Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
1999-11-24
2001-12-11
Smith, Jeffrey A. (Department: 3732)
Surgery
Instruments
Orthopedic instrumentation
C606S064000, C606S064000
Reexamination Certificate
active
06328738
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to orthopedic surgery, and more particularly to an anterior cervical fusion compression plate.
Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not the least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaptation is a relatively recent change, and as such has not benefitted from natural selection as much as have the horizontal backbones of other animals. As a result, stresses acting upon the human backbone (or “vertebral column”) are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.
The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five in the low back or lumbar region. There are also five bones in the pelvic or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves, and for providing structural support for the entire body.
Between the vertebral bones themselves exist soft tissue structures—discs—composed of fibrous tissue and cartilage which are compressible and act as shock absorbers for sudden downward forces on the upright column. The discs allow the bones to move independently of each other, as well. The repetitive forces which act on these intervertebral discs during repetitive day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time.
Presumably because of humans' upright posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect the more highly mobile areas of the spine. Disruption of a disc's internal architecture leads to bulging, herniation or protrusion of pieces of the disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements (spinal cord and nerves) cause pain, attended by varying degrees of disability. In addition, loss of disc space height reduces tension on the longitudinal spine ligaments, thereby contributing to spinal instabilites such as spinal curvature, and lithesis.
The time-honored method of addressing neural irritation and instability resulting from severe disc damage have largely focused on removal of the damaged disc and fusing the adjacent vertebral elements together. Removal of the disc relieves the mechanical and chemical irritation of neural elements, while osseous union (bone knitting) solves the problem of instability.
In the cervical spine, the most common type of fusion utilizes either bone dowels (Cloward Technique) or bone blocks (Smith Robinson Technique). These procedures have been used now for over four decades. One of the main causes of failure of these fusion techniques is the failure to fuse, or non-union, at the site where the bone is grafted between the vertebral bodies. In an attempt to circumvent this problem, various plate-type mechanisms have been used both to provide immediate stability, and to reduce or eliminate movement at the site of the fusion to allow successful bone knitting, much as a cast on a fractured limb provides support until healing can occur.
It is recognized that for bone knitting to occur, the interfaces of bone required to knit or heal must be held in close apposition and motion between the knitting or fusion interfaces must be restricted sufficiently for a certain minimal time period to permit stable bone growth to occur.
To achieve these ends, prior inventors have developed a variety of both external braces and internal fixation instruments, some in the form of plates. Internal fixation is advantageous in that is obviates the need for cumbersome external braces, collars or supports and ensures essentially total compliance. U.S. Pats. No. 5,041,113, 5,234,431, 5,344,421, and 5,681,311 provide examples of prior vertebral bone plate systems. U.S. Pat. No. 3,604,414 discloses a plate for setting fractures, having separate elements which are attached to respective bone fragments and have a toothed interface to maintain the position of the fragments after they have been drawn together.
While interface apposition and retardation of motion are known to enhance bone healing, it is also recognized that if the bony surfaces to be fused are held together under a compression force, osseous union is further enhanced. While many plating systems maintain bony apposition and provide stability, few provide sufficient compression to be called compression plates, and none provide adjustable degrees of compression of the grafted bone.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a cervical fusion compression plate that provides the desirable features of stabilizing grafted bone interfaces for sufficient time that bone union can occur. It is also the objective of this invention to provide adjustable degrees of compression of grated interfaces to further enhance the healing process and increase the probability of successful cervical bone graft fusion. It is also intended that this device be applicable to all generally accepted anterior cervical surgical approaches to the cervical spine, and that the device be sufficiently easy to apply such that is can be used readily even by surgeons with little experience with spinal instrumentation.
To achieve these objectives, a curved metal plate having roughly the curvature of a normal lordotic cervical spine is provided. At each end of the main body of the plate, there is a smaller U-shaped recess having a toothed surface at either side. Into this recess, a smaller, externally toothed secondary U-shaped insert is slid, so that the teeth mesh. The teeth are raked in opposition directions, as in a ratchet, so that relative motion can occur in only one direction (the compression direction), as long as the teeth are engaged. Incrementally increasing compression occurs as the plates are adjusted.
To facilitate placement of the fusion plate, a central screw is placed at the midline, below and parallel to the vertebral element's end face. This central screw is placed in each vertebral element above and below the fusion site. Placement of the central screw is aided by using a central screw guide which ensures that the screw is placed the correct distance from the end plate to avoid inadvertent placement of the screw in the disc space. The central screw guide has a needle-like probe which is pushed into the softer cervical disc material so that the exact location of the end plate can be determined. This allows placement of the central screw a predictable distance above or below a given vertebral end surface.
Once the central screws are placed, they can be utilized initially to distract the disc space to facilitate bone graft placement with any of the many available vertebral distraction instruments.
After the bone graft has been properly placed, the compression plate is then positioned so that the openings in the upper and lower U-shaped inserts fit over the upper central and lower central screws, respectively. Using a suitable pliers-like tool, the upper and lower U-shaped inserts are drawn toward the center of the plate. As the central screws are brought closer together, compression of the intervening bone graft results. Since the U-shaped inserts can move in only one direction with a ratcheting motion, gradually greater degrees of compression occur until optimal compression is deemed to have been achieved. The central screws are then tightened to firmly secure the entire plating system to the spine. Now, holes may be drilled for
Priddy Michael B.
Shoemaker and Mattare
Smith Jeffrey A.
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