Abstract:
A bileaflet heart valve comprising an annular base and pivoting leaflets. Each leaflet is “free-floating” within recesses without fixed rotational axis in order to increase translational movement and redistribute stresses. Each recess fluidly communicates with a groove extending at least partially around the inner surface of the annular base and fluid flow is directed through the recesses at different angles during antegrade circulation, retrograde circulation, and valve closure. A recess entrance angle to each of the recesses preferably being less than about 35° and the pivoting mechanism within the recess including first and second fulcrum edges of each leaflet shiftably engaged with side surfaces of the respective recesses. The leaflets have a beveled bottom side having two separate planar surfaces which lie at an angle to one another. In preferred embodiments, an upper planar surface of the bottom surface of each leaflet lies at an angle of greater than ninety (90) degrees with respect to a horizontal passing through a section of the annular base when the leaflet is in a fully open position.

Description:
CO-PENDING APPLICATIONS 
     The present application claims priority to U.S. Provisional application Ser. No. 60/060,922, filed Oct. 3, 1997, and relates to U.S. patent application Ser. No. 09/143669, filed Aug. 31, 1998, now abandoned which is a continuation of U.S. patent application Ser. No. 08/626,170, filed Mar. 29, 1996, now U.S. Pat. No. 5,824,062, which is a continuation-in-part of both U.S. patent application Ser. No. 08/412,696 filed Mar. 29, 1995, now abandoned, and U.S. patent application Ser. No. 08/546,210 filed Oct. 20, 1995, now abandoned, each of which is entitled BILEAFLET HEART VALVE. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to bileaflet hemodynamic heart valve prostheses of the type permitting translational and rotational movement of the leaflets, and particularly to a low-excursion prosthetic heart valve suitable for mitral valve replacement involving preservation of the papillary muscle and chordal structure wherein the valve may be oriented in either an anatomical or anti-anatomical configuration. 
     BACKGROUND OF THE INVENTION 
     The replacement of defective heart valves with hemodynamic prostheses is the most prevalent course of treatment for certain types of heart disease and dysfunction affecting the atrioventricular valves—namely the right AV (tricuspid) and the left AV (bicuspid) valves. Although a variety of tissue and prosthetic heart valve mechanisms have been developed, monoleaflet (tilting disc) and bileaflet valves currently hold the greatest measure of acceptance among practitioners. These valves include one or two pivoting leaflets or occluders retained within a seating collar or suture ring that is implanted in place of the physiological valve. 
     Replacement of a bicuspid (mitral) valve using a procedure that preserves portions of the papillary muscle and chordal apparatus is discussed herein for exemplary purposes. In that procedure, the anterior leaflet is bisected and detached from the annulus, and the two halves are groomed and then sutured to the posterior mitral annulus with the papillary muscle and chordal apparatus substantially intact. Such a procedure and its benefits are described in significant detail by H. Feikes, et al.,  Preservation of All Chordae Tendineae and Papillary Muscle During Mitral Valve Replacement with a Titling Disc Valve,  5 J. Cardiac Surg., No. 2 pp. 81-85 (1990). The authors conclude that this mitral valve replacement procedure can be practical using both monoleaflet and bileaflet valves. However, it is readily apparent to those skilled in reconstructive cardiac surgery that selection of a suitable valve type and proper orientation of the prosthesis can be important factors impacting the long term success of this procedure for a given patient. In particular, due to the position at which the valve tissue is sutured to the posterior mitral annulus, care must be taken to ensure that the peripheral edge of a leaflet does not contact the tissue during normal operation of the valve. Such contact can result in the intermittent, partial, or complete malfunction of the valve, as well as damage to or dislodgement of the valve tissue. 
     Four primary combinations of valve types and orientation are considered, as diagramed in FIGS. 25-28 herein. The four combinations ranked by ascending level of risk include: (1) monoleaflet valve M with anterior orientation (FIG.  25 ); (2) bileaflet valve with anti-anatomical orientation (FIG.  26 ); (3) bileaflet valve with anatomical orientation (FIG.  27 ); and (4) monoleaflet valve M with posterior orientation (FIG.  28 ). While the monoleaflet with posterior orientation is generally regarded as a high risk configuration and the monoleaflet with anterior orientation is considered to have little or no risk, the degree of risk associated with a bileaflet valve oriented in either the anatomical or anti-anatomical configuration depends upon the particular type of valve selected (particularly its range of excursions, radial exposure, and lateral exposure), the post-procedure anatomical characteristics of the annuls, and the patient&#39;s requirement for certain operational parameters associate with the valve. 
     While a monoleaflet valve may be preferred in order to achieve the lowest risk level with an anterior orientation, a physician may prefer to implant a bileaflet valve to obtain specific functional benefits associated with or unique to the particular bileaflet valve structure. 
     The bileaflet valve has been extensively developed and refined. However, there is still room for further improvement. Problems associated with the weakening or structural failure of critical components in the valve are linked both to dynamic mechanical stresses and cavitation. It is noted that a certain amount of antegrade and retrograde leakage is generally anticipated. However, the amount of leakage is preferably maintained within acceptable limits corresponding roughly to normal anatomical valves. In addition, minimizing the physical size of the valve prosthesis, particularly the longitudinal dimensions of the annular base, produces greater excursion along the peripheral edges of the leaflets, while simultaneously increasing the difficulty in raising the heights of the pivot axis. Furthermore, recesses, crevices, corners, and obstructions required to restrain the leaflets within the annular base and maintain pivotal movement also interfere with circulation, create turbulence, and produce zones of stagnation, each potentially providing a thrombogenic nidus that may eventually lead to an embolism. Although bileaflet valves are hemodynamic, spacing the fixed axis of rotation of the leaflets significantly apart from the secondary natural axis of rotation limits the maximum speed or angular rate which the leaflets may attain during opening and closing. 
     In regard to the selection of suitable materials, there is an inherent balancing between the selection of materials for ease of fabrication, biocompatibility, strength, and weight versus selection with respect to the acceptable level of fragility of the resulting components, particularly those involving delicate structures such as wire guides, cages, and pins that bear significant loads. In addition, the structure of many pivot mechanisms requires the annular bases to have opposing flat sides rather than a substantially or completely circular bore, thereby restricting the maximum flow volume and increasing the valve&#39;s nominal fluid pressure. 
     U.S. Pat. No. 4,276,658 to Hanson provides a representative example of a conventional bileaflet heart valve. That valve utilizes a pair of semicircular pivot “ears” disposes on opposing sides of each leaflet received within “hourglass-shaped” slots to control the pivotal movement of the leaflets—including the angular sweep between the open and closed positions, the tiling of the valve away from its restrained pivotal axis, and the translational movement of the leaflet both parallel with its normal plane and along the linear flow path through the bore of the annular base. The Hanson &#39;658 patent also describes the use of a pyrolytic carbon coating over a metallic or synthetic substrate for fabrication of the valve&#39;s components. 
     For comparison, U.S. Pat. Nos. 4,240,161 to Huffstutler and 3,859,668 to Anderson provide representative examples of the features, structure, and operation of monoleaflet or “titling disc” heart valves. 
