Biomedical Engineering Reference
In-Depth Information
5.4.3 Design stress analysis
Other key design considerations include the stresses that the design places on the
tissue. Minimizing stress is an important design objective that will improve
durability and long-term use. This consideration is so important, in fact, that it is
a required analysis of a new heart valve design to conduct a finite element
analysis (FEA) of the design (Sacks et al., 2006; Sacks and Yoganathan, 2007).
The PERIMOUNT
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valve has a central opening at rest, and this is because the
leaflet design has been developed to take into account the 5ë angulation of the
stent posts and the flexible nature of the stent material, which provides some
flexure during peak stresses, such as valve closure. Because the commissures
deflect inward under peak stresses (Drury et al., 1986), the leaflets move
forward, closing the central opening to create a competent valve during closure.
If the leaflets were designed to be fully closed at rest (zero pressure condition),
then once the valve was pressurized, the leaflets would be too large and thus
would necessarily rub against each other excessively, particularly in the center
(Fisher and Wheatley, 1987). This situation does not hold true for many
percutaneous valves, however, where now the stent is held rigidly to the aortic
wall and there are no commissures to flex under the load. In this case, the
leaflets have to be designed to be closed at rest or the valve would be
incompetent under pressurization (Webb et al., 2009).
5.4.4 Hemodynamics
Hemodynamics is another key attribute to a successful design, and the hemo-
dynamics of a valve can be evaluated both in vitro and in vivo. When tested
outside the bloodstream, this is called hydrodynamic testing, and it is
commonplace for every valve to be optimized utilizing a pulsatile flow loop
which exposes the valve to normal pressurization cycles and measures a variety
of valve parameters, including the resistance to valve opening, the amount of
fluid leaking through the valve before valve closure, and the pressure differential
across the valve (Walker et al., 1986; Yoganathan and Woo, 1986). These
parameters are shown diagrammatically in Fig. 5.10. Viewing the valve under
normal cycling conditions also enables the designer to observe the leaflet
motion, or kinematics, to ensure proper valve opening and closing behavior. As
discussed in the area of leaflet design and stent movement, it is important to
observe the valve design and ensure that the individual leaflets actually touch
each other during closure. The area where the leaflets touch is called the
coaptation area. If the leaflets do not coapt completely, the valve will leak even
when it is closed. Excessive coaptation area can result in leaflet `fluttering'
where the ends of the leaflet flutter in the flow, like a flag flutters in the wind.
Fluttering is very damaging to tissue and can result in a tattered and torn edge.
Figure 5.11 contains a series of photos of both a standard PERIMOUNT
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valve
