Combined numerical and experimental approach to determine numerical model of abdominal scaffold

A proper junction of the prosthesis and the abdominal wall is important in successful hernia repair. The number of tacks should be balanced to assure appropriate mesh fixation and not to induce post-operative pain. Numerical simulations help to find this balance. The study is aimed at creating a proper numerical model of a knitted surgical mesh subjected to boundary conditions and load occurring in the abdominal cavity. Continuous, anisotropic constitutive relation is considered to reflect the mesh behaviour. Different sets of material law parameters are determined on the basis of different bi-axial tests setups. Force- and displacement-controlled tests with different ratios are considered. Consequently, some numerical model variants are obtained featuring various reaction distributions in the scaffold fixation points. The proper variant is selected based on comparison of the position of maximal reaction force in the numerical model and in the reference physical model of operated hernia. Force-driven tests have shown anisotropic mesh behaviour, while equibiaxial displacement-driven test has demonstrated reduced anisotropic response. Within seven scenarios of constitutive parameters identification (based on single or combined experimental data), the equibiaxial force-controlled test appeared to produce the most relevant model to follow the prosthesis behaviour under pressure. The position of maximal reaction force in such model is similar to obtained in the physical hernia model. The equibiaxial force-driven test provides most suitable data for Gasser-Ogden-Holzapfel constitutive model identification of a considered surgical mesh to be used to model the mesh under pressure.

In vivo performance of intraperitoneal onlay mesh after ventral hernia repair

BACKGROUND

Ventral hernia repair needs to be improved since recurrence, postoperative pain and other complications are still reported in many patients. The behavior of implants in vivo is not sufficiently understood to design a surgical mesh mechanically compatible with the human abdominal wall.

METHODS

This analysis was based on radiological pictures of patients who underwent laparoscopic ventral hernia repair. The pictures show the trunk of the patient at rest in a standing position and under side bending. The change in the distance between different tacks due to trunk movement was analyzed, which allowed us to determine the in vivo elongation of the mesh incorporated into the abdominal wall.

FINDINGS

The relative elongations of the surgical mesh varied from a few percent to greater than 100% in two cases. The median of the median relative elongations obtained for all patients is 9.5%, and the median of the maximum relative elongations for all patients is 32.6%. The maximum elongation occurs between tacks that are next to each other. Trunk movement causes implant deformation, and this study provides quantitative information regarding changes in the distance between fasteners.

INTERPRETATION

The physiological movement of the human abdomen must be regarded as a very important factor in mesh deformation and should be considered in surgical practice to reduce the hernia recurrence rate and postoperative pain.

Using Hands-On Physical Computing Projects to Teach Computer Programming to Biomedical Engineering Students

Rapid advancements in the multidisciplinary field of biomedical engineering (BME) require competitive engineers with skill sets in a broad range of subjects including biology, physiology, mechanics, circuits, and programming. Accordingly, such a need should be reflected in the training of BME students. Among those skills, computer programming is an essential tool that is used in a wide variety of applications. In this paper, we have provided our experience in incorporating project-based learning, a promising approach in active learning, for teaching computer programming to BME students. We describe a low-cost method for using physical, hands-on computing that directly relates to BME. Additionally, we detail our efforts to teach multiple programming languages in one semester and provide a detailed analysis of the outcomes. We also provide basic materials for other instructors to adapt to fit their own needs.

Regional Quantification of Brain Tissue Strain Using Displacement-Encoding With Stimulated Echoes Magnetic Resonance Imaging

Intrinsic cardiac-induced deformation of brain tissue is thought to be important in the pathophysiology of various neurological disorders. In this study, we evaluated the feasibility of utilizing displacement encoding with stimulated echoes (DENSE) magnetic resonance imaging (MRI) to quantify two-dimensional (2D) neural tissue strain using cardiac-driven brain pulsations. We examined eight adult healthy volunteers with an electrocardiogram-gated spiral DENSE sequence performed at the midsagittal plane on a 3 Tesla MRI scanner. Displacement, pixel-wise trajectories, and principal strains were determined in seven regions of interest (ROI): the brain stem, cerebellum, corpus callosum, and four cerebral lobes. Quantification of small neural tissue motion and strain along with their spatial and temporal variations in different brain regions was found to be feasible using DENSE. The medial and inferior brain structures (brain stem, cerebellum, and corpus callosum) had significantly larger motion and strain compared to structures located more peripherally. The brain stem had the largest peak mean displacement (PMD) (187 ± 50 μm, mean ± SD). The largest mean principal strains in compression and extension were observed in the brain stem (0.38 ± 0.08%) and the corpus callosum (0.37 ± 0.08%), respectively. Measured values in percent strain were altered by as much as 0.1 between repeated scans. This study showed that DENSE can quantify regional variations in brain tissue motion and strain and has the potential to be utilized as a tool to evaluate the changes in brain tissue dynamics resulting from alterations in biomechanical stresses and tissue properties.

Anisotropic and nonlinear biaxial mechanical response of porcine small bowel mesentery

Intestinal malrotation places pediatric patients at the risk of midgut volvulus, a complication that can lead to ischemic bowel, short gut syndrome, and even death. Even though the treatments for symptomatic patients of this complication are clear, it is still a challenge to identify asymptomatic patients who are at a higher risk of midgut volvulus and decide on a suitable course of treatment. Development of an accurate computerized model of this intestinal abnormality could help in gaining a better understanding of its integral behavior. To aid in developing such a model, in the current study, we have characterized the biaxial mechanical properties of the porcine small bowel mesentery. First, the tissue stress-strain response was determined using a biaxial tensile testing equipment. The stress-strain data were then fitted into a Fung-type phenomenological constitutive model to quantify the tissue material parameters. The stress-strain responses were highly nonlinear, showing more compliance at the lower strains following by a rapid transition into a stiffer response at higher strains. The tissue was anisotropic and showed more stiffness in the radial direction. The data fitted the Fung-type constitutive model with an average R-squared value of 0.93. An averaging scheme was used to produce a set of material parameters which can represent the generic mechanical behavior of the tissue in the models.