     Various improvements directed toward correcting the deficiencies described above have been developed, each achieving varying degrees of success and accompanied by inherent tradeoffs with other beneficial features. 
     U.S. Pat. No. 3,903,548 to Nakib discloses an effort to utilize the beneficial features of the monoleaflet principle in a bileaflet valve that similarly omits fixed pivotal axis, however the resulting cage structure produces an unacceptably small effective bore and correspondingly high pressure gradient across the valve. 
     In a bileaflet valve structure such as disclosed in the Hanson &#39;658 patent, the leaflets may each pivot filly between the open and closed portions on the order of 80,000-120,000 times per day given a standard pulse of 60-80 beats per minute. Movement of the leaflets through a viscous aerated fluid such as blood may produce significant cavitation —the formation of partial vacuums caused by sudden movement of the flowing fluid away from the surface of the leaflets as a result of mechanical forces exerted by the leaflets. These partial vacuums produce “micro bubbles” on or near the surface of the leaflets, and when the pressure is released, vacuums change into positive pressure regions which lead to implosion of bubbles which can cause pitting of the surface of the leaflet. The cavitation potential is amplified greatly by the virtually instantaneous stopping and starting of the leaflets as they contact a rim along the annular base and also, in the case stopping, by the rate of speed at which the leaflet is traveling when it stops. Contact between the leaflet and the rim greatly increases the compressive forces on the adjacent fluid, and as the leaflet pivots away from the rim the corresponding effects of the expansions are magnified by increased negative pressures and stronger partial vacuums. Whereas standard cavitation produces pitting of metal surfaces due only to mechanical contact between the flowing fluid and moving object, introducing reciprocal movement and mechanical contact within the fluid cause the collapsing cavitation bubbles to strip or shear material from the leaflet surfaces at an accelerated rate. Although the surface pitting occurs at a near microscopic level, the result is surface degradation of the leaflet which can induce stress fractures and fragmentation leading to the premature failure of a leaflet. 
     U.S. Pat. No. 4,078,268 to Possis discloses a substantially circular bore through the annular base, as well as a nearly complete separation between the peripheral edges of the leaflets and the annular base around the circumference of the valve. While this design obviates certain cavitation problems, it permits high levels of antegrade and retrograde leakage and places the entire load of restraining each leaflet on a pair of pivot pins received within adjustable bearing plugs. The combination of increased torque, absorbed impact forces, vibration, and normal frictional contact are believed to exert undue mechanical stresses on the relatively delicate pivot pins and bearing plugs. 
     U.S. Pat. No. 5,080,669 to Tascon discloses an annular base that defines channels which intersect the pivot axis of the leaflets at various angles to direct flow of blood around enlargements in the leaflets that serve as the pivot axis, in an effort to cleanse the surfaces of the enlargements and prevent zones of thrombogenic stagnation from forming. However, the inward projections forming the channels and barriers restraining the leaflets in the Tascon &#39;669 design create obstacles to uniform blood flow through the bore of the annular base, and define acute corners and crevices which can accelerate the formation of a thrombus. In addition, the enlargements continuously block a majority of the potential flow through each of the channels, thereby minimizing any cleansing effect that is realized. 
     U.S. Pat. No. 4,892,540 to Vallana discloses a pair of vertical “chimneys” defined by the lobes of the annular base and communicating with the recesses in which the ears of the respective leaflets are received. In concept, blood flow in either the antegrade or retrograde direction passes between the pivot ears and the side wall of the annular base to cleanse the recess. However, the angled base portions forming each wedge-shaped separator body hold the pivot ears and leaflets in an elevated position proximate to the inlet from the chimney into the recess, thereby minimizing flow through the chimney. The pivot ears either reduce the flow rate within the recess or divert the flow away from portions of the recess where stagnation could occur, thus diminishing the effectiveness of any cleansing action. Whereas Tascon &#39;669 contemplates alternating between multiple flow paths oriented at diverse angles to enhance the “scrubbing” effect, Vallana &#39;540 only contemplates cleansing that is substantially repetitive and reciprocal along one path for both antegrade and retrograde flow. Finally, to the extent that Vallana &#39;540 would produce an acceptable retrograde cleansing action due to the pressure differential created within the recess feeding into the chimney, it is at the expense of a significantly restricted non-circular bore through the annular base accounting for a substantial reduction in antegrade circulation. 
     Although the Hanson &#39;658 patent discloses the pivot ears preventing blood stagnation in the area of engagement with the recesses, the use of transesophageal echocardiography in patients receiving mitral valve replacements has shown the formation of dangling fibrin strands along the interior surface of the valve in the areas between and proximate to the pivot recesses. These small filamentous abnormal echoes (SAE) are considered non-obstructive while within the valve, however their frequent disappearance strongly suggests a thrombotic origin and a significant correlation with the risk of early thrombogenic episode has been observed. 
     Many factors may be responsible for the formation of the fibrin strands, including regions of blood stagnation which provide a nidus for thrombogenic formations, or defects in the materials or structure of the valve that permit the direct attachment of blood cells. It may therefore readily be appreciated that two important goals when designing a bileaflet heart valve are maintaining optimal antegrade and retrograde circulation, and eliminating regions of reduced circulation within the valve that might foster the development of a thrombogenic mass. It is suggested that while the Hanson &#39;658 patent shows a relatively shallow semi-circular recess, in practice it has not been possible to achieve a workable commercial embodiment of a bileaflet valve having pivot ears with a suitably shallow recess to enhance cleansing of the recess by normal antegrade and retrograde circulation. For example, the commercially available embodiments of the Hanson &#39;658 valve have recesses forming entrance angles ranging from 35° to 48° measured between the lateral wall of the bore and the tangentially adjoining surface of the recess, depending upon overall size of the valve. Recesses forming an angle of 35° or less with the adjoining lateral wall have been achieved in monoleaflet valves, however the significantly different structure and operation of monoleaflet valves has not permitted the successful utilization of many comparable features in bileaflet valves. 
     Various adaptations have also been made in an effort to improve the pivot mechanism. One option is to eliminate the pivot ears or pins, and allow the leaflet to rock on projections extending inwardly from the annular base. These configurations generally require some engagement between the leaflet and the projections—either the projection being received within a notch or recess in the leaflet, or the leaflet forming a trapping flange that prevents egress from between two spaced-apart projections. For example, U.S. Pat. Nos. 4,863,459 to Olin and 4,935,030 to Alonso describe leaflets that include a swelled area or camming surface trapped between two projections. U.S. Pat. Nos. 4,373,216 to Klawitter, 4,692,165 to Bokros, 4,872,875 to Hwang, and 5,354,330 to Hanson each describe a variation in which the leaflet defines a peripheral notch or recess receiving a projection the annular base. While designs utilizing a notch in the leaflet are more secure than the trapped flange configurations, they are also more difficult to assemble without placing undue stress on the leaflets or projections. In addition, these designs similarly present flat-sided bores and projections which extend into the bore and obstruct antegrade flow. As the complexity of these projections increases, the opportunity for a crevice or recess providing a thrombogenic nidus also increases. Representative examples of relatively complex pivot structures that present several potential stagnation sites include U.S. Pat. Nos. 5,116,367 to Hwang and 5,123,920 to Bokros. 
