Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
1995-06-07
2001-04-03
Brown, Michael A. (Department: 3764)
Surgery
Instruments
Orthopedic instrumentation
C623S017120
Reexamination Certificate
active
06210412
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method for inserting interbody spinal fusion implants, and in particular to a method for inserting spinal fusion implants configured to restore and maintain two adjacent vertebrae of the spine in anatomical lordosis.
2. Description of the Related Art
Interbody spinal fusion refers to the method of achieving bony bridging between adjacent vertebrae through the disc space, the space between adjacent vertebrae normally occupied by a spinal disc. Numerous implants to facilitate such a fusion have been described by Cloward, Brantigan, and others, and are known to those skilled in the art. Generally, cylindrical implants offer the advantage of conforming to an easily prepared recipient bore spanning the disc space and penetrating into each of the adjacent vertebrae. Such a bore may be created by use of a drill. It is an anatomical fact that both the cervical spine and the lumbar spine are normally lordotic, that is convex forward. Such alignment is important to the proper functioning of the spine. Commonly, those conditions which require treatment by spinal fusion are associated with a loss of lordosis.
Michelson, in U.S. patent application Ser. No. 08/396,414, entitled APPARATUS AND METHOD OF INSERTING SPINAL IMPLANTS, teaches a method for restoring the anatomical lordosis of the spine while performing the interbody fusion procedure. While this has been a significant advance over prior methods, it has nevertheless been associated with a sometimes less than desirable consequence, that being the uneven removal of bone from each of the adjacent vertebrae relative to the vertebral endplates adjacent the disc space.
Therefore, there exists a need for spinal fusion implants and instrumentation that permits for the uniform depth of bone removal from each of the adjacent vertebrae while restoring anatomical lordosis.
SUMMARY OF THE INVENTION
The present invention is directed to a method for inserting a variety of interbody spinal fusion implants having at least a partially frusto-conical configuration to achieve a desired anatomical lordosis of the spine. In the preferred embodiment of the method of the present invention, the spinal fusion implants being inserted have an outer locus in which at least some of the points of the implant comprise a partially or fully frusto-conical shape substantially along the portion of the implant in contact with the adjacent vertebrae of the spine and have an insertion end and a trailing end. The spinal fusion implants may be further modified so that while the upper and lower surfaces are portions of a frusto-cone, or a cylinder at least one side portion may be truncated to form a planar surface that is parallel to the central longitudinal axis of the implant to form straight walls. These implants may have a more tapered aspect at the insertion end of the implant to facilitate insertion. The spinal fusion implants of the present invention may be relatively solid and/or porous and/or hollow, and may have surface roughenings to promote bone ingrowth and stability.
In the preferred method of the present invention, the diseased disc between two vertebrae is at least partially removed. The two vertebrae adjacent the diseased disc are then optimally distracted and placed in the desired amount of lordosis by any of a number of well known means including, but not limited to, those means that distract the vertebral bodies by engaging screws placed into the anterior aspect of the vertebral bodies, and disc space distractors that are placed from the anterior aspect of the spine into the disc space and are then used to urge the vertebral endplates away from each other and into lordosis. When the correct amount of distraction and lordosis have been achieved at the affected disc level, then a frusto-conical space is created from anterior to posterior between the adjacent vertebrae. The frusto-conical space that is created is greater in diameter than the disc space height, such that some bone is removed from each of the adjacent vertebrae. The created space is generally frusto-conical in shape, being greatest in diameter anteriorly and tapering to a lesser diameter posteriorly.
In an alternative method of implant insertion, the use of at least partially frusto-conical interbody spinal fusion implants allows for the creation of lordosis by the implant itself where none is present to begin with. The disc space which in the preferred circumstance would be fully distracted but need not be, but lacking lordosis, could have a bore drilled across that space such that equal arcs of bone are removed from each of the adjacent vertebrae using a drill or bone milling device capable of producing a cylindrical bore. The vertebrae, whether distracted from each other or not, are essentially lacking the full restoration of lordosis. The use of the substantially cylindrical bone drill provides for the removal of a generally uniform thickness of bone from each of the adjacent vertebrae from anterior to posterior. The insertion of a frusto-conical implant, having a larger diameter at its trailing edge than at its leading edge, then forces the anterior aspects of the adjacent vertebrae apart more so than the posterior aspects where the diameter is lesser. This utilizes the implant to produce the desired lordosis.
To further assist incorporation into the spinal fusion bone mass, the spinal fusion implants of the present invention may have wells extending into the material of the implant from the surface for the purpose of holding fusion promoting materials and to provide for areas of bone ingrowth fixation. These wells, or holes, may pass either into or through the implant and may or may not intersect. The spinal fusion implants of the present invention may have at least one chamber which may be in communication through at least one opening to the surface of the implant. Said chamber may have at least one access opening for loading the chamber with fusion promoting substances. The access opening may be capable of being closed with a cap or similar means. Still further, a variety of surface irregularities may be employed to increase implant stability and implant surface area, and/or for the purpose of advancing the spinal fusion implant into the fusion site such as a thread. The exterior of the spinal fusion implant of the present invention may have wholly or in part, a rough finish, knurling, forward facing ratchetings, threads or other surface irregularities sufficient to achieve the purpose described.
The spinal fusion implants of the present invention offer significant advantages over the prior art implants:
1. Because the spinal fusion implants of the present invention are at least partially frusto-conical in shape, those that taper from the leading edge to the trailing edge they are easy to introduce and easy to fully insert into the spinal segment to be fused. In the preferred embodiment, where the leading edge of the implant is larger than the trailing edge, the implant utilizes a tapered forward portion and an increasing thread height relative to the body from the leading edge to the trailing edge to facilitate insertion.
2. The shape of the implants of the present invention is consistent with the shape of the disc, which the implants at least in part replace, wherein the front of the disc is normally taller than the back of the disc, which allows for normal lordosis. The implants of the present invention are similarly taller anteriorly than they are posteriorly.
3. The spinal fusion implants of the present invention allow for a minimal and uniform removal of bone from the vertebrae adjacent the disc space while still providing for an interbody fusion in lordosis when properly inserted.
4. The spinal fusion implants of the present invention conform to a geometric shape, which shape is readily producible at the site of fusion, to receive said spinal fusion implants.
The spinal fusion implants of the present invention can be made of any material appropriate for human implantation and havi
Brown Michael A.
Loeb & Loeb LLP
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