Abstract:
A device for securing a spinal rod to the spine. The device includes an elongated spinal rod configured to be implanted adjacent to the spinal column of a patient spanning across several vertebral bodies. The device also includes at least one fixation element for engaging vertebral bodies at a number of vertebral levels, each of the fixation elements having a vertebral engaging portion and a stem portion extending from the vertebral engaging portion. A ball ring having an aperture extending through the center of the ball ring. The ball ring further including a through slot so as to be compressible and expandable about a central axis extending through the center of the ball ring. The aperture is adapted to receive the spinal rod. The device further includes a set screw and a rod connector.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to bone fixation devices and, in particular, to pedicle fixation assemblies and methods used in spinal fixation procedures.  
         [0002]     The spinal column is a highly complex system of bones and connective tissues that provide support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of stacked vertebral bodies, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord and nerves is located behind the vertebral bodies.  
         [0003]     A surgical technique commonly referred to as spinal fixation uses surgical implants for fusing together and/or mechanically immobilizing two or more vertebral bodies of the spinal column. Spinal fixation may also be used to alter the alignment of adjacent vertebral bodies relative to one another to change the overall alignment of the spinal column. Such techniques have been used effectively to treat a wide variety of conditions and, in most cases, to relieve pain.  
         [0004]     One spinal fixation technique involves immobilizing the spine using orthopedic stabilizing rods, commonly referred to as spinal rods, which run generally parallel to the spine. This technique involves exposing the spine posteriorly and fastening bone screws to the pedicles of vertebral bodies. The pedicle screws are generally placed at least one per vertebra and serve as anchor points for the spine rods. Clamping elements adapted for receiving a spine rod therethrough are then used to join the spine rods to the pedicle screws. The aligning influence of the spine rods forces the spinal column to conform to a more desirable shape. In certain instances, the spine rods may be bent to achieve the desired curvature of the spinal column.  
         [0005]     Most existing rod fixation systems require several components to build the systems. Each additional component or instrument required to assemble the fixation system adds to the complexity of the surgical technique. A need has thus arisen for improved fixation systems that minimize the assembly of small pieces of hardware during the surgical procedure. Thus, there remains a need for spinal fixation devices that facilitate simple and fast assembly of attachment of a spinal rod to a spine. It would be desirable to provide a device with pre-assembled components that will result in less time being required to assemble the components in the operating room.  
         [0006]     Additionally, many of the known existing rod fixation systems require at least a two-step locking procedure which not only lengthens the surgical procedure but also increases the complexity of the procedure. Although some existing rod fixation systems do offer one-step locking such as the spinal fixation system disclosed in U.S. Pat. No. 5,534,002, it would still be desirable to have a one-step-locking device with pre-assembled components and/or a one-step-locking device with increased friction surface area between assembled components.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed towards a device for securing a spinal rod to a spine. The assembly may include an elongated spinal rod, a bone fixation element, a ball ring, a set screw and a rod connector. The elongated spinal rod is configured to be implanted adjacent to a spinal column of a patient and span across vertebral bodies. The bone fixation element of the assembly includes a bone engaging portion, as well as a stem portion. The ball ring of the present invention has an aperture that extends through the center of the ball ring and can slidably receive the spinal rod. Furthermore, the ball ring also includes a through slot that allows the ball ring to be compressed and expanded about a central axis extending through the center of the ball ring. The rod connector of the present assembly is adapted for connecting the bone fixation elements to the spinal rod. The connector may include a body having a channel for housing the ball ring, a stem bore adapted to receive the stem portion of the bone fixation element and a set screw bore having threads which are engagable with the threads of the set screw. The channel is able to communicate with the set screw bore as well as the stem bore.  
         [0008]     In certain preferred embodiments, the spinal rod is generally cylindrical. Additionally, the stem portion of the bone fixation element may be smooth.  
         [0009]     In other preferred embodiments, the ball ring is captured within the channel and the connector, and the through slot of the ball ring permits the outer diameter of the ball ring to be reduced less than a minimum diameter of the channel. Furthermore, the ball ring may have a relaxed diameter smaller than the diameter of the channel of the rod connector, so as to permit polyaxial motion of the rod when the rod is housed within the aperture of the ball ring.  
