Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
2001-10-15
2004-02-24
Robert, Eduardo C. (Department: 3732)
Surgery
Instruments
Orthopedic instrumentation
C606S064000
Reexamination Certificate
active
06695845
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
not applicable
Reference to a Microfiche Appendix
not applicable
FIELD OF THE INVENTION
The invention relates generally to implantable medical devices and their methods of use for stabilizing skeletal bone, and relates more particularly to implantable medical devices fabricated of nonmetals and their use for stabilizing the cervical vertebrae of a human spine.
BACKGROUND OF THE INVENTION
The Stabilizer Need
In normal anatomy, the vertebrae of the cervical column are held together and to the skeleton by a complex arrangement of ligaments, tendons, and muscles. Degenerative diseases, deformities, or trauma may cause abnormal conditions. These problems generally cause or allow displacement or rotation of a vertebra relative to the adjacent vertebra. When spinal discs rupture or bulge, the intervertebral space between two adjacent vertebrae
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can decrease and cause discomfort to the patient. Frequently the bulging does no harm, but if it compresses against the spinal cord or a nerve, it may cause pain with loss of sensation, or weakness. Torn discs, torn ligaments, spinal fractures, and other conditions that affect the vertebral joints normal function can produce spinal pain. When surgery is needed, the discs are replaced with grafts that will heal or “fuse” with the vertebra. This implant, with its associated stabilization, maintains the vertebral position while healing takes place. This healing is referred to as “spinal fusion”. The objective of spinal implants is to facilitate realignment and/or fixation of spinal elements. Clinical studies have demonstrated that surgeries using spinal implants are more effective at maintaining alignment and providing rigidity to the spine than surgeries in which implants are not used. Since the introduction of stabilizers as crude plates, rods, and wires; these devices have been developed into sophisticated appliances, which can be assembled and configured to rigidize spines of any size or condition. These stabilizers provide mechanical fixation for restraint of an implanted graft material. With this fixation, displacement during healing is significantly reduced thereby reducing the failure rate.
Prior Technology
The majority of existing cervical stabilizers use plates that are bent in the axial plane to conform to the vertebrae, and along the spinal axes to maintain lordosis. Bicortical screw purchase (where the screw penetrates the near side and the far side of the vertebra) has been favored because of the increased strength of the construct and increased screw thread area within the bone. These screws are more technically challenging to place and implanting them adds an increased risk of morbidity from neural canal penetration. The reduced strength and decreased thread area of a unicortical screw purchase (where the screw penetrates only the near side of the vertebra) increases the probability of screw backout or loosening which may result in esophageal injury. Screw backout and loosening has led to the development of mechanisms for locking the screw head to the plate in unicortical screw plate designs. Such locking mechanisms not only prevent screw backout, they also reduce the tendency of the screw head to pivot within the plate. These devices contain many intricate components that increase the cost and reduce reliability of stabilizer systems. The unicortical metal devices presently available are relatively rigid devices.
Nonmetal stabilizers are preferred over metallic stabilizers because of the minimal interference with X-rays and magnetic resonant imaging (MRI) techniques used for postoperative evaluation. Bendability or precurvature of the plate is also desired to accommodate or restore the natural lordosis of the cervical spine. These, and other desirable features and advantages, are provided by the present invention, particular embodiments of which are described in the Detailed Description Of The Patent section of the present patent.
Once complete fusion has taken place the plate is no longer needed. Indeed it is undesirable because it may interfere with esophageal action or may later fracture resulting in esophageal injury. A fractured bone that has been fixed with a metallic stabilizer is much more likely to refracture if the stabilizer is removed or if the stabilizer breaks. Refracture may occur because the stress sharing or stress shielding, that the metal stabilizer provided during healing, has not allowed the bone to carry sufficient load to return to normal load bearing strength. The compression forces should be gradually transferred from the stabilizer to the healing bone. Bioabsorbable and biodegradable materials will reabsorbe into the bone and provide a gradual reduction of the plate and screw material after fusion. This allows temporal load shareing, promoting bony maturation and strengthening, and will eliminate possible internal injury, a second operation, refracture, and imaging artifacts.
The following patents are examples of the complications and stress raisers in effort to prevent screw backout. These stress raisers are not suitable for use in polymeric stabilization:
U.S. Pat. No. 5,578,034 to Estes discloses a bone screw with an enlarged head and an annular collar surrounding the bone screw shaft. The collar's inner diameter shrinks in response to a change in temperature, trapping the collar between head and the threads of the bone screw.
U.S. Pat. No. 5,275,601 to Gogolewski discloses an absorbable screw where a portion of the length of the screw head has a three-dimensional structure consisting of corrugations or serrations around the outer surface of the head portion. These serrations will cause stress raisers that may create cracks during fatigue cycling and will lead to screw and plate failure.
The following patents are examples of materials which may be used in the devices of this patent:
U.S. Pat. No. 5,522,895 to Mikos discloses biodegradable and bioresorbable materials and treatments that may be used in the device of this present patent.
U.S. Pat. No. 6,269,716 to Amis discloses a tapered screw head for biodegradable medical implants. The screw head has a star shaped outer circumference with external features for rotation. In the disclosed patent the resorbable fastener tapered head is connected to a threaded shaft. The stress raisers of both the threaded portion and tapered head are in the high stressed area at the plate/bone interface. This design is successfully used in non-load bearing bones in facial and cranial surgeries. However it does not have the required strength for load bearing applications.
The following patent is an example of stabilizing systems that disclose or claim tapered screws:
U.S. Pat. No. 6,228,085 to Theken discloses metallic bone fixation system with a three-dimensionally anatomically contoured plate to fit the anterior lateral profile of the vertebrae and forming a ledge to maintain the space between two vertebrae. The system is designed for use as a metal plate and is suited for thoracic and lumbar spines. It uses setscrews and threads in a portion of the hole. It has irregular surfaces in the plate such as steps, spines or teeth to bite into a bone. The screw may have a tapered outer surface adjacent to the threaded portion to provide pullout resistance of the screw in the plate.
Polymeric Stabilizers
Polymeric stabilizers have been patented and implanted in animal spines, however none have been successful, because of material failure. Making a polymeric stabilization system that will compete with present titanium plates is a challenge. Most previous polymeric stabilization systems have been designed similar to metal plate systems. The successful utilization of these polymers requires a novel design, which will operate within the limitations of polymeric material properties. The toughest bio-compatible polymers available have a tensile strength {fraction (1/25)} that of titanium and they are 50 times more elastic than titanium. Any successful polymeric systems must be designed to operate with
Dixon Robert A
Hackman Donald J
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