Abstract:
The present invention provides a bone screw which can be used for fixation and/or fastening of prosthetic devices or instruments to bone tissue whose structure or dimensions differ from one region to another. In particular, the present invention provides a bone screw which is designed to optimize purchase in both the cancellous and cortical regions of a vertebral body. In an exemplary embodiment, the bone screw has a distal portion and a proximal portion in which the diameter of the thread on the proximal portion of the screw is greater than the diameter of the thread on the distal portion of the screw.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of orthopedic surgery and more specifically to a bone screw for orthopedic use. 
       BACKGROUND OF THE INVENTION 
       [0002]    As is known in the field of orthopedic surgery, and more specifically spinal surgery, bone screws may be used for fixation or for the fastening of prosthetic devices or instruments to bone tissue. An exemplary use of bone screws may include using bone screws to fasten a prosthetic device, such as a bone plate or a spinal spacer, to a vertebral body for the treatment of a defect in a patient&#39;s spine, such as a fracture within a vertebral body or a degenerating intervertebral disc. Focusing on the bone plate example, bone screws can be used to fasten anchors to a number of vertebral bodies and a bone plate can then be connected to the vertebral bodies using the anchors to fuse a segment of the spine. In another exemplary use, bone screws can be used to fix the location of a spinal spacer once the spacer is implanted between adjacent vertebral bodies. 
         [0003]    The bone tissue that comprises the vertebral body, in terms of mechanical characteristics, can be divided into two distinct regions, namely, cancellous bone tissue, which is characterized by voids and a low density, and cortical bone tissue, which is a higher density, stronger bone region. Since the cortical bone tissue region is stronger than the cancellous bone tissue, the cortical bone tissue is better able to support a secure connection for screw fixation than the cancellous bone tissue. 
         [0004]    As such, there exists a need for a bone screw that is able to optimally purchase bone tissue where the mechanical characteristics of the bone tissue vary from one region to another to improve fixation and/or fastening of prosthetic devices or instruments to bone tissue. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a bone screw which can be used for fixation and/or fastening of prosthetic devices or instruments to bone tissue whose structure or dimensions differ from one region to another. In particular, the present invention provides a bone screw which is designed to optimize purchase in both the cancellous and cortical regions of the vertebral body. In an exemplary embodiment, the bone screw has a distal portion and a proximal portion, each portion having an approximately constant diameter over a portion of its length, in which the diameter of the thread on the proximal portion of the screw is greater than the diameter of the thread on the distal portion of the screw. 
         [0006]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred or exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0008]      FIG. 1  is a perspective view of an exemplary embodiment of the bone screw according to the present invention; 
           [0009]      FIG. 2  is a side perspective view of the bone screw shown in  FIG. 1 ; 
           [0010]      FIG. 3  is an enlarged partial cross-sectional view of the bone screw shown in  FIG. 1 ; and 
           [0011]      FIG. 4  is an enlarged partial cross-sectional second side view of the bone screw shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0013]    With reference to  FIGS. 1 and 2 , a preferred embodiment of a bone screw  10  according to the present invention is illustrated. The bone screw  10  preferably includes, concentric to a longitudinal axis  12 , a head portion  14 , a neck portion  18  and a shank portion  16 . The head portion  14  connects to the shank portion  16  through the neck portion  18 . The bone screw  10  is preferably constructed from any biocompatible material including, but not limited to, stainless steel alloys, titanium, titanium based alloys, or polymeric materials. 
         [0014]    In a preferred embodiment, the head portion  14  of bone screw  10  has a generally spherical shape and includes a recess  20  for receiving a driving instrument. As is well known in the art, the recess  20  may be configured and dimensioned to any shape that corresponds with the end of the driving instrument designed to engage the bone screw  10 . For example, the recess  20  may be any one of the following shapes: slot, cross, polygon, or multi-lobes. The generally spherical shape of the head portion  14  is configured and dimensioned to be received within a correspondingly shaped cavity in a receiving member (not shown) which may be part of a spinal fixation system. The shape of the head portion  14  allows the bone screw  10  to pivot, rotate and/or move with respect to the receiving member. In an exemplary use, the head portion  14  of the bone screw  10  is received in the cavity of the receiving member and the bone screw  10  is pivoted, rotated or moved until the desired orientation with respect to the receiving member is met. The bone screw  10  is then locked in place in the cavity of the receiving member. In a further preferred embodiment, the head portion  14  also includes texturing  22  that extends along at least a portion of the head portion  14 . The texturing  22  on the head portion  14  provides additional frictional surfaces which aid in locking the bone screw  10  in place with respect to the receiving member. 
