Patent Publication Number: US-2004059337-A1

Title: Bone preparation instruments and methods

Description:
[0001] The present application is a continuation-in-part of Ser. No. 09/611,237 filed Jul. 6, 2000. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This invention pertains to bone surgery. Specifically, the invention is directed to instrumentation and methods for preparing adjacent bones for receiving an implant therebetween. The invention is particularly advantageous for preparing an implant site for fusing vertebral bodies to facilitate fusion.  
       BACKGROUND OF THE INVENTION  
       [0003] Chronic back problems can cause pain and disability for a large segment of the population. Frequently, the cause of back pain is traceable to diseased disc material between opposing vertebrae. When the disc material is diseased, the opposing vertebrae may be inadequately supported, resulting in persistent pain.  
       [0004] Surgical techniques have been developed to remove the diseased disc material and fuse the joint between opposing vertebral bodies. Arthrodesis or fusion of the intervertebral joint can reduce the pain associated with movement of an intervertebral joint having diseased disc material. Generally, fusion techniques involve removal of the diseased disc and inserting a bone or non-bone implant between the opposing vertebral bodies to be fused.  
       [0005] Spinal fusion implants and related surgical instruments for implanting a fusion device are known and disclosed in, for example, U.S. Pat. Nos. 5,741,253; 5,722,977; 5,658,337; 5,609,636; 5,505,732; 5,489,308; 5,489,307; 5,458,638; 5,055,104; 5,026,373; 5,015,247; 4,961,740; 4,878,915; 4,834,757; 4,743,256; 4,501,269; and 3,848,601. The disclosure of each of these patents is incorporated herein by reference.  
       [0006] Often times, the degenerative changes of the diseased disc cause a collapse of the intervertebral disc space. Thus, prior to implantation, the intervertebral disc space may be distracted to restore the normal height of the disc space or the normal relationship between the vertebrae to be fused. Maintaining the restored disc space height and/or vertebral relationships throughout preparation of the implant site can be important for the ultimate stability at the fusion site.  
       [0007] The ease of use and efficiency of instruments and procedures used to prepare and place an implant at a fusion site is also very important to the overall success of the procedure. For example, in addition to other problems, removal of unequal amounts of bone on either side of the fusion site, particularly in vertebral fusion procedures, can result in over reaming of one vertebra relative to the adjacent vertebra and ultimately lead to a poorer surgical outcome.  
       [0008] Accordingly, there is a continuing need for instrumentation and methods that ensure precise placement of the implant as well as increasing the ease and efficiency of the implant procedure. The present invention is directed to this need.  
       SUMMARY OF THE INVENTION  
       [0009] The present invention is directed to bone cutting instruments and methods that provide efficient and precise preparation of a bore for receiving an implant between adjacent bones that are to be fused.  
       [0010] Throughout the specification, guidance may be provided through lists of examples. In each instance, the recited list serves only as a representative group. It is not meant, however, that the list is exclusive.  
       [0011] In one embodiment, the instruments of the invention include bone cutting instruments having paddles that can be inserted between adjacent bones to maintain a fixed spacing between the bones during preparation of the bones for fusion. In one embodiment, the bone cutting instruments include a cutting edge that is fixedly mounted to the spacing paddles. In an alternative embodiment, the paddles can be mounted to a channel guide that provides a track for slidably positioning the cutting edge at the site of bone preparation.  
       [0012] In a typical embodiment, a bone cutting instrument includes a cutting edge that extends beyond the height dimensions of the paddles with a portion of the cutting edge extending between the paddles. Depending on the configuration of the implant to be inserted between bones, the cutting edge can be circular, oval, rectangular, etc.  
       [0013] In another embodiment, a bone cutting instrument includes first, second, third, and fourth cutting edges that define an interior void, with the first and second cutting edges being opposite and extending beyond the third and fourth cutting edges.  
       [0014] In a further embodiment, the invention involves a rasp adapted to define a recess between two bone surfaces. The rasp includes a shaft and a rasp head with an arcuate distal surface. At least one of the transverse surfaces of the rasp is roughened. Examples of roughened surfaces include knurls, etchings, ridges, grooves, teeth, etc.  
       [0015] In a still further embodiment, the invention involves an implant insertion tool having a shaft with a pair of arms movable between a spaced apart holding position and a close together releasing position. The distal ends of the arms are shaped to fit inside an implant. The insertion tool also involves a sleeve operable on the arms to force the arms together towards the releasing position. The sleeve is hollow and is slidably mounted on the shaft. The sleeve forces the arms together to release the implant when the sleeve is in the engaging position. In an alternative embodiment, the sleeve is internally threaded and the shaft is externally threaded, with the threads matching. In this embodiment, the sleeve is rotated to move it along the shaft. The invention also provides kits comprising one or more instruments of the invention having various paddle and/or cutting edge heights, widths or shapes for preparing an implant site of a predetermined size or shape. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a side view of one embodiment of a bone cutting instrument according to the invention.  
     [0017]FIG. 2 is a close-up perspective view of the distal end of the bone cutting instrument of FIG. 1.  
     [0018]FIG. 3 is a distal end-on view of the bone cutting instrument of FIG. 1.  
     [0019]FIG. 4 is a perspective view of one embodiment of a non-bone implant suitable for use according to the invention.  
     [0020]FIG. 5 is a perspective of an alternative embodiment of a bone cutting instrument according to the invention, including a channel guide and first and second mandrels.  
     [0021]FIG. 6 is a view of the same bone cutting instrument of FIG. 5 with the first and second mandrels removed and a bone chisel in the place of the first mandrel.  
     [0022]FIG. 7 is a top plan view of the channel guide of FIG. 5 (the opposite side being substantially identical).  
     [0023]FIG. 8 is a side view of the channel guide of FIG. 7 (the opposite side view being substantially identical).  
     [0024]FIG. 9 is a distal end view of the channel guide of FIG. 7.  
     [0025]FIG. 10 is a proximal end-on view of the channel guide of FIG. 7.  
     [0026]FIG. 11 is a top plan view of an alternative embodiment of a channel guide according to the invention.  
     [0027]FIG. 12 is a side plan view of the channel guide of FIG. 11.  
     [0028]FIG. 13 is a distal end-on view of the channel guide of FIG. 11.  
     [0029]FIG. 14 is a proximal end-on view of the channel guide of FIG. 11.  
