Abstract:
A placement tool for positioning a multipart cannulated screw assembly to fixate first and second bone elements. The placement tool includes a stem and a head carried on the stem. The head has an outer shank engagement provision that engages the proximal end of an outer shank of a bone screw assembly and permits selective rotation of the outer shank. The head also has an inner shank engagement provision that engages the proximal end of an inner shank of the bone screw assembly and permits selective rotation of the inner shank. The inner shank and outer shank engagement provisions are connectable to one another such that the inner shank and outer shank engagement provisions rotate together.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of co-pending U.S. patent application Ser. No. 12/383,769, filed Mar. 25, 2009, which claims priority to and the benefit of U.S. Provisional Patent Application No. 61/070,795, filed Mar. 25, 2008, which applications are incorporated herein by reference in their entirety. 
     
    
     FIELD 
       [0002]    The present invention relates to a screw for the fixation of the facet joints of the human spine. More particularly, the invention relates to a device that is designed for use in the lower thoracic and lumbar spine, but may have wider application in general orthopedic uses including fracture fixation and implant fixation. 
       BACKGROUND 
       [0003]    The facet joints, or zygapophyseal joints, of the spine are located at two symmetrical locations at the posterior of the vertebral column. Each facet joint consists of two overlapping bony protrusions, the superior articular process of one vertebrae and the inferior articular process of the neighboring vertebrae.  FIG. 18  illustrates two vertebrae as they mate at the facet or zygapophyseal joints. 
         [0004]    In certain cases of degeneration of the spinal disk, instability of vertebral segments, arthritis of the facet joint, or trauma, partial or complete immobilization of one or more facet joints is desirable. Traditionally, immobilization is accomplished by anchoring orthopedic hardware into the vertebral bodies of adjacent segments, often through the pedicle, and interposing a plate or rod between the vertebrae to limit motion. Additionally, interbody devices are often placed into the disc space through a variety of techniques to further limit motion and promote bony fusion between adjacent vertebrae. However, for a number of reasons, it would be advantageous to eliminate motion and improve stability between two or more vertebrae by directly fastening one or both of the facet joints together. From a surgical perspective, the facet joint is easily accessible, thus reducing operative time, decreasing blood loss, decreasing incision size, reducing incidence of reoperation, and decreasing the risk of potential deleterious effects on nearby anatomic structures, including spinal nerve roots and the spinal cord itself. Further, fixation at the facet joints is more biomechanically desirable because the center of rotation of the lumbar spine for flexion and extension is located nearest to the facet joints. Thus, placing an immobilization device at or through the facet joint decreases the torque transmitted through the device, which in turn may prevent loosening or premature device failure. 
         [0005]    In order to provide effective fixation of the facet joint, a few challenges must be overcome. Most importantly, a fixation device must create compression between the two articular processes. The compression, which causes or enhances immobilization of the joint by encouraging stability throughout the joint, must be maintained for a significant period of time. Additionally, loosening of the device must be prevented. Because the facet joint is generally a mobile joint, forces will continue to be transmitted through the joint after the insertion of an immobilization device. Without a specific way to prevent loosening of the device, loosening will likely occur as the result of micromotion. Once a device has loosened, the device will often begin to protrude from or back-out of the bone, causing significant discomfort, damage to the joint, or danger to surrounding tissues. 
         [0006]    Other devices, such as various types of bone screws, have been offered as ways to fasten the facet joints together. However, each previously proposed fixation device has suffered significant shortcomings. For example, a standard fully threaded bone screw may be sufficient for merely adjoining two surfaces. However, a fully threaded screw is not capable of creating any significant amount of compression between two bone surfaces. Any compression generated between surfaces is limited to the compressive forces generated by the screw threads themselves. Further, there is currently no way to effectively prevent a bone screw from loosening over time. When a screw is over-tightened and threads are stripped within the bone, or when threads strip over time as a result of micromotion, the compressive force between the facet joint surfaces will be lost and loosening will likely occur. 
         [0007]    To prevent loosening, still other bone screws are designed such that a portion of the screw expands within the bone after the device is implanted. However, the hoop stress generated by expansion of the device within a bone makes this device ill-suited for use in the relatively small bones of the facet joint. 
         [0008]    In attempt to create compression and prevent loosening, nut-and-bolt type assemblies have been offered as another method for immobilization of the facet joint. Using this type of assembly, a screw or “bolt” is passed through the facet joint and a nut with mating threads is placed around the screw on the back side of the facet. This approach is successful in creating compression and likely at maintaining the compression over time. However, because the nut must be introduced to the back side of the facet joint, this approach mandates a procedure that is significantly more invasive than is otherwise required. 
