Artificial disc implant

Abstract

An artificial disc implant includes an upper shell, a lower shell, and a spacer therebetween. The spacer preferably has properties similar to that of a natural spinal disc, while the upper and lower shells form a rigid interface between the implant and the adjacent vertebral bodies. The upper and lower shells can be configured to prevent expulsion of the spacer from the disc space. The implant upper and lower shells may further be configured into partially cylindrical shapes for ease of insertion through an insertion tube as presently known for interbody fusion devices. The devices may further be configured for insertion through a double-barreled insertion tube. Methods and instruments for inserting an artificial disc implant are also provided.

1 - 53 . (canceled) 
   
   
       54 . An artificial disc implant, comprising:
 an upper member having a first surface to engage a first vertebra, said upper member including no more than one upper bone engaging member extending from said first surface along at least a portion of said upper member, said upper bone engaging member for engaging the first vertebra;   a lower member having a second surface to engage a second vertebra, said lower member including no more than one lower bone engaging member extending from said second surface along at least a portion of said lower member, said lower bone engaging member for engaging the second vertebra; and   a spacer between said upper member and said lower member.   
   
   
       55 . The implant of  claim 54 , wherein said upper bone engaging member extends between opposite ends of said upper member and said lower bone engaging member extends between opposite ends of said lower member. 
   
   
       56 . The implant of  claim 54 , wherein said spacer extends between and contacts said upper and lower members. 
   
   
       57 . The implant of  claim 54 , wherein said spacer is compressible. 
   
   
       58 . The implant of  claim 54 , wherein said upper member and said lower member are movable relative to one another. 
   
   
       59 . The implant of  claim 58 , wherein:
 said upper member defines a cavity;   said lower member defines a cavity; and   said spacer is positioned in said cavities of said upper and lower members.   
   
   
       60 . The implant of  claim 54 , wherein said upper member and said lower member together define a substantially cylindrical implant. 
   
   
       61 . The implant of  claim 54 , wherein said upper bone engaging member is an elongated rib and said lower bone engaging member is an elongated rib. 
   
   
       62 . An artificial disc implant, comprising:
 an upper member having a first surface to engage a first vertebra, said upper member including a single upper bone engaging member extending along at least a portion of said upper member, said upper bone engaging member for engaging the first vertebra;   a lower member having a second surface to engage a second vertebra, said lower member including a single lower bone engaging member extending along at least a portion of said lower member, said lower bone engaging member for engaging the second vertebra; and   a spacer between said upper member and said lower member.   
   
   
       63 . The implant of  claim 62 , wherein said upper bone engaging member is an elongated rib and said lower bone engaging member is an elongated rib. 
   
   
       64 . The implant of  claim 62 , wherein said upper bone engaging member and said lower bone engaging member extend longitudinally along a length of respective ones of said upper member and said lower member. 
   
   
       65 . The implant of  claim 62 , wherein said spacer is compressible. 
   
   
       66 . The implant of  claim 62 , wherein:
 said upper bone engaging member projects from said first surface away from said lower member; and   said lower bone engaging member projects from said second surface away from said upper member.   
   
   
       67 . The implant of  claim 66 , wherein said upper member is movable relative to said lower member. 
   
   
       68 . The implant of  claim 67 , wherein:
 said upper member defines a cavity;   said lower member defines a cavity; and   said spacer is positioned in said cavities of said upper and lower members.   
   
   
       69 . An artificial disc implant, comprising:
 an upper member having a first surface to engage a first vertebra, said upper member including a single upper bone engaging member extending along at least a portion of said upper member, said upper bone engaging member for engaging the first vertebra;   a lower member having a second surface to engage a second vertebra, said lower member including a single lower bone engaging member extending along at least a portion of said lower member, said lower bone engaging member for engaging the second vertebra; and   a spacer between said upper member and said lower member, wherein said upper member and said lower member are movable relative to one another with said spacer therebetween.   
   
   
       70 . The implant of  claim 69 , wherein:
 said upper bone engaging member projects from said first surface away from said lower member; and   said lower bone engaging member projects from said second surface away from said upper member.   
   
   
       71 . The implant of  claim 70 , wherein said upper bone engaging member is an elongated rib and said lower bone engaging member is an elongated rib. 
   
   
       72 . The implant of  claim 71 , wherein said upper bone engaging member and said lower bone engaging member extend longitudinally along a length of respective ones of said upper member and said lower member. 
   
   
       73 . The implant of  claim 72 , wherein:
 said upper member defines a cavity;   said lower member defines a cavity; and   said spacer is positioned in said cavities of said upper and lower members.   
   
   
       74 . An intervertebral implant insertable between adjacent vertebrae, comprising,
 an upper part having an upper surface for engaging a vertebrae and a lower surface which includes a rounded portion,   a lower part having a lower surface for engaging a vertebrae and an upper surface portion in operative engagement with the rounded portion of the upper part,   said implant being constructed to be the sole implant in its intervertebral space,   the implant having a lead end which leads as the implant is inserted along a path into the intervertebral space and a trailing end opposite the lead end, and lateral planes which pass through the outermost boundaries of the implant and parallel to the said path, and   a single anchor on each of the upper surface of the upper part and the lower surface of the lower part, each said anchor being elongated, having a height greater than its width, and located along a line parallel to said path, the two anchors lying essentially in the same vertical plane, which plane is essentially midway between said lateral planes, each said anchor being adapted to enter a groove in the adjacent vertebrae as the implant moves along said path into the intervertebral space, to anchor its respective part to the vertebrae which its surface engages.   
   
   
       75 . An intervertebral implant according to  claim 74 , wherein the rounded portion of the upper part is concave and the upper surface portion of the lower part is convex. 
   
   
       76 . An intervertebral implant according to  claim 75 , wherein the lower part is formed in two pieces including a lower piece which has the said lower surface and an upper piece which has the upper surface portion. 
   
   
       77 . An intervertebral implant according to  claim 74 , the length of at least one anchor in the direction along the midline being greater than one half of the entire front to rear dimension of its respective part. 
   
   
       78 . An intervertebral implant according to  claim 77 , wherein the length of both anchors in the direction along the midline are greater than one half of the entire front to rear dimension of their respective upper or lower part. 
   
