Abstract:
A spinal prosthesis system includes: an anterior portion having an upper component and a lower component, the upper and lower components capable of relative movement about a first point; a posterior portion separate from the anterior portion, the posterior portion having an upper portion and a lower portion, the upper and lower portions capable of relative movement concentric to the upper and lower components about the first point. In another form, a method includes: determining a motion profile of a vertebral motion segment; determining a point of rotation based upon the motion profile; and manufacturing a spinal stabilization system having an anterior portion and a posterior portion, the anterior and posterior portions each having a center of rotation located at the point of rotation.

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure relate generally to devices and methods for accomplishing spinal surgery, and more particularly in some embodiments, to devices and methods relating to spinal prosthesis systems having anterior and posterior portions with a concentric (i.e., same) center of rotation. 
     BACKGROUND 
     Spinal implants are often comprised of multiple components. For example, some spinal implants include anterior and posterior portions that are each configured to preserve at least some vertebral motion. However, the anterior and posterior portions can adversely affect each other during motion by placing undesirable loads on the opposing portion. Although existing devices and methods have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. 
     SUMMARY 
     In one embodiment, a spinal prosthesis having a motion-preserving anterior portion and a motion-preserving posterior portion is provided. The anterior and posterior portions function so as not to adversely affect one another during motion. 
     In a second embodiment, a spinal prosthesis having a motion-preserving anterior portion and a motion-preserving posterior portion is provided wherein the anterior and posterior portions have concentric centers of rotation. 
     In another embodiment, a method of inserting a spinal prosthesis having a motion-preserving anterior portion and a motion-preserving posterior portion with concentric centers of rotation is provided 
     Additional and alternative features, advantages, uses, and embodiments are set forth in or will be apparent from the following description, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic side view of an arrangement that embodies aspects of the present disclosure. 
         FIG. 2A  is a sectional view of the arrangement of  FIG. 1  taken along section line  2 - 2 . 
         FIG. 2B  is a sectional view of an arrangement similar to  FIG. 2A , but showing an alternative embodiment. 
         FIG. 2C  is a sectional view of arrangement similar to  FIG. 2A , but showing an alternative embodiment. 
         FIG. 3  is diagrammatic posterior view of the arrangement of  FIG. 1 . 
         FIG. 4  is a diagrammatic side view similar to  FIG. 1 , but showing an alternative embodiment. 
         FIG. 5  is a sectional view of the arrangement of  FIG. 4  taken along section line  5 - 5 . 
         FIG. 6  is a diagrammatic side view similar to  FIGS. 1 and 4 , but showing an alternative embodiment. 
         FIG. 7  is a sectional view of the arrangement of  FIG. 6  taken along section line  7 - 7 . 
     
    
    
     DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments, or examples, illustrated in the drawings and specific language will be used to describe the embodiments. It will nevertheless be understood that no limitation of the scope of the invention is intended. Any alterations and further modifications of the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
       FIG. 1  is a diagrammatic side view of an arrangement  10  that embodies aspects of the present invention. The arrangement  10  includes a vertebra  12 , a vertebra  14 , and an apparatus  15 .  FIG. 2A  is a sectional view of the arrangement  10  of  FIG. 1  taken along section line  2 - 2 .  FIG. 3  is diagrammatic posterior view of the arrangement  10  of  FIG. 1 . 
     Referring to  FIGS. 1-3 , the vertebra  12  includes a spinous process  16 , a left pedicle  18 , a right pedicle  19 , and an inferior endplate  20 . Similarly, the vertebra  14  includes a spinous process  22 , a left pedicle  24 , a right pedicle  26 , and a superior endplate  28 . 
     The apparatus  15  is a prosthesis system adapted to preserve at least some motion between the vertebrae  12  and  14 . The apparatus  15  includes an anterior portion  30 . The anterior portion  30  includes an inferior piece  32 . The inferior piece  32  includes an inferior surface  34  that is adapted to engage with the superior endplate  28  of vertebra  14 . The inferior surface  34  may include features adapted to enhance engagement with the endplate  28 , including projections, such as spikes, keels, ridges, or other surface textures; surface treatments, such as chemical etching, bead-blasting, sanding, grinding, serrating, diamond-cutting, coating with a biocompatible and osteoconductive material (such as hydroxyapatite (HA), tricalcium phosphate (TCP), or calcium carbonate), or coating with osteoinductive materials (such as proteins from the transforming growth factor (TGF) beta superfamily or bone-morphogenic proteins, such as BMP2 or BMP7); or other features. The inferior piece  32  also includes a projection  36 . The projection  36  extends out of a superior surface  37  of the inferior piece  32 . The projection  36  is curved to substantially coincide with a virtual sphere centered about a center point  38  with a radius  39 . As shown in  FIG. 1 , the projection  36  is positioned posteriorly from a central portion of the inferior piece  32  in the anterior/posterior direction. As shown in  FIG. 2A , however, the projection  36  is centered about the central portion of the inferior piece  32  in the lateral direction. 
