Patent Publication Number: US-2010114171-A1

Title: Multi-planar spinal fixation assembly with locking element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to, and the benefit of, U.S. Provisional Patent Application Ser. No. 61/198,374, filed Nov. 5, 2008, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to orthopedic surgery and in particular to devices and prosthesis for stabilizing and fixing the bones and joints of the body. 
     2. Background of Related Art 
     It is a common surgical procedure to stabilize and fix bones and bone fragments in a particular spatial relationship with fixation devices to correct the location of skeletal components due to injury or disease. This can be accomplished by using a number of fixation devices such as bone pins, anchors, or screws placed in bone across a discontinuity (e.g., a fracture) in the bone, bone fragments, adjacent vertebrae, or joints. These fixation devices can be connected by a rod to maintain a desired spatial relationship. In some cases, these fixation devices may be permanently implanted. In other cases, these fixation devices may be implanted only as a temporary means of stabilizing or fixing the bones or bone fragments. It is also common that fixation devices that are intended to be permanently implanted require subsequent modifications as the dynamics of a patient&#39;s condition warrant. 
     Spinal fixation devices are widely employed in surgical processes for correcting spinal injuries and diseases. These devices commonly employ longitudinal link rods (e.g., spinal rods) secured to vertebrae by spinal bone fixation fasteners such as pedicle screws, hooks and others. 
     On occasion, the rod may dislocate from the spinal fixation device under bodily forces experienced after implantation. Such dislocation can be caused either by axial slip, i.e., sliding of the rod end through the spinal fixation device along the axis of the rod, or radial displacement of the rod out of the screw. Either type of dislocation can happen with any type of spinal fixation device, including both taper lock style screws and set screw style screws. 
     SUMMARY 
     The present disclosure relates to a spinal fixation assembly including a spinal rod, one or more spinal fixation devices, and a locking element. The one or more spinal fixation devices include a coupling, a screw, and a collet. The coupling has an opening extending therethrough. The screw is mounted to the coupling and positionable within a first vertebral body. The collet is receivable in the opening of the coupling and defines a saddle for engaging the spinal rod. 
     The spinal rod defines a longitudinal axis. The spinal rod is positioned transverse to the screw upon engagement with the saddle of the collet. 
     The locking element is mounted to the spinal rod and is configured to prevent axial translation of the spinal rod in one or both directions along the longitudinal axis thereof. The locking element includes a body defining a passage adapted to receive the spinal rod therethrough in locking engagement therewith. The locking element includes a locking screw for selectively locking the locking element to the spinal rod. The locking screw rotates into locking engagement with the spinal rod. The locking screw is disposed in threaded engagement with a threaded bore defined within the body of the locking element. The width of the locking element is greater than the width of the saddle such that when the locking element abuts the saddle, the locking element prevents the spinal rod from axially translating in one or both directions. 
     The spinal fixation assembly may include a second spinal fixation device having a collet defining a saddle wherein the spinal rod is positionable within the saddles of each spinal fixation device. 
     In one aspect, a spinal fixation assembly includes one or more spinal fixation devices and a spinal rod. The one or more spinal fixation devices include a coupling, a screw, and a collet. The coupling has an opening extending therethrough. The screw is mounted to the coupling and positionable within a first vertebral body. The collet is receivable in the opening of the coupling and defines a saddle. The spinal rod has one or more locking elements configured to prevent axial translation of the spinal rod in one or both axial directions when the spinal rod is positioned within the saddle of the collet of the spinal fixation device. The one or more locking elements may be an annular ring around the spinal rod. The diameter of the one or more locking elements is greater than the width of the saddle such that when the one or more locking elements abut the saddle, the one or more locking elements prevent the spinal rod from axially translating in one or both axial directions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1A  is a top perspective view of one embodiment of a spinal fixation device having a taper lock; 
         FIG. 1B  is a front view of the spinal fixation device of  FIG. 1A ; 
         FIG. 1C  is an exploded side view of the spinal fixation device of  FIG. 1A  with parts separated illustrating a pedicle screw, a coupling, a collet, and a pin; 
         FIG. 2A  is a front view of the coupling; 
         FIG. 2B  is a top perspective view of the coupling of  FIG. 2A ; 
         FIG. 3A  is a front view of the collet; 
