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Claims  |
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What is claimed:
1. A fusion cage adapted for promoting fusion with one or more bone
structures when bone-growth-inducting substance is packed into the fusion
cage, comprising:
a cage body defining an internal cavity adapted to be packed with the
bone-growth-inducing substance, which cage body defining an outer surface;
means defined on the outer surface of the cage body for mating the fusion
cage to the bone structure, which means prevents the fusion cage from
backing out from the bone structure;
means for providing immediate contact between the one or more bone
structures and the bone-growth-inducing substance packed into the fusion
cage when the fusion cage is mated to the one or more bone structures so
that the one or more bone structures cross over the outer surface into the
internal cavity and into contact with the bone-growth-inducing substance
packed in the fusion cage, and the bone-growth-inducing substance packed
in the fusion cage.
2. The fusion cage of claim 1 wherein said internal cavity of said cage
body defines an inner surface; and
wherein said means for providing immediate contact includes a plurality of
sites where the inner surface meets the outer surface.
3. The fusion cage of claim 1,
wherein said internal cavity of said cage body defines an inner surface;
wherein said mating means includes a thread with a plurality of turns and
with valleys defined between said turns; and
wherein said means for providing immediate contact includes a plurality of
perforations defined in said valleys so that the inner surface contacts
the outer surface.
4. The fusion cage of claim 1 wherein:
said mating means including a thread with a plurality of turns, and with
valleys defined between said turns; and
said means for providing immediate contact including a plurality of
channels which communicate with said internal cavity, which channels
define an inner surface and which channels pierce through the valleys to
define a plurality of perforations whereby the inner surface contacts the
outer surface, such channels also adapted to be packed with
bone-growth-inducing substance so that there is immediate contact between
the one or more bone structures and the bone-growth-inducing substance.
5. The fusion cage of claim 1:
wherein said cage body includes a plurality of spaced elongate members
which define said outer surface and which elongate members have inner
surfaces which define said internal cavity;
said mating means including a helical thread member formed about and
connected to the outer surface of the spaced elongate member, which
helical thread member defines a plurality of adjacent turns; and
wherein said means for providing immediate contact includes a plurality of
apertures defined between adjacent turns which apertures are bordered by
the spaced elongate member.
6. The fusion cage of claim 1:
wherein said cage body includes a helical structure with an outer surface
and which helical structure has an inner surface, which inner surface
defines said internal cavity;
said outer surface of the helical structure being substantially V-shaped
and adapted for biting into the one or more bone structures; and
said helical structure defining a plurality of spaced turns communicating
with the internal cavity so that the inner surface contacts the outer
surface such that with the outer surface biting into the bone structure,
their is immediate contact between the one or more bone structures and
with the bone-growth-inducing substance packed into the internal cavity of
the fusion cage.
7. The fusion cage of claim 1 wherein:
said outer surface defines an external substantially continuous V-thread
adapted for mating to the bone structure;
which thread has a multiplicity of turns and valleys defined between said
turns; and
said means for providing immediate contact includes a multiplicity of
perforations located in said valleys and providing communication between
said outer surface and said internal cavity.
8. A fusion cage as defined in claim 7 wherein the V-thread is continuous
and the angle at the crown of the V-thread is not more than 90.degree.,
but not less than 45.degree..
9. A fusion cage as defined in claim 7 wherein the angle at the crown of
the V-thread is about 60.degree..
10. A fusion cage as defined in claim 7 wherein the V-thread has from 3 to
8 turns per cm.
11. A fusion cage as defined in claim 8 wherein the valleys of the V-thread
have fillets, the radius of which is from 0.35 to 0.75 mm.
12. A fusion cage as defined in claim 7 wherein, the internal cavity
defines an inner surface and wherein said perforations comprise at least
from 30% to 60% of said inner surface.
13. A fusion cage as defined in claim 1 which is fitted with removable
perforated end caps.
