Patent Publication Number: US-2005131539-A1

Title: Intervertebral implant with reduced contact area and method

Description:
FIELD OF THE INVENTION  
      This invention pertains to procedures for intervertebral stabilization. Specifically, the disclosure provides implants, instrumentation and methods to facilitate stabilization or fusion between two vertebrae.  
     BACKGROUND OF THE INVENTION  
      Chronic back problems cause pain and disability for a large segment of the population. Frequently, the cause of back pain is traceable to diseased disk material between opposing vertebrae. When the disk material is diseased, the opposing vertebrae may be inadequately supported, resulting in persistent pain.  
      Surgical techniques have been developed to remove the diseased disk material and fuse,the joint between opposing vertebral bodies. Stabilization and/or arthrodesis of the intervertebral joint can reduce the pain associated with movement of an intervertebral joint having diseased disk material. Generally, fusion techniques involve removal of the diseased disk and packing the void area with a suitable matrix for facilitating a bony union between the opposing vertebral bodies.  
      Surgical devices for facilitating interbody fusion have also been developed. These devices typically provide for maintaining appropriate intervertebral spacing and stabilization of the vertebrae during the fusion process. Generally, these devices are referred to as cages. Examples of such devices are disclosed in, for example, U.S. Pat. Nos. 5,458,638, 5,489,307, 5,055,104, 5,026,373, 5,015,247, 4,961,740, 4,743,256 and 4,501,269, the entire disclosures of which are incorporated herein by reference.  
      Generally, the fusion device is implanted within a site prepared between opposing vertebrae. Typically, the site is a bore formed in the disk material and extends through the cortical end plates and into the cancellous bone of the opposing vertebrae. Many of the present fusion devices have a chamber enclosed by a cylindrical or rectangular wall that substantially contacts the entire interior surface of the bore. After placement of the device into the bore, the enclosed chamber (interior of the cage) can be filled with bone chips or other suitable material for facilitating bony union between the vertebrae.  
      Most of the present fusion devices provide vertebral stabilization during the fusion process by contact of the entire outer wall of the fusion device with substantially the entire interior surface of the wall of the insertion bore. While support provided by contact of the device with the entire wall of the bore provides adequate vertebral stabilization during the fusion process, it also has many disadvantages. For example, the greater the overall contact area of the device with the surface of the bore, the slower the rate at which new bone can grow into the bore to stabilize the joint In addition, the greater the surface area of the device that contacts the surface area of the bore, the less continuity that can occur between the bone that is external to the device and the bone that is internal to the device. This lack of continuity of bone can translate into reduced structural integrity of the bony union. Furthermore, reducing the amount and continuity of the bone growth into the fusion site can cause the patient&#39;s body to rely on the device for long term stabilization rather than relying on the structural integrity of the new bony union. The potential orthopedic problems resulting from the body&#39;s reliance on orthopedic implants for structural support are well known.  
      Moreover, because most fusion devices are manufactured with materials that are radiopaque to typical diagnostic imaging modalities, assessment of the status of new bone growth during the fusion process can be limited.  
      Accordingly, there is a continuing need for improved intervertebral stabilizing devices and methods. The present invention is directed to addressing these needs.  
     SUMMARY OF THE INVENTION  
      The invention is directed to procedures for intervertebral stabilization of opposing vertebrae. The disclosure provides implants, instruments and methods for stabilization or fusion of opposing vertebrae.  
      At various locations throughout the specification, lists of examples are provided. It should be noted that the examples are provided for illustrative purposes and are not intended to limit the scope of the invention.  
      An implant according to the invention includes an implant body having a first and second end spaced apart by a longitudinal axis of the implant. The implant body includes a first transverse member and a second transverse member maintained in spaced apart relationship by a central support member. The transverse members each include a bearing surface oriented to contact opposing vertebral surfaces.  
