INSERTION OF OCCLUDER(S) IN HEART VALVE PROSTHESIS

Abstract: A method of installing occluders in a prosthetic heart valve by compressing an exterior surface (17) of an orifice housing (15) of the body (13) of the prosthetic heart valve. Compression of the orifice housing occurs at four discrete locations, causing the orifice housing to distend sufficiently for insertion of occluders between diametrically opposed pivot supports (23, 25) on the interior surface (19) of the orifice housing.

Description: Description Insertion of Occluder(s) in Heart Valve Prostheses Technical Field The invention relates generally to medical devices and particularly, to prosthetic mechanical heart valves. More particularly, the invention relates to a method for insertion of occluders in prosthetic heart valves and to an apparatus for inserting occulders in prosthetic heart valves. The human heart, as part of its pumping arrangement, includes valves to prevent back flow. The heart valves can be damaged or malfunction. A common surgical technique is to replace such a valve with a prosthetic heart valve. A typical prosthetic heart valve has an orifice or blood passageway through which blood can flow, one or more occluders or leaflets, and a rigid valve orifice housing to support the occluders. The occluders can occlude, or stop, blood flow. When the occluders are in an open position, blood can flow through the orifice of the valve. When the occluders are in a closed position, blood is prevented from flowing back. The occluders open and close in response to changes in blood pressure on either side of the valve. The result is that the occluders allow blood flow in only one direction. Blood flows through the orifice from the inflow side of the orifice housing and out the outflow side of the orifice housing. People of different ages and sizes have differently sized hearts and differently sized natural heart valves. Prosthetic heart valves also have various sizes to accommodate these differences. Prosthetic heart valves are available in several different types. A common type of prosthetic heart valve has a single occluder, generally placed off-center so that it pivots in response to changes in blood pressure and opens and closes. Bi-leaflet valves are also common in which two leaflets are each mounted on a pivot. A leaflet of a bileaflet valve often has a pair of ears or projections which are integral to the leaflet and extend to each side. Each pair of ears fits into a corresponding pair of slots or pivot supports which allow the leaflet to pivot. In either single leaflet or bileaflet valves, the outer surface of the valve body often has an annular channel which receives a metal stiffener ring which can be used for interconnecting with and surrounding the suture ring. The orifice housing of a typical bileaflet valve is annular or ring-shaped, and has a generally circular cross-section. The interior surface of the orifice housing has a pair of flat wall sections opposite each other, or diametrically opposed. These flat wall sections are referred to as the orifice flats. The flat surfaces are both secants of the annular shape, cutting off part of the circle on the interior surface of the orifice housing. The result is that this type of valve has its shortest interior diameter perpendicular to the flat wall sections. Generally the pivot supports for the leaflets are within the orifice flats. The most common method for installing leaflets in a valve body is to deform the orifice housing so that the pivot supports are spread far enough apart to permit the leaflets to be installed. Orifice housings are generally manufactured using materials which have enough elasticity to allow for some distention of the orifice housing. Examples are pyrolytic carbon and pyrolytic carbon- coated graphite. Force is applied to distend the orifice housing, and when the force is removed, the orifice housing returns to its annular shape, securing the leaflets in the pivot supports. After this installation, a stabilizing ring may be shrunk-fit around the exterior surface of the orifice housing to stabilize it and make sure it retains its circular shape. One common approach for installing leaflets in the pivot supports is to engage a set of pins or shoes with the interior surface of the orifice housing. The pins or shoes are placed against the orifice near the leaflet insertion recesses. By applying force to the pins or shoes, the orifice is deformed to provide clearance to install the leaflets. After the leaflets are placed into the pivot recess, the orifice is relaxed and the pins or shoes are retracted. The insertion of occluders into the valve orifice housing is known industry-wide to be one of the more difficult aspects of designing and manufacturing prosthetic heart valves. The installation of the occluders must meet several requirements. The first of these is to provide sufficient occluder capture within the orifice housing to prevent leaflet release. If a leaflet comes loose from a prosthetic heart valve after implantation in a patient, the loose leaflet may cause blood vessel embolization, posing a serious, even deadly health risk for the patient. The leaflets must therefore be very securely installed in the orifice housing and must remain in place even under high pressures. If the orifice housing is too flexible, when the heart flexes during its pumping cycle the leaflets may pop out. A stiff orifice housing helps to hold the leaflets in place as the heart contracts. Another requirement is for the installed leaflets to open and close reliably. If a leaflet sticks, or is jammed open or closed, the result may also create a grave health risk for the patient. A stiff orifice housing helps to prevent binding between the occluders and the valve orifice housing as the heart flexes, improving reliability. On the other hand, a stiff orifice housing makes leaflet insertion more difficult. The more flexible the orifice housing, the easier it is to spread the flat interior wall sections of the orifice housing apart to insert the