Abstract:
The invention relates to a spinal fixation system containing a rod positioned contiguous to and spanning a length of the spine. The rod is held in place by a bone screw that has a double offset at the proximal end, which is connected to a clamping mechanism. The bone screw is secured to the bone and the rod is secured in the clamping mechanism, whereby the clamping mechanism of the new double offset bone screw can be easily moved to accommodate the location of the rod.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to segmental spinal instrumentation systems, and more particularly to pedicle screws for such systems.  
         BACKGROUND  
         [0002]    In the last two decades, surgeons have moved toward systems that provide a secure grasp of individual vertebrae, and that enable intricate correction of complex spinal deformities. These systems are called segmental spinal instrumentation systems, because they can secure each segment (vertebra) of the spine.  
           [0003]    Such segmental systems include three main components, rods, hooks, and bone screws. The hooks are used to attach to the arches of the vertebrae, and come in several sizes to accommodate various sizes of vertebrae. The rods are long and thin, but strong enough to be fairly rigid. The bone screws are screwed directly into the vertebrae from the posterior aspect, or in some aspects are screwed into spaces between vertebrae. They are also called “pedicle screws” because they are typically inserted into the “pedicle” of the vertebrae. Bone screws come in a variety of shapes and sizes.  
           [0004]    Both the hooks and the screws are connected or clamped to the rods by various setscrews, clamps, nuts, collars, wedges, or brackets, to rigidly secure them to the rods.  
         SUMMARY  
         [0005]    The invention is based on the discovery that if the shaft of a bone screw is bent or offset to form an S-curve, the resulting screw can be used more effectively to secure a rigid rod to multiple vertebrae.  
           [0006]    In general, the invention features a bone screw that includes a screw shaft having a double offset or double bend at a proximal end; a head fixed to the shaft proximal to the double offset; and a clamping mechanism rotatably secured to the head. The central axes of the head and the screw shaft can be parallel or angled. The screw shaft can be threaded from the distal end to a distalmost offset of the double offset, or only part way up the screw shaft. The clamping mechanism can further include a fixation member, such as a setscrew, and the clamping mechanism can be a U-shaped body that defines a channel for receiving a rod. In the new bone screw, the head can be an integral part of the shaft distal to the double offset, or it can be a separate part that is fixed to the shaft.  
           [0007]    In another aspect, the invention features a spinal fixation system that includes a rod to be positioned contiguous to and spanning a length of the spine along multiple vertebrae; and a plurality of the new bone screws. Of course, such systems can also include various hooks, and the bone screws can be in a variety of sizes. The double offset of these bone screws can comprise two right-angle bends. In this case, the bone screw comprises a shaft having a first central axis and a proximal end having a second central axis, and the first and second central axes are parallel. In other embodiments, the bends are not quite right angles, and then these axes can be angled with respect to each other. In all embodiments, the screw shaft of these bone screws can rotate independently of the clamping mechanism.  
           [0008]    In some embodiments, the head of the bone screw includes a protrusion configured to contact the rod in use. This protrusion keeps the rod from sliding within the clamping mechanism after the rod is secured into the clamping mechanism. When the rod is secured, the screw shaft and the clamping mechanism are fixed at a selected angle. In the new systems, the clamping mechanism can comprise a U-shaped body that defines a channel for receiving the rod, and the base of the clamping mechanism can have an aperture through which a proximal end of the bone screw protrudes. In some embodiments, the channel is open at the top to allow insertion of the rod into the channel, and the arms of the U-shaped body have female threading which contact male threading on the fixation member to secure the rod.  
           [0009]    In another aspect, the invention also features a method for aligning a spinal column by obtaining two or more of the new bone screws; securing a first bone screw into a first vertebra; securing a second bone screw into a second vertebra; positioning a rod along the spinal column; rotating the bone screw shafts and clamping mechanisms to accommodate the rod and bring the vertebra into proper alignment; and securing the rod to each clamping mechanism. For example, the rod can be secured to the clamping mechanisms with setscrews.  
           [0010]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.  
           [0011]    Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic three-dimensional view of a new spinal fixation system.  
         [0013]    [0013]FIG. 2 is an oblique view of a new bone screw.  
         [0014]    [0014]FIG. 3 is a side view of a new bone screw.  
         [0015]    [0015]FIG. 4 is a three-dimensional view of a new spinal fixation system oriented and engaged to a vertebra. 
     
    
       [0016]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 shows the general elements of a spinal fixation system  10  in accordance with the present invention. The system includes a bone screw  12  configured to connect to a bone, e.g., a vertebra, a rod  14  that sets a bone structure, e.g., a spinal column, in a fixed position, and a fixation member  16  that clamps the rod  14  to the bone screw  12 . In a preferred embodiment, at least two or more bone screws  12  are used with one rod  14 .  
