The present invention relates to a bone screw assembly for correcting misaligned spinal vertebrae. In particular, the present invention provides an assembly where the bone screws can not only be set at different axes vis-à-vis their connecting rods, but can also be set at different heights vis-à-vis such rods.
Nerve compression and pain can be caused when vertebrae in the spine become misaligned. Spondylolisthesis, for example, is a condition where vertebrae become misaligned by slipping over one another either forwards (anterolisthesis) or backwards (retrolisthesis). Surgical techniques can be used to correct such misalignments. In one such surgical technique, bone screws are affixed to various vertebrae and, through suitable connectors, commonly attached to one or more rods. Often, two sets of bone screw/rod assemblies are created in this way on either side of the spinous process. In these assemblies, the rods are shaped to the desired orientation or angulation of the spinal vertebrae. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region, or a lordotic curvature for lumbar region of the spine. Over a period of time, the rods apply pressure to the vertebrae until the vertebrae realign themselves in a proper orientation. This is known as a rod-type spinal fixation system. It is system where the desired angulation of the spinal vertebrae is achieved by the shape of the rod. In other words, a plurality of fixation devices including hooks, clamps, bolts and screws simply attach segments of the spine to a fixed rod that over time and with adjustments, as needed, corrects the spine's angulation.
There have been many attempts to create bone screw assemblies for realigning spinal vertebrae. One bone screw assembly that has been in common commercial use is described in Sherman et al.'s U.S. Pat. No. 5,885,286 (“Sherman patent”), the disclosure of which is hereby incorporated by reference. The Sherman patent discloses a relatively simple bone screw assembly featuring a bone screw, a receiver member, a compression member and a connector rod. The bone screw in the Sherman patent is cast with a spherically shaped head at its top end so that it can pivot along different axes when it is affixed to Sherman's receiver member. Sherman's receiver member also holds a transverse rod that can be fitted into a number of other receiver members. By tightening a compression member, such as a set screw, at the top of the receiver member, one can simultaneously lock both the rod and bone screw in position.
While the Sherman bone screw assembly has been used in many surgeries, it has drawbacks and limitations. One such drawback is illustrated in
Another multi-axial bone screw assembly is illustrated in Altarac's U.S. Pat. No. 7,163,538 (“Altarac patent”). The Altarac bone screw assembly consists of a complicated arrangement of a bone screw, a post member, a locking cap, a rod connector, a locking nut and a rod. The Altarac patent allows the bone screw to pivot on multiple axes vis-à-vis the rod connector by forming a cage on top of the bone screw that receives a ball shaped engagement member at one end of the intermediate post member. The other end of the intermediate post member is then attached to the rod connector by a combination of a locking cap and locking nut. While, at first glance, it appears that the height of Altarac's bone screw could be adjusted vis-à-vis the rod connector, FIG. 4 of the Altarac patent appears to show that the bone screw will be locked at a fixed, unvarying height vis-à-vis the rod connector. Moreover, due to its complicated arrangement of parts, the Altarac bone screw assembly would be difficult to use during surgery and, if improperly used, could easily lead to dangerous problems.
The object of the present invention is to provide a variable height, multi-axial bone screw assembly that allows improved angulation of bone screws vis-à-vis a fixed cylindrical rod. Such a bone screw assembly can advantageously be used for correction of abnormal cervical, thoracic and lumber curvatures.
The present invention provides a simple, variable height multi-axial bone screw assembly that allows bone screws to engage a fixed cylindrical rod in any degree of angular orientation or direction. In a preferred embodiment, the bone screw assembly of the present invention includes a bone screw, a collet, a tulip shaped connector, a crown member, a locking washer, a rod and a set-screw. In this embodiment, the bone screw has a threaded shank at its lower end and a generally cylindrical shaft at its upper end. The collet has a cylindrically shaped lower end and a generally bulb shaped upper end which is designed to be received by the tulip shaped connector. This collet also has an interior bore which allows the collet to slide over the bone screw shaft like a sleeve. By changing the amount of the bone screw shaft covered by the collet, one can vary the height of the bone screw vis-à-vis the tulip shaped connector.
The tulip shaped connector has a central bore that is designed, at its lower end, to receive the collet bulb. Within the tulip shaped connector, the locking washer can be placed below the collet bulb and the crown member can be placed above the collet bulb to reliably secure the collet bulb into the lower portion of the tulip shaped connector bore. Once the locking washer, collet bulb and crown member are assembled in the lower portion of the tulip shaped connector bore, the rod can be transversely placed above the crown member in a U-shaped channel within the tulip shaped connector. A set screw can then be tightened above the rod in the tulip shaped connector to simultaneously lock the rod, crown member, collet, locking washer and bone screw into their desired positions. Before the set screw is tightened, the collet bulb can be turned within the tulip shaped connector bore along multiple axes to achieve a desired axial orientation of the bone screw vis-à-vis the tulip shaped connector. To help the collet fit snugly around the bone screw, longitudinal slots are provided along the surface of the collet allowing the solid sections of the collet bulb to collapse around the bone screw shaft when the set screw of the tulip shaped connector is tightened. As a further aid to preventing the bone screw from slipping out of the collet after the bone screw assembly of the present invention has been implanted, the bone screw can be tapered outward.
