Internal shear properties of fresh porcine aortic valve cusps: implications for normal valve function

Several types of stress act on aortic heart valve tissue during the cardiac cycle. When closed the valve is subjected to primarily tensile stress due to the diastolic pressure, and upon opening bending stress occurs near the attachment with the aortic root and throughout the body of the cusps. Smooth bending requires internal tissue shearing. To measure the internal shear properties of the tissue a testing device was created which combined a high-precision linear actuator with a sensitive load cell.

Circular punch biopsy specimens from fresh porcine aortic valve cusps (n = 32) were examined. The shear stress versus shear strain characteristics were measured both in the circumferential (n = 17) and the radial (n = 13) direction, and the stress relaxation characteristics were also examined circumferentially (n = 15) and radially (n = 15). In addition seven specimens were tested repeatedly in both radial and circumferential directions for tissue isotropy.

The results from the shear stress versus strain tests showed the tissue to behave non-linearly 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 20 kPa at the extreme strains. The circumferential direction yielded slightly higher average moduli than the radial direction but this difference 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.

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 closed positions. This behavior pattern has particular application in the creation of accurate mathematical models of the valve tissue and may be important in understanding the mechanism of tissue failure in bioprosthetic valves.