0’
`
`Europiilsches Patentamt
`European Patent Office
`Otflce européen des brevets
`
`‘
`® Publication number:
`
`0
`A1
`
`C2)
`
`EUROPEAN PATENT APPLICATION
`
`® Application number: 393105726
`
`63 int. c:.5: A61F 2/44
`
`® Date of filing: 16.10.89
`
`
`The title of the invention has been amended
`@ Applicant: ceonn SURGICAL, INC.
`15265 Minnetonita Boulevard
`(Guidelines ior Examination in the EPO. A-ill.
`Mlnnetonlra Minnesota 55345(U$)
`7.3).
`
`
`
`(9 Priority: 17.10.33 us 259031
`
`@ Date of publication of application:
`23.05.90 Bulletin 90121
`
`Designated Contracting States:
`DE FR GB NL SE
`
`@ inventor: Ray, Charles D.
`19550 Cedarhurst
`
`Deephaven lililnnesota 55391 (US)
`Inventor: Dlckhudt. Eugene A.
`801 Continental Drive
`New Brighton Minnesota ss112(us)
`
`® Representative: Darby, David Thomas et at
`Abel & lmray Northumberland House 303-306
`High i-loiborn
`London WC1V 7LH(GB)
`
`@ Fusion cage for bone ioints.
`
`E7) A fusion basket (10) having an external. substan-
`tially continuous helical V-thread (12) by which it can
`be screwed into a here after flrst forming in the bore
`mating female threads that bite into the canceiious
`regions. Mating of the threads ensures that
`the
`fusion basket remains securely in place without com-
`pressing or splitting the recipient bone. Eventually.
`the ingrowth of bone through perforations (13) in the
`valley (14) of the thread forms a permanent intercon-
`nection between the two bony structures. when
`used to create bone ingrowth between adjacent ver-
`tebrae. the V-thread fusion basket is implanted in
`pairs on opposite sides oi the disc space.
`
`I-
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`MSD 1009
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`< E
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`P0369603
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`1
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`EP 0 359 603 A1
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`2
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`FUSION CAGE
`
`The invention concerns method and apparatus
`for fusing two adjacent bony structures such as a
`bone joint. especially adjacent vertebrae of
`the
`spine.
`
`BACKGROUND 95 E mvenmon
`
`Subsequent to Injury. disease or other degen-
`erative disorder.
`the disc. a ligamentous cushion
`between vertebrae, may undergo a painful deterio-
`ration. The disc shrinks and flattens out. and the
`distance between the vertebral bodies begins to
`collapse. Subsequently.
`there may be a progres-
`sive degeneration leading to mechanical instability.
`where painful
`transiocations occur between adja-
`cent vertebrae. The rnovement-induced pain may
`be so disabling that in many such cases. the seg-
`mental motion must be eliminated. Thus. rigid lu-
`sions may be the only present means to stop the
`translocations and relieve the pain.
`it is generally held that successful fusions de-
`mand a contiguous growth of bone to create a solid
`mass that will unite the movable elements into one
`unit. Otherwise. the fusion cannot achieve the tasks
`of pain reduction. maintenance of
`intervertebral
`height. and immobility of the segment. When fusion
`bone is first placed. it is soft and movable. having
`no cohesive strength. Therefore a variety of appli-
`ances have been developed that attempt to hold
`the segments quite still under conditions of normal
`spinal activity and daily stress. Bone graft material
`is placed between the segments. the outer or cor-
`tical surfaces of which have been removed or
`deeply scarilied so as to promote the ingrowth of
`the graft into these recipient sites. Thus positioned,
`the bone graft slowly unites the segments. Such an
`appliance is not meant to permanently secure im-
`mobility of the segments. Bone ingrowth is re-
`quired for this.
`Dependency upon such an appliance as the
`sole stabilizer is ultimately unsuccessful due to the
`development of a mechanical gap or transition be-
`tween the bone and the appliance. leading to struc-
`tural failure of the bone and adjacent connective
`tissue. Such failure is seen in fractures. erosion.
`. and absorption of bone with potential further coi-
`lapse. The pain may also become progresively
`disabling.
