Patent Publication Number: US-8114155-B2

Title: Annuloplasty ring with offset free ends

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 10/868,470, filed Jun. 14, 2004, now U.S. Pat. No. 7,367,991, which is a continuation-in-part of both U.S. application Ser. No. 09/941,406 filed Aug. 28, 2001, now U.S. Pat. No. 6,749,630 and U.S. application Ser. No. 10/139,070, filed May 3, 2002, now U.S. Pat. No. 6,908,482. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to medical devices and particularly to a tricuspid annuloplasty ring and delivery template. 
     BACKGROUND OF THE INVENTION 
     In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary, and are each mounted in an annulus comprising dense fibrous rings attached either directly or indirectly to the atrial and ventricular muscle fibers. 
     Heart valve disease is a widespread condition in which one or more of the valves of the heart fails to function properly. Diseased heart valves may be categorized as either stenotic, wherein the valve does not open sufficiently to allow adequate forward flow of blood through the valve, and/or incompetent, wherein the valve does not close completely, causing excessive backward flow of blood through the valve when the valve is closed. Valve disease can be severely debilitating and even fatal if left untreated. 
     Various surgical techniques may be used to repair a diseased or damaged valve. In a valve replacement operation, the damaged leaflets are excised and the annulus sculpted to receive a replacement valve. 
     Another less drastic method for treating defective valves is through repair or reconstruction, which is typically used on minimally calcified valves. One repair technique that has been shown to be effective in treating incompetence is annuloplasty, in which the effective size of the valve annulus is contracted by attaching a prosthetic annuloplasty repair segment or ring to an interior wall of the heart around the valve annulus. The annuloplasty ring is designed to support the functional changes that occur during the cardiac cycle: maintaining coaptation and valve integrity to prevent reverse flow while permitting good hemodynamics during forward flow. The annuloplasty ring typically comprises an inner substrate of a metal such as stainless or titanium, or a flexible material such as silicone rubber or Dacron cordage, covered with a biocompatible fabric or cloth to allow the ring to be sutured to the heart tissue. Annuloplasty rings may be stiff or flexible, may be split or continuous, and may have a variety of shapes, including circular, D-shaped, C-shaped, or kidney-shaped. Examples are seen in U.S. Pat. Nos. 5,041,130, 5,104,407, 5,201,880, 5,258,021, 5,607,471 and, 6,187,040 B1. Most annuloplasty rings are formed in a plane, with some D-shaped rings being bowed along their anterior or straight side to conform to the annulus at that location. 
     The present application has particular relevance to the repair of tricuspid valve, which regulates blood flow between the right atrium (RA) and right ventricle (RV), although certain aspects may apply to repair of other of the heart valves. 
     The tricuspid valve  20  is seen in plan view in  FIG. 1  and includes an annulus  22  and three leaflets  24   a ,  24   b ,  24   c  (septal, anterior, and posterior, respectively) extending inward into the flow orifice defined by the annulus. Chordae tendineae  26  connect the leaflets to papillary muscles located in the RV to control the movement of the leaflets. The tricuspid annulus  22  is an ovoid-shaped fibrous ring at the base of the valve that is less prominent than the mitral annulus, but slightly larger in circumference. The septal leaflet  24   a  is the site of attachment to the fibrous trigone, the fibrous “skeletal” structure within the heart. The triangle of Koch  30  and tendon of Todaro  32  provide anatomic landmarks during tricuspid valve repair procedures. The atrioventricular (AV) node  34  is a section of nodal tissue that delays cardiac impulses from the sinoatrial node to allow the atria to contract and empty their contents first, and relays cardiac impulses to the atrioventricular bundle. In a normal heart rhythm, the sinoatrial node generates an electrical impulse that travels through the right and left atrial muscles producing electrical changes which is represented on the electrocardiogram (ECG) by the p-wave. The electrical impulse then continue to travel through the specialized tissue of the AV node  34 , which conducts electricity at a slower pace. This will create a pause (PR interval) before the ventricles are stimulated. Of course, surgeons must avoid placing sutures too close to or within the AV node  34 . C-rings are good choices for tricuspid valve repairs because they allow surgeons to position the break in the ring adjacent the AV node  34 , thus avoiding the need for suturing at that location. 
