The disclosures herein relate generally to prosthetic heart valves and more particularly to tri-leaflet prosthetic heart valves having polymeric valve leaflets.
Prosthetic heart valves for human patients have been available since the 1950s, following the advent of blood oxygenators, which made open heart surgery possible. Today, there are three general types of prosthetic heart valves, including mechanical valves, tissue valves and polymer valves. A heart valve prosthesis is implanted into an annular opening in a patient's heart following surgical removal of a diseased or damaged natural valve. The valve can be secured in the annulus of the opening through the use of sutures or pins that penetrate the host tissue and an outside edge of the valve. Alternatively, the valve can be secured in the annulus by suturing the host tissue to a sewing ring. Heart valves function essentially as one-way check valves for blood flow through the beating heart.
The term “mechanical valve” as used herein refers to bi-leaflet heart valves comprising a valve orifice fabricated at least in part of a rigid, biologically compatible material such as pyrolytic carbon, and comprising essentially no biological components. The term “bioprosthetic valve” refers to a bi-leaflet or tri-leaflet heart valve comprising at least some biological components such as tissue or tissue components. The biological components of tissue valves are obtained from a donor animal (typically bovine or porcine), and the valve may comprise either biological materials alone or biological materials with man-made supports or stents. The term “polymeric valve” refers to a tri-leaflet or bi-leaflet heart valve comprising at least some elastomeric polymer components, including at least elastomeric polymer valve leaflets.
A bi-leaflet mechanical valve typically comprises an annular valve body in which two opposed leaflet occluders are pivotally mounted. The occluders are typically rigid, although some designs incorporate semi-rigid leaflets, and the occluders move between a closed position, in which the two leaflets are mated and block blood flow in the reverse direction, and an open position, in which the occluders are pivoted away from each other and do not block blood flow in the forward direction. The energy of blood flow causes the occluders to move between their open and closed positions.
Flexible heart valves seal against reverse flow by having leaflets whose total surface area is greater than the area of the orifice. Sections of the leaflets, therefore, contact one another, or coapt, to close the valve and prevent blood backflow. Coaptive sealing occurs over an area on the leaflets, rather than merely along their edges. Two leaflets are unlikely to seal with any stability if they only contact line to line. This can cause T-boning, or prolapse. T-boning occurs when the end of one leaflet slips below the end of the mating leaflet during closing, forming a line-on-line contact rather than an area contact.
Although both tissue and polymer valves involve flexible leaflets, the degree of control possible for the shape of tissue valve leaflets is extremely small, since the leaflets are formed from tissue sheets that are trimmed and sewn to a valve stent. Polymer valves, on the other hand, may be fabricated by molding, casting, and other known techniques, and therefore allow much greater control of valve body and leaflet shape. By precise control of the leaflet shape, polymer heart valves may be fabricated with improved wear and performance characteristics. In particular, by providing leaflets having an analytic shape in a selected position which can be represented generally by analytic geometry. An analytic shape may include a portion of a cylindrical surface, of an ellipsoid, of a paraboloid, or of another shape that can be defined mathematically.
A tri-leaflet heart valve prosthesis typically comprises an annular valve body and three flexible leaflets attached thereto. The valve body comprises an annular base and three leaflet support posts. A sewing ring annularly coupled to the periphery of the valve body provides a place for sutures to be applied when the valve is implanted. The leaflets are attached to the three shaped posts along an attachment curve, and they also each have a free, unattached edge remote from the attachment curve. The place where two adjacent leaflets come together at one of the support posts is called the commissure, and the generally curved area on the leaflet between the free edge and the attachment curve is known as the belly of the leaflet. The free edges of the three leaflets come together at a “triple point” generally on the axis of the valve.
One aspect of the sealing problem for tri-leaflet polymer valves arises from the nature of the valve geometry. As already noted, it is desirable to provide leaflets defined by an analytical shape. Tradeoffs must be made, however, among various possible geometries. In particular, it is desirable to provide a coaption surface that seals efficiently and robustly. Many prior art approaches to the difficult problem of leaflet design have been made.
