Histological, scanning and transmission electron microscope studies were made of normal human dura mater and cardiac valvular
bioprostheses made of
glycerol-treated human dura mater recovered after having been implanted in the aortic position (8 patients) or the mitral position (1 patient) for periods up to 4 years. Human dura mater has two layers: an inner or meningeal layer and an outer or endosteal layer. The surface of the inner layer is smoother than that of the outer layer. Both layers are composed mainly of large, wavy
collagen fibrils (which are thought to correspond to
type I collagen) and are relatively poor in elastic fibers and
proteoglycans. Small calcific deposits were found in normal dura from older patients. Changes occurring in dura mater
bioprostheses within 2 days after implantation consisted mainly of small surface thrombi. Calcific nodules, degenerated
collagen and evidence of penetration of erythrocytes and
plasma proteins into the cusps were observed in
bioprostheses that had failed after being in place for 1-4 years. The calcific deposits and the degenerated
collagen appeared structurally similar to those in
glutaraldehyde-treated porcine aortic valvular
bioprostheses. However,
collagen fibrils in the latter were smaller than those in dura mater. Platelet aggregates on the cuspal surfaces were much less numerous in dura mater
bioprostheses than in porcine aortic valvular
bioprostheses. It is postulated that this difference is a function of the size of the
collagen fibrils in the
bioprostheses and that this accounts for the very low incidence of clinically evident
thromboembolism in patients with implanted dura mater valves.