Abstract:
A generally rectangular intra-spinous spacer preferably made of bone material advantageously has a low potential for bone fusion. The spacer has depressions sized and positioned on opposite sides to receive spinal processes. The depressions reduce the amount of bone-to-spinal process contact. A fixation strap of the spacer secured either to both the superior and inferior spinous processes or to only one of the superior or inferior processes laterally maintains the implanted position of the spacer and limits either flexion and extension, just flexion, or just extension.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This claims the benefit of U.S. Provisional Application No. 60/688,359, filed Jun. 6, 2005. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The invention relates to intra-spinous spacers that are inserted between two vertebrae to replace a damaged or degenerated spinal disc. More particularly, the invention relates to a spacer to be placed between the posterior spinous process of the spine and its method of use.  
       BACKGROUND OF THE INVENTION  
       [0003]     Degenerative disc disease often results in a loss of disc height, which in turn can cause facet and nerve impingement. One standard of care is to remove the disc and fuse the two vertebrae together. However, this can lead to problems at adjacent vertebra levels as those levels become hypermobile to compensate for the loss of mobility at the fused level. A number of devices have therefore been developed to restore height without fusion. Such known devices include artificial discs, pedicle screws with flexible rods, and intra-spinous spacers. Known intra-spinous spacers are inserted between the posterior spinous process and can be made of solid or flexible material. Typically, known intra-spinous spacers are placed in the spine in slight distraction to off load the weight of the disc. Intra-spinous spacers also typically serve as a stop for extension, and some have attached straps that limit flexion. Many known intra-spinous spacers are in the shape of an H, wherein the sides of the H prevent the spacer from sliding out from between the processes. Known spacers also are usually made of a metal or a polymer. Ideally, however, bone would be a more suitable material for a spacer, but typical H-shaped bone spacers are likely to result undesirably in the vertebrae fusing to the spacer.  
       SUMMARY OF THE INVENTION  
       [0004]     The device described herein is an intra-spinous spacer designed to be placed between the posterior spinous process. In one embodiment, the intra-spinous spacer is generally rectangularly-shaped and advantageously may be made of bone material, such as allograft or autograft. The spacer has top and bottom depressions for receiving the spinous process. The depressions advantageously reduce bone contact, which reduces, if not eliminates, the likelihood of bone fusion between adjacent vertebrae. The spacer may be formed solely of cortical bone and may be terminally sterilized to kill proteins.  
         [0005]     The spacer may also include one or more fixation straps to secure the spacer laterally in place between the vertebrae. In one embodiment, the intra-spinous spacer may have one or more holes through the top and bottom surfaces to receive fixation straps. The fixation straps may be polyethylene or other biocompatible polymer and may be braided or unbraided. Other materials may include tendons or ligaments, or gracilis or semitendinousis. The strap(s) may be pretensioned prior to implantation, and where more than one strap is used, the straps may be preferably no more than 10 mm apart preferably to prevent slippage or rotation of the device. The straps may wrap around or surround the spacer. The strap(s) may be placed around one of the processes by forming a loop and may be locked or fixed to the spacer. The strap(s) may also be tacked, cemented, or otherwise secured to one of the processes. The strap(s) may be preferably about 70 mm long to wrap around one process and preferably about 140 mm long to wrap around both superior and inferior processes.  
         [0006]     The spacer can be used to advantageously limit and preferably prevent both extension and flexion and may be secured to both the superior and inferior spinous processes, or limit and preferably prevent extension alone by being secured to only the superior spinous process, or limit and preferably prevent flexion alone by being secured to only the inferior spinous process.  
         [0007]     In one embodiment, the spacer may have one or more openings or holes on the surface of the spacer to receive the straps and may further include a cover plate secured to the spacer for the straps. In one illustrative embodiment, the cover plate may be formed of bone and secured with pins made from bone. The cover plate may also be screwed into position. Alternatively, pins or screws may be used without a cover plate to secure the strap(s).  
