Pericardial heterografts: a comparative study of suture pull-out and tissue strength

The force required to pull sutures out of glutaraldehyde fixed bovine pericardium, for four different suture bites: 0.5 mm, 1 mm, 1.5 mm and 2.0 mm, was compared with the tissue strength. The mean suture pull-out force was significantly lower than the tissue strength for all bites, with a minimum value of 2.86 +/- 1.02 N for the 0.5 mm bite and a maximum value of 6.32 +/- 0.77 N for the 2.0 mm bite. The mean force which produced failure of the chemically modified pericardium was 15.49 +/- 8.48 N. The mean force at pull-out of the sutures lay on a regression line: force at failure = 1.68 + 2.25 x Bite. A video film of the experiments showed that the suture does not cut through the pericardium. It pulls a V-shaped band of collagen fibre bundles through the stationary pericardium. Eventually this band breaks away from the free edge of the tissue specimen. The specimens under uniaxial load failed by laminate debonding of two layers of tissue, rupture of the serosal surface layer followed by shear and fibre slippage. These results indicate that any suture which bears load, during the normal functioning of a heart valve substitute, will be a source of weakness, compared to the overall tissue strength. As a consequence the alignment/holding suture of the Standard Ionescu-Shiley valve and the modified stitch of the low profile valve are likely to be potential sites of fatigue failure.