Nondestructive measurement of esophageal biaxial mechanical properties utilizing sonometry

Characterization of the layer, direction and time-dependent mechanical behaviour of the human oesophagus and the effects of formalin preservation

The mechanical characterization of the oesophagus is essential for applications such as medical device design, surgical simulations and tissue engineering, as well as for investigating the organ's pathophysiology. However, the material response of the oesophagus has not been established ex vivo in regard to the more complex aspects of its mechanical behaviour using fresh, human tissue: as of yet, in the literature, only the hyperelastic response of the intact wall has been studied. Therefore, in this study, the layer-dependent, anisotropic, visco-hyperelastic behaviour of the human oesophagus was investigated through various mechanical tests. For this, cyclic tests, with increasing stretch levels, were conducted on the layers of the human oesophagus in the longitudinal and circumferential directions and at two different strain rates. Additionally, stress-relaxation tests on the oesophageal layers were carried out in both directions. Overall, the results show discrete properties in each layer and direction, highlighting the importance of treating the oesophagus as a multi-layered composite material with direction-dependent behaviour. Previously, the authors conducted layer-dependent cyclic experimentation on formalin-embalmed human oesophagi. A comparison between the fresh and embalmed tissue response was carried out and revealed surprising similarities in terms of anisotropy, strain-rate dependency, stress-softening and hysteresis, with the main difference between the two preservation states being the magnitude of these properties. As formalin fixation is known to notably affect the formation of cross-links between the collagen of biological materials, the differences may reveal the influence of cross-links on the mechanical behaviour of soft tissues.

Mechanical experimentation of the gastrointestinal tract: a systematic review

The gastrointestinal (GI) organs of the human body are responsible for transporting and extracting nutrients from food and drink, as well as excreting solid waste. Biomechanical experimentation of the GI organs provides insight into the mechanisms involved in their normal physiological functions, as well as understanding of how diseases can cause disruption to these. Additionally, experimental findings form the basis of all finite element (FE) modelling of these organs, which have a wide array of applications within medicine and engineering. This systematic review summarises the experimental studies that are currently in the literature (n = 247) and outlines the areas in which experimentation is lacking, highlighting what is still required in order to more fully understand the mechanical behaviour of the GI organs. These include (i) more human data, allowing for more accurate modelling for applications within medicine, (ii) an increase in time-dependent studies, and (iii) more sophisticated in vivo testing methods which allow for both the layer- and direction-dependent characterisation of the GI organs. The findings of this review can also be used to identify experimental data for the readers' own constitutive or FE modelling as the experimental studies have been grouped in terms of organ (oesophagus, stomach, small intestine, large intestine or rectum), test condition (ex vivo or in vivo), number of directions studied (isotropic or anisotropic), species family (human, porcine, feline etc.), tissue condition (intact wall or layer-dependent) and the type of test performed (biaxial tension, inflation-extension, distension (pressure-diameter), etc.). Furthermore, the studies that investigated the time-dependent (viscoelastic) behaviour of the tissues have been presented.

3D biomechanical properties of the layered esophagus: Fung-type SEF and new constitutive model

BACKGROUND AND PURPOSE

Most current studies on the passive biomechanical properties of esophageal tissues directly use the exponential strain energy function (SEF) to fit and calculate the constants of the constitutive equation. In the context of the extensive application of exponential SEF, in-depth research on the exponential SEF is still lacking. The purpose of this study is to combine the exponential function with the polynomial SEF to obtain the most suitable constitutive equation to describe the three-dimensional passive behavior of the esophagus.

METHODS

fresh pig esophagus with a length of 13 cm in the middle position was selected as esophageal samples. The esophageal sample was separated into muscular layer and mucosal layer with surgical scissors. Stretch-inflation mechanical tests of the intact esophagus, esophageal muscular, and esophageal mucosa were carried out on a triaxial test machine. The external radius, axial force, and internal pressure were recorded simultaneously. The seven-parameter Fung-type SEF and several new SEFs combining polynomials and exponents were used to fit the experimental data curves.

RESULTS

The stretch-inflation test data and the morphometric parameters at the zero-stress state of the layered esophagus were obtained. The new SEF with polynomial and exponential combination is more suitable to describe describing the three-dimensional passive biomechanical properties of esophageal tissue. Among them, New-Fung13 SEF is more suitable for describing the passive biomechanical properties of intact esophageal tissue, Sokolis-Fung13 SEF is more suitable for the esophageal muscle layer, and New-Fung10 SEF is more suitable for the esophageal mucosa. The constitutive parameters of the optimal constitutive model for each layer of the esophagus were obtained.

Use of Shear Wave Ultrasound Vibrometry for Detection of Simulated Esophageal Malignancy in Ex Vivo Porcine Esophagi

Esophageal cancer is a malignant neoplasm with poor outcomes. Determination of local disease progression is a major determining factor in treatment modality, radiation dose, radiation field and subsequent surgical therapy. Discrimination of true tumor extent is difficult given the similarity of soft tissues of the malignancy compared to non-malignant tissues using current imaging modalities. A possible method to discriminate between these tissues may be to exploit mechanical properties to diagnostic advantage, as malignant tissues tend to be stiffer relative to normal adjacent tissue. Shear waves propagate faster in stiffer tissues relative to softer tissues. This may be measured by using ultrasound based shear wave vibrometry. In this method, acoustic radiation force is used to create a shear wave in the tissue of interest and ultrafast ultrasound imaging is used to track the propagating wave to measure the wave velocity and estimate the shear moduli. In this study we created simulated malignant lesions (1.5 cm length) using radiofrequency ablation in ex vivo esophageal samples with varied progression (partial thickness n = 4, and full thickness n = 5) and used normal regions of the same esophageal specimen as controls. Shear wave vibrometry was used to measure shear wave group velocity and shear wave phase velocity in the ex vivo specimens. These values were used to estimate shear moduli using an elastic shear wave model and elastic and viscoelastic Lamb wave models. Our results show that the group and phase velocities increase due to both full and mucosal ablation, and that discrimination may be provided by higher order analysis using viscoelastic Lamb wave fitting. This technique may have application for determination of extent of early esophageal malignancy and warrants further investigation using in vivo approaches to determine performance compared to current imaging modalities.