14. A fusion cage as defined in claim 7, the major diameter of which is
from 12 to 16 mm.
15. A fusion cage as defined in claim 1 made of implantable-grade stainless
steel.
16. A fusion cage as defined in claim 1 made of X-ray-transparent material.
17. A fusion cage as defined in claim 1 wherein the cage body has a modulus
of elasticity of about that of the bone structure.
18. The fusion cage as defined in claim 1:
wherein the cage body has a width and a length, which length is greater
than the width; and
wherein the cage body has a modulus of elasticity of about that of the bone
structure so that the cage body can flex with the bone structures along
the length of the cage body.
19. The fusion cage as defined in claim 1 wherein the cage body is
comprised of a biodegradable material.
20. The fusion cage as defined in claim 1 wherein the cage body is in part
comprised of bone-activating material.
21. The fusion cage as defined in claim 1 wherein the cage body is coated
with bone-activating material.
22. The fusion cage as defined in claim 1 wherein said means for providing
immediate contact is substantially uniformly distributed throughout the
outer surface of the cage body.
23. The fusion cage as defined in claim 7 wherein the V-thread are
substantially sharp with the valleys are substantially rounded.
24. The fusion cage as defined in claim 1 wherein said internal cavity has
at least one open end, and said fusion cage including an end cap and means
for securing said end cap to said cage body in order to cover said open
end.
25. A fusion cage of claim 1 made of magnetic signal transparent material.
26. A fusion cage of claim 1 wherein the internal cavity includes at least
one opening into which is introduced the bone-growth-inducing substances,
and including an end cap for covering the opening, which end cap is made
of a material which is transparent to x-rays and magnetic signals.
27. A fusion cage adapted for promoting fusion with one or more bone
structures when bone-growth-inducing substance is packed into the fusion
cage, comprising:
a cage body having an inner surface which defines a cavity adapted to be
packed with the bone-growth-inducing substance;
said cage body defining an outer surface;
means defined on the outer surface of the cage body adapted for allowing
the fusion cage to bite into the bone structure;
means for providing a plurality of sites where the inner surface contacts
the outer surface in order to allow immediate contact between the one or
more bone structures and the bone-growth-inducing substance packed into
the fusion cage.
28. A fusion cage adapted for promoting fusion of one or more bone
structures when bone-growth-inducing substance is packed into the fusion
cage, comprising:
a cage body defining a cavity with an inner surface, said cavity adapted to
be packed with the bone-growth-inducing substance;
said cage body defining an outer surface;
means for defining threads on the outer surface of the cage body and
adapted for biting into the bone structure;
said threads defining means including a plurality of threads which define
valleys therebetween;
a plurality of perforations provided in the valleys of the threads for
providing communication between the outer surface and the cavity in order
to allow immediate contact between the one or more bone structures and the
bone-growth-inducing substance packed into the fusion cage.
29. A fusion cage adapted for promoting fusion with one or more bone
structures when bone-growth-inducing substance is packed into the fusion
cage, comprising:
a cage body defining a cavity with an inner surface, said cavity adapted to
be packed with bone-growth-inducing substance;
said cage body defining an outer surface comprised of a substantially
continuous thread with a plurality of turns and valleys defined between
said turns;
a plurality of perforations defined in said valleys so that the inner
surface contacts the outer surface in order to allow immediate contact
between the one or more bone structures and the bone-growth-inducing
substance packed into the fusion cage.
30. A fusion cage adapted for promoting the fusion with one or more bone
structures when bone-growth-inducing substance is packed into the fusion
cage comprising:
a cage body defining a cavity with an inner surface, the cavity being
adapted to be packed with bone-growth-inducing substance;
said cage body defining an outer surface comprised of a substantially
continuous thread with a plurality of turns and valleys defined between
adjacent turns;
said cavity defining a plurality of channels which pierce through the
valleys to define a plurality of perforations whereby the inner surface
contacts the outer surface so that there is immediate contact between the
one or more bone structures and the bone-growth-inducing substance packed
into the fusion cage.