     Despite numerous designs presently available or proposed in the past, there is a need for a tricuspid ring that more closely conforms to the actual shape of the tricuspid annulus. 
     SUMMARY OF THE INVENTION 
     The present invention provides an annuloplasty ring including a ring body generally arranged about an axis and being discontinuous so as to define two free ends. The ring body has a relaxed configuration following a three-dimensional path such that the free ends are axially offset from an annulus reference plane through a midpoint of the ring body. In a preferred embodiment, the two free ends are axially offset between about 2-15 mm. The annuloplasty ring is particularly adapted to reinforce the tricuspid annulus, and as such has a curvilinear anterior side ending in one of the free ends, a curvilinear posterior side ending in the other of the free ends, and a relatively straight septal side extending between the anterior and posterior sides. The posterior side is shorter and has a smaller radius of curvature than the anterior side. 
     In accordance with one aspect of the present invention, the ring body comprises an inner structural support of multiple bands of elastic material. A low friction material may be interposed between each two adjacent bands to facilitate movement therebetween. The multiple bands may be embedded in a matrix of pliable material, preferably molded silicone. In one embodiment, there are two concentrically disposed bands embedded in the matrix of pliable material. To facilitate bending about axes that extend radially, each of the multiple bands of elastic material may have a relatively wider radial dimension than its axial dimension. 
     In accordance with a further aspect of the invention, the ring body comprises an inner structural support band surrounded by a pliable matrix, wherein the matrix includes a tubular inner portion that surrounds the band and an outer flange through which implantation sutures can pass. The outer flange may be curved so as to be convex on its outer surface. Preferably, the outer flange is connected to the inner tubular portion with a plurality of circumferentially spaced apart radial walls so as to create a celled structure. 
     A further aspect of the present invention provides an annuloplasty ring including a ring body generally arranged about the central axis and being discontinuous so as to define two free ends. The ring body has a construction that renders it more flexible in bending about axes that extend radially from the central axis than about the central axis itself. The ring body may comprise an inner structural support of multiple bands of elastic material. A low friction material may be interposed between each two adjacent bands. The multiple bands may be embedded a matrix of pliable material, preferably silicone. There may be two concentrically disposed bands embedded in the matrix of pliable material. Desirably, each of the multiple bands elastic material has a relatively wider radial dimension than its axial dimension. 
     In a still further aspect of the present invention, an annuloplasty ring template is provided. The template has a rigid body with a peripheral mounting ring generally arranged about an axis and being discontinuous so as to define two free ends. The mounting ring follows a three-dimensional path such that the free ends are axially offset. The template may include a central platform to which the peripheral mounting ring is connected via a plurality of generally radially extending spokes. A handle-receiving hub may extend generally away from central platform. Desirably, the peripheral mounting ring extends about three-quarters circumferentially about the axis. 
     In a preferred embodiment, the peripheral mounting ring of the template defines a radially outward groove therein for receiving an annuloplasty ring. The template further may include a plurality of cutting guides provided on the peripheral mounting ring. A pair of through holes in the mounting ring are provided on either side of each cutting guide such that a length of suture may extend through or about an annuloplasty ring positioned on the outside of the mounting ring, through one of the holes, over the cutting guide, through the other hole, and back into the annuloplasty ring. Each cutting guide may comprise a pair of intersecting slots, one of the slots being shallower than the other and positioned to receive a suture extending between the through holes. The deeper slot provides space into which a sharp instrument may extend to sever the suture at the cutting guide. 
