Numerical Assessment of Novel Helical/Spiral Grafts with Improved Hemodynamics for Distal Graft Anastomoses

Abstract

In the present work, numerical simulations were conducted for a typical end-to-side distal graft anastomosis to assess the effects of inducing secondary flow, which is believed to remove unfavourable flow environment. Simulations were carried out for four models, generated based on two main features of 'out-of-plane helicity' and 'spiral ridge' in the grafts as well as their combination. Following a qualitative comparison against in vitro data, various mean flow and hemodynamic parameters were compared and the results showed that helicity is significantly more effective in inducing swirling flow in comparison to a spiral ridge, while their combination could be even more effective. In addition, the induced swirling flow was generally found to be increasing the wall shear stress and reducing the flow stagnation and particle residence time within the anastomotic region and the host artery, which may be beneficial to the graft longevity and patency rates. Finally, a parametric study on the spiral ridge geometrical features was conducted, which showed that the ridge height and the number of spiral ridges have significant effects on inducing swirling flow, and revealed the potential of improving the efficiency of such designs.

Fig 1. (a) Schematics of the four geometric models used in the present study; (b) Dimensions of the four geometric models used in the present study; (c) Cross-sectional view of the ridge profile used in the ‘Spiral’ and ‘Helical+Spiral’ models (the dimensions shown are of the ellipse that was used to create the ridge.

The centre of the ellipse was located at the surface of the graft wall and kept perpendicular to the axis of the graft). All dimensions are in mm.

Fig 2. Images of the finite volume mesh used in the present study.

(a) Cross-sectional view of the host artery distal of the anastomosis (taken at Plane 1); (b) Cut-away of mesh at the anastomosis (taken at Plane 2). Note that while this image corresponds to Model 4, the mesh for the other 3 models have very similar resolution.

Fig 3. The waveform of the blood flow through the Superficial Femoral Artery (SFA) graft.

Fig 4. Qualitative comparison of the normalised secondary velocity magnitude against the experimental data of [31]: (a) the present numerical simulations; (b) the experimental data.

The results correspond to a plane positioned 5 mm distal from the toe of the anastomosis and Re = 1140.

Fig 5. Comparison of the secondary and axial velocity magnitude at approximately peak flow phase (t₁ = 0.25s) at four monitoring planes for all four models.

Fig 6. Comparison of the secondary and axial velocity magnitude at approximately reversed flow phase (t₂ = 0.41s) at four monitoring planes for all four models.

Fig 7. λ₂ iso-surface of vortices within the present models at peak flow phase (t₁ = 0.25s) and reversed flow phase (t₂ = 0.41s), with λ₂ iso-value thresholds of 130,000 and 5,000, respectively.

Cross-sectional view at monitoring plane 3 (i.e., 5mm distal from the toe of the anastomosis) is also shown next to each model.

Fig 8. Distributions of different hemodynamic parameters viewed from the host arterial wall, opened ventrally and shown en face: (a) Axial Time-Averaged Wall Shear Stress (TAWSS_Axial); (b) Radial Time-Averaged Wall Shear Stress (TAWSS_θ); (c) Radial Time-Averaged Wall Shear Stress Gradient (TAWSSG); (d) Oscillatory Shear Index (OSI); and (e) Relative Residence Time (RRT).

Note that the colour scale of the TAWSS maps in (a) and (b) are inverted for ease of comparison.

Fig 9. Distributions of normalised wall shear stress on an unfolded model of the host artery for different spiral ridge designs.