BILEAFLET HEART VALVE

Abstract: This invention is a bileaflet heart valve (10) comprising an annular base (12) and pivoting leaflets (14). Each leaflet (14) is "free-floating" within recesses (32) without fixed rotational axis, to increase translational movement and redistribute stresses. Each recess (32) fluidly communicates with a groove extending at least partially around the inner surface (30) of the annular base (12), and flow is directed through the recesses (32) 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 DEG and the pivoting mechanism within the recess (32) including first and second fulcrum edges (164, 166) of each leaflet (14) is shiftingly engaged with side surfaces of the respective recess (32). The leaflets have a beveled bottom surface (74) having two separate planor surfaces which lie at an angle to one another. In preferred embodiments, a central region of each leaflet is spaced apart from the annular base (12) when the leaflet (14) is in a fully closed position to minimize cavitation.

Description: BILEAFLET HEART VALVE BACKGROUND OF THE INVENTION This is a continuation-in-part of both copending U.S. Patent Application Serial No. 08/412,696 filed March 29, 1995, and U.S. Patent Application Serial No. 08/546,210 filed October 20, 1995, both of which have the same title as set forth above, and for which the benefit of priority is hereby claimed pursuant to 35 USC § 120. FIELD OF THE INVENTION This 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. PRIOR ART 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 diagrammed in Figures 21-24 herein. The four combinations ranked by ascending level of risk include: (1) monoleaflet valve M with anterior orientation (Figure 21); (2) bileaflet valve with anti- anatomical orientation (Figure 22); (3) bileaflet valve with anatomical orientation (Figure 23); and (4) monoleaflet valve M with posterior orientation (Figure 24). 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'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 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. There is an inherent balancing between the selection of suitable materials for ease of fabrication, biocompatibility, strength, and weight versus the acceptable level of fragility to 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's nominal fluid pressure. Finally, although not a functional limitation, the realities of the regulatory process and competitive marketplace for prosthetic heart valves also places a burden on designers to retain some conventional features and aesthetic characteristics for purposes of FDA approval and physician acceptance. United States Patent 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 tilting 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 '658 patent also describes the use of a pyrolytic carbon coating over a metallic or synthetic substrate for fabrication of the valve's components. For comparison, United States Patent 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. United States Patent 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 '658 patent, the leaflets may each pivot fully 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 "vacuum 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 molecular level, the result is surface degradation of the leaflet which can induce stress fractures and fragmentation leading to the premature failure of a leaflet. United States Patent 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. United States Patent 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 '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. United States Patent 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 '669 contemplates alternating between multiple flow paths oriented at diverse angles to enhance the "scrubbing" effect, Vallana '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 '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 '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 '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 '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, United States Patent 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. United States Patent 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 United States Patent 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 procedure discussed above, incursion can also encompass two more complex relationships. It may be readily appreciated that the bileaflet valves described above have relatively large incursions, even in situations where the pivot axis have been raised into lobes extending above the top surface of the annular base. In those instances where the leaflets are retained by inward projections from the annular base or a trapping configuration, the incursion is even greater. Representative examples include the Hwang '367, Bokros '920, Olin '459, and Alonso '030 patents. If the pivotal axis of the leaflets are moved upwardly, incursions both above and below the annular base are encountered, as shown in the Bokros '165 patent. Alternately, the overall height of the annular base can be increased rather than raising the pivotal axis on lobes, as shown in United States Patent No. 5,137,532 to Bokros. However, even given the additional height of the annular base a significant incursion is still displayed. United States Patent 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'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 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'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 fragile structures that increase the likelihood of valve failure. United States Patent 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 '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, United States Patent 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 may also be appreciated from analyzing bileaflet heart valves, such as disclosed by the Hanson '658 and Possis '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 '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 '268 valve presents a larger central passage with narrower cross- sectional passages on each side, thereby reversing the fluid dynamics compared with the Hanson '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 leaflet presents a substantially uniform peripheral edge without pivot ears, pins, or other fixed axis of rotation, thereby distributing mechanical stresses over a larger area of the peripheral edge. In preferred embodiments the leaflets have a beveled bottom surface which minimizes the travel angle between the open and closed positions. The lateral ends of each leaflet are received within recesses where the ends are"free floating". 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 angle 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 spaced apart from the annular base to minimize cavitation potential. 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 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. 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, but wherein cavitation potential is minimized or eliminated along the central portion of the peripheral edge of the leaflet. It is a further object of this invention to provide a bileaflet valve such that the height of the pivot axis is raised and the height of the corresponding lobes may be reduced while achieving the advantages of minimal excursion and exposures. It is a further 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 drawings, in which like reference numerals indicate corresponding parts throughout the several views; Figure 1 is a perspective view of a bileaflet heart valve of the present invention in a completely closed position; Figure 2 is a perspective view of the bileaflet heart valve of Figure 1 in a completely open position; Figure 3 is a cross-sectional view of the bileaflet heart valve of Figure 1 taken through line 3-3 in Figure 1; Figure 4 is a cross-sectional view of the bileaflet heart valve of Figure 1 taken through line 4-4 in Figure 2; Figure 5 is a lateral side elevation view of the bileaflet heart valve of Figure 1; Figure 6 is a lateral side elevation view of the bileaflet heart valve of Figure 1; Figure 7 is a top plan view of the bileaflet heart valve of Figure 1; Figure 8 is a transverse cross-sectional view of the annular base of the bileaflet heart valve of Figure 7 taken through line 8-8 in Figure 7; Figure 9 is a partially broken away lateral perspective view of the annular base of the bileaflet heart valve of Figure 7; Figure 10 is a bottom plan view of a prior art leaflet having semicircular pivot ears for a bileaflet heart valve; Figure 11 is a partially broken away bottom plan view of a first embodiment of a leaflet for use with bileaflet heart valves of the present invention; Figure 12 is a partially broken away bottom plan view of a second alternate embodiment of a leaflet for use with bileaflet heart valves of the present invention; Figure 13 is a bottom plan view of a third alternate embodiment of a leaflet for use with bileaflet heart valves of the present invention; Figure 14 is a partially broken away bottom plan view of a fourth alternate embodiment of a leaflet for use with bileaflet heart valves of the present invention; Figure 15 is a partially broken away bottom plan view of a fifth alternate embodiment of a leaflet for use with bileaflet heart valves of the present invention; Figure 16 is a cross-sectional view of the leaflet of Figure 11 taken through line 16-16 in Figure 11; Figure 17 is a cross-sectional view of an alternate embodiment of the leaflet of Figure 16; Figure 18 is a lateral side elevation view of a recess and leaflet having a semi-circular pivot ear of the prior art; Figure 19 is lateral side elevation view of a recess and leaflet having a trapezoidal pivot ear of the prior art; Figure 20 is a lateral side elevation view of a recess and leaflet of the present invention with the leaflet in the fully upward (or open) translational position; Figure 21 is a perspective view of a monoleaflet heart valve with anterior orientation as known to the prior art; Figure 22 is a perspective view of a bileaflet heart valve with anti- anatomical orientation as known in the prior art; Figure 23 is a perspective view of a bileaflet heart valve with anatomical orientation as known in the prior art; Figure 24 is a perspective view of a monoleaflet heart valve with posterior orientation as known to the prior art; Figure 25 is a diagrammatic view of a prior art leaflet and recess having a semi-circular pivot ear; Figure 26 is a diagrammatic view of a prior art