Association Between Aneurysm Shoulder Stress and Abdominal Aortic Aneurysm Expansion A Longitudinal Follow-Up Study
ABSTRACT Aneurysm expansion rate is an important indicator of the potential risk of abdominal aortic aneurysm (AAA) rupture. Stress within the AAA wall is also thought to be a trigger for its rupture. However, the association between aneurysm wall stresses and expansion of AAA is unclear.
Forty-four patients with AAAs were included in this longitudinal follow-up study. They were assessed by serial abdominal ultrasonography and computed tomography scans if a critical size was reached or a rapid expansion occurred. Patient-specific 3-dimensional AAA geometries were reconstructed from the follow-up computed tomography images. Structural analysis was performed to calculate the wall stresses of the AAA models at both baseline and final visit. A nonlinear large-strain finite element method was used to compute the wall-stress distribution. The relationship between wall stresses and expansion rate was investigated. Slowly and rapidly expanding aneurysms had comparable baseline maximum diameters (median, 4.35 cm [interquartile range, 4.12 to 5.0 cm] versus 4.6 cm [interquartile range, 4.2 to 5.0 cm]; P=0.32). Rapidly expanding AAAs had significantly higher shoulder stresses than slowly expanding AAAs (median, 300 kPa [interquartile range, 280 to 320 kPa] versus 225 kPa [interquartile range, 211 to 249 kPa]; P=0.0001). A good correlation between shoulder stress at baseline and expansion rate was found (r=0.71; P=0.0001).
A higher shoulder stress was found to have an association with a rapidly expanding AAA. Therefore, it may be useful for estimating the expansion of AAAs and improve risk stratification of patients with AAAs.
- SourceAvailable from: Efstratios Georgakarakos
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- "More interestingly, the authors demonstrated that PWS (hazard ratio:25), outweighed gender (hazard ratio:3) as a rupture predictive tool and that after accounting for wall stress and gender, diameter would not predict rupture . Furthermore, low stress values have been associated with slow growth rate in small AAAs , whereas rapid growth rate of small AAAs can be strongly related with high values of wall stress at the inflection site between the neck and the sac . "
ABSTRACT: Abdominal Aortic Aneurysms (AAAs) are focal dilation of the aorta that can lead to excessive enlargement and rupture over time. Current practice suggests intervention when the maximum diameter exceeds 5.5 cm, since in this diameter range the annual rupture risk outweighs the operative mortality. However, small AAA (<5.5 cm), though infrequently, may rupture or produce symptoms. Evidence from large randomized studies of small AAAs support the heterogeneity in patterns of growth and rupture potential among small AAAs. Elevated wall stress values have been implicated in AAAs rupture and rapid enlargement. Additionally, many studies have identified a strong correlation between certain geometric factors and elevated stress values. In this article we discuss the possibility that geometrical factors may have a predictive value to identify those small AAAs that have an increased risk of rupture or growth rate either during initial examination or during follow-up, making them amenable for early repair.Medical Hypotheses 04/2012; 79(1):71-3. DOI:10.1016/j.mehy.2012.04.003 · 1.07 Impact Factor
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ABSTRACT: Inflammation and the associated recruitment of monocytes/macrophages into the aortic wall is critical in the pathology of abdominal aortic aneurysm (AAA) disease. The mechanisms behind this process are ill defined though wall shear stress (WSS) is known to play a role in leukocyte recruitment. In this study patterns of monocyte deposition and the probability of recruitment in AAA are determined numerically and compared with distributions of WSS. Finite element computational fluid dynamic (CFD) modelling was used to model blood hemodynamics, track monocyte behaviour and determine the magnitude of wall shear stress in rigid walled geometries of generalised AAA dimensions. Pulsatile flow was simulated using a one-way coupled Eulerian-Lagrangian simulation with near-wall particle lift and drag modification. Particle tracking was combined with a non-dimensional distance-to-wall dependent residence time model to observe the probability of cell adhesion.Patterns of WSS were closely related to vortex formation and dissipation in the AAA and monocyte NWPRT was highest in areas of vortex dissipation. Since areas of high deposition overlap with sites of high WSS and WSS is a limiting factor in monocyte recruitment, a WSS-limiter was applied to the NWPRT model.
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ABSTRACT: It is well known that, in the TEM approximation, the electrical characteristics of microstrips couplers are easily obtained, if we assume the equality between the odd, even modes phase velocities. In the other hand the directivity of such couplers is considerably decreased when the gap between these velocities increased. So, it can be very useful to equalize them. This result can be achieve by using a dielectric overlay or an upper ground plaine symetrical of the lower one with regard to the conductors. This last result can be generalized to a n-parallel microstrip device. Advantages, disadvantages of these two technics are discussed, experimental results are given.Microwave Symposium Digest, 1978 IEEE-MTT-S International; 07/1978