US20060184172A1 - Fusion device formed of a porous biocompatible material - Google Patents

Abstract

An artificial spinal implant is disclosed for insertion between two adjacent vertebrae. Instrumentation and procedure is also disclosed.

Description

RELATED APPLICATIONS
• This application is a divisional of U.S. application Ser. No. 10/740,747, filed Dec. 19 2003; which is a continuation of U.S. application Ser. No. 10/685,776, filed Oct. 15, 2003; which is a continuation of U.S. application Ser. No. 08/480,684, filed Jul. 7, 1995; which is a divisional of application Ser. No. 07/968,240, filed Oct. 29, 1992, now U.S. Pat. No. 5,741,253; which is a continuation of U.S. application Ser. No. 07/698,674, filed May 10,1991, now abandoned; which is a divisional application of U.S. application Ser. No. 07/205,935, filed Jun. 13, 1988, now U.S. Pat. No. 5,015,247; all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
• The present invention relates to an artificial fusion implant to be placed into the intervertebral space left after the removal of a damaged spinal disc.
• The purpose of the present invention is to provide an implant to be placed within the intervertebral disc space and provide for the permanent elimination of all motion at that location. To do so, the device is space occupying within the disc space, rigid, self-stabilizing to resist dislodgement, stabilizing to the adjacent spinal vertebrae to eliminate local motion, and able to intrinsically participate in a vertebral to vertebra bony fusion so as to assure the permanency of the result.
• At present, following the removal of a damaged disc, either bone or nothing is placed into the space left. Placing nothing in the space allows the space to collapse which may result in damage to the nerves; or the space may fill with scar tissue and eventually lead to a reherniation. The use of bone is less than optimal in that the bone obtained from the patient requires additional surgery and is of limited availability in its most useful form, and if obtained elsewhere, lacks living bone cells, carries a significant risk of infection, and is also limited in supply as it is usually obtained from accident victims. Furthermore, regardless of the source of the bone, it is only marginal structurally and lacks a means to either stabilize itself against dislodgement, or to stabilize the adjacent vertebrae.
• A review of related prior art will demonstrate the novelty of this present invention.
• There have been an extensive number of attempts to develop an acceptable disc prosthesis (an artificial disc). Such devices by design would be used to replace a damaged disc and seek to restore the height of the interspace and to restore the normal motion of that spinal joint. No such device has been found that is medically acceptable. This group of prosthetic or artificial disc replacements, seeking to preserve spinal motion and so are different from the present invention, would include:
• U.S. Pat. No. 3,867,728 STUBSTAD—describing a flexible disc implant.
• U.S. Pat. No. 4,349,921 KUNTZ—describing a flexible disc replacement with file like surface projections to discourage device dislocation.
• U.S. Pat. No. 4,309,777 PATIL—describing a motion preserving implant with spiked outer surfaces to resist dislocation and containing a series of springs to urge the vertebrae away from each other.
• U.S. Pat. No. 3,875,595 FRONING—describing a motion preserving bladder-like disc replacement with two opposed stud-like projections to resist dislocation.
• U.S. Pat. No. 2,372,622 FASSIO (French)—describing a motion preserving implant comprising complementarily opposed convex and concave surfaces.
• In summary then, these devices resemble the present invention only in that they are placed within the intervertebral space following the removal of a damaged disc. In that they seek to preserve spinal motion, they are diametrically different from the present invention which seeks to permanently eliminate all motion at that spinal segment.
• A second related area of prior art includes those devices utilized to replace essentially wholly removed vertebra. Such removal is generally necessitated by extensive vertebral fractures, or tumors, and is not associated with the treatment of disc disease. While the present invention is to be placed within the disc space, these other vertebral devices cannot be placed within the disc space as at least one vertebrae has already been removed such that there no longer remains a “disc space.” Furthermore, these devices are limited in that they seek to perform as temporary structural members mechanically replacing the removed vertebra (not a removed disc), and do not intrinsically participate in supplying osteogenic material to achieve cross vertebrae bony fusion. Therefore, again unlike the present invention which provides for a source of osteogenesis, use of this group of devices must be accompanied by a further surgery consisting of a bone fusion procedure utilizing conventional technique. This group consisting of vertebral struts rather than disc replacements would include the following:
• U.S. Pat. No. 4,553,273 WU—describing a turnbuckle-like vertebral strut.