leaflet and recess having a trapezoidal pivot ear; Figure 27 is a diagrammatic view of a earless leaflet and recess of this invention; Figure 28 is an enlarged partially broken away detailed cross- sectional view of the embodiment of the bileaflet heart valve of Figure 3; Figure 29 is a perspective view of a bileaflet heart valve of the present invention implanted in an anatomical orientation; Figure 30 is a cross-sectional view of an alternate embodiment of the bileaflet heart valve of Figure 1 showing the leaflets in the closed position, the alternate embodiment having lateral sides and grooves in the annular base confined to the portion of the annular base adjacent the lateral regions of the leaflets; Figure 31 is a cross-sectional view of the bileaflet heart valve of Figure 30 showing the leaflets in the open position; Figure 32 is a transverse cross-sectional view of the annular base of the bileaflet heart valve of Figure 30 with the leaflets omitted and showing one of the lateral sides of the annular base with a separation between the grooves defined by the inner wall of the annular base; Figure 33 is a partially broken away lateral perspective view of the an alternate embodiment of the bileaflet heart valve prosthesis of Figure 1 showing the top surface of the separator body and the side surfaces of the recesses beveled away from a line perpendicular to the longitudinal axis; Figure 34 is a cross-sectional view similar to that shown in Figure 3 of the annular base of the bileaflet heart valve prosthesis of Figure 33 showing the orientation of the beveled surfaces; Figure 35 is a diagrammatic view of a recess in the bore of the bileaflet heart valve of Figure 1 showing the angle formed between the lateral wall of the bore and the tangential adjoining surface of the recess; Figure 36 is a bottom plan view of an alternate embodiment of a leaflet for use with the bileaflet heart valve prosthesis of this invention, that embodiment being similar to the embodiment of Figure 13 and including a generally rectangular extension or lip along the mating edge; Figure 37 is a diagrammatic view of the recess in the annular base receiving a leaflet shown in phantom pivoting between the open and closed positions, showing the beveled arcuate surfaces along which the edges of the leaflet roll as the leaflet pivots between the open and closed positions; Figure 38 is a side elevation view taken from line 38-38 in Figure 37 showing the beveled surfaces extending between the lateral wall and the bore and the floor of the recess, and the floor of the recess and the face of the separator body; Figure 39 is a top plan view of an alternate embodiment of a leaflet for use with a bileaflet heart valve of the present invention, that embodiment having portions adjoining the flat side surfaces removed along an opposingly angled path to form two sets of beveled notches in the opposing side edges of the leaflet; Figure 40 is a side evaluation view of the leaflet of Figure 39, with the leaflet inverted, showing a single set of two notches disposed on opposing sides of the leaflet; Figure 41 is a diagrammatic side elevation view of the annular base of a bileaflet heart valve of the present invention, showing a raised lateral wall and reduced lobe height compared with the annular base of Figure 3; Figure 42 is a perspective view of a preferred embodiment of the present invention showing the leaflets in a fully open position; Figure 43 is a lateral side view of the preferred bileaflet heart valve of the present invention shown in Figure 42; Figure 44 is a lateral side view of the preferred bileaflet heart valve shown in Figure 42; Figure 45 is a top plan view of the preferred bileaflet heart valve shown in Figure 42; Figure 46 is a bottom plan view of the preferred bileaflet heart valve shown in Figure 42; Figure 47 is a partially broken away elevated perspective view of the annular base of the preferred bileaflet heart valve shown in Figure 42; Figure 48 is a cross-sectional lateral view of the annular base of the preferred bileaflet heart valve shown in Figure 42; Figure 49 is a lateral side cross-sectional view of the annular base of the preferred bileaflet heart valve shown in Figure 42; Figure 50 is an elevated perspective view of the bottom side of the preferred leaflet shown in Figure 42; Figure 51A is a bottom plan view of a leaflet of the preferred bileaflet heart valve shown in Figure 42; Figure 51 B is a top plan view of a leaflet of the preferred bileaflet heart valve shown in Figure 42; Figure 52 is a vertical side view of a first lateral side of the leaflet of the preferred bileaflet heart valve shown in Figure 42; Figure 53 is a vertical side view of a second lateral side of the leaflet of the preferred bileaflet heart valve shown in Figure 42; Figure 54 is a horizontal side view of an upper edge, including the mating edge, of a leaflet of the preferred bileaflet heart valve shown in Figure 42; Figure 55 is a horizontal bottom view of the peripheral edge of the leaflet of the preferred bileaflet heart valve shown in Figure 42; Figure 56 is a diagrammatic cross-sectional view of the preferred bileaflet heart valve shown in Figure 42 with the leaflets in a fully open position; Figure 57 is a diagrammatic cross-sectional view of the preferred bileaflet heart valve shown in Figure 42 illustrating the transition of the leaflets from a fully open position to a fully closed position; Figure 58 is a partially broken away cross-sectional view of the recess as seen from the line 58-58 of Figure 48; Figure 59 is a partially broken away cross-sectional view of the recess as seen from the line 59-59 of Figure 48; Figure 60 is a partially broken away cross-sectional view of the recess similar to that shown in Figure 59 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 Figure 57; Figure 61 is a partially broken away cross-sectional view similar to Figure 60, but showing the leaflet in the fully open position as shown in Figure 42; Figure 62 is a diagrammatic drawing of an alternate valve having alternate recesses and an alternate relationship between the leaflets and the diamond surfaces on the lateral sides of the leaflets; Figure 63 is a diagrammatic drawing of another alternate valve having alternate recesses and yet another alternate relationship between the leaflets and the diamond surfaces on the lateral sides of the leaflets; Figure 64 is an elevated perspective view of the preferred bileaflet heart valve of the present invention similar to that shown in Figure 42, except that the leaflets are in a fully closed position; Figure 65 is a partially broken away top plan view of the preferred bileaflet heart valve shown in Figure 64 when the leaflets are in a fully closed position; Figure 66 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; and Figure 67 is a vertical side view of a first lateral side of an alternate leaflet similar to that shown in Figure 52, except that the alternate leaflet has beveled edges to create further surfaces so as to mate with the edges 134' and 138' of the recess 132' shown in phantom in Figure 48. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Bileaflet heart valve prostheses of the present invention are illustrated in Figures 1-67 of the present application and referenced generally by the numerals 10 and 110. A number of embodiments are disclosed, the most preferred of which are disclosed in Figures 42-67. The description of the most preferred embodiments follows the detailed description of prior embodiments presented immediately hereinbelow. Referring now to Figures 1-9, the valve 10 generally includes an annular base 12 and a pair of leaflets 14 received within the annular base and generally pivotable between a closed position, shown in Figures 1 and 3, and an open position, shown in Figures 2 and 4. The annular base 12 defines a generally circular bore 16 extending entirely through the base 12, and a groove 18 circumscribing the generally cylindrical exterior surface 20 of the base 12 such that the valve 10 may be mounted within a suture ring (not shown) and rotate therein in a manner that is well known to the art. The groove 18 preferably has a generally u- shaped cross-section as shown in Figures 3 and 4. The annular base 12 includes an inner wall 26 defining the bore 16, and a pair of lobes 22 extending above a top surface 24 of the annular base 12 proximate a midline of the valve 10 shown in Figure 2. The inner wall 26 may be tapered or angled radially outward as much as approximately 10° relative to a longitudinal axis 28 of the valve 10 shown in Figures 3 and 4, or may be generally vertical as shown in Figures 8 and 9. The longitudinal axis 28 of the valve 10 generally approximates the linear flow path through the bore 16. The lobes 22 of the annular base 12 are spaced apart on the diametrically opposing lateral sides of the valve 10. The inner wall 26 of the annular base 12 and the inner surfaces 30 of the lobes 22 define a pair of recesses 32 extending radially outward from the bore 16. Each recess 32 is divided into two halves, each half corresponding to one of the two leaflets 14. As may be seen in Figures 2, 4, 8, and 9, the adjacent halves of each recess 32 are divided by a center line 34. The annular base 12 includes a separator body 36 disposed between each half of each recess 32 and defining, in some embodiments, a generally triangular or pyramidal top. Referring again to Figures 3, 4, 8, and 9, the annular base 12 further defines an interior groove 40 extending from and fluidly communicating with the lower portion of each half of the recesses 32 around the circumference of the inner wall 26 of the annular base 12, thereby defining a continuous passage extending around the periphery of circumference of the bore 16. Referring specifically to Figure 8, the interior groove 40 has a minimum height at the central locations 42 disposed between and most distantly from the recesses 32 or lateral sides of the annular base 12, within two opposing regions that may be referred to as the lateral sides of the annular base 12. The two lateral sides of the annular base 12 are therefore oriented at right angles to the two lateral sides of the annular base, and divide the valve 10 and annular base into four equal quadrants. The interior groove 40 flares or tapers upwardly along its top edge or surface 44 to increase the height of the groove 40 as the groove 40 traverses the inner wall 26 toward the recesses 32, such that the groove 40 has a height approximately equal to the height of the separator body 36 where the top edge 44 of the