FLOW DYNAMICS IN PERIPHERAL BYPASS ANASTOMOSES: OPTIMUM DESIGN

Cooperative study
TU-Graz; Inst. of Mathematics: K. Perktold, A. Leuprecht, M. Prosi, T. Berk
Univ.of Vienna, Dept. Vascular Surgery: W. Trubel
Univ. of Vienna, Center of Biomed. Research: H. Schima

Experimental-surgical study (long term animal study) and Fluid dynamical / wall mechanical study (mathematical modeling, computer simulation)

Apart from cell biological and biochemical influences on Intima Hyperplasia (IH) in synthetic prostheses reactive mechanisms resulting from non-physiological conditions play a certain role in the disease process.

The aim of the study: Investigation of the relationship between

Flow patterns
Mass transport processes
Mechanical stresses

and

postoperative IH

in established types of distal end-to-side anastomoses using e-PTFE prostheses related to different surgical techniques.

Different graft-artery connector types should be classified with respect to long term function of vascular reconstructions.

COMPUTATIONAL GEOMETRIC MODELS, Mechanical structure

Conventional end-to-side-anastomosis PTFE graft is directly connected to the artery

Miller-cuff anastomosis cylindrical venous cuff interposed

Taylor-patch anastomosis venous patch interposed

Development based on data of luminal casts of femoral bypasses implanted in sheep.
Material properties:
determined from exp. deformation results.
Elastic moduli: E_v = 2×E_a, E_g = 18×E_a
Ž Compliance mismatch

AXIAL AND SECONDARY VELOCITY

Jet flow more pronounced in Miller-cuff a.
in-plane velocity reflects the non-planarity
vortex in the entire cross-section Ž wash-out effect (= qualitative difference)

MAXIMUM PRINCIPAL STRESSES

continuous suture p = 100 mmHg

Medium size titanium VCS clips, clips are modeled as stiff springs connecting two shell structures at discrete locations, between the clips the two structures are not attached simulating the situation prior any healing.


Conv. vein graft anastomosis	Conv. PTFE graft anastomosis

Taylor patch anastomosis

Miller-cuff anastomosis

Compliance mismatch Ž stress discontinuity at the suture lines:

PTFE graft - artery
PTFE graft - vein.

Higher stress occurs in the stiffer material.

Better matching: vein - artery

CONTINUOUS SUTURE

Model: 3D patch inserted in the shell structure (limitation of computational efforts) Appropriate constraints along the common b. guarantees the correctness.

Conventional anastomosis	Miller-cuff anatomosis

Suture material: Prolene 6.0 (d = 0.093 mm), Suture pre-stress: 2000 dyne

Numerical suture model: Truss elements (ABAQUS): transmit only axial force, 2 node straight truss, uses linear interpolation for position and displacement.

Miller-cuff anastomosis: p = 100 mm Hg


	log. scale

Suture stress:

PTFE graft - vein cuff: 3484 dyne
Vein cuff - artery: 4244 dyne
(PTFE graft - artery: 2950 dyne)

Consequence of larger deformation suture stress is higher.

CONCLUSION

IH: On the artery floor in the anastomotic region and along the sutureI lines of the junctions.

Flow field:
The simulation demonstrates the influence of asymmetries and local geometric irregularities on the flow and WSS patterns

The in-vivo data support the advantage of the flow field characteristics in the arterial floor area of the Miller cuff.

Stress field:
More compliant materials results in

lower principal stresses at the suture lines
lower IH around the suture lines can be observed.

The wall mechanics and the compliance mismatch of the materials seems to play the mayor role in the development of IH.

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