Flexible Leaflet Polymeric Heart Valves - Cardiovascular and Cardiac Therapeutic Devices - page 108

Fig. 7 Catheter deliverable polymeric heart valves, from left to right: a Mecora PU/Nitinol
trileaflet design [ 120 ], b Tilting disk PU/Z-stent design by Sochman [ 122 ], c Hashimoto PU/
Nitinol umbrella shaped valve [ 123 ], and d Claiborne SIBS/Dacron/Nitinol trileaflet valve [ 124 ]
(with permissions)
Although these are early examples of attempts to produce catheter based valves
from polymers as opposed to bioprosthetic tissue, the potential advantage of very
small potential crimp sizes (10-14F vs. 18-19F for pericardial valves) is clear.
5 Valve Geometry
As the manufacture of polymeric valves provides greater freedom in design than is
the case with pericardial valves (where leaflets are fashioned out of essentially flat
sheets of tissue), a discussion on the geometry of the natural valve, and attempts to
mimic it or provide other optimised geometries, is given in this section.
It is not surprising that the valves of the heart that are unsupported by cordae
(and one that is) are of trileaflet design, as this configuration has been shown to be
superior in stress distribution to bi- and quarto-leaflet analogues [ 125 ]. The central
flow of the trileaflet design also has better mechanical efficiency, hydraulic
characteristics and flow patterns that cause much less blood trauma than
mechanical devices [ 126 ]. In trileaflet designs, the total free edge length of a
closed leaflet (1x diameter) is very close to the cord that suspends the leaflet (pD/
3 & D) with a planar free edge design thereby allowing greater opening area than
in a bileaflet design, as pointed out by Roe et al. [ 26 ]. This is not to say that bi and
quatro-leaflet designs cannot be successful, as seen with the bileaflet PU valve [ 57 ,
58 ] and quatrovalve bioprosthetic design [ 3 ].
The elliptical natural aortic sinuses [ 127 ] cause recirculating flow and provide
rapid closure of leaflets with negligible backflow [ 38 ]. Aortic leaflets themselves
have historically been considered to be portions of spherical surfaces, and
described as semilunar by Philistion of Locri (ca. 427-ca. 347 BC), sigmoid by
Erasistratus of Ceos (ca. 304-ca. 250 BC), as three adjacent hemispheres by
Leonardo da Vinci (1452-1519) during the Renaissance, and by Retzius (in 1843)
as three interlocking circles as seen from above [ 128 ]. More recently, Mercer (in
1973) approximated the leaflet shapes as paraboloids of revolution with foci at the
base of the leaflets [ 72 , 128 ], while Hamid and co-workers described the geometry
as an elliptic paraboloid [ 129 ].