groove 40 intersects the adjacent recess 32. The bottom edge or surface 46 of the groove 40 remains generally horizontal throughout the lateral side of the annular base 12, and remains generally horizontal or tapers upward only slightly as the groove 40 traverses from the lateral side to the lateral side of the annular base 12 and intersects the bottom edge of the separator body 36. The bottom edge 46 of the groove 40 intersects the separator body 36 at a point where the inner face of the separator body 36, the innermost bottom edge 46 of the groove 40, and the inner wall 26 of the annular base 12 remain flush or coplanar with one another. Similarly, the top edge 44 of the groove remains flush or coplanar with the inner surface 30 of the lobe 22 where the top edge 44 of the groove 40 intersects the recess 32. As further described in detail below, the opposing ends of a leaflet 14 are received within the corresponding and aligned halves of the two recesses 32 on each of the lobes 22, and pivot freely within the recesses 32 between the closed and open positions. Referring now to Figures 10, 13, 16, and 17, leaflets 14 generally have a straight mating edge 48, curved peripheral edge 50 extending from and between the opposing ends 52, 54 of the straight mating edge 48, a top planar surface 56 and a bottom planar surface 58. From a center point 60 along the mating edge 48 of each leaflet 14 spaced equidistantly from each end 52, 54, the leaflet 14 may be divided into three equiangular regions including a first lateral portion 62, a second lateral portion 64, and a central portion 66 disposed between the first and second lateral portions 62, 64, respectively. The first and second lateral portions 62, 64 of each leaflet 14 define and encompass the opposing lateral ends of that leaflet 14. Each leaflet 14 also defines a longitudinal or planar axis 68 oriented generally parallel to the top and bottom planar surfaces 56, 58. Referring again to Figures 3, 4, 8, and 9, it will be appreciated that the surfaces defining the top and bottom of the interior groove 40 extend from the corresponding top edge 44 and bottom edge 46 adjacent the inner wall 26 outwardly to converge at a radial vertex 70 forming a generally right angle therebetween in the proximity of the vertex 70. The top surface and bottom surface of the groove 40 are therefore oriented such that the peripheral edge 50 of the corresponding leaflet 14 does not contact the bottom surface or bottom edge 46 of the groove 40 as the leaflet 14 pivots through an arc from the closed position to the open position. Correspondingly, sufficient clearance is also provided between the lower surface and lower edge 46 and the peripheral edge 50 of the leaflet 14 to permit translational movement of the leaflet 14 along its planar axis 68. As such the bottom surface of the groove 40 is preferably curved or arcuate to mirror the maximum arc transcribed by the peripheral edge 50 of the leaflet 14. Similarly, it may be appreciated that the top surface 44 of the interior groove 40 is oriented such that it is generally parallel to and coplanar with the top surface 56 of the leaflet 14 along the peripheral edge 50 of the leaflet 14 when the leaflet 14 pivots to the fully closed position as shown in Figure 3, and may therefore be substantially planar between the vertex 70 and junction between the top edge 44 and side wall 26. The top edge or surface 44 of the interior groove 40 forms a stop or seating surface against which the top planar surface 56 of the leaflet 14 rests to prevent retrograde flow through the bore 16 of the valve 10. However, as previously described, reciprocal movement and mechanical contact between the leaflet 14 and annular base 12 accelerates the structural degradation and weakening of the leaflet 14 cause by cavitation forces. To reduce the potential for deleterious cavitation effects, the top planar surface 56 and peripheral edge 50 of the leaflet 14 is disposed or spaced-apart a distance from the top plana surface 44 of the interior groove 40 along at least the central portion 66 or region of the leaflet 14. This may be accomplished in any one of a number of ways. First, the radius of the peripheral edge 50 of the leaflets 14 maybe foreshortened within the central region 66 of the leaflet 14, thereby preventing contact between the leaflet 14 and annular base 12 in that region 66, but also permitting increased retrograde leakage through the separation or gap 72 that is formed. A second method is to curve the top surface and top edge 44 of the interior groove 40 upwardly as the top edge 44 approaches the lateral sides of the annular base 12 and recesses 32, thereby limiting contact between the leaflets 14 and annular base 12 to within the lateral side regions 62, 64 of the leaflets 14. Although contact between the lateral side regions 62, 64 of the leaflet 14 and the annular base 12 may induce some cavitation, it is contained within regions 62, 64 of the leaflet 14 where the linear or angular speed of the peripheral edge 50 of the leaflet 14 through the fluid is significantly slower than in the central region 66. Since the most deleterious cavitation effects are proportionally related to the relative speed of the leaflet 14 through the fluid medium, and this relationship is not linear around the peripheral edge 50 of leaflet 14, the result is to diminish a substantial proportion of the cavitation effects induced for the entire leaflet 14 by eliminating contact between the leaflet 14 and annular base 12 along approximately one third of the peripheral edge 50 of the leaflet 14. These two methods will reduce the speed of the leaflet 14 as it closes. As the edge 50 of the leaflet 14 approaches surface 40 of the base 12, the decreasing gap 72 offers increased resistance to the fluid that squeezes through the gap 72. This resistance to "squeeze flow" results in reduced speed of the leaflet 14 at closing. A third method is to restrict contact between the leaflet 14 and the annular base 12 to the surfaces of the recesses 32 and separator body 36. While this method reduces the effects of reciprocal movement and contact along substantially all of the entire peripheral edge 50 except the regions of the lateral sides 62, 64 forming the pivotal contacts within the recesses 32— where cavitation effects are at a minimum due to the relatively slow movement of the leaflet 14 through the fluid medium and tangential contact does not significantly magnify those effects—it also focuses additional mechanical stresses on the lateral sides 62, 64 and peripheral edge 50 of the leaflet 14 in the areas directly adjacent to the pivot axis. The linear speed of the leaflets 14 is not diminished since there is no area to cause the resistance or "squeeze flow." Consequently, the advantage of redistributing mechanical stresses created be restraining the pivotal movement of the leaflet 14 over a larger area of the peripheral edge 50 must be balanced against the benefits of reducing deleterious cavitation effects along a segment of the peripheral edge 50. A fourth manner to prevent seat contact between the top surface 56 and peripheral edge 50 of the leaflet 14 and the annular base 12 would be to place discrete downward projections (not shown) along the top surface of the interior groove 40 against which the top planar surface 56 of the leaflet 14 would impinge. However, it is believed this method could focus cavitation forces at contact points along the surface 56 to peripheral edge 50 of the leaflet 14 corresponding to the locations of the projections, and lead to the creation of deeper structural fault lines within the leaflet 14 radiating from the contact points between the leaflet 14 and projections. It will be appreciated that the separation or gap 72 formed between the peripheral edge 50 of the leaflet 14 and the annular base 12 will permit some retrograde leakage. The flow rate and volume of that leakage is reduced (1) by having the peripheral edge 50 of the leaflet 14 remain in close proximity to the bottom surface of the interior groove 40, and (2) by receiving the peripheral edge 50 of the leaflet 14 within the interior groove 40 beneath a seating surface defined by the annular base 12, such that there are little or no observable spaces or openings between the side wall 26 of the annular base 12 and the peripheral edge 50 of the leaflet 14 when viewed from a perspective along the longitudinal axis 28 of the valve 10. As previously discussed, the pivotal movement of the leaflets 14 are at least in part constrained against movement by contact between the top and bottom surfaces 56, 58 of the leaflet 14 and portions of the recesses 32 and separator body 36. In particular, in the fully closed position the mating edges 48 of the leaflets 14 are disposed closely confronting or contacting one another, with upward movement of the leaflets 14 due to retrograde pressure adjacent the mating edges 48 being restrained by contact between the top surface 56 of the leaflet 14 and the top edge 44 of the interior groove 40 at the junction between the interior groove 40 and recess 32. Upward movement of the leaflets 14 along the lateral sides of the annular base 12 is also restrained by the top edge 44 of the interior groove 40, except in the central region 66 where the separation or gap 72 is maintained to reduce the effects of cavitation. When in the fully open position as shown in Figure 4, the bottom surface 58 of the leaflets 14 contact the tapered opposing sides 74 of the separator body 36, and the top surfaces 56 of the leaflets 14 adjacent the mating edges 48 contact the outer planar walls 76 of the recesses 32. As such, the tapered opposing sides 74 of the separator body 36 and outer planar walls 76 of the recesses 32 hold the leaflets 14 in position at a slightly acute angle relative to the longitudinal axis 28 and one another, thus preventing over-rotation of the leaflets 14 to a parallel position in which pressure exerted by retrograde circulation could be insufficient to initiate closure of the valve 10. As such, it may be appreciated that a top segment of each lobe 22 may be removed, or reduced in the area directly adjacent the recesses, without diminishing the operation of the valve 10. In addition, the top surface of the lobe 22 may be opened to fluid communication with the recesses 32 by the removal of material forming the lobe 22 along the inner surface 30 of the lobe 22. The top segment of the lobe 22 along the inner surface 30 may be removed down to a point at or slightly above a horizontal plane defined by the peaks or apexes 80 of