   
       79 . An intervertebral implant according to  claim 74 , wherein the rounded portion of the upper part and the upper surface portion of the lower part are spherical. 
   
   
       80 . An intervertebral implant according to  claim 74 , the upper and lower parts, in plan view, being generally rectangular. 
   
   
       81 . An intervertebral implant according to  claim 80 , wherein the single anchors are substantially parallel to the shorter sides of the generally rectangular shape of the upper and lower parts. 
   
   
       82 . An intervertebral implant according to  claim 74 , wherein the rounded portion of the upper part is partially spherical and concave, and the upper surface portion of the lower part is partially spherical and convex. 
   
   
       83 . An intervertebral implant according to  claim 82 , wherein the lower part is formed in two pieces, including a lower piece which has the lower surface thereon and an upper piece which fits into a recess in the lower piece and has the said partially spherical and convex portion. 
   
   
       84 . An intervertebral implant according to  claim 83 , wherein, in the absence of the upper piece of the lower part, the rounded portion of the upper part can nest into the recess in the lower piece. 
   
   
       85 . An intervertebral implant according to  claim 84 , wherein, in the nested condition, the overall height of the upper and lower parts is less than the additive total height of the upper and lower parts, taken separately. 
   
   
       86 . An intervertebral implant insertable between adjacent vertebrae, comprising,
 a generally rectangular upper part having an upper surface for engaging a vertebrae and a lower surface which includes a rounded portion,   a generally rectangular lower part having a lower surface for engaging a vertebrae and having an upper surface portion in operative engagement with the rounded portion of the upper part,   said implant being constructed to be the sole implant in its intervertebral space,   one of the longer sides of the generally rectangular upper and lower parts comprising a leading end and the other of the longer sides of the generally rectangular upper and lower parts being a trailing end as the implant is moved in an insertion direction to be inserted into the intervertebral space, and   a single anchor on each of the upper surface of the upper part and the lower surface of the lower part, the two single anchors being elongated, each having a height greater than its width, and the two single anchors lying in essentially the same vertical plane, which plane is located essentially midway between the short sides of the generally rectangular upper and lower parts and thus parallel to the insertion direction, and each anchor being adapted to enter a groove in the adjacent vertebrae as the implant moves in said insertion direction into the intervertebral space, to anchor its respective part to the vertebrae which its surface engages.   
   
   
       87 . An intervertebral implant according to  claim 86 , wherein the rounded portion of the upper part is concave and the upper surface portion of the lower part is convex. 
   
   
       88 . An intervertebral implant according to  claim 87 , wherein the lower part is formed in two pieces including a lower piece which has the said lower surface and an upper piece which has the upper surface portion. 
   
   
       89 . An intervertebral implant according to  claim 86 , the length of at least one anchor in the insertion direction being greater than one half of the entire dimension of its respective part in the insertion direction from said leading end to said trailing end. 
   
   
       90 . An intervertebral implant according to  claim 89 , wherein the length of both anchors in the insertion direction are greater than one half of the entire dimension of their respective upper or lower part in the insertion direction from said leading end to said trailing end. 
   
   
       91 . An intervertebral implant according to  claim 86 , wherein the rounded portion of the upper part and the upper surface portion of the lower part are spherical. 
   
   
       92 . An intervertebral implant according to  claim 86 , wherein the rounded portion of the upper part is partially spherical and concave, and the upper surface portion of the lower part is partially spherical and convex. 
   
   
       93 . An intervertebral implant according to  claim 92 , wherein the lower part is formed in two pieces, including a lower piece which has the lower surface thereon and an upper piece which fits into a recess in the lower piece and has the said partially spherical convex portion. 
   
   
       94 . An intervertebral implant according to  claim 93 , wherein, in the absence of the upper piece of the lower part, the rounded portion of the upper part can nest into the recess in the lower part. 
   
   
       95 . An intervertebral implant according to  claim 94 , wherein, in the nested condition, the overall height of the upper and lower parts is less than the additive total height of the upper and lower parts, taken separately. 
   
   
       96 . An intervertebral implant comprising:
 a first part having an outer surface adapted for engaging a first vertebra;   a second part having an outer surface adapted for engaging a second vertebra;   the first and second parts adapted for operative engagement with each other;   wherein the first and second parts each have midlines on their respective outer surfaces passing essentially through the center of the outer surface of the respective part;   an anchor extending outwardly essentially from a midline of the outer surface of the first part, the anchor being adapted to enter a groove in the first vertebra as the implant moves into an intervertebral space;   an anchor extending outwardly essentially from a midline of the outer surface of the second part, the anchor being adapted to enter a groove in the second vertebra as the implant moves into an intervertebral space; and   wherein at least one of the anchors has a height greater than its width.   
   
   
       97 . An intervertebral implant according to  claim 96 , wherein each anchor has a height greater than its width. 
   
   
       98 . An intervertebral implant according to  claim 97 , wherein at least one anchor is elongated along said midline of the outer surface of its respective part. 
   
   
       99 . An intervertebral implant according to  claim 98 , wherein each anchor is elongated along said midline of the outer surface of its respective part. 
   
   
       100 . An intervertebral implant according to  claim 99 , wherein each anchor is elongated in a plane parallel to a path of insertion of the implant. 
   
   
       101 . An intervertebral implant according to  claim 100 , the implant having an anterior and a posterior, and wherein each anchor is elongated in an anterior to posterior plane of the implant. 
   
   
       102 . An intervertebral implant according to  claim 97 , wherein the two anchors lie essentially along a common plane passing through each of the respective midlines. 
   
   
       103 . An intervertebral implant according to  claim 97 , wherein at least one anchor is perpendicular to the outer surface of its respective part. 
   
   
       104 . An intervertebral implant according to  claim 103 , wherein each anchor is perpendicular to the outer surface of its respective part. 
   
   
       105 . An intervertebral implant according to  claim 97 , wherein the first and second parts of the implant are adapted for implantation together along a path of insertion into an intervertebral space. 
   
   
       106 . An intervertebral implant according to  claim 105 , the implant having an anterior and a posterior, and wherein said path of insertion is in an anterior to posterior direction. 
   
   
       107 . An intervertebral implant according to  claim 105 , wherein the anchors are essentially coplanar and vertically aligned with each other, such that the anchors of the first and second parts of the implant can be inserted together along a path of insertion into respective essentially vertically aligned grooves in the first and second vertebrae. 
   
