Patent ID: 12186192

DETAILED DESCRIPTION

Referring now to the Figures, wherein the components are labeled with like numerals throughout the several Figures, and initially toFIG.1, one general configuration of a delivery system10including an implantable device12in accordance with the invention is illustrated, wherein specific details of this and other delivery systems of the invention will be described in further detail below. As shown in this Figure, the implantable device12is being delivered to the mitral valve area of a representative heart14. Although the implantable devices of the invention, such as implantable device12, are generally described herein as being used for mitral valve repair, it is understood that many of the features of these implantable devices can be used in other areas of the heart. For example, the implantable devices of the invention can be used in any area of the heart where it would be advantageous to utilize a device having the functionality described herein. In any case, the implantable devices of the invention desirably restore normal functioning of a cardiac valve of a patient's heart, and are implantable using surgical or interventional techniques that include minimally invasive or percutaneous delivery methods. However, it is understood that more traditional open-heart techniques can alternatively be used with the devices and methods of the invention.

An embodiment of an annuloplasty ring (which is also referred to more generally herein as an “implantable device”) is illustrated inFIG.2, both in an expanded and an unexpanded condition, represented by reference numerals20and20′, respectively. The annuloplasty ring20,20′ is particularly adapted to repair one of the atrio-ventricular valves, such as the mitral and tricuspid valves. As a point of reference, the annuloplasty ring or implantable device12illustrated inFIG.1is configured for mitral valve annulus repair, although it is understood that other shapes may be incorporated for other valve annulus anatomies (e.g., the tricuspid valve annulus). Thus, the present invention is not limited to mitral valve annuloplasty, but can also be used in other areas of the heart, although the dimensions and shape of the ring would then be different to meet different physical requirements of the heart anatomy area in which it will be positioned or implanted.

Available shapes for the annuloplasty ring20are described in greater detail below. In general terms, however, the ring20can be shaped to match the native or natural shape of a valve annulus in which the ring20is to be implanted, at least with respect to the desired size of a modified or corrected annulus or portion thereof. For example, one side of the ring20can be generally shaped to mimic the native natural mitral valve posterior annulus anatomy (i.e., generally symmetrical, horseshoe-like shape) for mitral valve annulus repair.

As illustrated, the ring20includes a first section22and an optional second section24, wherein when only a first section22is used, the ring20does not actually comprise a complete “ring” or closed circle of material. Note that the unexpanded version of the ring20′ includes a corresponding first section22′ and an optional second section24′. In this embodiment of the ring, the first section22is a generally arcuate or U-shaped component that includes first and second ends26,28spaced from each other by lateral spacing. In some embodiments, the implantable device will consist of only a first section22, which is implantable using the delivery systems and methods described below. Due to at least a small amount of flexibility of the first section22, the first and second ends26,28are moveable toward and away from each other. Further, this first section22is adapted generally for implantation on a posterior aspect of a mitral valve annulus, although it is possible that it instead is used for the anterior aspect of a mitral valve annulus. In some embodiments in which the ring20is adapted for repairing of a mitral valve, the arcuate shape of the first section22is configured to match the natural posterior aspect anatomy of the mitral valve annulus.

In some cases, the ring20will also include the second section24, which extends from the first end26to the second end28of the ring20and thereby essentially completes the closed shape of the ring20. This second section24can follow the same general curvature of the first section22, such that it makes the ring20have a generally circular shape. Alternatively, the second section24can have a different general curvature than that of the first section22, or may instead extend in a generally linear configuration between the first and second ends26,28of the first section22. In any case, the ring may be configured to be partial, segmented, or a full ring. In embodiments in which the ring20is adapted for repairing of a mitral valve, the shape of the second section24can be configured to match the natural anterior aspect anatomy of the mitral valve annulus.