         [0008]     In one embodiment, the spacer is placed through the spinous ligament and in position between the spinous process at adjacent vertebrae. The supra-spinous ligament is not removed in this illustrative method of implanting the spacer. After the spacer is in position, one of the fixation straps is threaded or placed around the spinous process. If necessary, the other fixation strap, if provided, is threaded or placed around the other spinous process. Preferably, the fixation strap is placed first about the superior spinous process. Appropriate tension is placed on the straps, and the straps are locked into position. In one embodiment, a set screw or fixation pin is provided to lock the fixation strap into position. The spacer is preferably placed without adding any agents to promote bone growth, and the spacer may be coated with a material to resist or impede bone fusion of the spacer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The detailed description will be better understood in conjunction with the accompanying drawings, in which like reference characters represent like elements, as follows:  
         [0010]      FIGS. 1-4  are perspective, front, side, and bottom views, respectively, of a first illustrative embodiment of a spinal process spacer according to the invention;  
         [0011]      FIG. 5  is a perspective view of the spinal process spacer of  FIGS. 1-4  illustrating a double-sided strapping;  
         [0012]      FIG. 6  is a perspective view of an alternative embodiment of the spinal process spacer of  FIGS. 1-5  illustrating a single-sided strapping;  
         [0013]      FIGS. 7 and 8  are front and bottom views of the spinal process spacer of  FIG. 6 ;  
         [0014]      FIGS. 9-12  are perspective, front, side, and bottom views, respectively, of a second illustrative embodiment of a spinal process spacer;  
         [0015]      FIG. 13  is a perspective view of the spinal process spacer of  FIGS. 9-12  illustrating a double-sided strapping;  
         [0016]      FIG. 14  is a perspective view of an alternative embodiment of the spinal process spacer of  FIGS. 9-13  illustrating a single-sided strapping; and  
         [0017]      FIGS. 15 and 16  are front and bottom views of the spinal process spacer of  FIG. 14 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     While an intra-spinous spacer is illustrated and described herein with reference to certain preferred or exemplary embodiments, the invention should not be limited to these preferred or exemplary embodiments. Furthermore, the features described and illustrated herein can be used singularly or in combination with other features and embodiments.  
         [0019]      FIGS. 1-4  show a first embodiment of an intra-spinous spacer. Intra-spinous spacer  100  is preferably generally rectangular and has a front side  102 , a nose side  103 , a top side  104 , an end side  105 , a bottom side  206  (which in this embodiment is identical to top side  104 ), and a back side  307 . (Note that the  FIG. 3  view of nose side  103  is rotated 90° with respect to the  FIG. 1  view to conform to the  FIG. 4  view of bottom side  206 —thus in  FIG. 3 , front side  102  is shown on top, top side  104  is shown on the left, and back side  307  is shown on the bottom.) Spacer  100  preferably has rounded edges  108   a - c  between the top side and the front, end, and back sides and between the bottom side and the front, end, and back sides. Radiuses  208   a ,  308   b , and  408   c  of rounded edges  108   a - c  are each preferably about 1.5 mm. Alternatively, radiuses  208   a ,  308   b , and  408   c  can be of other dimensions and do not have to be the same as each other.  
         [0020]     Top side  104  has a depression  112  extending laterally across top side  104 . Depression  112  is dimensioned and shaped to receive a spinous process, and preferably the spinous process contacts only depression  112  on top side  104 . In one embodiment of the invention, bottom side  206  has an identical depression  212 , which is also dimensioned and shaped to receive a spinous process, and also preferably the spinous process contacts only depression  212  on bottom side  206 . Advantageously, depressions  112 ,  212  result in reduced and preferably minimized bone contact between spacer  100  and the vertebrae. Such reduced and minimized bone contact advantageously lowers, if not eliminates, the potential for bone fusion. Depression  112  preferably has a radius  214  of about 6 mm and a depth  216  into top side  104  of about 0.5 mm. In this embodiment, depression  212  has the same radius and depth. Alternatively, depressions  112 ,  212  can have other radiuses and depths and can have radiuses and depths different from each other. Also in this embodiment, the centers  213 ,  215  of depressions  112 ,  212  are positioned a distance  217  of about 9 mm from end side  105 . Depressions  112 ,  212  alternatively may be positioned at other distances from end side  105 .  
