Patent Publication Number: US-2005143825-A1

Title: Intervertebral prosthesis

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an osteogenic interbody fusion implant device and, more particularly, to a non-threaded intervertebral bone implant having a plurality of expandable barbs configured to facilitate securement of the implant within the intervertebral space.  
      2. Prior Art  
      The spine is a flexible column formed of a plurality of bones called vertebra. The vertebrae are hollow and piled one upon the other, forming a strong hollow column for support of the cranium and trunk. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibro-cartilaginous bodies.  
      The intervertebral fibro-cartilages are also known as intervertebral disks and are made of a fibrous ring filled with pulpy material. The disks function as spinal shock absorbers and also cooperate with synovial joints to facilitate movement and maintain flexibility of the spine. When one or more disks degenerate through accident or disease, nerves passing near the affected area may be compressed and are consequently irritated. The result may be chronic and/or debilitating back pain. Various methods and apparatus, both surgical and non-surgical, have been designed to relieve such back pain.  
      One method, interbody fusion, involves stretching the spine into a natural position so that nerve root canal sizes are increased and nerve irritation is eliminated or reduced. The space between vertebrae is maintained by fusing the vertebrae in the affected area together at a fixed distance. Numerous prosthetic implants have been suggested to fill the void between vertebrae. For example, U.S. Pat. No. 4,936,848 describes a spherical cage implant made of metal or ceramics, which is inserted between adjacent vertebrae. The cage has an interior cavity within which bone fragments are inserted. Such bone fragments may be autogenic and are intended to promote subsequent bone growth and fusion of the vertebrae.  
      Another method of preventing contact of vertebrae is described in U.S. Pat. No. 5,011,484, wherein a stud-shaped insert is inserted longitudinally between two vertebrae and secured in position. U.S. Pat. No. 4,309,777 describes an artificial intervertebral disc having upper and lower discs, which are connected to each other by springs. The artificial disc is held in between adjacent vertebrae by spikes which project from the disc into the surface of the vertebrae in contact therewith. U.S. Pat. No. 4,743,256 describes a rigid, porous plug which can be inserted between vertebrae and held in place by prongs or screws. The porous nature of the plug is alleged to facilitate ingrowth of bone tissue.  
      An implantable bone plug for insertion between vertebrae is also described in U.S. Pat. No. 4,878,915, wherein, in one embodiment, the exterior of the plug is provided with external threading which will, when the plug is rotated, advance the plug into prepared sites between the vertebrae. A portion of the plug is provided with a slot designed to receive the end of a key, which is used to rotate the plug. U.S. Pat. No. 5,105,255 describes a method for forming a bored hole between two adjacent vertebrae and then inserting a graft medium such as finely chopped cortical or cancellous bone chips into the bored hole.  
      U.S. Pat. No. 4,961,740 is directed to a substantially open fusion cage, which is inserted between the opposing bony surfaces of adjacent vertebrae by screwing the cage into place. The cage may be filled with bone chips or other bone growth-inducing (osteogenic) substances and, when inserted into the intervertebral space, intimate contact between the bone inducing substance contained within the cage and the native bone occurs through the outer surface of the cage.  
      Ideally, a fusion graft should stabilize the intervertebral space and become fused to adjacent vertebrae. Moreover, during the time it takes for fusion to occur, the graft should have sufficient structural integrity to withstand the stress of maintaining the space without substantially degrading or deforming and have sufficient stability to remain securely in place prior to actual bone ingrowth fusion. Consequently, a fusion graft should contain some kind of anchor and, additionally, a bone inducing substance, which causes rapid bone growth and quick fusion of the graft to adjacent vertebrae. In addition, the material from which the fusion graft is made should be biocompatible. Further, the implant material should closely resemble host tissue and not elicit an immune response from the host.  