Referring now to
The bone screw 20 in this preferred embodiment has a threaded shank 22 at its lower end with threads configured to solidly anchor the bone screw within a bone. Preferably, the threads are cancellous threads, or threads readily adapted for solid fixation within the cancellous bone of the vertebral body. It is understood that the threaded shank 22 can have a variety of configurations depending upon the nature of the bone within which the bone screw 20 is engaged. Moreover, the length of the threaded shank 21 can be adjusted depending upon the bone within which the screw is driven. In one embodiment, the threaded shank 22 has a length of about 1.75 inches, and is configured with threads to engage the pedicle of a lumbar vertebra.
The bone screw 20 further includes a generally cylindrical shaft 24 at its upper end. The diameter of this generally cylindrical bone screw shaft 24 is selected to allow the shaft 24 to fit smoothly within the interior bore 31 of a collet 30 without leaving too much space between the interior surface of the collet 30 and the exterior surface of the bone screw shaft 24. As those of skill in the art will recognize, the bone screw shaft 24 can be formed in other shapes besides cylindrical. For example, the bone screw shaft 24 could also be formed in non-cylindrical shapes such as hexagonal, octagonal or oval cross-sectional shapes. In such case, it would also be advantageous to form the interior bore 31 of the collet 30 in a matching hexagonal, octagonal or oval shape.
Bone screw 20 and collet 30 can be separate pieces (see,
Turning now to
The distance between the threaded shank 22 of the bone screw 20 and the tulip shaped connector 40 when the bone screw assembly 10 of the present invention is fully assembled and tightened (see,
To help the collet 30 firmly grip the bone screw shaft 24 at a desired position and hold it there, a number of different slots 36 can be formed beginning at the upper end 34 of the collet and extend downward. In the preferred embodiment shown in
As shown in
Referring again to
The tulip shaped connector 40 further defines a central bore 47. The lowermost portion of the bore 47 defines a recess having a bottom 48 within which the collet bulb 34 resides when the bone screw assembly of the present invention is fully assembled (see,
As illustrated in FIGS. 4 and 5 of Sherman's U.S. Pat. No. 5,885,286, the disclosure of which is hereby incorporated by reference, a tulip shaped connector can have tool recesses in each of its branches 42 and 43. These tool recesses are configured to be engaged by an insertion tool, such as an insertion tool used to insert spinal hooks into the spine. The tulip shaped connector 40 can also define a number of gripping holes at laterally adjacent sides of its body. These gripping holes can be engaged by an appropriately configured gripping tool to support the tulip shaped connector during tightening of the bone screw assembly 10.
As shown in
A close-up view of the crown member 50 is shown in
The bone screw assembly 10 of the present invention can be assembled in at least two different ways, either through the top of the tulip shaped connector 40 or partially through the bottom of the tulip shaped connector 40. Referring now to
After the locking nut 65 (if needed) and collet bulb 34 have been inserted into the central bore recess 47 of the tulip shaped connector 40, the crown member 50 is placed on top of the collet bulb 34 as shown in
Next, the rod 70 is placed in the U-shaped channel 44 of the tulip shaped connector 40 so that it rests on top of the crown member 50. A set screw 60 can then be loosely screwed into the top of the U-shaped channel 44 of the tulip-shaped connector 40. At this point, the upper shaft 24 of the bone screw 20 can be slid into the inner bore 31 of the collet 30 to a desired height and angular orientation. As previously noted, the spherical nature of the collet bulb 34 allows the collet bulb 34 to pivot to a desired orientation within the tulip shaped connector 40. When the bone screw 20 has been set to a desired height and angular orientation vis-à-vis the tulip shaped connector 40, the set screw 60 can be tightened down to lock all the components of the bone screw assembly 10 in place. As shown by the arrows in
The partial bottom assembly method differs in several ways from the previously described top assembly method. In the partial bottom assembly method, the collet bulb 34 is inserted through the bottom 48 of the central bore 47 into the tulip shaped connector 40. In contrast to the top assembly method, the diameter of the bottom 48 of the central bore 47 always needs to be greater than the diameter of the collet bulb 34. The locking nut 65 is next squeezed down to a smaller diameter by virtue of gap 68 and also inserted through the bottom 48 of the central bore 47. After the locking nut 65 passes through the bottom 48 of the central bore 47, it is allowed to expand back to it normal size so that it can prevent the collet bulb 34 from dropping out of the tulip shaped connector 40. The remaining steps of the partial bottom assembly method would then be the same as the top assembly method with the crown member 50 being fitted on top of the collet bulb 34 followed by the rod 70 and set screw 60. As in the top assembly method, the shaft 24 of the bone screw 20 is inserted into the collet bore 31 at a desired height and orientation before the set screw is fully tightened.
In the foregoing specification, the invention has been described with reference to specific preferred embodiments and methods. It will, however, be evident to those of skill in the art that various modifications and changes may be made without departing from the broader spirit and scope of the invention. For example, while a set-screw 60 has been described for the preferred embodiment to lock the bone screw assembly 10 together, those of skill in the art will recognize that alternative types of locking compression members could also be used, such as a snap lock compression member. Also, while the bone screw 20/collet 30 combination of the present invention has been described in connection with one type of tulip shaped connector 40, those of skill in the art will readily recognize that the bone screw/collet combination of the present invention can be used with many different types of tulip shaped connectors, such as the tulip shaped connectors used by Depuy (MOUNTAINEER), Stryker (XIA), Medtronic (LEGACY, SOLARA, VERTEX) and Synthes (CLICK-X), among others. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than restrictive, sense; the invention being limited only by the appended claims.
This application claims the benefit of U.S. Provisional Patent Application No. 61/163,313, filed Mar. 25, 2009, which application is incorporated herein by reference in its entirety.
Number | Date | Country | |
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61163313 | Mar 2009 | US |