• U.S. Pat. No. 4,401,112 REZAIAN—describing a turnbuckle-like vertebral strut with the addition of a long stabilizing staple that spans the missing vertebral body.
• U.S. Pat. No. 4,554,914 KAPP—describing a large distractible spike that elongates with a screw mechanism to span the gap left by the removal of an entire vertebra and to serve as an anchor for acrylic cement which is then used to replace the missing bone (vertebra).
• U.S. Pat. No. 4,636,217 OGILVIE—describing a vertebral strut mechanism that can be implanted after at least one vertebra has been removed and which device consists of a mechanism for causing the engagement of screws into the vertebra above and the vertebra below the one removed.
• In summary then, this group of devices differs from the present invention in that they are vertebral replacements struts, do not intrinsically participate in the bony fusion, can only be inserted in the limited circumstances where an entire vertebra has been removed from the anterior approach, and are not designed for, or intended to be used for the treatment of disc disease.
• A third area of prior art related to the present invention includes all devices designed to be applied to one of the surfaces of the spine. Such devices include all types of plates, struts, and rods which are attached by hooks, wires, and screws. These devices differ significantly from the present invention in that they are not inserted within the disc space, and furthermore do not intrinsically participate in supplying osteogenic material for the fusion.
• Therefore, with these devices where permanent spinal immobilization is desired an additional surgery consisting of a spinal fusion performed by conventional means for the use of supplemental methylmethacrylate cement is required. Such devices applied to the spine, but not within the disc space, would include the following:
• U.S. Pat. No. 4,604,995 STEPHENS—describing a “U” shaped metal rod attached to the posterior elements of the spine with wires to stabilize the spine over a large number of segments.
• U.S. Pat. No. 2,677,369 KNOWLES—describing a metal column device to be placed posteriorly along the lumbar spine to be held in position by its shape alone and to block pressure across the posterior portions of the spinal column by locking the spine in full flexion thereby shifting the maximum weight back onto the patient's own disc.
• Other devices are simply variations on the use of rods (e.g., Harrington Luque, Cotrel-Dubosset, Zielke), wires or cables (Dwyer), plates and screws (Steffee), or struts (Dunn, Knowles).
• In summary, none of these devices are designed or can be used within the disc space, do not replace a damaged disc, and do not intrinsically participate in the generation of a bony fusion.
• Another area of related prior art to be considered is that of devices designed to be placed within the vertebral interspace following the removal of a damaged disc, and seeking to eliminate further motion at that location.
• Such a device is contained in U.S. Pat. No. 4,501,269 BAGBY—describing an implantable device and limited instrumentation. The method employed is as follows: a hole is bored transversely across the joint and then a hollow metal basket of larger diameter than the hole is then pounded into the hole and then filled with the bone debris generated by the drilling.
• While the present invention (device, instrumentation, and method) may appear to bear some superficial resemblance to the BAGBY invention, it is minimal, while the differences are many fold and highly significant. These differences include the following:
• 1. Safety—The present invention provides for a system of completely guarded instrumentation so that all contiguous vital structures (e.g., large blood vessels, neural structures) are absolutely protected. Said instrumentation also makes overpenetration by the drill impossible. Such overpenetration in the cervical spine, for example, would result in the total paralysis or death of the patient. In the thoracic spine, the result would be complete paraplegia. In the lumbar spine, the result would be paraplegia or a life-threatening perforation of the aorta, vena cava, or iliac vessels. The present invention is atraumatically screwed into place while the BAGBY device, in contradistinction, is pounded into position. BAGBY describes that the implant is significantly larger in size than the hole drilled and must be pounded in. This is extremely dangerous and the pounding occurs directly over the spinal cord which is precariously vulnerable to percussive injury. Furthermore, while it is possible, for example in the lumbar spine, to insert the present invention away from the spinal cord and nerves, the BAGBY device must always be pounded directly towards the spinal cord.