     One prominent feature of the bileaflet valves discussed above is the degree of exposure or incursion that is exhibited by the leaflets relative to the annular base. Excursion can be thought of as the maximum distance which the distal ends of the leaflets protrude from the bottom of the annular base when the valve is completely open, measured from the lowermost planar surface of the base to the most distal point on the peripheral edge of the respective leaflet. However, when comparing the anatomical and anti-anatomical orientation of a bileaflet valve with reference to the mitral valve replacement procure discussed above, incursion can also encompass two more complex relationships. 
     U.S. Pat. Nos. 5,246,453 to Bokros and 5,002,567 to Bona disclose alternate configurations in which the leaflets are not generally planar, and are supported by and pivot about fulcrums disposed on the lower portion of each leaflet. While these designs present an incursion both above and below the annular base, it allows the height of the annular base to be reduced somewhat relative to comparable bileaflet valves. While such a design is considered to be more responsive to reversal in the antegrade flow, it also relies upon shifting the axis of rotation relative to the leaflet&#39;s moment of inertia and therefore produces different operational characteristics than might normally be expected. 
     One factor previously alluded to which affects the speed at which the valve operates, is the displacement between the fixed axis of rotation and the corresponding En moment of inertia of the leaflet. Another factor is the shape of the leaflet. In this regard, optimization of several physical parameters must be contemplated. The leaflets must move through an arcuate path in response to fluid pressure applied from both the antegrade and retrograde directions, starting from differential initial orientations relative to the fluid pressure, and within an initially static versus initially dynamic environment. Consequently, valves having superior opening characteristics may be slow to close or resist complete closure, and vice versa. Leaflets having an angled, curved, or bicurved design to enhance the immediate responsiveness to changes in hemodynamic forces can be employed to address this problem. Other factors include reducing turbulence or backwash that might resist the leaflet&#39;s momentum or increase its apparent resting inertia, reducing the weight or thickness of the leaflet, allowing the leaflet to rock or cam differently in response to antegrade or retrograde pressures, maximizing the laminar flow through the valve body over the entire leaflet surface, and eliminating sources of friction, vibration, or misalignment that could adversely affect the mechanical operation of the valve. 
     Another approach mentioned above is to increase the translational movement of the leaflet within the annular body, thereby permitting the leaflet to pivot more naturally about its inertial axis in direct response to the hemodynamic forces. This approach can potentially be more beneficial than merely moving the fixed axis of rotation nearer to the moment of inertia, since it also serves to reduce frictional forces and other physical impediments to proper valve operation. One limitation is the need to maintain proper alignment and seating of the leaflet without encumbering the flow passage with obstructions or incorporating free structures that increase the likelihood of valve failure. 
     U.S. Pat. No. 4,535,484 to Marconi describes a bileaflet valve in which the leaflets are “free-floating”, thereby increasing translational movement and reducing the mechanical stresses imposed at localized pivot points and other load bearing surfaces. However, the Marconi &#39;484 design requires a complex and fragile cage structure to restrain the leaflets, thereby producing a significant risk of damage to the valve during manufacturing or handling and increasing the potential for catastrophic failure of a valve component that would result in death or severe injury to the patient, mitigating against the use of certain materials such as pyrolytic carbon, and greatly increasing the cost and complexity of fabrication. 
     For comparison, U.S. Pat. No. 4,689,046 to Bokros describes a trapezoidal pivot ear having beveled edges, arguably decreasing the translational freedom, but enhancing the “sweeping” effect of the pivot ear to prevent thrombogenic formations within the recesses and distributing lateral stresses over a wider surface area. 
     It will also be appreciated from analyzing bileaflet heart valves, such as disclosed by the Hanson &#39;658 and Possis &#39;268 patents, that the leaflets divide the bore into three passages having unequal cross-sectional areas, and that corresponding effects on fluid dynamics should be expected. Observation of these valves in operation shows that flow rates through the passages will vary generally inversely with the corresponding cross-sectional area. As such, in a valve such as Hanson &#39;658 which present a relatively narrow central passage, the flow rate of blood passing through that central passage is greater than through the two passages on opposing sides. The faster blood flow in the center, relative to the sides, can cause additional turbulence within or downstream of the valve, or produce a pressure differential or venturi effect within the valve that can impede or retard the optimal translational or pivotal movement of the leaflets. The Possis &#39;268 valve presents a larger central passage with narrower cross-sectional passages on each side, thereby reversing the fluid dynamics compared with the Hanson &#39;658 design. 
     While many common functional goals have been recognized among designers of bileaflet heart valve prostheses, there are strongly divergent opinions concerning the prioritization of those goals and how best to achieve specific results or advantages. Accordingly it will be readily appreciated that these competing factors significantly influence the design and optimization of all bileaflet heart valves and that further improvements may be made. The present invention provides advantages over the prior art bileaflet heart valves and solves problems associated therewith. 
     SUMMARY OF THE INVENTION 
     Briefly described, the bileaflet heart valve prosthesis of the present invention comprises an annular base defining a substantially circular bore, and a pair of pivoting leaflets; each of the respective leaflets having first and second sides, the first side being a top side and the second side being a bottom side, the bottom sides of the respective leaflets generally facing one another when the respective leaflets are in an open position; each bottom side having an upper half and a lower half, a major portion of the upper half providing an upper surface lying generally in a first plane and a lower half providing a lower surface lying generally in a second plane, the first plane lying at an angle to the second plane; a third plane passing through a horizontal cross-section of the annular base, the first and second planes lying at angles to the third plane when the leaflets are in either the open or closed positions; wherein the first plane of each of the respective leaflets extends beyond an angle of 90° with respect to the third plane when the leaflets go from the fully closed position to the fully open position. 
     The first plane can extend beyond a 90° angle with respect to the third plane when the leaflets go from the fully closed position to the fully open position without diminishing the leverage for closure of the leaflets when the leaflets are in the fully open position. This is because the lower portion of the bottom side of the leaflets remain at a suitable angle to allow for adequate leverage against the lower portion of the leaflet, to shift the leaflets within the respective recesses and pivot the leaflets to the closed position once the upper fulcrum edge comes into contact with the upper sidewall of the respective recess. 
     In preferred embodiments the leaflets have a beveled bottom side which minimizes the travel angle “k′” between the open and closed positions. The lateral ends of each leaflet are received within “open channel” recesses where the ends are “free floating”, permitting translational and rotational movement of the leaflets within the respective recesses. In preferred embodiments, each recess communicates with at least one groove extending around an inner peripheral surface of the annular base, and a cleansing flow is directed vertically or angularly through the recess to the groove during antegrade circulation, and from the grooves through the recess during retrograde flow and valve closure. The direction of this cleansing flow through the recesses varies depending upon the direction of circulation and the orientation of the leaflets, and is mostly unobstructed within the recesses by the leaflets. The peripheral edges of the leaflets present minimal incursion or exposure beneath the bottom of the annular base when the valve is completely open. When the leaflets of the valve are closed, the peripheral edge of each leaflet in the central region is preferably slightly spaced apart from the annular base to allow free movement of the leaflet and to avoid unsuccessful wear and/or stress. The peripheral edge of each leaflet in preferred embodiments only contacts the annular base adjacent the groove proximate the lateral regions of the leaflet. 
