Abstract:
A method and system for insertion of an implant is disclosed. One embodiment of a system for use in implanting a spinal prosthesis incorporating principles of the invention includes an insertion assembly housing with a channel extending from a distal end portion to a proximal end portion, a gripper having a prosthesis coupling portion for coupling with a spinal prosthesis and an end portion, and a coupler member having a gripper coupling portion rotatably positioned within the channel and configured to couple with the end portion of the gripper within the channel.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to surgical methods and devices and, more particularly, to methods and devices used to facilitate insertion of implants. 
       BACKGROUND 
       [0002]    The spine is made of bony structures called vertebral bodies that are separated by soft tissue structures called intervertebral discs. The intervertebral disc is commonly referred to as a spinal disc. The spinal disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motions between vertebral segments of the axial skeleton. The disc acts as a synchondral joint and allows some amount of flexion, extension, lateral bending, and axial rotation. 
         [0003]    The normal disc is a mixed avascular structure including two vertebral end plates, annulus fibrosis and nucleus pulposus. The end plates are composed of thin cartilage overlying a layer of hard, cortical bone that attaches to the spongy cancellous bone of the adjacent vertebral body. 
         [0004]    The discs are subjected to a variety of loads as the posture of an individual changes. Even when the effects of gravity are removed, however, the soft tissue connected to the spine generates a compressive force along the spine. Thus, even when the human body is supine, the compressive load on the lumbar disc is on the order of 300 Newtons (N). 
         [0005]    A spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the annulus fibers are weakened or torn and the inner material of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal annular confines. The mass of a herniated or “slipped” nucleus tissue can compress a spinal nerve, resulting in leg pain, loss of muscle strength and control or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and dehydrates with subsequent loss in disc height. Consequently, the volume of the nucleus decreases, causing the annulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping plies of the annulus buckle and separate, either circumferential or radial annular tears may occur, potentially resulting in persistent and disabling back pain. Adjacent, ancillary facet joints will also be forced into an overriding position, which may cause additional back pain. 
         [0006]    When the discs wear out or are otherwise injured, a condition known as degenerative disc disease results. With this condition, discs do not function normally and may cause pain and limit activity. Recently, efforts have been directed to replacing intervertebral discs which display degenerative disc disease. In one such procedure, the damaged intervertebral disc is replaced by a prosthetic disc. 
         [0007]    One well known intervertebral prosthetic disc is produced by DePuy Spine, Inc. of Raynaham, Mass. and is sold under the trademark CHARITÉ®. This disc prosthesis includes two metal endplates and a center polyethylene core. The center core includes a superior spherical bearing surface and an inferior spherical bearing surface. The superior endplate includes a concave surface that fits upon and is congruent with the superior bearing surface of the core. The inferior endplate includes a concave surface that fits under and is congruent with the inferior bearing surface of the core. 
         [0008]    During a CHARITE® ( artificial disc replacement procedure, the damaged disc is typically removed via an anterior surgical approach and the end surfaces of the exposed vertebrae are cleared of debris. The vertebrae are spread apart and the metal endplates are positioned on the respective vertebra and tapped into place. The polyethylene core is then inserted between the endplates and the vertebrae are returned to their normal position. The pressure of the spinal column further seats the endplates into the vertebral bones and secures the core in place. 
         [0009]    While the sequential implantation of components is effective, the amount of time required to position three separate components as opposed to implanting a single unit increases the duration of the procedure. Additionally, the increased number of steps increases the risk of the procedure. 
         [0010]    In response to the foregoing limitations, some instrumentation has been developed wherein a distraction instrument may also serve as an installation instrument. In particular, in addition to being configured to spread apart the two vertebrae, the instrument is also configured to slide the assembled artificial disc into place while the vertebrae remain separated. A central ramp is provided on the instrument to facilitate sliding of the implant between the vertebrae. Once the artificial disc is positioned, the installation instrument is decoupled from the artificial disc and removed. 