      The bearing surfaces of the implant can be linear, curved or other suitable configuration. In addition, the bearing surfaces can include a pattern for anchoring the implant and/or resisting displacement once the implant is inserted between opposing vertebrae.  
      An implant of the invention provides a reduced displacement volume relative to the insertion bore necessary to accommodate the implant. The central support member or transverse members can also include openings which further reduce the displacement volume of the implants. In addition to enhancing the continuity of the new bone growth between the stabilized vertebral bodies, the reduced displacement volume of the implant facilitates assessment of the fusion process using known imaging modalities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of an implant embodiment of the invention having a first and second curved bearing surfaces;  
       FIG. 2  is a side elevation view of the implant of  FIG. 1  (the opposite side being identical in appearance);  
       FIG. 3  is a top plan view of a first transverse member of the implant of  FIG. 1  (the top view of the second transverse member view being identical in appearance);  
       FIG. 4  is an elevation view of a trailing end of the implant of  FIG. 1 ;  
       FIG. 5  is an elevation view of a leading end of the implant of  FIG. 1 ;  
       FIG. 6  is a perspective view of a second embodiment of an implant according to the invention;  
       FIG. 7  is a top plan view of a first transverse member of the implant of  FIG. 6  (the top view of the second transverse member being identical in appearance);  
       FIG. 8  is a perspective view of a third embodiment of an implant according to the invention;  
       FIG. 9  is a side elevation view of an embodiment of a tapered implant according to the invention (the opposite side being identical in appearance);  
       FIG. 10  is a top plan view of the implant of  FIG. 9  taken 90° from the view of  FIG. 9  (the opposite side being identical in appearance);  
       FIG. 11  is a side elevation view of an implant according to the invention illustrating a first and second taper (the opposite side being identical in appearance);  
       FIG. 12  is a top plan view of the implant of  FIG. 11  taken 90° from the view of  FIG. 11  (the opposite side being identical in appearance);  
       FIG. 13  is a side elevation view of another embodiment of an implant according to the invention having a first and second taper;  
       FIG. 14  is an end view of two opposing vertebrae stretched apart and including two implants of  FIGS. 1-5  disposed therebetween;  
       FIG. 15  is a side elevation view of an insertion tool for use with an implant of invention;  
       FIG. 16  is a side view of a distal end of the insertion tool of  FIG. 15 ;  
       FIG. 17  is perspective view of an implant of  FIGS. 1-5  and the distal end of the insertion tool of  FIG. 15 ;  
       FIG. 18  is an end on view of the distal end of the insertion tool of  FIG. 15  with an implant of  FIGS. 1-5 ;.  
       FIG. 19  is a side elevation view of an alternative embodiment of an insertion tool according to the invention; and  
       FIG. 20  is an end on view of an implant of  FIGS. 1-5  loaded onto the distal end of the insertion tool of  FIG. 19 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention is directed to intervertebral stabilization and arthrodesis procedures that can provide for greater structural integrity of the bony union between fused vertebral bodies of the vertebral column. In addition, the devices and methods disclosed herein facilitate greater continuity between the bone formed at the fusion site and the remainder of the vertebral body. In some embodiments, the invention provides enhanced ability to assess new bone growth during the fusion process using typical diagnostic imaging modalities such as x-rays.  
      An implant of the invention can be prepared from known implant materials including, for example, titanium, stainless steel, porous titanium, bone or other suitable material used to manufacture orthopedic implants. Unlike prior implants, the present implants have no surrounding sidewalls and no chamber. The disclosed implants support the axial load of the vertebral column by a “central support member” that separates opposing bearing surfaces of the implant.  
      The “central support member” provides for stabilization of the vertebral bodies with a reduced area of contact between the exterior surface of the implant and the inside surface of a bore formed to accommodate the implant. In addition to promoting greater structural integrity and continuity of the bony union, the reduced contact area also reduces obstruction of assessment of the fusion process. Further reduction in obstruction of assessment of the fusion process can be provided by forming openings in the bearing surfaces and/or providing the central support member in the form of one or more columns having openings in between.  