         FIG. 3B  is a top perspective view of the collet of  FIG. 3A ; 
         FIG. 4A  is a side view of the pedicle screw; 
         FIG. 4B  is a top view of the pedicle screw of  FIG. 4A ; 
         FIG. 5  is a front perspective view of the pin; 
         FIG. 6  is a cross-sectional view of the spinal fixation device; 
         FIG. 7  is a side view of one embodiment of spinal fixation assembly including first and second spinal fixation devices, a spinal rod, and a locking element in accordance with the present disclosure; 
         FIG. 8A  is a front view of the locking element of the spinal fixation assembly of  FIG. 7 , the locking element including a body and a locking screw; 
         FIG. 8B  is an exploded front view of the locking element of  FIG. 8A   
         FIG. 9  is a perspective view of the body of the locking element of  FIG. 8A ; 
         FIG. 10  is a perspective view of the locking screw of the locking element of  FIG. 8A ; 
         FIG. 11  is a top view of one embodiment of a spinal fixation assembly including first and second spinal fixation devices and an embodiment of a spinal rod in accordance with the present disclosure; 
         FIG. 12  is a side view of the spinal rod of  FIG. 11 ; 
         FIG. 12A  is an enlarged side view of the detailed area A of  FIG. 12 ; and 
         FIG. 13  is a perspective view of another embodiment of a spinal fixation assembly in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various embodiments of the presently disclosed spinal fixation assembly will now be described in detail with reference to the drawings, wherein like reference numerals identify similar or identical elements. In the drawings and in the description that follows, the term “proximal,” will refer to the end of a device that is closest to the operator, while the term “distal” will refer to the end of the device that is farthest from the operator. In addition, the term “cephalad” is used in this application to indicate a direction toward a patient&#39;s head, whereas the term “caudad” indicates a direction toward the patient&#39;s feet. Further still, for the purposes of this application, the term “medial” indicates a direction toward the middle of the body of the patient, whilst the term “lateral” indicates a direction toward a side of the body of the patient (i.e., away from the middle of the body of the patient). The term “posterior” indicates a direction toward the patient&#39;s back, and the term “anterior” indicates a direction toward the patient&#39;s front. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. 
     Referring initially to  FIGS. 1A-1C , in which like reference numerals identify similar or identical elements, a spinal fixation device is generally designated as  100 . The spinal fixation device  100  includes a pedicle screw  10 , a pin  30 , an outer housing or coupling  50 , and an inner housing or collet  70 . One example of such a spinal fixation device is disclosed in International Application Number PCT/US2008/080682, the entire contents of which are hereby incorporated by reference herein. While a taper lock screw as shown and described herein is preferred, the locking element of the present disclosure may find application with the use of other types of spinal fixation devices, such as set screw type fixation devices. One example of such as set screw type fixation device is shown in U.S. Pat. No. 5,733,286, the entire contents of which are incorporated by reference herein. 
     Referring now to  FIGS. 2A and 2B , the coupling  50  includes an annular body portion  52  having an opening  54  extending axially therethrough. Additionally, the coupling  50  includes a plurality of fingers  56  that are located in opposing regions of the coupling  50  and define a saddle  58  having a generally U-shaped configuration. The U-shaped saddle  58  is configured and dimensioned for receiving a rod “R” (see  FIG. 7 ). 
     As shown in  FIGS. 3A and 3B , the collet  70  has a generally cylindrical body portion  72  with an opening  74  extending axially therethrough. A pair of upstanding wings  76  defines a saddle  78  having a generally U-shaped configuration. The saddle  78  is configured and dimensioned for receiving the rod “R.” The body portion  72  includes a slot  73  that extends from the nadir of the saddle  78  towards the bottom of the body portion  72  and essentially bisects the body portion  72  along a central axis, and defines left and right sections of the body portion as viewed in  FIG. 3A . Preferably, the slot  73  does not extend all the way through the body portion. Although less desirable, such a full slot could be used. This arrangement permits each of the wings  76  to flex towards and away from each other. The dimensions of the saddle  78  vary according to the flexure of the wings  76 . As the wings  76  are moved closer to each other, the saddle  78  decreases in size and when the wings  76  are moved away from each other, the saddle  78  increases in size. Allowing the saddle  78  to vary in size permits the collet  70  to accommodate rods having differing outside diameters. Alternatively, compressing the wings  76  towards each other increasingly engages the outer surface of a rod “R” located in the saddle  78 , thereby frictionally securing the rod “R” in a desired position. 