     [0030]FIG. 15 is a top plan view of one embodiment of a mandrel according to the invention.  
     [0031]FIG. 16 is a side plan view of the mandrel of FIG. 15.  
     [0032]FIG. 17 is a transverse cross-section view of the mandrel of FIG. 15 through line  16 - 16 .  
     [0033]FIG. 18 is a top plan view of one embodiment of a bone chisel according to the invention.  
     [0034]FIG. 19 is a longitudinal cross-section view taken through line  18 - 18  of the bone chisel of FIG. 18.  
     [0035]FIG. 20 is a distal end view of the bone chisel of FIG. 18.  
     [0036]FIG. 21 is a diagrammatical illustration of adjacent vertebrae having channels for receiving implants and prepared according to the invention.  
     [0037]FIG. 22 is a top view of another embodiment of a bone cutting instrument according to the invention.  
     [0038]FIG. 23 is a side view of the bone cutting instrument of FIG. 22.  
     [0039]FIG. 24 is a distal end-on view of the bone cutting instrument of FIG. 22.  
     [0040]FIG. 25 is a top view of a rasp according to the invention.  
     [0041]FIG. 26 is a side view of the rasp of FIG. 25.  
     [0042]FIG. 27 is a proximal end-on view of the rasp of FIG. 25.  
     [0043]FIG. 28 is an enlarged partial perspective view of the teeth on the rasp head of the invention.  
     [0044]FIG. 29 is an enlarged partial top view of the rasp head of FIG. 25.  
     [0045]FIG. 30 is a top view of an alignment pin of the invention.  
     [0046]FIG. 31 is a side view of the alignment pin of FIG. 30.  
     [0047]FIG. 32 is an exploded perspective view of the alignment pin of FIG. 30.  
     [0048]FIG. 33 is a top view of the handle of the alignment pin of the invention.  
     [0049]FIG. 34 is a side view of the handle of FIG. 33.  
     [0050]FIG. 35 is a distal end-on view of the handle of FIG. 33.  
     [0051]FIG. 36 is a perspective view of a collar of the alignment pin of the invention.  
     [0052]FIG. 37 is a top view of the collar of FIG. 36.  
     [0053]FIG. 38 is a side view of the collar of FIG. 36.  
     [0054]FIG. 39 is an end-on view of the collar of FIG. 36.  
     [0055]FIG. 40 is a top view of an implant insertion tool of the invention.  
     [0056]FIG. 41 is a side view of the implant insertion tool of FIG. 40.  
     [0057]FIG. 42 is a distal end-on view of the implant insertion tool of FIG. 40.  
     [0058]FIG. 43 is a side view of a sleeve of the invention.  
     [0059]FIG. 44 is a side cross-sectional view of the sleeve of FIG. 43.  
     [0060]FIG. 45 is an end-on view of the sleeve of FIG. 43.  
     [0061]FIG. 46 is a top view of an insertion tool handle of the invention.  
     [0062]FIG. 47 is a cross-sectional view of the handle of FIG. 46.  
     [0063]FIG. 48 is a cross-sectional view of the handle of FIG. 46.  
     [0064]FIG. 49 is a perspective view of a two-part implant.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0065] The present invention is directed to instruments and methods for preparing an implant site for receiving an implant between adjacent bones to be fused. The instruments of the invention can be advantageously used for fusion of joints. In some embodiments, the instruments and methods disclosed are particularly advantageous for preparing an implant site for fusing cervical, thoracic and/or lumbar intervertebral joints. Thus, for exemplary purposes, the instruments and methods of the invention will be described with reference to fusion of a lumbar intervertebral joint. However, it will be appreciated that the disclosed instruments and methods can be used for fusion of all types of bones and particularly bones adjacent to a joint space. In the case of fusing an intervertebral joint, the invention can be performed using an anterior, posterior or lateral approach to the patient&#39;s vertebrae.  
     [0066] As used herein, an “implant” includes any implant suitable for facilitating fusion between adjacent bones and includes implants prepared from known implant materials including, non-bone material such as titanium, stainless steel, porous titanium, ceramic, etc. or bone including heterologous, homologous, autologous, artificial bone, etc. The implants suitable for the invention also can be threaded implants or non-threaded.  
     [0067] An “implant site” refers to the location for placement of an implant between adjacent bones, such as adjacent vertebrae. In a typical embodiment for vertebral fusion, the implant site can be a channel prepared by removing a notch from the opposing end plates of first and second vertebral bodies adjacent the intervertebral joint space. Preferably the notches are made through the articular cartilage and cortical bone into the cancellous bone. It will be appreciated that the notches formed in the bone can be any shape suitable for receiving an implant of a particular shape including, for example, rectangular, circular, oval, etc. In the case of a circular channel, after forming the channel, the channel can be threaded, using known tap systems, for receiving a threaded implant.  
     [0068] Preparing an implant site according to the invention can be performed more quickly and easily than prior procedures and can significantly reduce surgery time and costs. Some cutting tools previously used to prepare implant sites are easy to use but lack certain precision characteristics. For example, the distal end of some cutting tools may be vulnerable to shifting from a desired location during cutting due to a lack of vertical stability, caused by, for example, irregularities or undulations at the surface of the vertebrae against which the distal end of the cutting tool is placed during cutting.  
     [0069] The disclosed devices can provide greater vertical stability and, in the case of vertebral fusion, help to ensure that an equal amount of bone is removed from the endplates of the vertebrae on either side of the joint space. Removing equal amounts of bone can facilitate greater coaptation between the implant and the implant channel, greater fusion stability, greater motion segment stability, faster fusion, reduced pain, reduced chance of migration, reduced chance of subsidence, etc.  
     [0070] Throughout the specification, unless stated otherwise, the terms “proximal” and “distal” are relative terms, the term “proximal” referring to a location nearest the surgeon and the term “distal” referring to a location farthest from the surgeon. So, in the case of performing a vertebral fusion from an anterior approach, the anterior surfaces of the vertebrae are “proximal” and the posterior surfaces of the vertebrae are “distal” relative to the surgeon performing the procedure. Likewise, in a posterior approach, the posterior vertebral surfaces are proximal and the anterior surfaces are distal.  
     [0071] Generally, when preparing an implant site instruments used to prepare the site are advanced into the disc space from a proximal to distal direction. That is, in an anterior approach the instruments are advanced from the anterior surface (proximal) towards the posterior surface (distal) and in a posterior approach the instruments are advanced from the posterior surface (proximal) towards the anterior surface (distal). Similar relative orientations also apply for lateral approaches.  
     [0072] As used herein, the “depth” of a vertebra is defined as the anterior posterior dimension of the vertebrae. The “width” of the vertebrae is the dimension from the right lateral edge to the left lateral edge. The “height” of the disc space is the dimension from the superior endplate to the inferior endplate of opposing vertebrae.  
     [0073] The implants can be sized for a particular application. For example, for stabilizing a lumbar disc space, the implant may have a height dimension “H” of about 2 mm to about 30 mm, a width dimension “W” of about 6 mm to about 40 mm and a length dimension “L” of about 10 mm to about 40 mm. Other sizes will be appreciated as being within the scope of the invention after review of the present disclosure. One instrument of the invention, a bone cutting instrument or channel guide, has a proximal end and a distal end with a pair of paddles extending from the distal end of the instrument. In use, the paddles are placed into the space between the bones to be fused to provide vertical stability of the device as well to maintain a selected spacing between the bones, which is determined by the height of the paddles.  
     [0074] In general, the instruments will be available having varied paddle heights and varied widths between paddles. For cervical vertebral applications a typical range of paddle heights can be approximately 2 mm to 12 mm, in 1 mm increments. For lumbar applications a typical range of paddle heights can be approximately 3 mm to 18 mm in 1 mm increments. However, larger or smaller widths with larger or smaller increments can be available as needed. Thus, for example, in the case of vertebral fusion, a range of paddle heights will be available to establish and maintain a selected joint space height between the vertebrae during preparation of the implant site.  
     [0075] Instruments having various widths or spacing between paddles will be available for different procedures. For example, if a single implant is to be used, it will typically have a greater width, and thus require a preparation instrument having a greater spacing between paddles, than if multiple implants will be used. A typical width between paddles for a bone cutting instrument for placing a single implant can be about 4 mm to 40 mm. A typical width between paddles for a bone cutting instrument for implanting two implants between lumbar vertebrae will be about 4 mm to 24 mm.  
     [0076] The distal tip of the paddles can be tapered to facilitate insertion of the paddles into the joint space. In addition, the opposing edges of the paddles can have a convergent or divergent taper from the distal tip to a proximal aspect of the paddle. A tapered paddle can provide a lordotic taper to the joint space to create a channel for receiving a tapered implant for restoring or creating a particular degree of lordosis between the adjacent vertebral bodies.  
     [0077] A bone cutting instrument of the invention also includes a cutting edge. As will be further described below, the cutting edge can be separable or non-separable from the paddles. In the case of a separable cutting edge, the instrument can include one or more tracks to guide the cutting edge to a particular location. The cutting edge can be rectangular or circular, oval, elliptical, oblong, etc. In one embodiment, the cutting edge is a three-sided rectangle and provides for removing a rectangular notch of bone.  
     [0078] The invention can be used with known starter guides, depth gauges, taps and implant drivers used for preparing or inserting an implant into an implant site. Examples of suitable instruments are disclosed in U.S. Pat. Nos. 5,722,977; 5,658,337; 5,609,636; 5,489,307; 5,484,638; 4,834,757; 3,848,601, etc., the entire disclosures of which are incorporated herein by reference.  
     [0079] In another embodiment, instruments for preparing a channel in adjacent bones and for inserting an implant are sized to be used with a particular sized implant or implants. In this embodiment, each different size of implant has a corresponding set of bone cutting and implant inserting instruments. The instruments of the invention can be provided in kits including guides having paddles of different lengths and widths and correspondingly sized bone cutting edges for spacing bones and preparing implant sites for implants of various shapes and sizes. In another embodiment, the bone cutting instrument, rasp, and implant insertion tool can be provided in a kit, with or without an implant sized and configured to be implanted using the instruments. In a further embodiment, a kit can be provided that includes a plurality of incrementally sized implants and incrementally sized bone cutting instruments, rasps, and implant insertion tools so the user can select the appropriate size needed for a particular procedure.  
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT  
     [0080] I. Instruments  
     [0081] The instruments and methods of the invention will now be described by reference to the accompanying drawings. The illustrated embodiments and description are provided only for exemplary purposes to facilitate comprehension of the invention and should not be construed to limit the scope of the invention.  
     [0082]FIG. 1 is a side view and FIG. 2 an enlarged perspective view of the distal end of one embodiment of a bone cutting instrument  10  according to the invention. Instrument  10  has a proximal end  15  and a distal end  16  spaced along longitudinal axis X-X. At the proximal end  15  of shaft  17  there is a handle  18  for operating instrument  10 . At the distal end  16 , instrument  10  includes a first paddle  20 , a second paddle  21  and a cutting edge  23 . In the illustrated embodiment, cutting edge  23  is at the distal end of chamber  25 . Proximal to cutting edge  23 , chamber  25  can include one or more openings  24  for passage of bone debris collected within chamber  25  during cutting.  
     [0083] Paddles  20  and  21  include a tapered distal tip,  20   a  and  21   a,  respectively, to facilitate insertion of instrument  10  between adjacent bones. Proximal to the tapered distal ends  20   a  and  21   a,  instrument  10  also includes markings  30  such as notches  31 - 34  at predetermined distances from distal tips  20   a  and  21   a.  During use, markings  30  provide the surgeon with an indication of the depth of distal penetration of instrument  10  between adjacent bones. Furrows  36  and  37  (not visible) are present along a portion of the sides  40  and  41 , respectively, of paddles  20  and  21 . Furrows  36  and  37  provide a reduced surface area of paddle sides  38  and  39  and thus facilitate removal of instrument  10  from between adjacent bones.  
     [0084]FIG. 3 is a distal end-on view of instrument  10  showing that paddles  20  and  21  each have the same height dimension P H  and a width dimension W P  between paddles  20  and  21 . A portion of cutting edge  23  is shown to extend beyond height dimension P H  at location  40  and  41  and a portion of cutting edge  23  is within the spacing between paddles  20  and  21  at locations  42  and  43 . The perimeter configuration of cutting edge  23  in FIG. 3 is a parallelepiped shape particularly suited for preparing a channel or implant bore between adjacent bones for insertion of an implant having a cross-sectional configuration such as that of the implant shown in FIG. 4. It will be appreciated, however, that the perimeter configuration of cutting edge  23  can be square, rectangular, circular, oval, etc., depending on the external configuration of the implant to be inserted into the channel. In the illustrated embodiment, paddles  20 , and  21  are fixedly attached to cutting edge  23 . The paddle length can vary to correspond with the depth of the vertebrae.  
