| 1. |
- Roos, Håkan, et al.
(författare)
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Displacement Forces in Iliac Landing Zones and Stent Graft Interconnections in Endovascular Aortic Repair: An Experimental Study
- 2014
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 47:3, s. 262-267
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: Stent graft migration influences the long-term durability of endovascular aortic repair. Flow-induced displacement forces acting on the attachment zones may contribute to migration. Proximal fixation of aortic stent grafts has been improved by using hooks, while distal fixation and stent graft interconnections depend on self-expansion forces only. We hypothesized that flow-induced displacement forces would be significant at the distal end, and would correlate with graft movements. Methods: As part of an experimental study, an iliac limb stent graft was inserted in a pulsatile flow model similar to aortic in vivo conditions, and fixed-mounted at its proximal and distal ends to strain gauge load cells. Peak displacement forces at both ends and pulsatile graft movement were recorded at different graft angulations (0-90°), perfusion pressures (145/80, 170/90, or 195/100 mmHg), and stroke frequencies (60-100 b.p.m.). Results: Flow-induced forces were of the same magnitude at the proximal and distal end of the stent graft (peak 1.8 N). Both the forces and graft movement increased with angulation and perfusion pressure, but not with stroke rate. Graft movement reached a maximum of 0.29 ± 0.01 mm per stroke despite fixed ends. There were strong correlations between proximal and distal displacement forces (r = 0.97, p < .001), and between displacement forces and graft movement (r = 0.98, p < .001). Conclusions: Pulsatile flow through a tubular untapered stent graft causes forces of similar magnitude at both ends and induces pulsatile graft movements in its unsupported mid-section. Peak forces are close to those previously reported to be required to extract a stent graft. The forces and movements increase with increasing graft angulation and perfusion pressure. Improved anchoring of the distal end of stent grafts may be considered. © 2013 European Society for Vascular Surgery.
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| 2. |
- Tong, J., et al.
(författare)
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Effects of Age on the Elastic Properties of the Intraluminal Thrombus and the Thrombus-covered Wall in Abdominal Aortic Aneurysms : Biaxial Extension Behaviour and Material Modelling
- 2011
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 42:2, s. 207-219
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Tidskriftsartikel (refereegranskat)abstract
- Objective: The intraluminal thrombus (ILT) present in the majority of abdominal aortic aneurysms (AAAs) plays an important role in aneurysm wall weakening. Studying the age-dependent elastic properties of the ILT and the thrombus-covered wall provides a better understanding of the potential effect of ILT on AAA remodelling. Materials and methods: A total of 43 AAA samples (mean age 67 6 years) including ILT and AAA wall was harvested. Biaxial extension tests on the three individual ILT layers and the thrombus-covered wall were performed. Histological investigations of the thrombi were performed to determine four different age phases, and to correlate with the change in the mechanical properties. A three-dimensional material model was fitted to the experimental data. Results: The luminal layers of the ILT exhibit anisotropic stress responses, whereas the medial and the abluminal layers are isotropic materials. The stresses at failure in the equibiaxial protocol continuously decrease from the luminal to the abluminal side, whereby cracks, mainly oriented along the longitudinal direction, can be observed in the ruptured luminal layers. The thrombi in the third and fourth phases contribute to wall weakening and to an increase of the mechanical anisotropy of their covered walls. The material models for the thrombi and the thrombus-covered walls are in excellent agreement with the experimental data. Conclusion: Our results suggest that thrombus age might be a potential predictor for the strength of the wall underneath the ILT and AAA rupture.
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| 3. |
- Fröjse, Rolf, et al.
(författare)
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Intestinal pHi studied with continuous saline tonometry during ischaemia and reperfusion in the pig.
- 2002
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 24:2, s. 150-155
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVE: To evaluate continuous saline tonometry for detection of progressive intestinal ischaemia and reperfusion in a porcine model. DESIGN: In eight anaesthetised pigs, small bowel mucosal pCO2 was recorded by means of two identical equipments for continuous saline tonometry and a standard tonometry balloon during ischaemia and reperfusion. RESULTS: Both systems of saline tonometry functioned stably during the four hour protocol ischaemia, although not significant until after 45 min for one of the tonometers. CONCLUSION: The equipment for continuous saline tonometry has a good reactivity, an accuracy comparable with standard tonometry.
