(Joanna Chikwe M.D., Farzan Filsoufi M.D.)
Prosthetic valves are divided into mechanical and bioprosthetic valves. The different types of mechanical valve are described according to the shape of the moving part (occluder) that opens and closes the valve orifice. During the last fifty years, three types of mechanical prostheses have been used in clinical practice: caged-ball, single tilting disc and bileaflet valves. Currently, the bileaflet valves are the most commonly used mechanical heart valves.
The first mechanical valves were the caged ball valves of which the Starr-Edwards valve (Baxter Healthcare Corp. Edwards division, Irvine, California) has been used most widely and was still available until recently. The pioneering work of Albert Starr and Lowell Edwards is extensively described in the Historic Review section. The unique characteristics of this design were firstly the fact that the occluder travels completely out of the orifice reducing the possibility of thrombus or pannus growing from the sewing ring to interfere with the valve mechanism; and secondly the continuously changing points of contact of the ball reduced wear and tear in any one area. The initial model of the valve underwent several minor modifications and the final design was produced in 1965 and remained unchanged for several decades. Caged ball valves had a very low incidence of mechanical failure. They were, however, associated with a high thrombogenic risk, with a linearized risk of thrombotic event of 4-6% per year and a recommended INR of 2.5 to 3.5, compared to 2.0-3.0 for newer generation of mechanical prostheses. Additionally, the high profile of the valve implanted in the mitral position could occasionally cause left ventricular outflow tract obstruction. A final consideration was the lack of central blood flow which was thought to facilitate clot formation. In 2007, Edwards Lifesciences discontinued manufacture of the Starr-Edwards valve.
Tilting disc prostheses use a single circular leaflet or disc that tilts to occlude or open the valve orifice. These valves were developed in the late 1960's and the most popular was the Bjork-Shiley valve (Shiley, Inc, Irvine, California) which first became available for clinical use in 1969. Bjork-Shiley valves were implanted in almost 300,000 patients worldwide in the aortic and/or mitral position between 1969 and 1986. The opening angle of these monoleaflet valves was between 60 and 70 degrees and was greater in the aortic than mitral position. These valves are no longer produced primarily because one model, Bjork-Shiley Convexo-Concave Tilting Disc Valve, displayed a mechanical failure mode in which the outlet strut could fracture allowing the occluder to embolize from its housing. Another tilting disc valve, the Medtronic-Hall valve (Medtronic) that gained FDA approval in 1981 was available on the market until a recent date. It consists of a pyrolytic carbon-coated disc retained in titanium housing by a strut through a central hole in the disc.
The most significant advantages of the tilting disc valves over the caged ball prostheses were lower profile, central blood ejection fraction and lower thrombogenicity. Disadvantages of tilting disc valves include suboptimal hemodynamic performances with stasis and turbulent flow, thrombus and pannus formation interfering with the motion of the disk, and the need for careful orientation of the disc during implantation to avoid outflow obstruction.
The first bileaflet valves were Gott-Daggett and Kalke-Lillehei which were developed in 1963 and 1967 respectively. These valves were implanted only in a few patients and associated with high thromboembolic events.
In 1977 a new generation of valves employing two semicircular disks (bileaflet valves) on a central hinge that tilted to open and close the valve orifice was introduced into clinical use by St. Jude Medical. The St. Jude Medical valve has become the most widely used mechanical valve prosthesis, and became the basis for models produced by other manufacturers. Modern bileaflet valves are constructed of two pyrolite carbon coated leaflets made of radiopaque tungsten-impregnated graphite substrate and a pyrolite carbon or pyrolite carbon coated titanium housing. The opening angle of leaflets is between 75 and 85 degrees. During the last three decades, several clinical studies have shown that bileaflet prostheses have demonstrated better effective orifice areas, lower transvalvular gradients, and lower rates of thromboembolism than caged ball and tilting disc valves.
St. Jude Medical produces several models of the basic valve: the HP model uses a smaller sewing ring so that a larger valve can be implanted into small aortic roots, and the Masters model allows the housing to be rotated by the surgeon within the sewing ring once it is implanted to facilitate orientation of the valve, reducing the chance of material such as residual subvalvular apparatus or suture material interfering with the leaflet motion. The Regent model further increases the effective orifice area by allowing supraannular insertion with only the pivot guards and not the housing projecting beyond the aortic annulus.
Since the introduction of the bileaflet valve over 2 million bileaflet prostheses have been implanted. The Sulzer CarboMedics valve and ATS Medical valve are the most popular bileaflet valve alternatives. The Carbomedics valve, approved by the FDA in 1993, is a bileaflet valve with flat pyrolytic carbon-coated leaflets and a solid carbon housing reinforced with a radiopaque titanium ring. The ATS Open Pivot valve, approved in 1995, attempts to reduce thromboembolism formation in the hinge area by locating the hinge socket on the leaflet rather than the housing. The ATS sewing ring is smaller to allow a large valve to be implanted for a given annular diameter without the need for concomitant annular enlargement, whereas the Sulzer CarboMedics valve achieves the same result through a supravalvular implantation.
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