31. A fusion cage adapted for promoting fusion with one or more bone
structures when bone-growth-inducing substance is packed into the fusion
cage comprising:
a plurality of spaced elongate members which define an internal cavity;
said elongate members having an inner surface that faces the internal
cavity and an outer surface that faces away from the internal cavity;
a helical thread member formed about and connected to the outer surfaces of
the spaced elongate members, which helical thread member defines a
plurality of turns; and
a plurality of apertures defined between turns, which apertures are
bordered by the spaced elongate members, wherein the thread member is
adapted to contact the bone structure and provide immediate contact
between the one or more bone structures and the bone-growth-inducing
substance packed into the fusion cage.
32. A fusion cage adapted for promoting the fusion with one or more bone
structures when bone-growth-inducing substance is packed into the fusion
cage, comprising:
a helical structure having an inner surface which defines an internal
cavity;
said helical structure having a substantially V-shaped outer surface
pointing in a direction away from the internal cavity and adapted for
biting into the one or more bone structures;
said helical structure defining a plurality of spaced turns communicating
with the internal cavity so that the inner surface intersects the outer
surface such that with the outer surface biting into the one or more bone
structures, there is immediate contact between the one or more bone
structures and the bone-growth-inducing substance packed into the internal
cavity of the fusion cage.
33. A surgical method for fusing a fusion cage with one or more bone
structures comprising the steps of:
forming a bore with an internal thread, in the one or more bone structures;
screwing the fusion cage into the bore which fusion cage includes:
(a) a cage body defining an internal cavity with an inner surface, and the
cage body having an outer surface defining an external thread that mates
with the internal thread, the external thread having a plurality of
adjacent turns which define valleys therebetween; and
(b) a multiplicity of perforations located in the valleys in such a manner
that the inner surface contact the outer surface;
said screwing step further causing the one or more bone structures to
extend through the perforations and into the internal cavity;
packing the fusion cage with bone-growth-inducing substance in such a
manner that the bone-growth-inducing substance contacts the one or more
bone structures and the outer surface.
34. A fusion cage adapted for promoting the fusion with one or more bone
structures when bone-growth-inducing substance is packed into the fusion
cage, comprising:
a helical structure having an inner surface which defines an internal
cavity;
said helical structure having an outer surface pointing in a direction away
from the internal cavity and adapted for contacting one or more bone
structures;
said helical structure defining a plurality of spaced turns communicating
with the internal cavity such that with the outer surface contacting the
one or more bone structures, there is immediate contact between the one or
more bone structures and the bone-growth-inducing substance packed into
the internal cavity of the fusion cage. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns method and apparatus for fusing two adjacent bony
structures such as a bone joint, especially adjacent vertebrae of the
spine.
2. Description of Related Art
Subsequent to injury, disease or other degenerative disorder, the disc, a
ligamentous cushion between vertebrae, may undergo a painful
deterioration. The disc shrinks and flattens out, and the distance between
the vertebral bodies begins to collapse. Subsequently, there may be a
progressive degeneration leading to mechanical instability, where painful
translocations occur between adjacent vertebrae. The movement-induced pain
may be so disabling that in many such cases, the segmental motion must be
eliminated. Thus, rigid fusions may be the only present means to stop the
translocations and relieve the pain.
It is generally held that successful fusions demand 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 appliances 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 cortical surfaces of which have been removed or
deeply scarified 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 immobility
of the segments. Bone ingrowth is required for this.
Dependency upon such an appliance as the sole stabilizer is ultimately
unsuccessful due to the development of a mechanical gap or transition
between the bone and the appliance, leading to structural failure of the
bone and adjacent connective tissue. Such failure is seen in fractures,
erosion and absorption of bone with potential further collapse. The pain
may also become progressively disabling.
