Patent Publication Number: US-2011077742-A1

Title: Expandable Vertebral Body Implants and Methods of Use

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/489,189, filed on Jul. 19, 2006, and herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Spinal implants are often used in the surgical treatment of spinal disorders such as degenerative disc disease, disc herniations, scoliosis or other curvature abnormalities, and fractures. Many different types of treatments are used, including the removal of one or more vertebral bodies and/or intervertebral disc tissue. In some cases, spinal fusion is indicated to inhibit relative motion between vertebral bodies. In other cases, dynamic implants are used to preserve motion between vertebral bodies. In yet other cases, relatively static implants that exhibit some degree of flexibility may be inserted between vertebral bodies. 
     Regardless of the type of treatment and the type of implant used, surgical implantation tends to be a difficult for several reasons. For instance, access to the affected area may be limited by other anatomy. Further, a surgeon must be mindful of the spinal cord and neighboring nerve system. The size of the implant may present an additional obstacle. In some cases, a surgeon may discover that an implanted device has an inappropriate size for a particular application, which may require removal of the implant and insertion of a different implant. This trial and error approach may increase the opportunity for injury and is certainly time-consuming. Expandable implants are becoming more prevalent as a response to some of these concerns. However, the expansion mechanism in these devices tends to be complex and large. Consequently, existing devices do not appear to address each of these issues in a manner that improves the ease with which the device may be surgically implanted. 
     SUMMARY 
     The present application is directed to a vertebral implant for insertion between vertebral bodies in a patient. The vertebral implant may include a spacer member and first and second end members positioned on opposing sides of the spacer member. Each end member may include a bone contact surface to contact one of the vertebral bodies, an opposing surface, and a perimeter surface extending between the bone contact surface and the opposing surface. The end members may further include overlapping first and second recesses that each extend inward from the perimeter surface and the opposing surface and are bounded by sidewalls formed by the end members. At least one attachment member may be coupled to each of the first and second end members and may be spaced away from the first and second recesses and the spacer member. The implant may include a longitudinal axis that extends through the spacer member and the first and second recesses of each of the end members. The second recesses may include a greater width measured perpendicular to the longitudinal axis than the first recesses with the second recesses extending outward beyond the first recesses in a direction perpendicular to the longitudinal axis. The attachment member may be positioned away from the longitudinal axis a greater distance than the sidewalls of the first and second recesses. 
     The vertebral implant may include a spacer member with a first end and a second end. A first end member may be positioned on the first end of the spacer member, and a second end member may be positioned on the second end of the spacer member. A longitudinal axis may extend through the spacer member, the first end member, and the second end member. The second end member may include an inner surface that faces towards the first end member with a recess extending into the inner surface, and may have a bottom surface opposite from the inner surface that contacts against the second end of the spacer member. The recess may be bounded by sidewalls that extend between the bottom surface and the inner surface. The recess may have a width measured perpendicular to the longitudinal axis that is greater at the inner surface than at the bottom surface. A first attachment member may include a first end attached to the first end member and a second end attached to the second end member to couple together the first and second end members. The first attachment member may be positioned a greater distance from the longitudinal axis than the sidewalls of the recess. 