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| 4. |
- Gasser, T. Christian, et al.
(författare)
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Biomechanical Rupture Risk Assessment of Abdominal Aortic Aneurysms : Model Complexity versus Predictability of Finite Element Simulations
- 2010
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 40:2, s. 176-185
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Tidskriftsartikel (refereegranskat)abstract
- Objective: Investigation of the predictability of finite element (FE) models regarding rupture risk assessment of abdominal aortic aneurysms (AAAs). Materials and materials: Peak wall stress (PWS) and peak wall rupture risk (PWRR) of ruptured (n = 20) and non-ruptured (n = 30) AAAs were predicted by four FE models of different complexities derived from computed tomography (CT) data. Two matching sub-groups of ruptured and non-ruptured aneurysms were used to investigate the usability of different FE models to discriminate amongst them. Results: All FE models exhibited a strong positive correlation between PWS and PWRR with the maximum diameter. FE models, which excluded the intra-luminal thrombus (ILT) failed to discriminate between ruptured and non-ruptured aneurysms. The predictability of all applied FE models was strengthened by including wall strength data, that is, computing the PWRR. The most sophisticated FE model applied in this study predicted PWS and PWRR 1.17 (p = 0.021) and 1.43 (p = 0.016) times higher in ruptured than diameter-matched non-ruptured aneurysms, respectively. Conclusions: PWRR reinforces PWS as a biomechanical rupture risk index. The ILT has a major impact on AAA biomechanics and rupture risk, and hence, needs to be considered in meaningful FE simulations. The applied FE models, however, could not explain rupture in all analysed aneurysms.
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| 5. |
- Polzer, S., et al.
(författare)
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The Impact of Intraluminal Thrombus Failure on the Mechanical Stress in the Wall of Abdominal Aortic Aneurysms
- 2011
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 41:4, s. 467-473
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: The role of the intraluminal thrombus (ILT) in abdominal aortic aneurysm (AAA) rupture is controversial, and it is still not clear if an ILT increases or decreases AAA rupture risk. Specifically, signs of bleeding in the ILT are considered to increase AAA rupture risk. to further explore this hypothesis, intact AAAs (n = 4) with clear signs of fissures in the ILT, identified by computed tomography angiography (CTA) were investigated. Methods: Two different cases of ILT fissuring were investigated, where (1) ILT fissures were extracted directly from the CTA data and (2) a hypothetical fissure was introduced in the otherwise-intact ILT tissue. Wall stress distributions were predicted based on detailed Finite Element (FE) models. Results: ILT fissures extracted from CTA data locally increase the mechanical stress in the underlying wall by up to 30%. The largest impact on wall stress was observed if the ILT crack reaches the aneurysm wall, or if it involves large parts of the ILT. By contrast, a concentric failure in the medial ILT, which does not reach the aneurysm wall, has almost no impact on wall stress distribution. Hypothetical ILT fissures that connect the lumen with the wall cause a twofold increase of the stress in the underlying wall. Conclusions: ILT fissures increase the stress in the underlying wall, whereas regions other than that remain unaffected. If ILT fissures reach the wall or involve large parts of the ILT, the resulting increase in wall stress could possibly cause AAA rupture.
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| 6. |
- Roos, Håkan, et al.