         [0010]     In some preferred embodiments, the set screw bore may be offset from the stem bore at an angle greater than or equal to 3 degrees with reference taken from a stem bore axis.  
         [0011]     Since the channel may intersect the stem bore, the ball ring, once inside the channel, is able to apply pressure against the stem of the fixation element when the stem is housed within the stem bore. In order to achieve greater lateral movement of the ball ring within the channel, the channel and the stem bore may have a non-circular geometric configuration, and preferably are elliptical. This non-circular configuration of the channel and stem bore create a situation for the device where, in a locked position, the ball ring may either apply pressure against or receive pressure from at least three contact points, while the stem may also apply pressure against or receive pressure from at least three contact points. The stem bore may also be offset at an angle between 3 degrees and 15 degrees relative to a second axis extending through the maximum distance of the channel. Furthermore, the set screw bore may be aligned with this second axis.  
         [0012]     In an alternate embodiment, the spinal rod assembly may further include a second ball ring. With the addition of the second ball ring, the connector may be constructed similar to the previous embodiment described, with the exception that the second ball ring is now positioned about the stem of the bone fixation element. Thus, instead of the first ball ring contacting the stem when it is moved in a lateral direction, the first ball ring applies pressure against the second ball ring. This causes the second ball ring to tighten around the stem while the first ball ring tightens around the rod as pressure is applied to the first ball ring by the set screw.  
         [0013]     In an additional embodiment, the set screw bore may be placed between the channel and the stem bore. In this configuration, as the set screw is translated downward through the set screw bore, the set screw comes in contact with the first and second ball rings, thus tightening both around the rod and stem respectively, until the stem and rod are locked relative to the connector. The rod channel and stem bore may have a non-circular configuration. Furthermore, the set screw may have a tapered end, and the set screw bore is configured to cooperate with the set screw having a tapered end. In this embodiment, the set screw bore and the stem bore may be along around a Zenith axis. Furthermore, the set screw may be pre-seated or captured within the connector.  
         [0014]     In an alternate embodiment, the set screw may have a configuration which prevents it from being backed all the way out of the set screw bore. Consistent with this approach, the set screw may have to be placed within the connector by maneuvering the set screw through the channel and up through the set screw bore.  
         [0015]     In a method of use of the present invention, the bone fixation element may first be engaged with a vertebral body. The connector is then placed over the stem portion of the bone fixation element, wherein the channel of the connector is in communication with both the stem bore and the set screw bore. The set screw and the ball ring may be placed within the connector, either before the connector is placed over the stem portion or after the connector is placed over the stem portion. The spinal rod then may be slid through the aperture of the ball ring and maneuvered until a correct position for the rod is achieved. In order to lock the rod and stem relative to one another, the set screw may then be translated downward through the set screw bore until it applies a force against the ball ring. This forces the ball ring to tighten around the rod and move in a lateral direction towards the stem. As the set screw is continually translated downward through the set screw bore, the stem comes in contact with the interior wall of the stem bore until it is locked between the ball ring and the set screw bore. Additionally, the ball ring is further tightened around the rod until the ball ring is locked between the set screw and the stem. At this point, the rod and stem are locked relative to the connector.  
         [0016]     The stem bore may have a larger diameter than the stem in order to allow polyaxial motion of the stem within the connector. Additionally, the set screw, as well as the bone fixation element, may have recesses for cooperating with a tool.  
         [0017]     The present invention as described herein may also be provided within a kit. The kit may include a plurality of connectors, a plurality of bone fixation elements, a plurality of spinal rods, a plurality of set screws, a plurality of ball rings and any combination thereof. Furthermore, different size elements may be included within the kit, as well as different types of these elements. For instance, with regard to the bone fixation element, the kit may include pedicle screws and/or hook assemblies or other variations thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  illustrates one embodiment of the present invention;  
         [0019]      FIG. 2  shows a blow-up prospective view of the embodiment shown in  FIG. 1 ;  
         [0020]      FIG. 3  is cross-sectional view of two embodiments shown in  FIG. 1  connected to a vertebral body;  
         [0021]      FIG. 4  is a prospective view of a bone fixation element;  
         [0022]      FIG. 5   a  is a prospective view of an embodiment of a connector used in the spinal assembly;  
         [0023]      FIG. 5   b  is a cross-sectional view of the connector shown in  FIG. 5   a;    
         [0024]      FIG. 6  is a prospective view of a ball ring;  
         [0025]      FIG. 7  is a prospective view of a set screw;  
         [0026]      FIG. 8   a  is a cross-sectional view of the embodiment of  FIG. 1  shown in an assembled state;  
         [0027]      FIG. 8   b  is a top view of the assembly shown in  FIG. 8   a ; and  
         [0028]      FIG. 9  is a cross-sectional view of an additional embodiment according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]     Before describing several exemplary embodiments of the present invention, it is to be understood that the present invention is not limited to the details of construction or process steps set forth in the following description. The present invention is capable of other embodiments and of being practiced or carried out in various ways.  
