Dominique Ambard

Experimental evaluation of stent retrievers' mechanical properties and effectiveness.

Background Five 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. Objective To 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 methods All 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. Results Mechanical 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). Conclusions None 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.

Influence of Location, Fluid Flow Direction, and Tissue Maturity on the Macroscopic Permeability of Vertebral End Plates

Study Design. We implemented a pilot study in a growing animal model. The macroscopic permeability of the vertebral endplates was measured. The influence of location, tissue maturity, and fluid flow direction was quantified. Objective. We hypothesized that the macroscopic permeability of vertebral endplate may decrease with maturity of the vertebral segment. Summary of Background Data. The alternation of loading induced by the diurnal cycle generates convective flux into the vertebral segment with the dominant flow path through the vertebral endplates. The alteration of mass transport at the disc-vertebrae interface may interrupt the mechanobiologic balance, and have an effect such as degenerative changes or scoliosis. Methods. A previously validated method for measuring permeability, based on the relaxation pressure caused by a transient-flow rate was used. Three specimens were extracted from each L1 to L5 endplate. Seventy-one specimens were frozen, and 64 were stored fresh in a standard culture media. A microscopic analysis completed the biomechanical analysis. Results. At 2, 4, and 6 months, the mean permeability (10−14 m4/N · s, flow-in/flow-out) of the central zone was respectively: 1.23/1.66, 1.03/1.29, and 0.792/1.00. Laterally, it was 1.03/1.19, 1.094/1.001, and 0.765/0.863. For all groups, cartilage endplate and growth plate were both thinner in the center of the plate. Weak differences of the vascular network were detected, except for a small increase of vascular density in the central zone. Conclusion. 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.

Mechanical Behavior of Annulus Fibrosus: A Microstructural Model of Fibers Reorientation

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.

Biomechanical Assessment of the Individual Risk of Rupture of Cerebral Aneurysms: A Proof of Concept

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.

Validation of an immersed thick boundary method for simulating fluid-structure interactions of deformable membranes

This paper constitutes an extension of the work of Mendez et al. (2014) 36, 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 finite-element 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.

CHEMO-HYDRO-MECHANICAL COUPLING IN ANNULUS FIBROSUS TISSUE

The study focus on biomechanical annulus fibrosus behaviour. On one hand, cyclic tensile tests are performed on annulus samples and bring out a non-linear behaviour with an important hysteresis on each cycle. On the other hand, digital image correlation allows to obtain surfacic strain fields in both anisotropic planes. Finally, a poro-elastic model in large strain is used to understand these observations.

Experimental analysis of the transverse mechanical behaviour of annulus fibrosus tissue

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.

A reactive poroelastic model to predict the periprosthetic tissue healing

Conditions influencing the implant osteointegration in the early post-operative period include the surgical technique and coupled mechanical and biochemical factors. We hypothesized that coupling deformable porous media mechanics to governing equations of cell migration, might help to predict the periprosthetic tissue healing and in particular the heterogeneous bone formation which is unfavourable to the implant survival. To proceed, a multiphasic model of porous tissue surrounding a loaded implant was coupled to osteoblast migration and immature bone deposit. A finite element resolution was implemented and the application concerned a canine implant. The sensitivity analysis using volume strain as variable showed that compression was rather unfavourable to homogenous distribution of periprosthetic bone healing.

Substructuring and poroelastic modelling of the intervertebral disc

We proposed a substructure technique to predict the time-dependant response of biological tissue within the framework of a finite element resolution. Theoretical considerations in poroelasticity preceded the calculation of the sub-structured poroelastic matrix. The transient response was obtained using an exponential fitting method. We computed the creep response of an MRI 3D reconstructed L(5)-S(1) intervertebral disc of a scoliotic spine. The FE model was reduced from 10,000 degrees of freedom for the full 3D disc to only 40 degrees of freedom for the sub-structured model defined by 10 nodes attached to junction nodes located on both lower and upper surfaces of the disc. Comparisons of displacement fields were made between the full poroelastic FE model and the sub-structured model in three different loading conditions: compression, offset compression and torsion. Discrepancies in displacement were lower than 10% for the first time steps when time-dependant events were significant. The substructuring technique provided an exact solution in quasi-static behavior after pressure relaxation. Couplings between vertical and transversal displacements predicted by the reference FE model were well stored by the sub-structured model despite the drastic reduction of degrees of freedom. Finally, we demonstrated that substructuring was very efficient to reduce the size of numerical models while respecting the time-dependant behavior of the structure. This result highlighted the potential interest of substructure techniques in large-scale models of musculoskeletal structures.

Sensitivity analysis of periprosthetic healing to cell migration, growth factor and post-operative gap using a mechanobiological model

A theoretical rationale, which could help in the investigation of mechanobiological factors affecting periprosthetic tissue healing, is still an open problem. We used a parametric sensitivity analysis to extend a theoretical model based on reactive transport and computational cell biology. The numerical experimentation involved the drill hole, the haptotactic and chemotactic migrations, and the initial concentration of an anabolic growth factor. Output measure was the mineral fraction in tissue surrounding a polymethymethacrylate (PMMA) canine implant (stable loaded implant, non-critical gap). Increasing growth factor concentration increased structural matrix synthesis. A cell adhesion gradient resulted in heterogeneous bone distribution and a growth factor gradient resulted in homogeneous bone distribution in the gap. This could explain the radial variation of bone density from the implant surface to the drill hole, indicating less secure fixation. This study helps to understand the relative importance of various host and clinical factors influencing bone distribution and resulting implant fixation.