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Objective: Cerebral contusions (CC) represent a frequent lesion in traumatic brain injury, with potential morbidity from mass effect and tissue loss. Better understanding of the mechanical etiology will help to improve head protection. The goal of this study is to investigate the threshold for mechanical impact parameters to induce CC in an in vivo porcine controlled cortical impact model. Methods: Thirty-four adult male pigs underwent craniotomy and controlled cortical impact with a hemispherical tip on intact dura under general anesthesia. Peak impact depth varied between 1.1 and 12.6 mm, and impact velocity between 0.4 and 2.2 m/s while the dwell time was kept at 200 ms. Two days following impact, the animals underwent magnetic resonance (MR) imaging of the brain, and were subsequently sacrificed for brain extraction. CC damage was investigated by magnetic resonance imaging and histology. Results: All animals recovered from the impact without overt neurological deficit. Provoked injuries were histologically confirmed to be CC. Decreasing probability of cortical damage and white matter edema volume was observed with decreasing impact depth and velocity. No CC could be demonstrated below a product of impact depth and velocity of 0.8 mm*m/s, whereas the probability for CC was one third below 15 mm*m/s. The threshold for CC development as estimated from the current series of experiments, was situated at an impact depth of 2.0 mm and impact velocity of 0.4 m/s Conclusion: Mechanical thresholds for CC development could be explored in the current porcine controlled cortical impact model. Findings will be used to further refine a cerebral contusion porcine model with volumetric histology data in light of future finite element cerebral contusion validation studies. more...

Controlled cortical impact (CCI) on porcine brain is often utilized to investigate the pathophysiology and functional outcome of focal traumatic brain injury (TBI), such as cerebral contusion (CC). Using a finite element (FE) model of the porcine brain, the localized brain strain and strain rate resulting from CCI can be computed and compared to the experimentally assessed cortical lesion. This way, tissue-level injury metrics and corresponding thresholds specific for CC can be established. However, the variability and uncertainty associated with the CCI experimental parameters contribute to the uncertainty of the provoked cortical lesion and, in turn, of the predicted injury metrics. Uncertainty quantification via probabilistic methods (Monte Carlo simulation, MCS) requires a large number of FE simulations, which results in a time-consuming process. Following the recent success of machine learning (ML) in TBI biomechanical modeling, we developed an artificial neural network as surrogate of the FE porcine brain model to predict the brain strain and the strain rate in a computationally efficient way. We assessed the effect of several experimental and modeling parameters on four FE-derived CC injury metrics (maximum principal strain, maximum principal strain rate, product of maximum principal strain and strain rate, and maximum shear strain). Next, we compared the in silico brain mechanical response with cortical damage data from in vivo CCI experiments on pig brains to evaluate the predictive performance of the CC injury metrics. Our ML surrogate was capable of rapidly predicting the outcome of the FE porcine brain undergoing CCI. The now computationally efficient MCS showed that depth and velocity of indentation were the most influential parameters for the strain and the strain rate-based injury metrics, respectively. The sensitivity analysis and comparison with the cortical damage experimental data indicate a better performance of maximum principal strain and maximum shear strain as tissue-level injury metrics for CC. These results provide guidelines to optimize the design of CCI tests and bring new insights to the understanding of the mechanical response of brain tissue to focal traumatic brain injury. Our findings also highlight the potential of using ML for computationally efficient TBI biomechanics investigations. Frontiers ispartof: Frontiers In Bioengineering And Biotechnology vol:9 ispartof: location:Switzerland status: published more...

Injurious mechanical loading of articular cartilage and associated lesions compromise the mechanical and structural integrity of joints and contribute to the onset and progression of cartilage degeneration leading to osteoarthritis (OA). Despite extensive in vitro and in vivo research, it remains unclear how the changes in cartilage composition and structure that occur during cartilage degeneration after injury, interact. Recently, in silico techniques provide a unique integrated platform to investigate the causal mechanisms by which the local mechanical environment of injured cartilage drives cartilage degeneration. Here, we introduce a novel integrated Cartilage Adaptive REorientation Degeneration (CARED) algorithm to predict the interaction between degenerative variations in main cartilage constituents, namely collagen fibril disorganization and degradation, proteoglycan (PG) loss, and change in water content. The algorithm iteratively interacts with a finite element (FE) model of a cartilage explant, with and without variable depth to full-thickness defects. In these FE models, intact and injured explants were subjected to normal (2 MPa unconfined compression in 0.1 s) and injurious mechanical loading (4 MPa unconfined compression in 0.1 s). Depending on the mechanical response of the FE model, the collagen fibril orientation and density, PG and water content were iteratively updated. In the CARED model, fixed charge density (FCD) loss and increased water content were related to decrease in PG content. Our model predictions were consistent with earlier experimental studies. In the intact explant model, minimal degenerative changes were observed under normal loading, while the injurious loading caused a reorientation of collagen fibrils toward the direction perpendicular to the surface, intense collagen degradation at the surface, and intense PG loss in the superficial and middle zones. In the injured explant models, normal loading induced intense collagen degradation, collagen reorientation, and PG depletion both on the surface and around the lesion. Our results confirm that the cartilage lesion depth is a crucial parameter affecting tissue degeneration, even under physiological loading conditions. The results suggest that potential fibril reorientation might prevent or slow down fibril degradation under conditions in which the tissue mechanical homeostasis is perturbed like the presence of defects or injurious loading. ispartof: Frontiers in Bioengineering and Biotechnology vol:9 ispartof: location:Switzerland status: published more...

The regional dynamic mechanical properties of mouse, rat, pig, and human brain tissue were compared directly in this first-of-its-kind study. Our results suggest the use of pig or mouse brain tissue as suitable surrogates to characterise human brain tissue. The importance of this work is highlighted by the extensive use of constitutive data from animal brain tissue in traumatic brain injury research in the absence of human brain tissue data without any direct evidence supporting their use. Indentation force-relaxation experiments were performed on mouse, rat, pig, and human brains at 10/s strain rate up to 35% strain to determine the dynamic mechanical properties of brain tissue. Finite element models have become useful tools to investigate the biomechanics of traumatic brain injury - a global leading cause of death and disability and a risk factor for developing neurodegenerative diseases. However, these models require accurate constitutive data for brain tissue to produce reliable results. The results presented here provide validation for the use of pig and mouse brain tissue data in such models. Statement of Significance The significance of this work is underscored by the extensive use of animal brain tissue as a surrogate for human brain tissue without any direct evidence supporting the validity of their use. For the first time ever, we demonstrate that porcine and murine brain tissue can be used as surrogates for human brain tissue under dynamic loading conditions. These findings will allow researchers to select appropriate animal surrogates for human brain tissue under dynamic loading conditions. Furthermore, our findings support the use of animal surrogate data to improve the fidelity of computational models of the human brain, and provide experimental data to develop constitutive models of brain tissue. more...

