US20030139815A1

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Publication number: US20030139815A1
Application number: US10/375,540
Authority: US
Inventors: Jamie Grooms; Kevin Carter; Tom Sander; David Dulebohn; Loic Josse; Lawrence Boyd; Charles Branch; Mingyan Liu
Original assignee: University of Florida Tissue Bank Inc
Current assignee: Regeneration Technologies Inc
Priority date: 1997-08-27
Filing date: 2003-02-27
Publication date: 2003-07-24
Also published as: US7048762B1

Abstract

An implant composed substantially of cortical bone is provided for use in cervical Smith-Robinson vertebral fusion procedures. The implant is derived from allograft or autograft cortical bone sources, is machined to form a symmetrically or asymmetrically shaped (e.g. a substantially “D”-shaped) implant having a canal running therethrough according to methods of this invention, and inserted into the space between adjacent cervical vertebrae to provide support and induce fusion of the adjacent vertebrae. Osteogenic, osteoinductive or osteoconductive materials may be packed into the canal of the implant to expedite vertebral fusion and to allow autologous bony ingrowth.

Description

1.0 BACKGROUND OF THE INVENTION

1.1 Field of the Invention[0001]

This invention relates to a cortical bone implant for use in cervical Smith-Robinson vertebral fusion procedures, as well as methods for the manufacture and use thereof.[0002]

1.2 Background Art[0003]

Since at least the mid to late 1950's anterior cervical spinal fusions have been performed in order to alleviate chronic neck, arm and shoulder pain caused by trauma, disc herniation, or spondylosis (Robinson and Smith, 1955; Smith and Robinson, 1958). The classic procedure referred to as the Smith-Robinson cervical fusion employs a horseshoe-shaped graft to promote vertebral fusion (Robinson et al., 1962). The Cloward technique employs a cancellous bone dowel (Cloward, 1958), and the Bailey-Badgley procedure uses a strut (Bailey and Badgley, 1960). In a study comparing the compressive load capacity of the various implants used according to these procedures, it was found that the Smith-Robinson graft could sustain loads up to 344 N, the Cloward dowel could sustain loads of up to 188 N, and the Bailey-Badgley type could sustain loads up to 195 N, (White and Hirsch, 1972). In a modified Smith-Robinson procedure, the horseshoe-shaped implant is inserted with the cortical end of the implant located posteriorly, which has been reported to increase the fusion rate while decreasing the graft extrusion and collapse sometimes experienced with the Cloward dowels (Whitecloud and Dunsker, 1993). However, in a recent study evaluating the success and relief rates achieved according to these procedures, it was found that less than 100% success rate (fusion, patient improvement and absence of complications) was achieved, regardless of which method or implant was used (Grooms et al, 1996).[0004]

U.S. Pat. No. 5,306,309, discloses a spinal disk implant comprising a solid body of biocompatible synthetic material arranged to define a right-rectangular solid having two opposed side faces and two opposed transverse faces, including a convexly curved anterior face and a posterior face, for implantation in the intervertebral space. The discussion of vertebral and intervertebral morphology is hereby incorporated by reference.[0005]

U.S. Pat. No. 5,609,635, discloses a lordotic interbody spinal fusion implant comprising a wedge shaped metallic cage for insertion into the intervertebral space.[0006]

U.S. Pat. No. 5,306,307, discloses a ceramic spinal disk implant having a serrated edge.[0007]

None of these references disclose a cortical bone intervertebral implant having a substantially “D”- or bread-loaf-shaped structure having a canal into which osteogenic, osteoinductive, or osteoconductive materials may be packed, which sustains spinal loads, and which is remodeled into the spine in the course of fusion. Accordingly, the present invention addresses the need in the art for improvements to both the implant and the avoidance of post-surgical complications from anterior cervical fusions. The present invention provides a new cortical bone implant for use in achieving anterior cervical fusions when implanted according to the Smith-Robinson procedure. In addition, in view of the peculiar characteristics of bone, the present invention comprises unique methods and apparatuses for the manufacture of the substantially “D”-shaped cortical bone implant.[0008]

2.0 SUMMARY OF THE INVENTION

An implant composed substantially of cortical bone is provided for use in cervical Smith-Robinson vertebral fusion procedures. According to methods of this invention, the implant is derived from allograft or autograft cortical bone sources, is machined to form a substantially “D”- or other appropriately shaped implant having a canal into which osteogenic, osteoinductive, or osteoconductive material may be packed. The implant is inserted into the space between adjacent cervical vertebrae to provide support and induce fusion of the adjacent vertebrae.[0009]

3.0 BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides several views of the fusion implant of this invention.[0010]

FIG. 2 provides views of the core cutter and drill assembly and the bone plug formed by cutting into the diaphysis of a long bone when such a core cutter and drill assembly is used.[0011]

FIG. 3 provides a view of broach as used according to this invention and an asymmetric canal formed by use of such a broach.[0012]

FIG. 4 provides several views of an apparatus for machining a profile on the exterior surface of the implant of this invention.[0013]

FIG. 5 provides a view of an apparatus for inscribing retention teeth in the upper surface, lower surface or both upper and lower surfaces of the implant.[0014]

FIG. 6 provides several views and dimensions for specific embodiments of the implant of this invention.[0015]

FIG. 7 provides a view of a stacked embodiment of the implant of this invention.[0016]

FIG. 8 provides several views of an implant of this invention formed by juxtaposition of mirror image halves of the implant, as well as an embodiment useful for posterior lumbar intervertebral fusion procedures (PLIFs).[0017]

FIG. 9 provides a view of a stacked embodiment of the implant of this invention wherein the stacked constituents thereof are retained in registered relationship by press-fitting or otherwise bringing more than one implant into contact with each other and having a cancellous plug or other biocompatible material located in the central canal of each stacked implant, thereby acting as a retention pin.[0018]

FIG. 10 shows an alternate method for producing bone stock for making the implant of this invention of essentially unlimited height from the anterior margin of the tibia or the linea aspera of the femur.[0019]

FIG. 11 shows dimensions and further processing of the implant of this invention produced according to the alternate method of FIG. 10.[0020]

FIGS.[0021]12-17 show final profiles for implants produced according to the alternate method of FIGS. 10 and 11.