• Furthermore, since the BAGBY device is pounded into a smooth hole under great resistance, and lacking any specific design features to secure it, the device is highly susceptible to forceful ejection which would result in great danger to the patient and a clinical failure. The present invention, in contradistinction, is securely screwed into place, and possesses highly specialized locking threads to make accidental dislodgement impossible. Because of the proximity of the spinal cord, spinal nerves, and blood vessels, any implant dislodgement as might occur with the BAGBY device might have catastrophic consequences.
• 2. Broad applicability—The BAGBY device can only be inserted from the front of the vertebral column, however, the present invention can be utilized in the cervical, thoracic, and lumbar spine, and can be inserted from behind (posteriorly) in the lumbar spine. This is of great importance in that the purpose of these devices is in the treatment of disc disease and probably greater than99 percent of all lumbar operations for the treatment of disc disease are performed from behind where the present invention can easily be utilized, but the BAGBY device, as per his description, cannot.
• 3. Disc removal—The BAGBY invention requires the complete removal of the disc prior to the drilling step, whereas the present invention eliminates the laborious separate process of disc removal and efficiently removes the disc and prepares the vertebral end plates in a single step.
• 4. Time required—The present invention saves time over the BAGBY invention in that time is not wasted laboring to remove the disc prior to initiating the fusion. Also, since with the present invention the procedure is performed through a system of guarded instrumentation, time is not wasted constantly placing and replacing various soft tissue retractors throughout the procedure.
• 5. Implant stability—Dislodgement of the implant would be a major source of device failure (an unsuccessful clinical result), and might result in patient paralysis or even death. As discussed, the BAGBY device lacks any specific means of achieving stability and since it is pounded in against resistance to achieve vertebral distraction, it is susceptible to forceful dislodgement by the tendency of the two distracted vertebrae, to return to their original positions squeezing out the device. The present invention however is screwed into place. As there is no unscrewing force present between the vertebrae and compression alone cannot dislodge the implant, the implant is inherently stable by its design. Furthermore, the threads of the present invention are highly specialized in that they are periodically interrupted such that the tail ends of each of the tabs so formed are blunted and twisted so as to resist accidental unscrewing. The removal of an implant with such “locking threads” requires the use of a special extractor included within the instrumentation. The stability of the present invention is still further enhanced, again in contradistinction to the BAGBY device, by the presence of a “bone ingrowth” surface texturing, which both increases the friction of the fit and allows for the direct growth of the vertebral bone into the casing of the implant itself.
• 6. Spinal stability—The present invention is not only self-stabilizing, it also provides stability to the adjacent vertebrae in at least three ways that the BAGBY device cannot. First, the BAGBY device is placed transversely across the joint in the center, leaving both vertebrae free to rock back and forth over this round barrel shaped axis, much like a board over a barrel, being used for a seesaw.
• Secondly, as the BAGBY device lacks any specific design features to resist sliding, it may actually behave as a third body allowing the translation of the vertebrae relative to the device and to each other.
• Thirdly, any device can only provide stability if it remains properly seated. The present invention is inherently stable, and therefore assures that it will stabilize the adjacent vertebrae; rather than, as with the BAGBY device, where the instability of the spine to be treated may instead cause a dislocation of the implant, with further loss of spinal stability.
• 7. The collapse of the interspace—While both the present invention and the BAGBY device can be fabricated to withstand the compression forces within the interspace, the interspace may nevertheless collapse under the superincumbent body weight as the implant settles into the vertebral bone. This is related to the load per unit area. Again the present invention is superior to the BAGBY device in at least four ways. First, the present invention offers considerably greater surface area to distribute the load. Secondly, while the BAGBY device is placed centrally, the present device is placed bilaterally where the bone tends to be more cortical and much stronger out towards the rim. Thirdly, the present invention supports the load achieving an “I” beam effect, whereas the BAGBY implant does not. Fourthly, it is not pressure alone that causes the collapse of the bone adjacent to the implant, but also bony erosion that is caused by the motion under pressure of the implant against the bone. As discussed in item #6 above, the present invention alone is highly resistant to such motion, again diminishing the likelihood of erosion and interspace collapse.