leaflets. The coatings of many orifice housings develop cracks when the fracture stress limitations of the materials are exceeded. If the force applied to distend the orifice housing is too great for the material, the fracture stress limitations may be exceeded. Since valve bodies with cracks generally must be discarded, preventing cracks can greatly reduce the expense of valve manufacture. A flexible orifice housing is less likely to crack during this operation. The problems of holding the leaflets in place, reliable leaflet opening and closing, and cracking are compounded by certain constraints associated with pyrolytic carbon manufacturing. Two possible approaches for pyrolytic carbon manufacturing are mandrel and substrate manufacturing. Mandrel refers to a core around which the pyrolytic carbon is dip-cast or otherwise shaped. The mandrel is removed after shaping. In substrate manufacturing the pyrolytic carbon is also shaped around a core, but the core is not removed after shaping. Mandrel products typically have stress concentration features on the outer surface as a result of coating over pivot detail from the mandrel. Substrate products typically are very stiff due to the greater sectional properties provided by having the substrate present under the pyrolytic carbon coating. These constraints tend to make leaflet insertion more difficult. This is particularly true for the smaller valve sizes, which generally show the greatest stiffness among a family of valve products. A desirable method for leaflet insertion should maximize the flat-to-flat deflection of the orifice housing within the constraint of its material strength, while not damaging the valve in any way. Such a method should work for all valve sizes with minimal changes, and work for various valve designs. The method should also be easy to use and reliably repeatable, have little or no dependence on the skill of an operator for success, and be easy to implement in the factory. U.S. Patent No. 5,336,259 describes a method of making a prosthetic heart valve where the valve body is made of pyrocarbon or a pyrocarbon-coated substrate. The method involves inserting the pivot supports while the annular valve body is compressed from two opposite sides by pressure pads or cables. This method distributes the compressive forces over relatively large areas of the valve body (at least 60 of arc on each side) and helps to eliminate cracks. Disclosure of Invention The invention provides a method and apparatus for installing occluders in the orifice housing of a prosthetic heart valve by compression at four points on the exterior surface of the orifice housing. According to the invention, the first step is determining four appropriate locations for application of compressive forces for a given prosthetic heart valve design and valve size. The next step is applying compressive forces to the exterior surface of the orifice housing, causing the orifice housing to distend. Next at least one occluder is installed in the orifice housing. Finally, the compressive forces are gradually removed to allow the orifice housing to return to a generally circular configuration with each occluder supported for movement between a pair of pivot supports located on an interior surface of the orifice housing. The four locations are selected so that the stress on an orifice housing at a given location is approximately in direct proportion to the strength of the orifice housing at the same location. The stress on and strength of the orifice housing may be determined by analytical means or experimental means. For example, the four locations for application of compressive forces to the exterior of a typical orifice housing are located at angles of approximately 20 160 200 and 340 from the centerline of the orifice housing. The centerline is a diameter of the orifice housing perpendicular to a line connecting a pair of support means for supporting a single occluder. The compressive forces are applied at four discrete locations on the exterior surface of the orifice housing by a first block and a second block, the first and second blocks being mounted in a fixture so as to be diametrically opposed to each other. The blocks may be removably attached to the fixture so that one fixture can accommodate various types and sizes of blocks. Two of the four locations are points of contact between the exterior surface of the orifice housing and the first block, and the remaining two of the four locations are points of contact between the exterior surface of the orifice housing and the second block. For each block the two locations are located symmetrically about a centerline of the orifice housing. At least one of the blocks is capable of one-dimensional movement toward and away from the other block. Preferably one of the blocks is stationary with respect to the fixture and the other block is movable with respect to the fixture. The movement of the blocks toward each other applies the compressive forces to the orifice housing. The movement of the blocks away from each other releases the compressive forces. The application of the compressive forces causes the orifice housing to distend from its unstressed generally circular configuration and allows a pair of support means to move sufficiently far apart from each other to allow an occluder to be installed between them. The orifice housing is aligned symmetrically with respect to the blocks before applying compressive forces to the orifice housing, using a locator. Preferably there will be a deviation of 4 degrees or less between the centerline of the orifice housing and the direction of block movement, so that compressive force is applied to the exterior surface of the orifice housing at the desired four discrete locations. The locator may have an angled top to hold one or more occluders in a neutral position while the compressive forces are released. Preferably the locator is located between the blocks and has stops to halt the deflection of the orifice housing when a desired deflection of the orifice housing