     The anterior portion  30  also includes a superior piece  40 . The superior piece  40  includes a superior surface  42  that is adapted to engage with the inferior endplate  20  of vertebra  12 . Similar to inferior surface  34  of the inferior piece  32 , the superior surface  42  may include features adapted to enhance engagement with the endplate  20 . The superior piece  40  also includes a recess  44 . The recess  44  extends into an inferior surface  45  of the superior piece  40 . The recess  44  is adapted to movably mate with the projection  36  of the inferior piece  32 . Thus, the recess  44  is substantially spherical about the center point  38  to match the projection  36 . However, when mated together the recess  44  does not fully envelope the projection  36 , leaving space between the pieces  32  and  40 , to allow for spherical, rotational movement about the center point  38  between the inferior piece  32  and the superior piece  40 . Thus, the anterior portion  30  of the apparatus  15  provides load-bearing support to the anterior portion of the vertebral joint while still allowing some movement to help preserve motion between the vertebrae  12  and  14 . In some embodiments the anterior portion  30  can be a Prestige cervical disc available from Medtronic, Inc. In some embodiments, the anterior portion  30  can be a Maverick lumbar disc also available from Medtronic, Inc. 
     The apparatus also includes two posterior portions  46  and  47  ( FIGS. 2 and 3 ). The posterior portion  46  includes an inferior part  48 . The inferior part  48  includes a lower section  50  and an upper section  52 . A surface  54  extends at least partially between the lower section  50  and the upper section  52  on the posterior side of the inferior part  48 . As shown, the surface  54  is curved to substantially coincide with a virtual sphere centered about center point  38  with a radius  55 . Thus, the surface  54  is curved in both the vertical plane (best seen in  FIG. 1 ) and the horizontal plane (best seen in  FIG. 2A ). 
     The inferior part  48  is secured to pedicle  24  of vertebra  14  by a mechanism  56 . The mechanism  56  receives a portion of the lower section  50  of the inferior part  48 . The mechanism  56  includes a screw portion adapted to engage and extend into the pedicle  24 . In some embodiments, the inferior part  48  includes an opening adapted to receive a pedicle screw directly. Further, in some embodiments the inferior part  48  may be secured to the pedicle  24  using appropriate means other than a pedicle screw. 
     The posterior portion  46  also includes a superior part  57 . The superior part  57  includes an upper section  58  and a lower section  60 . A surface  62  extends at least partially between the upper section  58  and the lower section  60  on the anterior side of the superior part  57 . The surface  62  is curved to movably mate with the surface  54  of the inferior part  48 . Similar to the surface  54 , the surface  62  is curved to substantially coincide with the virtual sphere centered about center point  38  with radius  55 . Thus, the surface  62  is also curved in both the vertical plane (best seen in  FIG. 1 ) and the horizontal plane (best seen in  FIG. 2A ). 
     The superior part  57  is secured to pedicle  18  of vertebra  12  by a mechanism  64 . The mechanism  64  receives a portion of the upper section  58  of the superior part  57 . The mechanism  64  includes a screw portion adapted to engage and extend into the pedicle  24 . In some embodiments, the superior part  57  includes an opening adapted to receive a pedicle screw directly. Further, in some embodiments the superior part  57  may be secured to the pedicle  18  using appropriate means other than a pedicle screw. 
     The surfaces  54  and  62  movably mate to allow for spherical, rotational movement about the center point  38  between the inferior part  48  and the superior part  57 . In this manner, the posterior portion  46  of the apparatus  15  provides load-bearing support to the posterior portion of the vertebral joint while still allowing some movement to help preserve motion between the vertebrae  12  and  14 . 