A reliable computational model of the human head is necessary for better understanding of the physical mechanisms of traumatic brain injury (TBI), car-crash investigation, development of protective head gear and advancement of dural replacement materials. The performance and biofidelity of these models depend largely on the material description of the different structures present in the head. One of these structures is the dura mater, the protective layer around the brain. We tested five human dura mater specimens, with samples at different locations, using planar biaxial tests. We describe the resulting stress-strain curves using both the anisotropic Gasser-Ogden-Holzapfel (GOH) model and the isotropic one-term Ogden model. The low-strain section of the curves is also described using a Neo-Hookean formulation. The obtained stress-strain curves reveal highly nonlinear but isotropic behaviour. A significant amount of inter- and intra-specimen variability is noticed, whereby the latter does not seem to be influenced by location. The GOH model achieves the best fit of the individual test data. A simple Neo-Hookean model can only be used with extreme caution, as it does not manage to capture the nonlinear effects present even at low strains. more...

OBJECTIVES Wrapping with various materials was an early treatment for aortic aneurysms. Wrapping with low-porosity vascular grafts has been associated with graft migration and vascular erosion. An alternative is to use a macroporous mesh (MPM) made of the same polymer (polyethylene terephalate). We compared the histological outcome 1 year after wrapping sheep aortas with low-porosity grafts versus MPM fabrics. METHODS The 2 different fabrics were wrapped around the aorta of 3 sheep. After 1 year the aortas were excised. The 2 wrapped segments of aorta were compared with each other and control aorta. Histological examinations and measurements were made of the layers of the aortic wall in 36 prespecified locations in each of the 3 sheep. RESULTS Both fabrics were consistently surrounded by foreign body reaction and well-vascularized fibrosis. This was more pronounced with the low-porosity vascular graft material which was poorly incorporated and caused buckling at the transition between wrapped and unwrapped aorta. Conversely, the MPM was fully incorporated, resulting in a composite mesh/biological aortic wall. There was reduction of medial thickness with both materials but it was locally more extreme due to the corrugations in the vascular graft material. The findings were consistent between sampled locations and were similar in the 3 animals. CONCLUSIONS The different porosity and rigidity of the materials influences their incorporation into the aortic wall. The incorporation of the pliable MPM precludes the complications of migration and erosion which are seen after wrapping with low-porosity prosthetic vascular graft material. more...

Cardiac surgeries may expose pulmonary arterial tissue to systemic conditions, potentially resulting in failure of that tissue. Our goal was to quantitatively assess pulmonary artery adaptation due to changes in mechanical environment. In 17 sheep, we placed a pulmonary autograft in aortic position, with or without macroporous mesh reinforcement. It was exposed to systemic conditions for 6 months. All sheep underwent 3 ECG-gated MRI’s. Explanted tissue was subjected to mechanical and histological analysis. Results showed progressive dilatation of the unreinforced autograft, while reinforced autografts stabilized after two months. Some unreinforced pulmonary autograft samples displayed more aorta-like mechanical behavior with increased collagen deposition. The mechanical behavior of reinforced autografts was dominated by the mesh. The decrease in media thickness and loss of vascular smooth muscle cells was more pronounced in reinforced than in unreinforced autografts. In conclusion, altering the mechanical environment of a pulmonary artery causes changes in its mechano-biological properties. ispartof: Scientific Reports vol:10 issue:1 ispartof: location:England status: published more...

As degenerative joint diseases such as osteoarthritis (OA) progress, the matrix constituents, particularly collagen fibrils and proteoglycans, become damaged, therefore deteriorating the tissue's mechanical properties. This study aims to further the understanding of the effect of degradation of the different cartilage constituents on the mechanical loading environment in early stage OA. To this end, intact, collagen- and proteoglycan-depleted cartilage plugs were cyclically loaded in axial compression using an experimental model simulating in vivo cartilage-on-cartilage contact conditions in a micro-MRI scanner. Depletion of collagen and proteoglycans was achieved through enzymatic degradation with collagenase and chondroitinase ABC, respectively. Using a displacement-encoded imaging sequence (DENSE), strains were computed and compared in intact and degraded samples. The results revealed that, while degradation with one or the other enzyme had little effect on the contact strains, degradation with a combination of both enzymes caused an increase in the means and variance of the transverse, axial and shear strains, particularly in the superficial zone of the cartilage. This effect indicates that the balance between cartilage matrix constituents plays an essential role in maintaining the mechanical properties of the tissue, and a disturbance in this balance leads to a decrease of the load bearing capacity associated with degenerative joint diseases such as OA. ispartof: JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS vol:98 pages:383-394 ispartof: location:Netherlands status: published more...

The excellent clinical outcomes of the Ross procedure and previous histological studies suggest that the pulmonary autograft has the potential to offer young patients a permanent solution to aortic valve disease. We aim to study the early mechanobiological adaptation of the autograft. To this end, we have reviewed relevant existing animal models, including the canine models which enabled Donald N Ross to perform the first Ross procedure in a patient in 1967. Two research groups recently evaluated the isolated effect of systemic pressures on pulmonary arterial tissue in an ovine model of a pulmonary artery interposition graft in the descending aorta. While this model is ideal to study the artery's biological response and the effect of external support, it does not recreate the complex environment of the aortic root. The freestanding Ross procedure has been performed in pigs and sheep before. These studies offered valuable insights into leaflet growth and histological remodeling, yet may be less relevant to adults undergoing the Ross procedure, as pronounced autograft dilatation was achieved by using small, rapidly growing animals. Therefore, a large animal model remains needed to determine the ideal conditions and surgical technique to ensure long-term autograft remodeling and valve function. We set out to develop an ovine model of the Ross procedure performed as a freestanding root replacement, acknowledging that the sheep's specific anatomy and the setting of an animal laboratory would mandate several modifications in surgical strategy. This article describes the development, surgical technique and early outcomes of our animal model while highlighting opportunities for further research. ispartof: Annals Of Cardiothoracic Surgery vol:10 issue:4 pages:1-10 ispartof: location:China status: published more...