The first and second sections22,24of the ring20can be made of any biocompatible material or combination of materials that provide for a desired level of stiffness when implanted in a patient, yet also allows for flexibility in a radial direction. Further, any materials from which the ring is made can allow for the ring20to be expanded in response to an outward radial force to contact the native valve annulus, but then to elastically contract in response to removal of the outward radial force to pull the annulus into a desired configuration. In one exemplary embodiment, the ring is made from silicone or another elastic polymer/material. It is understood that each of the first and second sections22,24may be made of the same or different material or combination of materials as the other of the first and second sections22,24. It is further contemplated that the material that provides the desired structure for the ring may be at least partially sheathed or surrounded by a cover that is made from polyester or another material. In addition, the ring may include a reinforcement substrate. Finally, the ring may include one or more areas or segments that have radiopaque characteristics to allow for visualization of the ring via fluoroscopy. For example, the structural portion of the ring and/or the covering material of the ring can be loaded with barium or another radiopaque material.

FIG.3illustrates another embodiment of an annuloplasty ring in both an expanded and an unexpanded condition, represented by reference numerals40and40′, respectively. Annuloplasty ring40includes a first section42and an optional second section44, wherein when only a first section42is provided, the ring40comprises an open configuration rather than comprising a closed circle of material. Note that the unexpanded version of the ring40′ includes a corresponding first section42′ and can also include an optional second section (not illustrated in the unexpanded condition). As with the annuloplasty ring20described above, the first section42is generally arcuate or U-shaped, with first and second ends46,48being spaced from each other. The first and/or second sections42,44are configured as a wire formed into a zigzag type of pattern, such that when the ring40is compressed, the wire will be configured so that the zigzag configuration is at least somewhat collapsed so that it is reduced in diameter from when it is in an expanded condition. In an alternative configuration, the structure of the ring40may include one or more portions that are configured as a coil rather than a zigzag. Due to at least a small amount of flexibility of the first section42, the first and second ends46,48are moveable toward and away from each other. Further, the first section42can be adapted generally for implantation on a posterior aspect of a mitral valve annulus, although it is possible that it instead is used for the anterior aspect of a mitral valve annulus. In some embodiments in which the ring40is adapted for repairing of a mitral valve, the arcuate shape of the first section42is configured to match the natural posterior aspect anatomy of the mitral valve annulus.

In some cases, the ring40will also include the second section44, which extends from the first end46to the second end48of the ring40and thereby essentially completes a closed shape of the ring40. This second section44can follow the same general curvature of the first section42, such that it makes the ring40have a circular shape. Alternatively, the second section44can have a different general curvature than that of the first section42, or may instead extend in a generally linear configuration between the first and second ends46,48of the first section42. In any case, the ring may be configured to be partial, segmented, or a full ring. In embodiments in which the ring40is adapted for repairing a mitral valve, the shape of the second section44can be configured to match the natural anterior aspect anatomy of the mitral valve annulus.

The first and second sections42,44of the wire that makes up the ring40can be made of any biocompatible material or combination of materials that provide for a desired level of stiffness when implanted in a patient, yet also allows for flexibility in a radial direction. In one exemplary embodiment, the wire can be constructed of Nitinol and/or another elastic material, such as a super-elastic material. It is further contemplated that the material that provides the desired structure for the ring may be fully or partially sheathed in a cover that is made from polyester or another material. In addition, the ring may include a reinforcement substrate. Finally, the ring may include one or more areas or segments that have radiopaque characteristics to allow for visualization of the ring via fluoroscopy. For example, the structural portion of the ring and/or the covering material of the ring can be loaded with barium or another radiopaque material.

With this embodiment, the distance between the peaks and valleys of the zigzag pattern can vary considerably in order to provide the desired performance of the ring40. That is, the configuration of the zigzag pattern of the ring40allow for expansion in response to an outward radial force to contact the native valve annulus, but then will allow it to elastically contract, or foreshorten, in response to removal of the outward radial force to pull the annulus into a desired configuration. In one exemplary embodiment, the ring may include a wire having a zigzag or accordion-like pattern around its perimeter. That is, the wire of the ring is arranged essentially as a pattern of wire segments that each terminate at a sharp turn from which the next wire segment extends in a generally opposite direction. In one exemplary embodiment, the wire segments within a particular zigzag pattern each have generally the same length as each other. In another exemplary embodiment, the length of at least some of the wire segments with a particular zigzag pattern can be different from the length of other wire segments within that same zigzag pattern.