     Thus, the anterior portion  30  and the posterior portion  46  allow for movement about the same center of rotation, namely center point  38 . Therefore, the anterior portion  30  will not adversely load or affect the posterior portion  46  during movement between the inferior and superior pieces  32  and  40 . Similarly, the posterior portion  46  will not adversely load or affect the anterior portion  30  during movement between the inferior part  48  and superior part  57 . Thus, the anterior portion  30  and posterior portion  46  of the apparatus  15  function together to provide load-bearing support and preserve vertebral motion between the vertebrae  12  and  14  without adversely affecting each other. 
     Referring more specifically to  FIGS. 2 and 3 , in the current embodiment the posterior portion  47  is substantially similar to posterior portion  46  described above and, therefore, various aspects will not be described in great detail. The posterior portion  47  includes an inferior part  68  with a surface  69  and a superior part  70  with a surface  71 . The inferior part  68  is secured to the pedicle  26  of vertebra  14  by mechanism  72 . The superior part  70  is secured to the pedicle  19  of the vertebra  12  by mechanism  74 . The surfaces  69  and  71  are curved to substantially coincide with a virtual sphere centered about center point  38  with a radius  76 . The radius  76  has substantially the same length as radius  55  for posterior portion  46 . Thus, the surfaces  69  and  71  are defined by the same virtual sphere that defines surfaces  54  and  62 . However, in other embodiments the radii of the posterior portions  46  and  47  may have different lengths, such that the surfaces of posterior portion  47  are not defined by the same virtual sphere as the surfaces of posterior portion  46 , but the posterior devices maintain the same center point. For example, as shown in  FIG. 2B , a radius  55 ′ of posterior portion  46  can be greater than a radius  76 ′ of posterior portion  47 . In other embodiments, the radius  55 ′ can be less than the radius  76 ′. In some embodiments, the difference in length is a result of non-symmetrical physical attributes of the patient. In other embodiments, the difference in length is a result of the center of rotation being off-center with respect to the vertebrae (as shown in  FIG. 2B ). 
     The surfaces  69  and  71  movably mate to allow for spherical, rotational movement about the center point  38  between the inferior part  68  and the superior part  70 . In this manner, the posterior portion  47  of the apparatus  15  provides load-bearing support to the posterior portion of the vertebral joint while still allowing some movement to help preserve motion between the vertebrae  12  and  14 . Thus, the anterior portion  30 , the posterior portion  46 , and the posterior portion  47  allow for movement about the same center of rotation, namely center point  38 . Thus, the anterior portion  30  will not adversely load or adversely affect the posterior portion  47  during movement, and vice-versa. Similarly, the posterior portion  46  will not adversely load or affect the posterior portion  47  during movement, and vice-versa. Thus, the anterior portion  30  and the posterior portions  46  and  47  of the apparatus  15  function together to provide load-bearing support and preserve vertebral motion between the vertebrae  12  and  14  without adversely affecting one another. In other embodiments, the apparatus  15  may be a unilateral rather than a bilateral. That is, in some embodiments the apparatus  15  includes only a single posterior portion. For example, as shown in  FIG. 2C , an apparatus  15 ′ includes the anterior portion  30  and the posterior portion  46 . Though currently illustrated as being positioned on the left side of the vertebrae, in other embodiments the posterior portion  46  is positioned on the right side. 
       FIG. 4  is a diagrammatic side view similar to  FIG. 1 , but showing an apparatus  78  that is an alternative embodiment of the apparatus  15  described above. Apparatus  78  is similar to apparatus  15  in many respects and, therefore, some aspects will not be described in great detail.  FIG. 5  is a sectional view of  FIG. 4  taken along section line  5 - 5 . 
     The apparatus  78  is a prosthesis system adapted to preserve at least some motion between the vertebrae  12  and  14 . The apparatus  78  includes an anterior portion  80 . The anterior portion  80  includes an inferior piece  82 . The inferior piece  82  includes an inferior surface  84  that is adapted to engage with the superior endplate  28  of vertebra  14 . The inferior surface  84  may include features adapted to enhance engagement with the endplate  28 . The inferior piece  82  also includes a projection  86 . The projection  86  extends out of a superior surface  87  of the inferior piece  82 . The projection  86  is curved to substantially coincide with a virtual sphere centered about a center point  88  with a radius  89 . In this embodiment, the projection  86  is centered about a central portion of the inferior piece  82  in both the anterior/posterior and lateral directions. 
