Developments in cardiovascular valve technology - Biomaterials and Devices for the Circulatory System - page 105

housing assembly and struts were carved out of a solid piece of Stellite. In
1981 the last design modifi cation consisted in building only one outfl ow
strut; accordingly, the prosthesis currently in use is called the Monostrut
Björk-Shiley valve. This change allows the disc to open at a 70° angle,
resulting in better hemodynamics. 78
Introduced in 1977, the St. Jude Medical prosthesis is a bileafl et design
composed of a pyrolite housing and leafl ets and a Dacron sewing ring (Fig.
4.8). The leafl ets are held in position by two lateral protrusions contained
in a butterfl y shaped recession in the inner side of the ring. In the closed
position the leafl ets meet the housing at 30° angles. The leafl ets open to 85°,
providing a central and two lateral orifi ces through which the blood passes
in a central laminar fl ow. The orifi ce : tissue annulus diameter ratio has excel-
lent hemodynamics. The leafl ets are impregnated with tungsten, a radio-
opaque material which allows visualization under X-ray or fl uoroscopy. 79

4.5.2 Thrombogenicity

It has been well recognized since the dawn of cardiac surgery that valve
replacement imposes a risk of thromboembolism. This complication is asso-
ciated with both prosthetic mechanical and biological valves. With progres-
sive improvements in design and materials over the last four decades, there
has been a signifi cant reduction in thromboembolic events. Moreover, not
all embolic episodes or cerebral ischemic events are valve related. Patient-
related risk factors, including atherosclerotic plaques of the great and
carotid arteries, atrial fi brillation, and cardiomyopathy, should be consid-
ered. Normal anatomic valves do not form thrombi, because the endothe-
lium is thromboresistant by virtue of its ability to produce prostacyclin. 80
All prosthetic cardiac valves, and even biological tissue valves to a certain
extent are thrombogenic because of the foreign material surfaces exposed
to circulating blood.
In the mid-1960s Davila, while conducting studies in calves of a new
prosthetic valve, detected growth of granulation tissue at the junction of the
prosthesis sewing ring and the exposed metal surface of the valve. Further
observation revealed that the clot originated at the site of granulation and
enlarged over time. There was a tendency for the granulation tissue and the
thrombus to propagate towards the valve orifi ce, with a risk of detachment
and embolization of the thrombus. Davila postulated that these phenomena
represented a xenogeneic reaction: the body was attempting to eliminate
or encapsulate a foreign object, the valve. Davila described the process as
the 'frustration of healing.' 81 These fi ndings engendered the novel idea of a
cloth-covered prosthesis, designed to stimulate tissue ingrowth and produce
a thromboresistant pseudo-intima. Braunwald and Bonchek at the National
Institutes of Health implemented this concept and designed a cloth-covered

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