Developments in cardiovascular valve technology - Biomaterials and Devices for the Circulatory System - page 89

The experimental work of Braunwald and Davila demonstrated that
covering the surfaces of the mechanical prosthesis with cloth would foster
the ingrowth of neointimal tissue, thereby coating the prosthesis with
thrombogenically inert host tissue. In 1968 the fi rst totally cloth-covered
valve with a hollow metallic ball was introduced for clinical use to reduce
the incidence of thromboemboli. Various models of cloth-cover valves were
designed and used clinically until the 1970s, when a series of unfavorable
clinical reports appeared which demonstrated that, although the cloth cov-
erings lowered the incidence of thromboembolism, transvalvular pressure
gradients were excessive. 25-27 Most surgeons using this valve abandoned it
in favor of the original uncovered prosthesis.
Smeloff and Cutter developed an open-cage ball-valve, which was intro-
duced clinically in 1964. It consisted of a double-cage full-fl ow orifi ce with
a silicon rubber poppet. The cage was machined from a solid piece of tita-
nium, which eliminated the potential for weld fractures. The open cage
design allowed the diameter of the valve orifi ce to be slightly larger than
the ball diameter. This design allowed the ball to move freely from one cage
to another, permitting a minimal amount of backfl ow which produced a
'washing' effect, dislodging adherent formed elements from the ball surface.
In the closed position the ball sat on the struts of the small cage. The ratio
of valve orifi ce : tissue annulus diameter was therefore superior to any of
the other ball valves available at that time. 28
A novel ball-valve design was introduced by Dr Nina Braunwald while
she was working at the National Institutes of Health in 1960; this valve
represented the prototype of a cloth-covered valve. The prosthesis consisted
of an open ended cage in which the three struts were covered with an ultra-
thin Dacron tubing and ultra-thin polypropylene mesh fabric. A modifi ed-
cure silicone ball functioned as the poppet. 29-31 The prosthesis was approved
for general clinical use in 1968. By 1977 approximately 2500 of these devices
had been implanted. The largest series reported were from the University
of Alabama and the Mayo clinic. The published results were not favorable;
the principal limitation of this prosthesis was the wear and tear of the cloth
cover that resulted in major complications, including thromboembolism of
debris and clot, poppet deformation, and loss of ball volume. The latter
defect resulted in escape of the ball from the cage followed by sudden
death. 32

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4.3.2 Caged-disc valves

The Beall-Surgitool prosthesis, designed by Dr Arthur Beall of Houston
and manufactured by the Surgitool Company, was a cage-disc, low profi le
device with a Tefl on occluder in the cage. The valve was cloth covered and
designed only for the mitral position. The prosthesis underwent several