4.0 DETAILED DESCRIPTION OF THE INVENTION

According to this invention, a substantially “D”-shaped cortical bone implant for cervical Smith-Robinson fusions is produced, preferably under aseptic conditions.[0022]

Class

10 clean room processing is desirable, and sterilization of all machining tools is likewise preferred, (particularly after switching from one allograft donor to the next), so that the finished product may be treated by standard techniques known in the art (alcohol, peroxide, or like treatments), prior to storage and shipment to physicians for use in implantation procedures. Because of the peculiarities of working with bone, and in particular, because of the desirability of maintaining aseptic conditions while working with this material, novel approaches have been adopted in the production of the product of this invention.

The implant is preferably formed from cortical bone obtained from tibia, femur or other source of strong cortical bone. The bone source may be autograft or, due to possible complications at the donor site (infection, pain, delayed healing), is preferably, allograft bone. In addition, it is critical that the source bone be derived from a donor whose medical history is well known (absence of transmissible diseases, cancer, osteoporosis), and that the donor bone be obtained under aseptic conditions according to accepted practices in the art of tissue banking. In addition, extensive in vitro testing should be conducted to ensure the absence of pathogenic agents.[0023]

The approach adopted in describing the implant of this invention is to first provide a narrative disclosure of preferred methods for making the implant, followed by a detailed description of the implant itself, followed by a detailed description of various apparatuses and aspects of the machining process, and finally, a detailed description of the method of using the implant.[0024]

4.1 Narrative Description of Implant Manufacture

While any shape of cortical bone may be used to begin with, we have found that for consistent production of cortical bone which may be reliably machined, it is advantageous to commence with a plug of bone which extends from the exterior of the diaphysis of a long bone toward the intramedullary canal (where, in vivo, the bone marrow resides). The result is a bone plug or dowel which has an outer substantially cortical end and an internal end which is composed largely of soft cancellous bone. In cutting the bone plug, we have discovered that the use of a core cutter is convenient. This device comprises an outer coring element of any desired diameter, whereby the diameter of the bone plug is defined, and a centrally located solid drill bit, which provides a canal through the center of the bone plug as well as stability for the core cutting element. The core cutter-drill assembly is preferably torqued by an air drill, driven by sterile air, and the source bone is preferably immobilized in a sterilized vice during the core-cutting process.[0025]

We have discovered that in the above-described manner, cortical bone implants may be fashioned having heights, widths and lengths which are practically useful in the Smith-Robinson cervical fusion method. According to this method, the height of the implant is only limited by the distance from the exterior of the bone diaphysis to the intramedullary canal. However, we have discovered that, by this method, final implant heights from about 7 mm to about 14 mm may be produced, depending on the choice of bone source and the location on the bone from which the bone plug is cored. Since it is extremely rare for the cervical intervertebral space to extend beyond these limits, this method is therefore capable of supplying implants of required or useful heights. Likewise, the length and width of the implant are defined by the diameter of the core-cutter, and final lengths and widths of between about 7 and 14 mm are easily provided for by this method. In addition, where the need arises for heights between about 10 mm and 14 mm, or if difficulty is experienced in obtaining donor bone having a sufficient width from the exterior of the bone to the intra-medullary canal to provide such heights, alternate methods of producing the implant of desired heights disclosed herein may be employed. For example, in a first such alternate method, implants of this invention are produced and then stacked to provide a unitary implant of the desired height dimensions. Such stacked implants may be maintained in a unitary association by drilling appropriate holes through the height of the implant, and inserting therein appropriate retention pins made from any desirable material, including cortical bone, bioabsorbable synthetic polymer, titanium or other metallic retention pins. Alternatively, the stacked implants may be retained in a unitary association by means of a plug of cancellous bone, hydroxyapatite or other biocompatible, osteoconductive or osteoinductive material, and press-fitting the stacked implants to achieve the desired height (see FIG. 9). In a further alternate method, a section of cortical bone along the long axis of a long bone may be machined according to methods known in the art. By then further shaping and cutting appropriate heights in such cortical bone, and bringing halves of the implant into juxtaposition with each other, implants of any desired shape and height are produced. In yet a further alternate procedure, (see FIGS.[0026]10-17), unitary implants of this invention of essentially unlimited height are produced by length-wise sectioning the anterior margin of the tibia or linea aspera of the femur, segmenting the substantially triangular cortical bone to desired heights, drilling a cannulation through the segments thus produced, and finally shaping the implants to desired dimensions, as defined below for the first principal method of making the implant of this invention.

Continuing with a description of the first method for making the implant of this invention, the cancellous bone on the internal side of the bone plug is removed by any convenient means, including with a saw, an abrasive means such as a diamond tipped rotary sander, or a tooling bit mounted in a lathe, to produce a “washer” shaped piece of substantially cortical bone. Both the internal and external ends of the bone plug should be machined flat, thereby forming a top face and a bottom face, each of which is substantially planar, and preferably parallel. While the cancellous bone is partially or completely removed by this process, there remains a slight difference in the density of the bone from the external (cortical) to the internal (cancellous or originally intra-medullary) aspect of the bone plug. It is desirable to record the orientation of the bone plug as subsequent machining steps proceed most efficiently when machined from the external aspect toward the internal aspect.[0027]

In order to accommodate subsequent machining steps and to provide an orientation to the implant according to which the surgeon may properly insert the implant, the circular internal canal formed by the centrally located solid drill bit of the core-cutter is modified to form an asymmetric shape, such as a key way. This may be achieved by any of a number of different means, including drilling a slot into an aspect of the internal canal closest to the external (more dense cortical) end of the dowel. In one embodiment of this invention, we have found that an implant of consistently good final quality may be machined by conversion of the circular canal into a substantially “D” shaped canal having three essentially rectangular walls and a fourth convexly curved wall. We have found that it is desirable for the curvature of the convexly curved wall to approximate the external curvature of the bone plug. This modification may be achieved by any of a variety of means. However, we have invented an efficient means by which consistently usable implants may be reproducibly machined. This is accomplished by immobilizing the implant, for example in an arbor press assembly, and, preferably from the originally cortical external (denser) end of the implant, slowly forcing a broach through the originally circular canal. The broach is preferably a hard metallic member having a plurality of spaced-apart ribs or rings machined therein, with indentations provided between each ring which thereby form the spacing between adjacent rings. In addition, the edges of each ring are desirably very precise, angular, and sharp, such that as the broach is forced through the originally circular internal canal, the sharp cutting edge of each ring shaves off an incremental amount of bone as the ring passes through the implant. Each ring of the plurality of rings has a shape which, starting at the insertion end of the broach is tapered from an essentially circular shape to any desired final shape for the canal. Accordingly, in one embodiment of this invention, the rings transition from a circular shape to a substantially “D”-shaped profile or any other desired shape over several inches and over a plurality of spaced apart rings. It will be appreciated that the length of the broach and the number of rings used is defined by the amount of bone that must be removed to form the new shape, the width of each ring and the width of the space between each ring. Removal of no more than about 0.004″ of bone by each ring has been found to be a sufficiently small transition to ensure that the vast majority of implant blanks survive this machining step. Broaches of approximately 6″ in length have been found adequate for most implant shapes, but for very asymmetric shapes (e.g. an implant which is 11 mm wide and 14 mm long), more bone would need to be removed to form the “D”-shaped canal than from a symmetric implant (e.g. a 14 mm wide by 14 mm long implant). This need may be accommodated by use of more than one broach, with the shape of the insertion end of each consecutive broach substantially matching the shape of the last ring on the previous broach.[0028]