     The vertebral implant may include a spacer member with a first end and a second end. The implant may include a first end member positioned on the first end of the spacer member, and a second end member positioned on the second end of the spacer member with a longitudinal axis extending through the spacer member, the first end member, and the second end member. First and second attachment members may couple together the first and second end members. Each of the first and second attachment members may have a first end attached to the first end member and a second end attached to the second end member. Each of the first and second end members may include an inner surface that faces inward, an outer surface that faces outward to contact against one of the vertebral bodies, and a perimeter that extends between the inner and outer surfaces. Each of the first and second end members may further include a recess extending into the inner surface and the perimeter and may have a bottom surface opposite from the inner surface that contacts against the spacer member. The recess may have sidewalls that extend between the bottom surface and the inner surface. The recess may have a width measured perpendicular to the longitudinal axis that is greater at the inner surface than at the bottom surface. Each of the first and second attachment members may be positioned away from the longitudinal axis, the spacer member, and the recesses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a lateral view of a vertebral implant according to one or more embodiments shown relative to vertebral bodies; 
         FIG. 2  is a lateral view of a vertebral implant according to one or more embodiments; 
         FIG. 3  depicts lateral views of a set of spacer members, each including a different height, of a vertebral implant according to one embodiment; 
         FIG. 4  is a perspective view of end members of a vertebral implant according to one embodiment; 
         FIG. 5  is a lateral view of end members of a vertebral implant according to one embodiment; 
         FIG. 6  is a lateral view of end members of a vertebral implant according to one embodiment; 
         FIGS. 7-10  illustrate a sequence of implantation steps to obtain a desired vertebral body spacing, each Figure depicting a lateral view of a distractor and components of a vertebral implant according to one or more embodiments shown relative to vertebral bodies; 
         FIG. 11  depicts a top view of a distractor according to one embodiment; 
         FIG. 12  is a perspective view of end members of a vertebral implant according to one embodiment; 
         FIG. 13  is a lateral view of a vertebral implant according to one or more embodiments; 
         FIG. 14  is a lateral view of a vertebral implant according to one or more embodiments; and 
         FIG. 15  is a lateral view of a vertebral implant according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments disclosed herein relate to a vertebral implant in which multiple components may be combined to establish a desired spacing between vertebral bodies in a patient. Advantageously, the implant components may be inserted at different times or in a compressed state with the components adjusted to the desired spacing in situ. Reference number  10  in  FIG. 1  generally identifies an exemplary implant. In one embodiment, the implant  10  is positionable within an intervertebral space S to span one or more vertebral levels along the longitudinal axis of the spinal column. Although the illustrated embodiment of the implant  10  spans one vertebral level, it should be understood that the implant  10  may be configured to span multiple vertebral levels, including two or more vertebral levels. 
     The implant  10  generally includes a first end member  22 , a second end member  24 , and one or more spacer members  26  coupled between the first and second end members  22 ,  24 . In the illustrated embodiment, end member  22  is disposed in a superior position relative to an inferior opposite end member  24 . In one embodiment, the end members  22 ,  24  and spacer member  26  are formed of a biocompatible material, such as, for example, a carbon fiber material, or non-metallic substances, including polymers or copolymers made from materials such as PEEK and UHMWPE. In further embodiments, the end members  22 ,  24  and spacer member  26  may be formed of other suitable biocompatible materials, such as, for example, stainless steel, titanium, cobalt-chrome, and shape memory alloys. 
     The end members  22 ,  24  are adapted to engage the endplates of upper and lower vertebral bodies V 1 , V 2 . The spacer member  26  is engaged between the end members  22 ,  24  to maintain an intervertebral axial space S between the upper and lower vertebral bodies V 1 , V 2  following the removal of one or more vertebral levels (shown in phantom). To facilitate insertion of the implant  10 , the spacer member  26  may be inserted separately from the end members  22 ,  24 . That is, the end members  22 ,  24  may be inserted during a first insertion step and the spacer member  26  may be inserted during a second, subsequent insertion step. Further details regarding exemplary insertion steps are provided below. 
       FIG. 2  shows an exemplary implant  10  including the end members  22 ,  24  and the spacer member  26  disposed therebetween. The spacer member  26  may have a fixed or adjustable height. In the illustrated embodiment, the spacer member  26  includes an extendable portion  28  that is adjustable in a longitudinal direction as illustrated by the arrows labeled E. The exemplary spacer member  26  is expandable in a direction that is substantially transverse to the bone contact surfaces  42 ,  44  of the end members  22 ,  24 . In one embodiment, the extendable portion  28  is movable relative to an outer portion  32 . The extendable portion  28  may include threads  34  that engage with a rotatable collar  30 . The rotatable collar  30  is rotatably coupled to the outer portion  32  such that rotation of the collar  30  produces a corresponding extension or retraction of the extendable portion  28 . Certainly, other types of extendable spacer members  26  may be used. Those skilled in the art will appreciate that other types of mechanical spacers may be used. Similarly, other spacers may implement pneumatic, hydraulic, or electric power to extend from a first compressed height to a desired second height to attain the desired amount of vertebral distraction. 
