Improving vascular grafts: the importance of mechanical and haemodynamic properties.

In the last 40 years, as techniques and materials have improved, the success rate of vascular prostheses with a diameter greater than 6mm has risen steadily, 5-year survival rates exceeding 95% in most centres. With smaller grafts no comparable improvement has occurred, the majority failing within 5 years, usually as a result of intimal hyperplasia and, ultimately atherosclerosis, in and around the downstream anastomosis. Clinical evidence suggests that the patency rates of small grafts are improved by matching the elastic properties of the graft to that of the artery into which it is placed. Although there is little reliable evidence that 'elastic mismatch' per se is the cause of intimal hyperplasia, it is generally accepted that mechanical factors are important in its genesis. These include disturbed flow at the anastomosis leading to fluctuations in shear stress at the endothelium (a known cause of intimal hyperplasia in normal arteries), injury due to suturing and stress concentration at the anastomosis. Few suitable materials or techniques have yet been developed to improve the long-term survival rates of small grafts. Recent advances in tissue engineering in which prostheses are manufactured by culturing vascular smooth muscle cells on a tubular scaffold of biodegradable polymer may ultimately make it possible to manufacture biologically and haemodynamically compatible grafts with diameters as small as 1mm.