         [0030]     Referring now to the drawings and particularly to  FIGS. 1-3 , a bone fixation assembly  10 , in accordance with certain preferred embodiments of the present invention is shown. The bone fixation assembly may include at least one fixation element  12  such as a bone screw, hook or anchor, a connector  14 , an elongated spinal rod  16  which is preferably cylindrical, a ball ring  18 , and a set screw  20 . The bone fixation assembly  10  may be secured to the pedicles  11  of the vertebral bodies of a spinal column, as shown in  FIG. 3 .  
         [0031]     Connector  14  of bone fixation assembly  10  is preferably made of a biological inert material, for example, any metal customarily used for surgical devices and particularly those used for bone screws and pins, such as titanium or stainless steel. Other suitable materials for the connecting element  14  include, but are not limited to, alloys, composite materials, ceramics, or carbon fiber materials.  
         [0032]     Fixation element  12  as shown in  FIG. 4  includes a stem  22  and a bone-engaging portion  24 . Stem  22  is preferably cylindrical and preferably has a smooth outer surface to facilitate sliding as will be discussed below. The stem can be constructed in other ways or in connection with other components to facilitate sliding of connector  14  with respect to fixation element  12 . Fixation element  12  could be any suitable fixation element for attachment to a bone, for example, a hook or a screw. In preferred embodiments, fixation element  12  comprises a screw or fastener having a stem portion  22  and threaded bone-engaging portion  24 . Bone-engaging threads  26  are adapted to be engaged in bone material. Portion  24  of fixation element  12  opposite stem  22  has a tip  28  for insertion into bone, and external screw threads  26  extending between tip  28  and stem  22 . Screw threads  26  have an inner root diameter and an outer diameter. Fixation element  12 , including screw threads  26  and stem  22 , is preferably made of a biological inert material, such as titanium or stainless steel.  
         [0033]     In the embodiment shown in  FIG. 4 , one end of bone fixation element  12  includes a top surface  30 , which preferably includes a recess  32 . In the embodiment shown, recess  32  is in the form of a hollow hexagonal recess adapted to receive an end of a hexagonal driver for rotating fixation element  12 . It should be understood, however, that other internal or external tool engagement recesses can be used according to the present invention.  
         [0034]     Connector  14 , which is shown in more detail in  FIGS. 5   a  and  5   b , has at least a first bore  40  having a longitudinal axis  41  extending in the Zenith axis adapted to receive stem  22  of fixation element  12  and to permit axial, sliding movement of connector  14  along the longitudinal axis of stem  22  with respect to fixation element  12 . Preferably, first bore  40  has a non-circular shape and most preferably is elliptical. Bore  40  may have a diameter larger than the diameter of stem  22  in order to allow polyaxial motion of fixation element  12  within bore  40 . Connector  14  further includes a ball ring receiving bore in the form of channel  42  for receiving and/or housing ball ring  18  which, in turn, receives rod  16 . Connector  14  has a set screw bore  44  which intersects channel  42  and extends along axis  45 . Channel  42  preferably has a non-circular geometry and may be elliptical in shape. Channel  42  has a longitudinal axis  43  extending in the Y-axis, that is substantially transverse to longitudinal axis  41  of first bore  40 . In a preferred embodiment, the angle between axis  45  and axis  41  is in a range between 3 degrees and 15 degrees as shown by arc  49  in  FIG. 5   a . Axis  41  is preferably at an angle to the plane containing axis  43 , extending parallel to the sagittal plane.  