         [0015]    With continued reference to  FIGS. 1 and 2 , in a preferred embodiment, the neck portion  18  of the bone screw  10  integrally connects the head portion  14  with the shank portion  16 . Preferably, the neck portion  18  includes a generally cylindrical region  24  and a truncated generally frustoconical region  26 . The diameter of the distal end  23  of the frustoconical region  26  is preferably dimensioned to match a major diameter (discussed below) of the bone screw  10  while the diameter of the proximal end  25  of the frustoconical region  26  is preferably dimensioned to match the diameter of the generally cylindrical region  24  of the neck portion  18 . In a preferred embodiment, the generally cylindrical region  24  will have a diameter that is at least as large as a minor diameter (discussed below) of the bone screw  10 , but the diameter of the generally cylindrical region  24  can be smaller than the minor diameter of the bone screw  10 . By having the diameter of the neck portion  18  dimensioned at least as large as a minor diameter of the bone screw  10 , the overall rigidity and strength of the bone screw  10  is increased. 
         [0016]    Turning to  FIGS. 1-4 , in a preferred embodiment, the shank portion  16  of the bone screw  10  includes a shaft  28 , having a length L, surrounded at least in part by a plurality of thread portions  30 ,  32 . The diameter of the shaft  28  is the minor diameter of the bone screw  10  and the diameter of the shaft  28  including the thread portions  30 ,  32  is the major diameter of the screw  10 . In a preferred embodiment, the diameter of the shaft  28  remains generally constant from the proximal end  27  toward the distal end  29  of the shaft  28 . However, the diameter of the distal end  29  of the shaft  28  preferably decreases towards the distal tip  34  of the bone screw  10 . The constant diameter of a majority portion of the shaft  28  allows for optimal screw positioning when the bone screw is inserted into a predetermined area in the bone tissue. The constant diameter also allows for varying the depth positioning of the bone screw in the bone. For example, if a surgeon places the bone screw  10  into bone tissue at a first depth and decides the placement is more optimal at a second, shallower depth, the bone screw  10  can be backed out to the second depth and still remain fixed in the bone. In another embodiment, the diameter of the shaft  28  may vary along its length, including increasing in diameter from the proximal end to the distal end or decreasing in diameter from the proximal end to the distal end. 
         [0017]    Looking at  FIGS. 1-2 , the plurality of threads  30 ,  32  surrounding the shaft  28  extend, in a preferred embodiment, from the distal tip  34  of the shaft  28  to the distal end  23  of the frustoconcial region  26  of the neck portion  18 . In another preferred embodiment, the threads  30 ,  32  may extend along only a portion of shaft  28 . As seen in  FIGS. 1-2 , the thread portions  30 ,  32  are preferably a Modified Buttress thread but the threads can be any other type of threading that is anatomically conforming, including, but not limited to Buttress, Acme, Unified, Whitworth and B&amp;S Worm threads. 
         [0018]    In a preferred embodiment, the diameter or depth or height (hereinafter, diameter) of the thread portion  30  remains substantially constant over its length L 1  and the diameter of the thread portion  32  remains substantially constant over a portion of its length L 2 . Preferably, the diameter of the thread portion  32  decreases towards the distal tip  34  of the bone screw  10 . By having a decreased diameter thread portion  32  near the distal tip  34  of the bone screw  10 , the bone screw  10  can be self-starting. In another preferred embodiment, bone screw  10  may also include at least one flute to clear any chips, dust, or debris generated when the bone screw  10  is implanted into bone tissue. 
         [0019]    In a preferred embodiment, the thread portion  30  also differs dimensionally from the thread portion  32 . More specifically, the thread portion  30  preferably has a larger diameter than the thread portion  32 . The diameter of the thread portion is determined by subtracting the minor diameter from the major diameter of the bone screw  10 . For example, if the minor diameter of the bone screw  10  is 4 mm and the major diameter of the bone screw  10  near the proximal end  27  of the shaft  16  is 7 mm, the diameter of the thread portion  30  around the proximal end  27  of the shaft  16  is 3 mm. A preferred difference in the diameter between the thread portion  30  and the thread portion  32  is 2.0 mm but a larger or smaller difference between the thread portion diameters is also contemplated. In a preferred embodiment, the ratio of the diameter of the thread portion  30  to the ratio of the thread portion  32  is approximately 1.2, but can vary from 1.0 to 1.5 
