Biomedical Engineering Reference
In-Depth Information
5.6 Prototypes of a new valve designed to fit within the aortic sinuses. Since
the sinuses are not circular, the valve is not circular. The trilobal design is
particularly evident from the inflow side of the valve. The advantages of such a
valve design are increased orifice area and improved hemodynamics. (Photos
reprinted with permission from Arbor Surgical Technologies, Inc. (now owned
by Medtronic).)
Stentless valves fabricated from isolated porcine aortic valves were the first
examples of commercial bioprostheses made from xenogeneic tissues. In some
aspects, porcine stentless valves are easier to fabricate because the manufac-
turing process does not alter the leaflet size, shape or alignment, as these issues
were determined by the pig itself. Stentless valves can also be fabricated
entirely from bovine pericardium. Porcine aortic valve isolations can be
trimmed down and fitted to a frame to create a porcine stented valve, although
care must be taken during the trimming process to avoid trimming too far and
thus weakening the attachment of the aortic leaflets to the wall tissue. In the
case of a stented porcine valve, the stent is there more for the surgical
implantation, rather than the leaflet support, as the leaflets are still attached to
the native aortic wall. Figure 5.7 shows a photo of a trimmed porcine aortic
valve isolation placed inside a metallic wire stent where the aortic wall was
trimmed too thin to permit long-term leaflet attachment during use. The high
pressure environment in the bloodstream placed a tremendous amount of force
on the leaflet at the tip of the commissure, and the trimmed aortic wall was
unable to provide sufficient attachment support to the leaflet under these
conditions. The solution to this problem is to avoid trimming the aortic wall so
thin during construction.
The most frequently implanted valve in use today is the Carpentier±Edwards
PERIMOUNT
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valve line (Lane, 1985). This valve design succeeded in part by
learning from others' failures, in particular, the failure of the Ionescu±Shiley
pericardial valve. Comparing these two valves highlights other important design
features, including the following:
