Abstract:
A trial implant assembly is provided that includes a threaded shaft having exterior threading adjacent a distal end and a trial implant that includes a trial base having an interiorly threaded through hole couplable to the distal end of the shaft and a trial head having a superior endplate and an inferior endplate and a support connecting the superior endplate to the inferior endplate. A variety of visualization windows are provided through the trial implant as well as a variety of mechanically adjustable stops to prevent over insertion of the trial head within the intervertebral disc space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/169,444 filed Apr. 15, 2009, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein. 
    
    
     BACKGROUND 
     When removing a disc from an intervertebral space disposed between adjacent vertebrae, the conventional procedure is to fuse the adjacent vertebrae together. More recently, there have been developments in the field of disc replacement, namely disc arthroplasty, which involves the insertion of an artificial intervertebral disc implant into the intervertebral space. The artificial disc then allows limited universal movement of the adjacent vertebrae with respect to each other. 
     One such intervertebral implant includes an upper part that can communicate with an adjacent vertebrae, a lower part that can communicate with an adjacent vertebrae, and an insert located between these two parts. An example of this type of implant is disclosed in U.S. Pat. No. 5,314,477 (Marnay), the disclosure of which is hereby incorporated as if set forth in its entirety herein. 
     Instruments have been developed for preparing an intervertebral space for receiving an artificial disc implant. These instruments include a set of different sizes of trial implants, different ones of which are inserted into a cleaned out intervertebral space until the correct size trial implant has been determined, thereby determining the size of the actual disc implant to be permanently inserted. 
     In disc arthroplasty procedures, proper implant location assists in determining the kind of motion obtained from the device. Because proper implant positioning assists in patient recovery and spinal motion, fluoroscopy is used to visualize the position of the prosthesis and implant trial throughout the procedure. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of the disclosure, a trial implant assembly is provided that can increase visualization of the implant and/or trial implant position while minimizing fluoroscopy, thereby reducing the amount of radiation exposure to operating room personnel and the patient. 
     In one embodiment, a trial implant assembly is provided that includes a trial implant configured to be inserted into an intervertebral space that is defined by a superior vertebral body and an inferior vertebral body. The trial implant includes a trial base and a trial head connected to the trial base. The trial base includes an engagement member configured to couple the trial implant to a shaft. The trial head is distally spaced from the trial base. The trial head defines a superior endplate and an inferior endplate configured to face the superior vertebral body and the inferior vertebral body, respectively. The trial head further defines at least one visualization window extending distally there-through between the superior and inferior endplates. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of example embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purposes of illustrating the trial implant assembly of the present application, there is shown in the drawings example embodiments. It should be understood, however, that the application is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
         FIG. 1  is a perspective view of a pair of vertebral bodies separated by an intervertebral space; 
         FIG. 2A  is a perspective view of a trial implant assembly including a shaft and a trial implant in accordance with one embodiment; 
         FIG. 2B  is a bottom plan view of the trial implant assembly illustrated in  FIG. 2A ; 
         FIG. 2C  is a side elevation view of the trial implant assembly illustrated in  FIG. 2A ; 
         FIG. 2D  is a sectional side elevation view of a portion of the trial implant assembly illustrated in  FIG. 2A ; 
         FIG. 3A  is a perspective view of a distal portion of the trial implant assembly illustrated in  FIGS. 2A-C ; 
         FIG. 3B  is a distal end elevation view of the distal portion of the trial implant assembly illustrated in  FIG. 3A ; 
         FIG. 4A  is a side elevation view of the trial implant assembly similar to  FIG. 2C , but showing the trial implant in a translated position relative to the shaft; 
         FIG. 4B  is a side elevation view of the distal end of the trial implant assembly illustrated in  FIG. 2C  inserted into an intervertebral disc space at a first insertion depth; 
         FIG. 4C  is a side elevation view of the distal end of the trial implant assembly illustrated in  FIG. 4A  inserted into an intervertebral disc space at a second insertion depth that is different than the first insertion depth; 
         FIG. 5  is a perspective view of the trial implant assembly of  FIG. 2  used in conjunction with a retainer distracter instrument and showing an anterior side of adjacent vertebrae; 
         FIG. 6  is perspective view of the trial implant assembly illustrated in  FIG. 5 ; 
         FIG. 7  is a proximal end elevation view of the trial implant assembly illustrated in  FIG. 5 ; 
         FIG. 8A  is a perspective view of a distal portion of a trial implant assembly constructed in accordance with an alternative embodiment; 
         FIG. 8B  is a top plan view of the trial implant assembly illustrated in  FIG. 8A ; 
         FIG. 8C  is a side elevation view of the trial implant assembly illustrated in  FIG. 8A  showing an angularly offset shaft; 
         FIG. 8D  is a distal end elevation view of the distal portion of the trial implant assembly illustrated in  FIG. 8A ; 
         FIG. 8E  is a sectional side elevation view of the angularly offset shaft as illustrated in  FIG. 8B  constructed in accordance with one embodiment; 
         FIG. 8F  is a sectional side elevation view of the angularly offset shaft as illustrated in  FIG. 8B  constructed in accordance with another embodiment; 
         FIG. 9A  is a perspective view of a trial implant assembly including a shaft and a trial implant in accordance with an alternative embodiment; 
         FIG. 9B  is a top plan view of the trial implant assembly illustrated in  FIG. 9A ; 
         FIG. 9C  is a side elevation view of the trial implant assembly illustrated in  FIG. 9A ; 
         FIG. 10A  is a perspective view of a distal portion of the trial implant assembly illustrated in  FIGS. 9A-C ; 
         FIG. 10B  is a distal end elevation view of the distal portion of the trial implant assembly illustrated in  FIG. 10A ; 
         FIG. 10C  is a proximal end elevation view of the distal portion of the trial implant assembly illustrated in  FIG. 10B ; 
         FIG. 11A  is a perspective view of a trial implant assembly in accordance with an alternative embodiment; 
         FIG. 11B  is a side elevation view of the trial implant assembly illustrated in  FIG. 11A ; 
         FIG. 11C  is a sectional side elevation view of the distal end of a shaft and a proximal end of a trial implant of the trial implant assembly illustrated in  FIG. 11A ; 
         FIG. 11D  is a sectional distal end elevation view of the trial implant assembly illustrated in  FIG. 11A ; 
         FIG. 12A  is a perspective view of a trial implant assembly including a shaft and a trial implant in accordance with an alternative embodiment; 
         FIG. 12B  is a top plan view of the trial implant assembly illustrated in  FIG. 12A ; 
         FIG. 12C  is a side elevation view of the trial implant assembly illustrated in  FIG. 12A ; 
         FIG. 13A  is a perspective view of a distal portion of the trial implant assembly illustrated in  FIGS. 12A-C ; 
         FIG. 13B  is a distal end elevation view of the distal portion of the trial implant assembly illustrated in  FIGS. 12A-C ; 
         FIG. 13C  is a sectional side elevation view of the distal end of a shaft and a proximal end of a trial implant of the trial implant assembly illustrated in  FIGS. 13A-B ; 
         FIG. 13D  is a distal end elevation view of the distal portion of the trial implant assembly similar to  FIG. 13B , but also showing a pair of superior tracks; 
