Abstract:
A variable height, multi-axial bone screw assembly including a bone screw, a collet, a tulip shaped connector, a crown member, a rod and a set-screw. The bone screw has threads at its lower end and a generally cylindrical shaft at its upper end. The collet has a cylindrically shaped lower end to hold and grasp the bone screw shaft as well as a generally bulb shaped upper end to be received by the tulip shaped connector. By changing the position of the bone screw shaft grasped by the collet, the height of the bone screw vis-à-vis the tulip shaped connector can be varied. All the elements of the bone screw assembly are combined together by placing the collet head, crown member and rod within a central bore of the connector and firmly tightening them in place using the set screw.

Description:
[0001]    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. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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&#39;s angulation. 
         [0004]    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.&#39;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&#39;s receiver member. Sherman&#39;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. 
         [0005]    While the Sherman bone screw assembly has been used in many surgeries, it has drawbacks and limitations. One such drawback is illustrated in  FIG. 1 . Because Sherman&#39;s spherically shaped head is formed integrally with the remainder of his bone screw, the height of Sherman&#39;s bone screw vis-à-vis Sherman&#39;s receiving member cannot be varied. This lack of variability can cause Sherman&#39;s receiving member to be twisted as it tries to grip onto the rod as shown in the left and right bone screw assemblies illustrated in  FIG. 1 . With this twisting, the compression member contacts the rod at an angle which leaves gaps between the compression member and the rod. These gaps can create undesirable looseness in the bone screw assembly. 
         [0006]    Another multi-axial bone screw assembly is illustrated in Altarac&#39;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&#39;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. 
         [0007]    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. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    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. 
         [0009]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a cross-section view of three multi-axial bone screw assemblies of the type shown in Sherman&#39;s U.S. Pat. No. 5,885,286 attached to a spinal rod; 
           [0011]      FIG. 2  shows an exploded view of a preferred bone screw assembly of the present invention; 
           [0012]      FIG. 3  shows a cross-section, close-up view of the  FIG. 2  bone screw assembly when fully assembled, but not yet tightened with the set screw. 
           [0013]      FIG. 4  shows the same cross-section, close-up view of the  FIG. 2  bone screw assembly after it has been fully assembled and tightened with the set screw. 
           [0014]      FIG. 5  shows three multi-axial bone screw assemblies of the present invention attached to a spinal rod where the bone screws are set at different heights. 
           [0015]      FIG. 6  shows a cross-section close-up view of an alternative embodiment of the bone screw assembly of the present invention where the bone screw is tapered at its upper end. 
           [0016]      FIG. 7  shows a top elevation view of a preferred tulip shaped connector. 
           [0017]      FIG. 8  shows a side elevation view of a preferred tulip shaped connector. 
           [0018]      FIG. 9  shows a top elevation view of a preferred crown member. 
           [0019]      FIG. 10  shows a side elevation view of a preferred crown member. 
           [0020]      FIG. 11  shows a perspective view of a preferred collet. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Referring now to  FIG. 2 , a preferred embodiment of the bone screw assembly  10  of the present invention is shown in exploded form. Components of this bone screw assembly  10  embodiment include a bone screw  20 , a collet  30 , a tulip shaped connector  40 , a crown member  50  (see  FIG. 3 ), an optional locking washer  65 , a rod  70  and a set screw  60 . The components of the bone screw assembly  10  can be metallic, such as titanium, titanium alloy or stainless steel, or non-metallic, such as PEEK or other types of plastics, or a combination thereof. 
         [0022]    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. 
         [0023]    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. 
         [0024]    Bone screw  20  and collet  30  can be separate pieces (see,  FIG. 2 ) or one-piece (not shown). As separate pieces, the collet  30  and tulip shaped connector  40  may slide over the bone screw  20  (see,  FIG. 3 ). In another separate piece embodiment, the bone screw  20  may possess a head (not shown) to help it attach to collet  30  and tulip shaped connector  40 . In a one-piece embodiment, the bone screw  20 , collet  30  and tulip shaped connector  40  may be one contiguous piece. 