     In preferred embodiments, the angle at which fluid washing the surfaces of the annular base flows into the recesses is less than 35° to permit better washing dynamics. The preferred valve also has a dynamic pivot constructed primarily on the lateral sides of the leaflets where two fulcrum edges are created by notches in the peripheral edge. The leaflets pivot on each of the respective fulcrum edges at different points in the opening and closing cycle of the valve. This swivel pivot mechanism also permits significant translational movement of the leaflets especially in the fully open position. This mechanism is believed to provide a pivot mechanism which permits the valve to open and close more rapidly than prior art bileaflet valves. 
     It is one object of this invention to design a bileaflet heart valve prosthesis of the type used for tricuspid or bicuspid (mitral) valve replacement, and particularly one which provides superior operating capabilities and minimizes the risk to the patient when implanted using a procedure involving preservation of the papillary muscle and chordal structure by fixation to the posterior mitral annulus. 
     It is a related object of this invention to design the above bileaflet valve for implantation in either the anatomical or anti-anatomical configuration, such that the peripheral edges of the leaflets present an extremely low incursion below the bottom surface of the annular base, and further present minimal radial and lateral exposure. 
     It is an additional object of this invention to design the above bileaflet valve such that the passages through the bore of the valve between the leaflets provide substantially equal relative flow rates, thereby mitigating against flow differentials, gradients, or venturi effects which would otherwise cause turbulence or impede the translational or pivotal movement of the leaflets. 
     It is another object of this invention to design the present bileaflet valve such that it utilizes a “free floating leaflet” configuration with no pivot ears or projections, to thereby reduce and redistribute mechanical or contact stresses otherwise focused on these pivot axis in conventional bileaflet valves. 
     It is a further object of this invention to design the above bileaflet valve such that it defines a cleansing channel or recess within the annular base in the region traversed by the lateral ends of the leaflets, and such that the cleansing channel is unobstructed within that region in a generally vertical direction, and induces or “steers” both vertical and angular fluid flow through that region during antegrade and retrograde circulation. 
     It is another object of this invention to provide a bileaflet valve such that a shallow angle of less than about 35° may be formed between the lateral surfaces of the annular bore and the adjoining surfaces of the recesses which restrain the leaflets. It is believed that this will enhance cleansing of the recesses by normal antegrade and retrograde circulation. Furthermore, because the recesses have unobstructed open channels permitting easy antegrade and retrograde flow through the recesses, the surfaces within the respective recesses will permit enhanced washing action. 
     It is a further object of this invention to provide a bileaflet valve such that the peripheral edge of each leaflet is received within a recess and beneath a seat defined by the annular base, such that there are no observable gaps between the annular base and peripheral edge in the contact regions between the leaflets and annular base when viewed from a perspective along the longitudinal axis of the valve. 
     It is a fisher object of this invention to design the above bileaflet valve such that the annular base of the valve defines beveled arcuate surfaces which contact the edges of the leaflets as the leaflets move between the open and closed positions, thereby creating a generally smooth and continuous arcuate path along which the leaflets roll when pivoting between the open and closed positions to distribute stress forces over an extended region of the leaflet and annular base. 
     The above-described features, advantages and objects, along with various other advantages and features of novelty are pointed out with particularity in the claims of the present invention annexed hereto and forming a part thereof. However, for a better understanding of the invention, its advantages, and objects attained by its use, reference should be made to the drawings which form a further part hereof and to the accompanying descriptive matter in which there is illustrated and described preferred embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawing, in which like reference numerals indicate corresponding parts throughout the several views: 
     FIG. 1 is a perspective view of a preferred embodiment of the present invention showing the leaflets in a fully open position; 
     FIG. 2 is a lateral side view of the preferred bileaflet heart valve of the present invention shown in FIG. 1; 
     FIG. 3 is a lateral side view of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 4 is a top plan view of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 5 is a bottom plan view of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 6 is a partially broken away elevated perspective view of the annular base of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 7 is a cross-sectional side view of the lateral side of the annular base of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 8 is a cross-sectional side view of the traverse side of the annular base of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 9A is an elevated perspective view of the bottom side of the preferred leaflet shown in FIG. 1; 
     FIG. 9B is a cross-sectional perspective view of the preferred leaflet shown in FIG. 1, in a manner similar to that shown in FIG. 9A, but providing a perspective view only of a cross-section of the leaflet as seen from the line  9 B- 9 B of FIG. 9A; 
     FIG. 10 is a bottom plan view of a leaflet of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 11 is a top plan view of a leaflet of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 12 is a vertical side view of a first lateral side of the leaflet of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 13 is a vertical side view of a second lateral side of the leaflet of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 14 is a horizontal side view of an upper edge, including the mating edge, of a leaflet of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 15 is a horizontal side view of the peripheral edge of the leaflet of the preferred bileaflet heart valve shown in FIG. 1; 
     FIG. 16 is a diagrammatic cross-sectional view of the preferred bileaflet heart valve shown in FIG. 1 with the leaflets in a fully open position; 
     FIG. 17 is a diagrammatic cross-sectional view of the preferred bileaflet heart valve shown in FIG. 1 illustrating the transition of the leaflets from a fully open position to a fully closed position; 
     FIG. 18 is a partially broken away cross-sectional view of the recess as seen from the line  18 — 18  of FIG. 7; 
     FIG. 19 is a partially broken away cross-sectional view of the recess as seen from the line  19 — 19  of FIG. 7; 
     FIG. 20 is a partially broken away cross-sectional view of the recess similar to that shown in FIG. 19 but generally showing a lateral side portion of a leaflet within the recess when the leaflet is in a fully closed position as shown diagrammatically in FIG.  17 ; 
     FIG. 21 is a partially broken away cross-sectional view similar to FIG. 20, but showing the leaflet in an open position as shown in FIG. 1; 
     FIG. 22 is an elevated perspective view of the preferred bileaflet heart valve of the present invention similar to that shown in FIG. 1, except that the leaflets are in a fully closed position; 
     FIG. 23 is a partially broken away bottom plan view of the preferred bileaflet heart valve shown in FIG. 22 when the leaflets are in a fully closed position; 
     FIG. 24 provides a graphic representation of the quantity of blood flowing through a bileaflet heart valve during a single heart contraction cycle wherein the positive quantity indicates blood flowing in an antegrade direction and the negative quantity below the “y” axis indicates the quantity of blood flowing in the retrograde direction; 
     FIG. 25 is a perspective view of a monoleaflet heart valve with anterior orientation as known to the prior art; 
     FIG. 26 is a perspective view of a bileaflet heart valve with anti-anatomical orientation; 
     FIG. 27 is a perspective view of a bileaflet heart valve with anatomical orientation; 
     FIG. 28 is a perspective view of a monoleaflet heart valve with posterior orientation as known to the prior art; and 
     FIG. 29 is a perspective view of a bileaflet heart valve of the present invention implanted in an anatomical orientation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, a preferred bileaflet heart valve prosthesis  110  of the present invention and parts thereof are illustrated. The bileaflet heart valve prosthesis  110  of the present invention is preferably fabricated from a metal such a titanium, a carbon compound (or carbon with a minor percentage of silicon) such as pyrolytic carbon or the like, a metal alloy, or a suitable substrate coated with pyrolytic carbon as are well known in the art. 
