be associated with increased stiffness as well as decreased viscoelastic damping behaviour. Porcine liver was stiffer than human liver with similar viscoelastic properties. Bovine liver exhibited similar stiffness as human liver, however lower viscoelastic damping. Conclusions The differences between human and animal liver tissue, concerning their mechanical properties, can be explained by their characteristic histology. Changes in mechanical properties due to Thiel preservation might stem from altered protein cross-linking and dehydration. The results illustrate that appropriate materials for medical training systems must be selected based on which mechanical properties are relevant for the respective application.

Tissue engineering strategies promote bone regeneration for large bone defects by stimulating the osteogenesis route via intramembranous ossification in engineered grafts, which upon implantation are frequently constrained by insufficient integration and functional anastomosis of vasculature from the host tissue. In this study, we developed a hybrid biomaterial incorporating decellularized cartilage extracellular matrix (CD-ECM) as a template and silk fibroin (SF) as a carrier to assess the bone regeneration capacity of bone marrow-derived mesenchymal stem cells (hBMSC’s) via the endochondral ossification (ECO) route. hBMSC’s were primed two weeks for chondrogenesis, followed by six weeks for hypertrophy onto hybrid CD-ECM/SF or SF alone scaffolds and evaluated for the mineralized matrix formation in vitro. Calcium deposition biochemically determined increased significantly from 4-8 weeks in both SF and CD-ECM/SF constructs, and retention of sGAG’s were observed only in CD-ECM/SF constructs. SEM/EDX revealed calcium and phosphate crystal localization by hBMSC’s under all conditions. Compressive modulus reached a maximum of 40 KPa after eight weeks of hypertrophic induction. μCT scanning at eight weeks indicated a cloud of denser minerals in groups after hypertrophic induction in CD-ECM/SF constructs than SF constructs. Gene expression by RT-qPCR revealed that hBMSC’s expressed hypertrophic markers VEGF, COL10, RUNX2, but the absence of early hypertrophic marker ChM1 and later hypertrophic marker TSBS1 and the presence of osteogenic markers ALPL, IBSP, OSX under all conditions. Our data indicate a new method to prime hBMSC’S into the late hypertrophic stage in vitro in mechanically stable constructs for ECO-mediated bone tissue regeneration.

The extensor mechanism is a tendinous network connecting intrinsic and extrinsic muscles of the finger and its function has not yet been fully understood. The goal of this study was to assess the effect of the extensor mechanism on the maximum isometric fingertip forces – a parameter which is essential for grasping. For this purpose, maximum fingertip forces in all directions (i.e. feasible force sets) of two musculoskeletal models of the index finger were compared: the wEM model included a full representation of the extensor mechanism, whereas in the noEM model the extensor mechanism was replaced by a single extensor tendon without connectivity to intrinsic muscles. The feasible force sets were computed in the flexion-extension plane for nine postures. Forces in four predefined directions (palmar, proximal, dorsal, and distal), and the peak resultant forces were evaluated. Averaged forces in all four predefined directions were considerably larger in the wEM model (+187.6%). However, peak resultant forces were slightly lower in the wEM model (−4.3% on average). The general advantage of the wEM model could be explained by co-contraction of intrinsic and extrinsic extensor muscles which allowed reaching larger activation levels of the extrinsic flexors. Only within a narrow range of force directions the co-contraction of intrinsic muscles limited the fingertip forces and lead to lower peak resultant forces in the wEM model. Rather than maximizing peak resultant forces, it appears that the extensor mechanism is a sophisticated tool for increasing maximum fingertip forces over a broad range of postures and force directions – making the finger more versatile during grasping.

Renal osteodystrophy affects the majority of patients with advanced chronic kidney disease (CKD) and is characterized by progressive bone loss. This study evaluated the effects of sclerostin knockout on bone in a murine model of severe, surgically induced CKD in both sclerostin knockout and wild-type mice. Mice of both genotypes with normal kidney function served as controls. Tibiae were analyzed using micro-computed tomography, and lumbar vertebrae were analyzed by histomorphometry. Results were tested for statistical significance by 2-way ANOVA to investigate whether bone of the knockout mice reacted differently to CKD compared with bone of wild-type mice. In the tibiae, there was no difference after creation of CKD between wild-type and knockout animals for cortical thickness or cross-sectional moment of inertia. Increases in cortical porosity induced by CKD differed significantly between genotypes in the tibial metaphysis but not in the diaphysis. In the trabecular compartment, no difference in reaction to CKD between genotypes was found for bone volume, trabecular number, trabecular thickness, and trabecular separation. In the lumbar vertebrae, significant differences in response to CKD between wild-type and knockout mice were seen for both bone volume and trabecular thickness. Osteoblast parameters did not differ significantly, whereas osteoclast numbers significantly increased in the wild-type but significantly decreased in knockout mice with CKD. No differences in response to CKD between genotypes were found for bone formation rate or mineral apposition rate. Thus, complete absence of sclerostin has only minor effects on CKD-induced bone loss in mice.