      All of the above-described implants are intended to support and maintain an appropriate intervertebral space. Unfortunately, most prior art implants do not fulfill one or more of these criteria for an ideal interbody fusion graft. For example, many of the implants, such as the one described in U.S. Pat. No. 4,936,848 are made of metals and ceramics and, while biocompatible, do not precisely mimic the body&#39;s natural bone tissue. U.S. Pat. No. 5,015,255 describes a graft in the form of bone chips that may eventually result in fusion between the vertebrae. If adequate fusion of the bone chips occurs, the final fused graft may closely mimic the body&#39;s naturally occurring tissues. However, when the bone chips are inserted, they are unconfined and may not remain contained between the vertebrae for a sufficient time to adequately fuse to each other and to adjacent vertebrae. The bone plug disclosed in U.S. Pat. No. 4,878,915 has a threaded outer surface to assist in placement of the implant between the adjacent vertebrae. The external threads, however, compromise the strength of the implant. In addition, the threaded bone implant may have a tendency of backing out of the prepared bore.  
      In U.S. Pat. Nos. 4,580,936, 4,859,128, 4,877,362, 5,030,050, 5,441,500, 5,489,210, 5,713,903, 5,968,044, 5,417,712, 5,501,695, 5,522,845, 5,571,104 and 6,290,701 there are disclosed a variety of anchors for attaching suture, bone and/or soft tissue to bone. The foregoing patents further disclose a number of installation tools for deploying the anchors disclosed therein. Complete details of the construction and operation of these anchors and their associated installation tools are provided in the above-identified patents, which patents are hereby incorporated herein by reference. Other prior art bone-engaging substrate fastening means often employ several straight or curved cantilevered barbs, where the barbs may be elastically deformed to permit insertion into a hole drilled in a bone. These fasteners are well known in medical applications wherein the need for high holding strength has lead to the development of anchors having multiple cantilevered barbs. In each case, the body, the attachment means, and the bone-engaging means mechanically cooperate with one another to fasten a suture, bone portion, soft tissue, prosthesis, post or other substrate to a bone.  
      There remains a need for improved intervertebral fusion implants with anchoring means, which more closely embody the ideal properties of a spinal fusion implant. In particular, there remains a need for an expandable intervertebral prosthesis capable of elevating the intervertebral spacing by rotation of the expansion cylinder. The ability of the prosthesis to control intervertebral elevation positions the tubular outer body of the expandable intervertebral prosthesis snugly between the vertebrae, pressing against the bone surfaces of the adjacent vertebra to promote fast bone growth and healing.  
      There further remains a need for an expandable intervertebral prosthesis for facilitating arthrodesis in the disc space between adjacent vertebrae with predictable and controllable initial anchorage strength sufficient to permit gradual load sharing and provide full repair and restoration of function during bone fusion. There exists a further need for a expandable intervertebral prosthesis device having elastically deformable expansion barbs on its exterior surface, wherein the outer ends of the barbs extend outwardly from the prosthetic body toward a surrounding bone when the prosthetic body, or a portion thereof, is controllably moved. There exists a further need for a expandable intervertebral prosthesis device having a movable expansion cylinder, wherein the outer ends of the barbs extend outwardly from the prosthetic body toward a surrounding bone thereafter to easily, rapidly and reliably anchor the prosthesis to the bone as the expansion cylinder is retracted from a fully extended position.  
     SUMMARY  
      An expandable intervertebral prosthesis for implantation within a hole drilled between adjacent vertebrae, thereafter promoting the fusion of the adjacent vertebrae to one another. In a first embodiment, the intervertebral prosthesis comprises: (a) a tubular outer body portion having a proximal end, a distal end and an axial bore therebetween; and (b) an expansion cylinder slidably mounted within the axial bore of the tubular outer body portion. The tubular outer body portion has a generally cylindrical outer surface with a plurality of apertures therewithin. The tubular outer body portion may further include a plurality of elastically deformable barbs on its exterior surface that may be elastically deformed from their normally outward projecting configuration. The expansion cylinder includes a plurality barbs located in circumferentially spaced relation on the outer surface of the cylinder and disposed in various angles and attitudes with respect to the longitudinal axis. When the expansion cylinder is advanced into the axial bore of the tubular outer body portion, the barbs deform to lie within slots on the outer surface thereof. The assembly comprising the tubular outer body portion and the expansion cylinder slidably mounted within the axial bore therof comprises a first embodiment of the intervertebral prosthesis.  