         [0025]      FIG. 6  illustrates a further alternative embodiment for the shape of the upper shaft  82  of the bone screw  80 . In this embodiment, the upper shaft  82  is tapered so that the diameter at the top of the shaft  84  is greater than the diameter at the bottom of the shaft  86  (i.e., where it meets the threaded shank  87 ). For this embodiment, the collet&#39;s lower end  92  should be formed with a matching reverse taper so that the upper shaft  82  of the bone screw  80  continues to fit comfortably, but snugly, into the central bore  93  of the collet  90 . This alternative tapered embodiment has the benefit of creating a wedge effect which encourages the bone screw  80  to remain firmly attached to the collet  90  after implantation. This retention benefit can also be achieved by placing a small flange, protrusion (e.g., bumps), threads, ridges, grooves, furrows, channels or indentations around the exterior circumference at the top  84  of the bone screw shaft and/or placing a similar flange, protrusion, threads, ridges, grooves, furrows, channels or indentations around the interior circumference at the bottom  94  of the collet  90 . Other gripping techniques may include creating a frictional surface by roughening or knurling the surface of the bone screw shaft  82  or collet  90 . As an additional alternative, rather than fitting around the outside of the bone screw shaft  82 , a central longitudinal bore (not shown) can be made inside the bone screw shaft  82  and the collet  90  can then be formed to adjustably fit inside such a central longitudinal bone screw bore. 
         [0026]    Turning now to  FIGS. 3-4 , a tool receiving recess  33  can be formed at the top of the bone screw shaft  24 . In the embodiments shown in  FIGS. 3-4 , this recess  33  is a hex recess to receive a hex end driving tool. It is understood, though, that the tool receiving recess  33  can have other configurations, such as a TORX® configuration. 
         [0027]    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,  FIG. 4 ) is determined by the collet  30 . A close-up view of this collet  30 , in a preferred form, is shown in  FIG. 11 . In this  FIG. 11  embodiment, the collet  30  has a cylindrically shaped lower end  32  and a generally bulb shaped upper end  34 . Inside the collet, a central bore  31  is made in a size and shape to comfortably, but snugly, fit the bone screw shaft  24 . Like the bone screw  20 , the collet  30  is preferably made of a strong, durable and non-infectious material, such as titanium, titanium alloy or stainless steel. 
         [0028]    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  FIG. 11 , one to four such equidistant slots  36  can be used from beginning at the top of the collet bulb  34  and extending well into the cylindrically shaped lower end  32  of the collet bulb  34  (but not completely to the bottom of that lower end  32 ). Furthermore, these slots  36  may not be contiguous but vary in length from one another. These slots  36  create movable sections  39  of the collet  30 , which are particularly movable at the bulb shaped upper end  34 . When pressure is applied to the outside of these movable sections  39 , they converge inwardly to grab the bone screw shaft  24 . 
         [0029]    As shown in  FIG. 11 , the generally bulb shaped upper end  34  of the collet is preferably flattened at the top. It has been found that making the top of the collet  30  generally bulb shaped is optimum for providing multi-axial angular variations to the position of the bone screw  20  relative to a spinal rod  70  when the bone screw assembly  10  of the present invention is fully assembled as shown in  FIGS. 3-4 . In one embodiment, the collet bulb  34  has a diameter of approximately 0.3 inches. As shown in  FIG. 11 , though, the collet bulb  34  does not form a complete sphere, but is instead preferably flattened at the top. This flattening is done to allow the collet bulb  34  to fit more securely in the tulip shaped connector  40 . 
         [0030]    Referring again to  FIGS. 2-4 , a tulip shaped connector  40  is provided to support both the collet  30  and the spinal rod  70 . A close-up of this tulip shaped connector  40  is provided in  FIGS. 7 and 8 . In the preferred embodiment shown in  FIGS. 7-8 , the tulip shaped connector  40  includes a U-shaped body  41  defining a first branch  42  and a second branch  43 . The branches form a U-shaped channel  44  between each other. The U-shaped channel  44  terminates in a trough bottom  45 . Preferably, the U-shaped channel  44  has a width that is slightly larger than the diameter of the spinal rod  70 . The U-shaped channel  44  has an opening  46  at the top where rod  70 , bone screw  20 , collet  30  and crown member  50  can be inserted. 
         [0031]    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,  FIG. 4 ). In addition to the collet bulb  34 , the central bore  47  also accommodates the crown member  50 . In one embodiment, the exterior of the crown member  50  and the crown member receiving portion of the central bore  47  may be threaded to more securely attach the crown member  50  to the tulip shaped connector  40 . The tulip shaped connector  40  is preferably sized for minimal bulk and minimum prominence above the spine. In one embodiment, the tulip shaped connector  40  has a height of about 0.6 inches. In this embodiment, a rod disposed within a U-shaped channel  46  can sit as low as 0.2 inches above the surface of the vertebra when the tulip shaped connector  40  contacts the bone. 
         [0032]    As illustrated in FIGS. 4 and 5 of Sherman&#39;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 . 
