Tapered bone fusion cages or blocks, implantation means and method

This application is a divisional of U.S. patent application Ser. No. 11/025,217 filed Dec. 29, 2004, and entitled “Tapered Bone Fusion Cages or Blocks, Implantation Means and Method” which claims the benefit of U.S. Provisional Patent Application Serial No. 60/533,622 filed Dec. 31, 2003 and entitled “Tapered Bone, Fusion Cages or Blocks, Implantation Means and Method”; and the entire teachings of which are incorporated herein by reference.

The present disclosure relates to fusions of the spine, more specifically to intradiscal or interbody fusions utilizing hollow, formed, perforated, threaded cages in severe, disabling discogenic back pain problems, with or without a herniation (protrusion) of the disc.

Degenerative changes of the human spinal column often are accompanied by severe, disabling back pain; one method of success in eliminating such pain originating from within the disc, called discogenic pain, is to surgically eliminate the disc and create a fusion or bony union between adjacent vertebrae, eliminating the offending, painful disc. Several successful surgical devices and methods are now available to obtain the desired bone or suitable substitute fusion. One such valid method utilizes fusion cages that are hollow, usually threaded devices, to contain and protect the bone graft material; the cage is driven or screwed into the prepared disc space to facilitate the development of a solid bony fusion. Into these cages morcelized bone graft or substitute fusion inducing material is placed with the fusion developing by a growth of the contained material from one vertebra, through the multiple perforations in the cage walls, into the adjacent vertebra. Such devices are nearly all cylindrical with parallel walls; however, at some disc spaces having a forward-opening taper, it is preferable to use inserts having the same taper so they will more closely conform to that disc space taper and provide an improved distributed attachment of the insert along essentially all of the tapering disc space. Further, tightness of the laminated, circumferential collagen fibers of the annulus, the outer part of the flexible disc structure, is essential for early stability of the movable spinal segment. Thus, the goal is to immediately stabilize the segment by the implant while the fusion slowly develops. The device or material used to facilitate a fusion formation must initially be able to support the vertical forces, roughly up to 1.8 times the body weight and to induce or conduct the fusion formation.

The threaded fusion cage system was designed to simplify the surgery for spine fusion. Each cage of the pair ordinarily used in the procedure is inserted into the surgically prepared and tapped or threaded hole formed between the two adjacent vertebrae, penetrating into the bone of each. Although shaped bone grafts or substitute material may be used instead of the cages to accomplish the fusion in many cases, the cages permit the use of disorganized bone chips being held into position by the cage structure. The optimal penetration, called purchase, of each cage into each of the opposing vertebral bodies to be united by bone growth, known as a fusion process, has led to substantial success in fusing the spine for over 14 years and cages of various manufactures have been implanted in perhaps 500,000 patients worldwide. Cages are hollow threaded titanium devices nearly always formed as straight non-tapering cylinders and are appropriate for most applications since the end plates of the vertebral bodies are generally quite parallel.

Anatomically, however, some of the disc spaces are not parallel, particularly at the lowest lumbar space which adjoins the top of the sacrum bone. This space, called Lumbar 5-Sacral 1 (or L5-S1) is commonly involved in the disabling, degenerative discogenic pain process. The L5-S1 disc space normally has a taper, with a larger opening at the front. When parallel-walled fusion cages are inserted into the L5-S1disc space, more commonly from a frontal or anterior approach through the abdomen, sections of the usual parallel walled cages maybe too deeply purchased towards the posterior portion of the disc space and essentially have little or no purchase into the more anterior or frontal portion of the tapering disc space. One solution to this problem to obtain good purchase along the majority of the disc space is to use a tapered fusion cage whose angle of taper is chosen to more closely match the forwardly widening angle of the disc. To suit a variety of anatomical variations, a range of tapering angles of cages is needed, usually 6°, 9° and 12°, larger towards the front. In addition, with this normally greater forward opening of the taper, it is extremely unlikely that it would be practical or safe (relative to posterior nerve issues) to utilize a posterior approach for implantation. Various tapered cage designs and methods of implant are described in Ray C D, Dickhudt E A: V-threaded fusion cage and method of fusing a joint. U.S. Pat. No. 4,961,740; and, Ray C D, Dickhudt E A: Surgical method and apparatus for fusing adjacent bone structures. U.S. Pat. No. 5,026,373; and Ray C D: Surgically implanting threaded fusion cages between adjacent low-back vertebrae by an anterior approach. U.S. Pat. No. 5,05.5,_04; and Ray C D: Instrumentation and method for facilitating insertion of spinal implant. U.S. Pat. No. 6,042,582; and Winslow C J, Mitchell S T, Jayne K, Ray C D: Open posterior lumbar fusion cage insertion set and method. U.S. Pat. No. 6,083,225 Systems presently manufactured by Stryker Spine, Inc., of Allendale, N J, as the Ray Threaded Fusion Cage and associate instruments are also instructive. Additionally, other tapered cage systems have been allowed US patent coverage by other inventors.

