BackgroundFive randomized controlled trials recently appeared in the literature demonstrating that early mechanical thrombectomy in patients with acute ischemic stroke is significantly related to an improved outcome. Stent retrievers are accepted as the most effective devices for intracranial thrombectomy.ObjectiveTo analyze the mechanical properties of stent retrievers, their behavior during retrieval, and interaction with different clots and to identify device features that might correlate with the effectiveness of thrombus removal.Materials and methodsAll stent retrievers available in France up to June 2015 were evaluated by mechanical and functional tests aimed at investigating the variation of their radial force and their behavior during retrieval. Devices were also tested during in vitro thrombectomies using white and red experimental thrombi produced with human blood. Functional tests and in vitro thrombectomies were conducted using a rigid 3D printed vascular model.ResultsMechanical tests showed a variation in radial force during retrieval for each stent. A constant radial force during retrieval was related to continuous cohesion over the vessel wall and a higher rate of clot removal efficacy. All stent retrievers failed when interacting with white large thrombi (diameter ≥6 mm).ConclusionsNone of the tested devices were effective in removing white clots of large diameter (≥6 mm). Constant radial force during retrieval allows constant cohesion to the vessel wall and pressure over the clot; such features allow for a higher rate of clot removal.

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The quality of the fixation orthopaedic implant to its surrounding bone determines its clinical longevity. Up to 20% of hip replacement operations are currently revisions for aseptic loosening. While this fixation quality is determined primarily by the bone and tissue anchoring the implant, conditions influencing bone growth in the early post-operative period include the surgical technique and coupled mechanical and biochemical factors. The aim of the study was to propose an original mechano-biological formulation of the healing process of periprosthetic tissue. The multiphasic porous model involved the solid osseous matrix, the extracellular fluid phase, the osteoblastic cellular phase responsible from the bone formation and the growth factor phase promoting the cellular activity. To derive the non-linear convective-diffuse governing equations, mass balance was associated to cell active haptotactic and chemotactic migration, growth factor diffusion, cell proliferation (logistic law) and bone formation (reactive medium). The in-vivo application concerned a canine axisymmetric implant which was stable and mechanically unloaded. Predictive numerical results were compared to ex-vivo data from a histologic study. The generic healing pattern involving two main oscillations of the radial bone formation was well predicted. In the future, the model could assist in evaluating the role of growth factor concentrations and their temporal delivering as far as the role of pertinent sources such as bioactive coating or additional biomaterials.

This paper constitutes an extension of the work of Mendez, Gibaud & Nicoud: An unstructured solver for simulations of deformable particles in flows at arbitrary Reynolds numbers, Journal of Computational Physics, 256(1): 465-483 (2014), for three-dimensional simulations of deformable membranes under flow.An immersed thick boundary method is used, combining the immersed boundary method with a three-dimensional modeling of the structural part. The immersed boundary method is adapted to unstructured grids for the fluid resolution, using the reproducing kernel particle method. An unstructured finite-volume flow solver for the incompressible Navier-Stokes equations, is coupled with a finiteelement solver for the structure. The validation process relying on a number of test cases proves the efficiency of the method, and its robustness is illustrated when computing the dynamics of a tri-leaflet aortic valve. The proposed immersed thick boundary method is able to tackle applications involving both thin and thick membranes/closed and open membranes, in significantly high Reynolds number flows and highly complex geometries. et al. [35] and has since been extensively studied and applied to a wide variety 30 of FSI problems. In a previous work, Mendez et al. [36] used the IBM to study fluid-structure interaction of deformable particles in flows at arbitrary Reynolds numbers, in the context of complex geometries often encountred when dealing with medical artificial devices. Since this work was only considering the 2D resolution, the main purpose of the present paper is to extend it to 3D. The IBM 35 being originally developed to deal with zero volume structures, a membranelike structure with an infinitely thin thickness can be considered, neglecting the bending stiffness of the membrane. Although suitable when modeling very thin capsules under flow [37, 38], this approach reaches its limitations when considering membranes having a significant bending rigidity. To capture bending 40 effects, an additional model based on the Helfrich energy [39] can be introduced (also used by Mendez et al. [36]), and has been notably used to model flowing capsules and red blood cells [40, 41]. Another approach is the one introduced by Le and Tan [32], where the IBM is combined with a thin-shell model to simulate the deformation of liquid capsules under flow [32, 42, 33]. 45In the present paper, another approach to simulate flowing deformable membranes in the context of the IBM is proposed, the immersed thick boundary method (ITBM). This approach is inspired from the extended immersed boundary method (EIBM), introduced by Wang and Liu [29], and later adapted to the immersed finite element method (IFEM) [43, 44]. Instead of the volumeless im-50 mersed boundary, a submerged solid which occupies a finite volume within the fluid domain is considered. This approach constitutes an alternative to the one consisting in combining the IBM with a thin shell model. Indeed, rather than representing implicitly the thickness of the membrane using a thin shell...