`fusions
`Approximately 150,000 lumbar spinal
`were performed in the USA during 1987. as re-
`ported by the American Hospital Association. There
`are many methods for intervertebral fusion. The
`most successful have achieved a success rate of
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`25
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`35
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`about 90% in random cases. However. several of
`these techniques. especially those requiring com-
`plex appiiances. are difficult
`to master and are
`hazardous to nerve and vessel structures normally
`lying close to the involved bones.
`From a biomechanical point of view. the most
`important location of a spinal fusion is at the rne-
`chanical center of rotation between the vertebrae.
`This point is centered within the disc space. There-
`fore. an interbody fusion is the most rigid and thus
`the most sought after method among surgeons.
`Current methods of interbody fusions are. however.
`the most hazardous of all spinal fusion methods.
`Both anterior
`(transabdominai) and posterior
`surgical approaches are used for interbody fusions.
`Typically. a plug. dowel. or segment of bone is
`driven tightly into a cavity carved inside the inter-
`body,
`intradiscal space. Since there must be a
`bone-to-bone bridge created during the fusion pro-
`cess. connective tissue and discal tissue must be
`removed. Therefore. deep cuts within the bone
`must penetrate into the softer. cancellous region to
`promote bone growth across the space.
`lntervertebral fusions using circular bone grafts
`have been reported in the orthopedic and neurosur-
`gical literature for some years. B. H. Vviitberger in a
`paper published in Clinical Orthopedics, Vol. 35.
`pp. 69-79. 1964. reviewed various methods of inter-
`vertebral body fusion using posterior bone dowels
`driven firmly into a suitably smaller hole between
`the adjacent vertebrae. Upon doing so the dowel
`can split or crack or collapse. The stretched bone
`might also split and it can be compressed by the
`dowel to the point that it will not grow normally due
`to collapse of
`formerly open pores or vascular
`channels.
`if this occurs. there may be a late ab-
`sorptlon of surrounding bone and the dowel might
`loosen, with a renewed danger of expulsion. See
`also a two-page brochure from Neurological Sur-
`gery Associates
`of Cincinnati.
`Inc.
`entitled
`"Posterior Lumbar interbody Fusion Made Simple"
`which shows. after the bone dowel placement. the
`"(a)pp|ication of 5 mm dacron suture around spi-
`nous processes.”
`_
`U.S. Patent 4.501.269 (Bagby) describes a sur-
`gical procedure for stabilizing the cervical spine of
`a horse and says that the procedure "is applicable
`to any human or animal joint formed by opposed
`contiguous bony surfaces which are covered and
`separated by intervening cartilage and are sur-
`rounded by ligaments which resist expansion of the
`joint. Specific examples of such joints are a spinal
`joint between adjacent vertebrae or the ankle joint.
`The process was developed to immediately sta-
`bilize the joint and to further promote ultimate
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`bone—lo—bone iuslon...The implanted structure is in
`the form of a perforated cylincrical bone basket
`which can be filled with bone fragments produced
`during the preparation of the joint. These bone
`fragments provide autogenous tissue to promote
`bone growth through the basket. as well as around
`it.
`
`"The process involves the initial steps of surgi-
`cally accessing the joint and removing intervening
`cartilage located between the contiguous bony sur-
`faces. A transverse cylindrical opening is
`then
`bored across the contiguous bony surfaces. imme-
`diate stabilization is achieved by driving into the
`cylindrical opening a hollow basket having a rigid
`perforated cylindrical wall whose outside diameter
`is slightly greater than the inside diameter of the
`cylindrical opening. The implanting of the basket
`spreads the bony surfaces apart in opposition to
`the resistance to expansion of the joint provided by
`the surrounding ligaments." (Col. 2. lines 26-55). _
`Web. J. Neurosurg Vol. 63, pp. 750-753 (1983)
`describes a means for cervical spine fusion. using
`an anterior approach. by surgically implanting a
`cylindrical bone graft. “Screw threads are placed in
`the graft with a small, previously sterilized die. The
`grooves of the thread can be made as deep as
`required. The vertebral cervical bodies are pre-
`pared according to CIoward's technique. After a
`cylindrical bed has been drilled in the appropriate
`intervertebral bodies.