is achieved. Preferably the locator is also capable of translating in one dimension parallel to the block movement so as to remain centered between the blocks. The initial positions of the blocks before compression of an orifice housing can be accurately determined by manual means, load measurement means, load transducer means, electrical circuit completion means, optical means, optical interference means, or displacement indicator means. According to the invention, one or more of the blocks may be a pin block. A pin block has two pins extending from the block, each pin having generally the shape of a right circular cylinder. The axis of each pin extends in a direction approximately perpendicular to a plane formed by the intersection of a line connecting a pair of pivot supports for a single occluder and the centerline. Each pin contacts the exterior surface of the orifice housing on a pin contact surface. Each pin contact surface contacts the exterior surface of the orifice housing at one of the four discrete locations for application of compressive forces. The pins apply compressive force to the exterior surface of the orifice housing as the blocks move. The pins may be removably attached to the block. The block may also have several pin attachment locations capable of accommodating various sizes of pins. One or more of the blocks may also be a v-block. A v-block has a contact side, which is the side that most closely approaches the other diametrically opposed block when the blocks are moved toward each other. The contact side of the block has an indentation which is bilaterally symmetrical about a line of symmetry. The line of symmetry is approximately perpendicular to a plane formed by the intersection of a line connecting a pair of pivot supports for supporting a single occluder and the centerline of an orifice housing in contact with the block. The indentation has two sides, one on each side of the line of symmetry. Each of the sides contacts the exterior surface of the orifice housing on an indentation contact surface. Each indentation contact surface forms one of the discrete locations for application of compressive forces by movement of the blocks to distend the orifice housing. Preferably each of the two sides of the indentation has approximately the shape of a flat plane, with the planes converging along the line of symmetry. The sides of the indentation may also have the shape of a continuous smooth curve. The curve is selected so that the indentation contact surfaces are capable of contacting the exterior surface of orifice housings of at least two different sizes of prosthetic heart valves at appropriate discrete locations for compression. According to the invention the pins or blocks have a stiffness sufficient to prevent significant bending or deformation (to the extent of 3 % or more of the exterior diameter of the orifice housing) of the pins or blocks during compression. The pins or blocks should also be stiff enough to prevent significant damage to the pins or blocks from repeated compressions of orifice housings. The pins or blocks preferably are made of glass, sapphire, ruby, or other materials having a modulus of 200,000 psi or greater. The pin contact or indentation contact surfaces are preferably polished to allow rotation of the orifice housing during compression of the orifice housing. The height of extension of the pins from the block, or the height of the v-block from its surface in contact with a base surface to the v-block' s top surface, is preferably at least 60 percent of the height of the orifice housing. The height of the orifice housing is measured from the inflow side of the orifice housing to the outflow side of the orifice housing. It is also an object of the invention to provide an apparatus for distending prosthetic heart valve orifice housings for mounting occluders between pairs of pivot supports. The apparatus includes a fixture, a stationary block and a movable block, movement means, and locator means. The stationary block and the movable block may be pin blocks or v-blocks. The stationary block is rigidly mounted on the fixture. The movable block is mounted on the fixture in a position diametrically opposed to the stationary block, and is mounted to allow one-dimensional movement of the movable block toward and away from the stationary block. Each of the blocks is adapted to contact the exterior surface of an orifice housing and apply compressive forces to it at two discrete locations. The orifice housing distends as the movable block moves toward the stationary block. The distention moves at least one pair of pivot supports on the interior surface of the orifice housing sufficiently far apart for an occluder to be installed between them. The compressive forces are released as the movable block moves away from the stationary block. The orifice housing is allowed to return to its unstressed generally circular cross section with the occluder supported for movement between the pair of pivot supports. The movement means is attached to the movable block and causes the movable block to move toward or away from the stationary block in a controlled manner. Each pair of discrete loading locations on the exterior surface of the orifice housing contacting the stationary block is located symmetrically about the centerline of the orifice housing. The locator is mounted on the fixture between the stationary block and the movable block and equidistant from the stationary block and the movable block. The locator serves to align the orifice housing symmetrically with respect to the blocks so the compressive forces are symmetrically applied. The locator is capable of translating in one dimension parallel to the motion of the movable block, so as to remain equidistant from the blocks during block movement. Brief Description of Drawings FIG. 1 is a perspective view of an apparatus for distending an orifice housing of a prosthetic heart valve in accordance with a method incorporating features of the present invention. FIG. 2 is a top view of