Traumatic brain injury (TBI) is an important cause of mortality and morbidity worldwide. Finite element models of the human head are used widely to simulate TBI loading scenar- ios, to improve the understanding of the mechanical pathogenesis of head trauma. The re- liability of such computational models depends strongly on the accuracy of the mechanical properties of the different components of the brain. Here, we address the shortage of high- quality data on the region-specific properties of human brain tissue at dynamic rates and under large deformation. We mechanically characterized 12 different regions of the human brain including cerebrum, cerebellum and brainstem through n = 595 force-relaxation ex- periments with a custom-built micro-indentation apparatus. We imposed up to 35% strain at 10/s strain rate, i.e. values representative for TBI. Pronounced differences in mechani- cal response were observed across the brain. This work both highlights and addresses the need to assign accurate, region-specific viscoelastic properties to different brain regions in finite element head models. ispartof: International Journal Of Engineering Science vol:155 status: Published online more...

To understand traumas to the nervous system, the relation between mechanical load and functional impairment needs to be explained. Cellular-level computational models are being used to capture the mechanism behind mechanically-induced injuries and possibly predict these events. However, uncertainties in the material properties used in computational models undermine the validity of their predictions. For this reason, in this study the squid giant axon was used as a model to provide a description of the axonal mechanical behavior in a large strain and high strain rate regime $$({\boldsymbol{\varepsilon }}={\bf{10}}{\boldsymbol{ \% }},\,\mathop{{\boldsymbol{\varepsilon }}}\limits^{\cdot }={\bf{1}}{{\boldsymbol{s}}}^{-{\bf{1}}})$$ ( ε = 10 % , ε ⋅ = 1 s − 1 ) , which is relevant for injury investigations. More importantly, squid giant axon membrane sheaths were isolated and tested under dynamic uniaxial tension and relaxation. From the lumen outward, the membrane sheath presents: an axolemma, a layer of Schwann cells followed by the basement membrane and a prominent layer of loose connective tissue consisting of fibroblasts and collagen. Our results highlight the load-bearing role of this enwrapping structure and provide a constitutive description that could in turn be used in computational models. Furthermore, tests performed on collagen-depleted membrane sheaths reveal both the substantial contribution of the endoneurium to the total sheath’s response and an interesting increase in material nonlinearity when the collagen in this connective layer is digested. All in all, our results provide useful insights for modelling the axonal mechanical response and in turn will lead to a better understanding of the relationship between mechanical insult and electrophysiological outcome. more...

Purpose: To compensate for the lack of haptic feedback by surgical robots, limitation of exerted forces could be implemented. The limits should be based on the observed relationship between tissue load and induced damage. This study examines whether age-related changes influence this relationship. Methods: Descending thoracic aortas of male C57BL/6J mice of 10, 25 and 40 weeks were clamped in vivo (no clamp, 0.5N or 2.0N) for 2 min. Functional integrity was tested in vitro by studying endothelium-dependent and -independent vasoreactivity. Results: Endothelium-dependent relaxation deteriorated with increased clamping force at all ages. Clamping did not influence endothelium-independent vasodilation. Age (10, 25 and 40 weeks) did not significantly impact on the effect of clamping on endothelium-dependent and independent vasoreactivity. Conclusions: Within the tested conditions, mechanical clamping induces damage to the vascular endothelium, but not to the smooth muscle cells. Age has no effect on the obtained results in mice from 10 to 40 weeks old. more...

A commonly heard concern in the Ross procedure, where a diseased aortic valve is replaced by the patient's own pulmonary valve, is the possibility of pulmonary autograft dilatation. We performed a biomechanical investigation of the use of a personalized external aortic root support or exostent as a possibility for supporting the autograft. In ten sheep a short length of pulmonary artery was interposed in the descending aorta, serving as a simplified version of the Ross procedure. In seven of these cases, the autograft was supported by an external mesh or so-called exostent. Three sheep served as control, of which one was excluded from the mechanical testing. The sheep were sacrificed six months after the procedure. Samples of the relevant tissues were obtained for subsequent mechanical testing: normal aorta, normal pulmonary artery, aorta with exostent, pulmonary artery with exostent, and pulmonary artery in aortic position for six months. After mechanical testing, the material parameters of the Gasser-Ogden-Holzapfel model were determined for the different tissue types. Stress-strain curves of the different tissue types show significantly different mechanical behavior. At baseline, stress-strain curves of the pulmonary artery are lower than aortic stress-strain curves, but at the strain levels at which the collagen fibers are recruited, the pulmonary artery behaves stiffer than the aorta. After being in aortic position for six months, the pulmonary artery tends towards aorta-like behavior, indicating that growth and remodeling processes have taken place. When adding an exostent around the pulmonary autograft, the mechanical behavior of the composite artery (exostent + artery) differs from the artery alone, the non-linearity being more evident in the former. ispartof: Journal of the Mechanical Behavior of Biomedical Materials vol:78 pages:164-174 ispartof: location:Netherlands status: published more...

OBJECTIVES The Ross procedure involves replacing a patient’s diseased aortic valve with their own pulmonary valve. The most common failure mode is dilatation of the autograft. Various strategies to reinforce the autograft have been proposed. Personalized external aortic root support has been shown to be effective in stabilizing the aortic root in Marfan patients. In this study, the use of a similar external mesh to support a pulmonary artery autograft was evaluated. METHODS The pulmonary artery was translocated as an interposition autograft in the descending thoracic aortas of 10 sheep. The autograft was reinforced with a polyethylene terephthalate mesh (n = 7) or left unreinforced (n = 3). After 6 months, a computed tomography scan was taken, and the descending aorta was excised and histologically examined using the haematoxylin–eosin and Elastica van Gieson stains. RESULTS The autograft/aortic diameter ratio was 1.59 in the unreinforced group but much less in the reinforced group (1.11) (P more...