the separator bodies 36. This segment is denoted by the horizontal phantom line 77 in Figure 28. Top surfaces 38 of the separator body 36, as well as the outer planar walls 76 of the recesses 32, may also be beveled, tapered, or angled slightly relative to the orientation shown. In Figures 34, 37, 38 and 41, these surfaces are beveled. Translational movement of the leaflets 14 is restrained by contact between the peripheral edge 50 of each leaflet 14 along a limited zone within the lateral side regions 62, 64 and the confronting curved radial surfaces of the recesses 32. Referring particularly to Figures 10, 13, 18-20, and 26-28, the relative degrees of translational movement between a leaflet 14 having a semi¬ circular pivot ear 84 (Figures 10, 18, and 25) a trapezoidal pivot ear 86 (Figures 19 and 26), and no pivot ear (Figures 13, 20, and 27) are diagrammed. It will be appreciated that the translational movement D for a semicircular pivot ear 84 disposed between two flat edges 88 and within a semicircular recess 32 will be limited by the spacing distance X between the edge 88 of the pivot ear 84 and the wall of the recess 32, which is dependent upon the radii of both the pivot ear 84 and the recess 32. Similarly, the translational movement D for the trapezoidal pivot ear 86 is controlled by the height and baseline width of the pivot ear 86 and the depth and surface width of the recess 32, plus the relative angles of the beveling of the recess 32 and pivot ear 86. In practice, the translational movement D of the trapezoidal pivot ear 86 is less than for a semicircular pivot ear 84, and may be selectively limited by increasing the width of the distal end of the pivot ear 84 relative to the recess 32, or decreasing the surface width of the recess 32 relative to the baseline width of the pivot ear 84. Adjusting the bevel angles will also control the minimum spacing X between the pivot ear 86 and recess 32. By comparison, the translational movement D of the lateral side region 62, 64 for an earless leaflet 14 is limited only by the height of the leaflet 14 along the longest secant line S that can be draw bisecting the limited zone within the lateral side regions 62, 64 of the leaflet 14 between two points along the peripheral edge 50 which contact the confronting curved radial surfaces of the recess 32. Consequently, the translational movement D for an earless leaflet 14 can be increased significantly over that for a leaflet 14 having either a semicircular or trapezoidal pivot ear 84, 86. The degree of translational movement permitted for an earless leaflet 14 is therefore defined by the depth, width, and arc shape of the curved radial surfaces 82 of the recess 32 confronting the peripheral edge 50 of the leaflet 14 proximate the corresponding lateral side region 62, and the corresponding arc shape of the peripheral edge 50 of the leaflet 14. Since the translational movement of the earless leaflet 14 can actually be controlled by adjusting the arc shape or curvature of the curved radial surfaces 82 of the recess 32 at either the top or bottom thereof, it is possible to define a recess 32 in which the profile of the peripheral edge 50 of the leaflet 14 mates substantially with the curved radial surfaces 82 of the recess 32 when the leaflet 14 is in the open position and disposed as high as possible on the annular base 12, as shown in Figure 20. In such an instance, the degree of translational freedom for the leaflet is controlled by adjusting the arc shape at the bottom of the recess 32 relative to the shape of the peripheral edge 50 of the leaflet 14. This maximizes the translational movement available to the leaflet 14 when pivoting between the open position (where the leaflet 14 is in its lowest position relative to the annular base 12) and the closed position, prior to the leaflet 14 contacting the top of the recess 32 as shown in Figure 20. It will be appreciated that the peripheral edge 50 of the leaflet 14 may form composite curves within and between the lateral side regions 62, 64 and the central region 66, so that the separation or gap 72 between the side wall 26 or interior groove 40 is not affected by the curvature of the peripheral edge 50 within the lateral side regions 62, 64. Similarly, the maximum incursion or radial exposure is not adversely affected by the curvature of the peripheral edge 50 of the leaflet 14 within the lateral side regions 62, 64, except to the extent that downward translational movement of the leaflets 14 when the valve 10 is in the open position may be responsible for some incursion depending upon the height of the annular base 12, pivot axis, and shape of the leaflet 14. Referring now to Figure 4, it will be appreciated that despite the increased translational movement of the leaflets 14 compared with those having pivot ears 84 and 86, the peripheral edges 50 of the leaflets 14 exhibit little or no incursion below the bottom plane 90 of the annular base 12 even when the valve 10 is in the completely open position. Figure 4 depicts a substantially zero incursion distance I measured from the bottom plane 90 of the annular base 12 and the peripheral edges 50 of the leaflets 14. Even in a configuration where minimal incursion I is incurred, as depicted by the phantom lines in Figure 4, it should be noted that radial exposure is confined to a region extending from a central plane (consistent with longitudinal axis 28) bisecting the lobes 22 of the valve 10 to a point approximately one-half the distance between the plane or longitudinal axis 28 and the side wall 26 of the bore 16. Similarly, that radial exposure is confined to approximately one-half or less of the distance between the plane or longitudinal axis 28 and the exterior surface 20 of the annular base 12. Lateral exposure is similarly negligible. Referring now particularly to Figures 11-15, several embodiments of the earless leaflet 14 are shown. For comparison, a conventional leaflet 14 having a semicircular pivot ear 84 disposed between two flat surfaces 88 such as disclosed in the Hanson '658 patent is shown in Figure 10. Figure 11 shows a leaflet 14 in which the peripheral edge 50 within the lateral side region 64 forms two composite curves 92, 94, the first curve 92 having a smaller radius r and being disposed more closely adjacent to the mating edge 48 than the second curve 94 having a greater radius R. The two curves 92, 94 meet at a junction 96 where the two curves 92, 94 are blended together smoothly. The primary distinction between this leaflet 14 and a conventional leaflet 14 having a pivot ear 84 as shown in Figure 10 is that the first curve 92 is not bounded on both sides by a flat surface 88. As shown in Figure 16, the mating edge 48 is angled from the top planar surface 56 to the bottom planar surface 58 at an angle of about 40° to about 60° relative to the planar axis 68 of the leaflet 14. Referring now to Figure 17, it may be appreciated that the exposed edges 48, 50 of the leaflet 14 extending between the planar surfaces 56, 58 may be rounded, radiused, or beveled as desired. Because the surfaces within the pivotal region sweep a plane and a spherical section when the leaflet 14 pivots, the configuration shown in Figure 11 is termed a flat spherical (FS) design. It will be further appreciated that while the peripheral edge 50 in the central region 66 can be completely curved along the radius of the second curve 94, the overall height of the leaflet 14 described above represents the truncation of the central region by a flat portion 98, thereby presenting a gap or separation 72 to mitigate against cavitation as described above. Figure 12 describes a variation to the leaflet shown in Figure 11 wherein the first curve 92 is connected to the first endpoint 52 by a line intersecting the first endpoint 52 and laying tangential to the first curve 92. The dimensions of this leaflet 14 are generally the same as the dimensions of the leaflet 14 of Figure 11, with the exception that the length of the mating edge 48. Because the surface within the pivotal region sweep out a conical and a spherical section when the leaflet 14 pivots, this configuration is termed a cone spherical (CS) design. Figure 13 describes a further embodiment termed a spherical flat (SF) design, in which the first curve 92 and second curve 94 are divided by a flat region 100. Figure 14 describes a similar embodiment termed a conical flat (CF) design, in which a first flat segment 102 extends from the first endpoint 52 of the mating edge 48 to a point tangential with the first curve 92, which is a small radius r into a second flat segment 104, and which progresses to a second curve 94. •Figure 15 describes a more complex embodiment termed a cone spherical cone (CSC) design that is similar to that of Figure 14 with the exception that the second flat segment 104 is not parallel with the centerline L bisecting the mating edge 48 at the center point 60. In this embodiment, the peripheral edge 50 extends from the first endpoint 52 of the mating edge 48 along a first flat segment 102, a first curve 92 having a radius r which progresses to a second curve 94 having a radius R. Alternately, the first flat segment 102 may also be a composite of two differently angled straight sections (not shown) with the section most closely adjacent the mating edge 58 flaring or tapering outwardly relative to the next adjacent flat section, thereby creating two conical zones adjacent to the mating edge 48. Referring again to Figures 21-24 and Figure 29, a bileaflet heart valve 10 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 Figures 22 and 23, 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 Figures 21 and 24. Referring now particularly to Figures 30-34 and 37-38, two alternate embodiments of the bileaflet heart valve 10 of this invention are shown. These embodiments have been generally described above with reference to the preferred embodiments, and are described separately herein in greater detail with the understanding that these alternate embodiments may be combined with other features and elements of the bileaflet heart valve 10 in the preferred embodiments. It will be appreciated that the inner wall 26 of the annular base 12 may be fabricated such that a smooth and continuous surface forms a separation between opposing sides of the groove 40, with the top edge 44 of the groove 40 contacting the leaflets 14 only within the lateral regions 62, 64 defined by lines oriented at an angle of approximately 60° relative to the centerline L of the corresponding leaflets 14. Referring now specifically