Abstract:
A disc preparation system includes an oscillating rasp for preparation of vertebral endplates and a central reamer for reaming a pair of kidney-shaped grooves into the vertebral endplate. The oscillating rasp is powered by a rotary power source. A linkage assembly is coupled to the rotary power source to convert the rotary motion into a reciprocating motion. A pair of rasps plates, which are linked to the linkage assembly, linearly reciprocate in opposite directions in response to the reciprocating motion of the linkage assembly. In one form, the central reamer includes a pair of cutting elements that are coupled to the rotary power source in order to rotate in response to rotational movement from the rotary power source. In another form, the central reamer includes a single cutting element coupled to an angled reamer handle and a reamer guide.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International PCT Application No. PCT/IB2003/000910, filed on Mar. 13, 2003 and published on Sep. 23, 2004 as International Publication No. WO 2004/080316, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to vertebral endplate tools, and more specifically, but not exclusively, concerns vertebral endplate tools that are used to form a centrally located cavity in vertebral endplates. 
     BACKGROUND 
     Intervertebral discs, located between the endplates of adjacent vertebrae, stabilize the spine, distribute forces between the vertebrae and cushion the vertebral bodies. An intervertebral disc may deteriorate due to trauma, aging, or disease resulting in pain or discomfort to a patient. One common procedure for relief of patient discomfort is a disectomy, or surgical removal of all or part of the intervertebral disc. Often, this is followed by implantation of a device or spinal implant between the adjacent vertebrae in order to maintain or restore disc space height. Through stabilization of the vertebrae, the risk of reoccurrence of the same disabling back pain due to persistent inflammation and/or instability is reduced. 
     During implantation of a spinal implant, the endplates of adjacent vertebrae are sometimes milled to ensure firm implantation of the spinal implant by promoting bone ingrowth. One problem faced with typical milling instrumentation is that it is unable to form precise cavities at desired locations in the endplates. If not precisely prepared, the formed disc space may result in the expulsion of the implant, which can lead to injury of the patient. 
     Moreover, precise control of the milling equipment is required in order to avoid damaging vital tissues along the spinal column, such as nerves. During milling, the surgeon has to apply force to the milling equipment in order to counteract the forces created by the milling equipment cutting into the vertebrae. If not counteracted, the resultant force can cause the milling equipment to cut into portions of the vertebrae not intended to be milled. 
     Thus, there remains a need for implant endplate preparation tools that are capable of precisely defining cavities for securing implants. 
     SUMMARY 
     The present invention contemplates intervertebral endplate tools that have a reduced profile and that can precisely prepare a cavity for insertion of a device for spacing adjacent vertebrae. 
     In one aspect, an oscillating rasp, which is used to prepare vertebral endplates, includes a rotary power source and a linkage assembly coupled to the rotary power source, which converts rotary motion of the rotary power source into a reciprocating motion. A pair of bilateral rasp plates is linked to the linkage assembly. The pair of bilateral rasp plates are adapted to linearly reciprocate in opposite directions in response to the reciprocating motion of the linkage assembly. 
     In another aspect, a central reamer is used to ream a pair of kidney-shaped grooves into a vertebral endplate. The reamer includes a rotary power source operable to rotate about a drive axis. A pair of cutting elements are coupled to the rotary power source. The cutting elements are adapted to rotate about a cutting axis in response to rotational movement of the rotary power source about the drive axis. The drive axis is arranged in a perpendicular arrangement with respect to the cutting axis. 
     In a further aspect, a kit includes an oscillating rasp to prepare surfaces of vertebral endplates. The kit further includes cutter configured to cut a guide slot into the vertebral endplates and a reamer. The reamer has a guide flange adapted to be slidably received in the guide slot, and the reamer has a cutting element adapted to cut a cavity into the vertebral endplates. 
     Another aspect concerns a method that includes rasping a generally flat surface on an endplate of a vertebrae with a rasp having a pair of bilateral rasp plates linearly reciprocating in opposite directions. A kidney-shaped central cavity is reamed within the flat surface with a cutting element of a central reamer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a diagrammatic view of a tool assembly according to one embodiment. 
         FIG. 2  is a partial cross-sectional, perspective view of a reciprocating rasp according to a further embodiment. 