      In some embodiments, in comparison to prior implants, the present implants have a reduced displacement volume relative to the cylindrical bore size necessary for insertion of the implant. For example, in some embodiments, the displacement volume of the implant takes up about 10% to 40% of the bore volume necessary to accommodate the implant between opposing vertebrae. In one preferred embodiment, the implant takes up about 24% or less of the bore volume necessary to accommodate the implant. Thus, in this embodiment, the remaining 76% of the bore volume can be filled with bone or other suitable bone support matrix. In contrast, the BAK implant (U.S. Pat. No. 5,489,308), commercially available from Sulzer Spine-Tech, Inc., takes up about 41% of the bore volume on a relative basis and the Proximity implant (U.S. Pat. No. 5,609,636), also available from Sulzer Spine-Tech, Inc., takes up about 30% of the bore volume on a relative basis.  
      According to the invention, the central support member is located between the bearing surfaces of the implant and typically does not extend to the lateral edges of the bearing surfaces. The term “central” includes an implant having a support member located away from the exact center of the bearing surfaces but providing the same function of a herein described centrally located support member. The “bearing surfaces” are the surfaces of the implant that directly contact the opposing vertebral bodies. The “lateral edges” of the bearing surfaces are the lateral most aspects of the bearing surfaces.  
      The implants also have a leading end and trailing end that are spaced apart along the longitudinal axis of the implant. In general, a transverse cross section taken through the longitudinal axis of the present implants has a substantially “I” shaped configuration. The “central support member” forms the vertical arm of the “I” and the “transverse members” form the horizontal arms of the “I”. In use, the central support member is typically oriented parallel to the longitudinal axis of the vertebral column and the transverse members are oriented perpendicular.  
      Each transverse member has a peripheral surface that is in direct contact with one of the opposing vertebral bodies. The traverse members also have an inner surface that is continuous with the lateral aspect of the central support member. A “channel” is present on either side of the central support member within the inner surface of the transverse member. As will be appreciated from the illustrated embodiment, the channel extends through the leading and trailing ends of the implant and opens laterally between opposing transverse members. As discussed below, after insertion of the implant between opposing vertebrae, the channel can be filled with a bone support matrix to facilitate new bone growth.  
      In some embodiments, the bearing surfaces are curved to provide an external surface configured for insertion of the implant into a circular bore formed between opposing vertebrae. In such embodiments, the opposing bearing surfaces can be parallel to one another along the longitudinal dimension of the implant from the trailing end to leading end. Alternatively, the implant can include a single or double taper including at least a first taper diverging from the longitudinal axis of the implant from the leading end to the trailing end. Implant embodiments having curved bearing surfaces can include a pattern for anchoring the implant between opposing vertebrae. The pattern can be, for example, knurls or other intermittently raised surface. Alternatively, the pattern can be a portion of a helical thread pattern which resists displacement of the implant from an insertion bore and also provides for threaded insertion of the implant into the bore.  
      In other embodiments, the bearing surfaces can be substantially linear. According to this embodiment, preferably, at least one of the bearing surfaces includes a pattern for anchoring the implant and reducing the chance of displacement of the implant from of the insertion bore.  
      The invention also provides a kit comprising a plurality of incrementally sized implants which can be selected by the clinician based on the size needed for a particular patient. In other embodiments kits are provided which include instrumentation for performing an implant procedure with or without a plurality of incrementally sized implants.  
      Instruments and methods suitable for insertion of an implant of the invention are disclosed in, for example, U.S. Pat. Nos. 5,489,308 and 5,458,638, and co-pending application U.S. Ser. Nos. 08/902,083 and 08/921,001, the entire disclosures of which are incorporated herein by reference. Additional instruments particularly advantageous for the implants disclosed herein are described in detail below.  
