Surgery – Instruments – Orthopedic instrumentation
Reexamination Certificate
2000-08-17
2003-12-30
Manahan, Todd E. (Department: 3732)
Surgery
Instruments
Orthopedic instrumentation
Reexamination Certificate
active
06669697
ABSTRACT:
The present invention relates to apparatus and methods of multi-axis internal spinal fixation. In more detail, the present invention relates to a connection for use in an internal spinal fixation system, and a method of stabilizing, or fixing, the spine for use with either bilateral rods or plates (such as the Steffee/variable screw placement (VSP) system) or a central rod and plurality of cross-bars or plates (such as the so-called Tacoma Monorail System), utilizing wedge-shaped and/or flat washers having concave surfaces and, optionally, off-set and/or centered openings therein to provide multiple axes for transfering load from the patient's spinal column to the stabilizer through the pedicle screws used to fix the rods, cross-bars, and/or plates to the vertebrae of the patient.
There are many systems available for internal fixation of the spine. Such systems are described in the patent literature (see, for instance, U.S. Pat. Nos. 4,696,290, 5,047,029, 5,092,866, 5,129,899, 5,201,734, 5,312,404, 5,531,747, and 5,743,907 and European Application No. EP 0 846 444 A1) and the scientific literature (see, for instance, D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviews—Spine: Pedicle Fixation of the Lumbar Spine (Philadelphia: Nanley & Belfus, Inc.) 1992 and H. S. An and J. M. Cotler (Eds.), Spinal Instrumentation (Baltimore: Williams & Wilkins) 1992), and are available from such vendors as AcroMed, Smith & Nephew, MOSS® Miami, Osteonics, Sofamor Danek, and others.
A problem with all such systems, however, is the connection between the screws used to affix the system to the pedicle and the rods, cross-bars, and/or plates of the system. As stated in J. M. Cotler, et al., Principles, Indications, and Complications of Spinal Instrumentation: A Summary Chapter, in H. S. An and J. M. Cotler, Spinal Instrumentation pp. 435-456 (Baltimore: Wiliams & Wilkins) 1992, “[a] significant problem in pedicular screw fixation appears to be at the site of linkage between the screw and rod or plate.”
It appears that the problems at the site of this linkage may result from the geometry of the connection between the screw and the rod or plate. This difficult geometry results from several factors, including the different angles of the pedicles of the vertebrae, the location of the vertebrae and their relative sizes, the shape of the vertebrae and the spacing between vertebrae, the placement of the screws, the lordosis of the spine, and the need to insert the screws into each vertebra at an angle. With regard to the angle of the pedicle screws, pedicle screws are angled inwardly and upwardly into the vertebra for maximum strength and, because the surfaces of the pedicles of each vertebrae are angled relative to each other, the screws rarely line up across the vertebral body into which they are screwed. Nor do they usually line up from one vertebra to the adjacent vertebra, even if the adjacent vertebrae are the same size and shape (which they generally are not). For a more complete, discussion of the biomechanics of the bone-implant interface, reference is made to H. A. pool and R. W. Gaines, Biomechanics of Transpedicular Screw Spinal Implant Systems, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviews—Spine: Pedicle Fixation of the Lumbar Spine 37-44 (Philadelphia: Nanley & Belfus, Inc.) 1992, M. R. Pinto, Complication of Pedicle Screw Fixation, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviews—Spine: Pedicle Fixation of the Lumbar Spine 45-54 (Philadelphia: Nanley & Belfus, Inc.) 1992, and M. H. Krag, Vermont Spinal Fixator, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviews—Spine: Pedicle Fixation of the Lumbar Spine 121-145 (Philadelphia: Nanley & Belfus, Inc.) 1992, which references are incorporated herein in their entirety by these specific references thereto. A rod (or rods depending upon the particular stabilizer utilized) running along the longitudinal axis of the patient's spinal column provides the structural rigidity required for the stabilizer to stabilize the spine. Because the pedicle screws do not line up, the rod(s) must either be bent to the location of each screw head or structure must he provided that can be adjusted and/or positioned to enable the head of the pedicle screw to contact the rod(s) to transfer load from he screw to the rod.
As a result of this difficulty, the literature includes comments such as the following tatement in R. M. Puno and J. A. Byrd III, Transpedicular Screw/Rod Fixation Using the Puno-Winter-Byrd (PWB) System, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviews—Spine: Pedicle Fixation of the Lumbar Spine 83-106 (Philadelphia: Nanley & Belfus, Inc.) 1992:
“Transpedicular fixation has been proved to be of value in the treatment of spinal disorders . . . However, experience has shown that this method of instrumentation places great demand on the surgeon's skill because of the anatomic constraints related mainly to the anatomy and morphometry of the spinal pedicle.”
Many of the above-listed systems, and many of the systems described in the literature, attempt to relieve this burden on the surgeon by providing angled screws (for instance, the AMSET® R-F reduction-fixation system), so-called polyaxial screws (available from MOSS® Miami), full-length, scalloped, open-slot plate designs with an undersurface complementary to the shape of the screw head for positioning of the screws and up to 15° medial-lateral and 30° craniocaudal angulation at the screw-plate interface, and infinitely variable couplers (the so-called Rogozinski spinal rod system, for exanple) that are said to allow rotation through a 130° arc to allow screw placement within the pedicle without requiring that each screw be aligned with the screw in the adjacent vertebrae.
Although they address these problems, as evidenced by the introduction of new systems by the same vendors marketing the above-listed systems, no system completely solves all the problems presented by the need for optimal screw placement, angulation of the screw, and effective load transfer from spinal column to stabilizer. An ideal system would (a) accomodate optimal screw placement, height, and angulation, (b) accomodate different sizes and shapes of vertebrae, (c) minimize (or not require) bending or other fabrication during surgery, (d) maintain an angle of approximately 90° at the connection between the screw head and the plate or cross-bar to which the screw is attached for optimal load transfer and to minimize the likelihood of slippage and/or gross failure, and (e) be strong enough to provide lasting and rigid fixation of the spine. Those skilled in the art will recognize that this list is not exhaustive, but instead illustrates some of the characteristics of an ideal internal fixation system. Other design criteria are also important, and some practicioners may consider some criteria so important that they might not even list others.
So far as is known, none of the above-listed internal fixation systems meets these criteria in every patient. The disadvantages and limitations of currently available systems are made clear from reports in the literature of failure rates (failure of the device, not such complications as infection, phlebitis, seroma, neurologic deficit, etc.) as high as 25% (see R. Roy-Camille, et al., 203 Clin. Orthop. 7 (1986)), 11% (see, S. F. Heim and E. R. Luque, Danek Plaste and Screw System, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviews—Spine: Pedicle Fixation of the Lumbar Spine 201-234 (Philadelphia: Nanley & Belfus, Inc.) 1992), 8% (see, R. M. Puno and J. A. Byrd III, Transpedicular Screw/Rod Fixation Using the Puno/Winter/Byrd (PWB) System, supra), and 2-7% D. M. Arnold and L. L. Wiltse, The Wiltse System of Internal Fixation for the Lumbar Spine, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviews—Spine: Pedicle Fixation of the Lumbar Spine 55-82 (Philadelphia: Nanley & Belfus, Inc.) 1992).
The currently available systems have other limitat
Manahan Todd #E.
Perumala Corporation
Wisner Mark R.
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