   
       108 . An intervertebral implant according to  claim 107 , wherein the first and second parts are operatively engaged to allow pivotability of said first part relative to said second part. 
   
   
       109 . An intervertebral implant according to  claim 107 , wherein the first and second parts are operatively engaged to allow articulating motion between said first and second parts. 
   
   
       110 . An intervertebral implant according to  claim 96 , wherein at least one outer surface of the respective parts is adapted for contacting its respective vertebra. 
   
   
       111 . An intervertebral implant according to  claim 96 , wherein at least one outer surface of the respective parts is adapted for anchoring to its respective vertebra. 
   
   
       112 . An intervertebral implant according to  claim 96 , wherein the first and second parts includes a convex surface on one of the parts adapted for engaging with a mating concave surface on the other part. 
   
   
       113 . An intervertebral implant according to  claim 112 , wherein one of the first and second parts comprises two pieces, including a first piece which includes the anchor of the part and a second piece which includes said convex or concave surface. 
   
   
       114 . An intervertebral implant according to  claim 96 , wherein the implant is adapted for use as a total disc replacement. 
   
   
       115 . An intervertebral implant according to  claim 96 , wherein the implant is constructed to be the sole implant in its intervertebral space. 
   
   
       116 . An intervertebral implant according to  claim 96 , wherein the anchors are essentially coplanar and vertically aligned with each other, such that the two anchors can be inserted into respective essentially vertically aligned grooves formed in the first and second vertebrae. 
   
   
       117 . An intervertebral implant according to  claim 96 , wherein the overall length of at least one anchor in the direction of a path of insertion of the implant is greater than one half of the overall dimension of its respective first or second part in the direction of insertion. 
   
   
       118 . An intervertebral implant according to  claim 117 , wherein the overall length of each anchor in the direction of the path of insertion of the implant is greater than one half of the overall dimension of its respective first or second part in the direction of insertion. 
   
   
       119 . An intervertebral implant according to  claim 96 , the implant having an anterior and a posterior, and wherein the overall anterior to posterior length of each anchor is greater than one half of the overall anterior to posterior dimension of its respective first or second part. 
   
   
       120 . An intervertebral implant according to  claim 96 , wherein at least one anchor is a single uninterrupted solid piece extending outwardly from the outer surface of its respective part. 
   
   
       121 . An intervertebral implant according to  claim 96 , wherein at least one anchor is a single piece elongated in a plane parallel to a path of insertion of the implant. 
   
   
       122 . An intervertebral implant according to  claim 121 , wherein both anchors are single pieces elongated in respective planes that are parallel to the path of insertion of the implant. 
   
   
       123 . An intervertebral implant according to  claim 96 , wherein at least one anchor is elongated along and straddles said midline of the outer surface of its respective part. 
   
   
       124 . An intervertebral implant according to  claim 123 , wherein both anchors are elongated along and straddle said midline of the outer surface of its respective part. 
   
   
       125 . An intervertebral implant according to  claim 96 , wherein one or more of the first part outer surface or the second part outer surface further comprises at least one protrusion oriented outwardly from the respective outer surface with a height no greater than the width of the protrusion. 
   
   
       126 . An intervertebral implant according to  claim 96 , wherein one or more of the first part outer surface or the second part outer surface further comprises at least one protrusion oriented outwardly from the respective surface with a height substantially less than the height of the anchor of the respective surface. 
   
   
       127 . An intervertebral implant according to  claim 96 , wherein at least one of the first and second parts is, in plan view, generally rectangular in shape. 
   
   
       128 . An intervertebral implant according to  claim 127 , wherein both of the first and second parts are, in plan view, generally rectangular in shape. 
   
   
       129 . A component for an intervertebral implant, which component is adapted to cooperate with at least one other component to form an intervertebral implant, the component comprising:
 an outer surface adapted to engage a first vertebra and an inner surface adapted to operatively engage another component of the implant, wherein the component has midlines on the outer surface passing essentially through the center of the outer surface; and   an anchor extending outwardly essentially from a midline of the outer surface with a height greater than its width and adapted to enter a groove in the first vertebra as the component moves into an intervertebral space.   
   
   
       130 . A component for an intervertebral implant according to  claim 129 , wherein the anchor is elongated in a plane parallel to a path of insertion of the component. 
   
   
       131 . A component for an intervertebral implant according to  claim 130 , wherein the overall length of the anchor in the direction of the path of insertion of the component is greater than one half of the overall dimension of the component in the direction of insertion. 
   
   
       132 . A component for an intervertebral implant according to  claim 129 , wherein the anchor is a single uninterrupted solid piece extending outwardly from the outer surface of the part. 
   
   
       133 . A component for an intervertebral implant according to  claim 129 , the component having an anterior and a posterior, and wherein said midline is at an anterior to posterior plane of the component. 
   
   
       134 . A component for an intervertebral implant according to  claim 129 , the component having an anterior and a posterior, and wherein the anchor is elongated in an anterior to posterior plane of the component. 
   
   
       135 . A component for an intervertebral implant according to  claim 134 , wherein the anterior to posterior plane is essentially at said midline. 
   
   
       136 . A component for an intervertebral implant according to  claim 135 , wherein the overall anterior to posterior length of the anchor is greater than one half of the overall anterior to posterior dimension of the component. 
   
   
       137 . A component for an intervertebral implant according to  claim 129 , wherein the anchor is elongated along and straddles the midline. 
   
   
       138 . A component for an intervertebral implant according to  claim 129 , wherein the anchor is perpendicular to the outer surface. 
   
   
       139 . A component for an intervertebral implant according to  claim 129 , wherein the outer surface of the component is adapted for contacting the vertebrae. 
   
   
       140 . A component for an intervertebral implant according to  claim 129 , wherein the outer surface comprises at least one protrusion oriented outwardly from surface with a height no greater than the width of the protrusion. 
   
   
       141 . A component for an intervertebral implant according to  claim 129 , wherein the outer surface comprises at least one protrusion oriented outwardly from the surface with a height substantially less than the height of the anchor. 
   
   
       142 . A component for an intervertebral implant according to  claim 129 , wherein the component is, in plan view, generally rectangular in shape. 
   