Objectives The current study quantified the influence of cartilage defect location on the tibiofemoral load distribution during gait. Furthermore, changes in local mechanical stiffness representative for matrix damage or bone ingrowth were investigated. This may provide insights in the mechanical factors contributing to cartilage degeneration in the presence of an articular cartilage defect. Methods The load distribution following cartilage defects was calculated using a musculoskeletal model that included tibiofemoral and patellofemoral joints with 6 degrees-of-freedom. Circular cartilage defects of 100 mm2 were created at different locations in the tibiofemoral contact geometry. By assigning different mechanical properties to these defect locations, softening and hardening of the tissue were evaluated. Results Results indicate that cartilage defects located at the load-bearing area only affect the load distribution of the involved compartment. Cartilage defects in the central part of the tibia plateau and anterior-central part of the medial femoral condyle present the largest influence on load distribution. Softening at the defect location results in overloading, i.e., increased contact pressure and compressive strains, of the surrounding tissue. In contrast, inside the defect, the contact pressure decreases and the compressive strain increases. Hardening at the defect location presents the opposite results in load distribution compared to softening. Sensitivity analysis reveals that the surrounding contact pressure, contact force and compressive strain alter significantly when the elastic modulus is below 7 MPa or above 18 MPa. Conclusion Alterations in local mechanical behavior within the high load bearing area resulted in aberrant loading conditions, thereby potentially affecting the homeostatic balance not only at the defect but also at the tissue surrounding and opposing the defect. Especially, cartilage softening predisposes the tissue to loads that may contribute to accelerated risk of cartilage degeneration and the initiation or progression towards osteoarthritis of the whole compartment., PLoS ONE, 13 (10), ISSN:1932-6203 more...

The aim of this study was to validate carotid artery strain assessment in-vivo using ultrasound speckle tracking. The left carotid artery of five sheep was exposed and sonomicrometry crystals were sutured onto the artery wall to obtain reference strain. Ultrasound imaging was performed at baseline and stress, followed by strain estimation using an in-house speckle tracking algorithm tuned for vascular applications. The correlation between estimated and reference strain was r = 0.95 (p < 0.001) and r = 0.87 (p < 0.01) for longitudinal and circumferential strain, respectively. Moreover, acceptable limits of agreement were found in Bland-Altman analysis (longitudinally: -0.15 to 0.42%, circumferentially: -0.54 to 0.50%), which demonstrates the feasibility of estimating carotid artery strain using ultrasound speckle tracking. However, further studies are needed to test the algorithm on human in-vivo data and to investigate its potential to detect subclinical cardiovascular disease and characterize atherosclerotic plaques. journal_title: Physics in Medicine and Biology article_type: paper article_title: Strain assessment in the carotid artery wall using ultrasound speckle tracking: validation in a sheep model copyright_information: © 2015 Institute of Physics and Engineering in Medicine license_information: cc-by Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. date_received: 2014-08-18 date_accepted: 2014-11-26 date_epub: 2015-01-13 ispartof: Physics in Medicine and Biology vol:60 issue:3 pages:1107-23 ispartof: location:England status: published more...

Development of aortic aneurysms includes significant morphological changes within the tissue: collagen content increases, elastin content reduces and smooth muscle cells degenerate. We seek to quantify the impact of these changes on the passive mechanical response of aneurysms in the supra-physiological loading range via mechanical testing and constitutive modeling. We perform uniaxial extension tests on circumferentially and axially oriented strips from five thoracic (65.6 years ± 13.4, mean ± SD) and eight abdominal (63.9 years ± 11.4) aortic fusiform aneurysms to investigate both continuous and discontinuous softening during supra-physiological loading. We determine the significance of the differences between the fitted model parameters: diseased thoracic versus abdominal tissues, and healthy (Weisbecker et al., J. Mech. Behav. Biomed. Mater. 12, 93-106, 2012) versus diseased tissues. We also test correlations among these parameters and age, Body Mass Index (BMI) and preoperative aneurysm diameter, and investigate histological cuts. Tissue response is anisotropic for all tests and the anisotropic pseudo-elastic damage model fits the data well for both primary loading and discontinuous softening which we interpret as damage. We found statistically relevant differences between model parameters fitted to diseased thoracic versus abdominal tissues, as well as between those fitted to healthy versus diseased tissues. Only BMI correlated with fitted model parameters in abdominal aortic aneurysmal tissues. publisher: Elsevier articletitle: Human thoracic and abdominal aortic aneurysmal tissues: Damage experiments, statistical analysis and constitutive modeling journaltitle: Journal of the Mechanical Behavior of Biomedical Materials articlelink: http://dx.doi.org/10.1016/j.jmbbm.2014.10.003 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved. ispartof: Journal of the Mechanical Behavior of Biomedical Materials vol:41 pages:92-107 ispartof: location:Netherlands status: published more...

OBJECTIVE: This study aims to characterize the deformations in articular cartilage under compressive loading and link these to changes in the extracellular matrix constituents described by magnetic resonance imaging (MRI) relaxation times in an experimental model mimicking in vivo cartilage-on-cartilage contact. DESIGN: Quantitative MRI images, T1, T2 and T1ρ relaxation times, were acquired at 9.4T from bovine femoral osteochondral explants before and immediately after loading. Two-dimensional intra-tissue displacement and strain fields under cyclic compressive loading (350N) were measured using the displacement encoding with stimulated echoes (DENSE) method. Changes in relaxation times in response to loading were evaluated against the deformation fields. RESULTS: Deformation fields showed consistent patterns among all specimens, with maximal strains at the articular surface that decrease with tissue depth. Axial and transverse strains were maximal around the center of the contact region, whereas shear strains were minimal around the contact center but increased towards contact edges. A decrease in T2 and T1ρ was observed immediately after loading whereas the opposite was observed for T1. No correlations between cartilage deformation patterns and changes in relaxation times were observed. CONCLUSIONS: Displacement encoding combined with relaxometry by MRI can noninvasively monitor the cartilage biomechanical and biochemical properties associated with loading. The deformation fields reveal complex patterns reflecting the depth-dependent mechanical properties, but intra-tissue deformation under compressive loading does not correlate with structural and compositional changes. The compacting effect of cyclic compression on the cartilage tissue was revealed by the change in relaxation time immediately after loading. ispartof: OSTEOARTHRITIS AND CARTILAGE vol:26 issue:12 pages:1699-1709 ispartof: location:England status: published more...