         [0021]     Top side  104  also has a pair of holes  118 ,  120  for use with a fixation strap (described further below). Holes  118 ,  120  preferably extend completely through spacer  100  to bottom side  206 . Alternatively, one or both of holes  118 ,  120  may be threaded to accept screwed fixation devices and may not extend completely through spacer  100 . In an embodiment where one or both holes  118 ,  120  do not extend completely through spacer  100  from top side  104 , bottom side  206  may have, for example, one or more threaded holes. In one embodiment of the invention, holes  118 ,  120  each have a diameter  422  of about 2.5 mm and are spaced apart by a center-to-center distance  424  of about 8 mm. Alternatively, holes  118 ,  120  can have other center-to-center distances and other diameters, which can be different from each other. Preferably, the center-to-center distances are not more than 10 mm apart. Moreover, spacer  100  alternatively can have other numbers of holes, and the holes need not be on top side  104  and/or bottom side  206 .  
         [0022]     Nose side  103  has a generally tapered shape and extends from the top, bottom, front, and back sides of spacer  100 . Nose side  103  tapers distally and inwardly (preferably toward the center of nose side  103 ) from the top, bottom, front, and back sides to form a generally pointed or rounded distal tip  226 . Nose side  103  preferably has four surfaces that smoothly and continuously join together from the top, bottom, front, and back sides, respectively, with preferably no sharp boundary edges. Representative radiuses of nose side  103  are as follows: radius  228 , which curves down from top side  104 , is about 10 mm. Preferably, radius  229 , which curves up from bottom side  206 , is also about 10 mm. Radius  230  is about 1 mm, while radius  432  is about 2 mm. Radius  434 , which curves inward from back side  307  is about 14 mm. The distally and inwardly tapering portion of nose side  103  has a length  236 , measured from front side  102 , of about 8 mm and a length  438 , measured from the top and bottom sides, of also about 8 mm. Alternatively, nose side  103  can be of other dimensions.  
         [0023]     Referring in particular to  FIG. 3 , top side  104  and bottom side  206  each has a portion (starting at preferably halfway on each side) that curves inwardly toward back side  307  at respective radiuses  340 ,  342  of about 20 mm each. Back side  307  has a width  344  of about 8 mm. In other embodiments of the invention, radiuses  340 ,  342  and width  344  can be of other values.  
         [0024]     Spacer  100  has a length  246 , a width  348 , and a thickness  350 . In the embodiment shown in  FIGS. 1-4 , length  246  is preferably about 24 mm. Width  348  and thickness  350  are preferably dimensionally paired and variable depending on the spinal application. Preferably, the spacers are provided in a variety of sizes with thickness  350  increasing in 2 mm increments. Illustrative representative thicknesses can be about 6 mm to about 16 mm. Preferably, width  348  is about 4 mm greater than the thickness. Illustrative representative widths can be about 10 mm to about 20 mm. For example, width  348  may be about 10 mm, while thickness  350  may be about 6 mm. Or, width  348  may be about 20 mm, while thickness  350  may be about 16 mm. Other possible dimensions include a width  348  of about 16 mm and a thickness  350  of about 12 mm. Alternatively, spacer  100  may be of other lengths, widths, and thicknesses.  
         [0025]     Spacer  100  may be made of biocompatible materials such as, for example, PEAK, polycarbonate urethane, silicon polycarbonate urethane, or other polymer and plastic materials. The spacer may also be made of metals, such as, for example, titanium or stainless steel, and may also be made of composites, ceramics, or combinations of materials. Preferably, spacer  100  is made from bone and more preferably solely from cortical bone. Cortical bone reduces and preferably minimizes the possibility of bone fusion. Spacer  100  may be provided with a coating to minimize, resist, or prevent the possibility of bone fusion.  
         [0026]      FIG. 5  illustrates spacer  100  having a fixation strap extending from top side  104  for the superior spinous process and a fixation strap extending from bottom side  206  for the inferior spinous process. A fixation strap  552  may be first looped around a superior spinous process over top side  104 , inserted through hole  118  or  120 , looped around an inferior spinous process under bottom side  206 , inserted through the other of hole  118  or  120 , and then tightened and tied together as shown at location  554 . Note that fixation strap  552  can be tied together at other suitable or preferred locations. In an alternative method, strap  552  may be first looped around the superior spinous process, one end of the strap may then be inserted through one of holes  118  or  120 , the other end of the strap inserted through the other of holes  118  or  120 , one end of the strap looped around the inferior spinal process, and the strap then tightened and secured together. In yet another alternative method, fixation strap  552  may be looped around one of the superior or inferior spinous processes, one end of the strap inserted through hole  118 , the other end inserted through hole  120 , and the ends secured to either the spacer or the other spinous process. While the spacer has been shown with a single fixation strap to secure both the superior and inferior spinous processes, note that separate and multiple fixation straps can be used instead. A double-sided strapping of spacer  100 , as shown in  FIG. 5 , limits and preferably prevents both flexion and extension.  