      In operation, a hole is drilled between adjacent vertebrae and the above-described assembly (i.e., the intervertebral prosthesis) is inserted into the hole. The expansion cylinder is then partially retracted, thereby driving the outwardly biased elastically deformable barbs through the holes in the outer surface of the tubular outer body portion and into the surrounding bone, thereby anchoring the prosthesis within the intervertebral space. This embodiment of the present invention is not elevatable.  
      In another embodiment, the tubular outer body portion is frangible—being formed from two mirror image hemicylinders attached together along the length thereof to form a frangible joint therebetween. The frangible tubular outer body portion has an axial bore and preferably a plurality of elastically deformable barbs on the outer surface thereof. An elevating cylinder having longitudinal flanges or ridges on the outer surface thereof is rotatably disposed within the axial bore of the tubular outer body portion. The longitudinal ridges on the elevating cylinder fit snugly into a mating set of longitudinal channels or grooves on the inner wall of the axial bore of the tubular outer body portion.  
      In operation, a hole is drilled between adjacent vertebrae and the frangible tubular outer body portion containing the elevating cylinder is inserted into the hole. The barbs, being elastically deformable, flatten out during insertion and expand into the surrounding bone when the prosthesis is partially retracted. The elevating cylinder is then rotated through a 90′ angle. As the flanges move out of the mating grooves on the inner surface of the axial bore, the flanges urge the hemicylinders apart thereby breaking the frangible joint therebetween and elevating the opposing hemicylinders to press tightly against the surrounding bone, forcing the barbs even deeper into the bone. When the 90° rotation is complete, the flanges engage a second, shallower set of grooves within the axial bore that serve as a detent position. The elevating cylinder may further include an axial bore that contains a bone graft material and a plurality of holes in the outer surface thereof.  
      In yet a further embodiment of the intervertebral prosthesis of the present invention, a longitudinally frangible, tubular outer body portion has an elevating cylinder rotatable mounted within the axial bore thereof, and further includes a barbed expansion cylinder slidably mounted within a second axial bore in the elevating cylinder. In operation, a hole is drilled between the adjacent vertebrae to be fused and the prosthesis is inserted into the hole. Rotation of the elevating cylinder through a 90° angle separates the hemicylinders comprising the tubular outer body portion, forcing the opposing surfaces thereof against the surrounding bone, After rotation of the elevating cylinder is complete, partial retraction of the expansion cylinder drives the barbs on the surface thereof through holes in the elevating cylinder and tubular outer body portion and into the bone to anchor the prosthesis within the hole. In all embodiments, the elevating cylinder and/or the expansion cylinder may include a bone graft material housed within an axial bore therewithin.  
      In yet a further embodiment of an intervertebral prosthesis in accordance with the present invention, the prosthesis comprises a single tubular outer body portion having a plurality of holes and barbs on the outer cylindrical surface thereof and an axial bore. The barbs are elastically deformable. The plurality of holes in the surface thereof extend inwardly to the axial bore. The axial bore contains a bone graft material. In operation, a hole is drilled between adjacent vertebrae and the tubular outer body portion is inserted into the hole and advanced thereinto. As the prosthesis is advanced, the barbs bend, lying against the surface of the prosthesis. When the prosthesis is fully inserted into the hole, retraction of the prosthesis drives the elastically deformable barbs into the surrounding bone thereby anchoring the prosthesis within the hole. The plurality of holes in the surface of the tubular outer body permit ingrowth of bone into the bone graft material housed within the axial bore thereby promoting fusion of the adjacent vertebrae.  
      The features of the invention believed to be novel are set forth with particularity in the appended claims. However the invention itself, both as to organization and method of operation, together with further objects and advantages thereof may be best understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of an intervertebral prosthesis comprising an expansion cylinder slidably and rotatably disposed within the axial bore of a tubular outer body portion in accordance with a preferred embodiment of the present invention.  
       FIG. 2  is a perspective view of the expansion cylinder of the intervertebral prosthesis of  FIG. 1 .  