`the graft
`is
`screwed into
`place with instruments especially developed for this
`purpose." (P. 750). The Fig. 2 legend points out
`that a threaded graft dowel has a larger contact
`surface than a plain dowel and a greater resistance
`to pressure and sliding.
`An additional desirable effect of an interver-
`tebral fusion is the restoration or maintenance of a
`normal
`interveltebral spacing. Spreading devices
`are generally required in order to restore all or a
`part of the normal intradiscal height, in the process
`of placing the fusion material or appllcance. when
`the procedure is performed using the commonly
`employed posterior approach, a variety of spread-
`ers may be placed between various posterior bony
`elements nonnally attached to he vertebrae. such‘
`as. dorsal spinous processes or
`larninas. Using
`such spreaders. a forward tilt or wedging of the
`discal space occurs. with the posterior aspect of
`the space becoming more open than the anterior.
`When a borle graft of any shape is driven into a
`cavity that is wedged more open posterlorly be-
`tween two opposing movable vertebrae. there is a
`strong propensity for the graft to be retropulsed
`during the postoperative recovery period as a re-
`sult of to and fro movement between the opposing
`vertebrae. Thus.
`to aid in the prevention of graft
`expulsion.
`it would be desirable to have the cavity
`either maintain parallelism or be slightly narrower
`
`at its most posterior portion. Ventral to this cavity,
`the stout iigamentous disc anulus remains and pre-
`vents ventral migration of the graft into the retro-
`peritoneal space. Further, there is value in restoring
`the original spinal
`lordotic curve. as the fusion
`grows: this requires that the cavity and the inter-
`body fusion element be placed to promote a nor-
`mal
`spinal anatomical position,
`that
`is. without
`wedglng of the space in either direction.
`
`BRIEF SUMMARY 2: T__l~jE INVENTION
`
`The invention provides a fusion basket or cage
`which.
`like the fusion basket of Bagby. is a perfo-
`rate rigid cylinder that can be surgically inserted
`into a bore that has been formed in two adiacent
`bony structures such as two vertebrae. The fusion
`cage is then packed with bone chips or other bone-
`inducing substance.
`thus inviting ingrowth of live
`bone. The fusion cage of the invention differs from
`the fusion basket of Bagby by an external. substan-
`tially continuous helical V-thread by which it can be
`screwed into the bore. after first forming mating
`female threads in the bore. Mating of the threads
`ensures that the fusion basket remains securely in
`place. there being much less danger of splitting or
`compression atrophy of the recipient bone. Even-
`tally. the ingrowth of bone through perforations in
`the valley of the thread forms a pennanent inter-
`connection between the two bony structures.
`By V-thread is meant that the crown of the
`thread is sharp. although its valley preferably is
`blunt or rounded to permit the mating peaks of the
`female threads to have adequate strength. when
`the angle of the V-thread at its crown is about so’.
`a preferred range of radii for the fillet in the valley
`is from 0.35 to 0.75 mm. The angle at the crown of
`the V-thread should be no more than 90' . because
`a sharper thread would increase the exposed inter-
`face surface of bone relative to the implant. thus
`increasing the opportunity for
`ingrowth. However,
`the angle at
`the crown should be at
`least 45'.
`because the pitch would be undesirably small if the
`angle were smaller. An unduly small pitch would
`email weak female bone threads and create a dan-
`ger of cross threading.
`The perforations should be as large as possible
`as long as the fusion basket has adequate struc-
`tural strength. when the surface of the fusion bas-
`ket is projected onto the inner face of a cylinder.
`the projected perforations should comprise from
`30% to 60% of the projected area. preferably about
`50%. Individual apertures should be at least one
`mm both axially and transversely to permit good
`ingrowth of fresh bone. whereas the fusion basket
`might be unduly weakened if the apertures were
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`EP 0 369 603 A1
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`substantially more than 2 mm axially and 3 mm
`transversety when the angle of the V-thread at its
`crown is about 60' .