         [0011]    Such instruments are very effective. Nonetheless, they do have various limitations. For example, because of the various functions performed with the instrument, the instruments are complicated in construction, resulting in increased costs. Additionally, as a particular instrument becomes more complicated, the potential for a mechanical failure increases. A further limitation is that the artificial disc is retained in such instruments using spring force which can be unreliable. 
         [0012]    Accordingly, it would be advantageous to provide a tool for implanting an artificial disc or other spinal implant which does not rely upon a spring to maintain a secure hold upon the artificial disc or other spinal implant. It would also be advantageous if the tool could be used in combination with a distraction tool. It would be further advantageous if such features could be provided while allowing an artificial disc or other spinal implant to be implanted as a unit. 
       SUMMARY 
       [0013]    A method and system for insertion of an implant is disclosed. One embodiment of a system for use in implanting a spinal prosthesis incorporating principles of the invention includes an insertion assembly housing with a channel extending from a distal end portion to a proximal end portion, a gripper having a prosthesis coupling portion for coupling with a spinal prosthesis and an end portion, and a coupler member having a gripper coupling portion rotatably positioned within the channel and configured to couple with the end portion of the gripper within the channel. 
         [0014]    One method incorporating principles of the invention includes identifying a vertebral implant, coupling a gripper member with the vertebral implant, rotating a coupler member within a housing to generate an axial force, translating the axial force to a compressive force on the gripper member, positioning the coupled vertebral implant, removing the compressive force from the gripper member and decoupling the gripper member from the vertebral implant after the vertebral implant has been positioned. 
         [0015]    Another system for use in implanting a spinal implant includes an insertion assembly housing having an axis, a gripper with a spinal implant coupling portion configured to couple with a spinal implant and a coupling member configured to apply force to the gripper along the axis of the insertion assembly housing by rotation of the coupling member within the insertion assembly housing. 
         [0016]    The above-described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention may take form in various system and method components and arrangement of system and method components. The drawings are only for purposes of illustrating exemplary embodiments and are not to be construed as limiting the invention. 
           [0018]      FIG. 1  shows a superior perspective view of a spinal implant in the form of an intervertebral disc prosthesis including a superior plate and a inferior plate separated by a core; 
           [0019]      FIG. 2  shows a side cross-sectional view of the intervertebral disc prosthesis of  FIG. 1  with the superior plate rotated to display flexion; 
           [0020]      FIG. 3  shows a perspective view of the disc prosthesis of  FIG. 1  held by a system including a distraction instrument and a prosthesis insertion assembly incorporating features of the invention; 
           [0021]      FIG. 4  shows a cross-sectional view of the prosthesis insertion assembly of  FIG. 3  with a gripper partially inserted into the housing of the prosthesis insertion assembly and coupled with a coupling member incorporating features of the invention; 
           [0022]      FIG. 5  shows a side plan view of the prosthesis insertion assembly of  FIG. 3 ; 
           [0023]      FIG. 6  shows a perspective view of the base and knob portion of the system of  FIG. 3  decoupled from the prosthesis insertion assembly and with the vertebra engaging members removed; 
           [0024]      FIG. 7  is a partial cross-sectional view of the prosthesis insertion assembly of  FIG. 3  with a gripper partially inserted into the housing of the prosthesis insertion assembly such that the neck portion of the gripper abuts the throat portion of the prosthesis insertion assembly; 
           [0025]      FIG. 8  shows a side plan view of the prosthesis insertion assembly of  FIG. 3  with a disc prosthesis coupled with a gripper which is partially inserted into the prosthesis insertion assembly and with the coupling member removed; 
           [0026]      FIG. 9  shows an exploded perspective view of the system of  FIG. 3  with the vertebra engaging members detached from the insertion assembly; 
           [0027]      FIG. 10  shows a side plan view of the system of  FIG. 3  with the fingers of the distraction instrument inserted into a space in which the disc prosthesis is to be implanted; 