         FIG. 3  is an enlarged perspective view of a head portion of the  FIG. 2  rasp. 
         FIG. 4  is a partial cross sectional view of the head shown in  FIG. 3 . 
         FIG. 5  is a cross sectional, perspective view of a central reamer according to another embodiment. 
         FIG. 6  is an enlarged perspective view of a head portion of the  FIG. 5  reamer. 
         FIG. 7  is an enlarged cross sectional view of the head shown in  FIG. 6 . 
         FIG. 8  illustrates the stages involved in preparation of a disc space for implantation of an implant according to one embodiment. 
         FIG. 9  illustrates one environment in which the  FIG. 5  reamer is used. 
         FIG. 10  illustrates an angled reamer assembly according to another embodiment. 
         FIG. 11  illustrates an enlarged view of a head portion of the  FIG. 10  reamer assembly. 
         FIG. 12  illustrates a technique for disc space preparation with the  FIG. 10  reamer assembly. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the insertion, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. One embodiment of the invention is shown in great detail, although it will be apparent to those skilled in the art that some of the features which are not relevant to the invention may not be shown for the sake of clarity. 
     A tool system or assembly  20  according to one embodiment is illustrated in diagrammatic form in  FIG. 1 . As shown, tool assembly  20  includes a disc preparation tool  22  that is coupled to a rotary drive  24 . As will be described in detail below, the tool  22  is used in preparing a disc space for insertion of an intervertebral spacer. The rotary drive  24  supplies power to the tool  22 , and the disc preparation tool  22  converts the rotary motion of the rotary drive  24  into a cutting motion in order to prepare the disc space. By way of a non-limiting example, the rotary drive  24  can include a pneumatic motor, an electric motor, and/or a manually operated, rotary drive handle, to name a few. In one embodiment, the rotary drive  24  is a pneumatic motor that is operable to supply rotary movement to the tool  22 . 
     One version of tool  22  that can be used in system  20  is illustrated in  FIG. 2 . As illustrated, oscillating rasp  26  includes a distal end portion  28  and a proximal end portion  30 . The proximal end portion  30  includes a drive shaft  32  that is adapted to connect to the rotary drive  24 . As depicted in  FIG. 2 , the drive shaft  32  has an attachment head  34  and a drive portion  36  that has a plurality of drive ridges  38  radially oriented along the drive shaft  32 . The drive ridges  38  are adapted to engage the rotary drive  24  such that the rotary drive  24  is able to rotate the drive shaft  32 . 
     Within housing  40 , the rasp  26  includes a linkage assembly  42  that converts the rotary motion R of the drive shaft  32  about drive axis D into a linear reciprocating motion along longitudinal skis L of the rasp  26 . As illustrated in  FIG. 2 , the linkage assembly  42  includes a crankshaft  44  with a pair of cranks  46  pivotally coupled to a pair of connecting rods  48  ( 48   a ,  48   b ). The linkage assembly  42  is connected to the drive shaft  32 . Opposite the drive shaft  32 , along the drive axis D, the linkage assembly  42  includes a bearing  50  rotatably mounted within the housing  40 , thereby allowing rotation of the crankshaft  44 . As should be appreciated, the linkage assembly  42  can include a second bearing  50  that is located between the drive shaft  32  and the cranks  46 . As shown, the connecting rods  48  are positioned one hundred and eighty-degrees (180°) out of phase with respect to one another on the crankshaft  44  such that when one connecting rod  48   a  is extended along the longitudinal axis L towards the distal end portion  28 , the other connecting rod  48   b  is retracted away from the distal end portion  28 . The linkage assembly  42  is received within a cavity  52  defined in the housing  40 . An end cap  54  of the housing  40  seals the linkage assembly  42  within the cavity  52 . A shaft portion  56  connects the distal end portion  28  of the rasp  26  to the proximal end portion  30 . The shaft portion  56  defines in the housing  40  a longitudinal cavity  57 , which communicates with cavity  52 . As shown, a pair of rod members  58 ,  60  is received within the cavity  57  of the shaft portion  56  of the housing  40 . Each of the rod members  58 ,  60  is pivotally connected to one of the connecting rods  48  near the proximal end  30 . 