     [0085] For any particular perimeter configuration, bone cutting instruments  10  will be available which have incrementally varied sizes of cutting edge  23  corresponding to a particular size implant. In addition, bone cutting instruments  10  having paddles with varied heights P H  will be available to permit the surgeon to select a paddle height corresponding to a particular disc space height. In addition, it will be appreciated that the illustrated paddle edges  44   a,    44   b  (and  45   a,    45   b ) are parallel. In alternative embodiments, edges  44   a,    44   b  (and  45   a,    45   b ) can form a converging or diverging taper.  
     [0086]FIG. 5 is a perspective view of an alternative embodiment of a bone cutting instrument  100 . According to this embodiment, bone cutting instrument  100  has a proximal end  101 , a distal end  102  and includes a channel guide  103 , first mandrel  104  slidably received within a first track  112  and a second mandrel  105  slidably received within a second track  113 . FIG. 6 illustrates bone cutting instrument  100  with first and second mandrels  104 ,  105  removed from tracks  112  and  113  of channel guide  103 , and a bone chisel  106  slidably passed into track  112 .  
     [0087]FIG. 7 is a top plan view of the channel guide  103 , FIG. 8 is a side view, FIG. 9 is a distal end-on view and FIG. 10 is a proximal end-on view. Channel guide  103  includes a distal end  110 , a proximal end  111  and a first track  112  and a second track  113  extending from the proximal end  111  to the distal end  110 . Track  112  includes a base  112   a,  a first rail  112   b  and a second rail  112   c.  Track  113  includes a base  113   a,  a first rail  113   b  and a second rail  113   c.  In the illustrated embodiment, base  112   a  of track  112  and base  113   a  of track  113  are on opposing surfaces of rail spacing member  114 .  
     [0088] Extending distally from distal end  110 , channel guide  103  includes a first paddle  120  and a second paddle  121 . Paddle  120  has a first edge  120   a,  a second edge  120   b  and a tapered distal end  120   c.  Likewise, paddle  121  has a first edge  121   a,  a second edge  121   b  and a tapered distal end  121   c.  Paddle spacing member  115  extends between paddles  120  and  121  and has a first base surface  115   a  continuous with base  112   a  of track  112 , a second base surface  115   b  continuous with base  113   a  of track  113  and a tapered distal tip  115   c  coterminus with tapered distal ends  120   c  and  121   c.  Paddles  120  and  121  also have a width dimension We therebetween, spaced apart by a width of spacing member  115 . Tapered distal tips  115   c,    120   c  and  121   c  facilitate insertion of the paddles between adjacent bones.  
     [0089] Paddle  120  has a major height dimension P H1  between edge  120   a  and  120   b.  Paddle  120  also has a minor height dimension P H2  between base surface  115   a  and edge  120   a  and an equal minor height dimension P H2  between base surface  115   b  and edge  120   b.  Paddle  121  has the same height dimensions P H1  and P H2  as paddle  120 .  
     [0090] In the illustrated embodiment, a portion of track  112  includes a wall  125  extending between rails  112   b  and  112   c  and parallel to base  112   a.  Shown best in FIG. 10, wall  125  extending between rails  112   b  and  112   c  forms an enclosed lumen  126  over a proximal portion of track  112 . In a similar manner, a portion of track  113  includes a wall  127  extending between rails  113   b  and  113   c  which forms enclosed lumen  128  in that portion of track  113  where wall  127  is present. Each of lumens  126  and  128  has a height dimension L H . As best seen in FIG. 10, if walls  125  and  127  are ignored, and channel guide  103  viewed from the proximal end with rail spacing member  114  oriented in a vertical plane, channel guide  103  can have an “I beam” shaped configuration.  
     [0091] At the junction of the distal end  110  of channel guide  103  with paddles  120  and  121 , shoulders  130   a,    130   b  and  131   a,    131   b  are formed. Shoulders  130   a - 131   b  provide an affirmative stop to stop distal advancement of bone cutting instrument  100  when paddles  120  and  121  are inserted into an intervertebral disc space between adjacent vertebrae.  
     [0092] FIGS.  11 - 14  illustrate an alternative embodiment of a channel guide  150  according to the invention. Channel guide  150  is substantially identical to channel guide  103  except that channel guide  150  has a circular cross-section. However, similar to channel guide  103 , channel guide  150  includes a first track  151  and a second track  152 . Track  151  includes a base  151   a,  a first rail  151   b  and a second rail  151   c.  Likewise, track  152  includes a base  152   a,  a first rail  152   b  and a second rail  152   c.  Extending a portion of the length of channel guide  150  from proximal end  153 , rails  151   b  and  151   c  are continuous with one another forming an enclosed lumen  155  over track  151 . A similar enclosed lumen  156  is present over track  152 . Each of hemi-circular lumens  155  and  156  has a maximum lumen height L H .  
     [0093] Paddles  157  and  158  extend distally from the distal end  160  of tracks  151  and  152  respectively. Paddles  157  and  158  have a curved cross-section and each has a major height dimension P H1  extending from edge  157   a  to edge  157   b  and from edge  158   a  to edge  158   b.  Each of paddles  157  and  158  have a first and second minor height dimension P H2  as described for channel guide  103 . Shoulders  170   a    170   b  are formed at the junction of distal end  160  and paddles  157  and shoulders  171   a  and  171   b  are formed at the junction of distal end  160  and paddle  158 . It will be appreciated that various cross-sectional configurations for channel guides are within the scope of the invention, in addition to the rectangular cross-section of channel guide  103  and the circular cross-section of channel guide  150 .  