Having formed an asymmetric shape, such as a key way, from the internal canal running through the implant, we have found it desirable to modify the external profile of the implant from a substantially circular shape to another desired form. In one embodiment of this invention, the external form of the implant is machined so as to proportionately match the shape of the substantially “D”-shaped internal canal. An external “D”-shaped profile has been used in implants known in the art (see for example U.S. Pat. No. 5,306,309; 5,522,899) made from materials other than bone, because of the ability of the convexly curved face of the implant to substantially match the curvature of the anterior aspect of the intervertebral disk into which the implant is to be inserted, as well as to provide efficient spinal load distribution over the remainder of the implant. However, due to the peculiar nature of bone, and the requirements of aseptic or sterile manufacturing, inventive methods and apparatuses were required to produce the desired external profile for the cortical bone implant. It will be recognized that, based on the instant disclosure, a substantially “D”-shaped external profile of the implant may be machined by a variety of means which vary from the precise methods disclosed herein. In addition, other external profiles than the “D”-shaped profile are likewise enabled by modifications of the methods and apparatuses disclosed herein for formation of the “D”-shaped external or internal profile. Thus, according to one alternate method of making the implant of this invention, for example where the linea aspera of the femur is sectioned or where the anterior margin of the tibia is sectioned, an external profile that substantially varies from a “D”-shaped device may be produced, (see FIGS.[0029]10-17).

We have found it convenient and reproducible to use either of two principal methods for machining the external profile. The implant, with the “D” or alternately shaped internal canal being used as a key way, is fitted onto the end of a spindle which precisely matches the shape of the internal canal of the implant, thereby providing purchase for machining of the external profile of the implant. In a first preferred method, as the implant is rotated on the spindle, it is contacted with an asymmetric generator (grinding) wheel attached to a cog which meshes at a known registration point with a cog to which the spindle with the implant is attached. The speed of rotation of the exterior of the spindle mounted implant, and the exterior of the generator wheels are designed to differ such that as the generator wheel and implant are contacted and are rotated in fixed registration, the generator surface (which is preferably an abrasive diamond plated surface), grinds bone from the external surface of the implant, to form a profile thereon defined by the asymmetric shape of the grinder wheel.[0030]

In a second external profile generation method, the implant, with the “D” or alternately shaped internal canal being used as a key way, is fitted onto the end of a spindle which precisely matches the shape of the internal canal of the implant, thereby providing purchase for machining of the external profile of the implant. In this method, the spindle is affixed to an asymmetric cam which rotates concentrically with the spindle, and therefore the implant. The thus mounted implant is contacted with a cutter means, such as a sharp bit having cutting edges which rotate about an adjacent axis. The implant mounted spindle riding on the asymmetric cam is biased to contact the rotating cutter, which thus traces a profile onto the exterior of the implant defined by the shape of the asymmetric cam. For purposes of this disclosure, use of the term “asymmetric cam” should be understood to mean any desirable shape such that upon production of the implant, the shape thereof is defined by that of the asymmetric cam. Shapes contemplated by this disclosure include, but are not limited to, elliptical shapes, D-shapes, partially curved shapes, and the like. The implants produced according to any of the alternate procedures are likewise shaped, although the final shape may vary depending on the size of the bone stock used (see for example the final shapes of the device shown in FIGS.[0031]12-17).

Once the external profile has been machined, the implant is removed from the spindle, and the machining of the implant may either be terminated, to provide a substantially “D”-shaped cortical bone implant with flat upper and lower surfaces, or an external feature may be machined into the upper and lower surfaces to prevent backing out of the implant upon insertion into the intervertebral space. This may be achieved by a number of means, such as by machining annular rings, indentations and projections, ribbing or teeth into the upper, lower, or both surfaces of the implant. In a preferred embodiment of this invention, the implant is passed through a set of opposing jaws bearing teeth which broach a tooth-shaped profile into the implant as it is forced through the jaws. Alternatively, the implant is passed several times over a ridged surface which cuts the desired tooth profile into the upper, lower or both surfaces of the implant. Preferably, the thus formed teeth angle toward the anterior (convexly curved) face of the implant to prevent backing out of the implant once it is inserted into an appropriately shaped cavity formed in the intervertebral space in an anterior aspect of the cervical spine. In order to accommodate the difficulty surgeons experience in forming precise angles when forming such cavities in the spine, (see for example U.S. Pat. No. 5,397,364 disclosing a beveled edge to reduce trauma upon insertion of a metallic spinal implant), a beveled edge of defined radius is preferably machined into three faces of the implant, but leaving the anterior face unbeveled. The sharp anterior edge, like the teeth in the upper and lower surfaces of the implant, retards backing out of the implant.[0032]