Referring now toFIG.4, one exemplary delivery system60is illustrated for delivering an expandable ring of the type illustrated inFIG.2to a desired implantation location (e.g., a mitral valve annulus). The delivery system60generally includes an elongated tube62having a distal tip64, a balloon system66spaced from the distal tip64, and a plurality of clip deployment tubes68. The balloon system66is configured to have proximal and distal balloons70,72, which are positioned on proximal and distal sides of the clip deployment tubes68, respectively. The balloon system66may further include a compliant sheath74that at least partially covers the proximal balloon70, the distal balloon72, and any space or gap provided between the proximal and distal balloons. With this embodiment, the balloon system66is used to reconfigure (e.g., radially expand) an expandable ring80, wherein each of the balloons70,72is made of a compliant polymeric material that can conform to the area of a mitral valve annulus. In one embodiment, the balloons in a multi-balloon system can include both compliant and non-compliant portions. In such an embodiment, it is possible for the system to include non-compliant sections on both on the proximal side of a proximal balloon and on the distal side of a distal balloon. When these balloons are inflated, the compliant sections can expand radially and inwardly toward the annulus to thereby “sandwich” the annulus at the correct implant site for the delivery system, while the non-compliant sections do not expand. In yet another embodiment, one or both of the balloons70,72may utilize a braided wire structure that can provide for blood flow through the mitral valve area during the procedure.

In one configuration of the delivery system60, the clip deployment tubes68can be housed within the elongated tube62to allow for smooth movement of the delivery system60to the area of the mitral valve. Thus, the elongated tube62is provided with an aperture to correspond to each of the tubes68. In this way, each of the tubes62can be retracted and extended from its respective aperture via a control mechanism that can be located at the proximal end of the delivery system60, for example. A clip82or pair of clips82can be positionable within each of the clip deployment tubes68, wherein each of the clips82is slideable relative to the length of its respective tube68so that it can be completely enclosed within the tube68and also at least partially extend from a distal end of its respective tube68, as desired in the process of delivering and implanting an expandable ring, as will be described in further detail below.

Each of the clips82can be made of a shape-memory material, such as Nitinol, and can be formed so that it includes a straight portion84and a U-shaped end86when not subjected to external forces, as is illustrated inFIG.4. However, in order to also be able to enclose the entire length of each of the clips82within one of the clip deployment tubes68, the material from which the clips are made can also be deformable by the application of a straightening force. In this way, the U-shaped ends86can be manipulated to be enclosed within one of the tubes68. The shape that the U-shaped ends86will take when not subjected to external forces (i.e., when they are outside of their respective clip deployment tubes) can be configured for engagement with an adjacent surface, such as the tissue of a mitral valve annulus, for example. In one embodiment, the shape of the U-shaped clip ends86essentially includes an extension of the straight portion84, an arc portion88, and a second portion90that extends from the arc portion88and includes a distal tip92. The distal tip92may be relatively sharp for engagement and/or penetration of tissue or other desired material.

It is understood that the end86of one or more of the clips82can have a different configuration than that illustrated inFIG.4, such as being a more continuous spiral or including portions having different curvatures, for example. In any case, the features of the clips82, such as their shape and length, can be selected for any desired engagement with adjacent surfaces, such as tissue. That is, any of the clips82discussed herein include features that provide a fixation function for securing a ring or other device in its desired location relative to a target area within a patient. In addition, it is contemplated that the distal tip92, or areas spaced from the distal tip92, can also include hooks, barbs, or the like that provide for additional securing of the clip to a structure of a valve orifice, for example.

The distal tip64of delivery system60can be tapered to ease the passage of the delivery system through the patient's vasculature, such as is illustrated inFIG.4. The delivery system60may further include a guidewire (not shown) which can extend from the distal tip64and can be used to guide the delivery system60to its desired implant location.

The delivery system60can further include an outer sheath (not illustrated) that is moveable relative to the length of the elongated tube62. This outer sheath can be used to cover the balloon system66and other components and hold them within an inner area of the sheath, such as during movement of the delivery system to a target area within a patient. Thus, the outer sheath can be sized and shaped so that when the balloon system66is deflated or compressed, the sheath is at a sufficiently small diameter to allow for insertion of the delivery system into a desired location within a patient.

In order to inflate or expand the proximal and distal balloons70,72, the balloon system66is connected for fluid communication with one or more lumen that extend through the length of the elongated tube62, for example. The lumen provides for inflation and deflation of the balloon system66with a fluid, such as a radio-opaque fluid, during the process of deploying an implantable device, such as an annuloplasty ring within a patient. In an alternative embodiment, the balloon system is inflatable and deflatable in response to a change in air pressure rather than in response to changes in fluid volume.