Objective Vaginal childbirth is believed to be a significant risk factor for the development of pelvic floor dysfunction later in life. Previous studies have explored the use of medical imaging and simulations of childbirth to determine the stretch in the levator ani muscle. A report in 2012 has recorded magnetic resonance images of a live childbirth of a 24 year old woman giving birth vaginally for the second time, using a 1.0 Tesla open, high-field scanner. Our objective was to determine the stretch ratios in the levator muscle using these magnetic resonance images of live childbirth. Study Design Three-dimensional magnetic resonance image sequences were obtained to visualize coronal and axial planes before and after the childbirth. These images were obtained before the expulsion phase without pushing and were used to reconstruct the levator muscle and the fetal head in 3 dimensions. The fetal head was approximated to be an ellipsoid, and it is assumed that its middle section is visible in dynamic magnetic resonance images. Assuming incompressibility, the full deformation field of the fetal head is then calculated. Real-time cine magnetic resonance images were acquired for the during the expulsion phase, occurring over 2 contractions in the midsagittal plane. The levator muscle stretch is estimated using a custom program. The program calculates points of contact between the fetal head ellipsoid and the levator ani muscle model as the head descends down the birth canal and moves them orthogonal to its surface. Circumferential stretch was calculated to represent the extension needed to allow the passage of the fetal head. Results Starting from a position in the preexpulsion phase, the levator muscle experiences a maximum circumferential stretch of 248% on the posterior-medial portion of the levator ani muscle, as shown in previously published finite element simulations. However, the maximal stretch was notably less than that predicted by finite element models. This is because our baseline 3-dimensional model of the levator muscle is created from images taken shortly before expulsion and thus is already in a stretched state. Furthermore, the finite element models are created from images of a healthy nulliparous woman, while this study uses images from a para 2 woman. Conclusion This study is the first attempt to estimate the stretch in levator ani muscle using magnetic resonance images of a live childbirth. The stretch was significant and the locations corroborate with previous findings of finite element models. more...

Patient-specific biomechanical modelling can improve preoperative surgical planning. This requires patient-specific geometry as well as patient-specific material properties as input. The latter are, however, still quite challenging to estimate in vivo. This study focuses on the estimation of the mechanical properties of the arterial wall. Firstly, in vivo pressure, diameter and thickness of the arterial wall were acquired for sheep common carotid arteries. Next, the animals were sacrificed and the tissue was stored for mechanical testing. Planar biaxial tests were performed to obtain experimental stress-stretch curves. Finally, parameters for the hyperelastic Mooney-Rivlin and Gasser-Ogden-Holzapfel (GOH) material model were estimated based on the in vivo obtained pressure-diameter data as well as on the ex situ experimental stress-stretch curves. Both material models were able to capture the in vivo behaviour of the tissue. However, in the ex situ case only the GOH model provided satisfactory results. When comparing different fitting approaches, in vivo vs. ex situ, each of them showed its own advantages and disadvantages. The in vivo approach estimates the properties of the tissue in its physiological state while the ex situ approach allows to apply different loadings to properly capture the anisotropy of the tissue. Both of them could be further enhanced by improving the estimation of the stress-free state, i.e. by adding residual circumferential stresses in vivo and by accounting for the flattening effect of the tested samples ex vivo. • Competing interests: none declared • Word count: 4716. ispartof: Medical Engineering & Physics vol:55 pages:16-24 ispartof: location:England status: published more...

Accurate estimation of peak wall stress (PWS) is the crux of biomechanically motivated rupture risk assessment for abdominal aortic aneurysms aimed to improve clinical outcomes. Such assessments often use the finite element (FE) method to obtain PWS, albeit at a high computational cost, motivating simplifications in material or element formulations. These simplifications, while useful, come at a cost of reliability and accuracy. We achieve research-standard accuracy and maintain clinically applicable speeds by using novel computational technologies. We present a solution using our custom finite element code based on graphics processing unit (GPU) technology that is able to account for added complexities involved with more physiologically relevant solutions, e.g. strong anisotropy and heterogeneity. We present solutions up to 17× faster relative to an established finite element code using state-of-the-art nonlinear, anisotropic and nearly-incompressible material descriptions. We show a realistic assessment of the explicit GPU FE approach by using complex problem geometry, biofidelic material law, double-precision floating point computation and full element integration. Due to the increased solution speed without loss of accuracy, shown on five clinical cases of abdominal aortic aneurysms, the method shows promise for clinical use in determining rupture risk of abdominal aortic aneurysms. publisher: Elsevier articletitle: Rupture risk in abdominal aortic aneurysms: A realistic assessment of the explicit GPU approach journaltitle: Journal of Biomechanics articlelink: http://dx.doi.org/10.1016/j.jbiomech.2017.02.019 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved. ispartof: Journal of Biomechanics vol:56 pages:1-9 ispartof: location:United States status: published more...

Reliable computer models are needed for a better understanding of the physical mechanisms of skull fracture in accidental hits, falls, bicycle - motor vehicle & car accidents and assaults. The performance and biofidelity of these models depend on the correct anatomical representation and material description of these structures. In literature, a strain energy criterion has been proposed to predict skull fractures. However, a broad range of values for this criterion has been reported. This study investigates if the impactor orientation, scalp thickness and material model of the skull could provide us with insight in the influencing factors of this criterion. 18 skull fracture experiments previously performed in our research group were reproduced in finite element simulations. Subject-specific skull geometries were derived from medical images and used to create high-quality finite element meshes. Based on local Hounsfield units, a subject-specific isotropic material model was assigned. The subject-specific models were able to predict fractures who matched visually with the corresponding experimental fracture patterns and provided detailed fracture patterns. The sensitivity study showed that small variations in impactor positioning as well as variations of the local geometry (frontal-temporal-occipital) strongly influenced the skull strain energy. Subject-specific modelling leads to a more accurate prediction of the force-displacement curve. The average error of the peak fracture force for all the 18 cases is 0.4190 for the subject-specific and 0.4538 for the homogeneous material model, for the displacement; 0.3368 versus 0.3844. But it should be carefully interpreted as small variations in the computational model significantly influence the outcome. ispartof: JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS vol:100 ispartof: location:Netherlands status: published more...

Acute subdural hematoma (ASDH) is a type of intracranial haemorrhage following head impact, with high mortality rates. Bridging vein (BV) rupture is a major cause of ASDH, which is why a biofidelic representation of BVs in finite element (FE) head models is essential for the successful prediction of ASDH. We investigated the mechanical behavior of BVs in the KTH FE head model. First, a sensitivity study quantified the effect of loading conditions and mechanical properties on BV strain. It was found that the peak rotational velocity and acceleration and pulse duration have a pronounced effect on the BV strains. Both Young's modulus and diameter are also negatively correlated with the BV strains. A normalized multiple linear regression model using Young's modulus, outer diameter and peak rotational velocity to predict the BV strain yields an adjusted [Formula: see text]-value of 0.81. Secondly, cadaver head impact experiments were simulated with varying sets of mechanical properties, upon which the amount of successful BV rupture predictions was evaluated. The success rate fluctuated between 67 and 75%. To further increase the predictive capability of FE head models w.r.t. BV rupture, future work should be directed towards improvement of the BV representation, both geometrically and mechanically. more...