• 8. Bone ingrowth surface texturing—The present invention has a surface treatment of known and conventional technology to induce the growth of bone from the vertebrae directly into the casing material of the implant itself. The BAGBY device has no similar feature.
• 9. Fusion mass—The BAGBY invention calls for removing the disc and then drilling a hole between the adjacent vertebrae. The bony debris so generated is then put into the device. The present invention takes a core of pure bone producing marrow from the iliac crest, and then by use of a special press forcibly injects the device with an extremely dense compressed core of that osteogenic material until the material itself virtually extrudes from every cell of the implant.
• 10. The Probability of achieving fusion—The fusion rate within the spine is known to be related directly to the amount of exposed vascular bone bed area, the quality and quantity of the fusion mass available, and the extent of the stabilization obtained with all other factors being held constant. It would then be anticipated, that the fusion rate would be superior with the present invention as compared to the BAGBY device, because of optimal implant stability (#5), optimal spinal stability (#6), bone ingrowth surface treatment (#8), superior fusion mass (#9), and the greater exposed vertebral bony surface area (#7).
• The last area of prior art possibly related to the present invention and therefore, to be considered related to “BONY INGROWTH,” and patents either describe methods of producing materials and/or materials or devices to achieve the same. Such patents would include:
• U.S. Pat. No. 4,636,526 (DORMAN), U.S. Pat. No. 4,634,720 (DORMAN), U.S. Pat. No. 4,542,539 (ROWE), U.S. Pat. No. 4,405,319 (COSENTINO), U.S. Pat. No. 4,439,152 (SMALL), U.S. Pat. No. 4,168,326 (BROEMER), U.S. Pat. No. 4,535,485 (ASHMAN), U.S. Pat. No. 3,987,499 (SCHARBACH), U.S. Pat. No. 3,605,123 (HAHN), U.S. Pat. No. 4,655,777 (DUNN), U.S. Pat. No. 4,645,503 (LIN), U.S. Pat. No. 4,547,390 (ASHMAN), U.S. Pat. No. 4,608,052 (VAN KAMPEN), U.S. Pat. No. 4,698,375 (DORMAN), U.S. Pat. No. 4,661,536 (DORMAN), U.S. Pat. No. 3,952,334 (BOKROS), U.S. Pat. No. 3,905,047 (LONG), U.S. Pat. No. 4,693,721 (DUCHEYNE), and U.S. Pat. No. 4,070,514 (ENTHERLY).
• However, while the present invention would utilize bone ingrowth technology, it would do so with conventional technology.
• In summary then, the present invention, instrumentation, and method, alone provides for a one stage discectomy, fusion and interbody internal spinal fixation; that being performed more quickly, with greater safety, and more effectively, for all of the aforementioned reasons, than is possible with any other known art.
BRIEF SUMMARY OF THE INVENTION
• The present invention comprises a series of artificial implants, the purpose of which is to participate in, and directly cause bone fusion across an intervertebral space following the excision of a damaged disc. Such implants are structurally load bearing devices, stronger than bone, capable of withstanding the substantial forces generated within the spinal interspace. Such devices have a plurality of macro sized cells and openings, which can be loaded with fusion promoting materials, such as autogenous bone, for the purpose of materially influencing the adjacent vertebrae to form a bony bond to the implants and to each other. The implant casing may be surface textured or otherwise treated by any of a number of known technologies to achieve a “bone ingrowth surface” to further enhance the stability of the implant and to expedite the fusion.
• Further, said devices are so configured and designed so as to promote their own stability within the vertebra interspace and to resist being dislodged, and furthermore, to stabilize the adjacent spinal segments.
• The apparatus for preparing the vertebrae for insertion of the implant is also disclosed, such instrumentation and method allowing for the rapid and safe removal of the disc, preparation of the vertebrae, performance of the fusion, and internal stabilization of the spinal segment.
DISCUSSION OF THE INSTRUMENTATION
• The concept of performing various aspects of this surgery are not entirely new. Drills are frequently placed through hollow, tubular guards to protect the adjacent soft tissues. A set of instruments developed by Ralph Cloward utilizes such a tubular drill guard on a larger scale, for the purpose of drilling into the cervical spine. However, this inventor is unaware of any set of instruments, system, or procedure designed to allow the entire surgical procedure beyond the initial exposure, to be performed blindly and with complete safety through a fixed sheath apparatus. Specific design features which combine to make this uniquely possible are as follows:
• 1. The availability of the specific implant.