FIGS.7-11illustrate one exemplary method of delivering an expandable ring80using a delivery system60of the type generally described above. The delivery system can be used to deploy the ring80to the annulus of a native mitral valve, for example. This delivery system60may be surrounded by an outer sheath, which is not illustrated but is generally described above. The delivery system60can be advanced to the desired valve implant site using a guidewire, for example, where the delivery system can generally be in the configuration illustrated inFIG.7for movement to the desired implantation location. After the delivery system is properly positioned within the patient, the sheath can be moved proximally, exposing the balloon system66, including the proximal and distal balloons70,72. The balloons70,72can then be expanded either simultaneously or sequentially to radially expand the ring80relative to the implantation site, as is illustrated inFIG.8. As can also be seen in this figure, clip deployment tubes68have also been extended radially relative to the elongated tube62so that they are in contact with the expandable ring80

FIG.9illustrates another sequential step of the expandable ring implantation process. As shown, after the expandable ring80is in its desired position, such as adjacent to a mitral valve annulus or adjacent to tissue in close proximity to the mitral annulus, the distal ends of the clips82can be pushed outwardly beyond the distal ends of the clip deployment tubes68and through the thickness of the ring80until the distal ends of the clips82extend beyond the outer surface of the ring80. Because the outer surface of the ring80would be adjacent to a tissue surface in this method of the invention, such as the tissue of the mitral valve annulus, the process of extending the clips80from the distal end of the clip deployment tubes68will also force the distal tips of the clips80into the adjacent tissue. Thus, the distal tips of those clips80can be sufficiently sharp to be able to penetrate the tissue without causing tearing or other damage to the tissue. Further, when the clips80are made of a shape memory material and preshaped into a U-shaped configuration, as described above, once the tips of the clips80are not constrained by their respective clip deployment tubes68, they will move toward their U-shaped configuration, thereby causing a more positive engagement with the tissue that it penetrates. Depending on the penetration depth of the clips into the tissue and the specific dimensions of the clips, the distal tips of the clips may be buried relatively far within the tissue, or may “hook” back through the tissue and actually end relatively close to the tissue surface.

Once the implantable device80is positioned and anchored within the target tissue, the balloons70,72may be deflated, thereby leaving the device80implanted in the tissue, such as the mitral valve annulus, as is illustrated inFIG.10. It is noted that due to the physical properties of the implantable device (e.g., elasticity), deflation of the balloons70,72can allow for radial contraction and foreshortening of the implantable device by a sufficient amount to cause some remodeling of the shape of the tissue in which it is implanted. The delivery system60can then be removed from the patient, leaving only the implantable device80within the implantation site of the patient, as is illustrated inFIG.11.

The implantable devices of the present invention may be positioned within a desired area of the heart via entry in a number of different ways. In one example, the implantable devices may be inserted transatrially, where entry may be done either percutaneously or in a minimally invasive technique on a beating heart in which access is through the atrium of the heart, or even through a standard open heart procedure using heart-lung bypass and sternotomy. In another example, the implantable devices may be inserted transapically, where entry again may be done either percutaneously or in a minimally invasive technique on a beating heart in which access is through the ventricle of the heart. In yet another example, the implantable devices may be inserted transeptally, where entry can be done percutaneously, such as via the venous system into the right atrium and across a small hole in the atrial septum to enter the left atrium. In yet another example, the implantable devices may be inserted transfemorally through the arterial system. It is also possible that the delivery approaches may facilitate the crossing of the mitral valve, thereby avoiding entanglement in the mitral apparatus.