     At distal end portion  28 , the rasp  26  includes a head portion  64  that has a pair of rasp plate or cutting members  66 ,  68  that are used to cut a rectangular cavity into an endplate of a vertebral body plate. As shown in greater detail in  FIGS. 3 and 4 , the first rasp plate  66  and the second rasp plate  68  are connected to the first rod member  58  and the second rod member  60 , respectively. First  70  and second  72  pins respectively secure the rasp plates  66  and  68  to the rods  58  and  60 . Upper  76  and lower  78  surfaces of the rasp plates  66 ,  68  are textured to have cutting members  80  that are configured to cut into a pair of opposing vertebral endplates. In the illustrated embodiment, the cutting members  80  have a pyramidal shape. As should be appreciated, the upper and lower surfaces  76 ,  78  can include other types of texturing in order to cut into the endplates. 
     To ensure that the first  66  and second  68  rasp plates do not separate from one another during use, the first rasp plate  66  has a tongue member  82  received within a groove  84  in the second rasp plate  68 . As illustrated, the rasp plates  66 ,  68  each have a generally rectangular shape, and when placed side by side, have an overall rectangular shape. Both rasp plates  66  and  68  are received within a cavity  86  defined by a u-shaped end member  88  of the housing  40 . At transition portion  90 , the head portion  64  gradually tapers to the shaft portion  56 . The transition portion  90  further includes a pair of opposing stop surfaces  92  that extend above and below the rasp plates  66 ,  68  in order to prevent the rasp  26  from penetrating too far into the vertebrae along the longitudinal axis L. Moreover, the u-shaped end member  88  has upper and lower surfaces  94  that limit the penetration depth of the rasp plates  66 ,  68  into the vertebrae. The u-shaped end member  88  includes a tapered insertion portion  96  that is tapered to make insertion of the rasp  26  between the vertebrae easier, and corners  98  of the u-shaped end member  88  are rounded to minimize tissue damage. 
     Referring again to  FIG. 2 , when the drive shaft  32  is rotated by the rotary drive  24 , the linkage assembly  42  converts the rotary motion R into a reciprocating linear motion along the longitudinal axis L. In the rasp  26 , the alternating reciprocating motion of the connecting rods  48   a ,  48   b  and the rod members  58 ,  60 , which is created by the rotation of the crankshaft  44 , alternatingly reciprocates the rasp plate  66  and  68  in opposite, distal D and proximal P directions along the longitudinal axis L. By having the rasp plates  66  and  68  oscillate in longitudinally opposite directions, the rectangular cavity formed by the rasp  26  can have a more precise shape because the forces imparted by the oppositely moving rasp plate  66 ,  68  counteract one another, thereby minimizing the resultant force imparted on the proximal end portion  30  of the rasp  26 . During surgery, the surgeon has to apply little or no force to counteract the cutting forces generated by the rasp plates  66 ,  68 . Thus, the oscillating rasp  26  according to this embodiment is able to convert rotary force into a linear force such that a precisely dimensioned finished surface can be formed. 
     Another version of tool  22  is a central reamer  100  that is illustrated in  FIGS. 5-7 . As shown, the central reamer  100  includes a distal end portion  28   a  and an opposite proximal end portion  30   a . The reamer  100  includes a number of components similar to the ones described above for the rasp  26 . Like rasp  26 , the central reamer  100  includes a drive shaft  32  linkage assembly  42  with a crankshaft  44 . The crankshaft  44  includes a pair of cranks  46  each connected to a respective connecting rod  48 . The crankshaft  44  is supported in housing  56  by bearings  50 . The housing  56  forms central cavity  52  that is enclosed by end cap  54 . The shaft portion  56  of the reamer  100  slidingly receives first  58   a  and second  60   a  rod members. The rod members  58   a  and  60   a  are connected to connecting rods  48   a  and  48   b , respectively. As shown, rod members  58   a  and  60   a  differ from the previous embodiment at head portion  64   a  of the reamer  100 . 