`The novel fusion basket preferably is fitted with
`end caps. a first of which may be in ptace before
`the fusion basket
`is screwed into the recipient
`bone. and thus should have a maximum diameter
`no greater than the minor diameter of the V—thread
`of the lusion basket. The first end cap retains the
`bone-inducing sustance when it is packed into the
`fusion basket. The openend of the fusion basket
`may then be closed with a second end cap to hold
`the bone chips securely in place. The end caps
`may be lmperforate but preferably have substan-
`tially the same perforation as does the fusion bas-
`ket to permit bone or other tissue ingrowth through
`the end caps. However, end caps may not be
`necessary or. if used. they can be made of biodeg-
`radable material. even when the fusion basket is
`not
`
`Currently the novel V-thread lusion basket pref-
`erably is made of implantabIe~grade stainless steel.
`Titanium and ceramics are also useful. as are
`super-strength polymers or composites of poly-
`mers and high-strength filaments such as super-
`high-density polyethylene, glass. or graphite. Non-
`metallic composites have the preferred ability to
`pass x rays or magnetic beams without distortion,
`thus enhancing the preparation of scan images as
`compared to metallic fusion baskets. The fusion
`basket can be biodegradable. because it no longer
`is needed after the bone lngrowth has matured.
`When the fusion basket is not biodegradable. it can
`remain in place permanently after the ingrowth has
`taken place.
`in contrast
`to the need to remove
`many types of metallic supports or appliances that
`have heretofore been used to promote rigid tu-
`sions.
`
`Useful bone-inducing substances include bone
`chips and bone substitutes or synthetic material.
`with or without bone activating matter, such as
`hydroxyapatite. bone morphologic protein, bone
`growth factor, or cartilage activation factor. instead
`of being mixed with the bone-inducing substance.
`bone-activating matter can be coated onto the nov-
`el fusion basket, e.g.. after being microencapsulat-
`ed in a wax. when the fusion basket is made of an
`organic material. bone activating matter can be
`combined with the organic material before It
`is
`fonned into the fusion basket.
`For implantation between vertebrae of a per-
`son's lower back.
`two sizes of the novel fusion
`basket should suffice. one having a V-thread major
`diameter of I6 mm and the other a major diameter
`of 12 mm. Because the anterior-posterior dimen-
`sion of a typical lower lumbar vertebra is about 30
`mm,
`the length of the fusion basket preferably
`does not exceed 25 mm but is at least 20 mm in
`
`length to give sufficient contact as well as a good
`platfonn when implanted in pairs.
`The crown of the V-thread of the novel fusion
`
`basket preferable is continuous, both for strength
`and for ease of insertion into the threaded bore.
`Preferably the V-thread has from 3 to 8 turns per
`cm. A smaller turn ratio may result in an undesir-
`ably Iarge thread depth. penetrating too deeply into
`the cancellous bone. A larger turn ratio may unduly
`restrict the size of the periormatlons.
`The novel V-thread fusion basket can be im-
`
`planted for fusing adjacent bony structures by the
`following method: (a) forming ain said bony struc-
`tures a bore with a female thread that penetrates
`into their cancellous regions.
`(b) lonning a rigid.
`perforate. cylindrical basket to have an external.
`substantially continuous helical V-thread that can
`mate with said temaie thread,
`(c) screwing the
`basket into said threaded bore. and (d) packing the
`basket with bone-inducing substance. When the
`bore to be formed in step (a) is to extend between
`adjacent vertebra. there should be prior to step (a)
`the added step of spreading the vertebrae apart.
`preferably in a manner that maintains their parallel-
`ism.
`the fusion basket is implanted in pairs on
`opposite sides of the disc space.
`a
`should have
`The
`novel
`fusion basket
`modulus of elasticity approximating that of the re-
`cipient bone.
`thus permitting it to flex along its
`length. consequently minimizing stresses at
`the
`bony interface between the graft and recipient
`bone. Although a fusion basket of substantially
`lower modulus of elasticity would provide the same
`desirable result.
`it might not have adequate struc-
`tural strength.
`The bore into which the V-thread fusion basket
`is to be inserted preferably is tapped by hand.
`using a slow motion to ensure against burning the
`bone. This freshens the bone margins of the bore
`so that if any bone had been burned by drilling to
`form the bore. it is now cut away slowly by hand.