FIGS.5and6illustrate another exemplary embodiment of a delivery system160of the invention for delivering an expandable ring of the type illustrated inFIG.2to a desired implantation location (e.g., a mitral valve annulus). The delivery system160generally includes an elongated tube162having a distal tip, a proximal balloon170, a distal balloon172spaced from the proximal balloon170, and a plurality of clip deployment tubes168extending from the tube162in the space between the balloons170,172.FIG.5illustrates the delivery system160without a compliant sheath covering the balloons170,172, whileFIG.6illustrates a sheath174that extends to cover both of the balloons170,172. This sheath174is relatively compliant to allow it to expand or deform along with the expansion of the balloons. However, because the area between the balloons does not actually inflate but instead expands in response to expansion of the balloons it surrounds, the sheath174can also include windows or apertures190through which the clip deployment tubes168, and/or any clips that will be pushed through those tubes168, can be extended. With this embodiment, the space between the balloons170,172creates a sort of “saddle” region178in which an expandable ring can be positioned and which can also conform to the mitral valve annulus, for example. This balloon/sheath system can therefore be used to reconfigure (e.g., radially expand) an expandable ring or other implantable device, wherein each of the balloons170,172is also made of a compliant material that can conform to the area of a mitral valve annulus. In another embodiment, one or both of the balloons170,172may utilize a braided wire structure that can provide for blood flow through the mitral valve area during the procedure. In yet another embodiment, the balloons in a multi-balloon system can include both compliant and non-compliant portions, as is described above, where such balloons can allow for inflation of certain sections in radial and/or inward directions to sandwich the annulus at the correct implant site for its delivery system. Such a configuration can provide the system with the ability to expand radially toward the annulus as well as the ability to expand inwardly, in order to ensure that the annulus is positioned in a location where the device will be deployed.

In one configuration of the delivery system160, each of the clip deployment tubes168can be housed within the elongated tube162to allow for smooth movement of the delivery system160to the area of the mitral valve without the tubes168protruding from its outer surface. Thus, the elongated tube162can be provided with multiple apertures, each of which corresponds to one of the tubes168. In this way, each of the tubes162can be retracted and extended from its respective aperture via a control mechanism that can be located at the proximal end of the delivery system160, for example. A clip or pair of clips can be positionable within each of the clip deployment tubes168, wherein each of the clips is slideable relative to the length of its respective tube168so that it can be completely enclosed within the tube168and also at least partially extend from a distal end of its respective tube168, as desired in the process of delivering and implanting an expandable ring, as was described above relative toFIG.4. Similar to the embodiment ofFIG.4, the clips used with the delivery system160ofFIGS.5and6can also be made of a shape-memory material, such as Nitinol, and may also be formed to include one or more straight and/or curved portions, along with a relatively sharp distal tip, to facilitate engagement with tissue.

It is noted that when a delivery system of the invention includes multiple balloons or balloon segments, the different balloons or balloon segments can comprise the same material properties so that they expand in generally similar manners when expanded or inflated. Alternatively, multiple balloons of a single delivery system can have different material properties from each other (e.g., different material thicknesses) so that they expand differently when subjected to increased fluid flow or air pressure. In this way, the balloons can expand differently into certain areas of the patient's anatomy where it is desired to expand one of the balloon portions more than the other within a particular patient.

FIGS.12and13illustrate a delivery system260that is positioning an expandable ring280within a mitral valve annulus200of a heart, in accordance with the invention, where the valve annulus200includes a posterior portion210and an anterior portion220as viewed from the left atrium.FIG.12shows the ring280in its generally unexpanded condition, as it would be during the process of properly positioning the ring relative to the valve annulus. In this embodiment, the ring280is a generally C-shaped device having its curved area located generally adjacent to the posterior portion210of the valve. In these illustrations, the expandable ring280is shown without clips.FIG.13shows the ring280in an expanded condition, where the outer surface of the ring is in contact with the inner surface of the native valve annulus. This embodiment of the expandable ring280further includes an optional suture or other structure270extending between the two ends of the C-shaped ring280. This structure270can be used to keep the ends of the C-shaped device in a specific position relative to each other (e.g., to limit the expansion of the expandable ring280by a specific distance).

FIG.14is a side schematic view of a portion of a delivery system300and annuloplasty device of the invention, which can be referred to as a “stepwise expandable mitral valve ring” delivery system embodiment. As shown, delivery system300includes a central tube302with a distal port304, a proximal port306, and a mitral valve ring or implantable device308. The delivery system300includes a central area310from which semi-rigid wires312radially extend. The wires312provide support when deploying the mitral valve ring308. Multiple clips314can be made of a shape memory material, such as Nitinol, and can extend from the distal ends of the semi-rigid wires312. The clips314are detachable from the wires312and can be deployed into the surrounding mitral valve annulus to secure the ring308to the tissue of the mitral valve annulus. The delivery system300can further be provided with one or more tethers that extend from the area of the proximal port306to the mitral valve ring308to provide additional support to the system. The central area310of the delivery system300provides a stepwise screw-like action to deploy the ring308radially in a controlled manner. After the ring is deployed, the clips314can be detached from the wires312to anchor the ring308to the annulus.

