Alessandro Nava

Determination of the Mechanical Properties of Soft Human Tissues through Aspiration Experiments

Mechanical models for soft human organs are necessary for a variety of medical applications, such as surgical planning, virtual reality surgery simulators, and for diagnostic purposes. An adequate quantitative description of the mechanical behaviour of human organs requires high quality experimental data to be acquired and analyzed. We present a novel technique for the acquisition of such data from soft tissues and its post processing to determine some parameters of the tissue’s mechanical properties. A small tube is applied to the target organ and a weak vacuum is generated inside the tube according to a predefined pressure history. A video camera grabs images of the deformation profile of the aspirated tissue, and a pressure sensor measures the correspondent vacuum level. The images are processed and used to inform the fitting of uniaxial and continuum mechanics models. Whilst the aspiration test device has been designed to fulfill the requirements for in-vivo applications, for measurements obtained during open surgery, initial experiments performed on human cadaveric tissues demonstrate the ability to both differentiate between different organs and also between normal and diseased organs on the basis of the derived mechanical properties.

Mechanical characterization of the liver capsule and parenchyma

Internal organs are heterogeneous structures, both on the macro- and on the micro-scale. However, they are often modeled as homogeneous solids with uniform material properties. In this light, this work investigates the impact of the liver capsule on the integral behavior of the organ by means of in vitro tests and computer simulations. The stiffness of bovine liver obtained in tissue aspiration experiments differed by a factor of 2 to 3 when the capsule was removed. As a first step, the capsule was implemented as separate structure in a finite element model of the organ undergoing tissue aspiration. The finite element simulations are in good agreement with the experimental results

Experimental Observation and Modelling of Preconditioning in Soft Biological Tissues

Constitutive models for soft biological tissues and in particular for human organs are required for medical applications such as surgery simulation, surgery planning, diagnosis. In the literature the mechanical properties of biosolids are generally presented in “preconditioned” state, i.e. the stabilized conditions reached after several loading-unloading cycles. We hereby present experiments on soft tissues showing the evolution of the mechanical response in a series of loading and unloading cycles. The experimental procedure applied in this study is based on the so called “aspiration experiment” and is suitable for in-vivo applications under sterile conditions during open surgery. In the present study this technique is applied ex-vivo on bovine liver. A small tube is contacted to the target organ and a weak vacuum is generated inside the tube according to a predefined pressure history. Several identical loading and unloading cycles are applied in order to characterize the evolutive behaviour of the tissue. The experimental data are used to inform the fitting of uniaxial and threedimensional continuum mechanics models. This analysis demonstrates that a quasi-linear viscoelastic model fails in describing the observed evolution from the “virgin” to the preconditioned state. Good agreement between simulation and measurement are obtained by introducing an internal variable changing according to an evolution equation.

In Vivo Characterization of the Mechanics of Human Uterine Cervices

Abstract:  The uterine cervix has to provide mechanical resistance to ensure a normal development of the fetus. This is guaranteed by the composition of its extracellular matrix, which functions as a fiber‐reinforced composite. At term a complex remodeling process allows the cervical canal to open for birth. This remodeling is achieved by changes in the quality and quantity of collagen fibers and ground substance and their interplay, which influences the biomechanical behavior of the cervix but also contributes to pathologic conditions such as cervical incompetence (CI). We start by reviewing the anatomy and histological composition of the human cervix, and discuss its physiologic function and pathologic condition in pregnancy including biomechanical aspects. Established diagnostic methods on the cervix (palpation, endovaginal ultrasound) used in clinics as well as methods for assessment of cervical consistency (light‐induced fluorescence, electrical current, and impedance) are discussed. We show the first clinical application of an aspiration device, which allows in vivo testing of the biomechanical properties of the cervix with the aim to establish the physiological biomechanical changes throughout gestation and to detect pregnant women at risk for CI. In a pilot study on nonpregnant cervices before and after hysterectomy we found no considerable difference in the biomechanical response between in vivo and ex vivo. An outlook on further clinical applications during pregnancy is presented.