4.2 Detailed Description of the Implant

Referring now to FIG. 1A, there is shown a top view, as if viewed from the top of the spinal column, of a substantially “D”-shaped[0033]

cortical bone implant

100. The implant has awall thickness101, alength103, awidth102, and aninternal canal104, which fall within desired tolerances (see discussion below). The implant comprises four contiguous walls, including a substantially straightrear wall105, substantially

straight side walls

106 and107, and a preferably curvedfront wall108. In FIG. 1B, there is shown a side view of theimplant100, revealing theheight109, of the implant. In addition, this view shows, in outline, theinternal side walls106′ and107′ of the internal canal,104. It also shows the top110 and bottom111 surfaces of the implant. In FIG. 1C, there is shown a top view of an embodiment of theimplant100 in which anexternal feature120 has been inscribed onto the top110 and bottom111 surfaces of the implant. In addition, a “radius” orbevel115 is shown on the two side and posterior edges of the implant. FIG. 1D shows a side view of theimplant100 in which the inscribedfeature120 can clearly be seen in the top110 and bottom111 surfaces of the implant. In this view, it can be seen that theexternal feature120 has the side profile of a set of teeth, all of which angle toward theanterior face108 of the implant. An outline of thebevel115 is also evident in this view, as is the roundedposterior edge105. As can be seen, in this embodiment of the invention, theanterior edge108 is maintained with a sharp edged. In FIG. 1E, there is shown a detail of one embodiment of the inscribedfeature120 on the portion of the implant indicated in FIG. 1D. In a preferred embodiment, thefeature120 defines a tooth-like structure, withteeth121 separated from each other byconcavities122. An angle θ defines the grade of the concavity as it ramps to the tooth. Thetooth height123, space betweenteeth124, and aperture of theconcavity125 may all be defined by the manufacturer to optimize retention of the implant within the cervical spine after proper placement.

4.3 Detailed Description of the Method of Manufacturing the Implant

Because of the peculiar nature of bone, and the desirability of sterile or aseptic manufacturing, specific and specialized procedures and apparatuses are required for successful formation of the implants of this invention. Those skilled in the art will recognize that, based on the methods and apparatuses disclosed herein, the implant of this invention may be manufactured by alternate means suggested by those described herein. Nonetheless, through careful design and knowledge of bone structure, instruments for the manufacture of the implant of this invention have been invented for this purpose. In what follows, specific details with respect to preferred method and apparatuses for making the implant of this invention are provided. It should be recognized that the invention should not be construed as being limited to these specifics.[0034]

Referring to FIG. 2, there is shown in side view in FIG. 2A a[0035]

core cutter

200, having acore bit201 which is affixed by aset screw203 to theshaft204 of adrill bit202, centrally located within and coaxial with the core cutter. In FIG. 2B, an end-on view of thecore cutter200 is provided showing theset screw203 in outline. FIG. 2C shows a side view of thebone plug210 which is formed by cutting a plug of bone from the diaphysis of a long bone using thecore cutter200. At one end,211, originally the external cortical surface of the bone shaft, there is a substantially cortical bone surface through which ahole213 is formed by thecentral bit202 of thecore cutter200. The other end,212, is an irregular and bone surface which, in vivo, formed part of the wall of the intramedullary canal. Cancellous bone or other microstructure at theend212 is removed, and both ends are ground, cut or otherwise machined to be substantially flat and parallel, to form the substantiallycortical bone plug210 shown in FIG. 2D.

Referring to FIG. 3, there is shown in FIG. 3A an internal canal[0036]

profile broaching tool

300. A plurality of spaced-apart ribs or rings301 are provided along the length of the broach which taper from a substantially circular shape at theinsertion end302 of the broach, to substantially “D”-shaped rings303 (or any other desired shape) at thecompletion end304 of the broach300 (intermediate ribs305 are not shown; rather, the outline of the taper angle is shown). A notch or groove306 is provided in the broach completion end304 for releasably affixing the broach into a means, such as a press, for forcing the broach through the implant canal. In FIG. 3B, there is provided an end-on view of thecancellous bone plug310 after the broaching procedure is completed. As can be seen, theinternal canal104 has been converted from a circular canal into a substantially “D”-shaped canal. As will be appreciated from this disclosure, any of a number of different asymmetric shapes in theinternal canal104 may be defined by this or analogous means, the principal goal being to provide a purchase (referred to herein as a “key way”) within the implant for external machining of the implant. In one embodiment (see FIG. 9), the canal may be retained as a substantially circular canal, and aslot904 is machined therein to provide the necessary asymmetry to form a key way.

Having formed a key way within the implant, it is possible to modify the external profile of the implant. In one aspect of this invention, referring to FIG. 4A, this is conveniently achieved by affixing the[0037]

implant

410 to thespindle420 of alathe400. Thespindle shaft440 extends, through bearings (not shown), to a means450 (such as a handle or a motor) for rotating the spindle. Affixed to the spindle-shaft is acam430, the shape of which defines the ultimate external profile of theimplant410. Thespindle shaft440 and bearings are mounted in across slide441, which translates in a first plane, referred to as the “Y-plane”. Motion in the Y-plane is limited by contact of thecam430 with a limiting means460 such as a cam follower, which remains in register with acarriage442 which translates along a plane, the “X-plane”, transverse to the Y-plane motion of the cross-slide. The cross-slide is mounted in a slide-way443 of thecarriage442, which in turn is slideably mounted on thebed444 of the lathe, such that thecarriage442 is permitted to translate along the X-plane. Travel of theslide442 along the X-plane is limited by means of astop screw470.

Further detail of this means for generating the external profile of the implant is provided in FIG. 4B, which provides a side view of one specific embodiment of the implant[0038]

external profile generator

400. An air driven turbine withinhousing401 provides a source of torque to turn ashaft402. A means for cutting or grinding the external surface of theimplant410, such as an appropriately fashioned cutter or bit having anon-cutting end403 for fixation to theshaft402. Extending from thenon-cutting end403 which has a first diameter, is a cuttingsurface404, having a second, smaller diameter. A “shoulder”405, forms a radius extending between the smaller diameter of the cuttingsurface404 and the larger diameter ofnon-cutting surface403. The cuttingsurface404 is contacted with the implant blank410, mounted onspindle420, to which, as described above is mounted anasymmetric cam430. The thus mounted implant blank410 is brought into contact with the cuttingsurface404, by virtue of translation in the X-plane of thecarriage442. Thespindle420, and thus theasymmetric cam430 are rotated, manually or by motor driven means, throughshaft440 and handle450 which are attached concentrically with thecam430. Preferably, theasymmetric cam430 is elastically biased toward astationary cam follower460. In this fashion, after several revolutions of thehandle450, the shape of theasymmetric cam430 generates the desired external profile of theimplant410 riding on thespindle420, through contact with therotating cutting surface404.

