Differences between heart valve allografts and xenografts in the incidence and initiation of dystrophic calcification

Following surgical removal because of primary tissue failure, 30 antibiotic-sterilized human aortic valve allografts and 27 glutaraldehyde-treated porcine aortic valve xenografts were examined for macroscopic and microscopic evidence of dystrophic calcification. These grafts had been mounted on stents and used for from 34 to 166 months to replace diseased mitral valves. After explantation the grafts were carefully examined then prepared for light microscopy, for transmission electron microscopy and for energy dispersive X-ray microanalysis. Gross calcification occurred significantly (p = 0.002) more frequently in xenografts (89%), and was more extensive than in allografts (53%). Calcification usually appeared as nodular excrescences on the cusps, although occasionally it formed plates within them. This reduced tissue pliability and was usually associated with either valvular stenosis or regurgitation. The calcified deposits contained calcium and phosphate in ratios approaching those of hydroxyapatite. In xenograft valves the smallest discrete deposits of calcification were spherical and usually associated with membranous debris of porcine donor fibroblasts, but allografts did not contain donor cell remnants and early calcification was linearly arranged along collagen fibres.