       FIG. 2   a  is an end view of the expansion cylinder of  FIG. 2 .  
       FIG. 3  is a perspective view of an elevatable and expandable intervertebral prosthesis in accordance with a second preferred embodiment of the present invention wherein a frangible tubular outer body portion has an elevating cylinder rotatably disposed within the axial bore thereof.  
       FIG. 4  is a perspective view of an elevating cylinder suitable for use with the frangible tubular body portion as shown in the intervertebral prosthesis of  FIG. 3 .  
       FIG. 5  is a perspective view of an expansion cylinder as shown in  FIG. 2  but further including a bone graft material in an axial bore thereof and a plurality of holes in the outer surface.  
       FIG. 5   a  is an end view of the expansion cylinder of  FIG. 5 .  
       FIG. 6  is a perspective view of an elevatable and expandable embodiment of an intervertebral prosthesis prior to elevation and expansion illustrating, in phantom, how the plurality of curved barbs extend outwardly from the frangible tubular outer body portion when the prosthesis is deployed within a hole drilled in or between adjacent vertebrae.  
       FIG. 7  is an end view of the elevatable and expandable intervertebral prosthesis of  FIG. 6  prior to the elevation and expansion of the barbs.  
       FIG. 8  is an end view of the elevatable and expandable intervertebral prosthesis of  FIG. 6  following the elevation and expansion of the barbs and illustrating the separation of the hemicylinders comprising the frangible tubular outer body portion following rotation of the elevating cylinder.  
       FIG. 8   a  is a perspective view of an embodiment of the intervertebral prosthesis of the present invention consisting of a tubular outer body portion wherein there are no expansion or elevating cylinders.  
       FIG. 9  is a partially cutaway elevational view of an expandable intervertebral prosthesis insertion tool operable for inserting the tubular outer body of expandable intervertebral prosthesis into a hole drilled in bone and for forcing a expansion cylinder into the axial bore of the tubular outer body.  
       FIG. 10  is a schematic left end view of the expandable intervertebral prosthesis insertion tool of  FIG. 9 .  
       FIG. 11  is a right end view of the expandable intervertebral prosthesis insertion tool illustrated in  FIG. 9 .  
       FIG. 12  is a side elevational view of an expansion cylinder insertion rod adapted for use with the expandable intervertebral prosthesis insertion tool of  FIG. 9 .  
       FIG. 13  is a plan view of an intervertebral prosthesis of the present invention inserted into a hole drilled between adjacent vertebrae. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      With reference to  FIG. 1 , the expandable intervertebral prosthesis  10  in accordance with a first preferred embodiment of the present invention comprises a tubular outer body portion  11  with an expansion cylinder  12  slidably disposed within an axial bore  13  in the tubular outer body portion  11 . The expandable intervertebral prosthesis  10  has a proximal end  14  and a distal end  15 . The wall of the tubular outer body portion has a plurality of holes  19  therein. The cylindrical axial bore  13  is coextensive with the length of the tubular outer body portion  11 . The expansion cylinder  12  having a guide track  18  and a plurality of elastically deformable barbs  20  disposed along the length thereof is shown in greater detail in  FIG. 2 .  
      In order to use the embodiment of the expandable intervertebral prosthesis indicated at numeral  10 , a hole is first drilled between adjacent vertebrae in a direction substantially transverse to the direction of the spine, the hole being centered between adjacent vertebrae. The tubular outer body portion  11  (without barbs) is inserted into the hole. The outer diameter of the expansion cylinder  12  is dimensioned to slidably fit within the axial bore  13  of the tubular outer body portion  11  of the expandable intervertebral prosthesis  10 . At least one longitudinal guiding track  16  and  17  on the interior wall of the axial bore  13  is dimensioned to fit snugly to at least one mating track  18  on the outer surface of the expansion cylinder  12 . The barbs  20  on the expansion cylinder  12  are depressed by the application of external pressure to the proximal end  14  of the expansion cylinder  12  as it is slidably guided down through the axial bore  13  to the distal end  14  of the tubular outer body portion  11 . As the barbed portion of the expansion cylinder enters the axial bore, barbs  20 , which are formed out of an elastically deformable material, are forced radially inwardly so as to be disposed entirely within the axial bore  13  of the outer tubular member  11 . When the distal end  15  of the expansion cylinder  12  is fully advanced into the axial bore  13 , the sharp tips  21  of the barbs  20  are adjacent to holes  19  and partially expand thereinto. The expansion cylinder  12  is then retracted and the sharp outer ends  21  of the barbs  20  are forced progressively outwardly thereby penetrating the cancellous bone. As the expansion cylinder is progressively retracted from within the axial bore, that is, pulled in a proximal direction, the sharp outer ends  21  of the barbs  20  enter and are forced into the cortical bone. When the barbs  20  are fully expanded, no further retraction of the expansion cylinder is possible and the intervertebral prosthesis is locked in position between adjacent vertebrae.  