Ascending thoracic aortic aneurysms (ATAAs) are a silent disease, ultimately leading to dissection or rupture of the arterial wall. There is a growing consensus that diameter information is insufficient to assess rupture risk, whereas wall stress and strength provide a more reliable estimate. The latter parameters cannot be measured directly and must be inferred through biomechanical assessment, requiring a thorough knowledge of the mechanical behaviour of the tissue. However, for healthy and aneurysmal ascending aortic tissues, this knowledge remains scarce. This study provides the geometrical and mechanical properties of the ATAA of six patients with unprecedented detail. Prior to their ATAA repair, pressure and diameter were acquired non-invasively, from which the distensibility coefficient, pressure–strain modulus and wall stress were calculated. Uniaxial tensile tests on the resected tissue yielded ultimate stress and stretch values. Parameters for the Holzapfel–Gasser–Ogden material model were estimated based on the pre-operative pressure–diameter data and the post-operative stress–stretch curves from planar biaxial tensile tests. Our results confirmed that mechanical or geometrical information alone cannot provide sufficient rupture risk estimation. The ratio of physiological to ultimate wall stress seems a more promising parameter. However, wall stress estimation suffers from uncertainties in wall thickness measurement, for which our results show large variability, between patients but also between measurement methods. Our results also show a large strength variability, a value which cannot be measured non-invasively. Future work should therefore be directed towards improved accuracy of wall thickness estimation, but also towards the large-scale collection of ATAA wall strength data. ispartof: Biomechanics and Modeling in Mechanobiology vol:16 issue:2 pages:705-720 ispartof: location:Germany status: published more...

In robotic and conventional minimally invasive surgery the risk of complications caused by collateral tissue damage remains high. This paper studies the concept of imposing damage thresholds on surgical instruments to avoid tissue overload. More specifically, the correlation between mechanical loading and damage in case of vascular clamping is investigated. With a computer controlled device, a high and a low clamping load were applied in vivo on the abdominal aorta of 43 rats. Samples of both loading levels were compared with zero load control samples and with samples clamped by a mosquito clamp w.r.t. functionality and histological integrity. Analysis of the samples shows that high clamping forces result in endothelial and smooth muscle cell destruction. Clamping with a mosquito clamp will cause even more damage to the elastic lamellae. Samples loaded at the lower load showed significantly less smooth muscle cell damage and a lower degree of endothelial damage. This paper is the first to statistically quantify the correlation between the degree of mechanical loading and the degree of tissue damage, thus setting the first steps towards tissue overload prevention during surgery. Future experiments will also include the effects of loading duration, recovery and patient-specificity. more...

Leakage of blood alongside the implant is a relatively frequent and life-limiting complication after transcatheter aortic valve implantation. The aim of this study is to develop and validate a workflow to simulate the implantation prior to the intervention. Based on the simulation outcome, the amount of leakage is estimated in order to evaluate the risk of a severe complication. A finite element model of the stent implantation in 10 patients was created based on a pre-operative computed tomography scan. All 10 patients also received a follow-up computed tomography scan, after the implantation. This scan was used to extract the deformed geometry of the stent and the position of the calcifications for validation of the simulation results. The maximal average perimeter difference between the simulated stent and the post-operative stent is 2.9±2.1mm, and occurs at the bottom of the device. The sensitivity of the simulation to the soft tissue material parameters and aortic root wall thickness was tested. The maximal diameter deviation of 6% occurred when the thickness of the aortic root was doubled. The result of the leakage analysis based on the distance between the simulated stent and the surrounding aortic root corresponded well when no regurgitation was observed. The developed tools have the potential to reduce the occurrence and severity of leakage by providing the clinician with additional information prior to the intervention. The simulated geometry and estimated leakage can help decide on the best implant type, size and position before treatment. publisher: Elsevier articletitle: A validated methodology for patient specific computational modeling of self-expandable transcatheter aortic valve implantation journaltitle: Journal of Biomechanics articlelink: http://dx.doi.org/10.1016/j.jbiomech.2016.06.024 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved. ispartof: Journal of Biomechanics vol:49 issue:13 pages:2824-2830 ispartof: location:United States status: published more...

Mechanical characterization of soft biological tissue is becoming more and more prevalent. Despite the growing use of planar biaxial testing for soft tissue characterization, testing conditions and subsequent data analysis have not been standardized and vary widely. This also influences the quality of the result of the parameter fitting. Moreover, the testing conditions and data analysis are often not or incompletely reported, which impedes the proper comparison of parameters obtained from different studies. With a focus on planar biaxial tests using rakes, this paper investigates varying testing conditions and varying data analysis methods and their effect on the quality of the parameter fitting results. By means of a series of finite element simulations, aspects such as number of rakes, rakes׳ width, loading protocol, constitutive model, material stiffness and anisotropy are evaluated based on the degree of homogeneity of the stress field, and on the correlation between the experimentally obtained stress and the stress derived from the constitutive model. When calculating the aforementioned stresses, different definitions of the section width and deformation gradient are used in literature, each of which are looked into. Apart from this degree of homogeneity and correlation, also the effect on the quality of the parameter fitting result is evaluated. The results show that inhomogeneities can be reduced to a minimum for wise choices of testing conditions and analysis methods, but never completely eliminated. Therefore, a new parameter optimization procedure is proposed that corrects for the inhomogeneities in the stress field and induces significant improvements to the fitting results. Recommendations are made for best practice in rake-based planar biaxial testing of soft biological tissues and subsequent parameter fitting, and guidelines are formulated for reporting thereof in publications. publisher: Elsevier articletitle: Planar biaxial testing of soft biological tissue using rakes: A critical analysis of protocol and fitting process journaltitle: Journal of the Mechanical Behavior of Biomedical Materials articlelink: http://dx.doi.org/10.1016/j.jmbbm.2016.01.011 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved. ispartof: Journal of the Mechanical Behavior of Biomedical Materials vol:61 pages:135-151 ispartof: location:Netherlands status: published more...