     The anterior portion  80  also includes a superior piece  90 . The superior piece  90  includes a superior surface  92  that is adapted to engage with the inferior endplate  20  of vertebra  12 . Similar to inferior surface  84  of the inferior piece  82 , the superior surface  92  may include features adapted to enhance engagement with the endplate  28 . The superior piece  90  also includes a recess  94 . The recess  94  extends into an inferior surface  95  of the superior piece  90 . The recess  94  is adapted to movably mate with the projection  86  of the inferior piece  82 . Thus, the recess  94  is substantially spherical about the center point  88  to match the projection  86 . However, when mated together the recess  94  does not fully envelope the projection  86 , leaving space between the pieces  82  and  90 , to allow for spherical, rotational movement about the center point  88 . In this manner, the anterior portion  80  of the apparatus  78  provides load-bearing support to the anterior portion of the vertebral joint while still allowing some movement to help preserve motion between the vertebrae  12  and  14 . 
     The apparatus  78  also includes two posterior portions  96  and  97 . The posterior portion  96  is seen in  FIGS. 4 and 5 . The posterior portion  97  is only presented in a partial, sectional view in  FIG. 5 . In the current embodiment the posterior portion  97  is substantially similar to posterior portion  96  and, therefore, will not be described in detail. The posterior portion  96  includes an inferior part  98 . The inferior part  98  includes a lower section  100  and an upper section  102 . A surface  104  extends at least partially between the lower section  100  and the upper section  102  on the posterior side of the inferior part  98 . As shown, the surface  104  is curved to substantially coincide with a virtual sphere centered about center point  88  with a radius  105 . Thus, the surface  104  is curved in both the vertical plane (best seen in  FIG. 4 ) and the horizontal plane (best seen in  FIG. 5 ). The inferior part  98  is secured to pedicle  24  of vertebra  14  by a pedicle screw  106 . The pedicle screw  106  sized to pass through an opening in the lower section  100  of the inferior part  98  and into the pedicle  24 . In other embodiments the inferior part  98  is secured to the pedicle  24  using means other than a pedicle screw. 
     The posterior portion  96  also includes a superior part  107 . The superior part  107  includes an upper section  108  and a lower section  110 . A surface  112  extends at least partially between the upper section  108  and the lower section  110  on the anterior side of the superior part  107 . The surface  112  is curved to movably mate with the surface  104  of the inferior part  98 . Similar to the surface  104 , the surface  112  is curved to substantially coincide with the virtual sphere centered about center point  88  with radius  105 . Thus, the surface  112  is also curved in both the vertical plane (best seen in  FIG. 4 ) and the horizontal plane (best seen in  FIG. 5 ). The surface  112  is bounded on the upper end by a stop portion  114 . The stop portion  114  extends anteriorly from surface  112  and, as described in more detail below, serves to limit the range of motion allowed by the posterior portion  96  of the apparatus  78 . The superior part  107  is secured to pedicle  18  of vertebra  12  by a pedicle screw  115 . The pedicle screw  115  is sized to pass through an opening in the upper section  108  of the superior part  107  and into the pedicle  18 . In other embodiments the superior part  107  is secured to the pedicle  18  using means other than a pedicle screw. 
     The surfaces  104  and  112  movably mate to allow for spherical, rotational movement about the center point  88  between the inferior part  98  and the superior part  107 . In this manner, the posterior portion  96  of the apparatus  78  provides load-bearing support to the posterior portion of the vertebral joint while still allowing some movement to help preserve motion between the vertebrae  12  and  14 . However, the degree of spherical, rotational movement allowed in extension is limited by stop portion  114 . As the motion segment moves further into extension, the upper section  102  of the lower part  98  moves closer and closer to the stop portion  114  until they make contact. The contact between the upper section  102  and the stop portion  114  serves as a hard stop to limit the degree of extension allowed by the posterior portion  96 . The amount of extension allowed can be tailored for the condition of the specific patient. 