         [0009]    Finally, many devices currently available for fixation of the facet joint do not contain a central hollow and therefore are not equipped for use with a guide wire, as is known in the art of orthopedic devices. Without a guide wire, placement of the device within the bones is less efficient and accuracy is more difficult. Further, small devices, such as bone screws, can be dropped and even lost within the soft tissue surrounding the site of insertion. 
         [0010]    Because of the shortcomings associated with the currently available facet immobilization devices, physicians have largely been hesitant to attempt immobilization of the facet joint, despite the significant biomechanical and surgical benefits of doing so. 
         [0011]    As such, there is a considerable need for a facet fixation device that can be easily and effectively inserted through a small incision and extend through the inferior and superior articular processes in order to create active compression across the facet joint and limit loosening over time. 
       SUMMARY 
       [0012]    Described below is a cannulated and partially threaded bone screw assembly for the fixation of a spinal facet joint. The assembly is inserted through a very small incision near the facet joint, and has a feature that provides for the expansion of the portion of the device protruding through the superior facet of the inferior vertebrae. The expansion feature, along with the partially threaded screw shaft, serve to increase compression between the facet joint surfaces and also prevent loosening of the device over time. 
         [0013]    In one preferred embodiment the device comprises three parts which can move with respect to one another. The three parts are an outer screw shank, an inner screw shank, and a tip. The inner shank is disposed within the outer shank. The external surface of the outer shank is partially threaded at the distal end. The external surface of the inner shank is partially threaded at the distal end, such that the distal end of the inner shank secures the tip to the screw assembly. The external surface of the tip is fully threaded. Additionally, the internal surface of the tip is threaded at the proximal end such that the inner shank can screw into the tip. Once the outer shank is positioned such that it extends through the bones, the outer shank is held stationary and the inner shank is turned. The tip is drawn into the outer shank via the threads on the inner shank which causes the outer shank to deform. The deformation caused by the tip serves to increase the size of the outer shank such that it becomes larger in diameter than the hole through which it was inserted. The deformation increases the compressive force across the facet joints and also prevents the screw from loosening or backing out of the hole through which it was inserted. 
         [0014]    In another preferred embodiment the inner screw shank and the tip are a unitary piece. Similarly, the inner shank and the tip are disposed within the outer shank. The external surface of the outer shank is partially threaded at the distal end. Additionally, the internal surface of the outer shank is partially threaded at the proximal end. The external surface of the inner shank is partially threaded at the proximal end. The external surface of the tip is fully threaded. Once the assembly is appropriately positioned through the bones, the outer shank is held stationary and the inner shank is turned with respect to the outer shank. Due to the mating of the threads on the internal surface of the outer shank and the external surface of the inner shank, the inner shank and the tip are drawn up into the outer shank, causing the distal end of the outer shank to deform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of the cannulated screw assembly, according to the first embodiment of the invention. 
           [0016]      FIG. 2  is a perspective view of the distal end of the cannulated screw assembly, according to the first embodiment of the present invention. 
           [0017]      FIG. 3  is a perspective view of the cannulated screw assembly, according to the first embodiment of the present invention, wherein the outer shank and tip are shown in solid lines and the inner shank is shown using hidden lines. 
           [0018]      FIG. 4  is a fragmentary side view of the distal end of the cannulated screw assembly, according to the first embodiment of the invention, wherein the outer shank and tip are shown in solid lines and the inner shank is shown using hidden lines. 
           [0019]      FIG. 5  is a perspective view from the distal end of the outer shank, according to the first embodiment of the invention. 
           [0020]      FIG. 6  is a perspective view from the proximal end of the outer shank, according to the first embodiment of the present invention. 
           [0021]      FIG. 7  is a perspective view of the inner shank, according to the first embodiment of the present invention. 
           [0022]      FIG. 8  is a perspective view of the tip, according to the first embodiment of the present invention. 
           [0023]      FIG. 9  is a further perspective view of the tip, according to the first embodiment of the present invention. 
           [0024]      FIG. 10  is a perspective view of the cannulated screw assembly, according to a different arrangement of the present invention. 
           [0025]      FIG. 11  is a cross-sectional view of the cannulated screw assembly, according to the first embodiment of the invention. 
           [0026]      FIG. 12  is a perspective view of a cannulated screw assembly, according to a second embodiment of the present invention. 
           [0027]      FIG. 13  is a perspective view of a cannulated screw assembly, according to the second embodiment of the present invention, wherein the outer shank and the tip are shown using solid lines and the inner shank is shown using broken lines. 
