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The pericardial bioprosthesis: altered tissue shear properties following glutaraldehyde fixation
D R Boughner1, M Haldenby, A J Hui
1John P. Robarts Research Institute, Department of Medical Biophysics, University of Western Ontario, London, Canada.
Background And Aim Of The Study:
Glutaraldehyde-fixed bovine pericardial tissue used in the construction of valvular bioprostheses undergoes repeated bending stress during the cardiac cycle. To bend smoothly, internal tissue shearing is required. The effect of glutaraldehyde fixation on internal shear 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 apparatus. 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 degrees C bath; similar measurements were made on six fresh and six fixed samples at 37 degrees C. In addition, 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-linear 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 results were identical in radial or circumferential directions. For glutaraldehyde-fixed pericardium (n = 12), shear modulus increased promptly to 15-20 kPa at a strain of 0.2, 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 minimal resistance developing until the shear strain exceeded 0.5, while glutaraldehyde-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 marked tissue stiffening may be a factor in collagen fiber disruption, leading to bioprosthetic heart valve failure.