Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-03-05
2001-06-05
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06241770
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to interbody spinal fusion implants that are securely placed into the intervertebral space created across the spinal disc between two adjacent vertebral bodies after the removal of damaged spinal disc material and preferably at least some vertebral bone from each of the adjacent vertebral bodies for the purpose of achieving interbody spinal fusion, which fusion occurs preferably at least in part through the spinal fusion implant itself. In particular, the present invention is directed to an improved, interbody spinal fusion implant having opposed arcuate surfaces for penetrably engaging each of the vertebral bodies adjacent a disc space in the human spine and having a trailing end configured to conform to the anatomic contour of the anterior and/or lateral aspects of the vertebral bodies, so as to not protrude beyond the curved contours thereof, and in one preferred embodiment of the present invention the above described implants are structurally adapted to be rotated for proper insertion.
2. Description of the Related Art
Surgical interbody spinal fusion generally refers to the methods for achieving a bridge of bone tissue in continuity between adjacent vertebral bodies and across the disc space to thereby substantially eliminate relative motion between the adjacent vertebral bodies. The term “disc space” refers to the space between adjacent vertebrae normally occupied by a spinal disc.
Human vertebral bodies have a hard outer shell of compact bone (sometimes referred to as the cortex) and a relatively softer, inner mass of cancellous bone. Just below the cortex adjacent the disc is a region of bone referred to herein as the “subchondral zone”. The outer shell of compact bone (the boney endplate) adjacent to the spinal disc and the underlying subchondral zone are together herein referred to as the boney “end plate region” and, for the purposes of this application, is hereby so defined to avoid ambiguity. A circumferential ring of dense bone extends around the perimeter of the endplate region and is the mature boney successor of the “apophyseal growth ring”. This circumferential ring comprises of very dense bone and for the purposes of this application will be referred to as the “apophyseal rim”. The spinal disc that normally resides between the adjacent vertebral bodies maintains the spacing between those vertebral bodies and, in a healthy spine, allows for the normal relative motion between the vertebral bodies.
Reference is made throughout this Background section to the attached drawings in order to facilitate an understanding of the related art and problems associated therewith. In
FIG. 1
, a cross-sectional top plan view of a vertebral body V in the lumbar spine is shown to illustrate the dense bone of the apophyseal rim AR present at the perimeter of the vertebral body V about the endplate region and an inner mass of cancellous bone CB. The structure of the vertebral body has been compared to a core of wet balsa wood encased in a laminate of white oak. From the top plan view in
FIG. 1
, it can be seen that the best structural bone is peripherally disposed.
FIG. 2
is a top plan view of a fourth level lumbar vertebral body V shown in relationship anteriorly with the aorta and vena cava (collectively referred to as the “great vessels” GV).
FIG. 3
is a top plan view of a fifth lumbar level vertebral body V shown in relationship anteriorly with the iliac arteries and veins referred to by the designation “IA-V”. The location of these fragile blood vessels along the anterior aspects of the lumbar vertebrae makes it imperative that no hardware protrude dangerously therefrom where the vessels could be endangered.
Implants for use in human spinal surgery can be made of a variety of materials such as surgical quality metals, ceramics, plastics and plastic composites, cortical bone and other materials suitable for the intended purpose, and further may be absorbable and or bioactive as in being osteogenic. Fusion implants preferably have a structure designed to promote fusion of the adjacent vertebrae by allowing bone to grow through the implant from vertebral body to adjacent vertebral body to thereby fuse the adjacent vertebrae. This type of implant is intended to remain indefinitely within the patient's spine or if made of bone or other resorbable material to eventually be replaced with the patient's bone.
Michelson, Ray, Bagby, Kuslich, and others have taught the use of hollow, threaded perforated cylinders to be placed across a disc space between two adjacent vertebrae in the human spine to encourage interbody spinal fusion by the growth of bone from one vertebra adjacent a disc to the other vertebra adjacent that disc through such implants. Michelson, Zdeblick and others have also taught the use of similar devices that either have truncations of their sides such that they are not complete cylinders, and/or are tapered along their longitudinal axis much like a cylinder which has been split longitudinally and then wedged apart. All of these implants have in common opposed arcuate surfaces for penetrably engaging into each of the vertebral bodies adjacent a disc space to be fused. Such implants now in common use throughout the spine, may be used individually or inserted across the disc space in side-by-side pairs, and may be insertable from a variety of directions.
It is commonly held by surgeons skilled in the art of spinal fusion that the ability to achieve spinal fusion is inter alia directly related to the vascular surface area of contact over which the fusion can occur, the quality and the quantity of the fusion mass (e.g. bone graft), and the stability of the construct. However, the overall size of interbody spinal fusion implants is limited by the shape of the implants relative to the natural anatomy of the human spine. For example, such implants cannot dangerously protrude from the spine where they might cause injury to one or more of the proximate vital structures including the large blood vessels.
With reference to
FIG. 4
, a top plan view of the endplate region of a vertebral body V is shown to illustrate the area H available to safely receive an implant(s) inserted from the anterior aspect (front) of the spine, with the blood vessels retracted.
As can be seen in
FIG. 5
, a top plan view of the endplate region of a vertebral body V with the outlines of two differentially sized prior art implants A and B installed, one on each side of the midline of the vertebral body V, are shown. The implantation of such prior art implants A and B is limited by their configuration and the vascular structures present adjacent anteriorly to the implantation space. For example, the great vessels GV present at the L
4
level and above are shown in solid line in
FIG. 5
, and for the L
5
and S
1
levels, the iliac artery and vein IA-V are shown in dotted line. As shown in
FIG. 5
, prior art implant A represents an attempt by the surgeon to optimize the length of the implant which is inhibited by a limiting corner LC. Implant A, the longest prior art implant that can be inserted without interfering with the great vessels GV adjacent the vertebral body V, leaves cross-hatched area X of a cross section the vertebral body at the endplate region wasted which would be a very useful surface for contact for fusion and for support of the implant by the vertebral body. Similarly, implant B is an attempt by the surgeon to optimize the width of an implant which is also inhibited by a limiting corner LC′. Implant B, the widest prior art implant that can be inserted without interfering with the great vessels GV adjacent the vertebral body V, leaves cross-hatched area Y of the cross section of the vertebral body adjacent the endplate region wasted which could otherwise be a very useful surface area for contact for fusion and for support of the implant by the vertebral body. The presence of limiting corners LC and LC′ on any such implants precludes the surgeon from safely utilizing a
Martin & Ferraro LLP
Willse David H.
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