The finite element method and the aorto coronary bypass: a new approach to correlate fluid dynamics and intimal hyperplasia.

Intimal hyperplasia and atherosclerosis have a predominant role in the failure of aorto coronary bypass procedure. Theoretical studies and in vivo observations have shown that these pathologies are much more likely to occur in the coronary artery close to the end-to-side anastomosis. In this region fluid dynamics are alterated by the surgical solution; this suggest that fluid dynamic conditions (in particularly the wall shear stress) may be included in the causes of intimal hyperplasia. To correlate intimal hyperplasia and wall shear stress, a model of plaque growth present in the literature was employed and applied to two-dimensional fluid dynamic models based on the finite element method (FEM). The obtained values of shear stress were used in the model to calculate the intima thickening in the coronary artery. To take into account the changes of flow field due to the growth of intima, the geometry of the artery was updated during the course of the simulation. The studied models differed for the angle of bypass insertion (45, 60 and 90°), the flow rate from the bypass and the degree of stenosis of the native coronary artery. Results showed the growth of intima in the area of anastomosis, especially on the wall opposite to the surgical suture and on the upper wall immediately downstream from the anastomosis. High angle of insertion (90°) led to a fast reocclusion of the vessels, while low angle (45°) induced an intimal thickness without sensible reduction of the vessel lumen. A three-dimensional model of aorto coronary bypass has been set up to study the correct distribution of shear stresses. Results showed that the highest shear stresses were observed on the lateral wall of the coronary artery immediately downstream of the anastomosis. The physiologic values of the shear stresses are obtained for the lowest angle of insertion (45°). All the results indicate that, under the hypoteses of the plaque growth model, the geometry of the anastomotic junction may be of crucial importance in the reocclusion of the coronary artery.