   
       143 . A component for an intervertebral implant according to  claim 129 , wherein the component has a second surface in opposed relation to said outer surface, the second surface being adapted to engage said other component of said intervertebral implant. 
   
   
       144 . An intervertebral implant comprising:
 a first part having an outer surface adapted for engaging a first vertebra;   a second part having an outer surface adapted for engaging a second vertebra;   the first and second parts adapted for operative engagement with each other;   wherein the first and second parts each have midlines on their respective outer surfaces passing essentially through the center of the outer surface of the respective part;   an anchor extending outwardly essentially from a midline of the outer surface of the first part, wherein said anchor is elongated along and straddles said midline;   an anchor extending outwardly essentially from a midline of the outer surface of the second part, wherein said anchor is elongated along and straddles said midline; and   wherein at least one of the anchors has a height greater than its width.   
   
   
       145 . An intervertebral implant according to  claim 144 , wherein at least one anchor is adapted to enter a groove in the first or second vertebra as the implant moves into an intervertebral space. 
   
   
       146 . An intervertebral implant according to  claim 145 , wherein each anchor is adapted to enter a groove in the first or second vertebra as the implant moves into an intervertebral space. 
   
   
       147 . An intervertebral implant according to  claim 144 , wherein each anchor has a height greater than its width. 
   
   
       148 . An intervertebral implant according to  claim 147 , the implant having an anterior and a posterior, and wherein each anchor is elongated in an anterior to posterior plane of the implant. 
   
   
       149 . An intervertebral implant according to  claim 147 , wherein the two anchors lie essentially along a common plane passing through each of the respective midlines. 
   
   
       150 . An intervertebral implant according to  claim 147 , wherein at least one anchor is perpendicular to the outer surface of its respective part. 
   
   
       151 . An intervertebral implant according to  claim 150 , wherein each anchor is perpendicular to the outer surface of its respective part. 
   
   
       152 . An intervertebral implant according to  claim 147 , wherein the first and second parts of the implant are adapted for implantation together along a path of insertion into an intervertebral space. 
   
   
       153 . An intervertebral implant according to  claim 152 , the implant having an anterior and a posterior, and wherein said path of insertion is in an anterior to posterior direction. 
   
   
       154 . An intervertebral implant according to  claim 152 , wherein the anchors are essentially coplanar and vertically aligned with each other, such that the anchors of the first and second parts of the implant can be inserted together along a path of insertion into respective essentially vertically aligned grooves in the first and second vertebrae. 
   
   
       155 . An intervertebral implant according to  claim 154 , wherein the first and second parts are operatively engaged to allow pivotability of said first part relative to said second part. 
   
   
       156 . An intervertebral implant according to  claim 154 , wherein the first and second parts are operatively engaged to allow articulating motion between said first and second parts. 
   
   
       157 . An intervertebral implant according to  claim 144 , wherein at least one outer surface of the respective parts is adapted for contacting its respective vertebra. 
   
   
       158 . An intervertebral implant according to  claim 144 , wherein at least one outer surface of the respective parts is adapted for anchoring to its respective vertebra. 
   
   
       159 . An intervertebral implant according to  claim 144 , wherein the first and second parts includes a convex surface on one of the parts for engaging with a mating concave surface on the other part. 
   
   
       160 . An intervertebral implant according to  claim 159 , wherein one of the first and second parts comprises two pieces, including a first piece which includes the anchor of the part and a second piece which includes said convex or concave surface. 
   
   
       161 . An intervertebral implant according to  claim 144 , wherein the implant is adapted for use as a total disc replacement. 
   
   
       162 . An intervertebral implant according to  claim 144 , wherein the implant is constructed to be the sole implant in its intervertebral space. 
   
   
       163 . An intervertebral implant according to  claim 144 , wherein the anchors are essentially coplanar and vertically aligned with each other, such that the two anchors can be inserted into respective essentially vertically aligned grooves formed in the first and second vertebrae. 
   
   
       164 . An intervertebral implant according to  claim 144 , wherein the overall length of at least one anchor in the direction of a path of insertion of the implant is greater than one half of the overall dimension of its respective first or second part in the direction of insertion. 
   
   
       165 . An intervertebral implant according to  claim 164 , wherein the overall length of each anchor in the direction of the path of insertion of the implant is greater than one half of the overall dimension of its respective first or second part in the direction of insertion. 
   
   
       166 . An intervertebral implant according to  claim 144 , the implant having an anterior and a posterior, and wherein the overall anterior to posterior length of each anchor is greater than one half of the overall anterior to posterior dimension of its respective first or second part. 
   
   
       167 . An intervertebral implant according to  claim 144 , wherein at least one anchor is a single uninterrupted solid piece extending outwardly from the outer surface of its respective part. 
   
   
       168 . An intervertebral implant according to  claim 144 , wherein at least one anchor is a single piece elongated in a plane parallel to a path of insertion of the implant. 
   
   
       169 . An intervertebral implant according to  claim 168 , wherein both anchors are single pieces elongated in respective planes that are parallel to the path of insertion of the implant. 
   
   
       170 . An intervertebral implant according to  claim 144 , wherein one or more of the first part outer surface or the second part outer surface further comprises at least one protrusion oriented outwardly from the respective outer surface with a height no greater than the width of the protrusion. 
   
   
       171 . An intervertebral implant according to  claim 144 , wherein one or more of the first part outer surface or the second part outer surface further comprises at least one protrusion oriented outwardly from the respective surface with a height substantially less than the height of the anchor of the respective surface. 
   
   
       172 . An intervertebral implant according to  claim 144 , wherein at least one of the first and second parts is, in plan view, generally rectangular in shape. 
   
   
       173 . An intervertebral implant according to  claim 172 , wherein both of the first and second parts are, in plan view, generally rectangular in shape. 
   
   
       174 . A component for an intervertebral implant, which component is adapted to cooperate with at least one other component to form an intervertebral implant, the component comprising:
 an outer surface adapted to engage a first vertebra and an inner surface adapted to operatively engage another component of the implant, wherein the component has midlines on the outer surface passing essentially through the center of the outer surface; and   an anchor extending outwardly essentially from a midline of the outer surface with a height greater than its width, wherein said anchor is elongated along and straddles said midline.   
   