Related Art-in the past various instruments and methods have been developed for anterior insertion of various appropriately shaped supportive materials that can induce or conduct the formation of a solid fusion. Such materials have included solid bone autografts (the patient\'s own shaped bone) or allografts (shaped cadaver bone), shaped artificial bone substitutes (bioceramics or ocean coral) or a variety of appropriately shaped cage-like devices, each of which is cut or formed to match the desired angle of forward taper. The greatest problems associated with the instrumentation used for the implantation of these materials or devices to be implanted have been: (1) rigidly attaching a guiding assembly (usually tubular) onto or within the disc space of adjacent vertebrae for subsequent preparation of the bed and subsequent insertion of the appropriately tapered devices or cages while maintaining the proper spacing (for a plurality of implants) and angulation of the devices to be used, (2) reaming (tapered drilling) the recipient bed while rigidly maintaining the direction and depth of this process, creating the appropriate recipient bed, (3) tapping the recipient bed, or utilizing self-tapping cages, while maintaining the same initial angulation and spacing used in the above earlier stages, and (4) appropriate tightening of the circumferential fibers of the annulus with stabilization of the operated segment through a means to expand the disc. For such procedures, most commonly utilized is an essentially tubular guiding means through which the preparation and steps of implantation are performed. Such tubular means are temporarily attached across or within the disc space. This tubular means, usually a singular or double-barreled device, is forcefully driven onto or into the disc space. Through this stabile tubular device, the bed for the paired cage is formed by boring or reaming, then the tapping, and followed by cage or device insertion. Subsequently or prior to insertion, the utilized cages are filled with appropriate bone chips or substitute. Importantly, all such procedures place the tapered cages or materials with their long axes parallel to each other. Since the disc space and matching angulated cages are larger in front, and after insertion may even touch, the placement of ancillary bone outside and between the implants, as many surgeons prefer, is inhibited. Further, because the overall width of the two adjacent implants is twice their diameters, the pair of implants may be excessively wide for that disc space.

The novel systems and methods in accordance with one embodiment utilizes a variety of related instruments. Following adequate anterior lumbar transabdominal surgical approach and preparation of the affected disc space, including the scraping of end plate cartilages away from the adjacent vertebral bodies, a rod stabilizing/guiding instrument is driven into the disc space using a hammer striking against a removable plate. The guide has means to expand vertically to appropriately tighten its grip inside the tapering disc space while minimally changing the disc taper angle, by its expanding/lifting upper and lower knurled surfaces engaging the end plates along the majority length of their angulated central disc space surfaces. Each desired angulation, preferably 6°, 9° or 12° has a set of reaming or boring, threading or tapping instruments coded for ease in matching their various angulated components. The stabilizer/guide of this novel set of instruments is adjustable to accommodate various disc taper angles. Once the guide device is driven and stabilized inside the disc space and tightened, appropriately guided ancillary rod-mounted instruments are used to ream and tap for the later insertion of suitable tapered cages with great precision. These rod-mounted components pass through lateral guiding members, fixably removable and changeable as the procedure progresses. These rod guiding members are appropriately marked as to depth of penetration into the disc space. At conclusion of the prepared threaded recipient bed, the stabilizing/guiding unit is unlocked and removed from the vertebrae. The tapered insert is attached to the insertion rod and following the visible tapped recipient bed, screwed into position freehand under direct observation and by intraoperative fluoroscopy, a technique familiar to skilled surgeons such as is performed routinely in the insertion of pedicle screws and the like. When a prepared cage with arcs cut into the outer sides is used, it is implanted as first of the pair where the second may or may not have such side cuts. The pair of cages will this be nestled closer together, with a narrower total width, than if cages with no such side cut arcs were used. Since the insert is tapered, the disc space does not require a support to keep it open after the guide unit is removed; the taper will reopen the disc according to the depth of insertion of the tapered insert. Additional bone may be packed posterior and lateral to the cages.