`The tapping process is quite safe. in that the sur-
`geon can feel the progress of the technique.
`The V-thread fusion basket preferably is
`screwed by hand into the threaded bore. again
`permitting the surgeon to feel if the resistance is
`too great and that rethreading of the bore might be
`required.
`In contrast. a bone dowel
`typically is
`driven into a bore using a hammer. and in order to
`guard against an overly tight fit. the surgeon listens
`to the sound of
`the striking hammer and also
`monitors the degree of resistance.
`When using the novel fusion basket to creat
`bone ingrowth between adjacent vertebrae. the fu-
`sion basket shouid be implanted in pairs an op-
`posite sides of the disc space. Each is held in
`place by its V-thread. biting into female threads
`that penetrate into the cancellous bone of the inter-
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`posed vertebral bodies. Gravity, muscle pull. and
`elastic recoil of the spread (or stretched) outer disc
`anulus together exert force against each of the
`fusion baskets. Thus the fusion baskets are held in
`place by compression forces between the adjacent
`vertebrae.
`
`To prevent distraction traces from possibly dis-
`lodging the fusion baskets. e.g.. when the patient
`forward flexes.
`thus separating the posterior mar-
`gins of
`the adjacent vertebrae.
`the dorsal pro-
`cesses may be tied or wrapped together. By an-
`other technique, screws placed through the appro-
`priate facet jackets limit both flexion and extension
`motions.
`interbody spreader in the form of a
`A novel
`scissors jack has been developed to maintain a
`desirable parallel attitude between the adjacent ver-
`tebrae while the bore is drilled and then tapped by
`a novel
`instrument. Another
`instrument
`that has
`been developed for use in the implantation of the
`novel
`fusion basket
`is a tapping instrument
`for
`forming helical threads in a bore in recipient bone.
`This noval tapping instrument comprises
`a hollow cylindrical shaft having a handle at one
`end and an external thread which is formed at the
`other end with at least one scallop that exposes a
`cutting edge. and
`a pilot rod that slidably fits into said bore. projects
`beyond said other end of the hollow shaft. and is
`formed with a central recess that communicates
`with the scallop in the hollow shaft and provides a
`reservoir for detritus remvoed by said cutting edge.
`thus permitting the detritus to be carried away by
`removing the pilot rod from the the hollow shaft.
`The portion of the pilot rod that projects be-
`yond said other end of the hollow shaft preferably
`is threaded to carry detritus upwardly to the reser-
`voir.
`tapping instrument to
`when using the novel
`form female threads for an interbody fusion, the
`hollow shaft should have an odd number of scal-
`lops and cutting edges. preferably three because
`an odd number provides more equal removal of
`recipient bone on both sides of the bore than would
`an even number.
`
`tapping instrument and a novel
`The novel
`wrench are illustrated in the drawing that also illus-
`trates two V-thread fusion baskets of the invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the drawing. all figures of which are sche-
`matic.
`
`Pig. 1 is an exploded isometric view of a first
`V-thread fusion basket of the invention and two
`
`perforated end caps;
`
`Fig. 2 is an isometric view illustrating the
`formation of a body that can be cut
`to form a
`series of second V-thread fusion baskets of the
`invention;
`Fig. 3 is an isometric view of a tap (partly
`cut away to reveal details of construction) for for-
`ming female thread in bores into which a V-thread
`fusion basket is to be inserted; and
`Fig. 4 is an isometric view of a wrench for
`screwing a V-thread fusion basket into a threaded
`bore.
`
`The fusion basket 10 of Fig. 1 was formed from
`a solid steel cylinder by drilling eight
`small.
`"equally-spaced holes 11 in the axial direction. each
`hole being centered on a circle concentric with the
`axis of the cylinder. Then a large hole was drilled
`centered on the axis and having a radius substan-
`tially identical to that of the aforementioned circle.