• 2. The end of all the penetrating instrumentation is blunt faced.
• 3. All of the instruments have been stopped out at a predetermined depth to avoid overpenetration.
• 4. The design of the external sheath conforms to the spacial limitations of the specific surgical site.
• 5. The design and use of a second or inner sheath allows for the difference in size between the inside diameter of the outer sheath, and the outside diameter of the drill itself. This difference being necessary to accommodate the sum of the distraction to be produced, and the depth of the circumferential threading present on the implant.
• 6. A specially designed drill bit with a central shaft recess allows for the safe collection of the drilling products, which can then be removed without disturbing the outer sheath by removing the drill bit and inner sheath as a single unit.
• 7. A specially designed trephine for removing a core of bone slightly smaller in diameter than the internal diameter of the implant cavity itself, however of a greater length.
• 8. A specially designed press for forcefully compressing and injecting the long core of autogenous bone into the implant, such that it extrudes through the implant itself.
• 9. A specially designed driver extractor, which attaches to the implant and allows the implant to be either inserted or removed without itself dissociating from the implant, except by the deliberate disengagement of the operator.
OBJECTS OF THE PRESENT INVENTION
• It is an object of the present invention to provide an improved method of performing a discectomy, a fusion, and an internal stabilization of the spine, and specifically, all three of the above simultaneously and as a single procedure.
• It is another object of the present invention to provide an improved method of performing a discectomy, a fusion, and an internal stabilization of the spine, which is both quicker and safer than is possible by previous methods.
• It is another object of the present invention to provide an improved method of performing a discectomy, a fusion, and an internal stabilization of the spine, to provide for improved surgical spinal implants.
• It is another object of the present invention to provide an improved method of performing a discectomy, a fusion, and an internal stabilization of the spine, which provides for an improved system of surgical instrumentation to facilitate the performance of the combined discectomy, fusion, and internal spinal stabilization.
• These and other objects of the present invention will be apparent from review of the following specifications and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a perspective view of the driver and sheath and its orientation to a vertebral structure.
• FIG. 1A is a perspective view of the driver member for the outer sheath.
• FIG. 2 is a perspective view of the outer sheath being inserted into the vertebrae structure.
• FIG. 3 is a perspective view of the outer sheath and inner sheath assembly, with the drill bit of the present invention.
• FIG. 3A is a side sectional view of the collar and drill bit ofFIG. 3.
• FIG. 4 is a perspective view of a cylindrical implant and vertebrae structure.
• FIG. 4A is a perspective view of one preferred embodiment of the implant.
• FIG. 4B is a cross sectional view of the implant ofFIG. 4A.
• FIG. 4C is the driving and insertion equipment for the implant ofFIG. 4A.
• FIG. 4D is a side sectional view of the driver and implant between vertebrae.
• FIG. 5 is a sectional view of the vertebrae structure, taken along lines5-5 ofFIG. 4.
DETAILED DESCRIPTION OF THE DRAWINGS
• Referring toFIG. 1 a vertebrae structure comprising two vertebrae V and a disc D between the two vertebrae, is shown. A hollowtubular drill sleeve10 hasteeth12 at its lower end. Thesleeve10 has an enlarged diameterupper portion14.
• Adriver16, shown inFIG. 1A, consists of a solidtubular member18 and an increaseddiameter head20. The external diameter of thesolid tubular member18 is slightly smaller than the inside diameter of the hollowtubular drill sleeve10 and has a length that is substantially shorter than the overall length of the hollowtubular drill sleeve10.
• Thedrill sleeve10 is made of metal in order to be driven into the vertebrae V and be held in place by theteeth12 of thedrill sleeve10.
• Referring toFIG. 2 thedrill sleeve10 with thedriver16 installed is shown being driven into two vertebrae V on either side of a disc D by hammer H.
• Referring toFIG. 3 and3A the drill assembly is shown. InFIG. 3 thedrill sleeve10 is illustrated in the two vertebrae V, straddling the disc D.
