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United States Patent 
Ray et al.
[li] Patent Number: 4,961,740  Date of Patent: Oct. 9, 1990
 V-THREAD FUSION CAGE AND METHOD OF FUSING A BONE JOINT
 Inventors: Charles D. Ray, Deephaven; Eugene A. Dickhudt, New Brighton, both of Minn.
 Assignee: Surgical Dynamics, Inc., Alameda, Calif.
 Appl. No.: 259,031
 Filed: Oct. 17, 1988
 Int. C1.5 A61F 5/04; A61F 2/28
 U.S. CI 606/61; 606/86;
 Field of Search 128/92 YJ, 92 C, 92 CA,
128/92 UP, 92 YP, 92 YZ; 623/16, 17, 18
 References Cited
U.S. PATENT DOCUMENTS
Re. 31,865 4/1985 Roux 623/18
2,537,070 1/1951 Longfellow 128/92 YJ
3,720,959 3/1973 Hahn 623/16
3,783,860 1/1974 Burstein et al 128/92 YZ
4,059,115 11/1977 Jumashev et al 128/92 YJ
4,492,226 1/1985 Belykh et al 128/92 YZ
4,501,269 2/1985 Bagby 128/92 YJ
4,522,200 6/1985 Stednitz 128/92 YZ
4,599,086 7/1986 Doty 128/92 R
4,657,550 4/1987 Daher 128/92 YM
4,677,972 7/1987 Tornier 128/92 VP
FOREIGN PATENT DOCUMENTS
2295729 12/1974 France .
Primary Examiner—Robert A. Hafer
Assistant Examiner—Kerry Owens
Attorney, Agent, or Firm—Fliesler, Dubb, Meyer &
A fusion cage 10 includes a cage body defining an internal cavity with an inner surface and an outer surface. The outer surface defines a helical thread 12 comprised of a plurality of turns which define valleys 14 therebetween. Located in the valleys 14 are perforations 13 which provide communication between the outer surface and the interior cavity. Thus, when the fusion cage 10 is mated to a bone structure and the internnal cavity is packed with bone chips or other bone-growth-inducing substances, there is immediate contact between the bone structure and the bone chips through the perforations 13.
34 Claims, 1 Drawing Sheet
V-THREAD FUSION CAGE AND METHOD OF
FUSING A BONE JOINT
BACKGROUND OF THE INVENTION 5
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 10 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 15 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 20 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. Other- 25 wise, 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 30 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 35 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 stabi- 40 lizer 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 45 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 meth- 50 ods 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 nor- 55 mally 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 60 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 65 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 "(application 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, 5 we sometimes threaded the receiving bed with a diestock 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 10 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 15 posterior bony elements normally attached to the vertebrae, such as, dorsal spinous processes or lam in as. 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 20 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 25 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 mi- 30 gration 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, 35 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 40 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 45 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 50 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. 55
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°, a preferred range of radii for the 60 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 interface surface of bone relative to the implant, thus increasing the opportunity for ingrowth. However, the 65 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 entail weak fe
male 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°.
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