`A V-thread 12 was then machined in the external
`
`surface of the cylinder. thus opening through that
`surface a perforation 13 extending through the
`rounded valley 14 of the V-thread at each crossing
`of the valley and one of the small holes 11. A
`screw thread I5 was then machined in the internal
`surface of the fusion basket to threadably receive
`an end cap 16 that has apertures 18 similar to
`those of a salt shaker. Snap-on end caps would
`also be useful.
`
`In making a fusion basket by the technique
`described in the preceding paragraph.
`the small
`holes 11 could be enlarged to intersect each other.
`thus making it unnecessary to drill a central hole.
`Enlarged small holes would result in larger perfora-
`tions 13.
`
`Referring to Fig. 2. a series of fusion baskets
`can be made from a plurality of rods 22 of rectan-
`gular cross-section that can be continuously ex-
`truded and fed into each of eight keyways 23 In the
`surface of-a mandrel 24. Simultaneously. a rod 26
`of
`triangular cross-section is extruded. wrapped
`helically around the rectangular rods 22. and sol-
`dered or welded to each of the rectangular rods 2
`at every crossing to provide an external V-thread.
`Upon emerging from the keyways.
`the resulting
`body is cut into individual fusion baskets each of
`which has a perforation 28 between adjacent turns
`of the V-thread-forming rod 26 wherever it bridges
`a gap between adjacent rectangular rods 22.
`A fusion basket identical to that of Fig. 2 can
`be made from a hollow cylinder by machining an
`external V-thread and broaching a plurality of rec-
`tangular internal keyways.
`1 and 2
`Each of the fusion baskets of Figs.
`could be made from a model by the lost-wax
`process.
`The tapping isntrument 30 of Hg. 3 has a
`hollow cylindrical shaft 31 with a T-handle 32 at
`one and and an external thread 33 at the other end.
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`Siidably received within the hollow shaft is a pilot
`rod 34. one end 35 of which protrudes beyond the
`hollow shaft 31 and slidably fits into a bore that has
`been drilled into the recipient bone. At the other
`end of the pilot rod is a knurled cap 35A. Projec-
`ting from the threaded and of the hollow shaft 31
`are cutting teeth 36 that enlarge the bore to the
`minor diameter of
`the external
`thread 33 of the
`hollow shaft 31. The threaded end of the hollow
`shaft also is formed with three symmetrical scal-
`lops 37 (one shown) to expose a cutting edge 38 at
`the leading edge of the external thread 33, which
`cutting edge forms female bone threads in the bore
`upon rotation of the hollow shaft.
`Detritus created by tapping instrument 30 is
`deposited through the scallops 37 into a reservoir
`provided by a central recess 39 in the pilot rod 34.
`The end 35 of the pilot rod which extends from the
`recess 39 into the bore has external threads which.
`when the threaded pilot rod 34 is turned. carry
`detritus upwardly to be deposited through the scat-
`lops into the reservoir.
`to form fe-
`Upon rotating the hollow shaft 3|
`male bone threads in the bore.
`the surgeon can
`feel
`increased back pressure when the reservoir
`becomes full and should grasp the knurled cap 35A
`to remove and clean out
`the pilot
`rod.
`if
`the
`gummy nature of the detritus were to prevent the
`pilot rod from being easily pulled out of the hollow
`shaft. the knurled cap 35A could be removed to
`permit the hollow shaft 31 to be unscrewed from
`the threaded bore.
`leaving the pilot rod in place.
`The pilot rod than serves. as a guide if the bore
`has not yet been completely tapped and it
`is
`necessary to reinsert the hollow shaft to complete
`the tapping.
`The wrench 40 of Fig. 4 has a cylindrical shaft
`41 with a T-handle 42 at one end and an octagonal
`protuberance 44 at the other end. The comers of
`the protuberance 44 fit into recesses In the fusion
`basket to permit the fusion basket to be rotated by
`rotating the wrench. A spring-loaded ball 46 fric-
`tionally holds the protuberance in place when it is
`inserted into the fusion basket.