To ensure that the implant blank is machined only up to the point that the[0039]

forward edge

411 of the implant approaches but does not contact the “shoulder”405 on the cutter, astop screw470 is provided. Thestop screw470 is set to prevent further advancement of the implant blank410 by stopping advancement of thecarriage442 when theleading edge471 of the stop screw comes into contact with a measuringscrew480. The appropriate setting of thestop screw470 is achieved at the start of the milling process by first placing theimplant410 between theend481 of the measuringscrew480 and ananvil482, and tightening the measuringscrew480 until it just makes contact with the implant. In this fashion, the measuringscrew480 andanvil482 essentially form a micrometer, with the gap being defined by the width of the implant. Both the measuringscrew480 andanvil482 are housed within a measuringslide483 which, when slid all the way to the left as shown in FIG. 4B, abuts arotateable stop cam490, retained within the same slide-way as the measuringslide483 by aretainer484. Therotateable stop cam490 may be set in either of two positions, which produces a difference in the stopping point of thestop screw470 of approximately 0.06″. The significance of this difference is that the first position arrests advancement of the stop screw470 (and therefore the carriage442) just before theimplant410 contacts theradius shoulder405 of the cuttingsurface404. In the second position, thestop cam490 allows the stop screw to advance the additional approximately 0.06″ to allow contact of theimplant410 with theshoulder405 of the cuttingsurface404 to thereby bevel the edges of theimplant410 that are thus contacted. Accordingly, in the pre-milling setup, thestop cam490 should be rotated such that thestop screw470 is forced to stop the extra 0.06″, following which a further processing step may be carried out in which thestop cam490 is rotated to the second position in which thestop screw470 is allowed to advance this additional approximately 0.06″.

In FIG. 4C, there is provided an end-on, rear view (i.e. looking from the[0040]

handle

450 toward the spindle420) of theasymmetric cam430, thespindle420 and theimplant410. In addition, in this detail view, an additional feature in theasymmetric cam430 is seen as a diminution in the thickness along threefaces431 of theasymmetric cam430 which is a relief in the rear of theasymmetric cam430. The significance of thisrelief431 is that it restricts the contact of theimplant410 with theshoulder405 to the extent defined by the relief in the rear of theasymmetric cam430. As noted above, in fashioning an implant site in the intervertebral space during a partial discectomy, surgeons are unable to produce perfectly sharp angles. To accommodate this imperfection, to prevent trauma upon insertion of an implant with sharp edges, and to create as tight-fitting an implant as possible, the fashioning of a bevel around the edges of the implant that are inserted into the intervertebral space created by the surgeon is desired. At the same time, in order to prevent backing out of the implant, it may be desirable to retain a sharp anterior implant edge, and therefore the relief in thecam430 does not extend completely around the cam. Thus, upon completion of the external profile of theimplant410 as described above, thecarriage442 is backed away from the cutter, thestop cam490 is flipped to its second position allowing advancement of thestop screw470 the additional approximately 0.06″ mentioned above. At the same time, ashot pin432 is advanced into therelief431 by means of ashot pin mover433, thereby allowing rotation of thecam430 only to the extent permitted by theshot pin432 as it rides within therelief431. With theshot pin432 riding in therelief431, the “shoulder”405 contacts theleading edge411 of the implant blank410, thereby rounding three edges of theimplant410. After machining theleading edge411 of theimplant410, the implant is removed from thespindle420, turned around, and re-positioned on thespindle420, to inscribe the bevel on three edges of the other side of the implant.

In FIG. 4D, a frontal view is provided of the[0041]

spindle

420, theimplant410, theasymmetric cam430, and thecam follower460. Also shown is thecam adapter461, by means of which thecam follower460 is affixed to thecarriage442, and by means of which thecam follower460 maintains the cutting surfaces404/405 in contact with theimplant410 as defined by the shape of theasymmetric cam430. Also shown is a part of thecross-slide441, which is preferably biased or which may be pushed manually toward thecam follower460.

In FIG. 4E, a side detail view is provided of the[0042]

stop cam

490. In this view, a stop cam handle491 is shown which allows the operator of the implant outside profile generator to fix thestop cam490 in a first position A, and a second position B, whereby additional travel of thestop screw470, and thereby advancement of thecarriage442, is provided in position B, of about 0.06″ due to the difference in the distances shown for these positions.

By means of the apparatuses and method described above, a[0043]

cortical bone implant

100 as shown in FIG. 1 having a substantially “D”-shaped external profile, and a substantially “D”-shaped internal canal is produced. Naturally, based on this disclosure, those skilled in the art will appreciate that other shapes, both for the external profile and internal canal of the implant may be produced. For example, an ellipsoid is produced by the above described methods simply by modification of the shape of the asymmetrically shapedcam430, and the internal canal shape may be modified by drilling, routing, or broaching using a broach that tapers to any desired shape. The thus formed implant may be used after machining as described, followed by appropriate cleaning methods known in the art (e.g. bathing in alcohol, peroxide treatment etc.). In addition, however, it may be desirable to inscribe an external feature on theupper surface110, thelower surface111, or both. Such a feature may take any desirable form, such as annular rings, indentations, projections, ribbing or teeth. In a preferred embodiment, teeth sloping toward theanterior aspect108 of the implant are inscribed onto the top110 and bottom111 surfaces of the implant by forcing the implant through opposed broaches bearing inscribing teeth. Alternatively, the upper110, lower111 or both surfaces in turn may be repeatedly run, manually or by a machine-driven means, over an appropriately fashioned jaw bearing abrasive teeth such that the required profile of teeth are inscribed into the surfaces of the implant. Desirably, the successive teeth of the jaw are incrementally raised in height such that each tooth is only required to remove a small amount of bone (about 0.004″ per tooth, to a total depth of 0.015″). In addition, it is preferred that the rake (angle of the teeth) be sufficiently sharp as to allow the implant to bite into the implantation site, without at the same time being so sharp as to be excessively brittle.