         [0035]     In a most preferred embodiment, set screw bore  44  is aligned with channel  42  in the Zenith-axis so as to be offset from bore  40  similarly. It is also within the scope of the present invention that only set screw bore  44  is offset a distance greater than 3 degrees from bore  40  and not channel  42 , i.e., the plane containing axis  45  being parallel to the sagittal plane is also parallel to a plane containing axis  41 .  
         [0036]     Channel  42  may include a recessed portion  46  located within the interior of connector  14  and beveled portion  48  located on both exterior sides  50  and  52  of connector  14 . Although recessed portion  46  and beveled portion  48  both may be elliptical in shape, the inner diameter of portion  48  is smaller than the diameter of portion  46 . Preferably, a ridge  54  extends between beveled portion  48  and recessed portion  46  on both sides of connector  14 . Additionally, beveled portions  48  may have entrances that are tapered inwards reducing the minimum diameter of the beveled portions as you approach recessed portion  46  from the exterior of the connector.  
         [0037]     This configuration enables ball ring  18  to be placed within recessed portion  46  by compressing the ball ring diameter and still have a certain degree of motion, because in its relaxed position, the ball ring has a diameter less than the minimum diameter of the recessed portion. However, beveled portion  48  along with ridge  54  prohibits ball ring  18  from being able to withdraw out of recessed portion  46  because the beveled portion has a diameter less than the diameter of ball ring  18  in its relaxed position.  
         [0038]     Channel  42  has an opening that intersects with set screw bore  44 . This allows a set screw in bore  44  to contact the outer surface of ball ring  18  once the ball ring is housed within recessed portion  46  of channel  42 . As shown in  FIG. 3 , set screw bore  44  includes interior threads and is adapted for receiving set screw  20 , which includes mating external threads.  
         [0039]     As illustrated in  FIG. 6 , in the preferred embodiment, ball ring  18  has a generally spherical outer surface having an opening  80  through its body and, on insertion, may be coaxial with channel  42 . Ball ring  18  is adapted to receive spinal rod  16 , specifically opening  80  of the ball ring receives the rod. As shown in  FIG. 6 , ball ring  18  includes a through slot  82  extending through the surface of the ball ring. The through-slot  82  allows ball ring  18  to be compressed such that the inner diameter of ball ring opening  80  is reduced when the ball ring is compressed as well as expanded if need be.  
         [0040]     As shown in  FIG. 7 , the threads of set screw  20  are engageable with the threads of set screw bore  44 , thus permitting set screw  20  to be threaded downward through set screw bore  44  in order to apply pressure against ball ring  18 .  
         [0041]     Set screw  20  also includes a recess  60  for engaging a tool. Although the recess is shown in the form of a hexagonal recess adapted to receive an end of a hexagonal driver for turning set screw  20 , it will be understood that other internal or external tool engagement features can be used according to the present invention. In a preferred embodiment, the set screw is cylindrical and the surface of set screw  20 , which applies pressure against the ball ring, may include a tapered end surface  62 . Tapered end  62  contacts ball ring  18  to compress the same as will be described below.  
         [0042]     In a preferred embodiment, set screw  20  is sealed or pre-seated within connector  14  such that the set screw cannot be inadvertently removed from the connector. The elimination of any inadvertent removal of set screw  20  can be accomplished by providing a flared portion or a lip  66  on the end of the set screw opposite tool engaging recess  60 . The flared portion or lip  66  has a diameter that is greater than the diameter of the threaded portion  70  of set screw  20 . Thus, prior to ball ring  18  being placed within channel  42  of connector  14 , set screw  20  may be maneuvered through channel  42 . Upon reaching the intersection point between channel  42  and set screw bore  44 , set screw  20  with the surface containing recess  60  proceeding first, is placed through the opening of the set screw bore and translated upwards along axis  45 . The entrance of set screw bore  44  may have a diameter larger than the rest of bore  44 . As for example, the diameter at the entrance of bore  44  may be slightly larger than the diameter of set screw lip  66 , but the diameter of the rest of bore  44  may be slightly smaller than the set screw lip. This enables bore  44  to receive the set screw, but also prevents the set screw from inadvertently being removed or becoming displaced by sliding through the opposite outer end of set screw bore  44 . Additionally, or in the alternative, set screw  20  may be provided with a radially extending groove  68  that is without threads. The external threads are disposed only on the part of the set screw above groove  68 . Below groove  68 , the set screw includes a smooth portion  69  or is at least without threads. Thus, even if the diameters of set screw  20  and bore  44  are similar throughout, smooth portion  69  is unable to cooperate with the threads of bore  44 , thus prohibiting set screw  20  from becoming displaced from the outer end of bore  44 . After set screw  20  is located within bore  44 , ball ring  18  may then be placed within recessed portion  46 , thus further locking the set screw within the bore. Set screw  20  may be prevented from backing out of bore  44 .  