`
`
`implanting £13 Fusion Basket
`
`the novel fusion basket
`In order to implant
`between adjacent vertebrae, soft. collagenous disc
`material
`is first removed from the intervertebral
`space. A small window is created in the overlying
`laminas of each side. namely. standard laminotom-
`ies. The neural tissues. dural sac and nerves. are
`retracted medially. The intervertebral space is
`cleaned of disc material
`in a standard surgical
`fashion. if the disc space has narrowed as a result
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`of degeneration. a scissors-iack type vertebral
`spreader or a hydraulically inflated bladder is in-
`serted on one (the first) side inside the disc space
`and opened until the space approximates the nor-
`mal. This may be confirmed by a lateral x ray. The
`height of the disc space is measured on the x ray
`so that the proper sizes of drills.’ tap. and fusion
`basket may be chosen.
`The opposite (second) side of the same disc
`space is then addressed. The nerve tissues on the
`first side are relaxed and then retracted medlalward
`on the second side. A pilot drill (e.g.. 5 mm or 8
`mm diameter depending upon discal space height)
`cuts a small channel
`in the face of each of
`the
`vertebrae, penetrating the interdiscal space to a
`depth of about mm (the normal disc space is about
`30 mm deep and 50 mm wide). A drill step may be
`applied to the drill to prevent overboring the hole. A
`solid rod pilot is then inserted into the pilot hole
`and a pilot cutter (7 mm or 10 mm) is passed over
`it and brought downward to enlarge the pilot chari-
`nels to slidably receive the pilot
`rod 35 of
`the
`tapping instrument 30 of Fig. 3. The cutting thread
`33 (12 mm or 16 mm major diameter) cuts female
`bone threads through the opposing vertebral end
`plates and into both cancellous regions that will
`invite the ingrowth of new bone.
`A V-thread fusion basket of the Invention. with
`one end cap in place, is snapped onto the wrench
`40 of Hg. 4 by which It Is screwed by hand into the
`threaded lntradiscal bore to its full depth. After
`removing the wrench.
`the basket
`is packed with
`bone chips or other bone-inducing substance. and
`the second end cap is applied to hold the bone
`chips securely in place.
`After removing the vertebral spreader, the dura
`and nerves on the second side are relaxed and
`attention is once’ again directed to the first side
`which is drilled and tapped to receive a second
`fusion basket by the same procedure.
`Over a period of several weeks. the bone from
`the vertebral bodies will grow through the pertora~
`tions in the fusion baskets and unite with the bone-
`inducing substance Inside them, creating a solid
`fusion.
`it is believed that the novel fusion baskets will
`primarily be implanted by a posterior approach to
`the spine, aithough an anterior approach may be
`utilized. especially when applied to the cervical
`spine.
`
`Example 1
`
`The fusion basket of Fig. 1 has been machined
`from a cylinder of surgically implantable stainless
`steel to have the following dimensions:
`
`181
`
`181
`
`

`
`11
`
`EP 0 369 603 A1
`
`12
`
`16 mm
`
`diameter of starting cylinder
`fength of cylinder
`25 mm
`3 mm
`diameter of each small hole 11
`diameter of circle on which holes 11 are centered
`11.5 mm
`diameter of central hole
`
`11 mm
`_
`2.5 mm/tum
`pitch of V-thread 12
`so‘
`angle at crown of thread 12
`0.4 mm
`fillet radius in valley of thread 12
`1.6 mm
`axial width of perforations 13
`circumferential breadth of perfs. 13
`2.8 mm
`when projected onto interior of a cylinder. % of
`area perforated 25%
`in appear-
`A V-thread fusion basket Identical
`ance to one produced as in Fig. 2 can be made
`from a hollow cylindrical tube. After machining an
`external thread. a plurality of rectangular keyways
`are broached in the inner surface to form perfora-
`tions through the valley of the thread. A continuous
`technique for making a novel fusion basket starts
`with a continuous helical spring made from a trian-
`gular rod such as the rod 26 used in Fig. 2. then
`welding or soldering the inner-facing surface of the
`spring to a plurality of cylindrical wires, each ex-
`tending paraliet to the axis of the spring.
`
`Claims
`
`1. A fusion cage which is a hollow perforate
`rigid cylinder that can be surgically inserted into a
`bore that has been formed in two adjacent bony
`structures and filled and packed with bone chips.