a valve body of a prosthetic heart valve. FIG. 3 is an exploded perspective view of the apparatus for distending an orifice housing of Fig. 1. FIG. 4 is a perspective view of a set of pin blocks showing an orifice housing in initial position, in accordance with a method incorporating features of the present invention. FIG. 5 is a partial top view of the set of pin blocks of Fig. 4. FIG. 6 is a perspective view of a set of v-blocks, showing an orifice housing in phantom in initial position, in accordance with a method incorporating features of the present invention. FIG. 7 is a partial top view of the set of v-blocks of Fig. 6. FIG. 8 is a top view of an apparatus for distending an orifice housing of a prosthetic heart valve in accordance with the present invention, showing an orifice housing in initial position. FIG. 9 is a top view of the apparatus of Fig. 8, showing the orifice housing distended for insertion of leaflets. FIG. 10 is a top view of the apparatus of Fig. 8 showing the distended orifice housing with the leaflets installed. FIG. 11 is a perspective view of a single pin block with three pairs of pins, in accordance with a method incorporating features of the present invention. FIG. 12 is a side view of the pin block of Fig. 11. FIG. 13 is a front view of the pin block of Fig. 11. FIG. 14 is a top view of the pin block of Fig. 11. Best Mode for Carrying Out the Invention The present invention is a method for installing leaflets in a prosthetic heart valve by compression at four points on the outer surface of the valve orifice housing. This method has the advantage of a relatively simple fixture design and a robust process. The method is simple to use and to implement in the factory. There are fewer moving parts than in prior art methods. Since compression is used rather than spreading apart at the orifice flats, cracks are less likely. Cracks are also avoided by careful selection of the four points for compression. Once selected, the compression points may be loaded by any suitable means. Possible loading means include four round pins in a pin block, or two v-blocks (providing two compression points per v-block), or one pin block and one v-block. For at least some valve designs, a single set of two blocks may be used for compression of all valve sizes, without any need to change blocks. The method of the present invention includes identifying critical regions of the orifice housing and measuring the strength of the housing in those regions. Next, the necessary location of four regions of load application on the outer surface is determined. A load is applied to each of the four locations, using controlled movement of a set of blocks. The blocks may be either pin blocks or v-blocks. The loading distorts the orifice housing for placement of the leaflets into the orifice housing. The leaflets are then placed in the pivot supports. Finally, the load is gradually released, allowing the orifice housing to return to its normal shape and securing the leaflets. For a particular valve design a minimum of two critical regions is identified. Typically one outer surface and one inner surface of the orifice housing are identified as critical. At these regions, the housing strength is measured by standard means, such as stress, deflection or load. Preferably the housing strength is determined by interpretation of orifice tension test data. Preferably, the test data is representative of the worst case, or the lowest deflection to failure for a given valve design. The loading locations may be determined by analytical or experimental techniques. Loading locations for a particular valve design and size are chosen such that the applied stress on the orifice housing is in direct proportion to the strength of the orifice housing in a given location. The stress on the orifice housing may include the residual effects of the manufacturing process as appropriate. The flat-to-flat deflection of the orifice housing to be expected for a given compressive force at four locations can also be determined by analytical or experimental techniques. FIG. 1 is a perspective view of an apparatus 1 for distending an orifice housing of a prosthetic heart valve in accordance with a method incorporating features of the present invention. An orifice housing is placed between a stationary v-block 3 and a movable v-block 5. The stationary v-block 3 is rigidly attached to the fixture 7, while the movable v-block 5 is mounted on the fixture 7 so that the movable v-block 5 is capable of moving in one dimension, toward or away from the stationary v-block 3. The v-blocks 3 and 5 are centered with respect to one another. The v-blocks 3 and 5 form a set of diametrically opposed blocks. The v-blocks 3 and 5 each have a contact side 6 and an indentation 8. The back of the v-block 3, or the side opposite the contact side 6, is in contact with a stationary jaw 9. The back of the v-block 5 is in contact with a movable jaw 10. The jaws 9 and 10 are centered with respect to one another. The v-blocks 3 and 5 may be mounted on the jaws 9 and 10, with the jaws in turn mounted on the fixture 7. Rotation of a screw mechanism 14 causes controlled movement of the movable jaw 10 and the movable v-block 5 in the desired direction. The movement of the movable jaw 10 and the movable v-block 5 may be controlled by any suitable means, including various standard manual or mechanical control methods. The v-blocks rest on a base surface 16. The height of the v-blocks 3 and 5 is the distance from the surface of the block resting on the base surface to the top surface 18 of the block. The v-blocks 3 and 5 and/or the jaws 9 and 10 may be either permanently attached to the fixture or interchangeable. For example, a set of pin blocks may also be mounted on the fixture in place of the v-blocks 3 and 5. Preferably a single fixture 7 is used for all valve sizes of a given design. The fixture 7 provides sufficient rigidity to prevent the v-block surfaces from becoming unparallel to each other or to the outer surface of the orifice housing. As