Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2000-04-24
2002-01-29
Smith, Jeffrey A. (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06342074
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to interbody spinal fusion implants and surgical procedures for implanting load bearing devices in intervertebral disc spaces after removal of damaged or diseased discs, with the implanted devices being securely connected to the adjacent vertebrae to stabilize the vertebrae while providing for proper transmission of loads therebetween, and being hollow to receive bone graft for fusion of the vertebrae.
More particularly, the present invention relates to interbody spinal fusion implants and a procedure for surgically implanting a device of novel design that 1) has a thickness selected to restore and maintain proper spacing between the end plates of two adjacent vertebrae of the spine, 2) has its top and bottom surfaces inclined to define a wedge shape when viewed in side elevation if such inclination is needed to restore and maintain normal spinal lordosis by restoring and maintaining the normal anatomic angular relationship of the vertebrae, 3) is rigidly connected to each of the adjacent vertebrae by large screws that angle upwardly and downwardly from an anterior or side region of the implant and that thread through the vertebral end plates and into the soft cancellous bone in the central regions of the adjacent vertebrae to prevent relative movement of the adjacent vertebrae, 4) defines a hollow central region for receiving bone graft material for fusing the vertebrae, and 5) provides annular load-carrying portions of the implant that surround the central region and function to transmit loads directly, in a physiologic manner, from the hard cortical periphery of one vertebral end plate to the hard cortical periphery of the adjacent vertebral end plate.
RELATED ART
The vertebrae of the human spine are arranged in a columnar array one atop another, with each horizontally extending space between adjacent pairs of the vertebrae being provided with a separate intervertebral disc that transmits between adjacent vertebrae such forces as are imposed on and carried by the spine, while also functioning to cushion and permit a limited amount of relative movement to take place between the adjacent vertebrae. In a healthy spine, 1) each vertebra is composed of a hard outer ring of cortical bone, and a softer center of cancellous bone; 2) each intervertebral disc is composed of a tough outer ring of fibrous material, and a softer center of jelly-like material; and, 3) the physiologic manner in which loads are carried by the spine makes use of the hard fibrous annuli of the discs to transmit force between the hard cortical periphery of the vertebrae.
The cervical and lumbar areas of the human spine are, in a healthy state, lordotic such that they curve convexly forward. Normal lordosis results, at least in significant measure, from the normal wedge-shaped nature of the spaces between adjacent pairs of the cervical and lumbar vertebrae, and the normal wedge-shaped nature of the intervertebral discs that fill these spaces. If disc damage or degeneration occurs, lordosis tends to be lost, at least in part, because the wedge-shaped character of the discs diminishes and may become lost—hence, the wedge-shaped character of the spaces between adjacent cervical and lumbar vertebrae is likewise caused to diminish, and normal spinal curvature is altered as a result. Moreover, if normal disc thickness diminishes, as is common in the presence of disc damage or disease, the normal spacing between the vertebrae is thereby caused to diminish, causing the height of the spinal column to be undesirably diminished. Loss of lordosis and loss of proper vertebral spacing disturb the overall mechanics of the spine, often causing cascading degenerative changes.
Disc degeneration and the spinal changes that result can bring pain. An accepted treatment for back pain caused by a degenerative disc is to remove the disc and fuse the adjacent vertebrae in a manner that maintains suitable spacing of the vertebrae while preventing the vertebrae from moving relative to each other, for example by filling the intervertebral space where the disc was removed with bone graft that will enable the adjacent vertebrae to grow together and become one solid piece of bone.
The main front (i.e., anterior) portions of adjacent vertebrae between which an intervertebral disc normally resides are referred to by the term “vertebral bodies.” The anterior space between adjacent vertebral bodies where a disc normally resides is referred to by the term “intervertebral disc space.” When bone graft material that has been inserted into the intervertebral disc space fuses the vertebral bodies of adjacent vertebrae, the process is referred to as “anterior interbody fusion.”
Anterior interbody fusion may require weeks, sometimes months to achieve a desirable result, and is likely to achieve unsatisfactory results if relative movement takes place between the adjacent vertebrae while fusion is underway. If relative movement of adjacent vertebral bodies takes place while fusion is underway, this will, as a minimum, slow the rate of fusion, and can prevent acceptable fusion results from being achieved.
Also, if relative movement takes place while fusion is underway, significant continued back pain after surgery may result. To stabilize the adjacent vertebrae and prevent relative movement from taking place, one traditional approach has been to place the patient in a body cast, a procedure that still is used in many instances. To avoid using a body cast and to improve the prospects for achieving satisfactory fusion by more directly immobilizing adjacent vertebrae, some surgeons have used steel rods and other devices that are secured to the posterior of the vertebrae. A significant drawback associated with the use of such posterior appliances is the need for posterior surgery to put them in place.
Operating on the spine from a posterior approach disrupts posterior muscles causing permanent muscle dysfunction, referred to as “fusion disease”—hence, posterior back surgery is to be avoided unless essential. Other drawbacks accompanying the use of such posterior immobilizers include the need for a second surgery to remove these temporary appliances once satisfactory fusion has been achieved, and the discomfort, inconvenience and danger that can result from having to live with this hardware for weeks or months following the original surgery.
While a variety of anterior interbody fusion techniques and implant devices have been developed for use in the cervical region of the spine where the vertebrae are smaller and the loads that must be transmitted between adjacent vertebrae are smaller than in the lumbar region of the spine, it has proven more elusive to achieve consistently good anterior interbody fusion results in the lumbar region of the spine without having to employ body casts or temporary installations of posterior immobilizers.
An approach that has been used with greater success to achieve anterior interbody fusion in the lumbar region of the spine than in the cervical region of the spine has been to install, within an intervertebral space where a degenerative disc has been removed, a pair of so-called “threaded fusion cages.” Threaded fusion cages are of generally cylindrical form and are available in a variety of diameters. If threaded fusion cages are to be installed in an intervertebral space from which a degenerated disc has been removed, cages are selected that have sufficiently large diameters to enable their threaded exteriors to “thread” or “bite” into underportions of the vertebral body of the upper vertebra, and into upper portions of the vertebral body of the lower vertebra as each of the cages are “threaded” into the intervertebral space.
After the threaded fusion cages are installed, their hollow interiors are filled with bone graft material so that bone growth will take place within and about the threaded fusion cages while the cages serve to maintain proper spacing between adjacent vertebrae.
While bone growth is taking place, spinal loads are transmitted between the adjacent vertebrae prin
Emerson Roger D.
Emerson & Skeriotis
Skeriotis John M.
Smith Jeffrey A.
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