Effects of fixation back pressure and antimineralization treatment on the morphology of porcine aortic bioprosthetic valves.

Calcification frequently causes failure of porcine aortic valve bioprostheses; changes in collagen configuration induced by high-back-pressure fixation have deleterious effects on porcine aortic valve mechanics. Although modified porcine aortic valve bioprostheses that include the use of lower-pressure fixation and antimineralization treatments are used clinically, the morphologic characteristics of these valves are not known. We evaluated, by light and scanning and transmission electron microscopy, the comparative structural details of clinically processed Hancock Standard (no antimineralization treatment, 80 mm Hg fixation), Hancock II (T-6 antimineralization treatment, fixed initially at 1.5 mm Hg, then 80 mm Hg) and Intact (toluidine blue antimineralization treatment, zero-pressure-fixed [0 mm Hg]) porcine aortic valve bioprostheses as well as true low-pressure (1.5 mm Hg) fixed valves, zero-pressure-fixed porcine aortic valves (with no further treatment), and freshly fixed porcine aortic valve cusps as controls. Commercially processed valves had near-complete loss of endothelium and amorphous extracellular matrix and autolytic changes in the cuspal connective tissue cells. Both 80 and 1.5/80 mm Hg fixed valves, but not zero-pressure-fixed cusps (Intact valves, zero-pressure-fixed porcine aortic valves or immediately fixed porcine aortic valve cusps), had overall flattening and compression with near-complete loss of transverse cuspal ridges and collagen crimp; valves fixed at 1.5 mm Hg had intermediate features. T-6 and toluidine blue treatments induced no definite incremental microscopic changes attributable to the antimineralization treatment. No degenerative changes in collagen were noted in any valves that underwent antimineralization treatment. These studies indicate that valves fixed at zero but not at 80 or 1.5/80 mm Hg pressure retain collagen architecture virtually identical to that of relaxed native porcine aortic valve cusps and that the antimineralization treatments studied do not adversely affect collagen morphology.