         [0033]    As shown in  FIGS. 3-4 , the bone screw assembly  10  on the present invention preferably includes a crown member  50  that is positioned in the central bore  47  of the tulip shaped connector  40  between the rod  70  and the collet bulb  34 . The purpose of this crown member  50  is to work in conjunction with the locking nut  65  or central bore bottom  48  to securely hold the collet bulb  34  in place and, when the set screw  60  is tightened, to exert pressure to push the sections  39  of the collect bulb  43  against the shaft  24  of the bone screw  20  to securely hold the bone screw  20  in place. 
         [0034]    A close-up view of the crown member  50  is shown in  FIGS. 9-10 . In the preferred embodiment, the crown member  50  is hollow and defines a conical bore  51  at its lower end. As shown in  FIGS. 3-4 , the collet bulb  34  at least partially resides within the conical bore  51  of the crown member  50  when the bone screw assembly  10  is assembled. The crown member  50  further define a tool insertion bore  52  that can be oriented directly over the tool receiving recess  33  of the bone screw  20  when the bone screw  20  is situated within the tulip shaped connector  40 . Crown member  50  may also defines a conical tool relief  57  at the top of the tool insertion bore  52 . This relief is oriented at an angle to permit positioning of a driving tool into the tool receiving recess  33  of the bone screw  20  even when the tulip shaped connector  40  is not directly aligned with the bone screw. In another embodiment, the crown member  50  defines a spherical bore at its lower surface for contacting the collet bulb  34 . 
         [0035]    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  FIGS. 3-4 , the top assembly method can begin by placing the locking nut  65  into the bottom  48  of the central bore recess  47 . The lower end  32  of the collet can then be dropped through the hole  66  in the locking nut  65  until the bottom of the collet bulb  34  contacts the inner periphery  67  of the locking nut. The locking nut  65  should be sized so that its hole  66  is smaller than the diameter of the collet bulb  34 . On the other hand, the hole  66  of the locking nut  65  should be larger than the cross-sectional diameter of the cylindrically shaped lower end  32  of the collet  30  and larger than the bottom  48  of the central bore  47  of the tulip shaped connector  40 . In this way, the locking nut  65  will be held inside the tulip shaped connector  40  and also serve to hold the collet bulb  34  inside the tulip shaped connector  40 , but not the lower end  32  of the collet  30 . As an alternative embodiment, if the diameter of the tulip shaped connector bore bottom  48  is small enough to firmly hold the collet bulb inside the tulip shaped connector, but not the lower end  32  of the collet  30 , the locking nut  65  can be dispensed with in this top assembly method. 
         [0036]    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  FIGS. 3-4 . In order to prevent pieces of the tulip shaped connector  40  from falling out during assembly, the crown member  50  can have peripheral screw threads which mate with threads on the interior wall of the tulip shaped connector bore  47 . By screwing the crown member  50  into the central bore  47 , the locking nut  65  (if needed), collet bulb  34  and crown member  50  are prevented from falling out of the tulip shaped connector  40  while other pieces of the bone screw assembly  10  are being assembled. 
         [0037]    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  FIG. 4 , the downward forces exerted by the set screw  60  are translated into angular forces by the crown member  50  to firmly press the sections  39  of the collet against the bone screw shaft  24  so that the bone screw shaft  24  is firmly held in place at its desired height and angular orientation. 
         [0038]    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. 
         [0039]      FIG. 5  illustrates how a plurality of bone screw assemblies  100 ,  110 ,  120  of the present invention can advantageously be assembled together during spinal surgery. Each of these bone screw assemblies  100 ,  110 ,  120  are connected together by a common rod  70 . In the preferred embodiment, the rod  70  can either be a metal, such as titanium, titanium alloy or stainless steel, or a resilient medical plastic. By comparing the prior art bone screw assemblies shown in  FIG. 1  with the bone screw assemblies of the present invention shown in  FIG. 5 , one can appreciate an important advantage of the present invention. As shown in  FIG. 5 , the interactions of the collet  30  with the bone screw  20  allows the bone screws to be fitted at different heights vis-à-vis the tulip shaped connectors  40 . In particular, the left and right bone screw assemblies  100 ,  120  in  FIG. 5  have the bone screw  20  closer to the tulip shaped connector  40  that the center bone screw assembly  110 . By permitting these variable height screws, the lower surfaces of the set screws  60  and upper surfaces of the crown members  50  make flush contact with the adjacent surfaces of the rod  70 . By contrast, as shown in  FIG. 1 , the prior art bone screw assemblies, which do not allow the bone screws to be at variable heights, can cause gaps to be formed between the adjacent surfaces of the set screw and rod as well as between the adjacent surfaces of the crown member and rod. As previously noted, these gaps can lead to a dangerous loosening of the bone screw assemblies. 
         [0040]    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.