In FIG. 5, FIG. 5A, there is provided a top view of one side of one embodiment of[0044]

blades

502 for use in abroach assembly500 for inscribing teeth into the top110, bottom111 or both surfaces of the implant. In outline, there is shown a lock-down handle501 for clamping the assembly ofblades502 to abase503. By bringing a mirror image jaw into register with the depicted broach, a space is formed between the opposingteeth502 at a distance sufficient to accommodate passage of the implant therebetween, provided that the teeth abrade recesses into the top and bottom surfaces of theimplant100. To ensure proper engagement of theblades502 and theimplant100, there is provided anon-cutting surface506 for contacting theimplant100 as it is introduced into thebroach assembly500. Thenon-cutting surface506 acts as a type of micrometer, forcing the cutting surfaces of theteeth502 sufficiently apart to properly engage the implant as it passes through thebroach assembly500. In FIG. 5B, there is provided a side view of animplant mounting device504 having a “D”-shapedcavity505 into which a “D”-shaped implant may be fitted for passage through the opposing jaws of the broachingjaw apparatus500. The resultant implant has the profile shown in FIGS.1C-1E.

In FIGS.[0045]5C-5E, there is shown an alternate apparatus and method for fashioning the retention teeth in the implant. In FIG. 5C, there is shown acarriage510 having an appropriately dimensionedslot520 for receiving the implant to be grooved. Atensioning screw530 brings aretention arm531 into juxtaposition withcarriage housing member532, thereby clamping the implant into position withinslot520. Through

carriage housing members

532 and533, there is aligned a guide-rod534 for guiding the carriage containing the implant as it is raked across ablade assembly540, over which saidcarriage510 is made to pass. Saidguide rod540 also conveniently acts as a hinge, allowing thecarriage510 to swing upward for implant loading and also permitting the carriage to move down toward the base as the implant surface is cut on each successive pass of the carriage over saidblade assembly540. Theblade assembly540 is bolted within abase550 over which saidcarriage510 slides. Saidbase550 also acts to receive

fixation screws

551 and552 which retain saidguide rod534 in place. A plurality ofindividual blades560 are placed in arecess554 in thebase550 and are maintained in registered position byretention screws552 passing throughretention holes553 in each blade. Eachblade560 has an initialnon-cutting surface561, which is approximately 0.015″ below the cuttingsurface562, which in combination with said plurality of blades, forms a flat loading area for implant insertion into saidslot520. FIG. 5D provides a side view of oneblade560, while FIG. 5E provides an end on view of thecarriage510 as it sits above thebase550. Accordingly, the implant is inserted into theslot520 with thecarriage510 swung up from thebase550. The carriage is then swung down into the starting position with the implant making contact with thenon-cutting surfaces561 of the plurality of blades. The implant is depressed so that it is forced snugly against the non-cutting surface, and then tensioned into place with theretention screw530. Thereafter, the carriage is slid several times over the base550 such that the cutting surfaces562 of the plurality of blades thereby inscribe the desired tooth structure into the top surface, the bottom surfaces or both (after switching the implant around) surfaces of the implant. When the metallic bottom of the carriage comes into contact with the base, the machining of the implant is complete.

In FIG. 6A-I, there is provided a view of three different cortical bone implants according to this invention having particular geometries by way of example and not limitation. In FIG. 56A, there is shown an example of an[0046]

implant

600 which has a height of 7 mm, a width of 11 mm, and a length of 14 mm. In addition, dimensions of various radii of the implant are provided. Note the effect of the “shoulder”405 of the cutter which produces the a 0.059″ radius andindent profile610 starting at the approximate center of the part and proceeding around to the opposite side of the implant, i.e. around three faces of the implant. In FIG. 6B, theimplant600 is shown as a side view, and in FIG. 6C, there is shown a detail view of the teeth. Identical descriptions apply to the 7 mm×11 mm×11 mm views of the implants of FIGS.6D-6F and the 7 mm×14 mm×14 mm implant of FIGS.6G-6I.

In FIG. 7, there is shown a further aspect of this invention in which an implant, either machined as described above, or prior to said machining, is further machined so as to allow stacking thereof to achieve implants of various heights. Commencing from a blank cortical plug at the stage shown in FIG. 2D has the advantage that if breakage of the implant occurs during machining, this will likely occur prior to completion of all of machining steps. According to this embodiment of the invention, two implant blanks of known height are selected such that a unitary implant composed of both starting implants can be produced of a new desired height (e.g. a 6 mm high implant may be stacked with a 7 mm high implant to produce a 13 mm implant). Each implant blank is placed in a drill jig, and by means of a drill press or like means, holes are drilled through the implants. With the implants still in the jig, the jig is placed on the table of an arbor press. Pins, composed of cortical bone, resorbable but strong biocompatible synthetic material, or metallic pins of the appropriate diameter are then impelled into the holes in the implants such that the implants are formed into a unitary body by these pins. In order to encourage bony ingrowth, channels may be cut into the adjacent surfaces of the implants. The embodiment shown in FIG. 7A is a top view of an[0047]

implant

700 into which four holes701-704 have been drilled. In FIG. 7B, there is shown the juxtaposition of two implants700A and700B, with the drilled holes701-704 in register to receive pins for maintaining the implants in register. In this view, the

adjacent surfaces

710A and710B have not been inscribed with teeth, while the

surfaces

711A and711B have been so inscribed. Based on this disclosure, those skilled in the art will recognize that a number of variations and modifications may be made to stack various forms of bone implants, or to maintain such implants in register with each other. These modifications are to be considered within the scope of this invention. Thus, as shown in FIG. 9, animplant900 is produced by producing two

implants

901 and902, each having a cavity orcanal903, including an asymmetrickey way904 machined therein. By press-fitting the two implants together using an appropriately shapedcancellous plug905 or a plug made from another biocompatible material, including but not limited to hydroxyapatite, cortical bone, synthetic materials, ceramic, optionally treated with growth factors such as bone morphogenetic protein and the like, the two

implants

901 and902 are retained in registered juxtaposition to form theimplant900.

In a further embodiment of this invention, shown in FIG. 8, a method for assembling the implant of this invention from component parts is provided. In FIG. 8A, there is shown an[0048]

implant

800 composed of two side-by-side halves,801A and801B. The two halves of the implant are brought into juxtaposition to form a unitary implant. The two halves may be implanted in juxtaposition, or holes may be formed in each half, and the halves maintained in contact by forcing pins through the holes, in a fashion analogous to that described above for maintaining stacked implants in contact with each other. For this embodiment of the invention, a portion of cortical bone may be harvested from any suitable source of cortical bone. As shown in FIG. 8B, a segment, in the form of a block or a column of cortical bone is harvested along the long axis of a long bone, such as the femur, tibia, or fibula. The shape of the bone may be inscribed into the thus-harvested cortical bone by routing, broaching or other means as described herein. The thus-machined cortical bone may then be sectioned into appropriate heights, as needed, to provide the implant halves801A and801B. Alternate sites for harvesting the cortical bone segment are shown in FIGS. 8B and 8C.