In Vivo Experiments to Characterize the Mechanical Behavior of the Human Uterine Cervix

The main purpose of the present study was to test the reliability and sensitivity of mechanical data obtained from human cervices with respect to a possible clinical application for diagnostic purposes. Future studies will be performed with the goal of using the proposed method for the detection of early cervical changes associated with pathologic conditions.

Inverse finite element characterization of the human myometrium derived from uniaxial compression experiments.

Abstract The low strain-rate behavior of the human myometrium under compression was determined. To this end, uniaxial, unconstrained compression experiments were conducted on a total of 25 samples from three excised human uteri at strain rates between 0.001 s-1 and 0.008 s-1. A three-dimensional finite element model of each sample was created and used together with an optimization algorithm to find material parameters in an inverse estimation process. Friction and shape irregularities of samples were incorporated in the models. The uterine specimens in compression were modeled as viscoelastic, non-linear, nearly incompressible and isotropic continua. Simulations of uniaxial, frictionless compressions of an idealized cuboid were used to compare the resulting material parameters among each other. The intra- and inter-subject variability in stiffness of specimens was found to be large and to cover such a wide range that the effect of anisotropy which is of minor influence under compressive deformations in the first place could be neglected. Material parameters for a viscoelastic model based on a decoupled, reduced quadratic strain-energy function were presented for the uterine samples representing a median stiffness. Zusammenfassung Diese Studie hatte zum Ziel, das Verhalten von menschlichem Myometrium bei geringen Dehnungsgeschwindigkeiten unter Kompression zu bestimmen. Dazu führten wir einaxiale Druckversuche an 25 Proben von 3 exzidierten Uteri mit niedrigen Dehngeschwindigkeiten zwischen 0,001 s-1 und 0,008 s-1 durch. Von jeder Probe wurde ein dreidimensionales Finite-Elemente-Modell erstellt, und mit einem inversen Optimierungsalgorithmus konnten daraus Materialparameter bestimmt werden. Unregelmäßigkeiten der Probengeometrie und die Reibung zwischen Probe und Platten fanden im Modell Berücksichtigung. Die Uterusproben wurden als viskoelastische, nicht-lineare, fast inkompressible und isotrope Kontinua modelliert. Die resultierenden Materialparameter wurden anhand von Simulationen einaxialer, reibungsfreier Druckversuche an einer idealisierten rechteckigen Probe untereinander verglichen. Die Resultate zeigten eine enorme Variabilität bezüglich der Steifigkeit sowohl innerhalb eines Organs als auch zwischen verschiedenen Uteri. Das Ausmass der Variabilität der Parameterwerte war so groß, dass der Einfluss der Anisotropie, welcher unter kompressiver Belastung ohnehin gering ist, vernachlässigt werden konnte. Schliesslich wurden Materialparameter von Samples mit einer medianen Steifigkeit präsentiert. Diese Parameter beschrieben ein viskoelastisches Modell, das auf einer entkoppelten, reduzierten, quadratischen Verzerrungsenergie-Funktion basiert.

In vivo characterization of the mechanics of human uterine cervices

The uterine cervix has to provide mechanical resistance to ensure a normal development of the fetus. This is guaranteed by the composition of its extracellular matrix, which functions as a fiber-reinforced composite. At term a complex remodeling process allows the cervical canal to open for birth. This remodeling is achieved by changes in the quality and quantity of collagen fibers and ground substance and their interplay, which influences the biomechanical behavior of the cervix but also contributes to pathologic conditions such as cervical incompetence (CI). We start by reviewing the anatomy and histological composition of the human cervix, and discuss its physiologic function and pathologic condition in pregnancy including biomechanical aspects. Established diagnostic methods on the cervix (palpation, endovaginal ultrasound) used in clinics as well as methods for assessment of cervical consistency (light-induced fluorescence, electrical current, and impedance) are discussed. We show the first clinical application of an aspiration device, which allows in vivo testing of the biomechanical properties of the cervix with the aim to establish the physiological biomechanical changes throughout gestation and to detect pregnant women at risk for CI. In a pilot study on nonpregnant cervices before and after hysterectomy we found no considerable difference in the biomechanical response between in vivo and ex vivo. An outlook on further clinical applications during pregnancy is presented.