   
       175 . A component for an intervertebral implant according to  claim 174 , wherein said anchor is elongated in a plane parallel to a path of insertion of the component. 
   
   
       176 . A component for an intervertebral implant according to  claim 175 , wherein the overall length of the anchor in the direction of the path of insertion of the component is greater than one half of the overall dimension of the component in the direction of insertion. 
   
   
       177 . A component for an intervertebral implant according to  claim 174 , wherein the anchor is a single uninterrupted solid piece extending outwardly from the outer surface of the part. 
   
   
       178 . A component for an intervertebral implant according to  claim 174 , wherein the overall anterior to posterior length of the anchor is greater than one half of the overall anterior to posterior dimension of the component. 
   
   
       179 . A component for an intervertebral implant according to  claim 174 , wherein the anchor is perpendicular to the outer surface. 
   
   
       180 . A component for an intervertebral implant according to  claim 174 , wherein the outer surface of the component is adapted for contacting the vertebrae. 
   
   
       181 . A component for an intervertebral implant according to  claim 174 , wherein the anchor is adapted to enter a groove in the first vertebra as the component moves into an intervertebral space. 
   
   
       182 . A component for an intervertebral implant according to  claim 174 , wherein the outer surface comprises at least one protrusion oriented outwardly from surface with a height no greater than the width of the protrusion. 
   
   
       183 . A component for an intervertebral implant according to  claim 174 , wherein the outer surface comprises at least one protrusion oriented outwardly from the surface with a height substantially less than the height of the anchor. 
   
   
       184 . A component for an intervertebral implant according to  claim 174 , wherein the component is, in plan view, generally rectangular in shape. 
   
   
       185 . A component for an intervertebral implant according to  claim 174 , wherein the component has a second surface in opposed relation to said outer surface, the second surface being adapted to engage said other component of said intervertebral implant. 
   
   
       186 . An artificial disc implant, comprising:
 an upper shell having an upper bone contacting surface for engaging an upper vertebral body, and having a first interior bearing wall including an arcuate portion;   a lower shell having a lower bone contacting surface for engaging a lower vertebral body, and having an upper surface portion in operative engagement with the arcuate portion of said upper shell, the upper vertebral body being adjacent the lower vertebral body to provide an intervertebral space;   said implant having a leading end, which leads as said implant is inserted into the intervertebral space, and a trailing end opposite the leading end, and having a longitudinal axis extending from the leading end to the trailing end; and   a single central rib on each of said upper shell and said lower shell, wherein each said single central rib extends longitudinally along the longitudinal axis of said implant.   
   
   
       187 . An artificial disc implant according to  claim 186 , wherein the location of said single central rib on said upper shell corresponds to the location of said single central rib on said lower shell. 
   
   
       188 . An artificial disc implant according to  claim 186 , wherein said single central ribs both lie in the same plane. 
   
   
       189 . An artificial disc implant according to  claim 188 , wherein the plane intersects the longitudinal axis of said implant. 
   
   
       190 . An artificial disc implant according to  claim 186 , wherein said single central ribs are single central anchors. 
   
   
       191 . An artificial disc implant according to  claim 186 , wherein at least one of said single central ribs has a height greater than its width. 
   
   
       192 . An artificial disc implant according to  claim 186 , wherein each said single central rib is adapted to retain its respective shell in the intervertebral space. 
   
   
       193 . An artificial disc implant according to  claim 186 , wherein each said single central rib is adapted to enter a groove in its respective vertebral body as said implant moves along a path into the intervertebral space, to anchor its respective shell to its respective vertebral body. 
   
   
       194 . An artificial disc implant according to  claim 186 , wherein said implant is constructed to be the sole implant in the intervertebral space. 
   
   
       195 . An artificial disc implant according to  claim 186 , wherein the arcuate portion is concave and the upper surface portion is convex. 
   
   
       196 . An artificial disc implant according to  claim 195 , wherein said lower shell is formed in two pieces including a lower piece which has the lower bone contacting surface and an upper piece which has the upper surface portion. 
   
   
       197 . An artificial disc implant according to  claim 186 , wherein the length of at least one said single central rib in the direction along the longitudinal axis is greater than one half of the distance from the leading end to the trailing end along the longitudinal axis. 
   
   
       198 . An artificial disc implant according to  claim 197 , wherein the length of both said single central ribs in the direction along the longitudinal axis is greater than one half of the distance from the leading end to the trailing end along the longitudinal axis. 
   
   
       199 . An artificial disc implant according to  claim 186 , wherein the arcuate portion of said upper shell is partially cylindrical. 
   
   
       200 . An artificial disc implant according to  claim 186 , wherein said upper and lower shells, in plan view, are substantially rectangular. 
   
   
       201 . An artificial disc implant according to  claim 186 , wherein the arcuate portion is partially spherical and concave, and the upper surface portion is partially spherical and convex. 
   
   
       202 . An artificial disc implant according to  claim 201 , wherein said lower shell is formed in two pieces including a lower piece which has the lower bone contacting surface and an upper piece which has the upper surface portion. 
   
   
       203 . An artificial disc implant according to  claim 202 , wherein, in the absence of the upper piece of said lower shell, the arcuate portion of said upper shell can nest into said lower shell. 
   
   
       204 . An artificial disc implant according to  claim 203 , wherein, in the nested condition, the overall height of said upper and lower shells is less than the additive total height of said upper and lower shells, taken separately. 
   
   
       205 . An artificial disc implant, comprising:
 a substantially rectangular upper shell having an upper bone contacting surface for engaging an upper vertebral body, and having a first interior bearing wall including an arcuate portion;   a substantially rectangular lower shell having a lower bone contacting surface for engaging a lower vertebral body, and having an upper surface portion in operative engagement with the arcuate portion of said upper shell, the upper vertebral body being adjacent the lower vertebral body to provide an intervertebral space;   said implant having a leading end, which leads as said implant is inserted into the intervertebral space, and a trailing end opposite the leading end, and having a longitudinal axis extending from the leading end to the trailing end; and   a single central rib on each of said upper shell and said lower shell, wherein each said single central rib extends longitudinally along the longitudinal axis of said implant, and both said single central ribs lie in the same vertical plane.   
   
   
       206 . An artificial disc implant according to  claim 205 , wherein at least one of said single central ribs has a height greater than its width. 
   