     Further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of the tricuspid valve and surrounding anatomy; 
         FIG. 2  is a perspective view of an exemplary annuloplasty ring of the present invention illustrating its axially-spaced free ends; 
         FIG. 2A  is a cross-sectional view of the exemplary annuloplasty ring taken along line  2 A- 2 A of  FIG. 2 ; 
         FIG. 2B  is a cross-sectional view similar to  FIG. 2A  of an annuloplasty ring of the prior art; 
         FIGS. 2C-2E  are cross-sectional views of alternative annuloplasty rings of the present invention taken through the rings in the same location as line  2 A- 2 A of  FIG. 2 ; 
         FIG. 3  is a perspective view of a tricuspid valve and surrounding anatomy; 
         FIG. 4  is a plan view of the annuloplasty ring of  FIG. 2  implanted around the tricuspid valve; 
         FIG. 5  is a perspective view of exemplary annuloplasty ring, holder template and delivery handle of the present invention; 
         FIG. 6  is a perspective view of the ring, template and handle combination of  FIG. 5  positioned above the tricuspid valve and showing an exemplary attachment method; 
         FIGS. 7A-7C  are perspective and elevational views of a further exemplary holder template of the present invention; 
         FIGS. 8A-8D  are perspective, elevational and plan views of a rigid yet elastic inner structural support of an alternative annuloplasty ring of the present invention; and 
         FIGS. 9A-9C  are sectional views through the structural support of  FIGS. 8A-8D , taken along the section lines indicated in  FIG. 8B . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a non-planar or three-dimensional (3D) annuloplasty ring that is shaped to conform to a 3D annulus. Some studies show that the tricuspid valve has such a non-planar annulus, and thus the present invention is particularly suited for repair of that valve. Of course, other valves may in some patients have 3D annuluses, and the annuloplasty ring of the present invention may also have use in those locations, if desired by the surgeon. Of course, all annuloplasty rings are three-dimensional to some extent, as they have a cross-sectional thickness. In the context of the present invention, a non-planar or three-dimensional annuloplasty ring has a nominal cross-sectional centerline that assumes a three-dimensional shape, or in other words does not lie in a single plane. Likewise, the exemplary ring of the present invention, as well as other shapes that may benefit from the features embodied herein, has a non-circular peripheral shape, but is shown having an axis. The term “axis” in reference to the illustrated ring, and other non-circular or non-planar rings, refers the line through the ring that passes through the area centroid of the ring when viewed in plan view. This “axis” can also be viewed as an imaginary line of blood flow within the valve orifice and thus within the ring when implanted therein. 
     It should be understood that the various constructional details of any one embodiment herein may be transferred to another embodiment, even if not explicitly mentioned. For instance, the inner ring structural support seen in  FIGS. 8A-8D  may be used in a ring that has a silicone sleeve and outer fabric covering as described elsewhere. 
     Despite numerous ring designs in the past, none has effectively accommodated the shape of the tricuspid valve. Prior C-shaped rings (i.e., those with a break in continuity around the periphery) are formed in a plane. When implanted, a planar ring will tend to conform a non-planar annulus to its own shape because of its relative stiffness. Unfortunately, this may interfere with optimum performance of the “repaired” valve. 
       FIG. 2  illustrates an exemplary annuloplasty ring  40  of the present invention having a ring body  42  generally arranged about an axis  44  and being discontinuous so as to define two free ends  46   a ,  46   b .  FIG. 3  shows a tricuspid valve  20  in perspective, and  FIG. 4  shows the annuloplasty ring  40  in plan view after having been implanted or otherwise affixed to the tricuspid valve  20 . When viewed in plan view, as seen in  FIG. 4 , the body  42  of the annuloplasty ring  40  defines a relatively straight septal side  50   a , a curvilinear posterior side  50   b , and a curvilinear anterior side  50   c . The posterior side  50   b  is shorter and has a smaller radius of curvature than the anterior side  50   c.    
     Again, the axis  44  in  FIG. 2  lies at the centroid of the ring or along of the axis of blood flow through the ring  40  when implanted, and it will be understood that the directions up and down are as viewed in the figure. The ring  40  is designed to be implanted in a tricuspid annulus such that blood will flow in the downward direction. 
       FIG. 2  illustrates an exemplary axial offset of the two free ends  46   a ,  46   b . Radial lines are shown from each free end  46   a ,  46   b  to the central axis  44 . The distance A between the intersections of these radial lines and the axis  44  represents the axial offset. The distance A may vary depending on the patient, but is typically between about 2.0 mm and 15.0 mm. In this embodiment, the curvilinear anterior side  50   c  lies generally in a plane all the way to the free end  46   a . Therefore, because the second free end  46   b  drops below the main part of the anterior side  50   c , which generally defines an annulus reference plane for the ring and host annulus, then it is axially offset from the first free end  46   a . However, the first free end  46   a  may not lie in the annulus reference plane, and may drop to the same elevation as the second free end  46   b . Either free end  46   a ,  46   b  may even be axially above the annulus reference plane. In short, the ring  40  is designed to be three-dimensional to conform to the native tricuspid annulus, and those of skill in the art will recognize the number of possible permutations. 