     Referring now to FIGS. 25-29, a bileaflet heart valve  10  similar to the preferred heart valve prosthesis  110  of the present invention is shown diagrammatically implanted within the heart  101  of a patient, with the valve  10  sutured in place proximate to the mitral annulus  103  of the anatomical coronary valve and disposed above the papillary muscle and tendineae chordae  105  fixed to the posterior mitral annulus as described previously. The bileaflet valve  10  may be implanted in either the fully anatomical orientation or the fully anti-anatomical orientation as shown in FIGS. 26 and 27, respectively, or adjusted between the fully anatomical and anti-anatomical orientations by rotating the valve  10  within the corresponding suture ring (not shown) as is well known to the art. These orientations may be compared with the anterior and posterior orientations of a monoleaflet valve M shown in FIGS. 25 and 28. 
     Referring now to FIGS. 1-5, a preferred embodiment of the bileaflet heart valve prosthesis  110  is described. The preferred bileaflet heart valve  110  of the present invention shown in FIG. 1 includes an annular base  112  and first and second leaflets  114 . The fist and second leaflets  114  are mounted within the annular base  112  for pivotal movement between a fully open position, shown in FIGS. 1-5 and diagrammatically in FIG. 16, and in phantom in FIG. 17, and in a fully closed position shown in FIGS. 22-23 and diagrammatically in FIG.  17 . Referring now also to FIGS. 68, the annular base  112  has a top surface  124  and an inner wall  126  which defines a generally circular bore  116  passing through the annular base  112  in a direction generally parallel with a longitudinal axis  128  oriented generally in parallel with a vertical path for circulation of fluid or blood through the generally circular bore  116 . 
     The top surface  124  of the annular base  112  is raised proximate opposing lateral sides  129 . On the inner wall  2  inner surface sidewall  126  of the annular base  112  proximate the opposing lateral sides  129 , are flat portions  130  of lateral surfaces  133  which define flat lateral sides of the generally circular bore  116 . The flat portions  130  of the lateral surfaces  133  include a pair of recesses  132  in each of the respective lateral sides  129  of the base  112 . Further lateral depressions  135  are centrally located in a lower portion of the inner surface  126  proximate each of the two lateral sides  129 , below and between the respective recesses  132  on each side, in the respective flat portions. In preferred embodiments, these depressions have a curvilinear surface which would define a portion of one side of a cone. The recesses  132  extend into the respective flat portions  130  of the lateral surfaces  133 , thereby displacing a cylindrical bottom surface  140  of the recess  132  from the respective lateral surface  133  proximate the respective lateral side  129 . In preferred embodiments of the present invention, each of the cylindrical bottom surfaces  140  of the respective recesses  132  pass through a cylindrical radius which is “feathered out” as the cylindrical surface  140  approaches a junction with the respective lateral surface  133 . 
     Referring now also to FIGS. 18-21, a line  181 , shown in FIG. 19, which is tangential with a point on the cylindrical bottom surface  140  of the recess  132  just prior to a further point at which the cylindrical surface  140  is “feathered out” to form a junction with the lateral surface  133 , lies at an angle “g′” to a tangent line  184  which intersects line  181  and is tangential to the lateral surface  133 . In order to properly measure the entrance angle “g′” to the recess  132 , a number of lines similar to line  181  which are tangential to a point on the cylindrical surface  140  must be considered. This may be an infinite number of lines. The entrance angle, “g′”, will be the angle between the lines  184  and  181  which will be the greatest angle that exists between the line  184  and any of the lines which can be drawn which intersect with line  184  and are tangential to a point on the cylindrical surface  140 . This angle “g′”, is representative of a recess entrance angle to the cylindrical recess  132 . In preferred embodiments the recess entrance angle is less than about 35°. Preferably, the recess entrance angle “g′” is between about 20° and about 35°. More preferably, the recess entrance angle “g′” is from about 25° to about 34°. In even more preferred embodiments, the recess entrance angle “g′” ranges from about 28° to about 33.5°. There is no preferred angle because the preferred angle may vary in response to changes in other parameters, especially the diameter of the annular base  112 . It will be appreciated that recesses to retain pivotal leaflets have existed in the bileaflet heart valve prostheses of the prior art for some time. It is believed, however, that a lower recess entrance angle will facilitate washing of the recess to minimize stagnation and potential for thrombogenic events in proximity to the recess  132 . Therefore, it is believed that diminishing the angle of entrance to the recess  132  will provide for better washing activity and lessen any potential for embolism which may exist in patients utilizing prosthetic heart valves. 
     Referring now also to FIGS. 9-15, the leaflets  114  have two sides, a top planar surface  142  and a beveled bottom side  143 . The bottom surface  143  has a peripheral bevel portion  144  proximate the peripheral edge  150  and a central portion or central bevel  145  proximate a mating edge  148 . The mating edge  148  has a narrow planar surface running nearly the entire width of the leaflet  114 . The respective leaflets are mirror images of one another in preferred embodiments so that when the respective leaflets  114  pivot to reside in the fully closed position, the mating edges  148  of the respective leaflets mate together to significantly obstruct blood flow through the very limited space between the respective mating surfaces  148 . 
     It will be appreciated that some blood will “regurgitate” between the mating edges  148  of the respective leaflets  114  when they are closed. However, this is to be expected. In fact, such blood flow, while it should be minimized, performs an important function of cleansing the mating edges  148  as the blood regurgitates between the respective edges  148 . 
     The central beveled portion  145  of the beveled bottom side includes a flat planar surface  146  which is flanked on either side along the width W of the leaflets  114 , by curvilinear side surfaces  147   a  and  147   b  which rise up proximate lateral sides  151  of the leaflets  114  to flat side bevels  147   c  and  147   d  which separate the mating edges  148  from the peripheral bevel  144  on the beveled bottom side  143  proximate the respective lateral sides  151 . The width Wps of the flat planar surface  146  is greater than one-half of the width W of the leaflet  114 , and is therefore a major portion of the central bevel  145 . As used herein, the phrase “a major portion” means a portion of the whole which has a width dimension which is at least as great as that of one-half of the width of the whole. 
