Karine Bruyere-Garnier

Mechanical characterization of liver capsule through uniaxial quasi-static tensile tests until failure

Accidentology data showed that liver is often injured in car crashes; three types of injuries occur: hematoma, laceration and vessel failure. This paper focuses on surface laceration, which involves liver capsule and hepatic parenchyma. Liver capsule behavior has been studied but its failure properties are still unclear, particularly on a local point of view. In the present study, tensile quasi-static tests are run on parenchyma and capsule samples until failure to characterize capsule failure. Normalized load as well as failure properties-ultimate load per width unit and ultimate strain-are determined. Digital image correlation is used to measure the full local strain field on the capsule. Mean values of failure characteristics for hepatic capsule are 47+/-29% for the ultimate local strain and 0.3+/-0.3 N/mm for the ultimate load per width unit. A comparison between human and porcine tissues is conducted based on Mann-Whitney statistical test; it reveals that capsule characteristics are close between these two species; however, freezing preservation significantly affects porcine capsule failure properties. Therefore using porcine instead of human tissue to determine failure characteristics of liver capsule seems satisfactory only on fresh tissues.

Characterization of the nonlinear behaviour and the failure of human liver capsule through inflation tests

This paper aims at describing an inflation test protocol on a human liver capsule using stereo-correlation. The biaxial tension created by the inflation test is comparable to the type of loading the capsule would be subjected to during a liver compression. Confocal microscopy associated to an anti-collagen coloration reveals that the tissue is isotropic at the meso-scale. Stereo-correlation provides the strain field of the capsule during the test. It emphasizes the boundary condition effects on the strain field. The measurement of the shape of the capsule is used to determine the parameters of two hyperelastic (polynomial and exponential) homogeneous models. The ultimate first principal strain before failure is measured locally and its value is 50.5%±10.8%. In this protocol, the light goes throughout the sample and makes the heterogeneities of the material appear as darker grey levels on the pictures. These heterogeneities also appear on the strain fields, so we can assume that they have different material properties.

Characterizing liver capsule microstructure via in situ bulge test coupled with multiphoton imaging.

The characterization of biological tissue at the microscopic scale is the starting point of many applications in tissue engineering and especially in the development of structurally based constitutive models. In the present study, focus is made on the liver capsule, the membrane encompassing hepatic parenchyma, which takes a huge part in liver mechanical properties. An in situ bulge test experiment under a multiphoton microscope has been developed to assess the microstructure changes that arise with biaxial loading. Multiphoton microscopy allows to observe the elastin and collagen fiber networks simultaneously. Thus a description of the microstructure organization of the capsule is given, characterizing the shapes, geometry and arrangement of fibers. The orientation of fibers is calculated and orientation distribution evolution with loading is given, in the case of an equibiaxial and two non equibiaxial loadings, thanks to a circular and elliptic set up of the bulge test. The local strain fields have also been computed, by the mean of a photobleaching grid, to get an idea of what the liver capsule might experience when subjected to internal pressure. Results show that strain fields present some heterogeneity due to anisotropy. Reorientation occurs in non equibiaxial loadings and involves fibers layers from the inner to the outer surface as expected. Although there is a fiber network rearrangement to accommodate with loading in the case of equibiaxial loading, there is no significant reorientation of the main fibers direction of the different layers.

Photobleaching as a tool to measure the local strain field in fibrous membranes of connective tissues

Connective tissues are complex structures which contain collagen and elastin fibers. These fiber-based structures have a great influence on material mechanical properties and need to be studied at the microscopic scale. Several microscopy techniques have been developed in order to image such microstructures; among them are two-photon excited fluorescence microscopy and second harmonic generation. These observations have been coupled with mechanical characterization to link microstructural kinematics to macroscopic material parameter evolution. In this study, we present a new approach to measure local strain in soft biological tissues using a side-effect of fluorescence microscopy: photobleaching. Controlling the loss of fluorescence induced by photobleaching, we create a pattern on our sample that we can monitor during mechanical loading. The image analysis allows three-dimensional displacements of the patterns at various loading levels to be computed. Then, local strain distribution is derived using the finite element discretization on a four-node element mesh created from our photobleached pattern. Photobleaching tests on a human liver capsule have revealed that this technique is non-destructive and does not have any impact on mechanical properties. This method is likely to have other applications in biological material studies, considering that all collagen-elastin fiber-based biological tissues possess autofluorescence properties and thus can be photobleached.

Finite element modeling of the head skeleton with a new local quantitative assessment approach

The present study was undertaken to build a finite element model of the head skeleton and to perform a new assessment approach in order to validate it. The application fields for such an improved model are injury risk prediction as well as surgical planning. The geometrical reconstruction was performed using computed tomography scans and a total of 4680 shell elements were meshed on the median surface of the head skeleton with the particular characteristic of adapted mesh density and real element thickness. The assessment protocol of the finite element model was achieved using a quasi-static experimental compression test performed on the zygomatic bone area of a defleshed isolated head. Mechanical behavior of the finite element model was compared to the real one and the assessment approach was divided into two steps. First, the mechanical properties of the anatomical structure were identified using the simulation and then the simulated displacement field was compared to local displacement measurement performed during test using a digital correlation method. The assessment showed that the head skeleton model behaved qualitatively like the real structure. Quantitatively, the local relative error varied from 8% up to 70%.

Non-destructive assessment of human ribs mechanical properties using quantitative ultrasound.