     Although the annuloplasty ring  40  may be constructed in a number of ways as defined in the prior art, one particularly useful construction includes some relatively rigid yet elastic inner structural support surrounded by a pliable core material and a fabric cover. For example, as seen in  FIG. 2A , the annuloplasty ring  40  may include an inner skeleton of multiple bands  60  of relatively rigid yet elastic material such as Elgiloy surrounded by a suture-permeable core material  62  such as silicone, and having an outer fabric cover  64 . The multiple bands  60  may be separated by plastic or other relatively low friction material (e.g., TEFLON) so as to be able to more easily flex with respect to one another. It will be noted by those of skill in the art that the multiple bands  60 , which limit the flexibility of the ring  40 , are aligned generally perpendicular to the axis  44  and thus the ring is least flexible in bending about the axis. Desirably, the ring  40  of the present invention is more flexible in bending about axes along radial lines from the central axis  44 . That is, for example, the free ends  46   a ,  46   b  are more easily flexed up and down parallel to the axis than toward or away from one another. There are a number of ways to accomplish this flexibility orientation, as seen in  FIGS. 2C-2E . 
     In contrast,  FIG. 2B  shows a cross-section of a ring  70  of the prior art that has an inner skeleton of multiple bands  72  of relatively rigid material such as Elgiloy surrounded by a suture-permeable core material  74  such as silicone, with an outer fabric cover  76 . In such prior devices, the bands  60  are oriented along or parallel to the axis  44 , and thus are more flexible in bending about the central axis. 
       FIG. 2C  shows a cross-section of an alternative ring of the present invention having a plurality of inner bands  78  embedded in a more pliable matrix  80  such as silicone. As before, the bands  78  are desirably more rigid than the matrix  80  and provide structural support to the annulus when implanted. The bands  78  are thin in the axial dimension, and wide in the radial direction, so as to provide the preferred flexibility characteristics of the ring of the present invention. The bands  78  are shown spaced apart with matrix  80  material therebetween to enhance the aforementioned flexibility. 
       FIG. 2D  illustrates a further embodiment in which there are two concentrically disposed structural bands  81  embedded in a matrix  82  of pliable material. The bands  81  together create the desired flexibility characteristics of the ring, as explained above, even if their respective cross-sections are square or circular. As seen, however, the bands  81  preferably have a larger radial than axial dimension which contributes to the flexibility of the ring about radial axes. 
       FIG. 2E  shows a cross-section of a ring having a circular reinforcing band  83  surrounded by a pliable matrix  84 . A tubular inner portion of the matrix  84  surrounds the band  83 , while an outer wall or flange  85  provides additional material through which implantation sutures can pass. In a preferred embodiment, the flange  85  is curved so as to be convex on its outer surface, and is connected to the inner tubular portion with a plurality of circumferentially spaced apart radial walls  86 . A series of circumferential cells  87  is thus created between the walls  86 . The celled structure of the matrix  84  renders it soft and compressible, which facilitates conformance of the ring to very uneven annuluses. A fabric covering (not shown) may also be used. 
     With reference again to  FIGS. 2 and 3 , the three-dimensional shape of the annuloplasty ring  40  is seen corresponding generally to the shape of the tricuspid annulus  22 . The first free end  46   a  registers with an area adjacent the septal leaflet  24   a , to the anterior side of the AV node  34 . The second free end  46   b  registers with an area adjacent the septal leaflet  24   a , but to the posterior side of the AV node  34 . The second free end  46   b  is axially offset with respect to the first free end  46   a  as is the respective anatomical attachment areas. Therefore, the annuloplasty ring  40  closely conforms to the 3-D shape of the annulus  22 , and minimal distortion of the tissue occurs when attaching the ring thereto. Furthermore, the oriented flexibility of the ring  40  facilitates the 3-D shape matching, between ring and tissue because the free ends  46   a ,  46   b  may be easily flexed with respect to one another along arcs that are generally parallel to the axis  44 . 