      After the implant is inserted into the bore, the volume of the bore not occupied by the implant, for example in the region of the channels, can be filled with a bone support matrix. As used herein, a “bone support matrix” is a material that facilitates new bone growth between the opposing vertebral bodies. Suitable bone support matrices can be resorbable or nonresorbable and osteoconductive or osteoinductive. Examples of suitable matrices according to the invention include synthetic materials, such as Healous™, available from Orquest, Mountain View, Calif.; NeOsteo ™, available from Sulzer Orthopedic Biologics, Denver, Colo.; or any of a variety of bone morphogenic proteins (BMPs). Suitable bone support matrices also include heterologous, homologous, or autologous bone and derivatives thereof. Preferably, the bone support matrix is radiolucent on x-rays.  
      The bone support matrix can be packed into the bore after insertion of the implant between the vertebral bodies. Alternatively, a bone support matrix can be configured to fit into the longitudinal channels on either side of the central support member before or after installation of the implant into the site of implantation. In one embodiment, the external surface of the bone support matrix can include a portion of a helical thread. According to this embodiment, when used with an implant having a portion of helical threads on a bearing surface, the helical threads of the bone matrix can be complimentary to the helical threads on the implant such that when placed into the channel a complete helical thread pattern is formed for threadedly inserting the implant into the prepared site.  
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS  
      The implants and instruments of the invention will now be described by reference to the several drawing figures. The illustrated embodiments are provided only for descriptive purposes and are not intended to limit the implants which are within the scope of the invention. It will be appreciated, however, that while the illustrated embodiments share the general configuration of an “I” in transverse cross section, each embodiment has additional unique and advantageous features.  
      A. Implants  
       FIGS. 1-5  illustrate a first embodiment of an implant of the invention having a first transverse member  1  and a second transverse member  2  spaced apart by a central support member  3 . When inserted between opposing vertebrae, each transverse member is oriented transverse to the longitudinal axis of the vertebral column and the central support member is oriented parallel to the longitudinal axis of the vertebral column. Thus, the transverse members can also be referred to as a “cranial transverse member” and a “caudal transverse member” to indicate that when inserted between opposing vertebrae, one transverse member is oriented cranially and the other transverse member is oriented caudally.  
      The first transverse member  1  has a first bearing surface  4  and the second transverse member  2  has a second bearing surface  5 . The first bearing surface  4  and the second bearing surface  5  include a pattern  7  for anchoring the implant within an insertion bore. The illustrated pattern  7  is a portion of a helical thread  7   a  which provides for threadedly inserting implant  10  into a bore prepared between opposing vertebrae. The helical thread  7   a  is generally rectangular in profile. However, a thread pattern having sharp surfaces or a combination of rectangular and sharp threads can be used. In addition, other surface patterns, such as knurls, could be provided on the bearing surface and the device implanted by impact into a bore.  
      The illustrated central support member  3  comprises a plurality of columns  8   a - 8   d  with openings  9   a - 9   c  therebetween. Columns  8   a - 8   d  of central support member  3  maintain transverse members  1  and  2  in a fixed spatial relationship and provide rigid support and stabilization of opposing vertebral bodies which contact bearing surfaces  4  and  5 . Openings  9   a - 9   c  between columns  8   a - 8   d  promote greater continuity of new bone growth through the central support member as well as reduce the presence of radiopaque material which can obstruct assessment of the fusion process using typical diagnostic imaging modalities.  
       FIG. 3  illustrates a top view of the bearing surface  4  of the first transverse member  1 . Rotation of the implant 180° would show the bearing surface  5  of the second transverse member  2  which is substantially identical in appearance. The bearing surface  4  (and  5 ) includes rigid transverse supports, or trusses,  13   a - 13   d  having openings  12   a - 12   c  therebetween. As illustrated, the portion of helical thread  7   a  can be continuous in the region of the transverse supports  13   a - 13   d . In addition to facilitating greater structural integrity of the bony union, the openings  12   a - 12   d  also enhance the ability to assess new bone formation during the fusion process.  