         FIG. 14A  is a perspective view of a trial implant assembly constructed in accordance with an alternative embodiment; 
         FIG. 14B  is a side elevation view of the trial implant assembly illustrated in  FIG. 14A ; 
         FIG. 14C  is a distal end elevation view of the trial implant assembly illustrated in  FIG. 14A ; 
         FIG. 14D  is a bottom plan view of a trial implant of the trial implant assembly illustrated in  FIG. 14A ; 
         FIG. 14E  is a top plan view of the trial implant illustrated in  FIG. 14D ; 
         FIG. 15A  is a perspective view of a distal portion of a trial implant assembly constructed in accordance with an alternative embodiment; 
         FIG. 15B  is a top plan view of the trial implant assembly illustrated in  FIG. 15A ; 
         FIG. 15C  is a side elevation view of the trial implant assembly illustrated in  FIG. 15A ; 
         FIG. 15D  is a distal end elevation view of the trial implant assembly illustrated in  FIG. 15A ; 
         FIG. 15E  is a sectional elevation view of the trial implant assembly illustrated in  FIG. 15A ; 
         FIG. 16A  is a perspective view of a distal portion of a trial implant assembly constructed in accordance with an alternative embodiment; 
         FIG. 16B  is a bottom plan view of the trial implant assembly illustrated in  FIG. 16A ; 
         FIG. 16C  is a side elevation view of the trial implant assembly illustrated in  FIG. 16A ; 
         FIG. 16D  is a sectional elevation view of a portion of the trial implant assembly illustrated in  FIG. 16A ; 
         FIG. 16E  is a proximal end elevation view of the trial implant assembly illustrated in  FIG. 16C ; 
         FIG. 17A  is a perspective view of a trial implant assembly constructed in accordance with an alternative embodiment; 
         FIG. 17B  is a sectional proximal end elevation view of the trial implant assembly illustrated in  FIG. 17A ; 
         FIG. 18A  is a perspective view of a trial implant assembly constructed in accordance with an alternative embodiment; and 
         FIG. 18B  is a proximal end elevation view of the trial implant assembly illustrated in  FIG. 18A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a superior vertebral body  12   a  defines a superior vertebral surface  13   a  of an intervertebral space  14 , and an adjacent inferior vertebral body  12   b  defines an inferior vertebral surface  13   b  of the intervertebral space  14 . Thus, the intervertebral space  14  is disposed between the vertebral bodies  12   a - b . The vertebral bodies  12   a - b  can be anatomically adjacent vertebral bodies, or can remain after a discectomy has been performed that removed a vertebral body from a location between the vertebral bodies  12   a - b . As illustrated, the intervertebral space  14  is illustrated after a discectomy, whereby the disc material has been removed or at least partially removed to prepare the intervertebral space  14  to receive a disc implant that can achieve height restoration. Prior to inserting the permanent disc implant in the intervertebral space, one or more trial implants of various sizes, such as the trial implant  22  of a trial implant assembly  20  illustrated in  FIG. 2 , are inserted into the intervertebral space  14  until the correctly sized trial implant has been determined, thereby determining the size of the actual disc implant to be permanently inserted. The intervertebral space  14  can be disposed anywhere along the spine as desired. 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower” and “upper” designate directions in the drawings to which reference is made. The words “inner” or “distal” and “outer” or “proximal” refer to directions toward and away from, respectively, the geometric center of the implant and related parts thereof. The words, “anterior”, “posterior”, “superior,” “inferior,” “medial,” “lateral,” and related words and/or phrases are used to designate various positions and orientations in the human body to which reference is made and are not meant to be limiting. The terminology includes the above-listed words, derivatives thereof and words of similar import. 
     The trial implant assembly  20  is described herein as extending horizontally along a longitudinal direction “L” and lateral direction “A”, and vertically along a transverse direction “T”. Unless otherwise specified herein, the terms “lateral,” “longitudinal,” and “transverse” are used to describe the orthogonal directional components of various components. It should be appreciated that while the longitudinal and lateral directions are illustrated as extending along a horizontal plane, and that the transverse direction is illustrated as extending along a vertical plane, the planes that encompass the various directions may differ during use. For instance, when the trial implant  20  is implanted into an intervertebral space, such as the intervertebral space  14 , the transverse direction T extends generally along the superior-inferior (or caudal-cranial) direction, while the plane defined by the longitudinal direction L and lateral direction A lie generally in the anatomical plane defined by the anterior-posterior direction, and the medial-lateral direction. Accordingly, the directional terms “vertical” and “horizontal” are used to describe the implant assembly  20  and its components as illustrated merely for the purposes of clarity and illustration. 
     Referring now also to  FIGS. 2A-2D , a trial implant assembly  20  is configured to be positioned within an at least a partially cleared out disc space, such as the disc space  14  disposed between the superior vertebral body  12   a  and the inferior vertebral body  12   b . The trial implant assembly  20  includes a trial implant  22  coupled to a shaft  24 . The shaft  24  can be formed from any desired material such as stainless steel, while the trial implant  22  can be formed from any desired material such as a titanium alloy. It should be appreciated that both the shaft  24  and the trial implant  22  can be formed from a range of biocompatible metals or polymers, such as cobalt chromium molybdenum (CoCrMo), titanium and titanium alloys, stainless steel, ceramics, or polymers such as polyetheretherketone (PEEK), polyetherketoneketone (PEKK), and bioresorbable materials. 
     The shaft  24  includes a shaft body  26  that defines a proximal end  26   a , and a distal end  26   b  that is separated from the proximal end  26   a  along a longitudinally extending central shaft axis S. The shaft  24  includes a handle  28  or gripping portion at the proximal end  26   a  of the shaft body  26 , and a trial implant engagement member  30  at the distal end  26   b  of the shaft body  26 . The handle  28  can be knurled or otherwise textured to facilitate an ergonomically friendly gripping surface. The shaft body  26  defines, and thus carries, a vertebral abutment surface  27  at the distal end  26   b . The engagement member  30  is configured to be coupled to a complementary engagement member  32  of the trial implant  22  so as to connect the shaft  24  to the trial implant  22 . 
     Thus, the proximal end  26   a  of the shaft body  26  defines a proximal end  20   a  of the implant assembly  20 , and the trial implant  22  defines an opposed distal end  20   b  of the implant assembly  20 . Accordingly, a distal spatial relationship is used herein to refer to a longitudinal direction from the proximal end  20   a  toward the distal end  20   b , and a proximal spatial relationship is used herein to refer to a longitudinal direction from the distal end  20   b  toward the proximal end  20   a.    
     The trial implant  22  generally includes a trial base  34  coupled to the shaft  24 , a trial head  36  that is disposed distally from the trial base  34 , and a pair of laterally spaced ribs  38  and  40  that are fixedly connected between the trial head  36  and the trial base  34 . Thus, the trial base  34  is connected indirectly to the trial head  36  via the ribs  38  and  40 , though it should be appreciated that the trial base  34  could alternatively be directly connected to the trial head  36 . The trial base  34  includes a trial base body  35  having transversely opposed upper and lower surfaces  35   a  and  35   b , and laterally opposed outer surfaces  35   c  and  35   d . As illustrated in  FIG. 2D , the trial base  34  defines an engagement member  32  that is configured to connect to the engagement member  30  of the shaft  24 . 