to Figures 33 and 34, the top surfaces 38 and side surfaces 74 of the separator body 36, the side surfaces 76 of the recesses 32, and the top edge 44 of the groove 40 adjacent the recesses 32 are each beveled or tapered away from a normal orientation shown in Figures 3, 4, 8, and 9 in which the surfaces 38, 74, 76, 44 and corresponding junction edges are oriented in planes which are generally parallel with radial lines (not shown) extending away from the annular base 12 in a direction perpendicular to, and intersecting with, the longitudinal axis 28. As such, in the beveled or tapered embodiment shown in Figures 33 and 34, those surfaces 38, 74, 76, 44 are oriented at an acute angle relative to those radial lines extending away from the annular base 12 in a direction perpendicular to and intersecting with the longitudinal axis 28, thus forming generally pyramidal or trapezoidal structures when viewed from the perspective of the longitudinal axis 28 as shown in Figure 34. Referring now particularly to Figures 37 and 38, the beveled or tapered surfaces 44, 76 between the inner or lateral side wall 26 of the bore 16 and the floor of the recess 32, and the beveled or tapered surfaces 38, 74 between the lateral side wall 32 of the separator body 36 and the floor of the recess 32, are shown diagrammatically in greater detail. Referring to Figure 37, it will be appreciated that the two pairs of beveled surfaces 44, 76, and 38, 74, respectively, are joined along a radius to form a generally smooth and continuously arcuate path 107 along which the corresponding regions of the peripheral edge 50 of the appropriate leaflet 14 will pivot or "roll" as the leaflet 14 pivots or moves between the open and closed positions, thereby distributing stress and frictional forces over an extended area of the peripheral edge 50 of the leaflet 14 and the generally smooth and continuously arcuate path 107. Referring to Figure 38, a bevel angle will range from 20° to 40°, with a preferred angle "x" of approximately 30°, measured relative to perpendicular with either the lateral side wall 26 of the bore 16 or the floor of the recess 32, will result in the junctions 109 between the lateral side wall 26 along the bore 16 or the face of the separator body 36, and the beveled surfaces 44, 76 and 38, 74 respectively, being spaced apart from the normal or perpendicular lines intersecting the beveled surfaces 44, 76, and 38, 74 and the floor of the recess 32 by a distance "a" on the order of .008"-.020" measured along a line parallel with the longitudinal axis 28. Referring now particularly to Figure 35, a recess 32 in the side or lateral wall 26 of the bore 16 is shown diagrammatically, the recess 32 forming an angle of 35° or less between the lateral wall 26 and a line "T" tangential with the surface of the recess 32 closely proximate to and adjoining or intersecting the lateral wall 26. It will be appreciated that the intersection between the lateral wall 26 and recess 32 may be radiused, angled, or tapered slightly, either intentionally or due to the limitations of fabrication, however such radiusing, angling, or tapering is disregarded in measuring the angle between the lateral wall 26 and the line "T" tangential in the region closely proximate to the lateral wall 26. This angle will be the same as the angle formed between the optimal laminar flow vector "V" of the blood circulating in either the antegrade and retrograde directions closely adjacent to and parallel with the lateral wall 26 within the bore 16 of the valve 10 in the regions approaching the recess 32, with a portion of the blood diverting from this optimal laminar flow vector "V" to traverse along the surface of the recess 32. An angle of approximately 30° has proven to be effective to achieve the desired effect in providing a cleansing blood flow within the recess 32 proximate to the intersection between the lateral wall 26 and surface of the recess 32. However, it will be appreciated that angles of varying degree measuring less than 35° may similarly be suitable or even more effective in other applications or embodiments. Referring now to Figures 36 and 39-40, alternate embodiments of the leaflet 14 shown in Figure 13 are described. The elements of the leaflet 14 of Figure 36 conform substantially to those of the SF design shown in Figure 13, with the addition of a generally rectangular flange or lip 118 disposed along and extending from the mating edge 58 of the leaflet 14 in Figure 13, thereby resulting in a displaced mating edge 48. The elements of the "twist" embodiment of the leaflet 14 shown in Figures 39 and 40 conform substantially to those of designs shown in Figures 11, 13 and 36, but further define two pairs of beveled notches 121, 122 formed by removing portions of the side edges 92, 94 of the leaflet 14 adjacent to and bounded by the two curved segments 92, 94 along angled paths extending between the two planar surfaces 56, 58 of the leaflet 14. It will be appreciated that the notches 121, 122 are formed in one or both planar faces 56, 58 of the leaflet 14 adjacent one or both side edges 92, 94, 100 of the leaflet 14, and may extend at any angle up to the angle defined by the angled paths extending completely between the planar faces 56, 58 of the leaflet 14. By incorporating such angled notches 121, 122 into regions of the leaflet 14 adjacent the two curved segments 92, 94 of the peripheral edge 50, and permitting the lateral stability of the leaflet 14 within the annular base 12 to be maintained by the intervening flat side edges 100 and the flange or lip 118, the angles of the contact surfaces 74, 76 defined by the recesses 32 into the lateral wall 26 of the annular base 12 and the separator body 36 will be reduced an amount generally proportional to the particular angle formed between the angled surfaces of the notches 121, 122 and the planar faces 56, 58 of the leaflet 14. Providing two sets of diagonally opposing notches 121, 122 on each planar face 56, 58 of each leaflet 14 corresponding to the regions of contact between the surfaces 74, 76 of the recess 32 which prevent pivotal movement of the leaflet 14 beyond the open position will permit the "top" of surface 76 and the "bottom" of surface 74 to be moved angularly inward and the "bottom" of surfaces 74 to be moved angularly outward relative to the corresponding positions of those surfaces 74, 76 if the leaflet 14 did not possess the notches 121, 122 to define a narrower recess 32. Since the increased angle of the surfaces 74, 76 relative to the longitudinal axis 28 means that the cutting tool 124 used to remove portions of the lateral wall 26 of the annular base 12 to form the recesses 32 need only pivot through a narrower or lesser arc, the height of the top surface 24 of the annular base 12 may be increased correspondingly as shown in a phantom in Figure 41 and still permit clearance as the cutting tool 124 pivots through the arc necessary to define the contact surfaces 74, 76 of the recesses 32. Raising the height of the top surface 24 of the annular base 12 has the effect of increasing the length and uniformity of the bore 16, and effectively lowering the relative height of the lobes 22, resulting in the advantages previously discussed. Similar results could be achieved regarding the contact surfaces 38, 44 defining the closed position of the leaflets 14, however, the generally open top of the annular base 12 and bore 16 mitigate against interference with the cutting tool 75 when pivoted to define those surfaces 38, 46, and this process would result in reducing the available angle of the notches 121, 122 aligned with the contact surfaces 74, 76 corresponding to the open position due to the presence of the aligned notches 121, 122 on the opposing planar surfaces 56, 58 of the leaflet. The bileaflet heart valve prostheses 10, 110 of the present invention are 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 Figures 42-67, preferred embodiments of the bileaflet heart valve prosthesis 110 are described. The preferred embodiment of the bileaflet heart valve 110 of the present invention shown in Figure 42 includes an annular base 112 and first and second leaflets 114. The first and second leaflets 114 are mounted within the annular base 112 for pivotal movement between a fully open position, shown in Figures 42-46 and diagrammatically in Figure 56, and in phantom in Figure 57, and a fully closed position shown in Figures 64-65 and diagrammatically in Figure 57. 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 path for circulation of fluid or blood through the bore 116. The top surface 124 of the annular base 112 is raised proximate opposing lateral sides 129. On the inner wall 126 of the annular base 112 proximate the opposing lateral sides 129, flow diverter protrusions 130 extend away from the inner wall 126 toward a center 182 of the annular base 112 and the circular bore 116. The flow diverter protrusions 130 are placed generally between a pair of recesses 132 in each of the respective lateral sides 129 of the base 112. Referring now to Figures 42-49 and 58-59, the recesses 132 extend into the respective lateral sides 129, thereby displacing a cylindrical bottom surface 140 of the recess 132 from the respective lateral surface 133 proximate the respective lateral side 129 and the respective recess 132. In preferred embodiments of the present invention, the recesses 132 extend away from the flow diverter protrusions 130. As the recess 132 extends away from the flow diverter protrusion 130, it passes through a cylindrical radius which is "feathered out" as the cylindrical surface 140 approaches a junction with the respective lateral surface 133. As shown specifically in Figure 59, a line 181 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 angel, "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 15° and about 35°. More preferably, the recess entrance angle "g' " is from about 18° to about 34°. In even more preferred embodiments, the recess entrance angle "g' " ranges from about 21° to about 33°. It will be appreciated that recesses to retain pivotal leaflets have existed in the bileaflet heart valve prosthesis of the prior art for some time. It is believed that a lower recess entrance angle will facilitate washing of the recess to minimize stagnation and potential for thrombogeni: 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. The flow diverter protrusion 130 also helps to divert blood passing through the circular bore 116 into the recesses 132. It will be appreciated that the protrusion 130 can channel fluids passing through the bore 116. Since the protrusion 130 lies at one end of the respective recesses 132, as is clearly illustrated in Figure 48, a portion of the blood flowing into the circular bore 116 when the leaflets 114 are in an open position will channel around the flow diverter protrusion 130 and into the adjacent recesses 132 and along the bottom side wall 138 of the cylindrical recess 132. In preferred embodiments of the present bileaflet heart valve prosthesis 110 of the present invention, the side wall 138 may be beveled in a manner demonstrated in phantom in Figure 58 to form a side wall "138' ". As shown particularly in Figures 50-55, the leaflets 114 have a top planar surface 142 and a beveled bottom surface having a peripheral bevel 144 proximate the peripheral edge 150 and a central bevel 146 proximate a mating edge 148 which is preferably a straight edge having a flat surface. 