           [0028]      FIG. 14  is a side view of the outer shank, according to the second embodiment of the present invention. 
           [0029]      FIG. 15  is a perspective view of the inner shank and the tip, according to the second embodiment of the present invention. 
           [0030]      FIG. 16  is a cross-sectional view of the cannulated screw assembly, according to the second embodiment of the present invention. 
           [0031]      FIG. 17  is a fragmentary perspective view of a tool used to insert the cannulated screw assembly into bone. 
           [0032]      FIG. 18  is a perspective view of a cannulated screw assembly disposed within a facet joint, according to an embodiment of the present invention. 
       
    
    
       [0033]    It should be appreciated that the above description is not meant to limit the shape of any interface surface and is presented by way of example only. For example, a hex-shaped surface of the screw and insertion tool could be modified to a Torx shape, square shape, or any other shape that provides an interference fit adequate to rotate the screw or a portion of the screw assembly. Similarly, threads may vary in length or be uniform or non-uniform in pitch, and are not limited by their depiction in the drawings. 
         [0034]    The following Detailed Description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background or Summary of the Invention or the following Detailed Description of the invention. Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. The same reference numbers are used throughout the drawings to refer to the same or like parts. 
       DETAILED DESCRIPTION 
       [0035]    Referring generally to  FIGS. 1 ,  2 ,  3 ,  4  and  11 , there is illustrated a first embodiment of a cannulated screw assembly  30 . The cannulated screw assembly  30  is composed of an outer shank  40 , an inner shank  50 , and a tip  60 . The tip  60  and a distal portion of the shaft  41  are threaded to such that the device may be inserted into and through two bones. It should be generally understood that while in the following description of a preferred embodiment the screw assembly  30  is described as inserted through two bones, the device may alternatively be inserted into and through two fragments of the same bone or an implantable device and a bone. 
         [0036]    As shown in  FIG. 5 , the outer shank  40  comprises a head  43 , an unthreaded portion  42  proximal to the head  43 , and the threaded portion  41  at the distal end of the outer shank  40 . The outer shank  40  further includes one or more slits  45  such that the tip  60  can move into the distal end of the outer shank  40  and cause outward deformation at the distal end. The outer shank  40  further includes one or more notches  44  (shown in  FIG. 5 ) for interaction with one or more projections  61  of the tip  60  (best seen in  FIG. 8 ) such that the tip  60  cannot rotate with respect to the outer shank  40 . Additionally, the outer shank  40  defines a chamfered edge  47 , as shown in  FIG. 5  and discussed below. As shown in  FIG. 7 , the outer shank  40  is hollow such that the inner shank  50  can be disposed within the outer shank  40 . 
         [0037]    When the screw assembly  30  is fully situated into and through two bones, the unthreaded portion  42  of the screw assembly  30  is disposed within the first bone (the bone most proximal to head  43 ). The threaded portion  41  is disposed within and threaded into the second bone. This arrangement provides for active compression between the bone surfaces. The length of the screw assembly  30  and the relative length of the threaded portion of the shank  41  may vary according to the variations in the anatomy of patients. 
         [0038]    Referring now to  FIG. 7 , the inner shank  50  comprises a head  53 , a threaded portion  51  at the distal end of the inner shank  50 , and an unthreaded portion  52  proximate to the head  53 . The inner shank  50  defines an interior cannulated portion  54  through which a guide wire may pass during placement of the assembly  30  within one or more bones. 
         [0039]    As shown in  FIG. 8 , the tip  60  of the cannulated screw assembly  30  is fully threaded  63 . The tip  60  also includes a chamfered edge  62 , and a self-drilling groove  64 . The tip  60  includes the one or more projections  61  for interaction with the one or more notches  44 , such that the tip  60  does not rotate with respect to the outer shank  40 . The chamfered edge  62  of tip  60  meets the mutually chamfered edge  47  of the outer shank  40  where they come together at seam  67  ( FIG. 4 ). The tip  60  further defines a cannulated center  65  for passage of a guide wire during implantation of the device. In a preferred embodiment, the tip  60  is self-drilling and self-tapping, as known in the art. 
         [0040]    According to the first embodiment of the invention, and as shown in  FIG. 9 , the tip  60  of the cannulated screw assembly  30  also contains internal threads  66  at its proximal end to receive the threaded portion  51  at the distal end of the inner shank  50 . When the inner shank  50  is rotated with respect to the outer shank  40  the mating of threads of  51  and  66  create a force that will cause the tip  60  to be drawn up into the distal end of the outer shank  40 . To facilitate the movement of the tip  60  into the distal end of the outer shank  40 , mutually chamfered edges,  62  and  47 , create a wedge force at the seam  67  as the tip  60  is drawn up by the mating of the threads  51  and  66 . Additionally, the interaction of one or more projections  61  and one or more notches  44  prevents the tip  60  from rotating with respect to the outer shank  40 , thus ensuring that the force between mating threads  51  and  66  serves to move the tip  60  into the outer shank  40 . 