           [0028]      FIG. 11  shows a side plan view of the system of  FIG. 3  with the disc prosthesis used to force the fingers of the distraction instrument into a distracted configuration in accordance with principles of the invention; 
           [0029]      FIG. 12  shows a side plan view of the system of  FIG. 3  after continued rotation of the knob of the distraction instrument has forced the fingers of the distraction instrument out of the space in the spine while the disc prosthesis remains in the space in accordance with principles of the invention; 
           [0030]      FIG. 13  shows a side plan view of the system of  FIG. 3  after the distraction instrument has been removed and while the gripper and the prosthesis insertion assembly are still coupled; 
           [0031]      FIG. 14  shows an alternate embodiment of a gripper that may be used with the prosthesis insertion assembly of  FIG. 5  in accordance with principles of the invention; 
           [0032]      FIG. 15  shows an alternate embodiment of a disc prosthesis that may be used with the gripper of  FIG. 14  in accordance with principles of the invention; 
           [0033]      FIG. 16  shows the disc prosthesis of  FIG. 15  coupled with the gripper of  FIG. 14 ; 
           [0034]      FIG. 17  shows a perspective view of an alternate implantation system incorporating the prosthesis insertion assembly of  FIG. 5  and with the fingers of a distraction instrument inserted into a space in which the disc prosthesis is to be implanted; 
           [0035]      FIG. 18  shows a side plan view of the system of  FIG. 17 ; 
           [0036]      FIG. 19  shows a perspective view of the system of  FIG. 17  after an extension handle has been used to position the disc prosthesis within the prepared space using guide members on the prosthesis insertion assembly to align the disc prosthesis in accordance with principles of the invention; and 
           [0037]      FIG. 20  shows a perspective view of the system of  FIG. 17  after the distraction instrument has been removed, leaving the disc prosthesis in the prepared space. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    With reference to  FIGS. 1-2 , a spinal implant which in this embodiment is an intervertebral disc prosthesis  100  includes a superior plate  102 , an inferior plate  104 , and a core  106 . The core  106  is sandwiched between the superior plate  102  and the inferior plate  104 . The superior plate  102  and the inferior plate  104  ride upon the core  106  and are operable to rotate relative to the core  106 . 
         [0039]    In one embodiment, the superior plate  102  is formed of metal. In particular, the superior plate  102  may be formed using a medical grade cobalt chromium alloy. The superior plate  102  includes an upper surface  108  and a lower surface  110 . An outer perimeter edge  112  defines the “footprint” of the superior plate  102  when the disc prosthesis is implanted. 
         [0040]    The upper surface  108  of the superior plate  102  is designed for engagement with a vertebral surface of a patient. To this end, the upper surface  108  of the superior plate  102  may be slightly convex for close engagement with the slightly concave vertebral surface of the patient. Additionally, teeth  114  are included on the upper surface  108  of the superior plate  102 . The teeth  114  are designed to penetrate into the vertebral surface, helping to secure the superior plate  102  to the vertebral surface. A groove  109  extends across the upper surface  108 . 
         [0041]    The lower surface  110  of the superior plate  102  is generally flat near the outer edge  112 . As shown more clearly in  FIG. 2 , a collar portion  116  protrudes from the lower surface  110  and defines an inner concave surface  118  at the center of the collar portion  116 . 
         [0042]    The inferior plate  104  is a mirror image of the superior plate  102  and is also made of a medical grade cobalt chromium alloy. The inferior plate  104  includes a slightly convex lower surface  120  and an outer perimeter edge  122 . A plurality of teeth  124  extend from the lower surface  120 . The teeth  124  are designed to help secure the inferior plate  104  to a vertebral surface. The lower surface  120  also includes a groove (not shown). The upper surface  126  of the inferior plate  104  includes a collar portion  128  with an inner concave surface  130 . 
         [0043]    The core  106  is arranged within an interior space of the prosthesis  100  between the lower surface  110  of the superior plate  102  and the upper surface  126  of the inferior plate  104 . In one embodiment, the prosthesis core  106  is made from a plastic material having a high resistance to wear, such as ultra high molecular weight polyethylene (UHMWPE), which allows the endplates  102  and  104  to slide easily on the core  106 . 
