Biomechanical changes during abdominal aortic aneurysm growth

Abstract

The biomechanics-based Abdominal Aortic Aneurysm (AAA) rupture risk assessment has gained considerable scientific and clinical momentum. However, such studies have mainly focused on information at a single time point, and little is known about how AAA properties change over time. Consequently, the present study explored how geometry, wall stress-related and blood flow-related biomechanical properties change during AAA expansion. Four patients with a total of 23 Computed Tomography-Angiography (CT-A) scans at different time points were analyzed. At each time point, patient-specific properties were extracted from (i) the reconstructed geometry, (ii) the computed wall stress at Mean Arterial Pressure (MAP), and (iii) the computed blood flow velocity at standardized inflow and outflow conditions. Testing correlations between these parameters identified several nonintuitive dependencies. Most interestingly, the Peak Wall Rupture Index (PWRI) and the maximum Wall Shear Stress (WSS) independently predicted AAA volume growth. Similarly, Intra-luminal Thrombus (ILT) volume growth depended on both the maximum WSS and the ILT volume itself. In addition, ILT volume, ILT volume growth, and maximum ILT layer thickness correlated with PWRI as well as AAA volume growth. Consequently, a large ILT volume as well as fast increase of ILT volume over time may be a risk factor for AAA rupture. However, tailored clinical studies would be required to test this hypothesis and to clarify whether monitoring ILT development has any clinical benefit.

Fig 1. Analysis method performed for each patient at each time point.

(a) Lateral Computed Tomography-Angiography (CT-A) slice with segmented Abdominal Aortic Aneurysm (AAA). Yellow, blue and green curves denote the luminal surface, exterior surface and wall-thrombus interface, respectively. (b) Rupture risk index plot derived from the structural biomechanics-based analysis at Mean Arterial Pressure (MAP). (c) Wall Shear Stress distribution at t = 0.25 s of the cardiac cycle derived from a Computational Fluid Dynamics (CFD) computation. At the inlet and the outlets, the indicated volume flow rate and pressure versus time responses were prescribed [36,37].

Fig 2. Development over time of the wall rupture risk index at Mean Arterial Pressure (MAP) in all four Abdominal Aortic Aneurysm (AAA) patients.

Fig 3. Development over time of the Wall Shear Stress (WSS) at t = 0.25 s of the cardiac cycle, i.e. at the time of peak blood inflow, in all four Abdominal Aortic Aneurysm (AAA) patients.

Note that this time point does not correlate with the time when WSS peaks within the aneurysmatic portion of the aorta.

Fig 4. Development of the maximum diameter d_max and the Peak Wall Rupture Index (PWRI) in Abdominal Aortic Aneurysm (AAA) patients A to D.

Each time point is labeled with the time in years from baseline. For comparison, the black solid curve denotes the PWRI versus d_max characteristics that in average is seen in AAA patients. Dotted curves denote the 5% and 95% confidence intervals, respectively.

Fig 5

Influence of the maximum diameter on (a) the diameter growth Δd_max, (b) volume growth ΔV_tot, (c) maximum Wall Shear Stress WSS_max over the cardiac cycle, and (d) Peak Wall Rupture Index PWRI at Mean Arterial Pressure (MAP). Influence of the Intra-luminal Thrombus (ILT) volume on (e) WSS_max over the cardiac cycle, and (f) PWRI at MAP.

Fig 6

Influence of the change of vessel volume ΔV_tot on (a) the Minimum shear rate ${\dot{\gamma }}_{\mathrm{m}\mathrm{i}\mathrm{n}}$ over the cardiac cycle, (b) Peak Wall Rupture Index PWRI at Mean Arterial Pressure (MAP). Influence of the Intra-Luminal Thrombus (ILT) volume growth rate ΔV_ILT on (c) maximum thickness of the ILT layer H_{ILT max}, and (d) PWRI at MAP.