       FIG. 4  is an elevation view of the trailing end  20  of implant  10 . The inner surfaces  21   a ,  21   b  of transverse member  1  oppose the inner surfaces  22   a ,  22   b  of transverse member  2 . The inner surfaces of the transverse members are continuous with the lateral surfaces  23   a ,  23   b  of the central support member  3 . On either side of the central support member  3 , there are two longitudinal channels  24   a  and  24   b . Channel  24   a  is defined by surfaces  21   a ,  22   a  and  23   a  and channel  24   b  is defined by surfaces  21   b ,  22   b  and  23   b . Channels  24   a  and  24   b  not only provide a large area for uninterrupted new bone growth around the implant, but they also provide an arrangement for attachment of an insertion tool described below.  
      Between each inner surface  21   a ,  22   a ,  21   b  and  22   b  and its respective lateral edge  25   a ,  26   a ,  25   b  and  26   b  of transverse members  1  and  2 , there are undercut segments  27   a ,  28   a ,  27   b  and  28   b . The angle A between undercut segments  27   a  and  28   a  and the angle B between undercut segments  27   b  and  28   b  can be different. As will be discussed below, asymmetry of angles A and B can provide for proper orientation of the helical threads  7   a  of implant  10  with complimentary threads of a below described insertion tool.  
       FIG. 5  is an elevation view of the leading end  30  of the implant  10 . In the illustrated embodiment, trailing column  8   d  of central support member  3  includes lateral tabs  31   a  and  31   b . Lateral tabs  31   a  and  31   b  render the leading end distinguishable from the trailing end such that implant  10  can only be loaded onto a below described implant insertion tool in a certain orientation.  
      Referring to  FIG. 2 , the leading end  30  and trailing end  20  are spaced apart along the longitudinal axis X-X of implant  10  to provide a length L. The implant  10  can be provided with different lengths L between leading end  30  and trailing end  20  as well as different heights H between the bearing surfaces  4  and  5  of transverse members  1  and  2 , respectively. Incrementally sized length and height implants  10  can be provided in a kit for selected use by the surgeon based on the particular patient&#39;s needs.  
      Once inserted into a prepared bore site, the channels  24   a ,  24   b  and any other area of the bore not occupied by the implant can be filled with a bone support matrix. Referring again to  FIG. 1 , one embodiment of a bone support matrix  40  is illustrated. According to this embodiment, the bone support matrix  40  can be a resorbable matrix  41  configured to fit within channels  24   a  or  24   b . The inner surface  42  of bone support matrix  40  can be shaped to follow the contours of channels  24   a  or  24   b . The outer surface  43  of bone support matrix  40  can include a portion of helical threads  43  which are complimentary to portions of helical threads  7   a  of implant  10 . According to this embodiment, the implant  10  can be threaded into a tapped insertion bore with bone support matrix  40  in place. In alternative embodiments, after placement of the implant  10  into a bore, a bone support matrix configured to follow the contours of channels  24   a  and  24   b  but without a threaded outer surface can be inserted into the channels  24   a  and  24   b  of the implant.  
      Referring now to  FIGS. 6-7 , a second embodiment of an implant  100  is illustrated. The implant  100  includes four generally linear thread segments  101 ,  102 ,  103  and  104 . Linear thread segments  101  and  103  provide a bearing surface  105  of a first transverse member  106  and linear thread segments  102  and  104  provide a bearing surface  107  of a second transverse member  108 . As illustrated best in the top view of  FIG. 7 , thread segments  101  and  103  (and  102  and  104 ) are maintained in spaced apart alignment by transverse supports  109 ,  110  and  111 . In the illustrated embodiment there are two openings  112  and  113  between transverse supports  109 ,  110  and  111 . (The relative arrangement of the second transverse member  108  having thread segments  102  and  104  is identical to that just described for the first transverse segment  106 ). Transverse members  106  and  108  are maintained in spaced apart alignment by central support member  120 . In the illustrated embodiment, central support member  120  comprises columns  121 ,  122  and  123  and has openings  124  and  125  therebetween.  