     In particular, the engagement member  30  of the shaft  24  is illustrated as including external threads  42  disposed in a threaded region  44  proximate to the distal end  26   b  of the shaft  26 . The trial implant  22  includes an aperture  46  that extends longitudinally through the trial base body  35 . The aperture  46  is sized to receive the threaded region  44  of the shaft  24 . The engagement member  32  of the trial base  34  includes internal threads  48  disposed about the periphery of the aperture  46  that are configured to mate with the external threads  42  of the shaft  24  so as to couple the shaft  24  to the trial implant  22 . 
     Referring now to  FIGS. 2A-3B , the trial head  36  includes an upper or superior endplate  50  that defines an upper or superior, or outer transverse, engagement surface  51  configured to contact the inferior endplate  13   a  of the superior vertebral body  12   a , and an inferior endplate  52  that defines a lower or inferior, or outer transverse, engagement surface  53  configured to contact the superior endplate  13   b  of the inferior vertebral body  12   b . The superior endplate  50  further defines a lower or inferior, or inner transverse, surface  55 , and the inferior endplate  52  defines an upper or superior, or inner transverse, surface  57 . The surfaces  55  and  57  are spaced vertically along the transverse direction T by a gap G as illustrated, though it should be appreciated that the endplates  50  and  52  could alternatively be connected at their inner transverse ends. Thus, reference to superior endplates and inferior endplates is not intended to be limited to a pair of spaced apart endplates unless otherwise indicated. 
     As described above, the trial implant assembly  20  includes a pair of ribs  38  and  40  that are connected between the trial base  34  and the endplates  50  and  52 , such that the ribs  38  and  40  define side walls of the trial head  36 . Some or all of endplates  50  and  52 , the trial base  34 , and the ribs  38  and  40  can be integrally connected or discretely connected as desired. The ribs  38  and  40  define respective proximal ends  38   a  and  40   a  that are connected to the opposed lateral sides  35   c  and  35   d  of the trial base  34 , and respective distal ends  38   b  and  40   b  that are connected to the endplates  50  and  52 . 
     The distal ends  38   b  and  40   b  are connected between the inner transverse surfaces  55  and  57 , and are laterally spaced apart so as to at least partially define an aperture or visualization window  60  that extends longitudinally through the trial head  36 . The visualization window  60  is defined between the inner transverse surfaces  55  and  57  of the superior and inferior endplates  50  and  52 , respectively, and the ribs  38  and  40 . Thus, the visualization window  60  is enclosed, and extends transversely between the inner transverse surfaces  55  and  57 , and laterally between the ribs  38  and  40 . The trial head  36  further defines first and second laterally opposed slots  39  and  41  that are disposed on opposite sides of the window  60 , and are separated from the window by the first and second ribs  38  and  40 , respectively. The slots  39  and  41  are thus closed on their laterally inner ends, but open at their laterally outer ends. Accordingly, the slots  39  and  41  can be referred to as being open. The distal ends of the first and second ribs  38  and  40  terminate proximal to the distal end of the trial head  36 , or endplates  50  and  52 , so as to provide increased visualization and allow improved access to posterior structures in the disc space using a conventional nerve hook or probe. 
     In this regard, it should be appreciated that the trial implant assembly  22  is devoid of structure that obstructs a straight visualization axis V from extending from a first location disposed proximal to the handle  28  to a second location that passes through the visualization window  60 . Otherwise stated, the trial implant defines a visualization window such as window  60 , at least a portion of which up to all of which is visually unobstructed. The visualization axis V can extend parallel to the central shaft axis S as illustrated, or can extend at an angle with respect to the central shaft axis S. 
     The distal ends  38   b  and  40   b  vertically offset with respect to the proximal ends  38   a  and  40   a  of the ribs  38  and  40 , such that the gap G between the endplates  50  and  52  is at least partially vertically offset with respect to the aperture  46  and the shaft  24 , as well as the upper surface  35   a  of the trial base  34 . In accordance with the illustrated embodiment, the distal ends  38   b  and  40   b  are disposed above the proximal ends  38   a  and  40   a  of the ribs  38  and  40 , such that the gap G between the endplates  50  and  52  is disposed at least partially above the aperture and shaft  24 , as well as the upper surface  35   a  of the trial base  34 . In this regard, it should be appreciated that the inner transverse surface  57  of the inferior endplate  52  can be disposed above or below the upper surface  35   a  of the trial base  34 . Accordingly, visualization is possible through the visualization window  60  along a distal direction from a location proximal of the trial head  36 , and further from a location proximal of the handle  28 . 
     Referring now also to  FIG. 4A , rotation of the shaft  24  relative to the trial implant  22  causes the threads  42  of the shaft  24  and the threads  48  of the trial base  34  to ride along each other, thereby causing the trial implant  22  to translate longitudinally relative to the shaft  24 . For instance, rotation of the shaft  24  in a first direction (e.g., counterclockwise) relative to the trial implant  22  causes trial implant  22  to translate distally in the longitudinal direction relative to the shaft  24  as indicated by Arrow A 1 , while rotation of the shaft  24  in a second opposite direction (e.g., clockwise) relative to the trial implant  22  causes the trial implant  22  to move proximally in the longitudinal direction relative to the shaft  24  as indicated by Arrow A 2 .  FIG. 2C  illustrates the trial head  36  in a retracted position relative to the shaft  24 , while  FIG. 4A  illustrates the trial head  36  in an extended position relative to the shaft  24 . 
     Furthermore, referring also to  FIGS. 4B-C , at least a portion of the distal end  26   b  of the shaft body  26  is vertically offset from the trial head. In accordance with the illustrated embodiment, at least a portion of the distal end  26   b  of the shaft body  26  is disposed below the outer transverse surface  53  of the inferior endplate  52 . Accordingly, the vertebral abutment surface  27  of the shaft  24  is configured to abut one of the vertebrae, such as the inferior vertebra  12   b , when the trial head  36  is inserted into the intervertebral space  14 . Accordingly, when the trial head  36  is in a retracted position as shown in  FIG. 4B , the trial head  36  is inserted into the intervertebral disc space  14  at a first insertion depth D 1  when the abutment surface  27  abuts the inferior vertebra  12   b . When the trial head  36  is in an extended position as shown in  FIG. 4C , the trial head  36  is inserted into the intervertebral disc space  14  at a second insertion depth D 2  when the abutment surface  27  abuts the inferior vertebra  12   b . The second insertion depth D 2  is greater than the first insertion depth D 1 . 
     Referring again to  FIG. 4A , the shaft  24  can include a stop member  62  that is configured to abut the proximal end of the trial base  34  when the trial implant  22  is fully retracted. The stop member  62  thus prevents the trial implant  22  from being retracted to a location where the threads  42  and  48  would become disengaged. The stop member  62  projects radially out from the shaft body  26  so as to define a cross-sectional dimension greater than that of the shaft body  26 , and greater than that of the aperture  46  that receives the shaft body  26 . The shaft  24  can include depth markings  66  or other indicia distal of the stop member  62  to indicate the position of the trial implant  22  relative the shaft  24 . 