The results from this animal study showed that the central zone of the vertebral endplate was more permeable than the periphery and the flow-out permeability was up to 35% greater than the flow-in permeability. Increase of permeability with decrease of cartilage thickness was noticed within the same age group. We also found a statistically significant decrease of the macroscopic permeability correlated with the tissue maturity.

Uniaxial tensile and relaxation tests were carried out on annulus fibrosus samples carved out in the circumferential direction. Images were shot perpendicularly to the loading direction. Digital image correlation techniques accurately measured the evolution of full displacement fields in both transverse directions: plane of fibres and plane of lamellae. In the fibre plane, strains were governed by the reorientation of fibres along the loading direction. This implies strong transverse shrinkage with quasi-linear behaviour. Conversely, a wide range of behaviour was observed in the lamella plane: from shrinkage to swelling. Strong nonlinear evolutions were generally obtained. The strain field in the lamella plane generally presented a central strip section with more pronounced swelling. Our physical interpretation relies on the porous nature of annulus tissue and its anisotropic stiffness. Indeed, the liquid over-pressure generated inside the sample by the strong shrinkage in the fibre plane discharges in the perpendicular direction since rigidity is lower in the lamella plane. Regarding the strain field measured in the lamella plane, this interpretation agrees with (a) symmetric strain distribution with respect to the longitudinal axis of samples, (b) the reversal in behaviour from shrinkage to swelling and (c) the decrease in strain during relaxation tests associated with outward flows. The variety of transverse behaviours observed experimentally could result from uncertainties regarding the initial reference state of tissue samples. Since the mechanical behaviour is highly nonlinear, experimental results underline that a slight uncertainty concerning the pre-stress applied to samples can lead to wide variability in the mechanical properties identified.

Experimental uniaxial tensile tests have been carried out on annulus tissue samples harvested on pig and lamb lumbar intervertebral discs. When subjecting the samples to loading cycles, the stress-strain curves exhibit strong nonlinearities and hysteresis. This particular behavior results from the anisotropic microstructure of annulus tissue composed of woven oriented collagen fibers embedded in the extracellular matrix. During uniaxial tension, the collagen fibers reorient toward the loading direction increasing its global stiffness. To describe this behavior, we propose a heuristic two-dimensional rheological model based on three mechanical and one geometrical characteristics. The latter one is the fibers orientation angle becoming the key parameter that govern the macroscopic mechanical behavior. The experimental results are used to identify the physical properties associated with the rheological model, leading to an accurate representation of the stress-strain curve over a complete loading cycle. In this framework, the fibers reorientation can solely account for the rigidity increase while the hysteresis is associated with liquid viscous flows through the matrix. Based on this representation, unusual coupling effects between strains and fluid flows can be observed, that would significantly affect the cell nutrients transport mechanisms.

This study is a step towards a new biomechanical-based measurement of the patient specific risk of rupture of cerebral aneurysms. Following a previous experimental investigation suggesting a correlation between the risk of rupture and the material properties of cerebral aneurysms, fluid-structure interaction simulations are performed to compare the deformations of a patient-specific aneurysm when using degraded or undegraded materials. Results show that material properties have a major impact on the magnitude of systolic/diastolic aneurysmal volume variations along the cardiac cycle. Changes in terms of aneurysmal volume variations depending on the tissue characteristics are shown to be measurable by medical imaging. A one-at-a-time data uncertainty analysis is also presented and shows the robustness of this result to input data uncertainties. The study thus suggests that aneurysmal volume variations may be used as the basis of a biomechanical index of rupture risk.