`thus invitin ingrowth of live bone. wherein the
`improvement comprises: the fusion cage (21) has an
`extenal. substantially continuous helical V-thread
`by which it can be screwed into mating female
`threads formed in the bore and (b) is perforated in
`the valley between adjacent turns of the thread.
`2. A fusion cage as defined in claim 1 wherein
`the V-thread is continuous and the angle at the
`crown of the V-thread is no more than 90‘ . but not
`less than 45'.
`3. A fusion cage as defined in claim 2 wherein
`the angle at the crown of the V-thread is about
`60 .
`
`4. A fusion cage as defined in claim 2 wherein
`the V-thread has from 3 to 8 turns per cm.
`5. A fusion cage as defined in claim 2 wherein
`the valley of the V-thread has a fillet, the radius of
`which is from 0.35 to 0.75 mm.
`6. A fusion cage as defined in claim 1 wherein.
`when the surface of the fusion cage is projected
`onto the inner face of a cylinder. the perforations
`comprise from 30% to 60% of the projected area.
`7. A fusion cage as defined in claim 1. which is
`fitted with removable perforated end caps.
`8. A fusion cage as defined in claim 1. the
`
`major diameter of which is fmm 12 to 16 mm.
`9. A fusion cage as defined in claim 1, made of
`implantable-grade stainless steel.
`10. A fusion cage as defined in claim 1. made
`of biodegradable material.
`11. A fusion cage as defined in claim 1. made
`of x-ray-transparent material.
`12. A fusion cage that is a hollow rigid cylinder
`that is suitable for insertion during surgery into a
`bore that has been formed in adjacent bony struc-
`tures and can contain bone inducing substances.
`thus Inviting ingrowth of live bone. the fusion cage
`having an external. substantially continuous screw
`thread by which it can be screwed into mating
`female threads formed in the bore, and. having
`openings in the valley between turns of the thread.
`13. A fusion cage as claimed in claim 12
`having any one or more of the features as defined
`in claims 2 to 11.
`
`14. A fusion cage as claimed in claim 12 or 13.
`wherein the screw thread is a V-thread.
`15. A fusion cage as claimed in any one of the
`preceding claims for use in fusing adjacent bony
`structures.
`
`16. A tapping Instrument comprising a hollow
`cylindrical shaft having a handle at one end and an
`external thread which is formed at the other end.
`with at
`least one scallop that exposes a cutting
`edge. and a pilot rod that slidably fitsfnto said
`bore. projects beyond said other end of the hollow
`shaft. and is formed with a central recess that
`communicates with the scallop in the hollow shaft
`and provides a reservoir for detritus removed by
`said cutting edge. thus permitting the detritus to be
`carried away by removing the pilot rod from the
`hollow shaft.
`
`10
`
`15
`
`25
`
`JD
`
`35
`
`182
`
`182
`
`

`
`EP 0 369 603 A1
`
`183
`
`183
`
`

`
`E"'°"°"“ "“‘°"‘
`
`Oflice
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`EUROPEAN SEARCH REPORT
`
`Appllatinn Number
`
`EP
`
`89 31 0572
`
`
`
`DOCUMENTS CONSIDERED TO BE RELEVANT
`Citation of dnmmmt with indication, what appropriau,
`of relevant p
`
`Relevant
`to claim
`
`G.ASSlI'-'ICA'.l'ION OF THE
`APPLICATION (Int. Cl. 5)
`
`
`
`(BAGBY)
`D,Y US-A-4 5D1269
`* Complete document *
`
`(VICI-I)
`DE-A-3 505 567
`* Claims; figures *
`
`W0-A-8 707 ggl
`
`(S + G IMPLANTS)
`
`EP-A-0 268 115
`
`(BIEDERMANN)
`
`A 61 F
`
`2/44
`
`TEGINICAL FIELDS
`SEARCQ-[ED (Int. CL5)
`
`A61!‘
`
`
`
`
`
`
`
`
`
`
`CATEGORY OF CIT!-I) DOCUM|'JN'l'S
`
`X : pm-Iicnlaliy I-clevanl H taken done
`Y: particularly relevan! if combined with another
`document of the; same utcgory
`A : technologiul background
`0 : non-