Background Thoracic aortic rupture and aortopulmonary fistulation are rare conditions in horses. It mainly affects Friesian horses. Intrinsic differences in biomechanical properties of the aortic wall might predispose this breed. The biomechanical and biochemical properties of the thoracic aorta were characterized in warmblood horses, unaffected Friesian horses and Friesians with aortic rupture in an attempt to unravel the underlying pathogenesis of aortic rupture in Friesian horses. Samples of the thoracic aorta at the ligamentum arteriosum (LA), mid thoracic aorta (T1) and distal thoracic aorta (T2) were obtained from Friesian horses with aortic rupture (A), nonaffected Friesian (NA) and warmblood horses (WB). The biomechanical properties of these samples were determined using uniaxial tensile and rupture assays. The percentages of collagen and elastin (mg/mg dry weight) were quantified. Results Data revealed no significant biomechanical nor biochemical differences among the different groups of horses. The distal thoracic aorta displayed an increased stiffness associated with a higher collagen percentage in this area and a higher load-bearing capacity compared to the more proximal segments. Conclusions Our findings match reported findings in other animal species. Study results did not provide evidence that the predisposition of the Friesian horse breed for aortic rupture can be attributed to altered biomechanical properties of the aortic wall. Electronic supplementary material The online version of this article (doi:10.1186/s12917-015-0597-0) contains supplementary material, which is available to authorized users. more...

The aim of this study is to characterise the mechanical properties of aortic and pulmonary arterial tissue, thereby comparing both tissue types and investigating the effect of lung-affecting disease on the mechanical behaviour of pulmonary arteries. Force-controlled, planar biaxial tensile tests were performed on human tissue samples collected from donors and receptors undergoing lung transplantation. In total 8 pulmonary donor, 6 pulmonary receptor and 6 aortic donor samples were tested and analysed. Donor samples are considered to be healthy, while receptors provided pathological tissue. The stiffness and strength of each sample were calculated from the stress-strain curves and a statistical analysis was performed between the three tissue groups (pulmonary donor, pulmonary receptor and aortic donor). The stiffness of aortic donor tissue was found to be significantly higher than for pulmonary donor tissue (p < 0.01) at physiological systolic stresses. The same could be observed for the strength (p < 0.05). Pulmonary samples were, however, significantly stiffer than aortic samples at stresses in the physiological range of aorta (p < 0.01). There was no significant difference found between the donors and receptors for pulmonary samples. The fact that the physiological pressure in the aorta is fivefold higher than in the pulmonary artery is also reflected in its stiffness and strength. ispartof: pages:387-390 ispartof: IFMBE Proceedings vol:45 pages:387-390 ispartof: European Conference of the International Federation for Medical and Biological Engineering location:Dubrovnik date:7 Sep - 11 Sep 2014 status: published more...

Assessment of strain in the longitudinal direction of the arterial wall has been suggested to improve the evaluation of arterial stiffness and atherosclerosis. Recently, we showed the feasibility of ultrasound speckle tracking to assess carotid longitudinal strain in-silico and in-vitro. However, validation in the more challenging in-vivo setting is still lacking. The aim of this study was to validate longitudinal strain assessment in the common carotid artery (CCA) in an animal setup. The left CCAs of five sheep were exposed during Isoflurane anesthesia and sonomicrometry crystals were sutured onto the artery wall to obtain reference longitudinal strain. Ultrasound long-axis images were recorded at baseline and hypertension (Phenylephrine) and an in-house speckle tracking algorithm was applied to estimate longitudinal strain. The estimated strain curves varied cyclically throughout the cardiac cycles, showing a lengthening of the arterial segment in systole. A significant correlation between peak systolic estimated and reference strain was found (r=0.95, p < 0.001). The results indicate the feasibility of arterial longitudinal strain assessment in-vivo using ultrasound speckle tracking. QC 20140103 more...

This paper describes a force-measuring manipulator for minimally invasive surgery. It employs an extracorporeal force measurement system combined with a passive trocar support to deal with the restrictions of a single insertion point in the body. The first contribution is the detailed description of the design and the involved kinematics of the device and the force measurement system, while the second contribution is an extensive performance evaluation including an in vivo animal experiment. During the latter, endoscopic video images were recorded simultaneously with motion and force information of the instrument tip. Breathing effects, contact with other instruments and nonlinear soft tissue stiffness could be detected by the system, indicating its potential as a MIS manipulator and as an in vivo tissue property measurement tool. more...

Robotic surgery is an attractive, minimally invasive and high precision alternative to conventional surgical procedures. However, it lacks the natural touch and force feedback that allows the surgeon to control safe tissue manipulation. This is an important problem in standard surgical procedures such as clamping, which might induce severe tissue damage. In complex, heterogeneous, large deformation scenarios, the limits of the safe loading regime beyond which tissue damage occurs are unknown. Here, we show that a continuum damage model for arteries, implemented in a finite element setting, can help to predict arterial stiffness degradation and to identify critical loading regimes. The model consists of the main mechanical constituents of arterial tissue: extracellular matrix, collagen fibres and smooth muscle cells. All constituents are allowed to degrade independently in response to mechanical overload. To demonstrate the modularity and portability of the proposed model, we implement it in a commercial finite element programme, which allows to keep track of damage progression via internal variables. The loading history during arterial clamping is simulated through four successive steps, incorporating residual strains. The results of our first prototype simulation demonstrate significant regional variations in smooth muscle cell damage. In three additional steps, this damage is evaluated by simulating an isometric contraction experiment. The entire finite element simulation is finally compared with actual in vivo experiments. In the short term, our computational simulation tool can be useful to optimise surgical tools with the goal to minimise tissue damage. In the long term, it can potentially be used to inform computer-assisted surgery and identify safe loading regimes, in real time, to minimise tissue damage during robotic tissue manipulation. ispartof: Biomechanics and Modeling in Mechanobiology vol:12 issue:1 pages:123-136 ispartof: location:Germany status: published more...

Endovascular clamping can be achieved by means of intraluminal occlusion with an Endoclamp balloon. Circumventing the need for opening the thoracic cage provides significant benefits in terms of reducing trauma for the patient. This procedure, however, induces risks of damage to the aortic lumen by way of over-expansion of the balloon, exposing the aortic tissue to unsafe levels of loading. Accurate estimates of the induced stress and consequent damage are required intraoperatively to warn the surgeon and mitigate the risk of injury.This paper proposes a method for intraoperative monitoring of the inflicted damage to the arterial tissue, by means of finite element modeling. The Total Lagrangian Explicit Dynamic (TLED) finite element (FE) formulation, capable of handling geometric and material nonlinearities, is used. A 2D plane strain formulation is used to additionally simplify and speed up the execution time by reducing the number of degrees of freedom involved, meanwhile retaining sufficient accuracy for the proposed application. A material model incorporating damage as an internal variable allowed tracking of the degree of injury to the artery. The large amount of computation needed to solve for the stress field is relieved by using dedicated massively parallel hardware. An Nvidia CUDA GPGPU (General Purpose Graphics Processing Unit) is employed to parallelize critical portions of the TLED algorithm. The accuracy of the solution is verified against an industry-proven FE package Abaqus. Results show significant speed-ups compared to Abaqus, thereby retaining a sufficient degree of accuracy. more...