     Referring now also to  FIGS. 5-7 , an instrument assembly  21  can include the trial implant assembly  20  in combination with a distracter retainer instrument  68 . The distracter retainer instrument  68  includes a superior anchor screw  70  configured to be temporarily driven or implanted into the superior vertebral body  12   a , an inferior anchor screw  72  configured to be temporarily driven or implanted into the inferior vertebral body  12   b . The distracter retainer instrument  68  further includes a superior retainer distracter tube  74  couplable to the superior anchor screw  70 , and an inferior retainer distracter tube  76  couplable to the inferior anchor screw  72 . The distracter retainer instrument  68  can be implemented to initially separate the superior and inferior vertebral bodies  12   a  and  12   b , respectively, and retain distraction prior to cleaning out the disc tissue and inserting the total disc replacement implant. The anchor screws  70  and  72  can be removed from the vertebral bodies  12   a  and  12   b  after the permanent disc implant has been implanted between the vertebral bodies  12   a  and  12   b.    
     During operation, and with continuing reference to  FIGS. 1-7 , at least a partial discectomy is performed and the intervertebral disc space  14  is decompressed, for instance using the distracter retainer instrument  68 . A surgeon selects a trial head  36  to assess the size of the disc space  14  and couples the trial head  36  to the shaft  24 . In particular, the threaded region  44  of the shaft  24  is inserted into the aperture  46  of the trial base  34 , and the shaft  24  is rotated relative to the trial base  34  so that the threads  42  of the shaft  24  mate with the threads  48  of the trial base  34 . The shaft  24  is continuously rotated relative to the trial head  34  until the trial implant  22  is fully retracted with respect to the shaft  24 . Alternatively, the trial head could already be assembled to its own shaft with the head fully retracted. 
     The surgeon can insert the trial head  36  into the intervertebral disc space  14 , for instance by tapping on the proximal end of shaft  24  with a small mallet to advance the trial head  36  into the intervertebral disc space  14  until the abutment surface  27  abuts the inferior vertebral body  12   b . Should the surgeon wish to place the trial head  36  deeper within the disc space  14  the shaft  24  can be rotated in a counter clockwise direction, causing it to back out of trial base  34 . Because the pitch of the thread  44  on shaft  24  is fixed, the surgeon can precisely control how much the shaft backs out. The trial head  36  can then be inserted deeper into the intervertebral disc space  14  by tapping on the proximal end of the shaft  24  until the abutment surface  27  contacts the vertebral body  12   b  again. The optional depth markings  66  can assist in determining the desired insertion depth of the trial head  36  in the disc space  14 . 
     As the trial head  36  is inserted into the disc space  14 , the visualization window  60  and the lateral slots  39  and  41  allow the surgeon to visually determine the position of the trial head  36  without using fluoroscopy or other radio imaging. More particularly, the window  60  allows for visualization of the posterior longitudinal ligament (PLL), while the first and second lateral slots  39  and  41  allow for visualization of the exiting nerve roots. Accordingly, as described above with respect to the visualization window, at least a portion up to all of the lateral slots  39  and  41  are visually unobstructed. In this regard, the visualization window  60  can define a primary visualization window, while the lateral slots  39  and  41  can define auxiliary visualization windows disposed adjacent the primary visualization window  60 . Thus, at least one rib, such as ribs  38  and  40 , can define at least one visualization window, such as visualization windows  60 ,  39 , and  41 . 
     Once the trial head  36  is positioned appropriately within the disc space  14 , the surgeon assesses the fit of the trial head  36  within the disc space  14 . If the trial head  36  is not properly sized for the disc space, the surgeon removes and replaces the trial head  36  with a differently sized trial head  36  and repeats the evaluation process. Once the properly size trial head  36  is in place, the trial head  36  is removed and a permanent total disc replacement implant having a size corresponding to the properly sized trial head  36  is permanently implanted in the disc space. 
     It should thus be appreciated that a kit can be provided that includes a plurality of trial implants  22 , each couplable to the shaft  24 , and each having trial heads  36  of incrementally larger volumes that are sized and configured to fill and provide the desired spacing between the superior and inferior vertebral bodies  12   a  and  12   b . For instance, the trial heads  36  can define at least one or a plurality of varying characteristics, including a shape and/or a dimension such an outer lateral dimension, an outer longitudinal dimension, and a height between the outer transverse engagement surfaces  51  and  53 . The trial head  36  of each trial implant  22  can be color coded, such as anodized or otherwise colored, such that a range of different colors is provided to distinguish between the range of different sizes of the corresponding trial heads  36 . 
     It should be appreciated that the trial implant assembly  20  has been described in accordance with one embodiment, and that the trial implant assembly  20  could alternatively be constructed in accordance with numerous alternative embodiments that provide at least one visualization window. Some of the alternative embodiments are described below, it being appreciated that the scope of the present disclosure is not intended to be limited to any or all of the specific embodiments described herein. 
     For instance, referring to  FIGS. 8A-D , a trial implant assembly  120  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  20  incremented by  100 . Thus, the trial implant assembly  120  can be constructed substantially as described with respect to the trial implant assembly  20  except as otherwise noted. 
     As described above and illustrated with respect to  FIGS. 2-4C , the shaft  24  of the trial implant assembly  20  can extend longitudinally, or perpendicular to the transverse direction T. As illustrated in  FIGS. 8A-D , it is further recognized that the shaft  124  can extend in a direction angularly offset with respect to the longitudinal direction L, and thus with respect to the visualization window  160  that extends longitudinally through the trial head  136 . In accordance with the illustrated embodiment, the shaft  124  is angled transversely downward with respect to the longitudinal direction L so as to define an angle θ with respect to the longitudinal direction L that can be any angle as desired, such as between 0° and 60°. Thus, the central shaft axis S can extend at a non-perpendicular angle with respect to the transverse axis T. As illustrated in  FIG. 8E , the trial base  134  can oriented such that the upper and lower surfaces  135   a  and  135   b  extend parallel to the central shaft axis S, and the aperture  146  that receives the shaft  124  extends parallel to the upper and lower surfaces  135   a  and  135   b  as illustrated in  FIG. 2D . Alternatively, as illustrated in  FIG. 8F , upper and lower surfaces  135   a  and  135   b  can extend longitudinally, and thus angularly offset with respect to the central shaft axis S, and the aperture  146  that receives the shaft  124  can extend through the trial base in a direction that is angularly offset with respect to the upper and lower surfaces  135   a  and  135   b  so as to receive the shaft  124  along the angularly offset shaft axis S. 
