Olivier Boiron

Experimental and numerical studies on the starting effect on the secondary flow in a bend

A numerical and experimental modelling study was carried out in a curved tube to analyse the behaviour of unsteady flows in a bend. Based on a test bench, with no mechanical disturbances, the flow behaviour was observed using fluorescein injection. Velocity measurements were performed using hot-film anemometry. In addition, a finite volume method was used to perform three-dimensional unsteady numerical simulations. Womersley parameter values between 8 and 21 and Dean number values between 110 and 420 were used to assess the parameters affecting the flow behaviour. Secondary motions were observed, experimentally and numerically, showing the complexity of the flow patterns. The initiation and subsequent development are explained quantitatively. Based on our analysis of the starting effect, the secondary patterns were found to be highly dependent on both the initial conditions and the flow waveforms.

Stereoscopically Observed Deformations of a Compliant Abdominal Aortic Aneurysm Model

A new experimental setup has been implemented to precisely measure the deformations of an entire model abdominal aortic aneurysm (AAA). This setup addresses a gap between the computational and experimental models of AAA that have aimed at improving the limited understanding of aneurysm development and rupture. The experimental validation of the deformations from computational approaches has been limited by a lack of consideration of the large and varied deformations that AAAs undergo in response to physiologic flow and pressure. To address the issue of experimentally validating these calculated deformations, a stereoscopic imaging system utilizing two cameras was constructed to measure model aneurysm displacement in response to pressurization. The three model shapes, consisting of a healthy aorta, an AAA with bifurcation, and an AAA without bifurcation, were also evaluated with computational solid mechanical modeling using finite elements to assess the impact of differences between material properties and for comparison against the experimental inflations. The device demonstrated adequate accuracy (surface points were located to within 0.07u2009mm) for capturing local variation while allowing the full length of the aneurysm sac to be observed at once. The experimental model AAA demonstrated realistic aneurysm behavior by having cyclic strains consistent with reported clinical observations between pressures 80 and 120u2009mm Hg. These strains are 1-2%, and the local spatial variations in experimental strain were less than predicted by the computational models. The three different models demonstrated that the asymmetric bifurcation creates displacement differences but not cyclic strain differences within the aneurysm sac. The technique and device captured regional variations of strain that are unobservable with diameter measures alone. It also allowed the calculation of local strain and removed rigid body motion effects on the strain calculation. The results of the computations show that an asymmetric aortic bifurcation created displacement differences but not cyclic strain differences within the aneurysm sac.

Realistic glottal motion and airflow rate during human breathing

The glottal geometry is a key factor in the aerosol delivery efficiency for treatment of lung diseases. However, while glottal vibrations were extensively studied during human phonation, the realistic glottal motion during breathing is poorly understood. Therefore, most current studies assume an idealized steady glottis in the context of respiratory dynamics, and thus neglect the flow unsteadiness related to this motion. This is particularly important to assess the aerosol transport mechanisms in upper airways. This article presents a clinical study conducted on 20 volunteers, to examine the realistic glottal motion during several breathing tasks. Nasofibroscopy was used to investigate the glottal geometrical variations simultaneously with accurate airflow rate measurements. In total, 144 breathing sequences of 30s were recorded. Regarding the whole database, two cases of glottal time-variations were found: static or dynamic ones. Typically, the peak value of glottal area during slow breathing narrowed from 217 ± 54 mm(2) (mean ± STD) during inspiration, to 178 ± 35 mm(2) during expiration. Considering flow unsteadiness, it is shown that the harmonic approximation of the airflow rate underevaluates the inertial effects as compared to realistic patterns, especially at the onset of the breathing cycle. These measurements provide input data to conduct realistic numerical simulations of laryngeal airflow and particle deposition.

Risk Factor Analysis of Bird Beak Occurrence after Thoracic Endovascular Aortic Repair

OBJECTIVESnThe aim was to analyze the role played by anatomy and stent graft in the incidence of incomplete apposition to aortic arch.nnnMETHODSnBetween 2007 and 2014 data including available and suitable computed tomographic angiography (CTA) imaging of patients who had undergone thoracic endovascular aortic repair were reviewed. The study included 80 patients (65 men, 54 ± 21 years) treated for traumatic aortic rupture (n = 27), thoracic aortic aneurysm (n = 15), type B aortic dissection (n = 24), penetrating aortic ulcer (n = 5), intramural hematoma (n = 2), aorto-oesophageal fistula (n = 2), and aortic mural thrombus (n = 5). Pre- and post-operative CTA images were analyzed to characterize bird beak in terms of length and angle, and to calculate aortic angulation within a 30 mm range at the proximal deployment zone.nnnRESULTSnBird beak configuration was detected in 46 patients (57%): mean stent protrusion length was 16 mm (range: 8-29 mm) and mean bird beak angle was 20° (range: 7-40°). The bird beak effect was significantly more frequent after traumatic aortic rupture treatment (p = .05) and in landing zone 2 (p = .01). No influence of either stent graft type or generation, or degree of oversizing was observed (p = .29, p = .28, p = .81 respectively). However, the mean aortic angle of patients with bird beak was higher in the Pro-form group than that in the Zenith TX2 group (62° vs. 48°, p = .13). Multivariate analysis identified the aortic angle of the deployment zone as the unique independent risk factor of malapposition (HR = 1.05, 95% CI 1-1.10, p = .005). The cutoff value of 51° was found to be predictive of bird beak occurrence with a sensitivity of 58% and a specificity of 85%.nnnCONCLUSIONSnAssessment of proximal landing zone morphology to avoid deployment zones generating an aortic angle of over 50° can be recommended to improve aortic curvature apposition with the current available devices.