Cardiovascular surgeons increasingly resort to catheter-based diagnostic and therapeutic interventions because of their limited invasiveness. Although, these approaches allow treatment of patients considered unfit for conventional open surgery, exposure to radiation and high procedural complexity could lead to complications. These factors motivated the introduction of robotic technology offering more dexterous catheters, enhanced visualization and opening new possibilities in terms of guidance and coordinated control. In addition to improvements of patient outcome, through teleoperated catheter control radiation exposure of surgeons can be reduced. In order to limit surgical workload, intuitive mappings between joystick input and resulting catheter motion are essential. This paper presents and compares two proposed mappings and investigates the benefits of additional visual guidance. The comparison is based on data gathered during an experimental campaign involving 14 novices and three surgeons. The participants were asked to perform an endovascular task in a virtual reality simulator presented in the first part of this paper. Statistical results show significant superiority of one mapping with respect to the other and a significant improvement of performance thanks to additional visual guidance. Future work will focus on translating the results to a physical setup for surgical validation, also the learning effect will be analyzed more in-depth. ispartof: Journal of Medical Robotics Research vol:1 issue:3 pages:1-16 status: published more...

An infant developed severe right heart failure early after truncal repair with a pulmonary homograft. A mechanical obstruction by narrowing could not be identified at the homograft or pulmonary arteries. However, functional obstruction was caused by an extreme windkessel effect in a massively dilated homograft that absorbed rather than transmitted the pulse wave. Effective treatment consisted of replacing the dilated homograft by a rigid aortic homograft of equal size as the initial homograft. When confronted with circulatory failure after allograft placement, the clinician should not only look for obstruction by narrowing, but also consider the windkessel phenomenon. ispartof: Interactive Cardiovascular and Thoracic Surgery vol:15 issue:1 pages:181-182 ispartof: location:England status: published more...

Minimally Invasive Surgery offers clear advantages over conventional surgery, but issues such as patient safety, surgeon dexterity and ergonomics remain. This paper describes different solutions with respect to these issues, which all fall under the common denominator of ‘smart instrumentation’, i.e., adding intelligence to the surgical instruments. This intelligence refers to the optimization of the design process, the online interpretation of measurements into higher level information, and the efficient and stringent feedback of this information to the surgeon. more...

Purpose To describe our experience with balloon dilatation and stenting of modified systemic-to-pulmonary artery (PA) shunts in relation to an assessment and interpretation of the mechanical properties of thin-walled expandable polytetrafluoroethylene (ePTFE) stretch vascular grafts. Methods Our pediatric cardiology/cardiac surgery database was reviewed to identify all infants and children with a modified systemic-to-PA shunt who underwent cardiac catheterization. Reports and images were reviewed. Thin-walled stretchable and regular Gore-Tex® vascular grafts were mechanically compared using tensiometry. Results 11 patients underwent dilatation or stenting procedures of a systemic-to-PA shunt. No major complications occurred and none of our patients died during or due to this intervention. High pressures in balloons and stents with diameters larger than the graft were used. Shunt diameters and oxygen saturation levels increased from 2.05 ± 1.25 mm to 4.75 ± 0.88 mm and with 12 ± 6.8%, respectively. In 6 patients re-catheterizations were performed. Four patients died, all with patent shunts. The fail-stress and the fail-strain in the circumferential direction of the stretchable graft were significantly higher than in the non-stretchable graft. Conclusions Dilatation and stenting of stenosed modified systemic-to-PA shunts is feasible and safe. Dilatation and stenting of these shunts to calibers larger than those provided by the manufacturer is possible. Results of our technical study posit a great advantage for the use of the thin-walled stretch configuration of ePTFE. more...

Mechanical heart valves carry the disadvantage of lifelong antithrombotic therapy, due to the high risk of thrombus formation on the valve surface. Current diagnostic methods are incapable of detecting thrombus formation in an early stage. This article investigates a new diagnostic method, based on the analysis of the acoustic signal produced by the valve. This method should be capable of early detection of malfunction, thus permitting targeted medication and reducing valve-related complications and mortality. A measurement setup assuring optimal signal quality was developed, and a signal analysis program was implemented and validated on an in vitro mock circulatory loop. Next, four sheep were implanted with a bileaflet mechanical valve. The signals of their valves developing thrombosis were assessed on a weekly basis before explantation. Three sheep were sacrificed shortly after detection of malfunction according to the newly developed method. In each case, thrombus or membrane formation was detected on the leaflets upon explantation. In one sheep, no malfunction was found in the analysis, which was also confirmed by the condition of the valve upon explantation. These preliminary results indicate that acoustical analysis of mechanical heart valves permits early detection of valvular malfunction. Further research with more in vitro and animal testing is required to statistically validate these findings. more...

Soft tissue modelling has gained a great deal of importance, for a large part due to its application in surgical training simulators for minimally invasive surgery (MIS). This article provides a structured overview of different continuum-mechanical models that have been developed over the years. It aims at facilitating model choice for specific soft tissue modelling applications. According to the complexity of the model, different features of soft biological tissue will be incorporated, i.e. nonlinearity, viscoelasticity, anisotropy, heterogeneity and finally, tissue damage during deformation. A brief summary of experimental methods for material characterisation and an introduction to methods for geometric modelling are also provided. The overview is non-exhaustive, focusing on the most important general models and models with specific biological applications. A trade-off in complexity must be made for enabling real-time simulation, but still maintaining realistic representation of the organ deformation. Depending on the organ and tissue types, different models with emphasis on certain features will prove to be more appropriate, meaning the optimal model choice is organ and tissue-dependent. more...

Despite lifelong antithrombotic therapy, patients with mechanical heart valves carry high risk of fatal thrombo-embolic events due to the large possibility of thrombus formation on the valve leaflets [1]. Existing detection techniques for valve thrombosis such as echocardiography or rontgen fluoroscopy are insufficient. A bileaflet mechanical heart valve produces a weak but characteristic repetitive sound [2], consisting of an opening click, a closing click and a hinge noise (Figure 1). The frequency spectrum of the signal is also characteristic for each valve and usually has a peak around 13 kHz. Early detection of valvular malfunction through acoustical analysis of this valve signal can significantly reduce valve-related mortality and allow a more targeted administration of anticoagulation medication. more...