         [0041]    According to another embodiment, as shown in  FIG. 4 , a seating surface  46  within the head  43  of the outer shank  40  prevents the head  53  of the inner shank  50  from migrating into the outer shank  40  when a compressive force is created by turning the inner shank  50  as to move the tip  60  into the outer shank  40 . Once the tip  60  begins to move into the outer shank  40  the distal end of the outer shank  40  will deform and become larger in diameter. To facilitate this enlargement, one or more slits  45  are created on the outer shank  40 . 
         [0042]    Referring next to  FIG. 10  there is illustrated a further embodiment of the cannulated screw assembly  30 . In this embodiment, the head portion  43  of the screw assembly  30  includes a collar  75 . The collar  75  further defines a securing surface  76 . The securing surface  76  may consist of any surface texture, such as smooth, rough, or ridged (as shown in  FIG. 10 ), selected to obtain the desired amount of friction. The collar  75  is desirable in certain applications in that it provides the surface  76  which can be brought into contact with an external bone surface (not shown), in order to distribute forces and loads across the bone. Collars  75  of varying dimension can be used. In addition, if desired, an ancillary structure such as a washer (not shown) can also be interposed between the collar  75  and the bone. A washer can additionally distribute loads across a wider surface of the bone. 
         [0043]    According to a second embodiment of the present invention, as shown in  FIGS. 12 through 16 , the inner shank  50  and the tip  60  are one unitary piece. The head  43  at the proximal end of the outer shank  40  comprises an internal threading  49 . As discussed with respect to the first embodiment, the shaft of the outer shank  40  further comprises the threaded portion  41 , and the unthreaded portion  42 . Still according to the second embodiment, and as shown in  FIGS. 13 and 15 , the inner shank  50  includes the unthreaded portion  52 , the tip  60 , and an additional external threaded portion  55  located on the external surface of the proximal end of the inner shank  50 . The tip  60  comprises the outer threads  63 , the self-drilling groove  64 , and the cannulated center  65 . 
         [0044]    According to the above described second embodiment, rotation of the inner shank  50  and tip  60  with respect to the outer shank  40  causes the internal threading  49  at the proximal end of the outer shank  40  and the external threading  55  on the proximal end of the inner shank  50  to engage as shown  FIG. 16 . The mating of the threads  49  and  55  creates a force that will cause the inner shank  50  and the tip  60  to be drawn up into the distal end of the outer shank  40 . 
         [0045]    The outer shank  40  of the second embodiment, like the outer shank  40  of the first embodiment, includes one or more slits  45  to facilitate deformation, and mutually chamfered tip and outer shank surfaces,  47  and  62 , which meet at the interface  67  to further facilitate deformation. And similarly, the force associated with drawing the tip  60  up causes the distal end of the outer shank  40  to deform. The embodiment may further include the previously discussed collar  75  with the securing surface  76  for distributing forces and loads across the surface of a bone. 
         [0046]    The foregoing embodiments are preferably comprised of surgical stainless steel, titanium, cobalt-chronium alloy, or any other biocompatible material as is known in the art of medical device manufacture. The cannulated screw assembly  30  may be treated with an adherent layer of hydroxyapetite, calcium phosphate, or other osteoinductive coatings as known in the art. Further, the screw assembly  30  may be treated with growth factors, stem cells, or any other device coating known in the art, to be selected based on the desired outcome of the procedure. 
         [0047]    Referring now to  FIG. 17 , a custom tool  70  may be used to insert the cannulated screw assembly  30  into and through the bones. The tool  70  consists of a male driver device  72 , which serves as a provision for engaging and mating with the inner shank, situated within a female driver device  73 , which serves as a provision for engaging and mating with the outer shank. In this case they are portrayed as a male and female hexagonal-shaped driver, but other shapes are viable options. The tool  70  is cannulated  71  to allow a guide wire, such as a KIRSCHNER or “K” wire, to pass through. The tool  70  will initially ensure that the inner and outer screw shanks,  40  and  50 , rotate at the same rate when inserting the screw assembly  30  into and through the bones. This will prevent premature and undesired rotation of the inner shank  50  with respect to the outer shank  40  as the outer shank  40  is placed into bone. Turning the inner shank  50  prior to final placement is undesirable because it will cause expansion of the outer shank  40 . Once the screw assembly  30  is in the desired orientation, the tool  70  having the male driver device  72  can be used to rotate the inner shank  50  with respect to the outer shank  40  and thereby cause the desired deformation of the outer shank  40 . 