         [0027]      FIG. 6  illustrates a single-sided strapping of an alternative embodiment of spacer  100  to a superior spinous process. A fixation strap  652  is looped around a superior spinous process over top side  104  of spacer  100   x , inserted through holes  118  and  620 , and then tightened and tied together under bottom side  606  at location  654 . Alternatively, strap  652  can be tied at other suitable or more preferred locations. A single-sided strapping of spacer  100   x  bypasses the inferior spinous process and accordingly limits, and preferably prevents, extension alone. Note that the same single-sided strapping spacer may be used for attachment to the inferior spinous process instead of the superior spinous process.  
         [0028]     Spacer  100   x  is substantially identical to spacer  100 . In spacer  100 , holes  118  and  120  extend substantially straight through from top side  104  to bottom side  206 . In spacer  100   x , as shown in  FIGS. 7 and 8 , hole  118  also extends substantially straight through from top side  104  to bottom side  606 . However, hole  620  extends diagonally through spacer  100   x  from top side  104  to bottom side  606  (as best seen in  FIG. 7  via front side  702 ). Note that diagonal hole  620  alternatively may have a common opening  719  with straight hole  118  on bottom side  606 . Diagonal hole  620  advantageously allows fixation strap  652  to be used with a superior spinous process without interfering with the inferior spinous process, which is received in depression  212 , or vice versa.  
         [0029]     Fixation straps  552  and  652  may be made of polyethylene or other biocompatible polymer. They may also be wires, monofilaments, or tendons or ligaments, such as the gracilis or semitendinousis (located around the knee). Tendon fixation straps may be pretensioned prior to implant, and fixation straps may also be braided. Furthermore, the straps may be elastic or inelastic, but preferably are flexible to assist with the placement around the spinous process. In general, fixation straps should be about 120-140 mm in length, but can be of other lengths in accordance with the application. For example, the fixation strap of  FIG. 5  may be about 140 mm, and the fixation strap of  FIG. 6  may be about 70 mm. Preferably spacers  100 / 100   x  are supplied with fixation straps  552 / 652 , respectively, pre-threaded through hole  118 , hole  120 / 620 , or both prior to implantation.  
         [0030]      FIGS. 9-12  show a second embodiment of an intra-spinous spacer. Intra-spinous spacer  900  also is preferably generally rectangular and has a front side  902 , a nose side  903 , a top side  904 , an end side  905 , a bottom side  1006  (which in this embodiment is not identical to top side  904 ), and a back side  1107 . (Note that the  FIG. 11  view of nose side  903  is rotated 90° with respect to the  FIG. 9  view to conform to the  FIG. 12  view of bottom side  1006  —thus in  FIG. 11 , front side  902  is shown on top, top side  904  is shown on the left, and back side  1107  is shown on the bottom.) Spacer  900  preferably has rounded edges  908   a - c  between the top side and the front, end, and back sides and between the bottom side and the front, end, and back sides. Radiuses  1008   a ,  1108   b , and  1208   c  of rounded edges  908   a - c  are each preferably about 1.5 mm. Alternatively, radiuses  1008   a ,  1108   b , and  1208   c  can be of other dimensions and do not have to be the same as each other.  
