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February 25, 2017
Mechanical Valves Mechanical Valves

Mechanical Valves

(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.

Caged-Ball 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 Valves

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.  

Bileaflet Valves

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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