         [0044]    The prosthesis core  106  is generally disc shaped with an outer radial flange  132 , an upper spherical surface  134 , and a lower spherical surface  136 . The upper spherical surface  134  and the lower spherical surface  136  act as bearing surfaces/articulating surfaces that slidingly engage the bearing/articulating surfaces of the endplates  102  and  104 . Namely, the inner concave surface  118  and the inner concave surface  130 , respectively. As shown in  FIG. 2 , a first groove  138  is formed between the flange  132  and the collar portion  116  of the superior plate  102 . A second groove  139  is formed between the flange  132  and the collar portion  128  of the inferior plate  104 . 
         [0045]    When the prosthesis  100  is assembled, the concave surface  118  of the superior plate  102  and the upper spherical surface  134  of the core  106  slidingly engage one another and form articular surfaces. Likewise, the concave surface  130  of the inferior plate  104  and the lower spherical surface  136  of the core  106  slidingly engage one another and form articular surfaces. 
         [0046]    A tool that may be used to position the prosthesis  100  within a patient is shown in  FIG. 3 . The intervertebral distraction instrument  140  includes a first vertebra engaging member  142 , a second vertebra engaging member  144 , and a prosthesis insertion assembly  146 . A base  148 , which in this embodiment further functions as a handle, is located between the first vertebra engaging member  142  and the second vertebra engaging member  144 . A knob  150  is located rearward of the engaging members  142  and  144 . 
         [0047]    In the embodiment of  FIG. 3 , the first vertebra engaging member  142  is provided as an upper elongated distraction arm. The vertebra engaging member  142  includes a proximal end portion  152  and a distal end portion  154 . A finger  156  extends from the vertebra engaging member  142  at the distal end portion  154 . The finger  156  is a relatively thin tab with a vertebra engaging member  158  on the upper portion of the tab. 
         [0048]    The second vertebra engaging member  144  is provided as a lower elongated distraction arm, and is generally symmetric to the upper vertebra engaging member  142 . Accordingly, the vertebra engaging member  144  includes a proximal end portion  160 , a distal end portion  162 , and a finger  164 . The finger  164  includes a vertebra engaging member  166 . 
         [0049]    The upper vertebra engaging member  142  and the lower vertebra engaging member  144  are configured such that the finger  156  and the finger  164  converge. In the embodiment of  FIG. 3 , this is accomplished by the provision of a bend  168  in the vertebra engaging member  142  and a bend  170  in the vertebra engaging member  144 . The bends  168  and  170  are located between the respective proximal end portion  152  or  160  and finger  156  or  164  such that the fingers  156  and  164  converge. 
         [0050]    The prosthesis insertion assembly  146  is positioned between the vertebra engaging member  142  and the vertebra engaging member  144 . The prosthesis insertion assembly  146  is shown with a gripper  172  in  FIG. 4  and includes an outer prosthesis insertion assembly housing  174  positioned outwardly of an inner sleeve  175 . The gripper  172  includes a stem  176  with a gap  178  that extends along a portion of the stem  176 . A pair of arms  180  and  182  are connected to the stem  176  through a neck portion  184 . A blind threaded bore  186  is located within the stem  176  at the end of the gripper  172  opposite to the arms  180  and  182 . 
         [0051]    The outer prosthesis insertion assembly housing  174  includes guide members  188 ,  190 ,  192  and  194  (see also  FIG. 5 ) while an inner channel  196  is defined by the inner sleeve  175 . A throat  198  is located within the inner channel  196  (see, e.g.,  FIG. 7 ) proximate to one end portion  200  of the prosthesis insertion assembly housing  174  while the other end portion  202  of the housing  174  is externally threaded. 