• The retainingsleeve15 has an outside diameter slightly smaller than the inside diameter of thedrill sleeve10, and a length substantially the same length as thedrill sleeve10. The retainingsleeve15 has a collar17 at its upper end for engaging the top of thedrill sleeve10.
• Thedrill22 comprises anupper portion24, a central recessedportion26 and a lowercutting drill portion28. The upper24 andlower portion28 of thedrill22 have the same outside diameter. Thedrill24 has a collar30 attached to theupper portion24 of thedrill22.
• The outside diameter of thedrill22 is slightly smaller than the inside diameter of the retainingsleeve15. The length of the drill, from the collar30 to the end of the drill bit, is such that a predetermined portion of thedrill bit22 extends beyond theend29 of the sleeve when fully inserted.
• Referring toFIG. 4, a cylindrical embodiment of the present invention is shown, one implant positioned in the opening in the vertebrae and disc formed by thedrill22, and a second implant shown prior to implantation.
• Thecylindrical implant50 comprises a hollow tubular member which in the preferred embodiment is made of an ASTM surgically implantable material, and preferably Titanium. Thecylindrical implant50 is closed at oneend52 and open at theother end54. The outercylindrical implant50 has a series ofmacro-sized openings56 through the side walls of thecylindrical member50. A series ofexternal threads53 are formed on the circumference of thecylindrical implant50. Thethreads53 are locking threads having a series of interjections, the ends of which are blunted and twisted so as to resist unscrewing.
• Theopen end54 of thecylindrical implant50 has aninternal thread51 for receiving acomplementary cap52 which has anexternal thread58 for engaging theinternal threads51 of thecylindrical member50. Thecap52 has ahexagonal opening59 for use with an Allen wrench for tightening the cap. Adriver engaging element70 is located on therear surface60 of the implant. Thedriving engaging element70 comprises a raisedrectangular portion63 and a central threadedopening65, for engaging the driver apparatus, shown inFIG. 4C andFIG. 4D. Thedriving equipment100 comprises a centraltubular rod102 having a thread fitting into opening65 in the implant. An enlarged knurled knob106 is affixed to the other end of therod102 for ease in turning. Thecentral rod102 is enclosed within a hollow tubular member108, having a narrowlower portion110 and an increased diameterupper portion112. At the end of thelower portion110 is anattachment member114, having a generallyrectangular depression116 for complementing thedriver engaging element70 of theimplant50. A pair ofhandles118 and120 extend perpendicular from theupper position112 of the tubular member108 to assist in turning thedriver100.
• The operation is performed in the following manner: (Example Lumbar Spine Posterior Approach) A skin incision is made directly over the interspace to be operated on. The dissection is carried down along side of the superspinous and intraspinous ligaments preserving those structures. A semi hemi laminotomy is performed at the upper level, removing sufficient bone to allow access into the interspace. The ligament flavum is removed and then the dural sac is protected by retracting it medially along with the traversing (inferior) nerve root. The superior nerve root or the root exiting beneath the pedicle at the level above is visualized and protected.
• At this time thedrill sleeve10 is placed into the spinal canal with both nerve roots directly inspected and protected. Thedrill sleeve10 is embedded byteeth12 spanning the disc space from the midline over and it is seated into the two vertebrae V across the disc D space by using adriver20. Once this is done, thedriver20 is removed and aretraining sleeve15 is placed through thedrill sleeve10. Once seated,sleeve10 provides absolute protection to the dural sac and nerve roots as the remaining surgery is performed entirely through this sleeve.
• The inner sleeve allows for the difference between the outside diameter of thedrill22 and the outside diameter of thethreads53 of thecylindrical implant50. This then makes it possible to perform the entire operation through the lumen of the embedded outer sleeve despite the differences in diameter between the drill and the implant.
• Adrill22 is then placed in the retaining sleeve17. Thedrill22 is of such a length that it cannot penetrate more than28 millimeters beyond the end of thedrill sleeve10. This, of course, could be varied and made smaller for enhanced safety. However, at the present time27 to28 millimeters seems to be safe for probably3 standard deviations of the population. Thedrill22 is attached to a power unit and the drilling takes place.