         [0031]     Top side  904  has a depression  912  extending laterally across top side  904 . Depression  912  is dimensioned and shaped to receive a spinous process, and preferably the spinous process contacts only depression  912  on top side  904 . Bottom side  1006  has a depression  1012 , which is also dimensioned and shaped to receive a spinous process, and also preferably the spinous process contacts only depression  1012  on bottom side  1006 . Advantageously, depressions  912 ,  1012  result in reduced and preferably minimized bone contact between spacer  900  and the vertebrae. Such reduced and minimized bone contact advantageously lowers, if not eliminates, the potential for bone fusion. Spacer  900  may be provided with a coating to minimize, resist, or prevent the possibility of bone fusion. Depression  912  preferably has a radius  1014  of about 3 mm and a depth  1016  into top side  904  of about 1 mm. Depression  1012  has a radius  1009  of about 2 mm and the same depth  1011  into bottom side  1006  of about 1 mm. Alternatively, depressions  912 ,  1012  can have other radiuses and depths, including radiuses and depths that are the same as or different from each other. Also in this embodiment, the centers  1013 ,  1015  of depressions  912 ,  1012  are positioned a distance  1017  of about 12 mm from end side  905 . Depressions  912 ,  1012  alternatively may be positioned at other distances from end side  905 .  
         [0032]     Top side  904  also has a pair of holes  918 ,  920  for use with a fixation strap (described further below). Holes  918 ,  920  preferably extend completely through spacer  900  to bottom side  1006 . Alternatively, one or both of holes  918 ,  920  may be threaded to accept screwed fixation devices and may not extend completely through spacer  900 . In an embodiment where one or both holes  918 ,  920  do not extend completely through spacer  900  from top side  904 , bottom side  1006  may have, for example, one or more threaded holes. In one embodiment of the invention, holes  918 ,  920  each have a diameter  1222  of about 2.5 mm and are spaced apart by a center-to-center distance  1224  of about 10 mm. Alternatively, holes  918 ,  920  can have other center-to-center distances and other diameters, which can be different from each other. Moreover, spacer  900  alternatively can have other numbers of holes, and the holes need not be on top side  904  and/or bottom side  1006 .  
         [0033]     Nose side  903  has a generally tapered shape and extends from the top, bottom, front, and back sides of spacer  900 . Nose side  903  tapers distally and inwardly (preferably toward the center of nose side  103 ) from the top, bottom, front, and back sides to form a generally pointed or rounded distal tip  1026 . Nose side  903  preferably has four surfaces that smoothly and continuously join together from the top, bottom, front, and back sides, respectively, with preferably no sharp boundary edges. Representative radiuses of nose side  903  are as follows: radius  1028 , which curves down from top side  904 , is about 10 mm. Preferably, radius  1029 , which curves up from bottom side  1006 , is also about 10 mm. Radius  1030  is about 1 mm, while radius  1232  is about 2 mm. Radius  1234 , which curves inward from back side  1107  is about 14 mm. The distally and inwardly tapering portion of nose side  903  has a length  1036 , measured from front side  902 , of about 8 mm and a length  1238 , measured from the top and bottom sides, of also about 8 mm. Alternatively, nose side  903  can be of other dimensions.  
         [0034]     Referring in particular to  FIG. 11 , top side  904  and bottom side  1006  each has a portion (starting preferably halfway on each side) that curves inwardly toward back side  1107  at respective radiuses  1140 ,  1142  of about 20 mm each. Back side  1107  has a width  1144  of about 8 mm. In other embodiments of the invention, radiuses  1140 ,  1142  and width  1144  can be of other values.  
         [0035]     Spacer  900  has a length  1046 , a width  1148 , and a thickness  1150 . In the embodiment shown in  FIGS. 9-12 , length  1046  is preferably about 24 mm. Width  1148  and thickness  1150  are preferably dimensionally paired and variable depending on the spinal application. Preferably, the spacers are provided in a variety of sizes with thickness  1150  increasing in 2 mm increments. Illustrative representative thicknesses can be about 6 mm to about 16 mm. Preferably, width  1148  is about 4 mm greater than the thickness. Illustrative representative widths can be about 10 mm to about 20 mm. For example, width  1148  may be about 10 mm, while thickness  1150  may be about 6 mm. Or, width  1148  may be about 20 mm, while thickness  1150  may be about 16 mm. Other possible dimensions include a width  348  of about 14 mm and a thickness  1150  of about 10 mm. Alternatively, spacer  900  may be of other lengths, widths, and thicknesses.  
         [0036]     As spacer  100 , spacer  900  also may be made of biocompatible materials such as, for example, PEAK, polycarbonate urethane, silicon polycarbonate urethane, or other polymer and plastic materials. The spacer may also be made of metals, such as, for example, titanium or stainless steel, and may also be made of composites, ceramics, or combinations of materials. Preferably, spacer  900  is made from bone and more preferably solely from cortical bone. Spacer  900  may be provided with a coating to minimize, resist, or prevent the possibility of bone fusion.  