typically the smallest valve size of a given design has the greatest stiffness, if the rigidity is sufficient for the smallest size generally it is sufficient for all other sizes. A locator 11 between the v-blocks 3 and 5 of the apparatus 1 enables the operator to align an orifice housing properly between the v-blocks. Proper alignment assures that the orifice housing will contact the v-blocks 3 and 5 at the correct four locations for compression. The load points must be approximately symmetric about the centerline of the orifice housing. The centerline is a diameter of an orifice perpendicular to a line between a pair of pivot supports for installation of a single leaflet. The alignment of the centerline of the orifice housing with respect to the v-blocks preferably is controlled to within l-2_ during compression. Otherwise the technique is no longer symmetric nor optimum. A locator 11 is generally required to control the alignment. The locator 11 minimizes the chance of asymmetric loading and simplifies the installation of the leaflets. The locator 11 may serve an additional purpose of aiding in proper leaflet placement with respect to the pivot supports, making the process more robust. The locator 11 preferably has an angled top 12 with two upper surfaces for leaflet positioning. For a bileaflet valve, the angled upper surfaces of the locator 1 1 can be used to place both leaflets in a neutral position while the orifice housing is distended. This neutral position helps to ensure that the leaflets do not slip out of the pivot supports as the compressive forces are relaxed and the orifice housing returns to its unstressed configuration. An angled top with a single upper surface may be used to place a single leaflet in a neutral position during insertion into a single leaflet valve. The locator 11 preferably is also designed to stop the deflection when the orifice housing has deflected the proper amount or when the proper compressive force has been applied. Stops may be placed as needed at the maximum distortion positions of the orifice housing. This would prevent an operator from accidentally overloading and perhaps damaging the orifice housing. This simplifies the operation for the operator. If the locator is used to stop deflection, the locator is preferably made of a material having a stiffness of at least 100,000 psi. This provides sufficient resistance to the compression forces for sensing the stops by manual or automated means. Otherwise, the locator may be made of any material which is dimensionally stable enough so that it does not bend or change size in service. The locator may also have indicators to show the extent of displacement incorporated into it. The locator may have any suitable surface finish. The locator 11 preferably is capable of sliding or otherwise translating relative to the v-blocks as the movable v-block 5 moves toward or away from the stationary v-block 3. This sliding ability allows the locator 11 to remain centered with respect to the pivot features of the orifice housing and to stop the deflection symmetrically, even though only one of the blocks is moving. A load measurement device 13 attached to the fixture 7 measures the load on the orifice housing. This allows for accurate determination of the initial position for a particular orifice housing and also prevents overloading. Determination of the starting point or initial position is important because this may be the difference between success and failure in some valves. However, an exact determination of the initial position is less important with the present invention than with some other processes, as the orifice housing is stressed in direct proportion to its strength in critical regions. The invention provides for several methods for establishing zero. Initial position may be set by manual means, by means of a load transducer, by electrical circuit, or by displacement indicators. Initial position of the v-blocks may be set by manual means by manual manipulation of the orifice housing between the v-blocks 3 and 5. The v-blocks assist in manual zero because they allow for manual manipulation of the orifice housing during the zeroing process. The locator 11 provides for re-alignment if necessary. Repeatability of the zeroing process may be assured through training. One technique for confirming a correct initial position of the v-block 5 is to lift the orifice housing slightly from the base surface on which it rests between the v-blocks. If the orifice housing slips back easily into place with only the force of gravity, the initial position is incorrect in that it is too loose. If considerable force is required to push the orifice housing down into place, the initial position is too tight. If only a minimal amount of force is required in addition to gravity, then the initial position is correct. Initial position of the v-blocks may be set by placing a load transducer in line with the jaws 9 and 10 and in line with the orifice housing. This takes the place of the manual load sensor. The fixture is at "zero" once the specified load reading is achieved. A load cell may be incorporated into the load train and utilized by the operator to zero the load by moving the movable v-block 5. Initial position of the v-blocks may be set by electrical circuit. If the v-blocks 3 and 5 are made of metal or conductive polymer the v-blocks will be conductive. The conductive blocks could be used to form a circuit with the orifice housing if the orifice housing is made of a conductive material such as pyrolytic carbon. An air gap between the orifice housing and the vblock provides an essentially infinite resistance, so that no current will flow when there is a gap. The circuit will be completed when at least two of the four loading locations, one on each block, are in contact with the v-blocks 3 and 5. Geometric tolerancing should insure the remaining gaps are sufficiently closed for successful operation. A completed circuit would indicate a correct initial position. Optical methods for establishing zero may