     In addition, the body portion  72  includes a plurality of grooves  75  that extend to the bottom of the body portion  72  and which are open at the bottom of the body portion  72 . The grooves  75  extend vertically into each of the wings  76 , and define front and rear portions of the body portion  72 . As configured, the grooves  75  permit the front and rear sections of the body portion  72  to flex relative to the grooves  75  along the axis defined by the slot  73 . The body portion  72  also includes a plurality of notches  77  that are open at the bottom surface of the body portion  72  and extend vertically towards the wings  76 . The notches  77 , in combination with the slot  73  and the grooves  75 , allow arcuate sections  72 a of the body portion  72  to flex inwards and outwards from an initial position in response to compressive and tensile forces applied to the sections  72   a.    
     Referring now to  FIGS. 4A and 4B , the pedicle screw  10  includes a shank  16  having a helical thread  14  formed thereon. A cutting portion  12  is formed at a distal end of the pedicle screw  10 . A head  18  is located at a proximal end of the pedicle screw  10 . The head  18  has an outer diameter that is greater than the outer diameter of the shank  16 . On the top surface of the head  18 , a recess  20  is formed. The recess  20  is illustrated with a six-pointed star configuration for receiving the operative end of a suitable driving tool, but it is contemplated that other configurations may be used. A neck  16 a extends between a bottom surface of the head  18  and the beginning of the helical thread  14 . As configured, the neck  16 a is unthreaded. As shown, at least a portion of the diameter of the neck  16   a  is less than the diameter of the bottom of the head  18  and the major diameter of the threaded portion of the shank  16 . 
     Referring again to  FIGS. 1A-1C , the spinal fixation device  100  will now be discussed as assembled for use. The collet  70  is seated atop the head  18  of pedicle screw  10 . The opening at the bottom of collet  70  is dimensioned and configured for receiving the head  18 . As such, the collet  70  and the head  18  are rotatable and pivotable in relation to each other, thereby allowing the pedicle screw  10  to be repositioned in a plurality of orientations relative to the collet  70 . The combination of the collet  70  and pedicle screw  10  is inserted into the coupling  50 . The pin  30  aligns the collet  70  and the coupling  50  for maintaining a fixed relationship between them ( FIGS. 1C and 5 ). As assembled, the pedicle screw  10  is rotatable and pivotable in relation to the collet  70  and the coupling  50 . 
     Referring now to  FIG. 6 , additional features of the assembled spinal fixation device  100  will be discussed. The coupling  50  includes an inner annular lip  55  that is beveled. The lip  55  extends upwards and inwards from a bottom outer edge of the coupling  50 . Additionally, the collet  70  includes an annular beveled lip  79  that also extends upwards and inwards from bottom outer edge of the collet  70 . As shown in  FIG. 6 , angle α measures the angle of the beveled lip  79  from centerline C to the beveled lip  79 . Angle α may measure between 25 and 65 degrees. In an embodiment, angle α is approximately equal to 45 degrees. Angle β measures the angle of the beveled lip  55  from the centerline C to the beveled lip  55 . Angle β may measure between 32 and 72 degrees. In an embodiment, angle β is approximately equal to 52 degrees. By providing the coupling  50  and the collet  70  with beveled lips  55 ,  79 , there is a reduced interaction between the head  18  and the coupling  50  and/or the collet  70 . In addition, the pedicle screw  10  has a neck  16   a  with a length and diameter that cooperate with the beveled lips  55 ,  79  for reducing interaction therebetween. That is, the length of the non-threaded neck portion  16   a  of the pedicle screw  10  extends a distance from the bottom of the head  18  to a point beyond the beveled lip  79  of the of the collet  70  and beveled lip  55  of the coupling  50 , which together with the selected diameter of the neck  16   a  permits maximum angular motion of the pedicle screw  10  relative to the collet  70  and coupling  50 . This creates a smooth transition zone between the unthreaded neck  16   a  and the collet  70  and the coupling  50 . By reducing the interference between the neck  16   a  and the beveled lips  55 ,  79  in combination with the reduced interaction between the head  18  and the beveled lips  55 ,  79 , the pedicle screw  10  defines a cone of at least 70° with respect to a centerline “C” of the spinal fixation device ( FIG. 6 ). In another embodiment, the pedicle screw  10  has a conical range of motion that is at least 90°. In a further embodiment, the pedicle screw  10  has a conical range of motion that is at least 95°. 