FIG.15is a perspective view of a portion of a delivery system400, in accordance with the invention, which can be used for delivering an expandable ring of the type illustrated inFIG.2to a desired implantation location (e.g., a mitral valve annulus). The delivery system400generally includes an elongated tube402having a distal port404, a proximal balloon portion406, a distal balloon portion408spaced from the proximal balloon406, and a saddle region410positioned generally between the proximal and distal balloon portions406,408in which an expandable ring can be positioned and which can also conform to the mitral valve annulus, for example. The system400further includes a plurality of tethers420extending from the tube402generally at an area of a proximal port412for attachment to an anchoring ring or implantable device430. The proximal port area412is the area of the system where the tethers420originate and is the area into which the tethers can be retracted into the delivery system after attachment clips are deployed, as is described below.

The tethers420generally include a first area422that can comprise a suture-like material or other material that provides strength to the system, but provides enough flexibility to avoid damaging the mitral valve area. Attachment clips424, which may be made of a shape-memory material such as Nitinol, can take on a curved or U-shaped configuration when deployed in order to engage with the native valve annulus. This may occur after it is detached from the first area422of the tethers420. The tethers420can be used to hold the anchoring ring430in position after the balloons are expanded in order to prevent dislodgement of the anchoring ring430. This can be accomplished by providing tension to the tethers420to seat the anchoring ring430at the annulus in which it is to be implanted. The distal port404can be used when the delivery system400is being withdrawn from the implantation site to contain instruments for manipulation or anchoring of the ring430, for example. The distal portion404can additionally contain or allow insertion of instrumentation (e.g., wires or catheters) to allow for manipulation and adjustment of the expandable ring. The proximal port412can contain part of the delivery system and ring deployment instrumentation, if desired.

The delivery system400can be used in accordance with the methods and components of the invention to reconfigure (e.g., radially expand) an anchoring ring430or other implantable device, wherein each of the balloon portions is also made of a compliant material that can conform to the area of a mitral valve annulus. In an alternative embodiment, one or both of the balloon portions may utilize a braided wire structure that can provide for blood flow through the mitral valve area during the procedure.

FIGS.16and17are top and perspective views, respectively, of a system500that generally includes an expandable ring502having multiple clips504projecting from its outer surface, and a stented valve508positioned within a central opening of the ring502. Such a configuration can be utilized in situations where an annuloplasty ring, such as ring502, has been implanted into a patient in such a way that either immediate or subsequent implantation of a transcatheter valve, such as valve508, is desirable. As is shown in these Figures, the ring502is configured to be an uninterrupted ring; however, the ring502can instead be C-shaped or U-shaped, for example, with a gap between its ends. In any case, the stented valve can be configured in a number of different ways, only one of which is illustrated in these figures. In the illustrated exemplary embodiment, stented valve508includes an outer stent that includes a first portion512adjacent to the expandable ring502and having a first diameter, and a second portion514that is spaced further from the expandable ring502and that has a second diameter that is at least slightly larger than the first diameter. In an exemplary embodiment, the shape and size of the outer stent is designed so that the first portion512includes a central opening in which a valve (e.g., a tissue valve)510can be attached, and so that the second portion514has an outer diameter for engagement with anatomical structure within the heart in order to provide additional anchoring for the valve508. However, the outer stent can instead have a different shape to accommodate a specific area of the heart into which it will be implanted.

It is understood that the above description of devices and methods that is directed to mitral valve repair and replacement can also be applicable other areas of the heart that include an annulus or rim of tissue, such as for transcatheter valve replacement for aortic insufficiency, mitral regurgitation or stenosis, tricuspid regurgitation or stenosis, along with treatments for other heart issues.

The present invention has now been described with reference to several embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures described herein, but only by the structures described by the language of the claims and the equivalents of those structures.