Developments in cardiovascular valve technology - Biomaterials and Devices for the Circulatory System - page 93

mitral position. The infl ow strut passes through a small hole in the middle
of the disc, affording free rotation, and the outfl ow strut keeps the disc
in place. The inner surface of the metal ring has two small projections
that prevent disc opening beyond the prescribed angle. The Medtronic-
Hall valve has a larger minor orifi ce than other tilting-disc valves, 46 a
design feature that allows the disc to move out of the housing and rotate
on opening, permitting a better 'washing effect' of areas of low fl ow
velocity.
The Medtronic-Hall valve has a long-term record of durability, and a low
incidence of valve-related complications. A prospective study by Akins
found actuarial freedom from reoperation at ten years was 97% for aortic
and 88% for mitral implants of tilting disc devices. 47 Two large recent ret-
rospective series in Europe 46,48 document the excellent hemodynamics of
the Medtronic-Hall valve and the low incidence of all valve-related com-
plications, especially the very low incidences of thromboemboli and valve
thrombosis and the total absence of mechanical valve malfunction over a
20-25 year period. Butchart et al. 46 argue that the superior hemodynamics
and durability of this prosthesis make it the gold standard against which
other prostheses should be judged.

4.3.4 Bileafl et valves

In the late 1970s the bileafl et valves, composed of pyrolytic carbon-based
leafl ets, were introduced. Currently the St. Jude, Carbomedics, ATS, On-X
and Sorin Bio-carbon devices are available for clinical implantation. These
models differ in the design of the hinge mechanism, as well as the opening
and closing angles of the leafl ets. The bileafl et valve has become the
mechanical prosthesis of choice for most surgeons (Table 4.1).
The St. Jude bileafl et valve (St. Jude Medical Inc., St. Paul, MN) design
is credited to D.M. Nicoloff at the University of Minnesota. The valve con-
sists of a pyrolyte carbon housing and a Dacron velour sewing cuff. It is
manufactured from one piece of machined graphite and coated with pyro-
lytic carbon without seams or welds. The hinge mechanism consists of two
leafl ets held in position by two lateral protrusions at the leafl et edges which
fi t into two corresponding butterfl y shaped pivot recesses located on the
inner aspect of the valve ring (Fig. 4.8). The leafl ets meet the housing at a
30° angle and open to an angle of 85°. There is a washing effect as the blood
fl ows across the space between the leafl ets, thereby reducing the possibility
of clot formation in the recesses. Since the St. Jude valve was fi rst released
on the market, its sewing cuff has been redesigned twice to reduce its bulk,
while the basic structure of the housing and leafl ets was unchanged. These
refi nements, which spawned the Hemodynamic Plus (HP) and Regent valve
series, expanded the effective valve orifi ce area to 1.5 times that of the

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