     Referring now to  FIGS. 9A-10C , a trial implant assembly  220  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  20  incremented by  200 . Thus, the trial implant assembly  220  can be constructed substantially as described with respect to the trial implant assembly  20  except as otherwise noted. As illustrated, the trial implant assembly  220  includes the shaft  224 , and a trial implant  222  configured to be removably coupled to the shaft  224 . The trial implant  222  includes a trial head  226  having a superior endplate  250  and an inferior endplate  252  configured to be inserted into an intervertebral space, a trial base  234  configured to be coupled to the shaft  224 , and laterally spaced ribs  238  and  240  connected between the trial head  236  and the trial base  234  as described above. Thus, the ribs  238  and  240  are spaced so as to provide a primary visualization window  260  and auxiliary visualization window in the form of first and second laterally opposed slots  239  and  241 , as described above. 
     The trial base  234  includes a trial base body  235  that defines a pair of laterally or horizontally spaced engagement members  232   a - b  that are each configured to connect to a complementary engagement member  230  of the shaft  224 . In particular, the engagement member  230  of the shaft  224  is illustrated as including external threads  242  disposed in a threaded region  244  proximate to the distal end of the shaft  226 . The engagement members  232   a - b  each includes a corresponding pair of laterally spaced apertures  246   a - b  that each extends longitudinally through the trial base  234 . The apertures  246   a - b  are each sized to receive the threaded region  244  of the shaft  224 . The engagement members  232   a - b  of the trial base  234  includes internal threads  248  disposed about the periphery of the apertures  246   a - b  that are configured to mate with the external threads  242  of the shaft  224  so as to couple the shaft  224  to the trial implant  222 . 
     The apertures  246   a - b  are laterally offset on opposite sides with respect to a lateral midpoint of the primary visualization window  260 . Each aperture  246   a - b  is further vertically or transversely offset with respect to a vertical or transverse midpoint of the primary visualization window  260 . Furthermore, one or both of the apertures  246   a - b  can extend longitudinally, or can be angled with respect to the longitudinal direction L as described above. The trial base  234  thus defines a pair of offset engagement members  232   a - b  that are configured to be coupled to the shaft  224 , such that the shaft  224  can be coupled to the trial implant  222  at multiple locations. In particular, the threaded region  244  of the shaft  224  can be mated with the threads  248  of either engagement member  232   a - b  so as to provide relative motion between the trial implant  222  and the shaft  224  in the manner described above. 
     Referring now to  FIGS. 11A-D , a trial implant assembly  320  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  20  incremented by  300 . Thus, the trial implant assembly  320  can be constructed substantially as described with respect to the trial implant assembly  20  except as otherwise noted. As illustrated, the trial implant assembly  320  includes a shaft  324 , which may or may not be cannulated, and a trial implant  322 , which may or may not be cannulated, configured to be removably coupled to the shaft  324 . The trial implant  322  includes a trial head  336  having a superior endplate  350  and an inferior endplate  352  configured to be inserted into an intervertebral space, a trial base  334  configured to be coupled to the shaft  324 , and a laterally central rib  338  connected between the trial head  336  and the trial base  334 . 
     The superior endplate  350  defines a lower or inferior, or inner transverse, surface  355 , and the inferior endplate  352  defines an upper or superior, or inner transverse, surface  357 . The surfaces  355  and  357  are spaced vertically along the transverse direction T by a gap G as illustrated, though it should be appreciated that the endplates  350  and  352  could alternatively be connected at their inner transverse ends. The rib  338  extends distally from the trial base  334 , and is connected between the surfaces  355  and  357  of the endplates  350  and  352 , respectively. First and second laterally opposed visualization slots  339  and  341  extend longitudinally through the trial head  336  on opposed lateral sides of the rib  338 . The distal end of the rib  338  can terminate proximal to the distal end of the trial head  336 , or endplates  350  and  352 , so as to provide increased visualization and allow improved access to posterior structures in the disc space using a conventional nerve hook or probe. 
     The trial base  334  defines an engagement member  332  that is configured to connect to the engagement member  330  of the shaft  324 . The trial implant  322  includes an aperture  346  that extends longitudinally through the trial base body  335 , and can further extend longitudinally through the rib  338 . The aperture  346  is sized to receive the threaded region  344  of the shaft  324 . The engagement member  332  of the trial base  334  includes internal threads  348  disposed about the periphery of the aperture  346  that are configured to mate with the external threads  342  of the shaft  324  so as to couple the shaft  324  to the trial implant  322 . 
     The trial base  322  further includes a pair of superior tracks  373   a  and  375   a  extending up, or transversely out, from the superior or upper surface  335   a  of the trial base body  335 , and a pair of inferior tracks  373   b  and  375   b  extending down, or transversely out, from the lower or inferior surface  335   b  of the trial base body  335 . The superior tracks  373   a  and  375   a  are laterally spaced from each other so as to define a superior longitudinally elongate guide channel  377   a  extending between the superior tracks  373   a  and  375   a . The inferior tracks  373   b  and  375   b  are laterally spaced from each other so as to define an inferior longitudinally elongate guide channel  377   b  extending between the inferior tracks  373   b  and  375   b.    
     The shaft  324  includes an adjustable mechanical stop  380  coupled to the distal portion of the shaft body  326  such that the stop  380  is rotatable with respect to the shaft body  326  but is translatably fixed relative to the shaft  324 . Thus, the stop  380  can rotate about the shaft body  326  but is unable to translate along the shaft body  326 . In particular, the stop  380  includes at least one circumferential collar  381  (a pair of longitudinally spaced collars  381  are illustrated) that nest within a corresponding at least one radial groove  383  (a pair of longitudinally spaced grooves  383  are illustrated) that extend into the shaft body  326 . Interference between the collars  381  and the portion of the shaft body  326  that is adjacent the grooves  383  prevent the stop  380  from translating along the shaft body  326 . Alternatively, a snap ring can be snapped onto the shaft  324  into a groove disposed between the collars  381 , such that the interference between the snap ring and the collars  381  prevent translation of the stop  380  along the shaft  324 . The collars  381 , and thus the stop  380 , are free to rotate about the shaft body  326  within the radial grooves  383 . 
     The distal surface of the distal-most collar  381  defines a stop member  362  configured to abut the proximal end of the trial base  334  when the trial implant  322  is fully retracted on the shaft  324 . The stop member  362  thus prevents the trial implant  322  from being retracted to a location where the threads  342  and  348  would become disengaged. The stop member  362  projects radially out from the shaft body  326  so as to define a cross-sectional dimension greater than that of the shaft body  326 , and greater than that of the aperture  346  that receives the shaft body  326 . 
     The stop  380  further includes a guide body  385  extending transversely outward and longitudinally distal from the collars  381 . The guide body  385  defines a lateral outer dimension substantially equal to or slightly less than that of the guide channels  377   a - b . Accordingly, the guide body  385  is configured to ride within and translate within a select one of the guide channels  377   a - b  as the shaft  324  is rotated relative to the trial base  334 , thereby causing the implant  322  to translate relative to the shaft  324 . 
