Russian Journal of Biomechanics

The conspicuous increase in obesity rate occurring in the last decades in industrialized countries, often accompanied by high morbidity and high mortality rate, have been made obesity a global health concern. Bariatric Surgery is the most effective treatment for severe obesity. However, there are still many issues related to surgical procedures not yet been overcome. The importance of experimenting with a new rational approach based on bioengineering methods could strongly improve surgical approach by avoiding drawbacks and complications. The aim of this work is the construction of patient-specific computational models of the resected stomachs after laparoscopic sleeve gastrectomy able to interpret the structural mechanical behaviour of human gastric tissues. A coupled experimental- computational approach was performed. Experimental insufflation tests were performed on nine resected stomachs from LSG. Through a reverse engineering approach, nine specific- patient computational models were developed, aiming at simulating the experimental activities. A double-layered fiber-reinforced anisotropic hyperelastic material formulation was chosen. The experimental evidences provided the pressure-volume behavior of the resected stomachs. The comparison between experimental and computational results permitted to identify the set of the constitutive parameters. The stress-strain distribution described the region and the layer mainly solicited. An engineering approach allows us to characterize the mechanical behavior of the human gastric tissues. Reliable computational models will be used in understanding the biomechanics of the human stomach and will provide a clinical tool to help medical staff in optimizing bariatric procedures.

To study the physical and mechanical properties of zirconium dioxide post stump structures made by computer modeling and milling (CAD/CAM), the stress-strain state and their strength characteristics, numerical and experimental studies were carried out. The test specimens were a system consisting of previously extracted premolar teeth with an absent anatomical crown, a prepared root canal and a post stump inlay fixed in it, an artificial zirconium dioxide crown fixed on the pin insert, and a part of the root. The prepared teeth for the placement of the posts were scanned in the dental laboratory, after which the pin insert of zirconium dioxide were milled. Investigated 2 groups of samples: specimens of the first group were processed in a sandblasting unit; the specimens of the second were not subjected to any mechanical and temperature treatment. To test the compressive strength of the specimens, we used an Instron 5982 universal floor electromechanical testing machine in static mode under conditions of a single increasing load. The specimens were tested with vertical load and at an angle of 15° with and without sandblasting with aluminum oxide powder. The value of the force for each variant was obtained, at which the samples were destroyed. Also, a numerical study was carried out by the finite element method on the developed mathematical model of the stress-strain state dependence of the sample on the applied functional load acting vertically on the occlusal surface of the tooth and at an angle of 15°. The problem was considered in an axisymmetric setting. The computer model included a premolar tooth with a crown, a root and a pin insert, taking into account the mechanical properties of the materials of each of these elements. With an increase in the angle of the load acting on the crown from 0° to 15°, the destruction of the sample occurs at its lower values, by about 10-12 %; the maximum values of stresses were determined in the upper part of the crown and the neck of the tooth (connection of the crown to the root) - in the experiments, destruction occurred precisely in these places. Comparison of the experimental averaged values of the loads at which the destruction of the samples occurs with the numerical values showed their good agreement.

This study describes an integrated approach biomechanics to modeling of a frameless prosthetic heart valve based on the analysis of medical graphic data (multispiral computed tomography), solid modeling (Abaqus/CAE) and numerical analysis of blood flow (OpenFOAM). The object of the study was the clinical case of patient D. (56 years old), who was surgically implanted with the “TiAra” aortic valve prosthesis. In this work, we reconstructed three-dimensional computer models of the functioning of the prosthesis for 10 segments of one cardiac cycle, followed by numerical experiments reproducing the movement of the key points of the product -commissural racks, leaflets. High asymmetric mobility of the bioprosthesis elements was shown during deformations of the heart cycle - one of the racks is much more mobile than the other two (up to 32% in movement). Solid modeling of the prosthesis deformation did not reveal significant malfunctioning or exceeding the stress-strain state of the components: Mises stress maximum reached 0.8 MPa. The distribution of stress indicators on the diagrams of three-dimensional models revealed the main concentration in the leaflet bell and commissural racks. In addition, qualitatively, the operation of the cusp apparatus corresponded to the functioning conditions - the dynamics of pressure changes in the structure of the cardiac cycle “systole-diastole”, area of cusps contact, and the elastic properties of the materials. The analysis of the flow simulation results determined the presence of several sections with a turbulent structure in the region of the Valsalva sinuses, which, however, did not demonstrate critical quantitative characteristics - wall shear stresses (up to 74 MPa), flow velocities (0.1-1.0 m/s).

In the paper, we present a model of skin in the form of viscoelastic layer-like structure that is inhomogeneous in thickness and consists in turn of three layers: subcutaneous fat, dermis and epidermis. We also assume the presence of an inhomogeneous uniaxial prestress in the skin layer arisen as a result of relaxation after the skin lifting (tightening) surgery procedure. We formulate the problem on the basis of the general problem statement on steady-state vibrations of an inhomogeneous body, taking into account the initial stress-strain state. Using the correspondence principle, the elastic moduli in the problem statement are replaced by complex analogs corresponding to the model of a standard viscoelastic body. The inverse problem is to determine the mechanical characteristics (complex analogs of the Lame parameters) and prestress using the data on the acoustic response for a periodic probing non- invasive effect on the layer surface. By using the Fourier transform in the longitudinal coordinate, we reduce the original problem to solving a number of simpler problems in transforms. Based on the combination of different loading modes, we present a two-stage scheme for identifying mechanical properties and prestress. Within this scheme, at the first stage, complex analogs of the Lame parameters are successively determined using the iterative approach and the Tikhonov regularization method. At the second stage, by using the functions found at the first stage, prestresses are determined using the proposed projection technique. We illustrate the approach developed to the considered skin integument problem by performing computational experiments showing the efficiency of the techniques proposed in the ranges corresponding to the real values of the sought-for parameters. Additionally, we provide some recommendations on the choice of frequency ranges for the best identification at each stage.