INTERNAL SHEAR PROPERTIES OF FRESH PORCINE AORTIC-VALVE CUSPS - IMPLICATIONS FOR NORMAL VALVE FUNCTION

Background and aims of the study: Several types of stress act on aorti c heart valve tissue during the cardiac cycle. When closed the valve i s subjected to primarily tensile stress due to the diastolic pressure, and upon opening bending stress occurs near the attachment with the ao rtic root and throughout the body of the cusps, Smooth bending require s internal tissue shearing. To measure the internal shear properties o f the tissue a testing device was created which combined a high-precis ion linear actuator with a sensitive load cell. Materials and methods: Circular punch biopsy specimens from fresh porcine aortic valve cusps (n = 32) were examined. The shear stress versus shear strain characte ristics were measured both in the circumferential (n = 17) and the rad ial (n = 13) direction, and the stress relaxation characteristics were also examined circumferentially (n = 15) and radially (n = 15). In ad dition seven specimens were tested repeatedly in both radial and circu mferential directions for tissue isotropy. Results: The results from t he shear stress versus strain tests showed the tissue to behave non-li nearly over the strain range between -0.9 and 0.9. The average moduli at the near zero strains were less than 300 Pa and increased to over 2 0 kPa at the extreme strains. The circumferential direction yielded sl ightly higher average moduli than the radial direction but this differ ence was not significant, The stress relaxation results indicated that valve tissue relaxation occurs in two distinct phases, an initial low slope region and a second high slope region with respective values of 7.5 log(s)(-1) and -15 log(s)(-1) and with no significant difference between test directions. Conclusions: Our results define and describe the pattern of internal shear properties of the aortic valve that are particularly important during the transition between the open and clos ed positions. This behavior pattern has particular application in the creation of accurate mathematical models of the valve tissue and may b e important in understanding the mechanism of tissue failure in biopro sthetic valves.