         [0020]    By having a larger diameter thread portion  30  and a smaller diameter thread portion  32 , the bone screw  10  can grip bone tissue having regions with varying mechanical characteristics in an optimal manner. The larger diameter thread portion  30 , which surrounds the proximal portion  27  of the shaft  16 , is better suited to grip the cancellous region of the bone. The larger diameter thread portion, having the larger threads and increased purchasing surface area, better engages the softer, less dense bone tissue. Correspondingly, the smaller diameter thread portion  32 , which surrounds the distal portion  29  of the shaft  16 , is better suited to grip the cortical region of the bone. Since the cortical region is harder and denser, a smaller thread is preferred for the bone screw  10  to optimally purchase that bone tissue region. The combination of the larger diameter thread portion  30  and the smaller diameter thread portion  32  provides for an improved bone screw having greater bone tissue purchasing as well as greater pull-out strength than a screw with a single diameter thread. In a preferred embodiment, the improved purchasing lowers the bone screw  10  toggling over time and the pull out-strength of bone screw  10  compared to a screw having a single outer diameter thread has been determined to be at least 20% higher than the pull-out strength of the screw having a single outer diameter thread. 
         [0021]    In a preferred embodiment, the bone screw  10  also has a transition portion  35  between thread portions  30 ,  32  to allow for easier insertion of the bone screw  10  in the bone tissue. Preferably, over the transition portion  35 , the major diameter of the bone screw  10  decreases gradually between the thread portion  30  and thread portion  32  when viewed from a proximal to distal direction. 
         [0022]    Looking now at  FIGS. 3-4 , enlarged cross-sectional views of the thread portions  30 ,  32  can be seen. In a preferred embodiment, the thread angle a of the thread portions  30 ,  32  is preferably 25°, but can be between 20°-30°. It has been determined that this range of thread angles is optimal for purchasing in the different regions of the bone tissue. The radius β of the thread portion  30 ,  32  is preferably 0.5 mm, but can be between 0.1 mm-1 mm. Again, it has been determined that this range for the radius is optimal for purchasing in the different regions of the bone tissue. 
         [0023]    Turning back to  FIGS. 1-4 , the thread portions  30 ,  32  on the shaft  28  of the screw  10  is preferably a multi-start thread. More specifically, in a preferred embodiment, thread portions  30 ,  32  of the bone screw  10  is a two-start thread. Multi-start threads have the advantage of providing a thread on a screw shaft that has a smaller thread pitch (discussed below) than would be the case if the thread is a single-start thread. A smaller thread pitch can enhance the security of the fixation in bone tissue as well as increase the rate of installation of the screw in the bone tissue. In other preferred embodiments, a single-start thread portion as well as three or more start thread portion is also contemplated. 
         [0024]    As mentioned above, the thread pitch is defined as the distance along the axis of the screw between adjacent thread peaks, shown in  FIG. 2  as y. The thread lead is defined as the distance that is travelled along the axis of the screw in one complete 360° revolution of the screw, shown in  FIG. 2  as x. In a preferred embodiment, the number of starts of the thread portions  30 ,  32  is equal to the ratio of the thread lead x to the thread pitch y. For example, for a two-start thread, the thread lead is preferably 5 mm and the thread pitch is preferably 2.5 mm. In a preferred embodiment, the bone screw  10  includes a two start thread. In another preferred embodiment, the thread pitch y is substantially constant over thread portions  30 ,  32 . 
         [0025]    Dimensions, in millimeters, of a preferred embodiment of the bone screw  10 , which is suitable for use as a bone screw in a vertebral body, are as follows: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 L 
                 L1 
                 L2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 30.0 
                 15.0 
                 15.0 
               
               
                 35.0 
                 20.0 
                 15.0 
               
               
                 40.0 
                 20.0 
                 20.0 
               
               
                 45.0 
                 25.0 
                 20.0 
               
               
                 50.0 
                 25.0 
                 25.0 
               
               
                 55.0 
                 30.0 
                 25.0 
               
               
                 60.0 
                 35.0 
                 25.0 
               
               
                 65.0 
                 40.0 
                 25.0 
               
               
                 70.0 
                 45.0 
                 25.0 
               
               
                 75.0 
                 50.0 
                 25.0 
               
               
                 80.0 
                 55.0 
                 25.0 
               
               
                 85.0 
                 60.0 
                 25.0 
               
               
                 90.0 
                 65.0 
                 25.0 
               
               
                 95.0 
                 70.0 
                 25.0 
               
               
                 100.0 
                 75.0 
                 25.0 
               
               
                   
               
             
          
         
       
     
         [0026]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.