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. Lateral sides 151 of the respective leaflets 114 have a cylindrical surface proximate the diamond surface 154. Generally V-shaped notches 153, 155 are located adjacent to the diamond surface 154. The inflow notches 153 are located generally between the diamond surface 154 and the angular flat surface 152. The generally V- shaped notch 153 is created by an inflow flat 160 and an inflow side wall 156 of the cylindrical diamond surface 154. The generally V-shaped notch 155, namely the outflow notch 155, is defined by an outflow flat 162 and an outflow side wall 158 of the diamond surface 154. As shown particularly in Figures 52 and 53, a beveled outflow side wall "158' " may be provided as shown in phantom in those Figures. The beveling of the outflow side wall "158' " also extends around to a portion of the side surface "161' " proximate the diamond surface 154 which would be part of the central bevel surface 146 or the portion of the surface 161 in not beveled. This beveling is designed to cooperate with the beveling of the side wall 138 to form the beveled side wall "138' " shown in Figure 48 and 58. The beveled portions of the respective surfaces "158' ", "161' " and "138' " will cooperate to retain the leaflets 114 within the recess 132 and permit greater cleansing of recesses 132 and greater "sweeping" of the surfaces in these areas. 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 and the flow diverter protrusion 130 is actively washed as it channels blood into the recesses 132 and particularly toward the side wall 138 or its beveled counterpart "138' ". The leaflets 114 are also actively washed as the blood flows in the antegrade direction through the bore 116. In addition, the flow diverted protrusion 130 channels blood into the recess 132 proximate the diamond surface 154 on the lateral side of the leaflet 114 which can rest against the cylindrical bottom surface 140 of the recess 132. 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. Each leaflet 114 also has an angular surface 152 which directs a certain amount of blood flowing in an antegrade direction, when the leaflets 114 are in an open position, to the inflow side wall 156 of the diamond surface 154. As shown particularly in Figure 64, 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 Figure 64, 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 Figure 64. The bottom of the flow diverter protrusion 130 also channels retrograde blood flow into the recesses 132 where it will be directed up against the seats 136 created by the separation between the cylindrical bottom surface 140 and the upper edge 134 of the recess 132. An outflow side wall 158 of the diamond surface 154 may also channel retrograde blood flow to the recess 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 arrow 191 in Figure 64. Additional retrograde blood flow will simply wash the flow diverter protrusion 130 and channel upwards proximate arrow 192 in Figure 64. 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. 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 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 Figures 56-57 and 60-61, 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 exhibited generally possible in any of the prior art valves which have "matched" or "parallel" surfaces in both the open and closed positions. This potential for translational movement is not present when the leaflets 114 are in the closed position. An axis 165, parallel with respective cylindrical surfaces on "diamond" 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 axis 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" on "parallel" with the diamond- shaped surfaces 154 of the respective lateral sides 151 of the respective leaflets 114. The angle "k" equal to the travel angle "k' ", when the leaflets 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 Figure 56 where the first axis 165 of the leaflet lies at an angle "k" with respect to the second axis 167 of the cylindrical recess bottom surface 140. As shown in Figure 61, significant room for translational movement is provided. This translational movement of the leaflet 114, when in the fully open position, allows 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 leaflet 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 Figure 60 and translational movement from end to end is more limited. The leaflet 114 is likely to slip from the upper side edge 134 toward the lower side sidewall 138, however, because 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, that the present 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 flow direction and the linear momentum is transferred into angular momentum as soon as the top side fulcrum edge or pivot 166 contacts the side ' wall 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. Similarly, from moving from the fully closed position to the fully open position, it is believed that the angular momentum is transferred into linear motion at the end of the leaflet 114 movement, resulting in quicker opening than is possible with the 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 a about 15-20° 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 seats 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 tra sverse 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. While the leaflets 114 are in a fully opened position a certain amount of the flow is believed to be channeled between the angular surface 152 of the leaflet 114 and the flow diversion protrusion 130 of the angular base 112. Thus, the lower surface 138 of the recess 132 is thoroughly washed while the leaflets 114 are in the open position. The bottom surface of the recess 132 is in the form of a curvilinear surface or cylindrical surface and is considered to have a generally cylindrical shape. As used herein, cylindrical surface or cylindircal shape means a surface form by linear translation of a curve, or has a radius similar to, a portion of a surface of a cylinder. The diamond surface 154 of the leaflets 114 have a cylindrical shape which is "consistent" with or "mates" with the curvilinear or cylindrical recess bottom surfaces 140 of the recesses 132. However, as shown in Figure 60, 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 Figure 61, 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 Figure 56, the central bevel 146 and the peripheral bevel 144 of the bottom surface of the leaflet lie generally in a plane respectively designated by tangent lines 172 and 174. As measured by 6/29957 _____„, PCT/US96/04385 - 59 - the -angle "a" between tangent lines 172 and 174, the peripheral bevel and the central bevel lie generally in planes which lie at an angle to one another. In preferred embodiments this angle will be about 5° to about 16° from 180°, or at an angle of about 164° to about 175°, more preferably about 166° to about 173°. In the most preferred embodiments, the angle "a" will be about 167° to about 172°. In the most preferred working model, the angle "a" is about 169°. This bevel in 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 central bevel 146. 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 central bevel 146 to a horizontal plane 170, which 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 80° to about 97°, preferably about 82° to about 94°, more preferably about 84° to about 88°, and in a most preferred embodiment, "a' " will be about 86°. Similarly, the angle between the peripheral bevel 144 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 68° to about 84°, preferably about 70° to about 80°, more preferably about 73° to about 77°, and most preferably, it will be about 75°. Similarly, the top planar surface 142 of the top side of the leaflet 114 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 preferably in a range of from about 80° to about 86°, preferably about 81° to about 83°. In the most preferred embodiment, "c' " is about 81.5°. As shown particularly in phantom in Figure 57, 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 can 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 in Figure 57, 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 extend 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 Figure 64, the amount of regurgitation of blood in the retrograde direction is relatively significant. 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. Figure 66 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 Figures 47 and 49 and demonstrated diagrammatically in Figure 65, 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 Figure 57. 