Double dowel spinal fusion implant

Details Double dowel spinal fusion implant Double dowel spinal fusion implant

2002-01-30

2003-11-04

Isabella, David J. (Department: 3738)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Bone

Reexamination Certificate

active

06641613

ABSTRACT:

FIELD OF INVENTION
This invention relates generally to the treatment of injured, degenerated, or diseased tissue in the human spine, especially failed intervertebral discs and diseased vertebrae. It further relates to the removal of damaged tissue and to the stabilization of the remaining spine by bony fusion of at least two vertebrae adjacent or nearly adjacent to the space left by the surgical removal of tissue. More particularly, this invention relates to the implantation of devices which can structurally replace the removed discs during healing and at the same time share compressive load to facilitate growth of new bone between adjacent vertebrae. This invention further relates to the implantation of devices which are compatible with the natural curvature of the spinal column.
BACKGROUND OF THE INVENTION
For many years a treatment, often a treatment of last resort, for serious back problems has been spinal fusion surgery. Disc surgery, for example, typically requires removal of a portion or the entirety of an intervertebral disc. In such cases the structural contribution of the removed disc must be replaced. The most common sites for such surgery, namely those locations where body weight most concentrates its load, are the lumbar discs in the L1-2, L2-3, L3-4, L4-5, and L5-S1 intervertebral spaces. In addition, a number of degenerative diseases and other conditions such as scoliosis require correction of the relative orientation of vertebrae by surgery and fusion.
In current practice, a surgeon will use one or more procedures known in the art to attempt to fuse remaining adjacent spinal vertebrae. Some of the prior art techniques used with at best mixed success have been described in Nicholson, et al., U.S. Pat. No. 6,096,080, issued Aug. 1, 2000, which is incorporated by reference as though fully set forth herein. That patent set forth a successful advance over such prior art.
Prior art methods of attempting to achieve fusion as described in Nicholson, et al. do not produce reliable and predictable results. For example, Fraser, R. D. points out in “Interbody, Posterior and Combined Fusions,” Spine, V20(24S):1675, Dec. 15, 1995: “[A]nalysis of the literature does not indicate that one form of fusion is significantly better than another for degenerative conditions of the lumbar spine.” Ray, Charles D., reported the results of the original IDE study involving his Ray Threaded Fusion Cage (Ray-TFC) in Spine V22(6):667, Mar. 15, 1997. The study reported that of the two hundred eight patients who had two year follow-up, 96% had fusion but with only 40% having excellent results and 25% having fair or poor results.
There is a strong current of opinion among leading spine surgeons that loading the material which is to become bone and produce fusion with a compressive mechanical load yields superior results both in strength and in brevity of healing time. Such loading works in conjunction with the standard procedure of preparing the vertebrae to be fused by breaking through, or cutting into, the hardened endplate surfaces of vertebral bone so as to allow an interposed bone graft or implant to come into direct contact with vascularized cancellous bone tissue. The latter enables blood flow through material placed in the intervertebral space which in turn initiates the growth of new bone across the intervertebral space. This process allows for the incorporation of inserted bone grafts or implants into the two respective adjacent vertebral surfaces such that they become one continuous and rigid segment of bone.
In addition, the restoration of normal anatomy is a basic principle of all orthopedic reconstructive surgery. Lordosis, a pronounced forward curvature of the lumbar region of the spine, is a factor relevant to the design of lumbar implants. It is known in the art that the preservation of the natural curvature of the lumbar spine requires a design having a modest taper approximately equivalent to the effective angularity of the removed tissue.
Therefore there is a perceived need for a device which simultaneously provides: mechanical stability upon implantation; a proper substrate such as allograft bone to induce the growth of bone across the implant region; compressive loading to the implant so as to enhance bone growth and calcification; and accommodation to the normal anatomical curvature of the spine. There is also a perceived need for an implant configuration that can be used in other regions of the spine in addition to the lumbar region, specifically in the cervical region.
Bone based implants may have cylindrical, rectangular, and generic shapes. In the past, Cloward, Wilterberger, Crock, Viche, Bagby, Brantigan, and others have taught various methods involving the drilling of holes across the disc space between two adjacent vertebrae of the spine for the purpose of causing an interbody spinal fusion upon placement of a dowel in the hole. Cloward, for instance, taught placing a dowel of bone within a drilled hole for the purpose of bridging the intervertebral gap and incorporating the dowel into the fusion. Viche taught the threading of that bone dowel.
Bone used in dowel implants can be obtained from the patient's own hip iliac crest or it can be obtained as allograft from a tissue processor such as Regeneration Technologies, Inc. Many prior art bone grafts required distraction of the intervertebral space and contouring of the bone graft to ride on the end plates of the vertebral bodies.
Dowel bone grafts have been described in, for example, the following U.S. Patents: Michelson in U.S. Pat. Nos. 5,015,247, 5,860,973, and 6,149,650; Grivas, et al., in U.S. Pat. No. 5,814,084; Zdeblick, et al., in U.S. Pat. Nos. 6,206,922 and 6,245,072; Koros in U.S. Pat. No. 6,217,579; and McKay in U.S. Pat. No. 6,261,586.
Michelson, in U.S. Pat. No. 5,015,247 (May 14, 1991), describes a “threaded spinal implant” which would plausibly be a dowel type design with threads. Grivas, et al., in U.S. Pat. No. 5,814,084 (Sep. 29, 1998), describe a dowel made from “a plug from the shaft (diaphysis) of various long bones.” Grivas et al. state that their dowel has improved biomechanical and vertebral fusion induction properties compared to standard dowels known in the art.
Michelson, in a later patent, U.S. Pat. No. 5,860,973 (Jan. 19, 1999), teaches a spinal implant design which is “at least in part cylindrical in shape.” However, the use of a single dowel bone graft implant tends to flatten the intervertebral space, causing it to be distracted in such a way as to induce an undesirable change in spinal curvature. Also, the instrumentation used to implant single dowels is bulky and tends to be difficult to use partly because it blocks direct view of the implantation site. The Cloward method is now mostly of historical interest and has been largely replaced by methods such as the Smith-Robinson technique in which a straightforward discectomy is followed by implantation of a square block of bone. Smith-Robinson has been until recently a standard, even though it has problems related to both (1) bone non-incorporation, mostly due to poorly interfaced surfaces since the end plate of the vertebral bodies is not a square configuration, and (2) collapse or telescoping of the graft within the patient's adjacent vertebral body causing pain. Such complications may be attributed to poor fit of the graft and inadequate preparation of the endplates to accept the graft.
The invention of Nicholson, et al., U.S. Pat. No. 6,096,080 made great strides in overcoming the many disadvantages of prior art devices and procedures. However, it nevertheless had some drawbacks which are addressed by the current invention. The dovetail implant of that invention can only be placed in one orientation if it is to achieve its full intended effect. Moreover, it requires a relatively wide distraction of the adjacent vertebrae to accommodate the height of the implant necessitated by the geometry of the dovetail configuration.
It is therefore an object of the current invention to provide a spinal fusion implant able

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