Soft Tissue Replacements - Biomaterials - page 197

FIGURE 9.6 Photographs showing pitting on pyrolytic carbon surface of a mechanical heart valve. (Courtesy of
Baxter Health Care, Irvine, CA.)
This study demonstrated that for the valves of the same geometry (e.g., tilting disk) and size, the leaflet
edge velocity as well as the negative pressure transients were similar. However, the presence of cavitation
bubbles depended on the local interaction between the leaflet and the seat stop. Hence, it was pointed
out that magnitudes of leaflet velocity or presence of pressure transients below the liquid vapor pressure
might not necessarily indicate cavitation inception with mechanical valve closure. Chandran et al. (1998)
have also demonstrated the presence of negative pressure transients in the atrial chamber with implanted
mechanical valves in the mitral position in animals, demonstrating that the potential for cavitation exists
with implanted mechanical valves. Similar to the in vitro results, the transients were of smaller magni-
tudes with the Chitra valve made of flexible leaflets, and no pressure transients were observed with tissue
valve implanted in the mitral position in vivo . The demonstration of the negative pressure transients with
mechanical valve closure also shows that this phenomenon is localized and the flow chamber or valve
holder rigidity with the in vitro experiments will not affect the closing dynamics of the valve.
The pressure distribution on the leaflets and impact forces between the leaflets and guiding struts
have also been experimentally measured in order to understand the causes for strut failure (Chandran
et al., 1994b). The flow through the clearance between the leaflet and the housing at the instant of valve
closure (Lee and Chandran, 1994a,b) and in the fully closed position (Reif, 1991) and the resulting
wall shear stresses within the clearance are also being suggested as responsible for clinically significant
hemolysis and thrombus initiation. Detailed analysis of the complex closing dynamics of the leaflets
may also be exploited in improving the design of the mechanical valves to minimize problems with
structural failure (Cheon and Chandran, 1994). Further improvements in the design of the valves based
on the closing dynamics as well as improvements in material may result in minimizing thromboembolic
complications with implanted mechanical valves.
9.1.2.2 Biological Heart Valves
The first biological valves implanted were homograts with valves explanted from cadavers within 48 h
after death. Preservation of the valves included various techniques of sterilization, freeze-drying, and
immersing in antibiotic solution. The use of homografts is not popular due to problems with long-
term durability and due to limited availability except in a few centers (Shim and Lenker, 1988; Lee and
Boughner, 1991). Attempts were also made in the early 1960s in the use of xenograts (valves made from
animal tissue), and porcine bioprostheses became commercially available after the introduction of the
glutaraldehyde (rather than formaldehyde, which was initially used) fixation technique. Glutaraldehyde
reacts with tissue proteins to form crosslinks and results in improved durability (Carpentier et al., 1969).