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 30° to about 55°, preferably about 33° to about 52°, more preferably about 34° to about 50°, even more preferably about 35° to about 49°, even more preferably about 37° to about 47°, and most preferably about 40° to about 45°. Cavitation potential is also reduced because the seats 136, extending from the respective recesses 32 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, the gap 171 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 Figure 65, 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 ")' " will equal twice either of the equal angles "i' " and "h' ". In preferred embodiments, the radial angle of "]' " will range from about 5° to about 55°, preferably about 10° to 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, ex expected to minimize cavitation potential by permitting more regurgitation through the gap 171. Referring now to Figures 62 and 63, the recesses 132 of the preferred valve 110 may be varied in the manner shown diagrammatically in these figures. The alternate leaflets "114' " shown in Figure 62 shows recesses "132' " which have no seat or extension and they turned radially from the position of the recesses 132 shown in Figure 48. The recesses "132" " shown in Figure 63 have been turned an intermediate amount. la each case, it will be appreciated that the cylindrical diamond-shape surface "154' ", "154" " will be reoriented on the respective lateral sides "151' ", "152" " of the leaflets "114' ", "114" " in order to accommodate the different planes occupied by the upper and lower side walls of the alternate recesses "132' ", "132" ". The overall mechanism has not changed from that discussed previously in relation to Figures 48 and 56-57. In each case, the recesses "132' ", "132" " permit some translational movement in the direction of the ends of the respective recesses "132' ", "132' " when the leaflets are in the open position, until the top side fulcrum edge "166' ", "166" " engages the upper side side wall "136' ", 136" ". When the alternate leaflets "114' ", "114" " are in the fully closed position, it is believed that translational movement at the beginning the antegrade flow cycle will be primarily limited to a "sideways" shifting of the diamond-shape surface "154' ", "154" " until the bottom side fulcrum edge "164' ", "164" " engages the lower side wall "138' ", "138" ", and the leaflet "114" " begins to pivot on the fulcrum edge "164' ", "164" ". As the leaflet gradually pivots to the fully open position, greater translation movement is permitted as is discussed elsewhere in this disclosure. While the preferred embodiments of the above bileaflet heart valve 10, 110 have been described in detail with reference to the attached drawing Figures, 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.

Claims: What is claimed is: 1. A bileaflet heart valve prosthesis for controlling a circulation of a fluid within a heart of a patient, said bileaflet heart valve prosthesis having: an annular base and first and second leaflets, the first leaflet and the second leaflet being separately mounted within the annular base for pivotal movement between a closed position and an open position, the annular base defining a bore extending through the base, the base having a longitudinal axis oriented generally in parallel with the bore and the circulation of the fluid through the bore, the annular base having a first lateral side having a first lateral surface, a second lateral side having a second lateral surface, and a pair of transverse sides individually disposed between the first and second lateral sides, the annular base defining a first recess extending radially outward into the annular base from and communicating with the bore, and a second recess extending radially outward into the annular base from and communicating with the bore, the first recess being disposed within the first lateral side, the second recess being disposed within the second lateral side, the first recess having a first recess bottom surface, the first recess bottom surface intersecting the first lateral surface of the base such that the first recess bottom surface and the first lateral surface form a first junction, a first recess entrance angle being the largest of a plurality of angles between a first line generally tangential with the first lateral surface proximate the first junction and any of an infinite number of second lines intersecting the first line and generally tangential with any portion of the first recess bottom surface proximate the first junction, the second recess having a second recess bottom surface intersecting the second lateral surface such that the second recess bottom surface and the second lateral surface form a second junction, a second recess entrance angle being the largest of a plurality of angles between a third line tangential with the second lateral surface proximate the second junction and any of an infinite number of fourth lines intersecting the third line and tangential with (a) any portion of the second recess bottom surface proximate the second junction; wherein at least a first side portion of the first leaflet is received within the first recess and at least a portion of the first leaflet is received within said second recess to retain the first leaflet within the annular base; the improvement characterized in that each of the first and second recess entrance angles are less than about 35°. 2. The bileaflet heart valve prosthesis of claim 1 wherein the first and second recess entrance angles range from about 18° and about 34°. 3. The bileaflet heart valve prosthesis of claim 1 wherein the first and second recess bottom surfaces are cylindrical surfaces. 4. The bileaflet heart valve prosthesis of claim 1 wherein the annular base further defines third and fourth recesses extending radially outward into the annular base from and communicating with the bore, the third recess being disposed within the first lateral side and the fourth recess being disposed within the second lateral side, the third recess having a third recess bottom surface intersecting the first lateral surface such that the third recess bottom surface and the first lateral surface form a third junction, a third recess entrance angle being the largest of a plurality of angles between a fifth line generally tangential with the first lateral surface proximate the third junction and any of an infinite number of sixth lines intersecting the fifth line and generally tangential with any portion of said third recess bottom surface proximate the third junction, the fourth recess having a fourth recess bottom surface intersecting the second lateral surface, the fourth recess bottom surface and the second lateral surface intersecting to form a fourth junction, a fourth recess entrance angle being the largest of a plurality of angles between a seventh line generally tangential with the second lateral surface proximate the fourth junction and any of an infinite number of eighth lines intersecting the seventh line and generally tangential with any portion of the fourth recess bottom surface proximate the fourth junction, each of the third and fourth recess entrance angles being less than about 35°, wherein at least a third side portion of the second leaflet is received within the third recess and at least a fourth side portion of the second leaflet is received with the fourth recess to retain the second leaflet within the annular base. 5. The bileaflet heart valve prosthesis of claim 3, the first bileaflet having a peripheral edge, wherein the respective side portions of the first leaflet which are received within the first and second recesses each have a cylindrical surface along the peripheral edge proximate the respective side portions received within the respective recesses. 6. The bileaflet heart valve prosthesis of claim 5, the first leaflet having a complimentary pair of notches in the peripheral edge proximate each of the respective side portions of the first leaflet which are received within the respective first and second recesses, wherein the complimentary pair of notches cooperate to permit the leaflet to pivot within the respective recess. 7. The bileaflet heart valve prosthesis of claim 1, wherein the first leaflet has first and second sides, the first side being the top side and the second side being the bottom side, the bottom side having an upper surface lying generally in a first plane and a lower surface lying generally in a second plane, the first plane lying at an angle to the second plane. 8. A bileaflet heart valve prosthesis for controlling a circulation of a fluid within a heart of a patient, said bileaflet heart valve prosthesis having: an annular base and first and second leaflets, the first and second leaflets each being separately mounted within the annular base for pivotal movement between a closed position and an open position, the annular base defining a bore extending through the base, the annular base having a first lateral side having a first lateral surface, a second lateral side having a second lateral surface, and a pair of transverse sides individually disposed between the first and second lateral sides, the annular base defining a first recess extending radially outward into the annular base from and communicating with the bore, and a second recess extending radially outward into the annular base from and communicating with the bore, the first recess being disposed within the first lateral side, the second recess being disposed within the second lateral side, wherein at least a portion of the first leaflet is received within the first recess and at least a portion of the first leaflet is received within said second recess to retain the first leaflet within the annular base, wherein the annular base further defines third and fourth recesses extending radially outward into the annular base from and communicating with the bore, the third recess being disposed within the first lateral side and the fourth recess being disposed within the second lateral side, wherein at least a portion of the second leaflet is received within the third recess and at least a portion of the second leaflet is received with the fourth recess to retain the second leaflet within the annular base; wherein each leaflet has an upper surface and a curved peripheral edge and each recess has an upper side and a lower side, the upper side extending from the respective recess proximate the respective lateral side to a location on an adjacent transverse side, the upper edge of each recess forming a seat against which an area on the upper surface, proximate the curved peripheral edge, of a leaflet engaged within the respective recess, can abut when the leaflet is in the closed position, the improvement characterized in that each of the respective upper edges forms a seat which extends from one recess toward another and diminishes to become a smooth surface which is flush with the respective lateral surface. 