be used by incorporating transparent or semitransparent blocks and light sources into the fixture. An optical interference technique could be used to determine contact between the v-blocks 3 and 5 and the orifice housing. Contact would indicate a correct initial position. Displacement indicators may be used to set both initial position and to measure the extent of deflection during compression. Displacement indicators may be used to indicate how far the orifice housing has been deflected inward under the loading points, or outward at the pivot supports. The distance the orifice housing must be deflected inward under the loading points for a desired outward deflection can be determined analytically or experimentally. Measuring the extent of outward deflection at the pivot supports has the advantage of being direct, since the purpose of the compression is to provide sufficient clearance for leaflet installation. For either type of measurement, the deflection indicators are preferably mounted so that they can slide relative to the v-blocks as the movable v-block 5 moves toward or away from the stationary v-block 3. This sliding ability allows the displacement indicators to remain centered with respect to the pivot features of the orifice housing and with respect to the v-blocks, even though only one of the v-blocks is moving. The displacement indicators preferably are incorporated into the locator. FIG. 2 is a top view of a valve body 13 of a prosthetic heart valve. The orifice housing 15 has an exterior surface 17 and an interior surface 19. The orifice 21 extends through the center of the orifice housing 15. In the unstressed orifice housing 15, the flat surfaces of the first orifice flat 23 and the second orifice flat 25 are perpendicular to the shortest diameter of the orifice housing 1 5. The centerline of the orifice housing 15 is the diameter of the orifice housing perpendicular to the shortest diameter. The method of the invention may be used for the installation of leaflets in any of a large number of heart valve designs. The orifice housing 15 to be distended may be made of pyrolytic carbon; graphite, titanium, titanium alloy, stainless steel, or other metals or substrates coated with pyrolytic carbon; or other biocompatible polymers and materials. If the orifice housing 15 were an actual ring, a compressive force applied from a single point on each of two opposite sides in a vertical direction would be expected to produce deflection in a horizontal direction with an expected ratio of vertical to horizontal deflection of 1: 1 . However, since actual orifice housings have flats, they do not behave in this ideal way. As the size of the valve decreases, the deformation generally becomes more and more non-ideal, with less flat-to-flat deflection for a given arc deflection. Since deflection of an arc generates large tensile stresses on the interior surface under the load point, distributing the load at four load points instead of two load points is desirable. FIG. 3 is an exploded perspective view of the apparatus 1 for distending an orifice housing of Fig. 1. The locator 11 fits in a slot 27 so that it is capable of sliding. FIG. 4 is a perspective view of a set 29 of pin blocks 31 and 33, showing an orifice housing 15 in initial position, in accordance with a method incorporating features of the present invention. The stationary pin block 31 has two pins 35 and 37 extending from it. The movable pin block 33 has two pins 39 and 41 extending from it. Each of the pins 35,37,39, and 41 has the shape of a right circular cylinder. The axis of each pin extends in a direction perpendicular to the plane formed by the intersection of a line connecting a pair of pivot supports for supporting a single leaflet and the centerline of the orifice housing. The movable pin block 33 is mounted in a fixture so as to be capable of one-dimensional movement toward and away from the stationary block 31. The pins 35,37, 39 and 41 in the pin blocks 31 and 33 may be arranged to accommodate one particular orifice housing size, or a range of valve sizes, or for all valve sizes for a given valve design. The pin blocks 31 and 33 may be interchangeable so that several different types or sizes may be used in the same fixture, or each pin block may be permanently attached to a fixture. The pins may be removably attached to the pin block. Several pin attachment locations may be available for attaching pins to a pin block. The height of the pins should preferably be at least 60% of the height of the orifice housing 15 (the distance from the inflow side to the outflow side). This pin height is sufficient to prevent the orifice housing 15 from slipping off the pins during compression. Most preferably the height of the pins is somewhat higher than the height of the orifice housing 15, to provide a margin for error. Most preferably the height of the pins is approximately 105-110% of the orifice housing height. The pins may be made of any of a variety of materials having sufficient stiffness to provide an optimum stress proportion. The stress preferably is in a 1 : 1 ratio with the local orifice housing strength, which maximizes the deformation of the orifice housing prior to failure. The pin stiffness should be sufficient so that repeated compressions of orifice housings do not damage the pins. The pin diameter should be large enough to minimize bending of the pins on contact with the orifice housing and to optimize contact with the deformed shape of the orifice. The material selected for the pins also affects the required pin diameter. The pin surfaces should be polished or finished to a level which will permit rotation of the orifice housing. Precise knowledge of the deformed shape of the orifice housing is not necessary for compression with pin blocks. The pin blocks 31 and 33 may be mounted in any suitable fixture. FIG. 5 is a partial top view of the set of pin blocks of Fig. 4, showing the pins 35, 37, 39, and 41 and the orifice housing 15. The pins 35, 37, 39, and 41 contact the exterior surface of the