     The stop  380  further defines a vertebral abutment surface  327  defined by the distal surface of the guide body  385 . When the guide body  385  is disposed in the superior guide channel  377   a , the abutment surface  327  is configured to abut the superior vertebra when the trial head  336  is inserted into the intervertebral space. When the guide body  385  is disposed in the inferior guide channel  377   b , the abutment surface  327  is configured to abut the inferior vertebra when the trial head  336  is inserted into the intervertebral space. Thus, the guide channels  377   a - b  are configured to maintain the alignment of the vertebral abutment surface  327  with a select one of the superior and inferior vertebrae based on the anatomy of the patient. Once the trial head  336  is positioned within the intervertebral disc space, the shaft  324  can be uncoupled, i.e., unscrewed, from the trial implant  322  and the aperture  346  can serve as a primary visualization window into the intervertebral disc space as described above with respect to the primary visualization window  60 . The shaft  324  can further be cannulated, such that the cannulation of the shaft  324  is aligned with the aperture  346 . Accordingly, the visualization window defined by the aperture  346  can be visually accessed through the cannulation of the shaft  324 . 
     Referring now to  FIGS. 12A-13B , a trial implant assembly  420  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  20  incremented by 400. Thus, the trial implant assembly  420  can be constructed substantially as described with respect to the trial implant assembly  20  except as otherwise noted. As illustrated, the trial implant assembly  420  includes a shaft  424 , and a trial implant  422  configured to be removably coupled to the shaft  424 . The trial implant  422  includes a trial head  436  having a superior endplate  450  and an inferior endplate  452  configured to be inserted into an intervertebral space, a trial base  434  configured to be coupled to the shaft  424 , and a laterally central rib  438  connected between the trial head  436  and the trial base  434 . 
     The superior endplate  450  defines a lower or inferior, or inner transverse, surface  455 , and the inferior endplate  452  defines an upper or superior, or inner transverse, surface  457 . The surfaces  455  and  457  are spaced vertically along the transverse direction T by a gap G as illustrated, though it should be appreciated that the endplates  450  and  452  could alternatively be connected at their inner transverse ends. The rib  438  extends distally from the trial base  434 , and is connected between the surfaces  455  and  457  of the endplates  450  and  452 , respectively. First and second laterally opposed visualization slots  439  and  441  extend longitudinally through the trial head  336  on opposed lateral sides of the rib  438 . The distal end of the rib  438  can terminate proximal to the distal end of the trial head  436 , or endplates  450  and  452 , so as to provide increased visualization and allow improved access to posterior structures in the disc space using a conventional nerve hook or probe. Furthermore, the laterally opposed outer walls of the rib  438  are concave and convergingly tapered along a direction from the proximal end of the rib  438  toward the distal end of the rib  438 , thereby increasing visualization of the middle of the disc space. 
     The trial base  434  defines an engagement member  432  that is configured to connect to the engagement member  430  of the shaft  424 . In particular, the trial implant  422  includes an aperture  446  that extends longitudinally through the trial base body  435 , and further extends longitudinally through the rib  438 . A portion of substantially all of aperture  446  is sized to receive the threaded region  444  of the shaft  424 . The engagement member  432  of the trial base  434  includes internal threads  448  disposed about the periphery of part or all of the aperture  446  that are configured to mate with the external threads  442  of the shaft  424  so as to couple the shaft  424  to the trial implant  422 . 
     The trial base  422  can further include a pair of superior tracks  473   a  and  475   a  (see  FIG. 13D ) that extend transversely out from, and flare laterally out from, the superior or upper surface  435   a  of the trial base body  435 , and a pair of inferior tracks  473   b  and  475   b  that extend transversely out from, and flare laterally out from, the lower or inferior surface  435   b  of the trial base body  435 . Thus, the tracks  473   a  and  475   a  flare laterally away from each other, and the tracks  473   b  and  475   b  flare laterally away from each other. The superior tracks  473   a  and  475   a  can be constructed substantially as described with respect to the inferior tracks  475   b  and  475   b  illustrated in  FIGS. 12A-13C , except that the superior tracks  473   a  and  475   a  are transversely inverted with respect to the inferior tracks  473   b  and  475   b.    
     The shaft  424  defines a cannulation  425  that extends longitudinally through the shaft body  426 . The proximal portion of the shaft  424  includes a mechanism  415  for connecting to tubing for siphoning out blood and/or other tissue or debris remaining from the discectomy as described below. 
     The shaft  424  further includes an adjustable mechanical stop  480  coupled to the distal portion of the shaft body  426  such that the stop  480  is rotatable with respect to the shaft body  426  but is translatably fixed relative to the shaft  424 . Thus, the stop  480  can rotate about the shaft body  426  but is unable to translate along the shaft body  426 . In particular, the stop  480  includes at least one circumferential collar  481  (a pair of longitudinally spaced collars  481  are illustrated) that nest within a corresponding at least one radial groove  483  (a pair of spaced grooves  483  are illustrated) that extend into the shaft body  426 . Interference between the collars  481  and the portion of the shaft body  426  that is adjacent the grooves  483  prevent the stop  480  from translating along the shaft body  426 . Alternatively, a snap ring can be snapped onto the shaft  424  into a groove disposed between the collars  481 , such that the interference between the snap ring and the collars  481  prevent translation of the stop  480  along the shaft  424 . The collars  481 , and thus the stop  480 , are free to rotate about the shaft body  426  within the radial grooves  483 . 
     The distal surface of the distal-most collar  481  defines a stop member  462  configured to abut the proximal end of the trial base  434  when the trial implant  422  is fully retracted on the shaft  424 . The stop member  462  thus prevents the trial implant  422  from being retracted to a location where the threads  442  and  448  would become disengaged. The stop member  462  projects radially out from the shaft body  426  so as to define a cross-sectional dimension greater than that of the shaft body  426 , and greater than that of the aperture  446  that receives the shaft body  426 . 
     The stop  480  is forked with a first guide body  485   a  and a second guide body  485   b  that flare laterally away from each other along a transversely outward direction. Each of the first and second guide bodies  485   a - b  are positioned such that the laterally inner surfaces of the guide bodies  485   a - b  ride along the laterally outer surfaces of the superior tracks  473   a  and  475   a  when the guide bodies  485   a - b  are aligned with the superior tracks, and ride along the laterally outer surfaces of the inferior tracks  473   b  and  475   b  when the guide bodies  485   a - b  are aligned with the inferior tracks. In particular, each guide body  485   a  and  485   b  can include a pocket  487   a  and  487   b , respectively, that at least partially receives the corresponding track  473  and is configured to ride along the track  473 . 
     During operation, the stop  480  is rotated about the shaft  424  as desired so as to align the stop  480  with a select one of the superior tracks  473   a  and  475   a , and the inferior tracks  473   b  and  475   b . The threads  442  of the shaft  424  are then engaged with the threads  448  of the trial implant  422 , which causes the guide bodies  485   a - b  to ride along the selected tracks  473  and  475 . Accordingly, the guide bodies  485  are configured to ride along the tracks  473  and  475  as the shaft  324   424  is rotated relative to the trial base  434 , thereby causing the trial implant  422  to translate relative to the shaft  424 . The space between the first and second guide bodies  485   a - b  allow the trial implant  422  to be placed around or in close proximity to a Caspar distraction pin or various elements of the distracter  68  as described above with respect to  FIGS. 5-7 . 