• The recessed central area between the reducedportion26 allows for the accumulation of the debris generated by the drilling. At this time, leaving the outer sleeve firmly embedded, the retaining sleeve17 is removed with thedrill22 as a single unit. All the vertebrae and disc debris that was generated during the drilling is contained within the recess and against the inside wall of the retaining sleeve17 and cannot come out within the spinal canal. Once the retaining sleeve17 anddrill22 is out of the patient's operative field, all of the material so generated can be removed.
• The next step is that a screw tap is put down through thedrill sleeve10. The tap also has a collar on it that will automatically stop the tap from extending beyond28 millimeters of penetration. The tap itself has a blunt nose that would also avoid any perforation. The tap is then removed. The tap size has deliberately been selected so that its inner root diameter is1.3 millimeters greater than the outside diameter of thedrill22. This insures that the interspace will be distracted by at least that much once the implant is placed. The tap has its outside-diameter1.2 millimeters greater than its root diameter. The tap is removed and the space is now prepared to accept thecylindrical implant50.
• Theimplant50 is prepared by utilizing the trephine, a hollow drill, to obtain a core of pure cancellous bone from the patients iliac crest of slightly smaller diameter than the internal diameter of the implant but approximately6mm longer. Theimplant50 is placed in a press like device like an ammo loader and the bone graft measuring approximately32 millimeters is then compressed into the hollow body of the implant (26 mm) so that the bone graft fills theopening54 and extends through theopenings56. Thecap60 is then screwed on to theimplant50 by use of an Allen driver/wrench, and the device is ready for implantation.
• The inserter/remover is such that it locks onto the implant, so that the implant can be moved either clockwise or counter-clockwise, screwed or unscrewed. The implant itself has for its root diameter the-same exact root diameter as the tap which as already noted is already 1.3 millimeters greater than the drill and has an outside diameter, 1.5 millimeters greater than its root. This is also 0.3 millimeters greater than the threads cut by the tap so that in inserting the device it is actually cutting through previously uncut bone, helping to insure that it locks in firmly. The threads on theimplant50 are locking threads so that it is easier to screw the device in than for it to be unscrewed. However, with sufficient torque it is possible to extract the device if one so desires.
• Once the implant has been seated it is able to be inserted only 28 millimeters. Since theimplant50 is only 26 millimeters in length, this virtually guarantees that theimplant50 will be recessed into the vertebral bodies more than 2 millimeters and cannot protrude into the spinal canal.
• Similarly, the implants shown inFIG. 4B can be implanted. The implant inFIG. 4B is a modified solid, having extensive channeling throughout, and has no cap. Acentral opening61 permits insertion of the bone graft material into the interior of the implant.
• These implants have a surface configuration such as to induce bone ingrowth through the implant, and into the wall of the vertebrae in effect inducing fusion from one vertebrae in joint to the other, thereby eventually making the implant itself superfluous as the bone would do the work.
• The implant itself, because of its being made of stronger material than bone, would provide structural support to the two vertebrae while awaiting bone ingrowth. Once the bone ingrowth occurred, however, the implant would be firmly and permanently fixed in place.
• As shown inFIG. 4, more than one implant is inserted into the disc space, thereby preventing the rocking motion that would result in the difficulties referred to above in the discussion of the Bagby patent.
• While the invention has been described with regards to the preferred embodiment, it is recognized that alternative embodiment may be devised which would not depart from the present invention.

Claims (4)

1. A fusion device for facilitating arthrodesis in a disc space between adjacent vertebrae, comprising:

an elongate body having a length and including a pair of oppositely disposed arcuate side walls extending along said length and adapted for engagement with the adjacent vertebrae, said arcuate side walls defining external threads extending substantially entirely along said length, said elongate body at least partially formed of a porous biocompatible material to permit bone tissue ingrowth into said arcuate side walls.

2. The fusion device ofclaim 1, wherein said elongate body has a hollow interior and at least one opening in communication with said hollow interior.

3. The fusion device according toclaim 2, further comprising a bone growth inducing material disposed within said hollow interior.

4. The fusion device ofclaim 2, wherein said at least one opening extends through a corresponding one of said arcuate side walls.

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