   
       207 . An artificial disc implant according to  claim 205 , wherein said implant is constructed to be the sole implant in the intervertebral space. 
   
   
       208 . An artificial disc implant according to  claim 205 , wherein each said single central rib is adapted to anchor its respective shell in the intervertebral space. 
   
   
       209 . An artificial disc implant according to  claim 205 , wherein the arcuate portion is concave and the upper surface portion is convex. 
   
   
       210 . An artificial disc implant according to  claim 209 , wherein said lower shell is formed in two pieces including a lower piece which has the lower bone contacting surface and an upper piece which has the upper surface portion. 
   
   
       211 . An artificial disc implant, comprising:
 an upper shell having an upper bone contacting surface for engaging an upper vertebral body;   a lower shell having a lower bone contacting surface for engaging a lower vertebral body, said upper and lower shells being adapted to operatively engage each other, the upper vertebral body being adjacent the lower vertebral body to provide an intervertebral space;   said implant having a leading end, which leads as said implant is inserted into the intervertebral space, and a trailing end opposite the leading end, and having a longitudinal axis extending from the leading end to the trailing end;   a first central rib on said upper shell, said first central rib extending longitudinally along the longitudinal axis of said implant, and being adapted to retain said upper shell in the intervertebral space; and   a second central rib on said lower shell, said second central rib extending longitudinally along the longitudinal axis of said implant, and being adapted to retain said lower shell in the intervertebral space.   
   
   
       212 . An artificial disc implant according to  claim 211 , wherein at least one of said first and second central ribs has a height greater than its width. 
   
   
       213 . An artificial disc implant according to  claim 211 , wherein at least one of (i) said first central rib is a single central rib and (ii) said second central rib is a single central rib. 
   
   
       214 . An artificial disc implant according to  claim 211 , wherein each of said first and second central ribs has a height greater than its width. 
   
   
       215 . An artificial disc implant according to  claim 214 , wherein the length of at least one of said first and second central ribs in the direction along the longitudinal axis is greater than one half of the distance from the leading end to the trailing end along the longitudinal axis. 
   
   
       216 . An artificial disc implant according to  claim 215 , wherein the length of both said first and second central ribs in the direction along the longitudinal axis is greater than one half of the distance from the leading end to the trailing end along the longitudinal axis. 
   
   
       217 . An artificial disc implant according to  claim 216 , wherein both said first and second central ribs lie in a plane intersecting the longitudinal axis of said implant. 
   
   
       218 . An artificial disc implant according to  claim 217 , said implant having an anterior and a posterior, and wherein each of said first and second central ribs is elongated in an anterior to posterior plane of said implant. 
   
   
       219 . An artificial disc implant according to  claim 214 , wherein both said first and second central ribs lie along a common plane passing through the longitudinal axis of said implant. 
   
   
       220 . An artificial disc implant according to  claim 214 , wherein at least one of said first and second central ribs is perpendicular to the bone contacting surface of its respective shell. 
   
   
       221 . An artificial disc implant according to  claim 220 , wherein each of said first and second central ribs is perpendicular to the bone contacting surface of its respective shell. 
   
   
       222 . An artificial disc implant according to  claim 214 , wherein said upper and lower shells of said implant are adapted for implantation together along a path of insertion into the intervertebral space. 
   
   
       223 . An artificial disc implant according to  claim 222 , said implant having an anterior and a posterior, and wherein the path of insertion is in an anterior to posterior direction. 
   
   
       224 . An artificial disc implant according to  claim 222 , wherein both said first and second central ribs are essentially coplanar and vertically aligned with each other, such that said first and second central ribs can be inserted together along the path of insertion into respective essentially vertically aligned vertebral bodies. 
   
   
       225 . An artificial disc implant according to  claim 224 , wherein said upper and lower shells are operatively engaged to allow pivotability of said upper shell relative to said lower shell. 
   
   
       226 . An artificial disc implant according to  claim 224 , wherein said upper and lower shells are operatively engaged to allow articulating motion between said upper shell and said lower shell. 
   
   
       227 . An artificial disc implant according to  claim 211 , wherein at least one of said upper and lower bone contacting surfaces is adapted to engage its respective vertebral body. 
   
   
       228 . An artificial disc implant according to  claim 211 , wherein at least one of said upper and lower bone contacting surfaces is adapted for anchoring to its respective vertebral body. 
   
   
       229 . An artificial disc implant according to  claim 211 , wherein said upper and lower shells includes a convex surface on one of said shells adapted for engaging with a mating concave surface on the other of said shells. 
   
   
       230 . An artificial disc implant according to  claim 229 , wherein one of said upper and lower shells comprises two pieces, including a first piece which includes its respective central rib and a second piece which includes the convex or concave surface. 
   
   
       231 . An artificial disc implant according to  claim 211 , wherein said implant is adapted for use as a total disc replacement. 
   
   
       232 . An artificial disc implant according to  claim 211 , wherein said implant is constructed to be the sole implant in the intervertebral space. 
   
   
       233 . An artificial disc implant according to  claim 211 , wherein both said first and second central ribs are coplanar and vertically aligned with each other, such that said first and second central ribs can be inserted into respective essentially vertically aligned vertebral bodies. 
   
   
       234 . An artificial disc implant according to  claim 211 , wherein the overall length of at least one of said first and second central ribs in the direction along the longitudinal axis is greater than one half of the distance from the leading end to the trailing end along the longitudinal axis. 
   
   
       235 . An artificial disc implant according to  claim 234 , wherein the overall length of each of said first and second central ribs in the direction along the longitudinal axis is greater than one half of the distance from the leading end to the trailing end along the longitudinal axis. 
   
   
       236 . An artificial disc implant according to  claim 211 , said implant having an anterior and a posterior, and wherein the overall anterior to posterior length of each of said first and second central ribs is greater than one half of the overall anterior to posterior dimension of its respective shell. 
   
   
       237 . An artificial disc implant according to  claim 211 , wherein at least one of said first and second central ribs is a single uninterrupted solid piece extending outwardly from its respective bone contacting surface. 
   
   
       238 . An artificial disc implant according to  claim 211 , wherein at least one of said first and second central ribs is a single piece elongated in a plane parallel to a path of insertion of said implant. 
   