     In one embodiment, the spacer member  26  includes a fixed height. Thus, as  FIG. 3  shows, spacer members  26 ,  26 A,  26 B and so on may belong to a set of spacer members  126 , with each spacer member  26 ,  26 A,  26 B including a different height H 1 , H 2 , H 3 . As the parts are formed, the measurable height H 1 , H 2 , H 3  may be indicated as a marking  90  on the exterior of the spacer member  26 ,  26 A,  26 B to provide surgeons an indication of the relative height of the spacer members  26 ,  26 A,  26 B. This marking may be provided as an alphanumeric indication as represented by the letters H 1 , H 2 , H 3  in  FIG. 3 . The marking may include an actual height in inches or millimeters. The marking may include a relative indication of the height of the spacer within the set  126 . For instance, the spacer members  26 ,  26 A,  26 B may be marked in ascending numerical or alphabetical characters. The marking may be stamped, whether by ink or metal deformation, engraved, or otherwise displayed on the spacer members  26 ,  26 A,  26 B. 
     During implantation, a surgeon may select a spacer member  26 ,  26 A,  26 B from the set based upon an estimated or calculated desirable implant height. This estimation or calculation may be based at least partly upon radiograph information, the patient size and age, and the location of the implanted device  10 . However, during such a procedure, a surgeon may determine that a different size is desirable. For instance, the surgeon may determine that a slightly smaller or larger implant height is desirable. Accordingly, the surgeon may remove a first implanted spacer member  26 ,  26 A,  26 B in favor of a more appropriate spacer member  26 ,  26 A,  26 B from the overall set  126 . 
     As suggested, the spacer member  26  is separable from the end members  22 ,  24 . To illustrate this characteristic,  FIG. 4  shows the end members  22 ,  24  in perspective view with the spacer member  26  removed. In the illustrated embodiment, the end members  22 ,  24  include a kidney shape, though other shapes may be used. In further embodiments, the end members  22 ,  24  may take on other types of configurations, such as, for example, a circular shape, semi-oval shape, bean-shape, D-shape, elliptical-shape, egg-shape, or any other shape that would occur to one of skill in the art. The end members  22 ,  24  may take on substantially solid configurations, such as, for example, block-like or plate-like configurations that do not define an open inner region. In other embodiments, the end members  22 ,  24  could also be described as being annular, U-shaped, C-shaped, V-shaped, horseshoe-shaped, semi-circular shaped, semi-oval shaped, or other similar terms defining an implant including at least a partially open or hollow construction. 
     The end members  22 ,  24  include respective bone-contact surfaces  42 ,  44 . Each end member  22 ,  24  further includes an opposing surface  46 ,  48  that faces towards the spacer member  26  when inserted between the end members  22 ,  24 . A peripheral wall  50 ,  52  extends about the perimeter of the end members  22 ,  24  between the respective bone contact surfaces  42 ,  44  and opposing, non-bone contact surfaces  46 ,  48 . Note that the peripheral wall  50 ,  52  may be part of one or both the bone contact surfaces  42 ,  44  and opposing surfaces  46 ,  48 , such as where the surfaces blend into one another. Thus, there is no express requirement that there be a sharp edge between the bone contact surfaces  42 ,  44 , the peripheral surfaces  50 ,  52 , or the opposing surfaces  46 ,  48  as illustrated. 
     In one embodiment, the end members  22 ,  24  have an outer profile that is substantially complementary to the size and shape of the peripheral portion or outlying region of the vertebral bodies V 1 , V 2 , such as the cortical rim or the apophyseal ring of the vertebral endplates. In this manner, some portion of the bone contact surfaces  42 ,  44  of end members  22 ,  24  may be engaged against the cortical region of the vertebral endplates, thereby minimizing the likelihood of subsidence into the relatively softer cancellous region of the vertebral bodies V 1 , V 2  following insertion of the implant  10  within the intervertebral space S. 
     Additionally, the exemplary end members  22 ,  24  include one or more apertures  36  disposed about the bone contact surfaces  42 ,  44 . The apertures  36  may have different size, quantity, and location that those illustrated. The apertures  36  may be blind holes in that they do not extend through the end members  22 ,  24 . The apertures  36  may be through-holes in that they do extend through the end members  22 ,  24 . The end members  22 ,  24  may be inserted in conjunction with bone growth materials (not shown) that may include, for example, bone graft, bone morphogenetic protein (BMP), allograft, autograft, and various types of cement, growth factors and mineralization proteins. These bone growth materials may be packed into the apertures  36  to promote osseointegration of the end members  22 ,  24  to the vertebral bodies V 1 , V 2 . In a further embodiment, the bone growth promoting materials may be provided in a carrier (not shown), such as, for example, a sponge, a block, a cage, folded sheets, or paste that may be inserted into the apertures  36 . 