     The annuloplasty ring  40  is seen in  FIG. 4  implanted in the annulus  22  using a plurality of sutures  88 , although those of skill in the art will understand that there are other attachment means. The sutures  88  are evenly distributed around the ring body  42  and tied off to present minimal surface roughness and reduce the chance of thrombi forming thereon. Again, the free ends  46   a ,  46   b  are shown on either side of the AV node  34 , which minimizes the risk of damaging the sensitive conduction system. 
     A preferred delivery template and method are also provided for the three-dimensional annuloplasty ring  40  of the present invention. With reference to  FIGS. 5 and 6 , the ring  40  is shown releasably secured to a template  90  that is in turn held at the distal end of a delivery handle  92 . The template  90  provides a suturing platform for the ring  40 , maintaining its advantageous shape while being implanted. In this regard, the template  90  includes a peripheral mounting ring  94  generally arranged about an axis coincident with the axis  44  of the ring  40 . The peripheral mounting ring  94  is discontinuous so as to define two free ends  96   a ,  96   b  and generally follows a three-dimensional path such that the free ends are axially offset. Desirably, the three-dimensional path of the peripheral mounting ring  94  is the same as that of the annuloplasty ring  40 . Sutures (not shown) or other similar expedient releasably secure the ring  40  to the template  90  to form the assembly seen in  FIG. 5 . A hub  98  of the handle  92  may be releasably attached to the template using sutures or quick-release clips or the like so that the handle may be removed during implantation for better visibility of the annulus. 
       FIG. 6  shows a step in an interrupted suture implant procedure. After exposing the annulus  22 , the surgeon secures a plurality of individual sutures  100  around the annulus  22  in the locations that the sutures will be arranged around the ring  40 . The free ends of each suture  100  are then passed through the corresponding positions in the suture-permeable outer portion of the ring  40 , as seen at  102 . After all of the sutures  100  have been pre-threaded through the ring  40 , the surgeon manipulates the ring using the handle  92  down the array of sutures and into position in the annulus  22 . The next steps that are not illustrated include severing each suture close to the ring  40  and tying them off as seen in  FIG. 4 . Again, the handle  92  may be detached from the template  90  for this operation. Finally, the template  90  is detached from the ring  40  and removed with any attaching sutures from the operating site. 
       FIGS. 7A-7C  are several perspective views of an exemplary template  110  for use in implanting the ring  40  of the present invention. The template  110  includes a peripheral mounting ring  112  connected to a central platform  114  via a plurality of spokes  116 . The template  110  may be constructed of a variety of materials, with a biocompatible plastic being preferred. Windows  117  exist between the spokes  116  for greater visibility of the implant site. A handle-receiving hub  118  projects upward from the platform  114  and generally defines a central axis  120  of the template  110 . The mounting ring  112  extends approximately three-quarters around the axis  120  and terminates in two axially-spaced free ends  122   a ,  122   b.    
     In a preferred embodiment, the mounting ring  112  includes a radially outwardly opening channel or groove  124 , which is sized to have about the same curvature as the ring  40 , and thus snugly retains the ring  40  in place around the template  90 . The groove  124  is shallow so that a majority of the ring projects outward therefrom to facilitate exposure to the annulus and attachment thereto. 
     A plurality, preferably three, of cutting guides  126  projects axially upward from the mounting ring  112  at regular intervals around its periphery. The cutting guides  126  each include a first relatively deep slot  128  and a second shallower slot  130  crossing the first slot. Sutures (not shown) desirably fasten the ring  40  to the template, and extend across the cutting guides  126  for easy severability. A plurality of passages  132  in the mounting ring  112  opening in the groove  124  permit passage of sutures directly from the ring body  42  through the mounting ring to the cutting guides  126 . As seen best in  FIG. 7A , there are two such passages  132  on either side of each cutting guide  126 . The passages  132  are desirably straight holes from the upper surface of the mounting ring  112  that intersect and thus open to the concave groove  124 . 