orifice housing 15 on pin contact surfaces 43, 45, 47, and 49. The surfaces of the four pins show local geometric tangency to the exterior surface of the orifice housing 15 at the four pin contact surfaces 43, 45, 49, and 47. Each pin contact surface contacts one of the four discrete locations on the exterior surface of the orifice housing for application of compressive forces by movement of the movable pin block 33. Each pin applies compressive force to the exterior surface of the orifice housing to distend the orifice housing as the movable pin block 33 moves toward the stationary pin block 31. FIG. 6 is a perspective view of a set 50 of v-blocks 3 and 5, showing an orifice housing 15 in phantom in initial position, in accordance with a method incorporating features of the present invention. The contact sides of the v-blocks 3 and 5 each have an indentation 51 which is bilaterally symmetrical about a line of symmetry 53. The indentation 51 of the stationary v-block 3 has two sides 55 and 57, one on each side of the line of symmetry 53. The indentation 51 of the movable v-block 5 has two sides 59 and 61. The v-blocks 3 and 5 may be sized to accommodate one particular orifice housing size, or a range of valve sizes, or for all valve sizes for a given valve design. For some valve designs the optimum for the smallest size valve may be adequate for all sizes. The v-block indentations may be of various shapes. The v-block surfaces are preferably straight, so that the sides 55, 57, 59, and 61 of the indentations are flat planes. For at least some heart valve designs, the flat plane configuration allows for the use of the same set of v-blocks for compression of several different sizes of valves. The depth of the indentations and the angle between the flat planes may vary for different valve designs and valve sizes. The optimal angle is determined by the strength of the orifice housing 15 and the specific geometry of the orifice housing. Preferably the orifice housing 15 should be stressed in a manner which is in a 1 : 1 ratio with the strength of the orifice housing. This maximizes the deformation of the orifice housing prior to failure. For example, if the loading points have an angular separation of 407 A v-angle of 140 would be required between the sides of the indentation. Other geometries for the indentations are possible. A higher order curve may be constructed to define the edge of the block. Such a block may be used in the same fixture as other types of blocks. The sides of the indentations may have parabolic surfaces or other smooth curves with faces parallel to the exterior surface of the orifice housing. The curve is preferably selected so that the same set of blocks can be used for all different sizes of valves of a given valve design. Parabolic or spline edge geometry is constructed by appropriate regression to the optimum load region centers for each valve orifice housing size. For either flat or curved indentation surfaces, the maximum deformed shape for a given range of valve sizes should be checked for fit into the block. However, precise knowledge of the deformed shape of the orifice housing is not necessary. The v-blocks 3 and 5 may be centered with respect to the height of the orifice housing (the distance from the inflow side of the orifice housing to the outflow side of the orifice housing). If so, the v-blocks preferably cover at least 25% of the orifice housing height. The block may also rest on the same base surface of the fixture on which the orifice housing rests. If so, the block should preferably cover at least 60% of the orifice housing height dimension. If the v-block is not centered with respect to the orifice housing height and also does not rest on the same base surface as the orifice housing, the block should preferably cover at least 60% of the height of the orifice housing. This coverage by the v-blocks is sufficient to prevent the orifice housing from slipping from between the v-blocks during compression. Most preferably the height of the v-block is somewhat higher than the height of the orifice housing, to provide a margin for error. Most preferably the height of the v-block is approximately 105-110% of the orifice housing height. The v-blocks 3 and 5 may be constructed from any number of materials of sufficient stiffness to provide an optimum stress proportion. Materials for the v-blocks 3 and 5 should also be sufficiently stiff to resist permanent deformation of the block surface in service. Use of such stiff materials avoids problems with irregularities in the v-block surfaces developing in use. Stiffer valve designs may require stiffer materials to carry the greater loads during insertion of the leaflets. The v-block materials will preferably have stiffness greater than 200,000 pounds per square inch (psi). Modulus is a measure of stiffness; a material with a high modulus is very stiff. For pyrolytic carbon orifice housings in particular, the v-block materials must have a modulus greater than 200,000 psi. Materials such as glass, sapphire, and ruby, which have a high modulus, are suitable. The v-block surfaces must be polished or finished to a level which will permit rotation of the orifice housing. The v-block material is preferably able to receive a standard surface finish or polish of 63 RMS or better, and should be finished to grade 63 RMS or better. The block size should be sufficient to remain parallel to the outer surface of the orifice housing at maximum deformation and/or maximum load. The local deformation of the block relative to the orifice housing should not exceed 3 % of the housing exterior surface local diameter. The v-blocks do not require special coatings. Several different blocks may be interchangeable on one fixture or a single block may be permanently mounted on a fixture. Interchangeable v-blocks have an advantage in that v-blocks optimized for various valve designs or sizes may be used in the same fixture. FIG. 7 is a partial top view of the set of v-blocks of Fig. 6, showing the two v-blocks 3 and 5 and the