     The end members  22 ,  24  include a spacer recess  38  that is sized and shaped to accept the spacer member  26 . Each member  22 ,  24  may have a similarly formed spacer recess  38 . Alternatively, end member  22  may have a spacer recess  38  that is different in size or shape than a corresponding spacer recess  38  in the opposite end member  24 . Each end member  22 ,  24  further includes a distractor recess  40  that is sized and shaped to accept a distractor as discussed below. In short, a distractor (see e.g.,  FIGS. 7-11 ) may be used to establish a desired spacing and/or a desired amount of distraction between the end members  22 ,  24  prior to inserting the spacer member  26  into the spacer recesses  38 . In the embodiment illustrated, the spacer recess  38  and the distractor recess  40  extend inward from the outer peripheral surfaces  50 ,  52 . In the exemplary embodiment, the spacer recess  38  and the distractor recess  40  extend substantially parallel to the bone contact surfaces  42 ,  44 . In certain other implementations, the bone contact surfaces  42 ,  44  are not necessarily planar and may include curvatures or angled orientations relative to the longitudinal axis of the spine. In these types of end members  22 ,  24 , the spacer recess  38  and the distractor recess  40  may extend inward from the outer peripheral surface  50 ,  52  in a direction that coincides with an insertion direction for the spacer member  26 . That is, the spacer member  26 , if fully extended to a desire height (or including a desired fixed height), may be inserted into the end members  22 ,  24  without having to over-distract the end members  22 ,  24 . Note that  FIG. 5  is shown according to the view lines provided in  FIG. 4  and that this direction coincides with the spacer member  26  insertion direction. This direction may also coincide with a distractor (as in  FIGS. 7-11 ) insertion and removal direction. 
       FIG. 5  illustrates that the spacer recess  38  and the distractor recess  40  intersect one another and occupy much of the same volume.  FIG. 5  further suggests that the spacer recess  38  extends a greater depth from the respective non-bone-contact surfaces  46 ,  48  of the end members  22 ,  24  than the distractor recess surface  40 . Specifically, the spacer recess  38  terminates at a spacer abutment surface  56 . The distractor recess  40 , on the other hand, terminates at a distractor abutment surface  54 . Lateral side  60  further defines the spacer recess  38  while lateral side  62  further defines the distractor recess  40 . In the present embodiment, lateral side  62  is disposed outside of lateral side  60 . As described below, this stair-stepped configuration permits the spacer member  26  and a distractor to remain simultaneously engaged to different portions of the end members  22 ,  24 . In an alternative embodiment, the spacer recess  38 A and distractor recess  40 A are separate from one another as depicted in  FIG. 6 . Similar to the embodiment in  FIG. 5 , the spacer recess  38 A and distractor recess  40 A extend inward from the respective peripheral walls  50 A,  52 A and the respective non-bone-contact surfaces  46 A,  48 A. 
     In one embodiment as illustrated in  FIGS. 4 and 5 , the end member  24  is a second end member. The outer peripheral surface  52  is part of an intermediate wall that extends between surfaces  44 ,  48 . The spacer recess  38  and distractor recess  40  form a second receptacle. The spacer abutment surface  56  forms a bottom wall that defines a depth of the second receptacle. The distracter abutment surface  54  forms a ledge positioned between the bottom wall and the surface  48 . The ledge is uncovered in a direction directly towards the first member. In another embodiment, the end member  22  is a second end member and includes these similar elements. 