         [0037]     When inserted, the shapes of spacers  100  and  900  preferably cause the spacers to be pushed back towards the spinal column for a better fit between the vertebrae.  
         [0038]      FIG. 13  illustrates a double-sided strapping of spacer  900  to a superior spinous process and an inferior spinous process. Fixation strap  552  may be first looped around a superior spinous process over top side  904 , inserted through one of holes  918  or  920 , looped around an inferior spinous process under bottom side  1006 , inserted through the other of holes  918  or  920 , and then tightened and secured together as shown at location  554 . Note that fixation strap  552  can be secured together at other suitable or preferred locations. In an alternative method, strap  552  may be first looped around the superior spinous process, one end of the strap may then be inserted through one of holes  918  or  920 , the other end of the strap inserted through the other of holes  918  or  920 , one end of the strap looped around the inferior spinal process, and the strap then tightened and secured together. In yet another alternative method, fixation strap  552  may be looped around one of the superior or inferior spinous processes, one end of the strap inserted through hole  918 , the other end inserted through hole  920 , and the ends secured to either the spacer or the other spinous process. While the spacer has been shown with a single fixation strap to secure both the superior and inferior spinous processes, note that separate and multiple fixation straps can be used instead. A double-sided strapping of spacer  900  limits, and preferably prevents, both flexion and extension.  
         [0039]      FIG. 14  illustrates a single-sided strapping of an alternative embodiment of spacer  900  to a superior spinous process. Fixation strap  652  is looped around a superior spinous process over top side  904  of spacer  900   x , inserted through holes  918  and  1420 , and then tightened and tied together. Strap  652  may be tied and joined under bottom side  1406  at location  654 . Alternatively, strap  652  can be tied at other suitable or more preferred locations. A single-sided strapping of spacer  900   x  bypasses the inferior spinous process and accordingly limits, and preferably prevents, extension alone. Note that the same single-sided strapping spacer may be used for attachment to the inferior spinous process instead of the superior spinous process.  
         [0040]     Preferably spacers  900 / 900   x  are supplied with fixation straps  552 / 652 , respectively, pre-threaded through hole  918 , hole  920 / 1420 , or both prior to implantation.  
         [0041]     Spacer  900   x  is substantially identical to spacer  900 . In spacer  900 , holes  918  and  920  extend substantially straight through from top side  904  to bottom side  1006 . In spacer  900   x , as shown in  FIGS. 15 and 16 , hole  918  also extends substantially straight through from top side  904  to bottom side  1406 . However, hole  1420  extends diagonally through spacer  900   x  from top side  904  to bottom side  1406  (as best seen in  FIG. 15  via front side  1502 ). Note that diagonal hole  1420  alternatively may have a common opening  1519  with straight hole  918  on bottom side  1406 . Diagonal hole  1420  advantageously allows fixation strap  652  to be used with a superior spinous process without interfering with the inferior spinous process, which is received in depression  1012 , or vice versa.  
         [0042]     Spacers  100 ,  100   x ,  900 , and  900   x  are preferably terminally sterilized, preferably by chemical or other appropriate processes, prior to implantation to kill proteins.  
         [0043]     Furthermore, the depressions of spacers  100 ,  100   x ,  900 , and  900   x  may be lined with an inert material, such as, for example, silicone.  
         [0044]     Also note that spacers of the invention may include a cover plate made preferably of bone material and secured to the spacer preferably with bone pins. Cover plates may include recesses for wrapping a fixation strap around the spacer. Additionally or alternatively, fixation straps may be fixed to a cover plate with one or more set screws.  
         [0045]     Further note that while holes are shown in the spacers, other means may be alternatively used to secure one or more fixation straps. For example, fixation straps may be secured or tacked to the spacer or spinous process using, for example, bone pins, screws, or other suitable means.  
         [0046]     While it is apparent that the illustrative embodiments of the invention disclosed herein fulfill the objectives stated above, it will be appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Features and structures, such as, for example, the size, shape, and location of the spacer depressions can be used singularly or in combination with other features and structures. Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments which come within the spirit and scope of the invention.