     [0094]FIG. 15 is a top plan view of mandrel  104  ( 105  being identical) shown in FIG. 5; FIG. 16 is side plan view of mandrel  104  and FIG. 17 is a transverse cross-section view of mandrel  104  taken through line  16 - 16 . Mandrel  104  includes a distal end  201 , a proximal end  202  and a shaft  203  extending therebetween. As best seen in FIG. 15, mandrel  104  has a gap surface  205  and a tapered distal tip  206  at distal end  201 . Mandrel  104  has a shaft height M S  along a portion of shaft  203 , a distal end height M D  at distal end  201  and a proximal end height Me at proximal end  202 . Preferably, distal end height M D  is substantially equal to minor height P H2  of paddles  120  and  121 . Thus, when height M D  of mandrel  104  is equal to minor height P H2  of paddles  120  and  121 , a flush surface is provided extending from edge  120   a  of paddle  120  across gap surface  205  and edge  121   a  of paddle  121  (see FIG. 5). A similar flush surface is formed between mandrel  105  and paddle edges  120   b  and  121   b.    
     [0095] With mandrels  104  and  105  inserted within tracks  112  and  113  the space between paddles  120  and  121  is filled out. Thus, when inserted into an intervertebral disc space, pressure exerted by the bone cutting instrument  100  on each of the opposing vertebrae is not localized only on the edges of the paddles, but rather the pressure is distributed across the entire surface area between the paddles and including the gap surfaces of the mandrels. It will be appreciated that if the distracting guide has a cylindrical cross-section as illustrated in, for example, FIG. 11, the mandrel will have a corresponding shape including the features described for rectangular shaped mandrel  104  and  105 .  
     [0096] Shaft height M S  of mandrel  104  is provided to pass within track height L H  in close tolerance within lumen  126  (or  128 ) of channel guide  103 . The proximal end height M P  of mandrel  105  at the proximal end  202  is selected to be greater than shaft height M S  of shaft  203  to form a shoulder  207 . Shoulder  207  affirmatively stops distal advancement of mandrel  104  along track  112  when shoulder  207  abuts rail spacing member  114  of channel guide  103 . Second mandrel  105  can be configured identical to mandrel  104  to pass into track  113  and from base surface  115   b  to edges  120   b  and  121   b.    
     [0097] In a typical embodiment, paddle major height dimension P H1  can be about 3 to 15 mm, paddle minor height dimension P H2  about 1 to 7 mm, lumen height dimension L H  about 2 to 13 mm and mandrel proximal height dimension M P  of about 1 to 2 mm greater than lumen height dimension L H . For example, in the illustrated embodiment 100, the paddle major height dimension can be P H1  is 8 mm, the paddle minor dimension P H2  can be about 3.5 mm, the lumen height dimension L H  5 mm and the mandrel proximal height dimension about M P  6 mm.  
     [0098]FIG. 18 is a top plan view of one embodiment of a bone chisel  106  shown in FIG. 6. FIG. 19 is a longitudinal cross-section view through line  19 - 19 , and FIG. 20 is a distal end-on view of bone chisel  106 . Bone chisel  106  includes a proximal end  301 , distal end  302  and shaft  303  therebetween. Cutting surface  304  is at distal end  302 . Cutting surface  304  is a rectangular cutting surface  305  including longitudinal cutting edge  306  and first lateral cutting edge  307  and second lateral cutting edge  308 .  
     [0099] Distal end  302  of chisel  106  has a first chisel height C 1 . In a typical embodiment, the difference between first chisel height C 1  and paddle minor dimension P H2  determines the amount of bone removed from a bone end during bone cutting. Thus, to remove about 1 mm of bone from the end of the bone, the difference between C 1  and P H2  is about 1 mm. C 1  is typically selected to be about 1 to 3 mm greater than paddle minor dimension P H2 . In the case of cutting bone from vertebral endplates, C 1  is preferably sufficient to cut deep enough into the endplate to remove the articular cartilage and cortical bone to expose cancellous bone.  
     [0100] The distal end  302  of bone chisel  106  also includes a groove  310  extending a distance proximally from cutting surface  304  between cutting edges  306 - 308 . As illustrated in the longitudinal cross-section view of FIG. 19, groove  310  has a depth less than chisel height C 1  and provides for proximal passage of bone, cartilage or other debris as chisel  106  is advanced distally to cut between adjacent bones.  
     [0101] At proximal end  301 , bone chisel  106  has a second chisel height C 2 . A shoulder  320  is formed where chisel heights C 1  and C 2  meet. Chisel height C 1  is selected to provide for bone chisel  106  to pass in close tolerance within lumen  126  (or  128 ) of channel guide  103 . Shoulder  320  affirmatively stops distal advancement of bone chisel  106  within tracks  112  or  113  when shoulder  320  abuts against wall  125  or  127  at the proximal end  111  of channel guide  103 .  
     [0102] FIGS.  22 - 49  disclose various components of an embodiment for implanting an implant (e.g., implant  900  of FIG. 49) into an intervertebral space. Generally, the system includes a set of differently sized rasps (e.g., see rasp  600  of FIGS. 25 and 26) for determining the implant size required to match an intervertebral space. The system also includes a set of differently sized cutting tools (e.g., see cutting instrument  510  of FIGS.  22 - 24 ) for cutting bone surrounding the intervertebral space. The size and shape of the cutting tool selected is determined by the size and shape of the rasp. Preferably, the cutting tool slides onto the rasp. Before or after cutting, the rasp can be manipulated to condition or roughen a portion of the site for the implant. Thereafter, the rasp and the cutting tool can be removed. Finally, the implant(s) is inserted into the space.  
     [0103]FIG. 22 is a top view and FIG. 23 a side view of another alternative embodiment of a bone cutting instrument  510  according to the invention. Instrument  510  has a proximal end  515  and a distal end  516  spaced along longitudinal axis X-X. At the proximal end  515  of shaft  517  there is a handle  518  for operating instrument  510 . The handle  518  has a roughened area  519  that can be in the form of knurls, etchings, grooves, ridges, or other suitable patterns to enhance manual gripping of the handle  518 . At the distal end  516 , instrument  510  includes a first cutting edge  520 , a second cutting edge  521 , and third and fourth cutting edges  522  and  523 . In the illustrated embodiment, cutting edges  520 ,  521 ,  522  and  523  are at the distal end of chamber  525 . First, second, third, and fourth cutting edges  520 ,  521 ,  522  and  523  are beveled  520   a,    521   a,    522   a,  and  523   a,  respectively, to facilitate cutting and removal of bone. An internal hollow bore  527  extends from the proximal end  515  through the instrument  510  to the distal end  516  to receive a rasp  600  and to receive bone.  