         [0018]    In particular, the bone screw  12  of the spinal fixation system  10  contains a clamping mechanism  20  at the proximal end  37  of the screw shaft  30 . The clamping mechanism  20  has the ability to rotate 360°, rock left-to right, and rock forward-to-backward on an enlarged head  32  on the proximal end  37  of the screw shaft  30 . The clamping mechanism  20  includes a U-shaped body  21  with two arms  22   a  and  22   b  forming a channel  24  designed to receive the rod  14 . The channel  24  ends in an edge on opposite sides of the U-shaped body  21 .  
         [0019]    In one embodiment, the channel  24  is open, halfway up the diameter and throughout the length of the channel  24  to receive the rod  14 . In another embodiment, the channel  24  is enclosed with openings at one or both ends for insertion of the rod  14 . An enclosed channel  24  will have an opening on the surface enabling communication with a fixation member  16 .  
         [0020]    In one embodiment, the rod  14  has a diameter that is slightly smaller than the inner dimensions of channel  24 . Therefore, for a snug fit, a sleeve may be inserted into the channel  24  that is configured to fit in the channel  24  and to accept the dimensions of the rod  14 . On the surface of the arms  22   a  and  22   b  facing the channel are female threads  26  that mate with male threads  18  on the fixation member  16 . The clamping mechanism is immobilized to the screw shaft  30  when the rod  14  is secured to the clamping mechanism  20 .  
         [0021]    The clamping mechanism  20  is connected to the proximal end  37  of the screw shaft  30  in an opening in the clamping mechanism  20 . The clamping mechanism  20  contains an aperture through a wall of the clamping mechanism where the distal end  38  of the screw shaft  30  extends or protrudes from the aperture. The aperture narrows or has a recess or restriction that is smaller than the head  32  and stops the screw shaft  30  from proceeding through the aperture. The screw shaft  30  retains the ability to rotate with respect to the clamping mechanism  20 , enabling the clamping mechanism  20  to be positioned into close proximity to the rod  14 , even after the screw shaft  30  is set in bone.  
         [0022]    In one embodiment, the clamping mechanism  20  has the form of a U-shaped body  21  with an open top. In other embodiments, the clamping mechanism  20  has a C-shaped body with an open side. The connection remains dependent on the presence of an aperture in the clamping mechanism  20  for the head  32  of the screw shaft  30  to engage the clamping mechanism  20 .  
         [0023]    The head  32  of the screw shaft  30  can be attached to the clamping mechanism  20  in a variety of ways. In one embodiment, the head  32  of the screw shaft  30  is spherical and fits into the clamping mechanism  20  in a manner that allows for a “ball-and-socket” motion. This type of connection allows for the greatest range of mobility for the clamping mechanism  20  to swivel on the head  32  of the screw shaft  30 . In another embodiment, the head  32  takes the form of a hemisphere to form a semi-“ball-and-socket” joint. The semi-“ball-and-socket” mechanism allows a lesser degree of mobility of the clamping mechanism  20  relative to the screw shaft  30 , but may provide greater stability in some circumstances. A third embodiment includes a loosely fitted connection between the head  32  and the clamping mechanism  20 , allowing for a more restricted degree of mobility, but complete rotation between the head  32  and the clamping mechanism  20  along the central axis of the screw shaft  30 .  
         [0024]    In a specific embodiment, the head  32  of the screw shaft  30  is restrained within the clamping mechanism  20  via congruent contact surfaces with shapes that are part of a sphere. This allows completely free rotation of the clamping mechanism  20  about the head of the screw, and tilting of the screw within the clamping mechanism  20  of about 15 to 45 degrees, e.g., 30 degrees, both laterally and longitudinally.  
         [0025]    [0025]FIG. 1 shows grooves  28   a  and  28   b  in the arms  22   a  and  22   b  respectively, in the U-shaped body  21 . In one embodiment, the grooves are recessed into arms  22   a  and  22   b  to a desired depth. In another embodiment, the grooves extend through the width of the clamping mechanism  20  and form apertures. Once the bone screw has been driven into the bone using a driving instrument, grooves  28   a  and  28   b  are engaged by a different instrument to position and hold the clamping mechanism  20  in place while guiding the rod  14  and the setscrew  16  into place.  
         [0026]    The bone screw  12  contains a distal pointed end  38  for engaging a bone, and a proximal end  37  that are separated by a threaded shank  36  of the screw shaft  30  that is designed for securing the screw in the bone. The type of threading, the diameter, and the length of the threaded shank  36  can vary as required for different sizes and types of bones.  