      It will be appreciated that the transverse members and central support member of an implant need not include any openings as described thus far. In addition, rather than comprising support columns, and openings as illustrated, the central support member can include several fine thickness support columns with several fine openings interspersed therebetween giving a profile appearance similar to the tines of a comb. A similar arrangement can be provided for the transverse members rather than having the trusses and openings illustrated.  
      Referring now to  FIG. 8 , another implant  200  is illustrated. Implant  200  has a more classic “I-beam” appearance in cross section. Similar to the previously discussed embodiments, first transverse member  201  and second transverse member  202  are maintained in spaced apart alignment by central support member  203 . Transverse member  201  also includes transverse supports  204 ,  205  and  206  having openings  207 - 210  therebetween. Transverse member  202  has an identical arrangement of transverse supports and openings. In the illustration, central support member  203  comprises columns  211 ,  212 , and  213  has openings  214 - 216  therebetween. Bearing surfaces  220  and  221  include a pattern  223  of intermittent raised edges  224  which reduce the chance of displacement of the implant  200  once inserted into a bore.  
      It should be noted that as an alternative to the helical threads present on the bearing surface of other implants described herein, a pattern such as intermittent raised surface  224  or other non-helical thread pattern can be present on the bearing surface. Thus, rather than threadedly inserting such an implant into an insertion bore, the implant can simply be impacted by driving it into the bore along the X-X axis of the implant.  
       FIGS. 9 and 10  illustrate an implant  300  having a first taper diverging from longitudinal axis X-X from leading end  301  to trailing end  302 . In the side view of  FIG. 9 , implant  300  has a substantially frusto-conical shape with a conical angle α equal to a desired lordosis between the vertebrae into which the implant  300  is to be placed as fully described in co-pending application U.S. Ser. No. 08/902,083, the entire disclosure of which is incorporated herein by reference. In the illustrated embodiment, angle α is 8°. However, it will be appreciated that as with other implants, implant  300  will be available in a wide variety of sizes. For example, such implants may be provided having angles a ranging from 1° to 20° in 1° increments to permit a physician to select a desired implant to attain a desired lordosis. Further, such implants can be provided in varying heights (i.e., the diameter of the implants) to accommodate desired distraction and lordosis between opposing vertebrae.  
      The first transverse member  304  and second transverse member  305  include a surface pattern  306  comprising a portion of helical threads  306   a  along first bearing surface  308  and second bearing surface  309 . The threads  306   a  are generally square in cross-section with their flat outer peripheral surfaces  310  set at an angle of one-half α with respect to the longitudinal axis X-X and defined valleys  311  between the threads  306   a . At the leading end  301 , the implant has a major diameter D M  measured between diametrically opposite outer radial surfaces  310  of the threads  306   a  at the leading end  301 . At the leading end  301 , the implant  300  has a minor diameter D m  measured as the distance across the implant  300  between the valleys  311  of the thread pattern  306   a.    
      At the trailing end  302 , the implant  300  has a major diameter D′ M  measured between diametrically opposite outer radial surfaces  310  of threads  306   a  at the trailing end  302 . Finally, at the trailing end  302 , the implant  300  has a minor diameter D′ m  measured between diametrically opposite valleys  311  at the trailing end  302 .  
      The central support member  320  of implant  300  comprises vertical columns  321 ,  322  and  323  including openings  324  and  325  therebetween. Referring to the top view of  FIG. 10 , it can be seen that the first transverse member  304  (and also second transverse member  305 ) include transverse supports  330 ,  331  and  332  and include openings  333  and  334  therebetween. As with all implants disclosed herein, the number of columns and transverse supports can vary. The objective being to provide rigid support with the greatest amount of free space.  