     Further, as best seen in  FIG. 5 , the lower part  98  includes a projection  118  adapted to slot in a recess  120  in the upper part  107 . The recess  120  is bounded laterally by stop portions  122  and  124 . There is space between the projection  118  and stop portions  122  and  124  to allow left and right rotational movement of the spine. However, the degree of rotational movement is limited by the stop portions. As the motion segment rotates, the projection  118  of the lower part  98  moves closer and closer to the stop portions  122  (in left rotation) and  124  (in right rotation) until they make contact. The contact between the upper section  102  and the stop portions  122  and  124  serves as a hard stop to limit the degree of rotation allowed by the posterior portion  96 . The amount of rotation allowed can be tailored for the condition of the specific patient. For example, in addition to simple limits on the amount of rotation allowed, the amount of allowed rotation left may be different than the amount of allowed rotation right. That is, the allowed degree of left rotation may be greater than the allowed degree of right rotation and vice-versa. 
     The posterior portion  97  contains similar stop portions to those described with respect to posterior portion  96 . In some embodiments, the stop portions of the posterior portion  97  are substantially identical to those of the posterior portion  96 . However, in some embodiments the stop portions are different. In that regard, the stop portions of the posterior portion  97  can be tailored for the condition of the specific patient. Though not shown in the current embodiment, the posterior portions  96  and  97  may include further stop portions to limit flexion and both left and right lateral bending. Further, as described below with respect to  FIGS. 6 and 7 , the stop portions can increase resistance to movement in a direction rather than being a hard stop to movement in that direction. 
     Thus, the anterior portion  80  and the posterior portions  96  and  97  allow for movement about the same center of rotation, namely center point  88 . Therefore, the anterior portion  80  will not adversely load or affect the posterior portions  96  and  97  during movement, and vice-versa. Thus, the anterior portion  80  and posterior portions  46  and  47  of the apparatus  78  function together to provide load-bearing support and preserve vertebral motion between the vertebrae  12  and  14  without adversely affecting one another. 
       FIG. 6  is a diagrammatic side view similar to  FIG. 4 , but showing an apparatus  125  that is an alternative embodiment of the apparatus  78  described above. Apparatus  125  is similar to apparatus  78  in many respects and, therefore, many aspects will not be described in great detail.  FIG. 7  is a sectional view of  FIG. 6  taken along section line  7 - 7 . 
     The apparatus  125  also includes two posterior portions  126  and  127 . The posterior portion  127  is only presented in a partial, sectional view in  FIG. 7 . In the current embodiment, the posterior portion  127  is substantially similar to posterior portion  126  and, therefore, will not be described in detail. The posterior portion  126  includes an inferior part  128 . The inferior part  128  includes a surface  130  that extends along the posterior side of the inferior part. As shown, the surface  130  is curved to substantially coincide with a virtual sphere centered about center point  88  with a radius  131 . A stop portion  132  extends posteriorly from the inferior part  128  adjacent the surface  130 . The stop portion  132  is curved to limit the range of motion allowed by the posterior portion  126 . 
     The posterior portion  126  also includes a superior part  134 . The superior part  134  includes a surface  136  that extends along the anterior side of the superior part. The surface  136  is curved to movably mate with the surface  130  of the inferior part  128 . Similar to the surface  130 , the surface  136  is curved to substantially coincide with the virtual sphere centered about center point  88  with radius  131 . A stop portion  138  extends anteriorly from the superior part  134  adjacent surface  136 . The stop portion  138  is curved and serves with stop portion  132  to limit the range of motion allowed by the posterior portion  126 . 
     The surfaces  130  and  136  movably mate to allow for spherical, rotational movement about the center point  88 . However, the degree of spherical, rotational movement allowed in extension is limited by stop portions  132  and  138 . As the motion segment moves further into extension, the upper section of the inferior part  128  moves closer and closer to the stop portion  138  until they make contact. At the same time, the lower section of superior part  134  moves closer and closer to the stop portion  132  until they make contact. In the current embodiment, the stop portions  132  and  138  are positioned such that they will be contacted simultaneously by the superior part  134  and the inferior part  128 , respectively. In alternative embodiments, the stop portions  132  and  138  may be staggered such that one is contacted prior to the other. 
     Unlike the hard stops  114 ,  122 , and  124  described above, the stop portions  132  and  138  are gradual. That is, the curved surfaces of the stop portions  132  and  138  provide a gradual increase in resistance to movement in a direction towards further extension. The further into extension the patient moves, the more resistance the stop portions  132  and  138  create. The degree of slope of the curved surfaces of the stop portions  132  and  138  determines the amount of resistance at different degrees of extension. Again the resistance and amount of extension allowed can be tailored for the condition of the specific patient. 
