Virtual ABI: A computationally derived ABI index for noninvasive assessment of femoro-popliteal bypass surgery outcome.

BACKGROUND AND OBJECTIVE: Peripheral arterial disease of the lower limbs, which affects 12-14% of the population, is often treated by bypassing a blocked portion of the vessel. Due to the limited ability of clinicians to predict the outcome of a selected bypass strategy, the five-year graft occlusion ranges from 50% to 90%, with a 20% risk of amputation in the first 5 years after the surgery. The aim of this study was to develop a computational procedure that could enable surgeons to reduce negative effects by assessing patient-specific response to the available surgical strategies. METHODS: The Virtual ABI assumes patient-specific finite element modeling of patients' hemodynamics from routinely acquired medical scans of lower limbs. The key contribution of this study is a novel approach for prescribing boundary conditions, which combines noninvasive preoperative measurements and results of numerical simulations. RESULTS: The validation performed on six follow-up cases indicated high reliability of the Virtual ABI, since the correlation with the experimentally measured values of ankle-brachial index was R² = 0.9485. CONCLUSION: The initial validation showed that the proposed Virtual ABI is a noninvasive procedure that could assist clinicians to find an optimal strategy for treating a particular patient by varying bypass length, choosing adequate diameter, position and shape.

OpenMandible: An open-source framework for highly realistic numerical modelling of lower mandible physiology.

OBJECTIVE: Computer modeling of lower mandible physiology remains challenging because prescribing realistic material characteristics and boundary conditions from medical scans requires advanced equipment and skill sets. The objective of this study is to provide a framework that could reduce simplifications made and inconsistency (in terms of geometry, materials, and boundary conditions) among further studies on the topic. METHODS: The OpenMandible framework offers: 1) the first publicly available multiscale model of the mandible developed by combining cone beam computerized tomography (CBCT) and µCT imaging modalities, and 2) a C++ software tool for the generation of simulation-ready models (tet4 and hex8 elements). In addition to the application of conventional (Neumann and Dirichlet) boundary conditions, OpenMandible introduces a novel geodesic wave propagation - based approach for incorporating orthotropic micromechanical characteristics of cortical bone, and a unique algorithm for modeling muscles as uniformly directed vectors. The base intact model includes the mandible (spongy and compact bone), 14 teeth (comprising dentin, enamel, periodontal ligament, and pulp), simplified temporomandibular joints, and masticatory muscles (masseter, temporalis, medial, and lateral pterygoid). RESULTS: The complete source code, executables, showcases, and sample data are freely available on the public repository: https://github.com/ArsoVukicevic/OpenMandible. It has been demonstrated that by slightly editing the baseline model, one can study different "virtual" treatments or diseases, including tooth restoration, placement of implants, mandible bone degradation, and others. SIGNIFICANCE: OpenMandible eases the community to undertake a broad range of studies on the topic, while increasing their consistency and reproducibility. At the same time, the needs for dedicated equipment and skills for developing realistic simulation models are significantly reduced.

Numerical and experimental analysis of factors leading to suture dehiscence after Billroth II gastric resection.

The main goal of this study was to numerically quantify risk of duodenal stump blowout after Billroth II (BII) gastric resection. Our hypothesis was that the geometry of the reconstructed tract after BII resection is one of the key factors that can lead to duodenal dehiscence. We used computational fluid dynamics (CFD) with finite element (FE) simulations of various models of BII reconstructed gastrointestinal (GI) tract, as well as non-perfused, ex vivo, porcine experimental models. As main geometrical parameters for FE postoperative models we have used duodenal stump length and inclination between gastric remnant and duodenal stump. Virtual gastric resection was performed on each of 3D FE models based on multislice Computer Tomography (CT) DICOM. According to our computer simulation the difference between maximal duodenal stump pressures for models with most and least preferable geometry of reconstructed GI tract is about 30%. We compared the resulting postoperative duodenal pressure from computer simulations with duodenal stump dehiscence pressure from the experiment. Pressure at duodenal stump after BII resection obtained by computer simulation is 4-5 times lower than the dehiscence pressure according to our experiment on isolated bowel segment. Our conclusion is that if the surgery is performed technically correct, geometry variations of the reconstructed GI tract by themselves are not sufficient to cause duodenal stump blowout. Pressure that develops in the duodenal stump after BII resection using omega loop, only in the conjunction with other risk factors can cause duodenal dehiscence. Increased duodenal pressure after BII resection is risk factor. Hence we recommend the routine use of Roux en Y anastomosis as a safer solution in terms of resulting intraluminal pressure. However, if the surgeon decides to perform BII reconstruction, results obtained with this methodology can be valuable.