In addition to use for cervical Smith-Robinson type fusion, implants comprising each element,[0049]801A or801B alone, modifications and variations thereof, optionally in combination with another vertebral fusion implant, may be implanted, for example, to assist in induction of posterior lumbar intervertebral fusion (PLIF). In such a case, adevice810, such as that shown in FIGS.8D-8G is machined from bone stock as shown in FIGS. 8B, 8C or another appropriate bone stock, and is inserted, according to methods known in the art for insertion of PLIF implants. Preferably, the device as used for PLIF applications has the following dimensions similar to the following, see side view FIG. 8D: awidth811 of approximately 7 to 12 mm, and preferably about 9.4 to about 10 mm; atop dimension812 of about 4 to 5 mm; abottom dimension813 of about 4-6 mm and preferably about 5 mm; a flat surface of814 of about 4-7 mm, and preferably about 5.5 mm; awidth815 of about 5-7 mm and preferably about 5 mm; a curvature that defines anangle816 of between about 60 and 75 degrees, and preferably about 67 degrees. See FIG. 8E, rear view: a length L of about 20 to 26 mm; a height H of between about 7.5 and 14.5 mm; preferably, heights of about 8, 10, 12, and 14 mm are produced with lengths of about 20 and 26 mm; desirably, the side faces817 are machined to display a rough, ridged or grooved surface so that when theanterior end818 of the PLIF implant is properly seated in place, ridges directed to theposterior end819 of the PLIF implant prevent backing out of the implant. A detail of one embodiment of such a ridged surface is shown in FIG. 8F, wherein the following dimensions are preferred: anangle820 for each tooth of between about 30 and 40 degrees, preferably about 35 degrees; a distance between tooth crests821 of about 1-2 mm, preferably about 1.5 mm; atooth crest width822 of about 0.1 to about 0.2 mm, preferably about 0.125 mm; and atooth height823 of between about 0.1 to about 1 mm and preferably about 0.5 mm; returning to FIG. 8E, the implant preferably has ananterior end width824 of about 7-13 mm, preferably about 9-13 mm, with ataper angle825 from the height H of about 30 to 40 degrees, preferably about 35 degrees; an instrument attachment means,826, such as a tapped instrument attachment hole, is provided in the posterior face of the PLIF implant; this feature is best seen from the posterior view8G, which shows: aninstrument attachment hole826 having a diameter of about 1.5 to about 2.5 mm, preferably about 2 mm, and a depth of about 4-5 mm, preferably about 4.5 mm; an edge to center of the instrumentattachment hole dimension827 is carefully defined to match dimensions of any implant insertion device used in combination with this embodiment of the PLIF implant; a center of the instrument attachment hole to edgedimension828 is about 4-6 mm, preferably about 5 mm, with aridge829 of about 1 mm running along three edges of the posterior face of the implant. In displaying the section A-A from FIG. 8D in FIG. 8E, aslight air gap830 is shown as the section would exit bone on the concave surface of the implant and then reenter bone.

In use, the[0050]

implant

810 is inserted on either side of lumbar intervertebral spaces to thereby stabilize and assist in fusion of adjacent lumbar vertebrae. This is accomplished by distraction of the lumbar vertebrae, removal of an appropriate amount and shape of intervertebral disc matter, and insertion of theimplant810, preferably on each side on a posterior approach, according to methods known in the art. The concave surface of eachimplant810 is set to face inwardly, toward the center of the vertebral body, while the convex surface of theimplant810 is set to match, as much as possible, the natural external curvature of the lumbar vertebrae.

In an analogous but alternate method for production of the cervical implant, unitary implants may be produced by appropriately sectioning and machining the anterior margin of the tibia or linea aspera of the femur. Thus, as shown in FIG. 10A, a left femur[0051]1000 (posterior aspect), or in FIG. 10B, a left tibia1001 (anterior aspect), is sectioned at1004 and1005 to remove the head, neck andgreater trochanter1002 and internal andinternal condyles1006 of the femur, or tubercle andtuberosity1003

malleolus

1007 of the tibia. The result from such sectioning is the production of a shaft, or diaphysis, of thefemur1008 ortibia1009. Further processing according to this aspect of the invention involves theline asper1010 of the femur or the anterior margin of thetibia1011, as shown in FIG. 10C. Whether produced from the femur or tibia, adiaphysial shaft1012, extending as shown at1016 to a length permitted by the length of the shaft produced by the sectioning at1004/1005. The shaft comprises thenatural intramedullary canal1013. The thus produced shaft is then further sectioned in a plane shown at1014 to produce a shaft of bone removed from thenatural intramedullary canal1013 having a cylindrical but somewhat triangular external shape. Into this shaft may be drilled acannulation1015, as shown in FIG. 11.

FIG. 11 shows the substantially triangular shaft of substantially[0052]

cortical bone

1017 produced by sectioning the shaft of the long bone down the plane1014. Into the shaft ofbone1017 may be drilled a bore to produce acannulation1015 of appropriate dimensions. Thecannulation1015 may be introduced into the unitary shaft ofbone1017 or it may be introduced into sub-segments thereof by first cutting theshaft1017 at1035. In either case, the diameter of thecannulation1015 should be limited such that at the narrowest portion between the cannulation and the wall of thedevice1020 never falls below about 2 mm. When sectioned at1035, for example by mounting theshaft1017 on a lathe and contacting the spinning shaft with a very narrow blade (i.e. a parting tool of about 1 mm or less width),

implant blanks

1030 and1040 are produced which may be further machined to achieve desired external and internal profiles and key way features, as described for the implant of this invention produced by alternate methods described hereinabove. Implants of any desired height, for example 5 mm to about 14 mm, may thus be produced. FIGS.12-17 show specific embodiments of the implant of this invention produced according to this aspect of the invention.