         [0052]    The prosthesis insertion assembly  146  further includes a depth control member  204  and a coupling member  206 . The depth control member  204  is rotatably engaged with the inner sleeve  175  and includes an internally threaded bore  208  which extends completely through the depth control member  204 . The coupling member  206  includes a stem  210  with a threaded portion  212 . The coupling member  206  is rotatably connected to the depth control member  204  and further includes an internal bore  214  and a release mechanism  216  which extends into the internal bore  214 . The internal bore  214  is configured to receive a coupling portion  218  of a shaft  220  shown in  FIG. 6 . The shaft  220  extends through the base  148  and is connected to the knob  150 . A threaded portion  222  of the shaft  220  threadingly engages the base  148 . 
         [0053]    Operation of the insertion distraction instrument  140  may begin with the upper and lower vertebra engaging members disconnected and the prosthesis insertion assembly  146  decoupled from the shaft  220 . In such a procedure, the desired insertion depth is set by rotation of the depth control member  204 . The desired depth, which may be shown on an indicator  223  (see  FIG. 8 ), may be established with the depth control member  204  at any time. 
         [0054]    As the depth control member  204  is rotated, the threads of the internally threaded bore  208  engage the threads of the threaded end portion  202  of the outer prosthesis insertion assembly housing  174  causing relative movement between the outer prosthesis insertion assembly housing  174  and the inner sleeve  175  to which the depth control member  204  is rotatably engaged. Accordingly, the axial position of the guides  188 ,  190 ,  192  and  194  with respect to the throat  198  within the inner channel  196  may be adjusted. The insertion depth thus identifies the desired positioning of a disc prosthesis within a spinal column along the longitudinal axis of the instrument  140  when the disc prosthesis is inserted as discussed below. 
         [0055]    Continuing with the present example, once the desired depth setting has been established, the stem  176  of the gripper  172  is inserted into the inner channel  196  through the end portion  200 . The stem  176  is sized to pass through the throat  198 . The neck portion  184 , however, is tapered from a diameter somewhat smaller than the diameter of the throat  198  to a diameter somewhat larger than the throat  198  as shown in  FIG. 7 . Accordingly, once the neck portion  184  contacts the throat  198 , further axial movement of the stem  176  into the inner channel  196  is inhibited. 
         [0056]    The disc prosthesis  100  is then coupled with the gripper  172 . In this embodiment, the gripper  172  is sized to provide a friction fit for a prosthesis  100  of a specific size. Specifically, the arms  180  and  182  are sized and shaped to frictionally engage the prosthesis  100  by insertion of the arms  180  and  182  into the slot  109  and the slot (not shown) on the lower surface  120  of the inferior plate  104 . Accordingly, a kit may include a number of different grippers for use with differently sized and/or configured disc prostheses. In this embodiment the prosthesis  100  is a modular disc prosthesis and the arms  180  and  182  are configured to hold the assembled modular disc prosthesis together as a unit. 
         [0057]    If desired, the foregoing steps may be performed in a different order if desired. For example, the gripper  172  and the disc prosthesis  100  may be coupled prior to insertion of the gripper  172  within the prosthesis insertion assembly housing  174 . The gripper  172  may then be positioned within the housing  174  as shown in  FIG. 8 . 
         [0058]    Next, the threaded portion  212  of the shaft  210  is engaged with the threads of the threaded blind bore  186  (see, e.g.,  FIG. 4 ). In the embodiment of  FIG. 4 , the coupling member  206  is rotationally coupled with the outer prosthesis insertion assembly housing  174  through the depth control member  204 . In alternative embodiments, the coupling member may be separately provided. In such embodiments, the shaft of the coupling member is inserted into the inner channel  196  to allow coupling of the coupling member and the threaded blind bore  186 . 
         [0059]    Once the gripper  172  is coupled with the coupling member  206  within the housing  174  and with the disc prosthesis  100 , rotation of the coupling member  206  causes the axial force with which the neck portion  184  is forced against the throat  198  to increase. When sufficient axial force is provided, the axial force is translated to a compressive force by the neck portion  184  being pressed against the throat  198 . The gap  178  allows the arms  180  and  182  to move toward each other as indicated by the arrows  224  and  226  in  FIG. 7  in response to the compressive force. As the arms  180  and  182  move toward each other, the diameter of the neck portion  184  narrows, allowing the stem  176  to move axially in the direction of the arrow  228 , further into the inner channel  196 . 