Approximately 150,000 lumbar spinal fusions were performed in the USA
during 1987, as reported by the American Hospital Association. There are
many methods for intervertebral fusion. The most successful have achieved
a success rate of about 90% in random cases. However, several of these
techniques, especially those requiring complex appliances, 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 mechanical center of rotation between the vertebrae. This
point is centered within the disc space. Therefore, 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 (transabdominal) 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 interbody, intradiscal
space. Since there must be a bone-to-bone bridge created during the fusion
process, 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.
Intervertebral fusions using circular bone grafts have been reported in the
orthopedic and neurosurgical literature for some years. B. R. Wiltberger
in a paper published in Clinical Orthopedics, Vol 35, pp 69-79, 1964,
reviewed various methods of intervertebral 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 absorption of surrounding bone and the dowel might loosen, with a
renewed danger of expulsion. See also a 2-page brochure from Neurological
Surgery Associates of Cincinnati, Inc. entitled "Posterior Lumbar
Interbody Fusion Made Simple" which shows, after the bone dowel placement,
the "(a)pplication of 5 mm dacron suture around spinous processes."
U.S. Pat. No. 4,501,269 (Bagby) describes a surgical 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 surrounded 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 stabilize the
joint and to further promote ultimate bone-to-bone fusion. . . . The
implanted structure is in the form of a perforated cylindrical 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 surgically accessing the joint
and removing intervening cartilage located between the contiguous bony
surfaces. A transverse cylindrical opening is then bored across the
contiguous bony surfaces. Immediate 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).
Otero-Vich, 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 prepared according to Cloward'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).
Otero-Vich's 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. Otero-Vich also says:
"When grafts with a diameter of 14 mm were used, we sometimes threaded the
receiving bed with a die-stock of 13 mm to facilitate the insertion" (p.
751).
An additional desirable effect of an intervertebral fusion is the
restoration or maintenance of a normal intervertebral 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 appliance. When the procedure is performed using the commonly employed
posterior approach, a variety of spreaders may be placed between various
posterior bony elements normally attached to the vertebrae, such as,
dorsal spinous processes or laminas. 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 bone graft of any shape
is driven into a cavity that is wedged more open posteriorly between two
opposing movable vertebrae, there is a strong propensity for the graft to
be retropulsed during the postoperative recovery period as a result 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 ligamentous disc anulus remains
and prevents ventral migration of the graft into the retroperitoneal
space. Further, there is value in restoring the original spinal lordotic
curve, as the fusion grows; this requires that the cavity and the
interbody fusion element be placed to promote a normal spinal anatomical
position, that is, without wedging of the space in either direction.
SUMMARY OF THE INVENTION
The invention provides for a fusion cage which has a threaded outer surface
and an internal cavity which is adapted to be filled with bone chips.
Perforations are provided in valleys between adjacent turns of the thread,
which perforations provide communication between the outer surface and the
internal cavity. The cage can be screwed into a threaded bore provided in
the bone structure at the surgical site and then packed with the bone
chips. Once done, there is immediate contact between the bone structure
and the bone chips in order to promote fusion.
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. Eventually, the ingrowth of bone through perforations
in the valley of the thread forms a permanent interconnection 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 60.degree., 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.degree., because a sharper thread
would increase the exposed interface surface of bone relative to the
implant, thus increasing the opportunity for ingrowth. However, the angle
at the crown should be at least 45.degree., because the pitch would be
undesirably small if the angle were smaller. An unduly small pitch would
entail weak female bone threads and create a danger of cross threading.
The perforations should be as large as possible as long as the fusion
basket has adequate structural strength. When the surface of the fusion
basket 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
substantially more than 2 mm axially and 3 mm transversely when the angle
of the V-thread at its crown is about 60.degree..
The novel fusion basket preferably is fitted with end caps, a first of
which may be in place 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 fusion basket. The first end cap
retains the bone-inducing substance when it is packed into the fusion
basket. The open end 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
imperforate but preferably have substantially the same perforation as does
the fusion basket 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 biodegradable material, even when the fusion basket is not.