         [0027]    Proximal to the threaded shank  36  lies the double offset  34   a  and  34   b  adjacent the head  32 . The first offset or bend  34   a  from the distal end  38  of the bone screw  12  is angled away from the axis of the bone screw  12 . The second offset or bend  34   b  from the distal end  38  of the bone screw  12  angles back in the direction of, e.g., to become parallel to, the axis of the bone screw  12 . The double offset enables the clamping mechanism  20  to cover a greater circumference when the screw shaft  30  is rotated relative to the circumference generated if the screw shaft  30  was straight. The greater circumference allows a greater opportunity for positioning the clamping mechanism  20  with respect to the rod  14 .  
         [0028]    The rod  14  has a generally uniform cylindrical cross-section and is manufactured from a medically inert substance, e.g., a metal such as titanium or stainless steel. Other materials that have the same characteristics as titanium or steel may also be used. The rod  14  is configured to fit into the channel  24  of the clamping mechanism  20 .  
         [0029]    The fixation member  16  has male threads  18  on its outer surface that mate with the female threads  26  on the inner surface of the U-shaped body  21 . The fixation member  16  can be a setscrew and can have a plurality of socket configurations, e.g., hexagonal or octagonal. The fixation member is inserted into the U-shaped body of the clamping mechanism  20  by using a driving instrument, for example, a screwdriver or a wrench. For example, the fixation member  16  can be configured with a hexalobe shaped, e.g., Torx® socket, and turned with a hexalobe shaped, e.g., Torx® driver, or other conventional sockets.  
         [0030]    [0030]FIG. 2 is an oblique view of the screw shaft  30  absent the clamping mechanism. The pointed distal end  38  is separated from the proximal end  37  by the threaded shank  36 . The threaded shank  36  is followed proximally by the offsets  34   a  and  34   b . The screw shaft  30  is completed at the proximal end  37  by the enlarged head  32 . Preferably, the head  32  will have a partial spherical bottom  33  and a flattened or conical upper surface  31 . This configuration allows for significant mobility of the screw shaft  30  relative to the clamping mechanism  20 . The head  32  can, in some embodiments, have a recess for receiving an engaging tool. In most embodiments, a specially designed driving instrument wraps around the S-shaped double offset of the screw shaft  30 .  
         [0031]    [0031]FIG. 3 is an orthogonal view of the screw shaft  30 . It shows another angle to view the head  32  and the first and second offset  34   a  and  34   b . In FIG. 3, the screw shaft  30  has a small projection  35  on the conical surface  31  on the top of the head  32 . In one embodiment, this projection  35  contacts the rod  14  when the rod  14  is placed into the U-shaped body  21  of the clamping mechanism  20 . Upon securing the rod  14  with the fixation member  16 , the compression of the rod  14  onto the projection  35  secures the head  32  into the aperture of the clamping mechanism  20 , keeps the rod from sliding laterally within the U-shaped body  21 , and prevents angular changes in the shaft position relative to the U-shaped body  21 .  
         [0032]    Referring again to FIG. 3, the double offset  34   a  and  34   b  is visible as an S-shaped curve. In another embodiment, the offsets  34   a  and  34   b  can be at different angles to each other and to the screw shaft  36 . For example, as seen in FIG. 3, the central axes of a small section  36   a  of the shaft adjacent the head  32  (which coincides with the central axis of this head  32 ) and the main threaded shaft  36  are parallel to each other. However, by varying the angle, the central axes of the small section  36   a  and the threaded shank  36 , one can adjust the reach of the clamping mechanism  20  as it rotates about the main shaft  36 . By increasing the angle, the reach is increased, and decreasing the angle, the reach is decreased.  
         [0033]    [0033]FIG. 4 illustrates a method of using the spinal fixation system. A first vertebra  42  is to be fixed to a second vertebra  44 . The distal end  38  of the bone screw  12  is driven into the first vertebra at the pedicle region  46  to a predetermined depth using a driving instrument. A second bone screw is driven into the second vertebra  44  in a similar fashion. Rod  14  is placed into the channel  24  of the clamping members  20 , which are rotated to accommodate the rod. Once the rod is inserted into each clamping mechanism, the fixation members  16  are tightened into their respective clamping mechanisms  20 , thereby securing the spinal fixation system. The rod  14  is secured to at least two bone screws  12  that are engaged on different vertebrae.  
         [0034]    The spinal fixation system is manufactured and machined by standard techniques well known in the art, e.g., molding, milling, and threading. The materials used are medically approved and biologically inert. Such materials can include metals, e.g., titanium or steel. See, e.g., U.S. Pat. Nos. 5,797,911, 6,083,227, and 6,187,005.  
       OTHER EMBODIMENTS  
       [0035]    It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.