Morphological Analysis of Healthy Aortic Arch

OBJECTIVE/BACKGROUNDnThis study aimed to describe an arch morphology protocol in a healthy population, and to assess the impact of age and sex.nnnMETHODSnA retrospective morphology evaluation was conducted in a population with no personal history of thoracic aorta surgery or pathology, through computed tomography (CT) imaging analysis, using a standardised protocol. Based on centreline three dimensional coordinates, a single investigator calculated a series of parameters in the arch zones and in the total arch, using Matlab scripts. These were categorized as: (i) morphometric data: diameter, length and aortic angle of each zone, total arch angle, and length; (ii) geometric data: tortuosity index (TI), arch width, assimilated curvature radius (CRi), and attachment zone angles. Student or Mann-Whitney tests were used to compare parameter means. Their variability with age and sex was assessed through univariate and multivariate regression analysis.nnnRESULTSnCT images from 123 subjects (meanxa0±xa0SD age 53xa0±xa019 years) were reviewed. Significant correlation between age and morphology was found. The aorta expanded homogeneously and stretched heterogeneously with age because of posterior arch elongation. TI decrease, CRi, and attachment zone angle increase were also observed with aging. Age remained significantly associated with these morphological parameters, independently of body surface area and hypertension. Sex also affected morphology: longer total arch length and higher CRarch in men; lower zone 3 attachment angle in women CONCLUSION: Using mathematical algorithms, and with a view to improving endovascular arch treatment, this study provides a standardised arch morphology protocol and objectively identifies both age related evolution and sex related variation in the different zones.

New experimental protocols for tensile testing of abdominal aortic analogues

This work proposes an in vitro tensile testing protocol that is able to characterize abdominal aortic (AA) analogues under physiologically inspired mechanical loadings. Kinematic parameters are defined in agreement with in vivo measurements of aortic dynamics. A specific focus is given to the choice of the applied loading rates, deriving from the knowledge of aortic Peterson modulus and blood pressure variations from diastolic to systolic instants. The influence of physiological elongation rates has been tested on both porcine AAs and a thermoplastic polyurethane (TPU) material used to elaborate AA analogues. The diastolic and systolic elongation rates estimates vary between orders of magnitude O(10(-2)) and O(10(-1))s(-1). Negligible differences are obtained when comparing stress-elongation responses between both physiological elongation rates. In contrast, a noticeable stiffening of the TPU mechanical response is observed compared to that obtained under the common low traction rate of O(10(-3))s(-1). This work shows how relevant physiological elongation rates can be evaluated as a function of age, gender and pathological context.

Biaxial tensile tests of the porcine ascending aorta

One of the aims of this work is to develop an original custom built biaxial set-up to assess mechanical behavior of soft tissues. Stretch controlled biaxial tensile tests are performed and stereoscopic digital image correlation (SDIC) is implemented to measure the 3D components of the generated displacements. Using this experimental device, the main goal is to investigate the mechanical behavior of porcine ascending aorta in the more general context of human ascending aorta pathologies. The results highlight that (i) SDIC arrangement allows accurate assessment of displacements and so stress strain curves, (ii) porcine ascending aorta has a nearly linear and anisotropic mechanical behavior until 30% of strain, (iii) porcine ascending aorta is stiffer in the circumferential direction than in the longitudinal one, (iv) the material coefficient representing the interaction between the two loading directions is thickness dependent, (v) taking into account the variability of the samples the stress values are independent of the stretch rate in the range of values from 10(-3) to 10(-1)s(-1) and finally, (vi) unlike other segments of the aorta, 4-month-old pigs ascending aorta is definitely not a relevant model to investigate the mechanical behavior of the human ascending aorta.