U.S. Pat. No. 4,888,009 shows a prosthetic heart valve comprising leaflets of a spherical section, with no additional coaption surface. While this design is simple to fabricate, provides relatively good fabrication control, and has a small gap between leaflets, the vertical component of the angle between the surface tangents of opposed leaflets is not constant. For example, at the triple point and commissures, the leaflet surface tangent is nearly vertical, so the angle between the surface tangents of opposed leaflets is small and an effective and robust seal is facilitated in these regions. However, at the midpoint of the leaflet free edge between the commissures and the triple point, the leaflet surface tangent is much further from vertical. Consequently, the angle between the surface tangents of opposed leaflets is large, and the seal may not be effective or robust. Small deviations in position or load might disrupt the sealing of the leaflets and cause one free margin to slide below the other. The leaflets would have a line of contact instead of an area of contact.
Coaptive surfaces at the ends of the leaflet can be used to prescribe the angle between the surface tangents at the ends of opposing leaflets. The simplest shape for a coaptive surface is to have a vertical surface (i.e., a surface oriented generally parallel to the direction of blood flow) at the end of each leaflet. Such surfaces appear to be vertically aligned when the valve is in the closed position. For a tri-leaflet valve with identical leaflets, two vertical coaption surfaces are actually needed on each leaflet because each leaflet covers 120 degrees (not 180), and the leaflets must bend inward from the commissure to the triple point before again bending back to the other commissure (see FIG. 3). Tri-leaflet valves having vertical coaption surfaces, therefore, all have three general surface areas: the belly of the leaflet and the two coaptive surfaces. Many leaflet belly surface configurations have been proposed (with and without vertical coaption surfaces). Tri-leaflet valves having vertical coaption surfaces all suffer from a particular problem. Although the sealing of two vertical surfaces is effective, the discontinuous crease which transitions the coaptive surface to the leaflet belly resists the reverse buckling needed to open the valve. The result is high opening pressures and high pressure drops across the open valve.
In addition to leaflets comprising a single analytical shape, attempts have been made to improve valve performance by fabricating leaflets comprising more than one analytical shape. In this regard, WO 98/32400 provides a valve having leaflets comprising a cylindrical section and having a spherical coaption end. The transition from the leaflet belly to the coaption surface is made by revolving an arc around an axis to form a spherical coaption area. In addition, the specific shape chosen allows the surface tangencies at the leaflet free edges to be vertical. The designers conclude that bidirectional curvature in the leaflet belly produces poor opening characteristics, and that leaflets with only one degree of curvature in the belly are superior. Although the WO 98/32400 valve provides better performance than a fully spherical leaflet or a fully cylindrical leaflet, the valve has relatively large gaps at the triple point and the commissure.
General engineering experience with tissue and polymer heart valves have established a number of criteria for these valves, including:
1) A coaption surface which extends from the triple point to the commissure.
2) A coaption surface which is tangent to the belly geometry at its bottom and nearly vertical at its top.
3) A simple, singly curved leaflet belly.
4) A height short enough to fit into the natural anatomy.
5) A small gap area between leaflets to reduce regurgitation.
Cylindrical leaflets with revolved leaflet end sections e.g. spheres and toroids, produce adequate topological solutions for only a limited range of valve heights and gap areas. Given the limitations of existing leaflet geometries, it is desirable to have a valve leaflet defined by an analytic shape that provides a smooth transition surface from the leaflet belly to the coaption area, but which avoid large gaps at the commissures and the triple point. Analytical shapes suggested in the prior art have not achieved these goals. Therefore, what is needed is a new valve surface topology with more degrees of freedom so that a shorter valve with a small gap area, a cylindrical leaflet, and a tangent coaptive surface can be produced.
SUMMARY OF THE INVENTION
It has been discovered that a heart valve with leaflets having a helical swept coaption surface provides advantages not obtained from prior art analytical leaflet shapes. In one embodiment, accordingly, the present invention provides a valve leaflet having a base portion geometry comprising a cylindrical section and a top portion geometry comprising a swept helix. To this end, a heart valve includes a plurality of flexible leaflets. Each leaflet includes a top portion and a bottom portion. The bottom portion is a cylinder having an axis, a radius and an axial section. A first section of the top portion is a surface defined by a first arc having a first radius swept along a first helix. The first helix is a right handed helix having the same radius and axis as the cylinder. A second section of the top portion is a surface defined by a second arc having a second radius swept along a second helix. The second helix is a left handed helix having the same radius and axis as the cylinder.
A principal advantage of this embodiment is that a leaflet formed by the combination of a base cylinder geometry and a swept helical top geometry provide a valve with improved closure characteristics and which can be made of a small enough size to provide a good fit into the natural anatomy.