orifice housing 15. The sides 55, 57, 59, and 61 of the indentations 51 in each of the v- blocks 3 and 5 contact the exterior surface of the orifice housing 15 on indentation contact surfaces 63, 65, 67, and 69. The sides 55, 57, 59, and 61 of the v-blocks show local geometric tangency to the exterior surface of the orifice housing 15 at the indentation contact surfaces 63, 65, 67, and 69. Each indentation contact surface contacts one of the four discrete locations for application of compressive forces to the exterior surface of the orifice housing by movement of the movable v-block 5. Each v-block applies compressive force to the exterior surface of the orifice housing to distend the orifice housing as the movable v-block 5 moves toward the stationary v-block 3. FIG. 8 is a top view of an apparatus 71 for distending an orifice housing of a prosthetic heart valve in accordance with the present invention, showing an orifice housing 15 with the movable v- block 5 in initial position. FIG. 9 is a top view of the apparatus 71 of Fig. 8, showing the orifice housing 15 distended by compression between v-blocks 3 and 5 for insertion of leaflets. FIG. 10 is a top view of the apparatus 71 of Fig. 8 showing the distended orifice housing 15 with the leaflets installed. FIG. 11 is a perspective view of a single pin block 73 with three pairs of pins, in accordance with a method incorporating features of the present invention. Locations for pin installation are located at three different distances, 75, 77, and 79, from the contact side 81 of the pin block. Each pair of pins has a different diameter and height. Any of the pairs of pins may be installed at distances 75, 77, or 79 from the contact side of the pin block. This arrangement permits a great deal of flexibility in the locations of the pins so as to easily accommodate a variety of heart valve designs and valve sizes. The pin block 73 may be mounted in any suitable fixture. FIG. 12 is a side view of the pin block 73 of Fig. 1 1. FIG. 13 is a front view of the pin block 73 of Fig. 1 1. FIG. 14 is a top view of the pin block 73 of Fig. 1 1. Small size valves are the most difficult valves to manufacture as it is the most likely to be lost during leaflet insertion. During compression small size valve by opposing forces, the orifice housing often fails from the interior surface under one of the load points. Distributing the load lowers the stresses under each load point. Compression at two load points on each side of the orifice housing provides adequate load distribution. Orifice housing strength is determined by interpretation of orifice tension test data. The orifice housings tested are part of a lot of valves known to have a low deflection to failure, so that the worst case is represented. Based on the results for housing strength, optimum loading locations are at approximately 20 to each side of the centerline of the orifice housing. A pair of loading locations is located on each of the diametrically opposed sides of the orifice housing. This results in an angular separation of about 40 between each pair of loading locations on each side of the valve. The optimal angle between the loading points remains the same for all valve sizes. The preferred embodiments, drawings, and other disclosures in this specification are not to be taken as limiting the present invention. The scope of the invention is defined by the claims and their equivalents.

Claims: WHAT IS CLAIMED IS: 1. A method of installing an occluder in a prosthetic heart valve, said valve comprising an orifice housing (15) with an exterior surface (17), the method comprising the steps of and characterized by: (1) applying compressive force at a plurality of locations along the exterior surface of the orifice housing so that the orifice deforms; (2) installing an occluder in the orifice housing; and (3) removing the compressive forces to allow the orifice to substantially return to its original shape. 2. The method in accordance with claim 1 wherein the compressive force is applied simultaneously at four different locations. 3. The method in accordance with claim 2, wherein the four locations are on the exterior of the orifice housing at angles of approximately 20 degrees, 160 degrees, 200 degrees, and 340 degrees from the centerline of the orifice housing. 4. The method in accordance with claim 2, wherein the compressive force is symmetrically applied to the exterior of the orifice housing. 5. The method in accordance with claim 4, wherein the compressive force on the orifice housing at each of the four locations is approximately in direct proportion to the strength of the orifice housing at each of the four locations. 6. The method in accordance with claim 2, wherein the step of applying the compressive force further comprises the steps of: positioning the orifice housing on two pin blocks (31, 33); providing two pins (35, 37) to extend upwardly from a first of the pin blocks and providing two other pins (39, 41) to extend upwardly from a second of the pin blocks; and moving one of the pin blocks toward the other pin block so that the four pins contact and compress the orifice housing. 7. The method in accordance with claim 6, wherein the height of the pins is at least 60 percent of the height of the orifice housing. 8. The method in accordance with claim 2, wherein the step of applying the compressive force further comprises the steps of: positioning the orifice housing on two V-blocks (50); providing two contact side (55, 57) on a first of the V-blocks and two other contact sides (59, 61) on a second of the V-blocks; and moving one of the V-blocks toward the other V-block so that the four contact sides contact and compress the orifice housing. 9. The method in accordance with claim, wherein the height of the V-blocks are at least percent of the height of the orifice housing.

Inventors: HODGE JERRY L, RYDER JOHN K, HODGE, JERRY, L., RYDER, JOHN, K.
Applicants: SULZER CARBOMEDICS INC, SULZER CARBOMEDICS INC.
Application year: 1999
Main classification: A61F2
IPC codes: A61F2