     As is shown in  FIGS. 6 and 7 , the head portion  64   a  of the reamer  100  includes a cutting assembly  102 , which is used to cut kidney-shaped cavities into a pair of adjacent endplates. As illustrated, the cutting assembly  102  includes a pair of bilaterally oriented cutting members  104  mounted on a rotatable cutting shaft  106 . A pinion gear  108  is mounted on the cutting shaft  106  between the cutting members  104 . Both rod members  58   a  and  60   a  have rack members  110  and  112 , respectively, with teeth that engage the pinion gear  108 . As shown, the rack members  110  and  112  are positioned at opposite sides of the pinion gear  108 . As the drive shaft  32  is rotated, the linkage assembly  42 , through camshaft  44  and connecting rods  48 , convert the rotary motion R of shaft  32  into a reciprocating linear motion along the longitudinal axis L of the central reamer  100 . The rod members  58   a  and  60   a  reciprocate in an alternating manner along the longitudinal axis of the reamer  100 . With the rack members  110  and  112  moving in an alternating manner, the pinion gear  108  is rotated in an oscillating fashion such that the cutting members  104  are rotated in an oscillating direction, as indicated by arrows O in  FIG. 5 , about a cutting axis C and cutting shaft  106 . As illustrated, the cutting axis C is oriented perpendicular to both the longitudinal axis L as well as the drive axis D. 
     With continued reference to  FIGS. 6 and 7 , the cutting members  104  have a generally rectangular cross-sectional or box shape. Openings  116  are formed in the cutting members  104  between the individual cutting blades  114  so as to allow bone chips to be removed during cutting. Cutting members  104  along with the cutting shaft  106  are received in a cutting shaft cavity  118  defined in the head portion  64   a . A shaft support member  120  rotatably supports the cutting shaft  106 , and one end of the cutting shaft  106  is received within a shaft opening  122  defined in support member  101 . As shown, a pair of screws  124  secure the shaft support member  120  to the head portion  64   a . The other end of the cutting shaft  106  is supported by a second shaft opening  122  formed in the head  64   a . Referring to  FIG. 7 , the head portion  64   a  includes a pair of opposing vertebrae engaging surfaces  126  that are adapted to fit between and engage adjacent vertebrae. A pair of guide members  128 , which guide and center the reamer  100  when inserted between the adjacent vertebrae, bisect the vertebrae engaging surfaces  126 . At the end opposite of surface  122 , the engaging surfaces  126  and the guide member  128  respectively have tapered portions  130  and  132 , which aid in the insertion of the head portion  64   a  between the adjacent vertebrae. 
       FIG. 8  illustrates how the tools  22  according to the present invention are used to progressively shape a vertebra V. The illustrated vertebra V has been numbered V 1 -V 5  in order to show the progression of the cavities formed for insertion of a spacer into the vertebra V. However, it should be understood that the progressive numbering of vertebrae V 1 -V 5  is for explanation purposes only in order to merely show the progression in which a single vertebra V is shaped during a shaping technique according to the present invention. Moreover, although only a single vertebra V is illustrated, it should be understood that the illustrated cavity forming technique can occur at the same time on both opposing vertebrae in a disc space. As depicted, the oscillating rasp  26  removes a cartilage layer and prepares a regular flat surface or cavity  150  in the endplate  152  without cutting into spongy bone of the vertebrae V. Cavity  150  in the illustrated embodiment has a substantially rectangular shape, but it should be appreciated that the shape of cavity  150  can be different in order to accommodate differently shaped implants. The u-shaped end member of the rasp  26  ensures that the rasp  26  does not cut too deeply into the endplate  152  of the vertebrae V 1 . Moreover, the stop surfaces  92  prevent the rasp  26  from being inserted at an excessive depth into the vertebra V 1 . Following cutting of the rectangular cavity  152 , as shown with vertebra V 2 , a midline cutter support  154  is inserted into central cavity  152 . As mentioned above, the rasp  26  removes the cartilage layers and prepares regular flat surface or cavities  150  without cutting into spongy bone in the vertebrae V. As depicted, the midline cutter support  154  has a pair of vertebrae engaging surfaces  156 , each of which are bisected by a cutter slot  158 . A connecting member  160  connects the two halves of the head portion  64   b  of the midline cutter support  154 . The midline cutter support  154  further includes a handle portion  162  that defines a midline cutter cavity  164 . As shown with vertebra V 3 , cutter  166  is received within the midline cutter cavity  164  and cutter slot  158  in order to cut a midline center slot  168 . The cutter  166  has a blade portion  170  that extends above the vertebrae engaging surfaces  156  in order to cut the slot  168  in the vertebrae V 3 . As illustrated, the blade  170  has a tapered end  172  to coincide with tapered portion  132  on guide member  128  of the central reamer  100 . 