     [0104] In the illustrated embodiment, elongated openings  550  and  551  extend through the handle  518  and shaft  517 , respectively, of the instrument  510 . As described later in the specification, opening  550  allows for alignment of the cutting instrument  510  with rasp  600 . Opening  551  provides additional access to the internal bore  527  for cleaning the instrument and reduces the weight of the instrument.  
     [0105]FIG. 24 is a distal end-on view of instrument  510  showing that first and second cutting edges  520  and  521  define a height dimension C H  and the cutting edges  522  and  523  define a width dimension W C . The perimeter configuration of cutting edges  520 ,  521 ,  522 , and  523  in FIG. 24 is a rectangular shape particularly suited for preparing a channel or implant bore between adjacent bones for insertion of a two-part implant having a configuration such as that of the implant  900  shown in FIG. 49.  
     [0106] In the illustrated embodiment of FIG. 49, implant  900  is shown with growth component  901 , such as cancellous bone, and support component  902 , such as cortical bone. The growth component  901  has a similar size and shape as the distal end of the cutting instrument  510  (e.g., dimension A of growth component  901  corresponds to dimension W C  of cutting instrument  510  and dimension B of growth component  901  corresponds to dimension C H  of cutting instrument  510 ). Also, the rounded nose of the growth component  901  corresponds to the curvature of edges  520  and  521  of the cutting instrument  510 . The support component  902  has a similar size and configuration as the rasp head (see for example FIGS. 25, 26). The support component  902  of the implant may be the same size as the rasp head, or it can be larger than the rasp head. The support component  902  of the implant can be about 0 mm to about 4 mm larger in height than the rasp head. The height dimension C H  of the bone cutting instrument can be from about 0 mm to about 10 mm taller overall than the support component of the implant. It will be appreciated, however, that the perimeter configuration of cutting edges  520 ,  521 ,  522 , and  523  can be square, circular, oval, etc., depending on the external configuration of the implant to be inserted into the channel. The length of the first and second cutting edges  520  and  521  can vary to correspond with the depth of the vertebrae.  
     [0107] To cut different sized channels, a set of bone cutting instruments  510  will be available which has instruments with incrementally different sizes of cutting edges  520 ,  521 ,  522 ,  523  corresponding to a particular size implant. For example, bone cutting instruments  510  having first and second cutting edges  520 ,  521  with different heights C H  will be available to permit the surgeon to select a cutting edge height corresponding to a particular disc space height. In addition, it will be appreciated that the illustrated cutting edges  520  and  521  (and  522  and  523 ) are parallel. In alternative embodiments, cutting edges  520  and  521  (and  522  and  523 ) can form a converging or diverging taper.  
     [0108]FIG. 25 is a top view and FIG. 26 a side view of a rasp  600  according to one embodiment of the invention. The rasp  600  is inserted into the intervertebral space, and functions both as a trial sizer, i.e. for a particularly sized and shaped implant, and rasp. Rasp  600  has a proximal end  601  and a distal end  602  spaced along longitudinal axis X-X. At the proximal end  601  of shaft  603  there is a roughened area  604  that can be in the form of knurls, etchings, grooves, ridges, or other suitable patterns to enhance manual gripping of the shaft  603 . An opening  605  for receiving a portion of an alignment pin extends transversely through the proximal end  601  of the shaft  603 , and allows for alignment of the rasp with cutting instrument  510 .  
     [0109] At the distal end  602 , rasp  600  includes a rasp head  606 . In the illustrated embodiment, rasp head  606  includes an outer wall  607 , an inner wall  608  and has a generally “C-shaped” configuration with a first arm  609  continuous with a second arm  610 . The inner wall  608  defines a pocket or receptacle which is sized to complement and receive the distal end of the cutting instrument  510 . The first arm  609  and second arm  610  are spaced apart from the shaft  603 . Rasp head  606  includes a first engaging surface  611  and a second engaging surface  612 . In the illustrated embodiment, the first and second engaging surfaces  611 ,  612  have ridges  613 .  
     [0110] As illustrated best in FIG. 27, in this embodiment, rasp head  606  has a major height H M  and minor height H m . The taper from the major height to the minor height can be from about 0° to about 16°. The shape and configuration of the rasp head  606  corresponds to the shape and configuration of an implant. In one embodiment, the rasp head  606  corresponds in size and configuration with the support component  902  of a two-part implant  900 . The space between the first and second arms  609 ,  610  of the rasp head  606  corresponds with the growth component  901  of the implant. It will be appreciated, however, that the configuration of the rasp head  606  can be square, rectangular, circular, oval, etc., depending on the configuration of the implant(s) to be inserted into the channel.  
     [0111] As a trial sizer, the rasp  600  provides a means for determining the appropriate size bone cutting instrument and implant to use for a particular implant site. Multiple rasps  600  are provided, with incrementally different sized, shaped, and/or tapered rasp heads  606  corresponding to different sized, shaped, and/or tapered implants. The surgeon inserts and removes the various rasps  600  and determines which one is the correct size for the intervertebral space. The ridges  613  on the upper and lower surfaces of the rasp head act as a rasp to condition the upper and lower surfaces of the channel between the vertebrae.  
     [0112] Proximal to the distal end  602 , the shaft  603  of the rasp  600  also includes markings  614  at predetermined distances from the distal edge  615  of the rasp head.  
     [0113] During use, markings  614  provide the surgeon with an indication of the depth of distal penetration of rasp  600  between adjacent bones.  
     [0114] In one embodiment, the rasp shaft  603  is slidably received within the internal bore  527  of the bone cutting instrument  510 , with the opening  605  in the rasp shaft  603  accessible through the opening  550  in the bone cutting instrument handle  518 . An alignment pin  700  (see FIGS.  30 - 39 ) is inserted through openings  550 ,  605  to align the rasp within the bone cutting instrument. When the alignment pin  700  is in place, the shaft  603  of the rasp rides within the opening  550  in the bone cutting instrument  510  to maintain rotational alignment and limit the axial movement to within a predetermined range corresponding to the length of opening  550 . The aligmnent pin  700  also can function as a handle when the bone cutting instrument/rasp combination is forced between (e.g. hammered into) the adjacent bones to create the implant channel.  