     The respective lateral sides  151  of the respective leaflets  114  each have a cylindrical surface proximate the diamond-shaped cylindrical surface  154 . Notches  153 ,  155  are located adjacent to the diamond-shaped cylindrical surface  154 . The inflow notches  153  are located generally between the diamond-shaped cylindrical surface  154  and the top edge of the leaflet  114 . The generally V-shaped notch  153  is created and defined by an inflow flat  160  and an inflow side wall  156  of the diamond-shaped cylindrical surface  154 . The generally V-shaped notch  155 , called the outflow notch  155 , is created and defined by an outflow flat  162  and an outflow side wall  158  of the diamond surface  154 . 
     As previously discussed herein, washing of the various surfaces, crevices and the like by blood fluid passing through the heart valve prosthesis  110  is believed to be particularly important to reduce stagnation and potentially thrombogenic activity. The present bileaflet heart valve  110  is designed with this in mind. All of the surfaces of the present valve  110  are actively washed at one time or another in the pumping cycle of the heart in which the valve  110  is implanted. When the valve  110  is in the fully opened position all of the surfaces of the side wall  126  are actively washed by blood flowing over the surfaces, as are the recesses  132 . The leaflets  114  are also actively washed as the blood flows in the antegrade direction through the bore  116 . 
     The diamond-shaped cylindrical surface  154  also has a cylindrical radius generally consistent with the cylindrical radius of the bottom surface  140  of the recess  132 . As shown particularly in FIG. 22, when the leaflets  114  are in a fully closed position, some regurgitation of blood through the bileaflet valve  110  occurs in the retrograde direction. The regurgitation is desirable to a certain degree, so long as the energy efficiency of the pumping activity of the heart is not compromised. The regurgitation occurs in a number of areas. Referring now also to FIG. 22, and the other illustrations of the preferred bileaflet heart valve  110 , retrograde blood flow may pass between the mating surfaces  148  of the respective leaflets  114  as demonstrated by arrows  194 ,  195  and  196  in FIG.  22 . The bottom of the leaflets  114  also channel retrograde blood flow into the recesses  132  by directing the blood against the seats  136  created by the separation between the cylindrical bottom surface  140  and the upper edge  134  of the recesses  132 . An outflow side wall  158  of the diamond surface  154  may also channel retrograde blood flow to the recesses  132  and particularly to the seat  136 . This flow will then regurgitate between the leaflet  114  and the side wall  126  after it flows over the seat  136  and come out proximate the regurgitation representation arrows  191 ,  192  and  193 . It will be appreciated that flow through areas where the top planar surface  142  meets the seat  136  will be minimized and that this flow can be further minimized by widening the seat  136  further into the transverse side  131 . Additional retrograde blood flow will wash other portions of the valve  110 , especially portions of the inner wall  126 , including the lateral depressions  135  and the flat portions of the lateral surfaces  133 , and channel upwards proximate arrow  192  in FIG.  22 . It will be appreciated that there will almost always be at least some separation between the peripheral edge  150  of the leaflet  114  and the side wall  126 . This enables retrograde blood flow to regurgitate between the peripheral edge  150  and the side wall  126  proximate the entire peripheral edge  150 . Even where the top planar surface  142  of the respective leaflets  114  are pressed against the respective seats  136 , there is at least some space between the opposing surfaces for a very limited amount of “regurgitating” retrograde blood flow. The regurgitation is particularly significant proximate the transverse sides  131 . This is particularly true because of the side wall surface  126  proximate the center of the peripheral edge  150  is flush, thereby providing no obstruction to the retrograde flow of blood. It will be appreciated that the seat  136  is fully diminished to nothing in this area in preferred embodiments. A further discussion of the seats  136  follows a further description of the leaflets  114  immediately below. 
     Referring now particularly to FIGS. 16-21, a certain amount of “play” exists between the respective surfaces in the area of the diamond surface  154  and the recess  132  when the leaflets  114  are in the open position. This “play” permits a significant amount of translational movement. Because of the increased potential for translational movement between these surfaces when in the open position, the leaflets  114  have greater freedom for translational motion than is either exhibited or generally possible in any of the prior art valves which have “matched” or “parallel” surfaces in both the open and closed positions. As shown diagrammatically in FIG. 17, when the leaflets  114  are in the fully closed position, the top planar surface  142  is pressed against the seat  136  proximate the upper edge  134  of the recess  132 . Although considerable separation appears to exist between these surfaces in FIG. 17, this separation is exaggerated for clarity. During use of the valve  110 , the top planar surface  142  abuts against the seat  136 . In actual fact, the spacial relationship between the top planar surface  142  and the seat  136 , when the leaflets  114  are in the closed position, is that shown in FIG. 22, where the seat  136  cannot be separately called out because it is not visible in the view. 
     An axis  165 , parallel with respective cylindrical surfaces on diamond-shaped cylindrical surfaces  154  of the respective leaflets  114 , and perpendicular the top surface  142  will lie at an angle “k” to an axis  167 , parallel with the respective cylindrical bottom surface  140  of respective recess  132 , and perpendicular with the upper edge  134  of the recess  132 , when the leaflets  114  are in the fully opened position. When the leaflets  114  are in the fully closed position these respective axes  165  and  167  will be either superimposed upon one another, or in parallel with one another and the angle “k” will generally be about zero. In this position, therefore, the cylindrical surfaces  140  will be “matched” or “parallel” with the diamond-shaped surfaces  154  of the respective lateral sides  151  of the respective leaflets  114 . The angle “k”, shown diagrammatically in FIG. 17, is equal to the travel angle “k′”, when the leaflets  114  are in the fully open position. 
     It will be appreciated that significant translational movement is permitted when the leaflets  114  are in the open position. This can be seen in FIG. 16 where the first axis  165  of the leaflet  114  lies at an angle “k” with respect to the second axis  167  of the cylindrical recess bottom surface  140 . This translational movement of the leaflet  114 , when in the fully open position, is believed to allow the leaflet  114  to move from its fully open position to its fully closed position much faster than prior art devices. This is because the initial movement, when a retrograde flow of fluid begins, is an upward translational movement of the diamond-shaped surface  154  within the recess  132 , until the top side fulcrum edge  166  engages the upper edge sidewall or seat  136  within the recess  132 . When the top side fulcrum edge  166  engages the seat  136  within the recess  132 , the leaflet  114  has already overcome any inertia it may have had when “resting” in the fully opened position. The translational movement will subsequently give way to pivotal movement of the leaflet toward the fully closed position. This pivotal movement will occur rapidly since the initial translational movement will provide some momentum which will be translated into pivotal or annular movement toward closure of the leaflets  114 . 
     When the leaflet  114  is in the fully closed position, the initial movement of the leaflet is more likely to be followed immediately by a pivotal movement, because the cylindrical diamond-shaped surface  154  and the cylindrical recess bottom surface  140  are more closely mated as shown in FIG.  20  and the separation allowing translational movement from end to end is more limited. The leaflet  114  is likely to slip quickly from the upper side edge  134  toward the lower side sidewall  138  of the leaflet  114 . The leaflet will only begin to pivot after the bottom side fulcrum edge  164  is engaged with the lower side sidewall  138 . It will be appreciated, however, that the mechanism employed by the respective leaflets for pivoting is still a matter of inquiry and is not fully understood at this time. It is believed, however, this dynamic pivot mechanism allows for faster opening and closing of the respective valves  110 . When the valve is in the open position, and the flow direction changes from antegrade to retrograde, it is believed that the leaflet  114  begins its linear motion immediately with the change in the flow direction and the linear momentum is transferred into angular momentum as soon as the top side fulcrum edge or pivot  166  contacts the seat  136  proximate the upper edge  134  of the recess  132 . This is believed to result in quicker closing than is exhibited by prior art devices. 