      Referring to  FIGS. 11-13 , another embodiment of an implant  400  is shown. According to this embodiment, the first transverse member  401  and second transverse member  402  are maintained in spaced apart relationship by central support member  403 . Central support member  403  includes columns  420 ,  421  and  422  with openings  423  and  424  therebetween. First transverse member  401  includes transverse supports  425 ,  426  and  427  with openings  428  and  429  therebetween. The second transverse member  402  has an identical arrangement.  
      Implant  400  has a first and second taper and a longitudinal axis X-X extending from a leading end  404  to a trailing end  405 . The trailing end  405  of the present embodiment comprises a “trailing end rise” (TER)  406  and a terminal end  407 . The first taper of implant  400  diverges from the axis from the leading end  404  to the trailing end rise  406  of the trailing end  405 . The second taper diverges from the axis from the terminal end  407  to the TER  406 . The trailing end rise  406  is the region of greatest diameter of the implant  400 .  
      The first taper provides the bi-tapered implant  400  with a substantially frusto-conical shape with a conical angle a equal to a desired lordosis between selected vertebrae. The angle a of the illustrated embodiment, measured from the leading end  404  to the TER  406  is 8°, however, as previously stated, the herein disclosed implants will be available with a variety of angles and sizes. Referring to  FIG. 11 , the leading end  404  has a major diameter D M  measured between diametrically opposite outer radial surfaces  410  of the threads  411  at the leading end  404 . The leading end  404  also has a minor diameter D m  measured between diametrically opposite inner radial surfaces  412  of the valleys  413  of the thread pattern  411  of implant  400 .  
      At the trailing end  405 , the implant  400  has a major diameter D′ M  measured between diametrically opposite outer radial surfaces  414  of the threads  411  at the trailing end rise  406 . The trailing end  405  also has a minor diameter D′ m  measured across terminal end  407 .  
      The second taper of the implant  400  has a second angle, δ, extending from the terminal end  407  to the TER  406 . The angle δ will vary with the diameter D′ M  of the TER  406 , the diameter D′ m  of the terminal end  407 , and the longitudinal distance L E  therebetween. In the illustrated embodiment, the diameter D′ m  of the terminal end  407  is equal to the major diameter D M  of the leading end  404 .  
      The longitudinal distance L E  can be about 5% to 25% of the overall length L of the implant. Generally, L E  is less than 15% of the overall length L, typically about 8-10%.  
      It will be appreciated that the slope “m” of the second taper, relative to the longitudinal axis X-X, can be calculated by the equation:
 
D′ M −D′ m /L E 
 
 In the illustrated embodiment, m is about 1 (45°). However, the actual slope dimensions m can vary, typically, between 0.58 (30°) and 1.73 (60°). 
 
      The helical threads  411  can extend along the second taper as illustrated at  415  of  FIGS. 11-12 . Alternatively, as illustrated in  FIG. 13 , the threads  411  can stop at the terminal end rise  406  and the second taper comprise a flat  416 , undulating or other non-threaded surface, from trailing end rise  406  to terminal end  407 .  
      Implant  400  can also include other features as previously described for an implant.  
      B. Instrumentation and Insertion  
      Instrumentation and methods for preparing an insertion bore for placement of an implant between opposing vertebrae are known. U.S. Pat. Nos. 5,458,638 and 5,489,308 and co-pending applications U.S. Ser. Nos. 08/921,001 and ______ (M&amp;G Docket No. 6683.22USI1 filed Mar. 6, 1998) describe preferred instrumentation and methods for preparing an implant bore and inserting an implant therein. The methods include the use of a distraction spacer, boring tools and tapping tools. In addition, copending U.S. Ser. Nos. 08/902,083, 08/902,407 and 08/902,431 disclose distraction spacers, boring tools and tapping tools for preparing a tapered insertion bore suitable for insertion of single tapered implant  300  or double tapered implant  400 . The disclosure of each of these patents and patent applications are incorporated herein by reference.  
