Abstract:
A left-handed bone screw for use in orthopedic spinal surgical procedures. The left-handed bone screw reverses the pitch angle of a conventional right-handed bone screw. In reversing the thread, the present invention permits the left-handed screw to reuse a previously tapped cortical and cancellous bone hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/536,501, filed on Sep. 19, 2011, the disclosure of which is incorporated herein by reference in its entirety for all purposes. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to bone screws used in orthopedic spinal surgical procedures. Specifically, the invention is directed to a left-handed bone screw. 
       BACKGROUND OF THE INVENTION 
       [0003]    Several techniques and systems have been developed for correcting and stabilizing the spine and facilitating spinal fusion. Over the years, spinal and orthopedic implants have evolved toward progressively stronger and stiffer devices, since it is presumed that increased construct rigidity optimizes the bone fusion and provides more rapid and robust healing. The most widely used systems use a bendable rod that is placed longitudinally along the length of the spine. In such a procedure, a rod is attached to various vertebrae along the length of the spinal column by a number of bone anchor assemblies. A bone anchor element may be a hook that engages the vertebra laminae or a bone screw threaded into the vertebral bone. 
         [0004]    In present day bone screw assemblies, rods are typically situated on opposite sides of the spine or spinous processes. Numerous bone screws are screwed into the pedicles of the vertebral bodies. Rods are then affixed to these bone screws through various connectors so corrective and stabilizing forces are applied to the spine. When stabilized, the vertebra is decortified where the outer cortical bone is removed to provide a foundation for bone grafts. Over time, these bone grafts fuse the damaged vertebrae together. 
         [0005]    Bone screws are well known in the art. The threads, for example, of a bone screw anchor it to the bone and keep the screw from being axially pulled out of the bone. Such threads also cut a helical path into the bone as the screw rotates into the bone. 
         [0006]    As shown in  FIG. 1 , a right-handed bone screw  2  has male threads  4 , while its matching nut or substrate possess corresponding female threads. Almost all threads are so oriented that when a matching nut is turned clockwise, the nut moves down the screw. This is known as right-handedness. When turned counter-clockwise, the nut moves up the screw; this is known as left-handedness. Right-handedness is the default for bone screw threads. Returning to  FIG. 1 , a screw consists of a major diameter  6  and a minor diameter  8 . The major diameter  6  is the large diameter of the thread. The minor diameter  8  is the small diameter of the thread. Pitch  12  is the distance from the crest  14  of one thread to the next. The root  16  of the male thread  4  touches the inside diameter  8 . Screw threads are almost never made perfectly sharp but instead are truncated. This truncation is known as the thread depth  18 . Threads are strongest when they are truncated about 60% so that the increased material thickness is achieved at the crest  14  of the screw. The angle of the cross-sectional shape is called the pitch angle  20 . 
         [0007]    There are many aspects of thread shape in the art. The thread can have a sharp or blunt apex. The pitch of the thread can be varied. The height of the thread can be deep or shallow. The thickness of the thread can change. In all screws, however, threads have a superior surface facing the screw head and an inferior surface facing the screw tip. The engagement of the superior surface with the bone provides resistance to screw pull-out. The screw thread can be further modified by varying the cross-sectional shape of the thread from the tip to the head of a screw. In most screws, the thickness of the thread near the tip has a narrow cross-section to cut into the bone as the screw rotates into the bone. The thread and core become thicker toward the head of the screw to increase thread strength and to displace bone matter downward against the superior thread surface. This is particularly advantageous in bones because of the hard cortical bone shell. The cortical bone is harder and more compact than the spongy cancellous bone in the bone center. The cortical bone provides the bulk of the bone&#39;s resistance to screw pull-out forces (axial load forces). The thread and core near the screw head engages the cortical bone and, thus, carry much of axial load on the screw. 
         [0008]    A common problem among all bone screw assemblies is bone screw pull-out. When bone screws are weakened or begin to pullout, bone anchor or screw assemblies may slip. When the bone is strong and healthy, the initial fixation of the spinal and orthopedic screw is usually excellent, with more than adequate pull-out strength. With dense, sclerotic or osteoporotic bone, micro-motions resulting from the normal range of motion within the skeletal system may lead to a progressive degradation from the initially implanted state. In cases where the bone fails to heal, these micro-motions persist and cause the metallic screw to oscillate within the softer cancellous bone. When subjected to persistent toggling with the modulus mismatch of the metal to cortico-cancellous bone, the bone screw becomes loose. 
