Franck Jourdan

Biomechanical wall properties of human intracranial aneurysms resected following surgical clipping (IRRAs Project)

BACKGROUND AND PURPOSEnIndividual rupture risk assessment of intracranial aneurysms is a major issue in the clinical management of asymptomatic aneurysms. Aneurysm rupture occurs when wall tension exceeds the strength limit of the wall tissue. At present, aneurysmal wall mechanics are poorly understood and thus, risk assessment involving mechanical properties is inexistent. Aneurysm computational hemodynamics studies make the assumption of rigid walls, an arguable simplification. We therefore aim to assess mechanical properties of ruptured and unruptured intracranial aneurysms in order to provide the foundation for future patient-specific aneurysmal risk assessment. This work also challenges some of the currently held hypotheses in computational flow hemodynamics research.nnnMETHODSnA specific conservation protocol was applied to aneurysmal tissues following clipping and resection in order to preserve their mechanical properties. Sixteen intracranial aneurysms (11 female, 5 male) underwent mechanical uniaxial stress tests under physiological conditions, temperature, and saline isotonic solution. These represented 11 unruptured and 5 ruptured aneurysms. Stress/strain curves were then obtained for each sample, and a fitting algorithm was applied following a 3-parameter (C(10), C(01), C(11)) Mooney-Rivlin hyperelastic model. Each aneurysm was classified according to its biomechanical properties and (un)rupture status.nnnRESULTSnTissue testing demonstrated three main tissue classes: Soft, Rigid, and Intermediate. All unruptured aneurysms presented a more Rigid tissue than ruptured or pre-ruptured aneurysms within each gender subgroup. Wall thickness was not correlated to aneurysmal status (ruptured/unruptured). An Intermediate subgroup of unruptured aneurysms with softer tissue characteristic was identified and correlated with multiple documented risk factors of rupture.nnnCONCLUSIONnThere is a significant modification in biomechanical properties between ruptured aneurysm, presenting a soft tissue and unruptured aneurysms, presenting a rigid material. This finding strongly supports the idea that a biomechanical risk factor based assessment should be utilized in the to improve the therapeutic decision making.

Numerical model of bone remodeling sensitive to loading frequency through a poroelastic behavior and internal fluid movements

In this article, a phenomenological numerical model of bone remodeling is proposed. This model is based on the poroelasticity theory in order to take into account the effects of fluid movements in bone adaptation. Moreover, the proposed remodeling law is based on the classical Stanford law, enriched in order to take into account the loading frequency, through fluid movements. This coupling is materialized by a quadratic function of Darcy velocity. The numerical model is carried out, using a finite element method, and calibrated using experimental results at macroscopic level, from the literature. First results concern cyclic loadings on a mouse ulna, at different frequencies between 1 Hz and 30 Hz, for a force amplitude of 1.5 N and 2 N. Experimental results exhibit a sensitivity to the loading frequency, with privileged frequency for bone remodeling between 5 Hz and 10 Hz, for the force amplitude of 2 N. For the force amplitude of 1.5 N, no privileged frequencies for bone remodeling are highlighted. This tendency is reproduced by the proposed numerical computations. The model is identified on a single case (one frequency and one force amplitude) and validated on the other ones. The second experimental validation deals with a different loading regime, an internal fluid pressure at 20 Hz on a turkey ulna. The same framework is applied, and the numerical and experimental data are still matching in terms of gain in bone mass density.

A 3D finite element model for hyperthermia injury of blood-perfused skin.

The objective of this study is to propose a numerical model of thermal damage to the skin. This model simulates the propagation of a burn and suggests treatments to prevent it from spreading. In order to achieve this goal, we developed a 3D multi-layer finite element model of the skin coupled with a model presenting hyperthermic damage. The numerical model of the skin takes account of not only the thermal properties of various layers, but also blood perfusion and veins. The model of thermal damage is based on the Arrhenius’ law. We tested two various quick intervention treatments so as to prevent the burn from spreading. The first treatment consists of cooling the burned zone with a flow of cool water at 10°C, whereas the second solution simulates the apposition of ice on the burn. The results show that, according to the severity of the burn, the second treatment seems to be the most appropriate. Moreover, our model opens interesting prospects in the analysis of hyperthermic damage.

Influence of gravity on the skin thermal behavior: experimental study using dynamic infrared thermography

To better understand the thermomechanical behavior of the skin and its direct environment, we present an experimental study using dynamic infrared thermography. This experimental study aims to highlight quantitatively some effects of blood flow on the heat diffusion.

Rupture limit evaluation of human cerebral aneurysms wall: Experimental study

BACKGROUND AND PURPOSEnRupture risk of intracranial aneurysms is a major issue for public healthcare. A way to obtain an individual rupture risk assessment is a main objective of many research teams in the world. For many years, we have investigated the relationship between the mechanical properties of aneurysm wall tissues and the rupture risk. In this work, we try to go further and investigate rupture limit values.nnnMETHODSnFollowing surgical clipping, a specific conservation protocol was applied to aneurysmal tissues in order to preserve their mechanical properties. Thirty-nine intracranial aneurysms (27 females, 12 males) were tested using a uniaxial tensile test machine under physiological conditions, temperature, and saline isotonic solution. These represented 24 unruptured and 15 ruptured aneurysms. Stress/strain curves were then obtained for each sample, and a fitting algorithm was applied following a Yeoh hyperelastic model with 2 parameters. Moreover, uniaxial tensile tests were conducted until rupture of samples to obtain values of stress and strain rupture limit.nnnRESULTSnThe significant parameter a C2 of the hyperelastic Yeoh model, allowed us to classify samples rigidity following the terminology we adopted in previous papers (Costalat et al., 2011; Sanchez et al., 2013): Soft, Stiff and Intermediate. Moreover, strain/stress rupture limit values were gathered and analyzed thanks to the tissue rigidity, the status of the aneurysm (initially ruptured or unruptured) and the gender of the patient.nnnCONCLUSIONnStrain rupture limit was found quite stable around 20% and seems not to be correlated with the status of the aneurysm (initially ruptured or unruptured), neither with the gender of the patient. However, stretch and stress rupture limit seems not to be independent on the rigidity. The study confirms that ruptured aneurysms mainly present a soft tissue and unruptured aneurysms present a stiff material.

Experimental measures of human intracranial arteries wall properties

Stroke disease is the main cause of disability in France, with 130,000 cases a year. Most of them are ischemic, secondary to a brain vessel occlusion. Twenty per cent of the strokes are haemorrhagic, with a large proportion of intracranial aneurysm rupture (80%). Recently, it has been shown that numerical simulations based on patientspecific image data may be used to estimate wall shear stress (WSS) and pressure in the circle of Willis (Cebral et al. 2005). Others have used computations to analyse the flow in cerebral aneurysms, with focus on WSS, which is also thought to be associated with aneurysm formation and rupture (Ishida et al. 2005; Cebral et al. 2011). Few research teams (Monson et al. 2005, 2008) studied the material behaviour of human cerebral arteries. They highlighted, using uniaxial test, the difference between arteries and veins and the influence of vessel size. They also carried out inflation test; they showed the change of behaviour between longitudinal and circumferential mechanical properties. In order to improve the numeric modelling of brain vasculature, investigation and comparison of mechanical behaviour of human cerebral arteries in the location of the Willis circle have been done.