         [0060]    The movement of the arms  180  and  182  is limited by the physical structure of the disc prosthesis  100 . Thus, while some amount of movement may occur, once the arms  180  and  182  are firmly positioned against the disc prosthesis  100 , continued rotation of the coupling member  206  primarily increases the gripping force which the arms  180  and  182  exert against the disc prosthesis  100 , thereby providing a firm coupling. In embodiments wherein the disc prosthesis does not stop movement of the stem further into the inner bore, stops may be provided on the stem to restrict such axial movement after the desired gripping force is achieved. 
         [0061]    Because the axial location of the prosthesis  100  is fixed with respect to the neck portion  184  at this point, and because the throat  198  provides a stop for the neck portion  184 , the axial position of the prosthesis  100  with respect to the guides  188 ,  190 ,  192  and  194  is established by the depth established with the depth control member  204 . 
         [0062]    When the disc prosthesis  100  is firmly coupled with the prosthesis insertion assembly  146 , the prosthesis insertion assembly  146  is coupled to the shaft  220  by insertion of the coupling portion  218  into the internal bore  214  resulting in the configuration shown in  FIG. 9 . Next, the vertebra engaging members  142  and  144  are connected to the base  148 . As the vertebra engaging member  142  is connected, it is positioned within a space bordered by the guide members  188  and  190 . Similarly, as the vertebra engaging member  144  is connected, it is positioned within a space bordered by the guide members  192  and  194  resulting in the configuration shown in  FIG. 3 . 
         [0063]    Referring to  FIG. 10 , once a space (S) has been prepared for receipt of the disc prosthesis  100  using any acceptable procedure, the fingers  156  and  164  are inserted into the space (S). Insertion of the fingers  156  and  164  into the prepared space (S) continues until the vertebra engaging members  158  and  166  contact the vertebras located adjacent to the prepared space (S) as shown in  FIG. 10 . Next, the knob  150  is rotated in the direction of the arrow  232  while the vertebral engaging members  158  and  166  are pressed against the vertebras adjacent to the prepared space. Because the threaded portion  222  of the shaft  220  is threadingly engaged with the base  148 , rotation of the knob  150  causes the shaft  220  to move forwardly in the direction of the arrow  234  as well as rotate in the direction of the arrow  232 . 
         [0064]    The coupling portion  218  of the shaft  220  is free to rotate within the internal bore  214 . Accordingly, as the shaft  220  rotates, the coupling member  206  does not rotate. The axial movement of the shaft  220 , however, forces the prosthesis insertion assembly  146  to move forwardly in the direction of the arrow  234 . As the prosthesis insertion assembly  146  moves, alignment with the vertebra engaging members  142  and  144  is maintained by the guides  188 ,  190 ,  192  and  194 . 
         [0065]    The axially forward movement of the prosthesis insertion assembly  146  forces the disc prosthesis  100  against the distal end portions  154  and  162  of the vertebra engaging members  142  and  144 . This forces the fingers  156  and  164  against the vertebra adjacent to the prepared space (S), causing the vertebra to be forced apart and allowing the disc prosthesis  100  to move into the prepared space (S) as shown in  FIG. 11 . 
         [0066]    As the disc prosthesis  100  moves into the space (S), the guide members  188 ,  190 ,  192  and  194  come into contact with the vertebrae adjacent to the space (s). Accordingly, further forward movement of the prosthesis insertion assembly  146  is restricted. Thus, as the knob  150  continues to be rotated in the direction of the arrow  232 , the threaded portion  222  of the shaft  220  forces the base  148  to move in the direction of the arrow  236 , thereby pulling the fingers  156  and  164  out of the space (S) while the disc prosthesis  100  remains in the space (S) as shown in  FIG. 12 . 