       FIG. 14  diagrammatically illustrates two implants  10  inserted into a threaded bore between opposing vertebral bodies  450 ,  451 . It should be noted that in a preferred method, the openings  12   a - 12   c  of implants  10  are beyond the cortical end plates  452 ,  453  and provide exposure to cancellous bone  454 ,  455 . A bone support matrix can be packed around the implants  10 .  
       FIGS. 15-18  illustrate one preferred insertion tool  500 . Insertion tool  500  includes a tool body  502  extending from a proximal end  504  to a distal end  506 . In the illustrated embodiment, an internal bore  508  extends completely through the tool from the proximal end  504  to the distal end  506 . At the proximal end  504 , the bore can be provided with internal threads  510 . A handle  508  is provided at the proximal end  504  to permit a surgeon to manipulate the tool  500 .  
      At the distal end  506 , a plurality of grips are provided as best illustrated in  FIGS. 16 and 17 . The grips include threaded grips  522 , 523 . The threaded grips  522 ,  523  have opposing interior surfaces  524 ,  525  configured to slide into channels  24   a  and  24   b  of implant  10 . The exterior surfaces of the grips  522 ,  523  are provided with threads  526  and valleys  527  which are complimentary to helical thread portions  7   a  of the implant  10 .  
       FIG. 17  illustrates a perspective view of implant  10  and the distal end  506  of insertion tool  500 . The thread pattern  526  of the threaded grips  522 ,  523  matches the helical thread pattern of the threaded portions  7   a  of the implant  10  to defined a generally continuous thread pattern through the combination of the implant  10  and the tool  500 .  
      Referring now to the distal end view of tool  500  in  FIG. 18 a  preferred feature for assuring thread alignment between an implant  10  and insertion tool  500  is described. As illustrated, the lateral aspects  550  and  551  of each gripper  522  and  523 , respectively, each include a pair of tapered ridges  560  and  561 . The angle A formed between tapered ridges  561  is different than the angle B formed between tapered ridges  560 . However, angle A between tapered ridges  561  is identical to angle A of implant  10  and angle B of tapered ridges  560  is identical to angle B of implant  10  (see  FIG. 5 ). Thus, by providing different angles A and B on the distal end  506  of tool  500  which match with angles A and B of implant  10  only in a particular orientation, proper alignment of thread portions  7   a  of implant  10  and threads  526  of tool  500  is assured for proper insertion of the implant into a tapped insertion bore. In the illustrated embodiment, the opposing interior surfaces  524 ,  525  of the distal end  506  of grips  522  and  523  also include notches  570   a  and  570   b  which receive tabs  31   a  and  31   b  of implant  10 , respectively.  
      Referring now to  FIGS. 19 and 20 , an alternative embodiment of the distal end  506  of a tool  500  is illustrated. According to this embodiment, unthreaded grips  570  and  571  have opposing interior surfaces  572 ,  573  that provide for sliding grips  570  and  571  into channels  24   a  and  24   b  of implant  10 . However, as visualized best in  FIG. 20 , the lateral aspects  574 ,  575  of grips  570 ,  571 , respectively, do not include threads and do not extend to the lateral edges  25   a ,  25   b ,  26   a  and  26   b  of implant  10 .  
      The insertion tool  500 , with threaded or unthreaded grips as just described, can also to include two additional grips that slide into the regions between thread segments  101  and  103  and  102  and  104  of implant embodiment  100 . Such additional grips are illustrated, for example, in  FIGS. 24, 27 ,  28  and  31  of co-assigned U.S. Pat. No. 5,609,636, the entire disclosure of which is incorporated herein by reference.  
      Finally, an insertion tool as described above can also be prepared for tapered implants  300  and  400 . The difference being that grips  522  and  523  or  570  and  571  are tapered from the proximal end to the distal end as disclosed in co-pending application U.S. Ser. No. 08/902,083.  
      Having now described the present invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made in the invention without departing from the spirit or scope of the appended claims.