Per FIGS.[0053]12-17, there is provided views of five different cortical bone implants according to this invention having particular geometries by way of example and not limitation. In each figure, view A is a top view, view B is a side view, view C is a detail of the grooves which angle toward the posterior aspect of the implant, and view D is a sectional view through the line A-A shown in view A. In addition, where an osteogenic plug, such as a cancellous plug is present, this is shown in view E as a top view and view F as a side view of the cancellous plug. In FIG. 12, there is shown an example of an implant having a height H1 between about 5 mm and about 9 mm, a width W1 of about 11 mm, and a width W2 of about 11 mm. An outer dotted profile provides a means for comparing the shape of the implant produced according to this alternate manufacturing method with the external profile of the implant of FIG. 6. As can be seen, the implant produced according to this aspect of the invention has a substantially diamond-shaped external profile, as a result of the geometry of the startingshaft1017 of bone stock.

FIG. 13 shows a device similar to that of FIG. 12, with a cancellous plug inserted therein. FIG. 14 shows a device having a width W[0054]1 of about 14 mm and a height H1 of between about 5 mm and about 14 mm. FIG. 15 shows a device similar to that of FIG. 14 with a cancellous plug inserted therein. FIG. 16 shows a device having a width W1 of about 14 mm, a width W2 of about 14 mm, and a height of between about 5 mm and 11 mm. FIG. 17 shows a device similar to that of FIG. 16 having a cancellous plug inserted therein. Table I below summarizes the various features and provides examples of specific dimensions for various embodiments of the implant of this invention shown in FIGS.12-17:

	TABLE I


	Figure #

Code	Description	12	13	14	15	16	17

D1	Inner Hole Diameter	5.5	5.5	5.5	5.5	5.5	5.5
D2	Hole Centerline Distance	5.5	5.5	5.5	5.5	5.5	5.5
D3	Key Way Centerline Distance	2.9	2.9	2.9	2.9	4.4	4.4
G1	Groove Depth	0.5	0.5	0.5	0.5	0.5	0.5
G2	Top Flat Width	0.5	0.5	0.5	0.5	0.5	0.5
G3	Groove Pitch width without G5	1.375	1.375	1.375	1.375	1.375	1.375
G4	Groove Pitch width with G5	1.5	1.5	1.5	1.5	1.5	1.5
G5	Flat Width	0.125	0.125	0.125	0.125	0.125	0.125
G6	Groove Angle (degrees)	30	30	30	30	30	30
H1	Overall Height	5-9	5-9	5-13	5-13	5-11	5-11
R1	Keyway Radius	0.8	0.8	0.8	0.8	0.8	0.8
R2	Corner Radius	1.5	1.5	1.5	1.5	1.5	1.5
R3	Outside Arch Radius	11	11	11	11	14	14
W1	Outside Width	11	11	14	14	14	14
W2	Outside Width	11	11	11	11	14	14
W3	Wall Thickness	≧2	≧2	≧2	≧2	≧2	≧2
CP1	Cancellous Plug Height	N/A	5-9	N/A	5-13	N/A	5-11
CP2	Cancellous Plug Diameter	N/A	5.7-5.8	N/A	5.7-5.8	N/A	5.7-5.8

4.4 Manner of Using the Implant

In use, the[0055]

implant

100 is inserted into a space formed between adjacent vertebrae that are required to be fused. This may be accomplished by the surgeon removing portions of the intervertebral disk, (partial discectomy) and retracting the adjacent vertebrae to allow insertion of an appropriately dimensioned implant. Therear end105 of the implant is inserted first, and where present, theexternal feature120 prevents backing out of the implant. Where noexternal feature120 has been inscribed into the top and bottom surfaces of the implant, it may be necessary to affix the implant in position with plate and screw retention systems known in the art. According to this invention, implants are provided having a height of between about 7 and 14 mm, a length of between about 11 and 14 mm and a width of between about 11 and 14 mm. Any permutation or combination of these dimensions may be envisioned, for example (in order of height, length, width): 7×11×11, 8×11×11, etc.; 7×14×14, 8×14×14, etc.; 7×11×14, 8×11×14, etc.

Preferably, the surgeon performing the implantation saves the autologous material and debris produced in the course of the partial discectomy for packing into the canal of the present implant. In addition, or alternatively, the canal may be packed (either during the surgical procedure or the canal may be pre-packed) with osteogenic, osteoinductive, or osteoconductive materials, including but not limited to: allograft bone, autograft bone, autogenous osteogenic materials including bone marrow cancellous bone and the like, demineralized bone, freeze-dried demineralized bone, Grafton® (demineralized bone in glycerol), bone powder, bone derivatives, bone morphogenetic protein (purified or recombinant), antibiotic, bioactive glass, hyrdorxyapatite, bioactive ceramics, or combinations thereof.[0056]

Following implantation, the recipient (whether human or animal) is monitored for implant stability and success in fusion. Fusion is achieved over the course of several weeks to several months, during which time increasing levels of load may be placed on the spine.[0057]

5.0 REFERENCES

Bailey, R. W., and Badgley, L. E. (1960)[0058]Stabilization of the Cervical Spine by Anterior Fusion, J. Bone and Joint Surg. 42A:565-594.

Cloward, R. B. (1958)[0059]The Anterior Approach for Removal of Ruptured Cervical Discs, J. Bone and Joint Surg. 15:602-617.

Grooms et al., (1996)[0060]Success of Surgery on the Anterior Cervical Spine: Smith-Robinson Technique vs. Internal Plates, Clinical Performance of Skeletal Prostheses, L. L Hench and J. Wilson, Eds., Chapman & Hall.

Robinson, R. A., et al., (1962)[0061]The Results of Anterior Interbody Fusion of the Cervical Spine, J. Bone and Joint Surg. 44A:1569-1587.

Robinson, R. A. and Smith, G. W. (1955)[0062]Anterolateral Cervical Disc Removal and Interbody Fusion for Cervical Disc Syndrome, Bull. Johns Hopkins Hosp. 96:223-224.

Smith, G. W. and Robinson, R. A., (1958),[0063]The Treatment of Certain Cervical-Spine Disorders by Anterior Removal of the Intervertebral Disc and Interbody Fusion, J. Bone and Joint Surg. 40A:607-623.

White, A. A. III, and Hirsh, C. (1972)[0064]An Experimental Study of the Immediate Load Bearing Capacity of Some Commonly Used Iliac Bone Grafts, Acta Orthop. Scandanav. 42:482-490.

Whitecloud, T. S. III and Dunsker, S. (1993)[0065]Anterior Cervical Spine Surgery, Principles and Techniques in Spine Surgery, Raven Press, N.Y.

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