     The stop  480  further defines a vertebral abutment surface  427  defined by the distal surface of the guide body  485 . When the guide body  485  engages the superior tracks  473   a  and  475   a , the abutment surface  427  is configured to abut the superior vertebra when the trial head  436  is inserted into the intervertebral space. When the guide body  485  engages the inferior tracks  473   b  and  475   b , the abutment surface  427  is configured to abut the inferior vertebra when the trial head  436  is inserted into the intervertebral space. Thus, the tracks  473  and  475  are configured to maintain the alignment of the vertebral abutment surface  427  with a select one of the superior and inferior vertebrae based on the anatomy of the patient. Once the trial head  436  is positioned within the intervertebral disc space, the shaft  424  can be uncoupled, i.e., unscrewed, from the trial implant  422  and the aperture  446  can serve as a primary visualization window into the intervertebral disc space as described above with respect to the primary visualization window  60 . The cannulation  425  of the shaft  424 , which is in alignment with the aperture  446 , provides visual access to the visualization window defined by the aperture  446  prior to removal of the shaft  424  from the trial implant  422 . The cannulation  425  of the shaft  424  further allows blood and other matter to be siphoned out from the disc space. 
     Once the trial head  436  is inserted into the disc space, a vacuum source (not shown) is connected to the connection mechanism  415  on the proximal end of the shaft  424  and suction is applied to remove debris, such as blood and/or tissue debris remaining from the discectomy from the disc space, through the through hole  425 , and out through the cannulated interior of the shaft  424 . Thus, the cannulated shaft can provide a suction passageway for the removal of debris under an applied vacuum pressure. The removal of tissue debris and blood increases visualization and, further, provides a better suited intervertebral environment for implantation of the total disc replacement implant. 
     Referring now to  FIGS. 14A-E , a trial implant assembly  520  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  20  incremented by  500 . Thus, the trial implant assembly  520  can be constructed substantially as described with respect to the trial implant assembly  20  except as otherwise noted. As illustrated, the trial implant assembly  520  includes a shaft  524 , and a trial implant  522  configured to be removably coupled to the shaft  524 . The trial implant  522  includes a trial head  536  having a superior endplate  550  and an inferior endplate  552  configured to be inserted into an intervertebral space, a trial base  534  configured to be coupled to the shaft  524 , and a laterally central rib  538  connected between the trial head  536  and the trial base  534 . 
     The superior endplate  550  defines a lower or inferior, or inner transverse, surface  555 , and the inferior endplate  552  defines an upper or superior, or inner transverse, surface  557 . The surfaces  555  and  557  are spaced vertically along the transverse direction T by a gap G as illustrated, though it should be appreciated that the endplates  550  and  552  could alternatively be connected at their inner transverse ends. The rib  538  extends distally from the trial base  534 , and is connected between the surfaces  555  and  557  of the endplates  550  and  552 , respectively. First and second laterally opposed visualization slots  539  and  541  extend longitudinally through the trial head  536  on opposed lateral sides of the rib  538 . The distal end of the rib  538  can terminate proximal to the distal end of the trial head  536 , or endplates  550  and  552 , so as to provide increased visualization and allow improved access to posterior structures in the disc space using a conventional nerve hook or probe. 
     The trial base  534  further includes a pair of superior tracks  573   a  and  575   a  extending up, or transversely out, from the superior or upper surface  535   a  of the trial base body  535 , and a pair of inferior tracks  573   b  and  575   b  extending down, or transversely out, from the lower or inferior surface of the trial base body  535 . The superior tracks  573   a  and  575   a  are laterally spaced from each other so as to define a superior longitudinally elongate guide channel  577   a  extending between the superior tracks  573   a  and  575   a . The inferior tracks  573   b  and  575   b  are laterally spaced from each other so as to define an inferior longitudinally elongate guide channel  577   b  extending between the inferior tracks  573   b  and  575   b.    
     The shaft  524  includes an adjustable mechanical stop  580  coupled to the distal portion of the shaft body  526  such that the stop  580  is rotatable with respect to the shaft body  526  but is translatably fixed relative to the shaft body  526 . Thus, the stop  580  can rotate about the shaft body  526  but is unable to translate along the shaft body  526 . In particular, the stop  580  includes a pair of longitudinally spaced collars  581 . The shaft  524  includes a snap ring  597  that is disposed in a groove  583  extending into the shaft body  526 , such that the ring  597  is disposed between the collars  581 . Thus, interference between the ring  597  and the collars  581  prevent translation of the stop  580  along the shaft  524 . The collars  581 , and thus the stop  580 , are free to rotate about the shaft body  526 . 
     The distal surface of the distal-most collar  581  defines a stop member  562  configured to abut the proximal end of the trial base  534  when the trial implant  522  is fully retracted on the shaft  524 . The stop  580  further includes a guide body  585  extending transversely outward and longitudinally distal from the collars  581 . The guide body  585  defines a lateral outer dimension substantially equal to or slightly less than that of the guide channels  577   a - b . Accordingly, the guide body  585  is configured to ride within and translate within a select one of the guide channels  577   a - b  as the shaft  524  is rotated relative to the trial base  534 , thereby causing the trial implant  522  to translate relative to the shaft  524  in the manner described above. 
     The stop  580  further defines a pair vertebral abutment surfaces  527   a - b  defined by the distal surface of the guide body  585 . In particular, the guide body  585  defines a pair of legs  591   a - b  that extend forward from the guide body  585 . The legs  591   a - b  are laterally separated from each other. Thus, the vertebral abutment surfaces  527   a - b  are defined by the distal surfaces of the legs  591   a - b , such that a gap  593  extends laterally between the abutment surfaces  527   a - b . The abutment surfaces  527   a - b  are configured to abut the same vertebra when the trial head  536  is inserted into an intervertebral space. 
     For instance, when the guide body  585  is disposed in the superior guide channel  577   a , the abutment surfaces  527   a - b  are configured to abut the superior vertebra when the trial head  536  is inserted into the intervertebral space. When the guide body  585  is disposed in the inferior guide channel  577   b , the abutment surfaces  527   a - b  are configured to abut the inferior vertebra when the trial head  536  is inserted into the intervertebral space. Thus, the guide channels  577   a - b  are configured to maintain the alignment of the vertebral abutment surface  527  with a select one of the superior and inferior vertebrae based on the anatomy of the patient. 
     Referring now to  FIGS. 15A-E , a trial implant assembly  620  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  520  incremented by  100 . Thus, the trial implant assembly  620  can be constructed substantially as described with respect to the trial implant assembly  520  except as otherwise noted. As illustrated, the trial implant assembly  620  includes a shaft  624 , and a trial implant  622  configured to be removably coupled to the shaft  624 . The trial implant  622  includes a trial head  636  having a superior endplate  650  and an inferior endplate  652  configured to be inserted into an intervertebral space, a trial base  634  configured to be coupled to the shaft  624 , and a laterally central rib  638  connected between the trial head  636  and the trial base  634 . 
     The stop  680  includes a guide body  685  extending transversely outward and longitudinally distal from the collars  681 . The guide body  685  defines a lateral outer dimension substantially equal to or slightly less than that of the guide channels  677   a - b . Accordingly, the guide body  685  is configured to ride within and translate within a select one of the guide channels  677   a - b  as the shaft  624  is rotated relative to the trial base  634 , thereby causing the trial implant  622  to translate relative to the shaft  624  in the manner described above. 