     [0115] The alignment pin  700  has a handle  701 , a collar  702 , and a pin  703 . The handle  701  can have a roughened surface that can be in the form of knurls, etchings, grooves, ridges, or other suitable patterns to enhance manual gripping of the handle  701 . Alternatively, the handle  701  can be a shape that enhances manual gripping. In the illustrated embodiment, the handle  701  has indentations  704  to enhance manual gripping. The collar  702  is mounted to the handle  701  and the pin  703  is mounted to the collar  702 . The collar  702  can be of any shape. In the illustrated embodiment, the collar  702  has a circumferential ridge  705 . The ridge  705  has opposing flattened areas  706  that allow for unimpeded hammering of the bone cutting instrument and rasp while manually grasping the alignment pin.  
     [0116] Once a channel is cut into adjacent bones, an implant or implants are inserted to fuse the adjacent bones and provide stability to the site. In one embodiment, a two part implant  900  such as that shown in FIG. 49 is used. The support component  902  is inserted into the channel, and then the growth component  901  is inserted. FIGS. 40 and 41 illustrate one embodiment of an implant insertion tool  800  suitable for use with embodiments of the bone cutting instrument  510  and rasp  600  of the invention. As illustrated, implant insertion tool  800  has a proximal end  801  and a distal end  802  having a working end  803 . Working end  803  includes tabs  804  and  805  that fit cooperatively within grooves  903  of the support component  902  of an implant (see FIG. 49). In addition, the working end  803  includes a slot  806  that permits resilient/elastic arms  807  and  808  to flex or expand laterally away from axis A T .  
     [0117] In a typical embodiment, arms  807  and  808  are spring biased to expand away (e.g., laterally) from axis AT in the normal, relaxed position. A sleeve  820  (FIGS.  43 - 45 ) can then be slid from the proximal end  801  of the insertion tool  800 , over the slot  806 , to force arms  807  and  808  towards (e.g. medially) axis A T . That is, when the sleeve is advanced distally it brings arms  807  and  808  together towards axis A T . In this position, the working end  803  of implant insertion tool  800  can be inserted into an implant. Similarly, where useful for additional control, tabs  804  and  805  can be inserted into grooves  903  of an implant. The sleeve can then be slid towards the proximal end to allow arms  807  and  808  to expand away from axis A T  to provide friction holding of an implant on the working end  803 . After placement of an implant, the sleeve can be slid distally to bring arms  807  and  808  back toward axis A T  to remove implant insertion tool  800 , leaving the implant in place. Other arrangements providing for expansion and contraction of arms  807 ,  808 , relative to axis A T  also are contemplated by this disclosure  
     [0118] Thus, an implant can be mounted on the working end  803  of implant insertion tool  800  allowing the surgeon to manipulate an implant via tool  800  into a suitable position at the fusion site.  
     [0119] In one embodiment, the implant insertion tool  800  includes a sleeve  820  (FIGS.  43 - 45 ) and a handle  850  (FIGS.  46 - 48 ). In the illustrated embodiment, the insertion tool  800  has a threaded region  809  at the proximal end  801 . The threaded region  809  mates with the distal end  851  of the handle  850 . The handle has a roughened area  852  that can be in the form of knurls, etchings, grooves, ridges, or other suitable patterns to enhance manual gripping of the handle  850 . In one embodiment, the distal end  851  of the handle  850  has exterior threading to match internal threading  821  on a sleeve  820 .  
     [0120] The sleeve  820  is hollow and has a bore  822  extending from the proximal end  823  to the distal end  824 , and which is sized to fit over the proximal end  801  of the implant insertion tool  800 , and to threadibly advance, i.e. distally, over slot  806 , to force arms  807  and  808  towards axis A T .  
     [0121] II. Methods  
     [0122] The instruments of the invention can be used to prepare a channel of a selected configuration between adjacent bones, and to insert an implant or implants into the prepared channel. For exemplary purposes, the methods of the invention will be described with respect to preparing a channel between adjacent vertebral bodies. However, it will be appreciated that the principles and methods can also be applied to preparing a channel between other bones.  
     [0123] The present invention will first be described with reference to use in a posterior approach. In a posterior approach, a surgeon seeks access to the spine through the back of the patient. An alternative approach is the lateral, approach where the patient is on his side. Another alternative approach is an anterior approach where the surgeon seeks access to the spine through the abdomen of a patient. The approaches can be done through an open or laparoscopic procedure.  
     [0124] With reference to FIG. 21, once a surgeon has identified two vertebrae that are to be fused, e.g., lumbar vertebrae V 1  and V 2 , the surgeon determines the size of the desired implant and the desired amount of distraction of the intervertebral disc space IVS required before placement of the implant.  
     [0125] Exposure of the intervertebral disc can be obtained through any suitable technique known in the art. Preferably, the facet of the vertebrae is removed in as limited amount as possible to permit insertion of the implant site preparation instruments and the implant. Single or multiple implants can be used. If a single implant is used, the implant is typically positioned centrally within the lateral margins of the disc space. If a pair of implants is used, they are positioned on either side of the midline of the vertebrae and within the lateral margins of the disc space. If a single implant is used, the transverse (width) dimension of the implant will generally be greater than the transverse dimension of a single one of a pair of implants. A single implant is more likely to be used in a lateral or anterior approach than a posterior approach due to restrictions on the amount of lateral retraction that can be applied to the spinal cord.  
     [0126] Continuing with the posterior approach to lumbar vertebrae V 1  and V 2 , after lateral retraction of the cauda equina, a partial or full discectomy can be performed using known methods, being careful to maintain as much of the annulus as possible. A bone cutting instrument  100  (including mandrels  104 ,  105 ) having paddles with a major height dimension P H1  approximating the desired disc space height is passed into a first side of the intervertebral disc space between adjacent vertebrae V 1  and V 2 . In one embodiment, a distraction spacer, such as shown in FIG. 28 of U.S. Pat. No. 5,489,307, or similar device, can be used to maintain distraction of the disc space on a second side (i.e., contralateral to the first side being prepared) of the vertebral bodies V 1  and V 2 . If a distraction spacer is used, after preparation of the first side, the implant can be inserted into the channel prior to preparation of the channel on the second side. Alternatively, after preparing the channel on the first side, the bone cutting instrument can be removed and the cauda equina retracted over the first side and the channel on the second side prepared before inserting the implants.  