     It is believed that the preferred bileaflet heart valve prosthesis  110  of the present invention provides for a lowered thrombus potential due to the consideration given to access for washing in both the antegrade and retrograde directions. Furthermore, the dynamic pivot mechanism of the preferred leaflets  114  in cooperation with the preferred recesses  132  are believed to provide for faster opening and closing of the valve and less friction in the pivot area due to the use of a “rolling” pivot mechanism wherein the pivot activity changes focus from the top side fulcrum edge  166  to the bottom side fulcrum edge  164 . The preferred valve  110  also provides for a minimized travel angle “k′” between the fully opened position and the fully closed position. It is believed that the travel angle provided in the preferred valve  110  may represent at least about 15-10° reduction in the travel angle as compared to many of the prior art devices. This reduction in the travel angle is believed to minimize angular velocity, wear, cavitation potential, and regurgitation volume, while increasing overall efficiency. 
     The upper edges  134  for the preferred leaflets  114  are believed to slow the leaflet  114  just before closure due to the presence of significant amounts of fluids which may be “squeezed” or compressed against the sidewall  126  of the annular base  112 . Because the seats slow the leaflet  114  just before closure, they are believed to have a minimizing effect on the cavitation potential. It is also believed that the use of discontinuous seats, or seats which diminish prior to continuing into a seat extending from an opposite recess allows for a slight increase in regurgitation potential proximate the center portion of the leaflet where cavitation potential is generally highest due to the likelihood that this area is likely to be subjected to a greater angular velocity as it comes toward closure against the sidewall  126 . The seats  134  also decrease leakage or regurgitation proximate the lateral sides  129  of the annular base  112  when the leaflets  114  are in the closed position. The seats  134  are also believed to provide for increased antegrade flow to wash the flow channels or recesses  132  as the leaflets  114  close. As the leaflets  114  close the fluid in the recesses  132  begins to be “squeezed” or compressed within an upper portion of the recess distal to the transverse sides  131  of the annular base  112 . The width of the seats  134  decreases as they extend from the recess  132  to the transverse side  131 . Since there is no seat  134  in the center most region of the transverse side  131  in the preferred bileaflet heart valve  110 , the fluid “squeezed” or compressed against the seats  134  is generally believed to be released through the bore  116  after it washes at least a portion of the seat  134 . While the leaflets  114  are in the closed position, the seats  134  serve to reduce retrograde leakage or regurgitation and at least a portion of the retrograde flow is channeled around the diamond surface  154 , so as to thoroughly wash these areas when the leaflets  114  are in a closed position. 
     The bottom surface of the recess  132  is in the form of a curvilinear cylindrical surface and is considered to have a generally cylindrical shape. As used herein, cylindrical surface or cylindrical shape means a surface formed by linear translation of a curve, or a surface which has a radius similar to a portion of a surface of a cylinder. The diamond surface  154  at the lateral sides  151  of the leaflets  114  have a cylindrical shape which is “consistent” with or “mates” with the cylindrical recess bottom surfaces  140  of the recesses  132 . However, as shown in FIG. 20, the diamond surface  154  is consistent with and mates with the bottom surface  140  of the recess  132  only when the leaflet  114  is in the closed position. However, when the leaflet is in the open position, as shown in FIG. 21, and as previously discussed, significant room for translational movement is provided. Furthermore, it will be appreciated that the bottom surface of the recess  140  and the matched cylindrical diamond surface  154  of the leaflet  114  will not be in alignment when the leaflet is in any position other than a fully closed position, thus allowing for significant clearance between the extreme edges of the diamond surfaces  154  and the extreme edges of the recesses  132 . Because of the increased potential for translational movement when the leaflets  114  are in positions other than the fully closed position, the leaflets  114  will exhibit greater translational freedom for motion than is possible with prior art valves having parallel or matched surfaces in all positions as described and defined in descriptions of the prior art devices. 
     As shown particularly in FIG. 16, the flat planar surface  146  of the central bevel  145  and the peripheral bevel  144  of the bottom surface of the leaflet each lie generally in a plane respectively designated by tangent lines  172  and  174 . As measured by the angle “a” between tangent lines  172  and  174 , the peripheral bevel  144  and the flat planar surface  146  of the central bevel  145  lie generally in planes which lie at an angle to one another. In preferred embodiments this angle will be less than 180°, or preferably at an angle of about 161° to about 178°, more preferably about 166° to about 173°. In preferred embodiments, the angle “a” will be about 167° to about 172°. In the most preferred embodiment under consideration, the angle “a” is about 169°. This bevel in the bottom surfaces of the leaflet  114 , allows the angle of incidence for a flow of blood in the retrograde direction parallel with the longitudinal axis  128  to be a greater angle of incidence in respect to the peripheral bevel  144  than with the flat planar surface  146  of the central bevel  145 . This is believed to be advantageous for at least two reasons. First, since there is a greater angle of incidence, the force of the blood flowing in the retrograde direction will have greater impact upon the leaflet  114  and cause it to pivot toward the fully closed position more rapidly than might otherwise be expected. Furthermore, the difference between the respective bevels, and the angle of the tangent line  176  to the top planar surface  142  allow the peripheral edge  150  to have a shorter radial closing distance to travel before the leaflet  114  is in the fully closed position than might be expected for a leaflet having parallel surfaces. 
     In preferred embodiments, the angle of the plane in which the flat planar surface  146  of the central bevel  145  rests, to a horizontal plane  170 , which angle is consistent with the angle between tangent line  172  and the plane  170 , will be an angle “a′”. In preferred embodiments, “a′” may range from about 84° to about 97°, preferably about 86° to about 95°, more preferably about 88° to about 94°, more preferably about 90° to about 92°, more preferably more than 90°, and in the most preferred embodiments, “a′” will be either 91°, or 91° or more. Similarly, the angle between the plane in which the peripheral bevel  144  rests, and the horizontal plane  170  may be measured by taking the angle “b′” between the tangent line  174  and the horizontal plane  170 . In preferred embodiments, the angle “b′” will be less than 87°, preferably less than 86°. In preferred embodiments, “b′” will range from about 78° to about 84°, preferably about 80° to about 82°, and most preferably, it will be about 81°. Similarly, the angle of the plane in which the top planar surface  142  of the top side of the leaflet  114  rests, will lie at an angle “c′” to the horizontal plane  170  as measured between the tangent line  176  and the horizontal plane  170  when the leaflet is in the fully open position. In preferred embodiments, “c′” is greater than about 78° and less than 90°, and preferably in a range of from about 82° to about 89°, preferably about 84° to about 88°. In the most preferred embodiment, “c′” is about 86°. 