         [0009]    During bone screw pullout, dislodgement and breakage occur because of extreme load, shear, stress and/or torsion. When bone screw failure occurs, it not only weakens the mechanical strength of the bone screw assembly but also lowers the biological potential for bone healing. When bone screws loosen, removal may be necessary because the earlier threads made in the bone by the bone screw are no longer usable. Given the importance of screw threads, it is surprising that there is no or little reference known on the use of right-handedness or left-handedness bone screws. 
         [0010]    In summary, there is a need in the industry for improvements in bone screw and bone screw thread design. The present invention describes such an improvement. 
       BRIEF SUMMARY OF THE INVENTIONS 
       [0011]    The present invention provides an improvement to bone screws, such as vertebral pedicle bone screws, used in orthopedic spinal surgical procedures. Specifically, the present invention reverses the pitch angle to left-handedness from right-handedness. 
         [0012]    When bone screw failure necessitates bone screw removal, the reverse thread of the present invention permits the screw to return through the previously tapped cortical and cancellous bone substrate and, more importantly, cuts and taps through the new bone providing a new bone screw with added strength and integrity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a side view of a prior art right-handed screw. 
           [0014]      FIG. 2  is a side view of left-handed screw of the present invention. 
           [0015]      FIG. 3  shows a close-up side view of the left-handed bone screw. 
           [0016]      FIG. 4  is an example of the reverse thread of the present invention threaded through new bone substrate that was tapped earlier by a right-handed screw. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The present invention is shown in  FIGS. 2 ,  3  and  4 . The male threads  4  shown in  FIGS. 1 &amp; 2  are shifted from a negative slope on the vertical (x-y axis) in  FIG. 1  to a positive slope on the vertical (x-y-axis) in  FIG. 2 . In other words, the pitch angle  20  of the male threads  4  in  FIG. 1  is simply reversed to left-handedness from right-handedness in  FIG. 2 . 
         [0018]    When the male threads  4  are reversed, the pitch angle  20  is shifted either one thread up or down. As the male threads  4  are reversed, the male threads  4  are shifted into new bone substrate  22 . This is illustrated in  FIG. 3 . When the thread is reversed, the crest  14  and, perhaps, a portion of the root  16  is now surrounded by new bone substrate  22 . When the male threads  4  are reversed, a void  24  left over from the previous position of the female thread  26  is also created. 
         [0019]    As shown in  FIG. 4 , the tip  28  of the reverse thread  30  passes through the hole  32  left by the previous right-handed bone screw  2 . The tip  28  passes through the cortical bone  34  without cracking or damaging the bone. The reverse thread  30  of the present invention displaces the cancellous bone  36  by cutting and pushing new cancellous bone  36  downward. The leading edge  38  of the left-handed bone screw  40  cuts a new passageway in the bone for the reverse thread  30 . 
         [0020]    As the left-handed bone screw  40  is rotated downward into the bone, the core thickness of the left-handed bone screw  40  increases. This core continually pushed cancellous bone  36  radiallly outward and, thus, slightly compresses the cancellous bone  36  in between the reverse threads  30 . The cancellous bone  36  immediately below the inferior surface is displaced downward against the superior reverse thread  30  compacting the cancellous bone  36  between the reverse threads  30  and against the left-handed bone screw  40 . As it does so, it fills in the void  24  left previously by the right-handed bone screw  2 . The new cancellous bone  36  in the void  24  now has an opportunity to heal and strengthen the void  24 . New cancellous bone  36  may even replace and fill in the void  24  left in the cortical bone  34 , thereby, also strengthening it. This compression of the cancellous bone  36  between the reverse threads  30  also orients the bone to oppose screw pull-out. The compaction and orientation of the new bone improves the support provided by the cancellous bone  36 , thereby increasing the pull-out resistance of the reverse thread  30  and the left-handed bone screw  40 . 
         [0021]    The left-handed bone screw  40  and its reverse thread  30  described herein can be customized for particular bone applications. For example, the taper, diameter and cross-sectional shape of the bone screw can be modified on the left-handed bone screw  40  just as the right-handed bone screw  2 . The reverse thread  30  may also possess different pitch, height and thickness as other right-handed bone screws  2 . 
         [0022]    With the present invention, a surgeon can now use the previous right-handed bone screw hole with a left-handed screw having similar or the same size bone screw and similar pitch. The present invention avoids the use of a larger size right-handed bone screw that, in most cases, will further crack and damage the bone. The present left-handed invention also provides the new bone screw with increased strength and integrity. Furthermore, it increases the mechanical strength of the bone screw assembly and may also increase the biological potential for bone healing. 
         [0023]    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. 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.