   
       239 . An artificial disc implant according to  claim 238 , wherein both said first and second central ribs are single pieces elongated in respective planes that are parallel to the path of insertion of said implant. 
   
   
       240 . An artificial disc implant according to  claim 211 , wherein at least one of the upper bone contacting surface and lower bone contacting surface further comprises at least one protrusion oriented outwardly from the respective bone contacting surface, the at least one protrusion having a height no greater than the width of the at least one protrusion. 
   
   
       241 . An artificial disc implant according to  claim 211 , wherein at least one of the upper bone contacting surface and lower bone contacting surface further comprises at least one protrusion oriented outwardly from the respective bone contacting surface, the at least one protrusion having a height substantially less than the width of the at least one protrusion. 
   
   
       242 . An artificial disc implant according to  claim 211 , wherein at least one of said upper and lower shells is, in plan view, generally rectangular in shape. 
   
   
       243 . An artificial disc implant according to  claim 242 , wherein both said upper and lower shells are, in plan view, generally rectangular in shape. 
   
   
       244 . A component for an artificial disc implant, said component being adapted to cooperate with at least one other element to form the artificial disc implant, said component comprising:
 a bone contacting surface for engaging a vertebral body, and an interior bearing wall adapted to operatively engage another element of said implant, said component having a leading end, which leads as said component is inserted into an intervertebral space, and a trailing end opposite the leading end, and having a longitudinal axis extending from the leading end to the trailing end; and   a central rib extending outwardly from said bone contacting surface, wherein said central rib lies along said bone contacting surface in the direction of the longitudinal axis, has a height greater than its width, and is adapted to embed into the intervertebral space as said component moves into the intervertebral space.   
   
   
       245 . A component for an artificial disc implant according to  claim 244 , wherein said component comprises a single central rib extending outwardly from said bone contacting surface. 
   
   
       246 . A component for an artificial disc implant according to  claim 244 , wherein said central rib is elongated in a plane parallel to a path of insertion of said component. 
   
   
       247 . A component for an artificial disc implant according to  claim 246 , wherein the overall length of said central rib in the direction along the longitudinal axis is greater than one half of the distance from the leading end to the trailing end along the longitudinal axis. 
   
   
       248 . A component for an artificial disc implant according to  claim 244 , wherein said central rib is a single uninterrupted solid piece extending outwardly from said bone contacting surface. 
   
   
       249 . A component for an artificial disc implant according to  claim 244 , said component having an anterior and a posterior, and wherein the longitudinal axis extends from the anterior to the posterior of said component. 
   
   
       250 . A component for an artificial disc implant according to  claim 244 , said component having an anterior and a posterior, and wherein said central rib is elongated in an anterior to posterior plane of said component. 
   
   
       251 . A component for an artificial disc implant according to  claim 250 , wherein the anterior to posterior plane parallels and intersects the longitudinal axis of said component. 
   
   
       252 . A component for an artificial disc implant according to  claim 251 , wherein the overall anterior to posterior length of said central rib is greater than one half of the overall anterior to posterior dimension of said component. 
   
   
       253 . A component for an artificial disc implant according to  claim 244 , wherein said central rib is elongated along and straddles a midline of said bone contacting surface. 
   
   
       254 . A component for an artificial disc implant according to  claim 244 , wherein said central rib is perpendicular to said bone contacting surface. 
   
   
       255 . A component for an artificial disc implant according to  claim 244 , wherein said bone contacting surface is adapted for contacting the vertebral body. 
   
   
       256 . A component for an artificial disc implant according to  claim 244 , wherein said bone contacting surface comprises at least one protrusion oriented outwardly from said bone contacting surface, the at least one protrusion having a height no greater than the width of the at least one protrusion. 
   
   
       257 . A component for an artificial disc implant according to  claim 244 , wherein said bone contacting surface comprises at least one protrusion oriented outwardly from said bone contacting surface, the at least one protrusion having a height less than the height of said central rib. 
   
   
       258 . A component for an artificial disc implant according to  claim 244 , wherein said component is, in plan view, generally rectangular in shape. 
   
   
       259 . An artificial disc implant, comprising:
 an upper shell having an upper bone contacting surface for engaging an upper vertebral body;   a lower shell having a lower bone contacting surface for engaging a lower vertebral body, said upper and lower shells being adapted to operatively engage each other, and having midlines on their respective bone contacting surfaces passing through the center of the bone contacting surface of said respective shell, the upper vertebral body being adjacent the lower vertebral body to provide an intervertebral space;   a first central rib extending outwardly from the midline of the upper bone contacting surface of said upper shell; and   a second central rib extending outwardly from the midline of the lower bone contacting surface of said lower shell,   wherein each of said first and second central ribs extends from a location adjacent a leading end of said implant toward another location adjacent a trailing end of said implant and lies along the midline of the bone contacting surface of its respective shell.   
   
   
       260 . An artificial disc implant according to  claim 259 , wherein at least one of (i) said first central rib is the only rib on said upper shell and (ii) said second central rib is the only rib on said lower shell. 
   
   
       261 . An artificial disc implant according to  claim 259 , wherein at least one of said first and second central ribs has a height greater than its width. 
   
   
       262 . An artificial disc implant according to  claim 261 , wherein at least one of said first and second central ribs is adapted to anchor said implant in the intervertebral space. 
   
   
       263 . An artificial disc implant according to  claim 262 , wherein each of said first and second central ribs is adapted to anchor said implant in the intervertebral space. 
   
   
       264 . An artificial disc implant according to  claim 261 , wherein each of said first and second central ribs has a height greater than its width. 
   
   
       265 . An artificial disc implant according to  claim 264 , said implant having an anterior and a posterior, and wherein each of said first and second central ribs is elongated in an anterior to posterior plane of said implant. 
   
   
       266 . An artificial disc implant according to  claim 264 , wherein both said first and second central ribs lie essentially along a common plane passing through each of the respective midlines. 
   
   
       267 . An artificial disc implant according to  claim 264 , wherein at least one of said first and second central ribs is perpendicular to the bone contacting surface of its respective shell. 
   
   
       268 . An artificial disc implant according to  claim 267 , wherein each of said first and second central ribs is perpendicular to the bone contacting surface of its respective shell. 
   
   
       269 . An artificial disc implant according to  claim 264 , wherein said upper and lower shells of said implant are adapted for implantation together along a path of insertion into the intervertebral space. 
   