(författare)
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Displacement Forces in Stent Grafts: Influence of Diameter Variation and Curvature Asymmetry
- 2016
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 52:2, s. 150-156
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: Long-term durability after endovascular aortic repair is influenced by stent graft migration causing types I and III endoleaks. Flow induced displacement forces have been shown to have the potential to cause migration. In this study, the influence of the distal diameter of iliac limb stent grafts and the shape of graft curvature on flow induced displacement forces, were investigated. Methods: In an experimental pulsatile flow model mimicking aortic conditions in vivo, flow induced displacement forces at the proximal and distal ends of iliac limb stent grafts were studied at different angles (0-90 degrees) and perfusion pressures (145/80, 170/90, 195/100 mmHg). Bell-bottomed, tapered, and non-tapered stent grafts and also asymmetric stent graft curvatures at 90 bend were studied. Measurements of graft movement were performed at all studied angulations and graft shapes. Results: For all stent graft diameters, flow induced displacement forces increased with higher pressure and increased stent graft angulation. Forces in the bell-bottom graft were considerably higher than in tapered and non-tapered grafts, with a markedly elevated peak force at the distal end (proximal end, 2.3 +/- 0.06 N and distal end, 6.9 +/- 0.05 N compared with 1.7 +/- 0.08 N and 1.6 +/- 0.08 N in non-tapered grafts; p <.001 both). Peak forces in tapered and non-tapered grafts were not significantly different between the proximal and distal end. In asymmetric stent graft curvatures, a significant increase in displacement forces was observed in the attachment zone that was closest to the stent graft bend. Graft movement increased with greater displacement forces. Conclusion: Flow induced displacement forces in iliac limb stent grafts are significant and are influenced by distal stent graft diameter and the shape of the graft curvature. The displacement forces are particularly high at the large distal end of bell-bottom grafts. Wide iliac arteries treated with bell-bottom stent grafts may require more vigilant surveillance and improved stent graft fixation.
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| 7. |
- Roos, Håkan, et al.
(författare)
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Fluid Pressure Derived Force is the Main Contributor to Iliac Limb Displacement Forces – Shear Force and Redirection of Flow are Negligible
- 2019
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Ingår i: European Journal of Vascular and Endovascular Surgery. - : Elsevier BV. - 1078-5884 .- 1532-2165. ; 58:6 (supplement 1), s. e18-e19
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Tidskriftsartikel (refereegranskat)abstract
- Introduction - Complications after EVAR, including aortic rupture, continue to be a problem also in long term follow-up1. There is still a need for life-long surveillance and late re-interventions raises the question about long term EVAR-durability. Additional iliac stent grafts due to distal endoleak type I are among the more frequent among these late re-interventions2,3,4. There are indications that graft migration at iliac landing zones and graft interconnections are among the most common causes and that wide iliac arteries predispose for these complications5. Flow induced displacement forces have, in an experimental model, been shown to be significant also at distal landing zones in EVAR stent grafts6 and particularly high in grafts with large distal diameters7. These findings indicate that late EVAR failures are linked to flow induced displacement forces and subsequent graft migration. We have therefore conducted a study of fluid structure interaction with the aim to differentiate the magnitude of the different force components and numerically evaluate the forces acting on iliac limb stent grafts in different graft configurations. Methods - The displacement forces in iliac limb stent grafts were numerically evaluated using a finite volume approach for fluid-structure interaction (FSI) with the open source tool FOAM-extend-3.1. The grafts were modelled with homogeneous properties in three configurations; tapered (16 mm proximal diameter, 12 mm distal diameter), non tapered (16-16 mm) and bellbottom (16-27 mm), all at 90° angulation (see 16-27 in figure). Experimentally determined pulsatile conditions were applied for different pressures; 145/80, 170/90, 195/100 mmHg. Results - Stent graft displacement forces increased with higher fluid pressure in all graft configurations. Maximum forces in bellbottom grafts (proximal end 2.3 N, distal end 8.1 N) were particularly high compared to tapered (proximal end 2.5 N, distal end 1.2 N) and non tapered grafts (proximal end 2.5 N, distal end 2.5 N). The effects of shear stress and redirection of flow were together less than 2.8 % of the total forces in all graft configurations, whereas pressure derived forces were the main contributor to the forces acting on the stent graft. The flexibility of the stent graft absorbed up to 15 % of the forces. Conclusion - The forces acting on stent grafts during pulsatile flow are significant and particularly high in the distal end of grafts with large distal diameter. The main contributor to these displacement forces is pressure while the contribution of shear and redirection of flow are negligible. The flexibility of the stent graft tends to absorb a significant part of the forces.
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