     Specifically, the pedicle screw  10  is capable of being repositioned from a first position ( FIG. 6 ) throughout a plurality of angular positions with respect to the centerline “C”. The angular displacement with respect to the centerline “C” is shown as angleθ. Angle θ is at least 70°. In other embodiments, angle θ is in a range between about 80° and about 95°. As such, the pedicle screw  10  moves relative to the centerline “C” (i.e. off axis) in a range of about 35° to about 47.5°. 
     Referring now to  FIGS. 7-10 , one embodiment of a spinal fixation assembly is generally designated as  200 . The spinal fixation assembly  200  includes a spinal rod “R”, one or more spinal fixation devices  100 , and a locking element  210 . The spinal rod “R” may be positioned transverse to the screw  10  upon engagement with the saddle  78  of the collet  70  of each spinal fixation device  100 . 
     The locking element  210  is mounted to the spinal rod “R” and is configured to prevent axial translation of the spinal rod “R” along the longitudinal axis “L” thereof. The width of the locking element  210  is greater than the width of the saddle  78  and/or saddle  58  such that when the locking element  210  abuts saddle  78  and/or saddle  58 , the locking element  210  prevents the spinal rod “R” from axially translating in one or both directions. Where one locking element  210  is positioned on one end of the spinal rod “R”, the spinal rod “R” is prevented from moving in one of the axial directions such as the direction illustrated by arrow “A” in  FIG. 7 . In embodiments, a plurality of locking elements  210  may be positioned on the spinal rod “R” on opposing sides of one or more spinal fixation devices  100  such that the spinal rod “R” is prevented from moving in both axial directions. The locking element  210  includes a body  220  defining a passage  222  adapted to receive the spinal rod “R” therethrough in locking engagement therewith. The locking element  210  includes a locking screw  230  for selectively locking the locking element  210  to the spinal rod “R.” The locking screw  230  rotates into locking engagement with the spinal rod “R.” The locking screw  230  includes a head  232  and a shaft  234 . As best shown in  FIG. 10 , the head  232  defines a recess  232   a . The recess  232   a  is illustrated with a six-pointed star configuration for receiving the operative end of a suitable driving tool (not shown), but it is contemplated that other configurations may be used. As best shown in  FIG. 9 , the locking screw  230  is disposed in threaded engagement with a threaded bore  224  defined within the body  220  of the locking element  210 . In particular, the shaft  234  of the locking screw  230  includes threads for threadably engaging the threaded bore  224  of the body  220 . 
     With reference to  FIGS. 11-12A , one embodiment of a spinal fixation assembly is generally designated as  300 . The spinal fixation assembly  300  includes one or more spinal fixation devices  100  and a spinal rod  350 . The spinal rod  350  has one or more locking elements  360  configured to prevent axial translation of the spinal rod  350  when the spinal rod  350  is positioned within saddle  78  and/or saddle  58  of the spinal fixation device  100 . The locking element  360  prevents the spinal rod  350  from axially translating in one direction. In this embodiment, the spinal rod  350  is prevented from moving in the axial direction illustrated by arrow “B” in  FIG. 11 . In embodiments, the spinal rod  350  may include a plurality of locking elements  360  positioned on opposing sides of the one or more spinal fixation devices  100  such that the spinal rod  350  is prevented from moving in both axial directions. The one or more locking elements  360  may be an annular ring disposed around the spinal rod  350  ( FIG. 12A ). The diameter of the one or more locking elements  360  is greater than the width of saddle  78  and/or saddle  58  such that when the one or more locking elements  360  abut saddle  78  and/or saddle  58 , the one or more locking elements  360  prevent the spinal rod  350  from axially translating. 
     Referring now to  FIG. 13 , one embodiment of a spinal fixation assembly  400  includes two or more spinal fixation devices  100 , two or more spinal rods “R”, and one or more locking elements  410 . Locking element  410  includes first and second rod grasping members  420 ,  430  interconnected by a locking screw  440 . Each rod grasping member  420 ,  430  is adapted to removably attach to the spinal rods “R” by grasping arms  422 ,  432  and lock thereto by locking screws  424 ,  434  for preventing each spinal rod “R” from axially translating. One example of such a locking element  410  is disclosed in commonly owned U.S. patent application Ser. No. 12/125,612, the content of which is hereby incorporated by reference herein. 
     It will be understood that various modifications may be made to the embodiments of the presently disclosed device. While the present description relates primarily to taper lock screws, it will be understood that the principles of the disclosure also apply to other types of screws, including set screw rod locking mechanisms. Therefore, the above description should not be construed as limiting, but merely as exemplifications of embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the present disclosure.