   
       270 . An artificial disc implant according to  claim 269 , said implant having an anterior and a posterior, and wherein the path of insertion is in a direction from the anterior to the posterior of said implant. 
   
   
       271 . An artificial disc implant according to  claim 269 , wherein both said first and second central ribs are essentially coplanar and vertically aligned with each other, such that said first and second central ribs can be inserted together along the path of insertion into respective essentially vertically aligned vertebral bodies. 
   
   
       272 . An artificial disc implant according to  claim 271 , wherein said upper and lower shells are operatively engaged to allow pivotability of said upper shell relative to said lower shell. 
   
   
       273 . An artificial disc implant according to  claim 271 , wherein said upper and lower shells are operatively engaged to allow articulating motion between said upper shell relative to said lower shell. 
   
   
       274 . An artificial disc implant according to  claim 261 , wherein at least one bone contacting surface is adapted to engage its respective vertebra. 
   
   
       275 . An artificial disc implant according to  claim 261 , wherein at least one bone contacting surface is adapted for anchoring to its respective vertebral body. 
   
   
       276 . An artificial disc implant according to  claim 261 , wherein said upper and lower shells includes a convex surface on one of said shells adapted for engaging with a mating concave surface on the other of said shells. 
   
   
       277 . An artificial disc implant according to  claim 276 , wherein one of said upper and lower shells comprises two pieces, including a first piece which includes its respective central rib and a second piece which includes the convex or concave surface. 
   
   
       278 . An artificial disc implant according to  claim 261 , wherein said implant is adapted for use as a total disc replacement. 
   
   
       279 . An artificial disc implant according to  claim 261 , wherein said implant is constructed to be the sole implant in its intervertebral space. 
   
   
       280 . An artificial disc implant according to  claim 261 , wherein both said first and second central ribs are essentially coplanar and vertically aligned with each other, such that said first and second central ribs can be inserted into respective essentially vertically aligned vertebral bodies. 
   
   
       281 . An artificial disc implant according to  claim 261 , wherein the overall length of at least one of said first and second central ribs in the direction of a path of insertion of said implant is greater than one half of the overall dimension of its respective upper or lower shell in the direction of insertion. 
   
   
       282 . An artificial disc implant according to  claim 281 , wherein the overall length of each of said first and second central ribs in the direction of a path of insertion of said implant is greater than one half of the overall dimension of its respective upper or lower shell in the direction of insertion. 
   
   
       283 . An artificial disc implant according to  claim 261 , said implant having an anterior and a posterior, and wherein an overall anterior to posterior length of each of said first and second central ribs is greater than one half of an overall anterior to posterior dimension of its respective upper or lower shell. 
   
   
       284 . An artificial disc implant according to  claim 261 , wherein at least one of said first and second central ribs is a single uninterrupted solid piece extending outwardly from the bone contacting surface of its respective shell. 
   
   
       285 . An artificial disc implant according to  claim 261 , wherein at least one of said first and second central ribs is a single piece elongated in a plane parallel to a path of insertion of said implant. 
   
   
       286 . An artificial disc implant according to  claim 285 , wherein both said first and second central ribs are single pieces elongated in respective planes that are parallel to the path of insertion of said implant. 
   
   
       287 . An artificial disc implant according to  claim 261 , wherein at least one of the upper bone contacting surface and lower bone contacting surface further comprises at least one protrusion oriented outwardly from the respective bone contacting surface, the at least one protrusion having a height no greater than the width of the at least one protrusion. 
   
   
       288 . An artificial disc implant according to  claim 261 , wherein at least one of the upper bone contacting surface and lower bone contacting surface further comprises at least one protrusion oriented outwardly from the respective bone contacting surface, the at least one protrusion having a height less than the height of at least one of said first and second central ribs. 
   
   
       289 . An artificial disc implant according to  claim 261 , wherein at least one of said upper and lower shells is, in plan view, generally rectangular in shape. 
   
   
       290 . An artificial disc implant according to  claim 289 , wherein both said upper and lower shells are, in plan view, generally rectangular in shape. 
   
   
       291 . A component for an artificial disc implant, said component being adapted to cooperate with at least one other element to form the artificial disc implant, said component comprising:
 a bone contacting surface for engaging a vertebral body, and an interior bearing wall adapted to operatively engage another element of said implant, wherein said component has a midline on said bone contacting surface passing through the center of said bone contacting surface; and   a central rib extending outwardly from the midline of said bone contacting surface, wherein said central rib has a height greater than its width and is elongated along and straddles the midline.   
   
   
       292 . A component for an artificial disc implant according to  claim 291 , wherein said central rib is elongated in a plane parallel to a path of insertion of said component. 
   
   
       293 . A component for an artificial disc implant according to  claim 292 , wherein the overall length of said central rib in the direction of the path of insertion of said component is greater than one half of the overall dimension of said component in the direction of insertion. 
   
   
       294 . A component for an artificial disc implant according to  claim 291 , wherein said central rib is a single central rib comprising an uninterrupted solid piece extending outwardly from said bone contacting surface. 
   
   
       295 . A component for an artificial disc implant according to  claim 291 , wherein an overall anterior to posterior length of said central rib is greater than one half of an overall anterior to posterior dimension of said component. 
   
   
       296 . A component for an artificial disc implant according to  claim 291 , wherein said central rib is perpendicular to said bone contacting surface. 
   
   
       297 . A component for an artificial disc implant according to  claim 291 , wherein said bone contacting surface is adapted to engage the vertebral body. 
   
   
       298 . A component for an artificial disc implant according to  claim 291 , wherein said central rib is adapted to anchor said component in an intervertebral space. 
   
   
       299 . A component for an artificial disc implant according to  claim 291 , wherein said bone contacting surface comprises at least one protrusion oriented outwardly from said bone contacting surface, the at least one protrusion having a height no greater than the width of the at least one protrusion. 
   
   
       300 . A component for an artificial disc implant according to  claim 291 , wherein said bone contacting surface comprises at least one protrusion oriented outwardly from said bone contacting surface, the at least one protrusion having a height substantially less than the height of said central rib. 
   
   
       301 . A component for an artificial disc implant according to  claim 291 , wherein said component is, in plan view, substantially rectangular in shape.