9. The bileaflet heart valve prosthesis of claim 8, wherein each upper side has an end point where the seat diminishes into the respective lateral surface and no longer provides a seat against which the upper surface proximate the peripheral edge can abut, wherein the annular base has an inner wall which includes the lateral surface and the annular base has a radial distance which extends 360° around the inner wall, wherein the upper side extending from the first recess is the first upper side and the upper side extending from the second recess is the second upper side and the radial distance between the respective end points of the first and second upper sides is from about 5° to about 55° 10. A bileaflet heart valve prosthesis for controlling a circulation of a fluid within a heart of a patient in an antegrade direction and a retrograde direction, said bileaflet heart valve prosthesis having: an annular base and first and second leaflets, the first leaflet and the second leaflet being mounted within the annular base for pivotal movement between a closed position and an open position, the annular base defining a bore extending through the base, the annular base having a first lateral side having a first lateral surface, a second lateral side having a second lateral surface, and a pair of transverse sides individually disposed between the first and second lateral sides, the annular base defining a first recess extending radially outward into the annular base from and communicating with the bore, and a second recess extending radially outward into the annular base from and communicating with the bore, the first recess being disposed within the first lateral side, the second recess being disposed within the second lateral side, wherein at least a first side portion of the first leaflet is received within the first recess and at least a second side portion of the first leaflet is received within said second recess to retain the first leaflet within the annular base, wherein the annular base further defines third and fourth recesses extending radially outward into the annular base from and communicating with the bore, the third recess being disposed within the first lateral side and the fourth recess being disposed within the second lateral side, wherein at least a third portion of the second leaflet is received within the third recess and at least a fourth portion of the second leaflet is received with the fourth recess to retain the second leaflet within the annular base; the improvement characterized in that each of the respective lateral sides have a flow diversion protrusion extending away from the respective lateral surface toward a center of the bore, each of the flow diversion protrusions presenting an obstacle to the circulation of fluid through the bore such that fluid can be directed by the respective flow diversion protrusions into each of two adjacent recesses. 11. The bileaflet heart valve prosthesis of claim 10, wherein a substantial portion of the respective flow diversion protrusion separates the respective recesses on each of the respective lateral sides and each of the recesses have a recess surface which has a cylindrical surface. 12. The bileaflet heart valve prosthesis of claim 10, wherein each of the respective first and second leaflets have a mating edge and a peripheral edge, the peripheral edge having a flat surface proximate respective junctions between the mating edge and the peripheral edge, wherein the respective flat surfaces can divert a portion of the circulation of the fluid passing through the bore in the antegrade direction when the leaflets are in a fully open position into the respective recesses in which adjacent portions of the respective leaflet is received so as to wash the respective recesses with the diverted fluid. 13. A bileaflet heart valve prosthesis for controlling a circulation of a fluid within a heart of a patient, said bileaflet heart valve prosthesis having: an annular base and first and second leaflets, the first leaflet and the second leaflet being mounted within the annular base for pivotal movement between a fully closed position and a fully open position, the annular base defining a bore extending through the base, the annular base having a first lateral side having a first lateral surface, a second lateral side having a second lateral surface, and a pair of transverse sides individually disposed between the first and second lateral sides, the annular base defining a first recess extending radially outward into the annular base from and communicating with the bore, and a second recess extending radially outward into the annular base from and communicating with the bore, the first recess being disposed within the first lateral side, the second recess being disposed within the second lateral side, wherein at least a first side portion of the first leaflet is received within the first recess and at least a second side portion of the first leaflet is received within said second recess to retain the first leaflet within the annular base, wherein the annular base further defines third and fourth recesses extending radially outward into the annular base from and communicating with the bore, the third recess being disposed within the first lateral side and the fourth recess being disposed within the second lateral side, wherein at least a third side portion of the second leaflet is received within the third recess and at least a fourth side portion of the second leaflet is received with the fourth recess to retain the second leaflet within the annular base; wherein each of the respective leaflets have 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; the improvement characterized in that each bottom side has an upper surface lying generally in a first plane and a lower surface lying generally in a second plane, the first plane lying at an angle to the second plane. 14. The bileaflet heart valve prosthesis of claim 13, wherein the first and second planes lie at first and second angles respectively to a horizontal plane passing through a cross-section of the annular base when the respective leaflets are in the fully open position, the first angle being from about 84° to about 88°and the second angle being from about 68° to about 84°. 15. The bileaflet heart valve prosthesis of claim 13, wherein the angle between the first plane and the second plane is from about 164° to about 175°. 16. The bileaflet heart valve prosthesis of claim 13, wherein the respective top sides have top surfaces which lie generally in respective top planes and each of the top planes changes position from a first position to a second position when the respective leaflet changes position from the fully open position to the fully closed position, wherein a travel angle is an angle between the respective top plane when the leaflet is in the first position and the respective top plane when the leaflet is in the second position, and wherein the travel angle for each of the first and second leaflets is from about 35° to about 49°. 17. The bileaflet heart valve prosthesis of claim 16, wherein a horizontal plane, which is perpendicular to the lateral sides of the annular base, passes through a horizontal cross-section of the base, and each of the respective top planes lie at a greater angle to the horizontal plane than the second plane on the respective lower surface. 18. The bileaflet heart valve prosthesis of claim 13, wherein a horizontal plane, which is perpendicular to the lateral sides of the annular base, passes through a horizontal cross-section of the base, and the second plane lies at an angle to the horizontal plane when the respective leaflet is in the fully open position, the angle ranging from about 68° to about 84°. 19. A bileaflet heart valve prosthesis for controlling a circulation of a fluid within a heart of a patient, said bileaflet heart valve prosthesis having: an annular base and first and second leaflets, the first leaflet and the second leaflet being mounted within the annular base for pivotal movement between a fully closed position and a fully open position, the annular base defining a bore extending through the base, the annular base having a first lateral side having a first lateral surface, a second lateral side having a second lateral surface, and a pair of transverse sides individually disposed between the first and second lateral sides, the annular base defining a first recess extending radially outward into the annular base from and communicating with the bore, and a second recess extending radially outward into the annular base from and communicating with the bore, the first recess being disposed within the first lateral side, the second recess being disposed within the second lateral side, wherein at least a first side portion of the first leaflet is received within the first recess and at least a second side portion of the first leaflet is received within said second recess to retain the first leaflet within the annular base, wherein the annular base further defines third and fourth recesses extending radially outward into the annular base from and communicating with the bore, the third recess being disposed within the first lateral side and the fourth recess being disposed within the second lateral side, wherein at least a third side portion of the second leaflet is received within the third recess and at least a fourth side portion of the second leaflet is received with the fourth recess to retain the second leaflet within the annular base; the improvement characterized in that the first, second, third and fourth side portions of the respective leaflets each have a plurality of recess engagement surfaces, two of the plurality of recess engagement surfaces meeting to form a first fulcrum edge and two of the plurality of recess engagement surfaces meeting to form a second fulcrum edge removed from the first fulcrum edge, each of the recesses having an upper and lower recess side surface, wherein each of the respective first and second fulcrum edges engage an upper or lower recess side surface of the recess in which the respective side portion is engaged when the respective leaflet pivots either from the fully open position to the fully closed position or from the fully closed position to the fully open position such that engagement between one of the respective fulcrum edges and the respective side surface of each of the respective recesses is changeable from engagement with a side surface by the first fulcrum edge to engagement with a side surface by the second fulcrum edge as the respective leaflet pivots from one position to the other. 20. The bileaflet heart valve prosthesis of claim 19, wherein each of the 6/29957 _,____, PCT/US96/04385 - 76 - 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 surface lying generally in a first plane and a lower surface lying generally in a second plane, the first plane lying at an angle to the second plane.

Inventors: JOHNSON SHELLEY N, MIKHAIL ADEL A, PATKE NANDKISHOR G, STOBBS GENE E, JOHNSON, SHELLEY, N., MIKHAIL, ADEL, A., PATKE, NANDKISHOR, G., STOBBS, GENE, E.
Applicants: CV DYNAMICS INC DOING BUSINESS, CV DYNAMICS, INC., DOING BUSINESS AS MEDICAL INCOR
Application year: 1996
Main classification: A61F2
IPC codes: A61F2