Advanced finite element models of the thorax have been developed to study, for example, the effects of car crashes. While there is a need for material properties to parameterize such models, specific properties are largely missing. Non-destructive techniques applicable in vivo would, therefore, be of interest to support further development of thorax models. The only non-destructive technique available today to derive rib bone properties would be based on quantitative computed tomography that measures bone mineral density. However, this approach is limited by the radiation dose. Bidirectional ultrasound axial transmission was developed on long bones ex vivo and used to assess in vivo health status of the radius. However, it is currently unknown if the ribs are good candidates for such a measurement. Therefore, the goal of this study is to evaluate the relationship between ex vivo ultrasonic measurements (axial transmission) and the mechanical properties of human ribs to determine if the mechanical properties of the ribs can be quantified non-destructively. The results show statistically significant relationships between the ultrasonic measurements and mechanical properties of the ribs. These results are promising with respect to a non-destructive and non-ionizing assessment of rib mechanical properties. This ex vivo study is a first step toward in vivo studies to derive subject-specific rib properties.

Relationship between human rib mechanical properties and cortical bone density measured by high-resolution quantitative computed tomography.

Thoracic injuries are among the most serious injuries in vehicle crash accidents. To improve the safety of vehicle occupants, it is essential to have a better knowledge on biomechanical responses of human rib cage (Vezin and Berthet 2009). A better knowledge of the bone mechanical properties should be of great interest to take into account the inter-individual differences in the population. For such purpose, a non-invasive technique is requested. Quantitative computed tomography (QCT) is a clinical modality that can be used to measure bone density. A literature review by Helgason et al. (2008) demonstrated that bone density (measured by QCT) was a good predictor of bone mechanical properties. Kopperdahl et al. (2002) and Duchemin et al. (2008) showed the relationships between bone density assessed by QCT and mechanical properties on vertebral cancellous bone and femoral cortical bone respectively. In this context, the question is: do such relationships exist on human ribs? No study has demonstrated such relationships on this anatomical site. Thus, the goal of this study is to assess the relationship between human rib mechanical properties and the density.

Non-invasive assessment of human ribs mechanical properties

Despite the current safety systems, thoracic injuries in car crash accidents are among the most important. To improve the users’ safety, a better knowledge of the biomechanical response of the rib cage is necessary (Stitzel et al. 2003; Kemper et al. 2005; Vezin and Berthet 2009). One approach is based on finite element modelling of the thorax. Such models are already available for ‘standard’ subjects. However, to study the whole range of transport users (children, elderly, etc.), their customisation is needed. It involves acquiring data in vivo and thus the development of non-destructive methods. Before considering in vivo studies, it is essential to assess these methods ex vivo. In this context, a first question is: is it possible to estimate the ribs mechanical properties by non-invasive measurements? Currently, no method exists to measure the rib strength in clinical examination. Nonetheless, a method estimating the elasticity of bone tissues by ultrasonic axial transmission is under development. It was initially designed for the in vivo measurements on the radius (Talmant et al. 2009) and is applied here for the first time to the ribs.

Mechanical behaviour of the in vivo paediatric and adult trunk during respiratory physiotherapy

The load–deflection response of the human trunk has been studied using various methods. The different shapes observed may be due to the methodology and the population. The purpose of this study is to quantify and explain the in vivo mechanical response of paediatric and adult trunks during respiratory physiotherapy. Eight children aged 5–15 months and eight healthy adult volunteers aged 30–87 years participated in this study. The force applied by the physiotherapist and the displacement of the targets on his hands were recorded. Parameters were also measured and calculated to compare against other studies. Time lags between force time histories and displacement time histories were observed on both children and adults. Different time lags resulted in different shapes of the force–displacement curves. Factors including respiration, muscle contraction and loading pattern are part of the assumptions used to explain this phenomenon. The maximum displacements of the paediatric and adult trunks were 18 and 44 mm, respectively, with a maximum load of 208 and 250 N, respectively. This study provides a better explanation of the peculiar force–displacement characteristics of both living and active children and adults under a non-injurious, low-rate compression condition. Complementary data (e.g. muscle activity and breathing) should be collected in the future to go towards in vivo human trunk modelling.

Imaging of the human Glisson's capsule by two-photon excitation microscopy and mechanical characterisation by uniaxial tensile tests

Liver mechanical properties have been well investigated through many studies at the macroscopic level. These studies allowed elaborating more or less sophisticated models providing material data. The rupture phenomenon has been studied as well, for example by Brunon et al. [Brunon et al. 2011] to collect data on how the liver, and more precisely the Glissons capsule, responds to mechanical loading. With the emergence of multi-modal microscopy, these phenomenon have been considered to a whole different scale, as we can now investigate the behaviour of these materials at the microscopic level. This way, we can have a better understanding of damage and rupture mechanisms of these tissues, linking them to microstructure organisation, and thus develop realistic models based on microstructure. Recent studies have therefore focused on imaging the microstructure organisation of biological materials during or after loading. For example Goulam Houssen et al. [Goulam Houssen et al., 2011] have monitored the rat tail collagen fibers behaviour during uniaxial loading using Second-Harmonic Generation (SHG) from TwoPhoton Excitation Microscopy (TPE). Keyes et al. [Keyes et al., 2012] used SHG and autofluorescence from TPE to image pressurised porcine coronary arteries and observed fibers realignment. Thus, the aim of this study is to observe how the different constitutive fibers of the Glissons capsule are organized and how they react when subjected to uniaxial tensile loading, in order to understand how microstructure organisation impacts macro mechanical properties. In a first step, we worked on the imaging of the Glissons capsule by TPE to enforce our knowledge of its components and their state before loading. In a second step, we performed preliminary tensile tests using an in situ micro tensile stage to estimate mechanical parameters such as apparent modulus, ultimate stress and ultimate strain. 2. Methods