     Further, as best seen in  FIG. 7 , the superior part  134  includes stop portions  140  and  142 . Similar to the stop portions  132  and  138  described above, the stop portions  140  and  142  are gradual stops. The curved surfaces of the stop portions  140  and  142  provide a gradual increase in resistance to movement in a direction towards further rotation. As the motion segment rotates left or right, the inferior part  128  moves closer and closer to the stop portions  140  (in left rotation) and  142  (in right rotation) until they make contact. Once in contact, the stop portions  122  and  124  serve to increase resistance to further rotation. The further into a left or right rotational position the patient moves, the more resistance the stop portions  132  and  138  create. Again, the degree of slope of the curved surfaces of the stop portions  132  and  138  determines the amount of resistance at different degrees of extension. The resistance and amount of extension allowed can be tailored for the condition of the specific patient. 
     The posterior portion  127  contains similar stop portions to those described with respect to posterior portion  126 . In some embodiments, the stop portions of the posterior portion  127  are substantially identical to those of the posterior portion  126 . In other embodiments the stop portions are different. Though not shown in the current embodiment, the posterior portions  126  and  127  may include further stop portions to limit flexion and both left and right lateral bending. Further, the posterior portion  126  may include hard stops in addition to the gradual stops to provide an ultimate limit on the amount of allowed extension, rotation, flexion, and lateral bending. 
     The various components of the anterior and posterior portions of the embodiments described above may be formed of any suitable biocompatible material including metals such as cobalt-chromium alloys, titanium alloys, nickel titanium alloys, or stainless steel alloys. Ceramic materials such as aluminum oxide or alumina, zirconium oxide or zirconia, compact of particulate diamond, or pyrolytic carbon may also be suitable. Polymer materials may also be used, including any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); or cross-linked UHMWPE. Further, the components may each be formed of different materials, permitting metal on metal, metal on ceramic, metal on polymer, ceramic on ceramic, ceramic on polymer, or polymer on polymer constructions. Further, the articulating surfaces described above may be treated to limit friction and resulting wear debris caused by rotational movement. 
     In addition to spherical, rotational movement about a center point, the anterior and posterior portions of the apparatus may be configured for additional movement patterns without adversely affecting one another. For example and without limitation, in some embodiments the anterior and posterior portions may be adapted for corresponding helical movement, translational movement, circular movement, and combinations thereof. Further, the precise motion profile for the apparatus may be determined by monitoring the 3-D motion of a motion segment. Once the 3-D motion has been measured, a corresponding motion profile for the apparatus can be determined. The anterior and posterior portions of the apparatus can then be designed to match the specific motion profile of the motion segment. In this manner, the apparatus can be tailored to the precise motion profile of the motion segment without having the anterior and posterior portions work against each other. 
     The anterior and posterior portions can also include features to facilitate alignment of the portions about the common center of rotation. For example, in one embodiment the anterior portion includes fiducial markers. In one aspect, the anterior portion is inserted into the disc space and the fiducial markers are visualized using fluoroscopy, CT scan, ultrasound, or other imaging techniques to determine the center of rotation for the anterior portion of the device. The fiducial markers may include any type of marker capable of indicating the center of rotation including, but not limited to, radiopaque markers (e.g., tantalum beads), RFID tags, and other known markers. The position of the anterior portion can be adjusted to align the center of rotation with a predetermined desired point of rotation. In some embodiments, the center of rotation of the anterior portion may be determined without the use of fiducial markers. For example, the center of rotation may be determined based upon the shape of the anterior portion itself. 
     Once the anterior portion has been positioned and its center of rotation determined, the posterior portion can be inserted and positioned to align its center of rotation with that of the anterior portion. In one embodiment, the known shape of the posterior portion is used as a guide for aligning the centers of rotation. In another embodiment, the posterior portion includes fiducial markers that can be visualized to determine its center of rotation and align it with the center of rotation of the anterior portion. In another embodiment, the posterior portion is inserted prior to the anterior portion and the anterior portion is positioned so as to align the centers of rotation of the two portions. 
     Other modifications of the present disclosure would be apparent to one skilled in the art. Accordingly, all such modifications and alternatives are intended to be included within the scope of the invention as defined in the following claims. Those skilled in the art should also realize that such modifications and equivalent constructions or methods do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. It is understood that all spatial references, such as “horizontal,” “vertical,” “top,” “upper,” “lower,” “bottom,” “left,” and “right,” are for illustrative purposes only and can be varied within the scope of the disclosure. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.