         [0067]    Once the fingers  156  and  164  are clear of the space (S), the natural forces applied to the spinal column by the soft tissue attached to the spinal column will press the vertebrae adjacent to the space (S) against the disc prosthesis  100 . Thus, the teeth  114  are imbedded into the adjacent vertebrae, fixing the disc prosthesis  100  in place. If desired, the prosthesis insertion assembly  146  may be decoupled from the disc prosthesis  100  simply by forcing the distraction instrument  140  away from the spine to overcome the friction lock. 
         [0068]    Alternatively, the prosthesis insertion assembly  146  may be detached from the rest of the distraction instrument  140  as shown in  FIG. 13  by depression of the release mechanism  216 , which allows the shaft  220  to be removed from the internal bore  214 . Next, the coupling member  206  is rotated in the direction indicated by the arrow  238 . Such rotation of the coupling member  206  causes the neck portion  184  of the gripper  172  to be forced away from the throat  198  of the inner sleeve  175 . Thus, the resilient nature of the gripper  172  forces the arms  180  and  182  in a direction away from the disc prosthesis  100 . 
         [0069]    The rotation of the coupling member  206  reduces the coupling force between the gripper  172  and the disc prosthesis  100 . Accordingly, the gripper  172  may be decoupled from the disc prosthesis  100  by pulling on the coupling member  206 . 
         [0070]      FIG. 14  shows an alternative gripper  240  which may be used with the prosthesis insertion assembly  146  of  FIG. 5 . The gripper  240  includes a coupling portion  242 , a neck portion  244  and a stem  246  in an unstressed condition. The coupling portion  242  includes a slit  248  and a slit  250  which extend through the coupling portion  242  and the neck portion  244  into the stem  246 . The slits  248  and  250  define two opposing pairs of arms  252  and  254  in the coupling portion  242  (only one arm of arm pair  254  is shown in  FIG. 14 ). The neck portion  244  tapers from a larger diameter at the coupling portion  242  to a smaller diameter at the stem  246 . The stem  246  includes a threaded inner bore  256  which is configured to be engaged with the threaded portion  212  of the coupling member  206 . 
         [0071]    The coupling portion  242  of the gripper  240  is configured to mate with an artificial disc such as the artificial disc  260  shown in  FIG. 15 . The artificial disc  260  includes two endplates  262  and  264  which are separated by a core  266 . Each of the two endplates  262  and  264  include a number of engagement members  268 . In the embodiment of  FIG. 15 , the engagement members  268  are generally in the shape of a cone, with the apex  270  of the engagement members  268  spaced apart from the respective endplate  262  or  264 . In alternative embodiments, the engagement members may be pyramidal, conical, or another shape. Preferably, the portions of the engagement members farthest away from the endplates, such as the apex of the engagement members  268 , are relatively sharp. 
         [0072]    The endplates  262  and  264  further include four notches including notches  272  and  278  and two notches including the notch  280  and another notch not shown) that are symmetrical and spaced apart from the notches  272  and  278  to form two notch pairs. By way of example, the notch  280  which is shown in  FIG. 16  in shadow form, is the symmetrical to and spaced apart notch for the notch  272 . Thus, the notch  272  and the notch  280  are a notch pair. 
         [0073]    The four notches,  272 ,  278 ,  280 , and the notch not shown, are sized and shaped to snugly mate with the arms in the arm pairs  252  and  254 . Moreover, the distance between each of the notches in the notch pairs is substantially the same as the distance between the opposing arms of the arm pairs  254  and  256  when the arm pairs  252  and  254  are in an unstressed condition. The configuration of the notches including shape and location, may be modified to optimize the control over the implant based upon the approach being used. For example, some implants may be configurec to be used in an anterior approach whereas other may be configured for use in posterior or other approaches. 
         [0074]    The gripper  240  is used in much the same manner as the gripper  172  described above. One difference, however, is that the configuration of the gripper  240  and the artificial disc  260  allows for a tighter coupling. Specifically, as depicted in  FIG. 16 , the arm pairs  252  and  254  engage the notches,  272 ,  278 ,  280 , and the notch not shown, in a positive engagement as the individual arms are positioned within the notches,  272 ,  278 ,  280 , and the notch not shown. Accordingly, when the neck portion  244  is pulled against the throat  198  by rotation of the coupling member  206  as described above, the axial force is translated to a compressive force whereby the arm pairs  252  and  254  engage the notches,  272 ,  278 ,  280 , and the notch not shown more tightly and a very tight coupling is achieved between the gripper  240  and the artificial disc  260 . Thus, the potential for unintentional decoupling is reduced. 