      To remove the embedded intervertebral prosthesis from the bone, a pushpin (not shown) is inserted into the proximal end of axial bore  13  to contact the proximal end of the expansion cylinder  12 . When pressure is applied to the pushpin, the expansion cylinder is forced in a distal direction until the distal end of the expansion cylinder underlies the distal end of the tubular outer body portion. In this fully depressed position, the barbs  20  are retracted through the holes  19  from within the surrounding bone and folded against the outer surface of the expansion cylinder  12  to lie within the axial bore  13  in a space between the outer surface of the expansion cylinder  12  and the inner surface of the tubular outer body portion  11 . The expandable intervertebral prosthesis  10  may then be removed from the hole by applying traction to the tubular outer body portion  11 .  
      An elevatable embodiment of an intervertebral prosthesis in accordance with the present invention is shown in perspective view at numeral  30  in  FIG. 3 . In the elevatable embodiment  30 , the tubular outer portion  31  comprises two hemicylinders  32  and  33  attached along the length thereof by frangible attachment means  34  to form a tube having an axial bore  35  coextensive with the length thereof. The outer surface of the tubular outer portion  31  preferably includes a plurality of relatively short spikes  36  projecting outwardly therefrom. When elevating cylinder  37  is rotated within the axial bore  35 , camlike expansion flanges  38  and  39  on the cylindrical outer surface of the elevating cylinder are forced out of mating detent grooves  38   a  and  39   a  in the wall of the axial bore and urge the hemicylinders  32  and  33  apart, breaking the frangible connection  34  therebetween and forcing the hemicylinders against surrounding bone (not shown) until the expansion flange(s) come to rest in relatively shallow detent grooves  40  and  41  within the axial bore, thereby elevating the adjacent vertebrae upon which the opposing hemicylinders are pressed. The pressure forces spikes  36  into the surrounding bone thereby providing positive attachment of the outer tubular body  31  to the bone.  
      The rotatable elevating cylinder  37 , shown in perspective view in  FIG. 4 , may, in turn, have a second axial bore  42  coextensive with the length thereof through which a barbed expansion cylinder, such as the expansion cylinder shown at  12  in  FIGS. 1 and 2 , may be inserted. Slots  43  in the wall of the elevating cylinder  37  accommodate the folded barbs  20  during insertion of the expansion cylinder  12  into the axial bore  42  of the elevating cylinder  37 . When the expansion cylinder  12  is retracted, the barbs  20  expand through the holes  19  in the tubular outer body portion  31  and enter the surrounding bone thereby firmly anchoring the prosthesis to the bone.  
      In a further embodiment of an intervertebral prosthesis in accordance with either of the two foregoing embodiments, the expansion cylinder  50  may be modified by hollowing it out to provide an axial bore  51  that can be used to contain bone graft material  52  as shown in  FIG. 5 . The bone graft material  52  may be bone chips or a suitable osteogenic material. The expansion cylinder  50  has a plurality of holes  53  therein and an outer diameter dimensioned to be received within the axial bore  42  of elevating cylinder  37  ( FIG. 4 ). The holes  53 , together with the slots  43  in the extending cylinder, enable bone ingrowth into the core of the expansion cylinder  50 .  