     The implant  10  may be inserted into a patient according to the process steps illustrated in  FIGS. 7-10 . In  FIG. 7 , the end members  22 ,  24  are inserted and positioned within an intervertebral space formed after the removal of one or more vertebrae or discs. Next, a distractor  100  is inserted into the respective distractor recesses  40  in the end members  22 ,  24 . The exemplary distractor  300  extends along a longitudinal axis L and generally includes a first distractor arm  302  and a second distractor arm  304 . The first and second distractor arms  302 ,  304  are coupled to one another via a hinge mechanism  306  which provides for pivotal movement between the distractor arms  302 ,  304  about the hinge mechanism  306 . As should be appreciated, an inward compression force exerted onto the proximal portions  302   a ,  304   a  of the distractor arms in the direction of arrows A will cause the distal end portions  302   b ,  304   b  to be outwardly displaced in the direction of arrows B. Thus, distraction of the vertebral bodies V 1 , V 2  is achieved along longitudinal axis X, which corresponds at least generally with the longitudinal axis of the spine and of the implant  10 . 
     In the illustrated embodiment, the distractor  300  includes a threaded rod  320  having a first end portion  320   a  rotatably coupled with the proximal end portion  304   a  of the distractor arm  304 , and a second end portion  320   b  engaged within a threaded aperture (not specifically shown) extending through the proximal end portion  302   a  of the distractor arm  302 . As should be appreciated, the position of rod  320  may be adjusted relative to the distractor arm  302  by threading the rod  320  with a rotary knob  312  to correspondingly control the amount of distraction provided by the distractor arms  302 ,  304 . In one embodiment, the distractor  300  is provided with a gauge or stop member  310  that is adapted to limit outward displacement of the distal end portion  302   b ,  304   b , which in turn correspondingly limits that amount of distraction provided by the distractor arms  302 ,  304 . In this manner, over distraction of the intervertebral space S is avoided. 
     The distal end of the distal end portions  302   b ,  304   b  include geometry that engages the distractor recesses  40  of the end members  22 ,  24 .  FIG. 11  depicts a top view of the exemplary distractor  300 . In one embodiment, the distal end portions  302   b ,  304   b  of the distractor arms  302 ,  304  define a lateral offset relative to the longitudinal axis L. Further, the distal end portions  302   b ,  304   b  include an arcuate-shaped configuration defining a C-shaped lateral offset. However, other shapes and configurations of the distal end portions  302   b ,  304   b  are also contemplated. The shape of the distal end portions  302   b ,  304   b  define an open area  330  that is sized to fit around a spacer member  26 , which may be inserted after the end members  22 ,  24  are distracted a desired amount as shown in  FIGS. 8-10 . Notably,  FIGS. 7-10  illustrate one particular type of distractor  300  that uses a threaded rod  320  to achieve a mechanical advantage and distract the vertebral bodies V 1 , V 2  a desired amount. However, those skilled in the art will appreciate that different types of distractors may be used, including but not limited to devices incorporating pneumatic, hydraulic, electrical, or mechanical displacement forces. However, it is generally contemplated that the distractor ( 300  or otherwise) include engaging features with a size and shape that engages the distractor recesses  40 , yet provides access to the spacer recess  38  while the distractor  300  is engaged with the distractor recesses  40  as described herein. 
     With the distractor  300  engaged in the distractor recesses  40  as shown in  FIG. 7 , the rotary knob  312  may be rotated to force the distal end portions  302   b ,  304   b  in the direction of arrows B.  FIG. 8  shows the proximal ends  302   a ,  304   a  in closer proximity to one another. Correspondingly, the distal ends  302   b ,  304   b  are spaced farther apart compared to the position shown in  FIG. 7 . With the desired amount of distraction achieved, the distractor  300  may be maintained in the position shown and the spacer member  26  introduced into the space formed between the end members  22 ,  24  as shown in  FIG. 9 . In one implementation, the spacer member  26  is expandable and may be extended to the position shown in  FIG. 10 . However, in another implementation, the spacer member  26  includes a fixed height and may be introduced into the spacer recess  38  as described above. That is, since the end members  22 ,  24  include a spacer recess  38  that is accessible from the peripheral walls  50 ,  52 , additional distraction is not required. Furthermore, because the distractor  300  includes the open configuration shown in  FIG. 11 , the spacer member  26  may be introduced into the end members  22 ,  24  while the distractor  300  engages and maintains the desired amount of distraction through engagement with the respective distractor recesses  40 . 