         [0048]    The male driver device  72  and the female driver device  73  are preferably comprised within a single tool  70  including an additional locking mechanism (shown as a simple set screw  74 ) such that the user has the capacity to lock and unlock the ability of the male driver portion  72  to rotate with respect to the outer female driver portion  73 . The above described tool is desirable because (i) it ensures that while the outer shank  40  is being deformed by the motion of the inner shank  50  and tip  60 , the outer shank  40  does not turn within the bone, and (ii) it eliminates the added step of changing tools during insertion of the screw assembly  30 . Other equivalent locking and unlocking mechanisms for this purpose will be apparent to those skilled in the art. For example, the male driver portion  72  and the female driver portion  73  may be mounted on coaxial cannulated stems relatively axially moveable to a small degree to engage teeth or other interengageable blocking parts interfitting between portions  72  and  73  to prohibit and permit relative rotation with respect to each other. Less desirable, but still within the inventive concept, two different tools may be used—one in which the portions  72  and  73  are fixed with respect to each other for placing the screw assembly  30 , and one in which the portions  72  and  73  are rotatable with respect to each other to allow the inner shank to be rotated with respect to the outer shank in order to expand the tip, once the screw assembly is in place. In another alternative and less desirable embodiment, the male driver portion  72  and the female driver portion  73  may be two separate tools. 
         [0049]    The cannulated screw assembly  30  can be inserted into and through two bones as follows. First, a guide wire, such as a small diameter “K” wire, is inserted into and through the two bones, generally using fluoroscopic imaging, as is known in the art. Second, the cannulated screw assembly  30  is placed over the guide wire. Alternatively, the guide wire may be placed through the screw assembly  30  before the wire is threaded into and through the bones. With the custom tool  70 , using the self-drilling and self-tapping capacity if necessary, the screw assembly  30  is inserted into and through the bones until the distal tip  60  of the screw assembly  30  is protruding through the second bone. Finally, the inner shaft  50  is rotated with respect to the outer shank  40  using the custom tool  70 , such that the tip  60  is moved up into the outer shank  40  causing deformation of the distal end of the outer shank  40 . The deformation of the outer shank  40  causes the distal end of the outer shank  40  to permanently expand in diameter outside of and adjacent to the external surface of the more distal bone, thus providing additional compression between the bones and limiting or preventing loosening over time. 
         [0050]    Referring again to  FIG. 18 , it is noted that neighboring vertebrae come into proximity at the facet or zygapophyseal joint  84 . The facet joint  84  includes a superior articular process  82  of one vertebra and an inferior articular process  83  of a second vertebra. One manner of connecting the vertebrae through the zygapophyseal joint using the cannulated screw assembly  30  is illustrated. In the illustrated embodiment, the screw assembly  30  links two proximate vertebrae. The screw assembly  30  generally extends beyond the external surface of each of the bone protrusions that are linked by the assembly  30 . In the illustrated embodiment, the screw assembly  30  is positioned with the head situated in a generally medial orientation. Alternatively, a screw assembly can be positioned with a generally lateral configuration. In other embodiments, the screw assembly  30  can have a different position and need not protrude from the bone to the extent illustrated. It is also to be appreciated that other sections of the same two vertebrae can also be joined. For example, the second facet joint  84 , which is positioned opposite the first facet joint  84  may be joined by a second screw assembly  30 . In many circumstances it may be preferable to use the cannulated screw assembly  30  in connection with an interbody device (not shown) in order to achieve maximum immobilization of the spinal section. 
         [0051]    The above described embodiments provide significant advantages over the devices found in the prior art. Specifically, the deformation of the distal end of the outer shank  40  outside and adjacent to the external surface of the bone provides enhanced compression of the bones and prevents the screw assembly  30  from loosening within the bone, preventing discomfort and damage to the joint or surrounding tissues. The partially threaded shaft  41  of the screw assembly  30  further aides in creating compression and maintaining the compression over time. The cannula that extends throughout the center of the device allows for use of a guide wire, enabling effective and efficient placement of the device. As such, the screw assembly  30  is useful in a variety of applications, including fixation of the facet joint, orthopedic fracture fixation, and anchoring an implantable orthopedic device to bone. 
         [0052]    While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to a particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the general description.