     [0127] During insertion of the paddles  120 ,  121 , of bone cutting instrument  100  (FIG. 5), it may be advantageous to initially insert a paddle having a smaller than desired paddle height dimension P H1  and sequentially insert instruments having increasing paddle heights P H1  until the desired disc space height is achieved. Once the tapered distal ends of the paddles are inserted into the disc space, the proximal ends of channel guide  103  and mandrels  104  and  105  can be tapped (i.e., typically as a single unit), for example, with an orthopedic hammer, to advance the paddles into the disc space until the shoulders  130 ,  131  abut the posterior surfaces of the vertebral bodies.  
     [0128] The first mandrel  104  can then be removed and replaced with bone chisel  106  that is passed along track  112 . The proximal end  301  of chisel  106  is then tapped into first vertebrae V 1  to cut a first notch  400  into endplate  401 . Chisel  106  is then removed and can be replaced by first mandrel  104 . Second mandrel  105  can then be removed and replaced with bone chisel  106  that is passed along track  113  into second vertebrae V 2  to cut a first notch  402  into endplate  403  of second vertebrae V 2 . The bone channel guide  103  with mandrel  104  and bone chisel  106  can then be removed leaving channel  404  defined by notches  400  and  402  as indicated with broken lines in FIG. 21. An implant, such as a rectangular bone plug can then be inserted into channel  404  on the first side and the above procedure repeated on the second side. If the bone cutting instrument has a circular cutting edge, and a threaded implant is to be used, the channel formed can be threadedly tapped using known tapping instrumentation.  
     [0129] In an alternative embodiment, after lateral retraction of the cauda equina and discectomy, a bone cutting instrument  10  (FIGS.  1 - 3 ) having a preselected paddle height P H  can be inserted into the first side of the intervertebral disc space. As described above, cutting instruments  10  having paddles of increasingly greater paddle height dimension P H  can be sequentially inserted into the disc space until the appropriate disc height is established. A distraction spacer may be used on the contralateral side as previously described. After the tapered distal ends  20   a,    21   a  of paddles  20  and  21  having the appropriate height dimension P H  are inserted into the intervertebral disc space (IVS), the bone cutting instrument  10  is advanced until bone cutting surface  23  contacts the posterior surfaces of vertebrae V 1  and V 2 . At this point, without removing the bone cutting instrument, the proximal end  15  of instrument  10  can be tapped to further advance cutting edge  23  to simultaneously remove bone from the endplates of the adjacent vertebrae V 1  and V 2 . Bone and disc material cut by cutting surface  23  will pass into chamber  25  and out opening  24 . Advancement of cutting instrument  10  into the intervertebral disc space is continued until the paddles (or cutting edge) have reached a predetermined depth that can be indicated by marks  30 . Alternatively, the depth to which cutting instrument  10  is inserted into the disc space can be determined by visualization methods such as x-ray, MRI, CT scan, etc.  
     [0130] Once the appropriate depth has been reached, bone cutting instrument  10  is removed and any debris remaining in the channel can be removed using a rongeur, osteotome, forceps, etc. If a threaded implant is to be used, the channel formed by cutting instrument  10  can be tapped using known methods for tapping an implant bore. If a second implant site is to be prepared, the first implant can be inserted prior to preparation of the second implant site or both implants inserted after both implant sites are prepared.  
     [0131] Again with reference to FIG. 21, once a surgeon has identified two vertebrae that are to be fused, e.g., lumbar vertebrae V 1  and V 2 , the surgeon can use the rasp to determine the size of the desired implant and the desired amount of distraction of the intervertebral disc space (IVS) required before placement of the implant. Exposure of the intervertebral disc can be obtained through any suitable technique known in the art, such as by using a distractor. The distractor is inserted between the adjacent bone surfaces, and a rasp is passed through the adjacent bone surfaces to space the bone surfaces apart a predetermined distance. The distractor provides an exposure window through which the bone cutting instrument, rasp, and implant inserting tool with the implant can be inserted, as is more thoroughly described in U.S. Pat. No. 6,224,599 to Baynham, which is incorporated herein by reference.  
     [0132] The surgeon can determine the size of the desired implant  900  and can select the appropriately sized bone cutting instrument  510 , and implant inserting tool  800  using a series of rasps  600 . The rasp  600  functions as a trial sizer in that the rasp heads  606  correspond to various incremental implant sizes and shapes. The dimensions of the rasp head  606  are proportional to the dimensions of the bone cutting instrument  510 , implant inserting tool  800  and the implant  900 . Once a rasp  600  is found that corresponds to the size of the intervertebral space for a particular patient between particular vertebrae, the rasp  600  can be moved into and out of the space to prepare the vertebrae for receiving the implant  900 . The roughened surfaces  611 ,  612  on the rasp head  606  function as a rasp to provide a more uniform and osteoconductive/osteoinductive site for the implant.  
     [0133] Leaving the rasp  600  in place, the surgeon selects the appropriately sized bone cutting instrument  510  and slides it over the rasp shaft  603 , aligning the openings  550 ,  605  in the proximal shafts of the bone cutting instrument and rasp. The alignment pin  700  is inserted into the openings to maintain the alignment, and the bone cutting instrument  510  is forced, e.g. forced, into place. The openings  550 ,  605  in the shafts of the bone cutting instrument and rasp provide a stop when aligned with the alignment pin  700  to prevent the bone cutting instrument  510  from cutting too deep into the intervertebral space. The inner surface  608  of the rasp head  606  also acts as a stop for the third and fourth cutting edges  522 ,  523  of the bone cutting instrument  510 .  
     [0134] Once the channel has been cut, a slap hammer can be attached to the shaft  517  of the bone cutting instrument  510  to remove the instrument and the rasp  600 . The appropriately sized support component  902  of the implant  900  is positioned on the insertion tool  800  with the sleeve  820  in the proximal position, and the implant is inserted into the prepared channel. The sleeve  820  is then advanced distally to force the arms  807 ,  808  of the insertion tool toward one another to release the implant. If a two-part implant is used, the second part of the implant (growth component  901 ) is then inserted.  
     [0135] From the foregoing detailed description and exemplary embodiments, it will be evident that modifications and variations can be made in the devices and methods of the invention without departing from the spirit or scope of the invention. Therefore, it is intended that all modifications and variations not departing from the spirit of the invention come within the scope of the claims appended hereto.