     As shown in  FIG. 8  and in greater detail in  FIG. 9 , the guide member  128  of the central reamer  100  is slid into the center slot  168  formed in vertebra  14 . Slot  168  along with the guide member  128  ensures that the reamer  100  is properly centered over vertebra V 4 . The stop surface  92  on the central reamer  100  ensures that the cutting members  104  are positioned at the right penetration depth over vertebra V 4 . To insert the central reamer into the disc space, the cutting members  104  of the reamer  100  are oriented such that the cutting blades  114  are flush with vertebrae engaging surfaces  126 . Next, the reamer  100  is slid between the vertebrae V and positioned at the proper depth through stops  92 , which engage a side of vertebra V 4 . Once fully inserted, the cutting members  104  can be rotated in order to form a kidney-shaped or curved surfaced cavity  174 . After the kidney-shaped cavity  174  is reamed, a multi-axial spacer  180  can be inserted into cavity  174  and slot  168 . To prevent expulsion of spacer  180 , the spacer  180  includes opposing curved portions  182  that are conjugate with the formed kidney-shaped cavities  174  and fin portions  184  that are received within slot  168 . 
     A curved reamer and guide assembly  190  for forming cavity  174  into vertebrae V according to another embodiment is illustrated in  FIGS. 10-12 . As illustrated, assembly  190  includes a curved reamer  192  slidably received within a reamer guide  194 . The curved reamer  192  includes drive shaft  32  that is rotatably received within handled portion  96   196  and hollow curved shaft  198 . The drive shaft  32  is configured to connect to rotary drive  24 . The curved reamer  192  has a proximal end portion  200  at which the drive shaft  32  is received, and a distal end portion  202  at which a rotary cutting bit tool  204  is positioned. In the illustrated embodiment, the rotary cutting bit  204  is connected to the drive shaft  32  through a cable such that when the drive shaft  32  rotates, the rotary cutting bit  204  rotates. As illustrated, the curved shaft  198  includes a straight portion  206  and a bent or curved portion  208 , which is angled in an oblique angle with respect to longitudinal axis L of the curved reamer  192 . 
     The reamer guide  194  includes a handle end portion  210  with a cavity  212 , a guide end portion  214  and a solid shaft  216  connecting the handle end portion  210  to the guide end portion  214 . The guide end portion  214  of guide  194  defines a guide cavity  218  in which curved portion  208  of reamer  192  is received. The guide cavity  218  is further subdivided into a insertion portion  220  in which the reamer  192  is initially inserted outside the vertebrae V, and a reamer portion  222  at which the curved reamer  192  reams kidney-shaped cavity  194  in the vertebrae V. The insertion portion  220  includes a pair of oppositely disposed guide flanges  224 , which prevent the reamer  192  from accidentally cutting into the vertebrae V at the wrong location. At the reamer portion  222  notches  226  are formed in flanges  222  to act as a template for the rotary cutting tool  204  such that tool  204  is able to move and cut cavity  174  into the vertebrae. Guide fins  228  extend from the reamer guides  194  in order to align the reamer guide  194  in the center slot  168 . Stop members  230  are oriented perpendicular to the guide fins  228  to prevent the reamer guide  194  from being inserted too deeply into the disc space. 
     During use, the guide fins  228  are inserted into slots  168  formed in the vertebrae V. Once the stop members  230  prevent further insertion of the guide  194  into the disc space, the curved portion  208  of the reamer  192  is inserted into the insertion portion  220  of the guide cavity  218 . Next, the curved portion  208  is slid through the guide cavity  218 , past the guide flanges  224 , and into notches  226  of the reamer portion  222  at which the rotary cutting member  204  can be moved in order to ream out kidney-shaped cavity  228 . As previously mentioned, the guide flanges  224  as well as the notches  226  ensure that cavity  174  is formed at the proper location. Once cavity  174  is reamed, the reamer  192  along with the guide  194  can be removed from the disc space, and implant  180  can be implanted between the vertebrae V. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.