      The operation of an intervertebral prosthesis comprising a frangible tubular outer body portion  30 , an elevating cylinder  37  and the expansion cylinder  50  is best understood with reference to  FIG. 6 . In  FIG. 6 , an elevatable, expandable embodiment of an intervertebral prosthesis is illustrated in perspective view at numeral  60 . The prosthesis  60  has an outermost diameter dimensioned to be inserted into a hole drilled between adjacent vertebrae. The prosthesis  60  includes a tubular outer body portion  30  comprising a pair of mirror-image hemicylinders  32  and  33  joined along the length thereof by a frangible joint  34 . An elevating cylinder  37  having a pair of elevating flanges  39  projecting laterally from the outer surface of the elevating cylinder and coextensive with the length thereof is rotatably disposed within the axial bore of the tubular outer body portion  30 . After the tubular outer body portion  30  is inserted within the hole previously drilled between adjacent vertebrae, the elevating cylinder  37  is rotated ninety degrees. During rotation, the flanges  39  are forced out of the detent grooves  38   a  and  39   a  and urge the hemicylinders  32  and  33  apart thereby breaking frangible joint  34  and pressing the outer surface of the hemicylinders comprising the tubular outer body portion against surrounding bone (not shown). When the 90° rotation is complete, a pair of detent grooves  40  and  41  ( FIGS. 3 and 7 ) on the inner diameter of the tubular outer body portion engage the flanges  38  and  39  thereby locking the elevating cylinder in a position that creates a space between the hemicylinders as shown in  FIG. 8 . After the elevating cylinder is rotated and locked into position, a barbed expansion cylinder  50 , slidably disposed within an axial bore  42  of the elevating cylinder  37 , is partially retracted; forcing the barbs  20 , which were previously disposed within the slots  43  of the elevating cylinder  37 , outwardly through holes  19  and into the surrounding bone thereby anchoring the prosthesis  60  into the intervertebral hole.  
       FIG. 7  is an end view of the distal end of an expandable intervertebral prosthesis  60  prior to separation of the hemicylinders  32  and  33  and expansion of the barbs  20 .  FIG. 8  is a distal end view of the prosthesis  60  after rotation of the extending cylinder and partial retraction of the expansion cylinder to extend the elastically deformable barbs  20  into the surrounding bone (not shown).  
      It is preferred that the barbs  20  of expansion cylinders  12  or  50  and the spikes  36  of the tubular outer body portion  30  are formed out of polymer blends of glycolide and/or lactide homopolymer, copolymer and/or glycolide/lactide copolymer and polycaprolactone copolymers, and/or copolymers of glycolide, lactide, poly (L-lactide-co-DL-lactide), caprolactone, polyorthoesters, polydioxanone, trimethylene carbonate and/or polyethylene oxide or any other bioabsorbable material. A pseudoelastic shape memory alloy of the type disclosed in U.S. Pat. No. 4,665,906 entitled “Medical Devices Incorporating SIM Alloy Elements”, issued May 19, 1987 to Jervis, which patent is specifically incorporated herein by reference, may also be used to fabricate the barbs  20 . By way of example, one such pseudoelastic shape memory alloy might be a nickel titanium alloy such as Nitinol, which is available from Flexmedics of Minneapolis, Minn., among others. The use of such a material, in combination with the normal orientation of the barbs relative to the anchor body, permits the barbs to initially deflect inwardly to the extent required to permit the tubular outer body portion to be advanced into the drilled hole, or for the expansion cylinder  12  to be advanced into the axial bore of the tubular outer body portion  11 , yet resiliently “spring back” toward their normal, outwardly projecting position so as to prevent the prosthesis  10  or  60  from withdrawing from the drilled hole after being deployed therein. Other implantable (biocompatible) materials that may be used to fabricate an intervertebral prosthesis in accordance with any of the embodiments of the present invention include stainless steel, titanium and cobalt-chrome alloy.  