     Once the spacer member  26  is seated as desired in the spacer recesses  38 , the distractor  300  may be disengaged along the longitudinal axis L. Again, since the distractor recess  40  is open to the peripheral walls  50 ,  52  of the end members  22 ,  24 , the distractor  300  does not necessarily have to be compressed by turning the rotary knob  312  prior to removal. Once the distractor  300  is removed, the implant  10  remains in the vertebral space S as shown in  FIG. 1 . 
     The embodiments described above have contemplated a vertebral implant  10  including one end member  22 ,  24  at each end of a spacer member  26 . Further, the insertion of the implant  10  has been depicted using an exemplary anterior approach as is known in the art. However, other implant devices may be inserted using known posterior or trans-foraminal approaches. Accordingly, the end members  122   a ,  122   b ,  124   a ,  124   b  shown in  FIG. 12  may be incorporated in an implant inserted using posterior or lateral approaches. The exemplary end members  122   a ,  122   b ,  124   a ,  124   b  include features similar to the above-describe embodiments, including bone-contact surfaces  142 ,  144 , opposing non-bone-contact surfaces  146 ,  148 , peripheral surfaces  150 ,  152 , bone-growth apertures  136 , spacer recesses  138  and distractor recesses  140 . The function and characteristics of these features may be similar to those described above, with the size and shape of the features modified according to the different size of the end members  122   a ,  122   b ,  124   a ,  124   b.    
     The end members  22 ,  24  described above were embodied as separate members. In embodiments depicted in  FIGS. 13 ,  14 , and  15 , the end members are coupled to one another to ease installation. In the illustrated embodiments, most features of the end members  22 ,  24 , and spacer member  26  are similar to that described above. However, in the embodiments of  FIGS. 13-15 , each end member is coupled to one or more attachment members. For instance, in  FIG. 13 , the end members  22 A,  24 A of implant  10 A include sliding and telescoping attachment members  70 ,  72 , respectively. In one embodiment, the attachment members  70 ,  72  are rigid and capable of axially sliding relative to one another. Further, because the attachment members  70 ,  72  are rigid, the end members  22 A,  24 A are maintained in a predetermined alignment relative to each other. In one embodiment, attachment members  70 ,  72  are flexible members that are capable of axially sliding relative to one another. Consequently, the end members  22 A,  24 A remain coupled, but are movable relative to each other in multiple directions. 
       FIG. 14  depicts an embodiment of an implant  10 B that includes attachment members  76 ,  78  extending respectively from the end members  22 B,  24 B. The attachment members  76 ,  78  are disposed in sliding contact with one another and permit relative movement between the end members  22 B,  24 B in an extension direction. However, the attachment members  76 ,  78  prevent relative lateral motion between the end members  22 B,  24 B.  FIG. 15  depicts an embodiment of an implant  10 C that includes flexible attachment members  80  that are coupled between the end members  22 C,  24 C. In one embodiment, the attachment members  80  are tethers that permit motion of the end members  22 C,  24 C relative to each other in multiple directions. The attachment member  80  may be implemented as braided cable, including metal or non-metal materials, tubing, or other suitable variants and may include fewer or greater numbers of attachment members  80  than that depicted in  FIG. 15 . 
     Furthermore, embodiments disclosed above have not included any particular surface geometry, coating, or porosity as are found in conventionally known vertebral implants. Surface features such as these are used to promote bone growth and adhesion at the interface between an implant and a vertebral end plate. Examples of features used for this purpose include, for example, teeth, scales, keels, knurls, and roughened surfaces. Some of these features may be applied through post-processing techniques such as blasting, chemical etching, and coating, such as with hydroxyapatite. The bone interface surfaces, including the osteoconductive inserts, may also include growth-promoting additives such as bone morphogenetic proteins. Alternatively, pores, cavities, or other recesses into which bone may grow may be incorporated via a molding process. Other types of coatings or surface preparation may be used to improve bone growth into or through the bone-contact surfaces. However, the inserts that include these types of features may still be formed and characterized by the aspects disclosed herein. 
     Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc and are also not intended to be limiting. Like terms refer to like elements throughout the description. 
     As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
     The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. For instance, the end member embodiments disclosed herein have included a single spacer recess and a single distractor recess. In alternative implementations, each end member may include multiple spacer recesses or multiple distractor recesses for implementation in different locations or different implantation procedures. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.