Risk Factor Analysis for the Mal-Positioning of Thoracic Aortic Stent Grafts

OBJECTIVEnThe present study aimed at quantifying mal-positioning during thoracic endovascular aortic repair and analysing the extent to which anatomical factors influence the exact stent graft positioning.nnnMETHODSnA retrospective review was conducted of patients treated between 2007 and 2014 with a stent graft for whom proximal landing zones (LZ) could be precisely located by anatomical fixed landmarks, that is LZ 1, 2, or 3. The study included 66 patients (54 men; mean age 51 years, range 17-83 years) treated for traumatic aortic rupture (nxa0=xa027), type B aortic dissection (nxa0=xa021), thoracic aortic aneurysm (nxa0=xa08), penetrating aortic ulcer (nxa0=xa05), intramural hematoma (nxa0=xa01), and floating aortic thrombus (nxa0=xa04). Pharmacologic hemodynamic control was systematically obtained during stent graft deployment. Pre- and post-operative computed tomographic angiography was reviewed to quantify the distance between planned and achieved LZ and to analyze different anatomical factors: iliac diameter, calcification degree, aortic angulation at the proximal deployment zone, and tortuosity index (TI).nnnRESULTSnPrimary endoleak was noted in seven cases (10%): five type I (7%) and two type II (3%). Over a mean 35 month follow up (range 3-95 months), secondary endoleak was detected in two patients (3%), both type I, and stent graft migration was seen in three patients. Mal-positioning varied from 2 to 15xa0mm. A cutoff value of 11xa0mm was identified as an adverse event risk. Univariate analysis showed that TI and LZ were significantly associated with mal-positioning (pxa0=xa0.01, pxa0=xa0.04 respectively), and that aortic angulation tends to reach significance (pxa0=xa0.08). No influence of deployment mechanism (pxa0=xa0.50) or stent graft generation (pxa0=xa0.71) or access-related factors was observed. Multivariate analysis identified TI as the unique independent risk factor of mal-positioning (OR 241, 95% CI 1-6,149, pxa0=xa0.05). A TI >1.68 was optimal for inaccurate deployment prediction.nnnCONCLUSIONnTI calculation can be useful to anticipate difficulties during stent graft deployment and to reduce mal-positioning.

Nutrient convection and diffusion modelling in the intervertebral disc

The role of the intervertebral disc (IVD) is to transmit and distribute loads through the spine. Negatively charged hydrophilic macromolecules called proteoglycans are responsible for maintaining osmotic pressure in the disc, which partly counterbalances the loads applied to it. They are synthesised by the discs cells through a metabolic process that is mainly anaerobic dependent on the pH and quantity of nutrients. Under certain genetic, biochemical or mechanical factors, the IVDs undergo a degenerating process that consists of structural and constitutional changes that at the end alter the disc function. In a degenerated disc, the water content decreases resulting in a modification in the transport of nutrients and pH. Consequently, the synthesis of proteoglycans is altered. As the disc is not vascularised, the transport of nutrients into the cells is essentially done by diffusion from the intervertebral plates and the surrounding tissues. If we consider that a healthy disc loses and regains about 10% of its water content per day, the Peclet number that compares the convective and diffusive effects is around 1. This justifies the approach of considering diffusion as the main means of transport for discs being poorly solicitated mechanically. On the other hand, when the disc is exposed to important variations in mechanical loadings and movements, the dynamics of the disc’s aqueous phase is significant. We make the hypothesis that in this case convection caused by interstitial fluid flow adds to the diffusional effects of nutrients transport. With a numerical approach, the incidence of uniaxial loadingof the disc during a circadian cycle and the influence of the resulting convection on the local concentration of solutes are investigated. Convective movements are modellised by generalised Darcy’s law and the transport of nutrients is calculated by a convection–diffusion equation dependent on the electro-osmotic effect induced by the charge concentrations of the disc. The resulting system of partial differential equations is nonlinear and was solved using Comsol Multiphysics 3.5a. 2. Methods

Quantitative MRI water content mapping of porcine intervertebral disc during uniaxial compression

Abstract Background: Intervertebral disc (IVD) diseases are major public health problem in industrialized countries where they affect a large proportion of the population. In particular, IVD degeneration is considered to be one of the leading causes of pain consultation and sick leave. The aim of this study was to develop a new method for assessing the functionality of IVD in order to diagnose IVD degeneration. Methods: For this purpose, we have designed a specific device that enables to mechanically load porcine IVD ex vivo in the 4.7-Tesla horizontal superconducting magnet of a magnetic resonance (MR) scanner. Proton density weighted imaging (ρH-MRI) of the samples was acquired. Findings: The post-processing on MR images allowed (1) to reconstruct the 3D deformation under a known mechanical load and (2) to infer the IVD porosity assuming an incompressible poroelastic model. Interpretation: This study demonstrates the ability to follow the change in morphology and hydration of an IVD using MR measurements, thereby providing valued information for a better understanding of IVD function.