The pericardial bioprosthesis: Altered tissue shear properties following glutaraldehyde fixation

Background and nim of the study: Glutaraldehydefixed bovine pericardial tis sue used in the construction of valvular bioprostheses undergoes repeated b ending stress during the cardiac cycle. To bend smoothly, internal tissue s hearing is required. The effect of glutaraldehyde fixation on internal shea r properties of this material was examined. Methods: Pericardium from each of 12 bovine hearts was cut into two pieces; one piece was retained as fresh tissue, the other was glutaraldehyde-fixed . Circular samples were then mounted and installed in a shear testing appar atus. For each sample, the shear stress versus shear strain characteristics were measured in circumferential and radial directions at strain rates of 1.0, 0.1 and 0.02 s(-1) while immersed in a 20 degreesC bath; similar measu rements were made on six fresh and six fixed samples at 37 degreesC. In add ition, the stress relaxation properties were measured by holding the tissue at maximum shear for 100 s after each of the three shear deformations, and recording force generated with time. Results: The shear stress-strain test on fresh tissue (n = 12) showed non-l inear behavior at the three shear rates. The shear modulus for fresh tissue increased from <1.0 kPa to 5 kPa at a shear strain approaching 1.0, and re sults were identical in radial or circumferential directions. For glutarald ehyde-fixed pericardium (n = 12), shear modulus increased promptly to 15-20 kPa at a strain of 0.5 and did not vary with strain rate. Shear relaxation was similar in fresh and fixed tissue. Conclusion: Fresh pericardium sheared easily at low shear stresses, with mi nimal resistance developing until the shear strain exceeded 0.5, while glut araldehyde-fixed tissue displayed a marked resistance to shearing, with an immediate rise in shear stress at low strain. No differences were detected in shear properties between radial and circumferential directions. Such mar ked tissue stiffening may be a factor in collagen fiber disruption, leading to bioprosthetic heart valve failure.