     The stop  680  further defines a pair vertebral abutment surfaces  627   a - b  defined by the distal surface of the guide body  685 . In particular, the guide body  685  defines a pair of superior and inferior legs  691   a - b  that extend forward from the guide body  685 , and are transversely separated from each other Thus, the vertebral abutment surfaces  627   a - b  are defined by the distal surfaces of the legs  691   a - b , such that the abutment surfaces  627   a - b  are transversely separated. Thus, the abutment surfaces  627   a - b  are configured to abut different vertebrae when the trial head  636  is inserted into an intervertebral space. Specifically, the abutment surfaces  627   a - b  are configured to abut the adjacent vertebrae that define the intervertebral space into which the trial head  636  is inserted. Thus, the superior abutment surface  627   a  is configured to abut the superior vertebra, and the inferior abutment surface  627   b  is configured to abut the inferior vertebra. It should be appreciated that the superior and inferior legs  691   a - b  could be split so as to each define a pair of vertebral abutment surfaces in the manner illustrated in  FIGS. 14A-D . 
     Referring now to  FIGS. 16A-E , a trial implant assembly  720  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  520  incremented by  200 . Thus, the trial implant assembly  720  can be constructed substantially as described with respect to the trial implant assembly  520  except as otherwise noted. As illustrated, the trial implant assembly  720  includes a shaft  724 , and a trial implant  722  configured to be removably coupled to the shaft  724 . The trial implant  722  includes a trial head  736  having a superior endplate  750  and an inferior endplate  752  configured to be inserted into an intervertebral space, a trial base  734  configured to be coupled to the shaft  724 , and a laterally central rib  738  connected between the trial head  736  and the trial base  734 . 
     The stop  780  includes a guide body  785  extending radially outward and longitudinally distal from the collars  781 . The stop  780  defines a channel  792  extending into the radially inner end of the guide body  785 . The channel  792  can extend longitudinally through the proximal end of the guide body  785 , and can terminate prior to the distal end of the guide body, or can extend longitudinally through the distal end of the guide body. The trial base  734  further includes a track  773  extending obliquely out from the trial base body  735 . In accordance with the illustrated embodiment, the track  773  extends out from the collars of the trial base body  735  in a direction angularly offset with respect to both the lateral and the transverse directions. 
     Thus, the track  773  is laterally offset with respect to a laterally central midline of the endplates  750  and  752 . It should be appreciated that the track can extend along a direction having an upper transverse directional component, or a downward transverse directional component. It should be further appreciated that the trial base can include more than one track  773  extending out from the trial base body, each having an upper transverse directional component or a downward transverse directional component. The guide body channel  792  is configured to receive a select one of the at least one track  773  of the trial base  734  before the shaft  724  rotatably engages the trial base  734 . 
     The guide body channel  792  defines a cross sectional dimension that is substantially equal to or slightly greater than that of the track or tracks  773 . Accordingly, the track or tracks  773  are configured to ride within and translate within the channel  792  as the shaft  724  is rotated relative to the trial base  734 , thereby causing the trial implant  722  to translate relative to the shaft  724  in the manner described above. 
     The stop  780  further defines a vertebral abutment surface  727  defined by the distal surface of the guide body  785 . Thus, the vertebral abutment surfaces  727  is configured to abut the vertebrae that is aligned with the track  773  that is received in the channel  792  or otherwise engages the guide body  785 . For instance, if the guide body  785  engages a track  773  that has an upper transverse directional component, the vertebral abutment surface  727  can abut the superior vertebra. If the guide body  785  engages a track  773  that has a downward transverse directional component, the vertebral abutment surface  727  can abut the inferior vertebra. It should be appreciated that the stop  780  could include a pair or a plurality of guide members  785 , each configured to engage a track  773  of the trial base  734  as desired. 
     Referring now to  FIGS. 17A-B , a trial implant assembly  820  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  20  incremented by  800 . Thus, the trial implant assembly  820  can be constructed substantially as described with respect to the trial implant assembly  20  except as otherwise noted. As illustrated, the trial implant assembly  820  includes a shaft  824  and a trial implant  822  configured to be removably coupled to the shaft  824 . The trial implant  822  includes a trial head  836  having a superior endplate  850  and an inferior endplate  852  configured to be inserted into an intervertebral space, a trial base  834  configured to be coupled to the shaft  824 , and a laterally central rib  838  connected between the trial base  834  and the trial head  836 . It should be appreciated, however, that the trial base  834  could be connected directly to the trial head  836 . 
     In particular, the rib  838  is connected between the distal end of the trial base  834  and one of the endplates  850  and  852  of the trial head  836 . As illustrated, the rib  838  is connected to the inferior endplate  852 , and is not connected between the endplates  850  and  852 . The trial head  836  includes a pair of laterally outer endplate walls  888  that are connected between the endplates  850  and  852 , such that an enclosed visualization window  860  is defined between the outer endplate walls  888  and the inner transverse surfaces  855  and  857  of the endplates  850  and  852 . The trial base  834  is vertically offset from the visualization window  860 . 
     Referring now to  FIGS. 18A-B , a trial implant assembly  920  constructed in accordance with an alternative embodiment is illustrated including reference numerals corresponding to like elements of the trial implant assembly  820  incremented by  100 . Thus, the trial implant assembly  920  can be constructed substantially as described with respect to the trial implant assembly  820  except as otherwise noted. As illustrated, the trial implant assembly  920  includes a shaft  924  and a trial implant  922  configured to be removably coupled to the shaft  924 . The trial implant  922  includes a trial head  936  having a superior endplate  950  and an inferior endplate  952  configured to be inserted into an intervertebral space, a trial base  934  configured to be coupled to the shaft  924 , and a rib  938  that is central with respect to trial base  934  connected between the trial base  934  and the trial head  936 . It should be appreciated, however, that the trial base  934  could be connected directly to the trial head  936 . 
     In particular, the rib  938  is connected between the distal end of the trial base  934  and one of the laterally outer endplate walls  988  that are connected between the endplates  950  and  952 , such that an enclosed visualization window  960  is defined between the outer endplate walls  988 , and further defined by the inner transverse surfaces  955  and  957  of the endplates  950  and  952 . The trial base  934  is laterally offset from the visualization window  960 . 
     It should be appreciated that the trial implants  822  and  922  illustrate that trial implants usable in connection with any of the trial implant assemblies described herein can be configured so as to provide a range of numerous possible geometries and configurations of visualization windows. The visualization windows as described herein can be rectangular, square, round, elliptical in shape, and can be geometrically regular or irregular, and may be centered with respect to the trial head or offset with respect to the trial head. The windows can further be open or enclosed. 
     While the trial implant instrument assemblies of the present invention have been described in reference to surgical procedures for replacing a damaged intervertebral disc with a total disc replacement implant, it is understood that the teachings of the present invention are easily configurable for surgical procedures for fusing a damaged disc space using an interbody spacer. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. Furthermore, it should be appreciated that the structure, features, and methods as described above with respect to any of the embodiments described herein can be incorporated into any of the other embodiments described herein unless otherwise indicated. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present disclosure.