     The overall shape of the mounting ring  112  is three-dimensional, as explained above, with the two free ends  122   a ,  122   b  being axially spaced apart. The three-dimensional may be a gentle spiral, or other similar shape as dictated by the particular patient, or by a representative sample of patients. In the illustrated embodiment, and as best seen in  FIG. 7C , a majority of the mounting ring  112  lies in a plane, with one side that terminates in the second free end  122   b  being formed in a gentle curve or spiral so as to be axially spaced from the first free end  122   a . The annuloplasty ring is arranged on the mounting ring  112  so that the portion that will lie adjacent the septal leaflet (see  24   a  and  50   a  in  FIG. 4 ) extends along the spiral segment of the mounting ring. In general, it is believed that many patients have a relatively planar tricuspid annulus around the anterior and posterior sides, but a depressed septal side. The shape of the mounting ring  112  thus mimics the presumed anatomical contour, and thus the ring can be sewn into place without unduly distorting the annulus. 
       FIGS. 8A-8D  illustrate an exemplary inner structural support  150  for a tricuspid annuloplasty ring of the present invention. The structural support  150  is ultimately is covered with one or more outer flexible layers as described above, and therefore the final ring body assumes the shape of the support. The structural support  150  may be made of a relatively rigid material yet elastic material such as Elgiloy. 
     When viewed in plan view, as seen in  FIG. 8B , the structural support  150  defines a relatively straight septal side  156   a  ending in one of the free ends  154   b , a curvilinear posterior side  156   b , and a curvilinear anterior side  156   c  ending in the other of the free ends  154   a . The posterior side  156   b  is between the other two sides. As in the earlier embodiment, the posterior side  156   b  is shorter and has a smaller radius of curvature than the anterior side  156   c.    
     The structural support  150  is generally arranged about an axis  152  and is discontinuous so as to define two free ends  154   a ,  154   b . A majority of the structural support  150  is located generally in an annulus reference plane  151  (see  FIG. 8D ) perpendicular to the axis  152 , and the two free ends  154   a ,  154   b  curve away from the plane so as to be offset therefrom. The annulus reference plane  151  is defined as the plane that is perpendicular to the axis  152  at the elevation of the tricuspid annulus. That elevation, in turn, is represented in the drawings by the midpoint of the anterior side  156   c , or at least the midpoint of the larger cross-section portion thereof (as detailed below).  FIG. 8C  illustrates a midpoint M in the anterior side  156   c  that represents the nominal elevation of the host annulus. A perpendicular line to the axis  152  intersects point R. The reference plane is thus perpendicular to the axis  152  through R. 
     As seen best in  FIG. 8D , the two free ends  154   a ,  154   b  are thus axially offset in the same direction from the reference plane  151 . Of course, the free ends  154   a ,  154   b  need not be axially offset from each other as shown, and one or both may even curve upward above the reference plane  151 . The particular three-dimensional configuration is modeled to fit the natural shape of a tricuspid valve, or at least to approximate that shape as best as possible. 
     With regard to FIGS.  8 B and  9 A- 9 C, the cross-sectional shape of the structural support  150 , at least along most of the anterior side  156   c , is designed so as to have more flexible in bending at the free ends  154   a ,  154   b .  FIG. 9A  is a cross-section through the anterior side  156   c  and shows a generally C-shaped cross-section with an outwardly-facing groove  160  formed between an upper web  162  and a lower web  164 , both extending from an inner base portion  166 . The upper web  162  extends slightly farther radially outward than the lower web  164 . 
     The cross-sectional shape of the structural support  150  changes along its length, from the midpoint M to the free ends  154   a ,  154   b . The transition between the cross-section at the middle of the anterior side  156   c  and the cross-sections at the two free ends  154   a ,  154   b  is gradual, and is reflected in  FIGS. 9A-9C . The webs  162 ,  164  gradually diminish in radial dimension until all that is left is the rectangular base portion  166 , as seen in  FIG. 9C . Because at both free ends  154   a ,  154   b  the radial dimension is smaller than the axial, the ends are more flexible in bending about the central axis  152 . It should be mentioned that the properties of the inner ring structural support seen in  FIGS. 5A-8D  may be attained with other structures, for example, with the multiple concentric bands as described above. 
     While the foregoing is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Moreover, it will be obvious that certain other modifications may be practiced within the scope of the appended claims.