Currently the novel V-thread fusion basket preferably is made of
implantable-grade stainless steel. Titanium and ceramics are also useful,
as are super-strength polymers or composites of polymers 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 ingrowth 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 fusions.
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 novel fusion
basket, e.g., after being microencapsulated 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 formed into the fusion
basket.
For implantation between vertebrae of a person's lower back, two sizes of
the novel fusion basket should suffice, one having a V-thread major
diameter of 16 mm and the other a major diameter of 12 mm. Because the
anterior-posterior dimension 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 platform when implanted in pairs.
The crown of the V-thread of the novel fusion basket preferably 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 undesirably large thread depth, penetrating too
deeply into the cancellous bone. A larger turn ratio may unduly restrict
the size of the perforations.
The novel V-thread fusion basket can be implanted for fusing adjacent bony
structures by the following method:
(a) forming in said bony structures a bore with a female thread that
penetrates into their cancellous regions,
(b) forming a rigid, perforate, cylindrical basket to have an external,
substantially continuous helical V-thread that can mate with said female
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 vertebrae, there should
be prior to step (a) the added step of spreading the vertebrae apart,
preferably in a manner that maintains their parallelism, the fusion basket
is implanted in pairs on opposite sides of the disc space.
The novel fusion basket should have a modulus of elasticity approximating
that of the recipient 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 structural 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 surgeon 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 create bone ingrowth between adjacent
vertebrae, the fusion basket should be implanted in pairs on opposite
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
interposed 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 forces from possibly dislodging the fusion baskets,
e.g., when the patient forward flexes, thus separating the posterior
margins of the adjacent vertebrae, the dorsal processes may be tied or
wrapped together. By another technique, screws placed through the
appropriate facet jackets limit both flexion and extension motions.
A novel interbody spreader in the form of a scissors jack has been
developed to maintain a desirable parallel attitude between the adjacent
vertebrae 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 novel 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 removed by said cutting edge, thus permitting the detritus to
be carried away by removing the pilot rod from the hollow shaft. The
portion of the pilot rod that projects beyond said other end of the hollow
shaft preferably is threaded to carry detritus upwardly to the reservoir.
When using the novel tapping instrument to form female threads for an
interbody fusion, the hollow shaft should have an odd number of scallops
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.
The novel tapping instrument and a novel wrench are illustrated in the
drawing that also illustrates two V-thread fusion baskets of the invention
.
THE DRAWING
In the drawing, all figures of which are schematic,
FIG. 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 forming female threads 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
substantially 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 15 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 perforations 13.
Referring to FIG. 2, a series of fusion baskets can be made from a
plurality of rods 22 of rectangular cross-section that can be continuously
extruded 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 soldered or welded
to each of the rectangular rods 22 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
rectangular internal keyways.
Each of the fusion baskets of FIGS. 1 and 2 could be made from a model by
the lost wax process.
The tapping instrument 30 of FIG. 3 has a hollow cylindrical shaft 31 with
a T-handle 32 at one end and an external thread 33 at the other end.
Slidably 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. Projecting from the threaded end 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 scallops 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 scallops into the
reservoir.
Upon rotating the hollow shaft 31 to form female 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 then 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 corners 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 frictionally holds the protuberance in place when it is inserted into
the fusion basket.
IMPLANTING THE FUSION BASKET
In order to implant the novel fusion basket 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 laminotomies. 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 of degeneration, a scissors-jack type vertebral spreader or a
hydraulically inflated bladder is inserted on one (the first) side inside
the disc space and opened until the space approximates the normal. 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 medialward
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 25 mm
(the normal disc space is about 30 mm deep and 50 mm wide). A drill stop
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 channels
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 FIG. 4 by which it is screwed by hand into
the threaded intradiscal bore to its full depth. After removing the
wrench, the bask | | |