     As shown particularly in phantom in FIG. 17, when the leaflet  114  begins to pivot from the fully open position to the fully closed position in response to force exerted upon the peripheral bevel  144 , the force is believed to result in an initial translational movement of the leaflet to lift leaflet  114  within the recess  132 . When the leaflet  114  has reached the fully closed position shown diagrammatically in FIG. 17, an area on the top planar surface  142  proximate the peripheral edge  150  generally proximate the respective lateral sides  151  will abut against the seat  136  on either lateral side  129  and extending at least partially into the adjacent transverse side  131 . When the leaflet  114  is in the fully closed position, the respective mating edges  148  will generally rest against one another while generally allowing at least some retrograde regurgitation of blood between the respective mating surfaces  148 . 
     It will be appreciated that the preferred embodiment of the bileaflet heart valve prosthesis  110  of the present invention will not have any sharp edges and that all edges will in fact be polished, smoothed or feathered so as to minimize shearing of blood as it passes over any of these edges. These smooth “transitions” between surfaces of all kinds will be obtained by shaving and polishing all edges so that the edges are rounded and have a smooth transition from one plane to another. Any radial surfaces of course will be polished as well. 
     As shown in FIG. 22, the amount of regurgitation of blood in the retrograde direction is believed to be significant enough to provide appropriate cleansing of the valve  110 . Heart valves are generally designed with at least some regurgitation in mind so long as the regurgitation does not reduce the efficiency of the heart. It is believed that the regurgitation is important to permit the washing of the various surfaces of the present prosthetic device. FIG. 24 generally provides a representation of the quantity (Q) of blood flowing through a bileaflet heart valve during a contraction cycle when the valve is in the aortic position. During systole, the quantity of blood passing through the valve in the antegrade direction (+) is fairly significant. As the force from the contraction diminishes from its highest point, indicated at the apex of the curve (Qsys), until the antegrade flow ends and blood begins to flow in the retrograde direction (−), the leaflets  114  remain in an open position. The retrograde flow then begins to push the leaflets  114  toward the closed position at the lowest point of the curve below the “y” axis (Qcl). As the leaflets  114  close, most of the retrograde flow is obstructed, but not all of it. The remaining retrograde flow is due to leakage around the leaflets  114 . The retrograde leakage (Ql) has been discussed herein and is believed to have a positive effect in respect to washing the various surfaces of the prosthetic heart valve, in that this “regurgitation” will “wash” the surfaces to reduce stagnation of blood as a measure against potential thrombus. 
     As shown particularly in FIGS. 6 and 7 and demonstrated diagrammatically in FIG. 23, the upper edge  134  blends or “feathers” into the inner wall  126  of the annular base  112 , as does the seat  136 , in preferred embodiments. It is believed that this has a very positive effect upon preservation of the integrity of the top planar surface  142  of the respective leaflets  114  by reducing cavitation potential. This is particularly true in an area approximately 15° to either side of a center line  184  bisecting a leaflet  114 , and in the areas most proximate to the peripheral edge  150 . The potential for negative effects of cavitation upon the top planar surface  142  is also reduced by the shortened travel angle “k′” between the location of the top planar surface  142  when the leaflet is in the fully open position, and the top planar surface  142  when the leaflet is in the fully closed position as represented by tangent line  179  of FIG.  17 . Because the preferred leaflet  114  of the present invention has a “double-beveled” bottom surface, the position of the top planar surface  142  in relation to the side wall  126  can be minimized to reduce the radial distance “k′” traveled by the top planar surface  142  in moving to the closed position. In this way, the angular speed of the movement of the most distal portion of the top planar surface  142  proximate the peripheral edge  150 , where the cavitation potential is generally believed to be the greatest, is diminished gradually when the leaflet  114  approaches the closed position. Cavitation potential is also minimized because the distance is minimized by the beveled design of the leaflets  114 . In this regard, it will be appreciated that the leaflet will continue to gain speed as it pivots through a greater radial distance. Therefore, by minimizing the radial distance between the open position and the closed position, the radial speed of the leaflet  114  can be minimized. In preferred embodiments, the travel angle “k′” will be from about 37° to about 58°, preferably about 39° to about 56°, even more preferably about 40° to about 55°, and most preferably about 45° to about 50°. Cavitation potential is also reduced because the seats  136 , extending from the respective recesses  132  on the respective lateral sides of the leaflet  114 , help to slow the closure or “cushion” the closure of the leaflet against the side wall  126  because the blood between the peripheral edge  150  and the proximate portions of the top planar surface  142  must be “squeezed” out of the intervening space adjacent the respective seat  136  as the leaflet  114  is pivoting toward the fully closed position. Furthermore, a gap  171  (shown in FIG. 8) between the seats  136  of the opposing lateral sides extending into the transverse side permits a continuing flow of blood in the retrograde direction which also helps to prevent the formation of a vacuum on the top planar surface  142  proximate the peripheral edge  150  which is generally the genesis of cavitation damage on the planar surfaces of a leaflet  114 . The “cushioning” effect of the partial or “discontinuous” seats  136  also helps to prevent stress to other portions of the leaflet  114  as they collide with the side wall  126  or the seat  136 . 
     In FIG. 23, a center line  184  extending from a center point  182  is shown superimposed upon a bottom surface of a leaflet  114 . In preferred embodiments, the respective seats  136  extending from respective recesses  132  will extend only as far as the radius lines  185  and  186  which are radially equidistance from the center line  184 . For this reason, the radial angle “i′” will equal the radial angle “h′” between the radius lines  186 ,  185  and the center line  184 , respectively, and the radial angle “j′” will equal twice either of the equal angles “i′” and “h′”. In preferred embodiments, the radial angle of “j′” will range from about 5° to about 55°, preferably about 10° to about 50°, more preferably about 15° to about 45°, even more preferably about 20° to about 40°, even more preferably about 25° to about 35°, and even more preferably about 30°. The reason for limiting the extension of the seats  136  entirely through the inner wall  126  proximate the transverse surface  131  is in part because of a desire to minimize the cavitation potential which is generally greatest within 15° on either side of a center line  184  bisecting the top planar surface  142  of a pivotal leaflet  114  of a bileaflet heart valve. It will be understood that the area having the greatest cavitation potential is likely to be at the most distal portion of the top planar surface  142  from the center point  182 , because it is this portion of the leaflet  114  which gains the most angular speed when the leaflet is pivoting toward closure and is most capable of generating the force required to create cavitation bubbles on the top planar surface  142 . Therefore, eliminating the seat  136  in this particular area, is expected to minimize cavitation potential by permitting more regurgitation through the gap  171 . 
     While the preferred embodiments of the above bileaflet heart valve  10 ,  110  have been described in detail with reference to the attached drawings, it will be understood that various changes and adaptations may be made in the bileaflet heart valve  10 ,  110  without departing from the spirit and scope of the appended claims. It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only and changes may be made in detail, especially in matters of shape, size and arrangement of parts, within the principles of the present invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.