         [0075]    The prosthesis insertion assembly  146  may also be used with other distraction instruments such as the distraction instrument  290  shown in the system of  FIGS. 17 and 18 . The distraction instrument  290  includes a first vertebra engaging member  292  and a second vertebra engaging member  294 . A handle  296  is provided on the instrument  290  along with a pivot assembly  298 . A ratchet assembly  300  is located on the handle  296 . 
         [0076]    Fingers  302  and  304  extend from the vertebra engaging members  292  and  294 , respectively. The upper vertebra engaging member  292  and the lower vertebra engaging member  294  are configured such that the finger  302  and the finger  304  converge from an insertion opening  306  defined by the upper vertebra engaging member  292  and the lower vertebra engaging member  294 . 
         [0077]    In the system of  FIG. 17 , a gripper  308  is coupled with a disc prosthesis  310  in the manner described above. An extension handle  318  is configured similarly to the coupling member  206  except that the extension handle  318  is longer than the coupling member  206 . Thus, in a manner similar to the coupling member  206 , the extension handle  318  is coupled to the disc prosthesis  310 . 
         [0078]    Operation of the system of  FIG. 17  proceeds in a manner similar to the procedure described above with respect to the distraction instrument  140 . One difference is that as shown in  FIG. 17 , the insertion assembly  146  need not be coupled with the distraction instrument  290  prior to the insertion of the fingers  302  and  304  into the space prepared for the disc prosthesis  310 . Thus, after positioning the fingers  302  and  304  into the space prepared for the disc prosthesis  310 , the extension handle  318  is used to manipulate the gripper  308  into position within the insertion opening  306 . 
         [0079]    When the gripper  308  is positioned within the insertion opening  306 , the guide members  188  and  190  engage the upper vertebra engaging member  292  and the guide members  192  and  194  engage the lower vertebra engaging member  294 . Thus, the disc prosthesis  310  is placed into the desired alignment. Either before or after the guide members  188 ,  190 ,  192  and  194  engage the upper vertebra engaging member  292  and the lower vertebra engaging member  294 , the handle  296  is compressed, causing the upper vertebra engaging member  292  and the lower vertebra engaging member  294  to separate, thereby distracting the vertebra adjacent to the prepared space. As the handle  296  is compressed, the ratchet assembly  300  maintains the handle  296  in a compressed condition. 
         [0080]    The disc prosthesis  310  is then positioned in the prepared space by guiding the gripper  308  toward the space with the guide members  188 ,  190 ,  192  and  194  engaging the upper vertebra engaging member  292  and the lower vertebra engaging member  294  until the guide members  188 ,  190 ,  192  and  194  contact the vertebra adjacent to the prepared space as shown in  FIG. 19 . In this condition, the disc prosthesis  310  is positioned within the spine at the depth set using the depth control member  304 . 
         [0081]    Next, the distraction instrument  290  is removed by releasing the ratchet assembly  300 . This allows the compressive force exerted on the spine by the surrounding soft tissue to force the vertebras adjacent to the space with the disc prosthesis  310  toward each other. This in turn forces the fingers  302  and  304  toward each other as the adjacent vertebras are pressed onto the teeth on the endplates of the disc prosthesis  310  resulting in the configuration of  FIG. 20 . Removal of the distraction instrument  290 , the prosthesis insertion assembly  146  and the gripper  308  may then be accomplished in like manner to the previously set forth description. 
         [0082]    While the present invention has been illustrated by the description of exemplary processes and system components, and while the various processes and components have been described in considerable detail, applicant does not intend to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will also readily appear to those ordinarily skilled in the art. The invention in its broadest aspects is therefore not limited to the specific details, implementations, or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.