      In yet a further embodiment of an intervertebral prosthesis in accordance with the present invention, indicated generally at numeral  80  in  FIG. 8   a , the prosthesis  80  comprises a single tubular outer body portion  81  having a plurality of holes  19  and barbs  20  on the outer cylindrical surface thereof and an axial bore  82 . The barbs  20 , having sharp, outwardly biased tips  21 , are elastically deformable. The plurality of holes  19  in the surface thereof extend inwardly to the axial bore  82 . The axial bore  82  contains a bone graft material  52 . In operation, in order to implant the intervertebral prosthesis  80 , a hole is drilled between adjacent vertebrae and the tubular outer body portion  81  is inserted into the hole and advanced thereinto. As the prosthesis is advanced into the drilled hole, the (elastically deformable) barbs  20  bend, lying against the outer surface of the prosthesis  80 . When the prosthesis  80  is fully inserted into the hole, retraction of the prosthesis drives the elastically deformable barbs  20  into the surrounding bone (not shown) thereby anchoring the prosthesis within the hole. The plurality of holes  19  in the surface of the tubular outer body  81  permit ingrowth of host bone into the bone graft material housed within the axial bore thereby promoting fusion of the adjacent vertebrae.  
      A tool useful for inserting an expandable intervertebral prosthesis  10 ,  60  or  80  into a hole drilled in bone in accordance with another aspect of the present invention is shown in elevational cross-sectional view at  90  in  FIG. 9  and front and rear end views in  FIGS. 10 and 11  respectively. The tool  90  has a distal bone fastener-grasping end  91  and a proximal end  92  and a barrel  93  there between having an axial bore  94  dimensioned to slidably accommodate the proximal end of the expansion cylinder therewithin. With alternate reference to the embodiment  60  of the expandable intervertebral prosthesis shown in  FIG. 6 , the proximal end of the tubular outer body  53  of the expandable intervertebral prosthesis  50  is held securely within the distal end  91  of the tool  90  by suitable bone fastener grasping means, and the opposing (distal) end of the expandable intervertebral prosthesis is inserted into a hole drilled in a bone ( FIG. 13 ). Squeezing pivotally mounted trigger  95  forces the expansion cylinder  37  into the axial bore of the outer tubular body  30  comprising the expandable intervertebral prosthesis  60 . A clutch (not shown) rotates the expansion cylinder  37  disposed within the axial bore of the outer tubular body  30  thereby elevating and separating the hemicylinders comprising tubular body  30 . When the trigger  95  is released, a spring (not shown) retracts the extension cylinder  50  thereby expanding barbs  20  into the surrounding bone. The expandable intervertebral prosthesis  60  is released when the trigger  75  returns to its initial position. A pushrod  100 , dimensioned to fit within the axial bore  94  of the tool barrel  93 , is used for removing the prosthesis  60  from the hole. The prosthesis  60  is removed by placing the pushrod within the axial bore  94  of the tool  90  and placing the distal end  101  of the pushrod  100  against the extension pin and advancing it forward to retract the barbs. With the extension cylinder fully advanced and the barbs retracted, the expansion cylinder, if necessary, can then be rotated ninety degrees to bring the hemicylinders into juxtaposition along the length thereof, and the tubular outer body portion extracted from the hole by traction.  
       FIG. 13  is a plan view of an intervertebral prosthesis  60  of the present invention inserted into a hole  132  drilled between the bodies  133  and  134  of two adjacent vertebrael  30  and  131 . The transverse processes  135  and  136  of vertebrae  130  and  131  are unaffected by the presence of the prosthesis  60  within the hole  132 . The holes  19  enable bone growth between the vertebral bodies  133  and  134  to extend into the bone graft material  52  housed within the axial bore of the extension cylinder thereby fusing the vertebrae to one another.  
      While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. For example, axially elevating the expandable intervertebral prosthesis may be perform by other mean such as conical shape cylinders, screw, nail or wedge driven expander, collapsing, reducing or expanding diameter or any other expansion driven design. Other example, the outer tubular member  20  can be either expanded partially, fully or remain un-deformed when the expansion cylinder is advanced into the axial bore  21  of the outer tubular member  22  in a distal direction. Similarly, the outer surface of the outer tubular member is disclosed as cylindrical in the preferred embodiment, but may be hexagonal or have another polygonal cross sectional profile. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.