Computer modelling of maximal displacement forces in endoluminal thoracic aortic stent graft.

The objective of this study was to determine the orientation and magnitude of maximal displacement forces (DFs) in the thoracic aortic aneurysm endograft (TAA endograft) in three-dimensional (3D) space. Theoretical computer model representing the anatomically worst-case scenario with respect to DF magnitude was used to calculate the magnitude and orientation of maximal DF. A patient-specific anatomical computer model of typically seen, average size anatomy was used to analyse the progression of DF throughout the cardiac cycle. Maximal DFs were 35.01 and 37.32 N in standing and supine position, respectively, in 46-mm diameter TAA graft with 90° bend. A patient-specific model shows that a maximal DF magnitude is achieved at the peak systolic flow. In both models, the orientation of the DF vector was perpendicular to the greater curvature of the aorta, with upward (cranial) and sideways components. The effect of shearing force on the total DF that acts on the TAA endograft was found negligible due to the several orders of magnitude stronger contribution of pressure forces to the total DF relative to the wall shear stress contribution, resulting in aortic diameters and angulation being the main drivers of DF. It was discovered that the TAA endografts can be subjected to much stronger DF than previously suspected. The magnitude of maximal DF in thoracic aorta in the worst-case scenario could be as high as 35.01 N (standing) and 37.32 N (supine). This new information should be used in the process of designing new generations of TAA endografts with better migration resistance properties.

Numerical simulation of the flow field and mass transport pattern within the coronary artery.

In this study, the steady and pulsatile flow field with mass transport analysis in an anatomically correct model of coronary artery is simulated numerically using a specific patient data from a 64-multislice computed tomography scanner. It is assumed that the blood flow is laminar and that the Navier-Stokes equations of motion are applied. Downstream of the bifurcation, a strong skewing occurs towards the flow divider walls as a result of branching. For the low-density lipoprotein (LDL) transport analysis where a specific boundary condition at the arterial walls is applied, LDL is generally elevated at locations where shear stress distribution is low, but it does not co-locate at whole domain. This numerical simulation gives an insight, as well as detailed quantitative data, of haemodynamic conditions in the left coronary artery as well as mass transfer patterns for a specific patient.

Impact of aortic repair based on flow field computer simulation within the thoracic aorta.

Purpose of this computational study is to examine the hemodynamic parameters of velocity fields and shear stress in the thoracic aorta with and without aneurysm, based on an individual patient case and virtual surgical intervention. These two cases, case I (with aneurysm) and II (without aneurysm), are analyzed by computational fluid dynamics. The 3D Navier-Stokes equations and the continuity equation are solved with an unsteady stabilized finite element method. The vascular geometries are reconstructed based on computed tomography angiography images to generate a patient-specific 3D finite element mesh. The input data for the flow waveforms are derived from MR phase contrast flow measurements of a patient before surgical intervention. The computed results show velocity profiles skewed towards the inner aortic wall for both cases in the ascending aorta and in the aortic arch, while in the descending aorta these velocity profiles are skewed towards the outer aortic wall. Computed streamlines indicate that flow separation occurs at the proximal edge of the aneurysm, i.e. computed flow enters the aneurysm in the distal region, and that there is essentially a single, slowly rotating, vortex within the aneurysm during most of the systole. In summary, after virtual surgical intervention in case II higher shear stress distribution along the descending aorta could be found, which may produce more healthy reactions in the endothelium and benefit of vascular reconstruction of an aortic aneurysm at this particular location.