         [0043]     Compression of ball ring  18  occurs when set screw  20  is translated downward within set screw bore  44  until it begins to apply a pressure against ball ring  18 . Prior to this, rod  16  is first placed within ball ring  18  and is slidably received by the opening  80 . At this point, rod  16  may be angled within connector  14  with respect to the spinal column. Once aligned, ball ring  18  and rod  16 , housed in the ball ring, are moved laterally by the force applied by the set screw. They continue to move laterally until ball ring  18  contacts stem  22  of bone fastener  12  located in first bore  40 . By continuing translation of set screw  20  downward against ball ring  18 , the ball ring, rod  16  and stem  22  of fixation element  12  move laterally until stem  22  abuts against an interior wall  51  of bore  40  of connector  14 . At this point, stem  22  can no longer translate laterally, the continued pressure applied by the set screw against ball ring  18  causes ball ring  18  to compress and tighten around spinal rod  16  until a point is reached where ball ring  18  is fully tightened about spinal rod  16 . Stem  22 , spinal rod  16  and ball ring  18 , which is part of the connector body, are now locked within the connector body relative to one another. The lateral movement of ball ring  18  and rod  16  is aided by the elliptical shape of channel  42 . Additionally, the elliptical shape of channel  42  and bore  40  results in at least three points of contact for locking both ball ring  18  and stem  22 , as shown in  FIG. 3 . First, ball ring  18 , when in a locked position, may receive a force from or translates force to: (1) set screw  20 ; (2) stem  22 ; and (3) an interior wall  47  of channel  42  at positions A, B and C respectively in  FIG. 8A . When in the locked position, stem  22  also may have at least three points of contact or three locations where it applies or receives pressure from: (4) ball ring  18 ; and (5) and (6) two points located on interior wall  51  of bore  40 , D, E and F, respectively in  FIG. 8B .  
         [0044]     The use of ball ring  18  in the assembly also provides a greater friction surface area between not only set screw  20  and rod  16 , but also the friction surface between stem  22  and ball ring  18  as compared with an assembly devoid of a ball ring. The cylindrical interface  84  of ball ring  18  has a radius about equal to that of rod  16 , in such a way that ball ring  18  can slidingly receive rod  16 . The spherical outer face  86  of ball ring  18  has a radius which is adapted such that, when positioned within connector  14 , the walls or recessed portion  46  may contact outer face  86 . The angular position of rod  16  engaged in ball ring  18  can be controlled in two mutually perpendicular planes over an amplitude of, for example, 15 degrees on either side of a mean position of the rod, in which the rod is perpendicular to the sagittal plane.  
         [0045]     In the preferred method of operation of the bone fixation assembly, bone fixation element  12  is first engaged to a vertebral body  2  as shown in  FIG. 1 , preferably into a previously-drilled pilot hole in the bone. The bone fixation element is then screwed into the bone using a driver or other appropriate device, advancing the bone fixation element along its longitudinal axis into the vertebral bone. Bore  40  of connector  14  is aligned with stem  22  of bone fixation element  12 , and the connector is moved in an anterior direction with first bore  40  receiving stem  22 . At this point, connector  14  is still capable of being moved axially as well as rotationally about bone fixation element  12 . Additionally, since ball ring  18  and set screw  20  are preassembled as previously described, ball ring  18  is located in channel  42  and ready to receive spinal rod  16 , which is easily inserted through the ball ring opening  80 . Prior to tightening the assembly, the angle of the fixation element is adjusted by moving connector  12  and the first bore  40  with respect to stem  21  of element  12 . Due to the larger diameter of first bore  40  as compared to stem  22  of the bone fixation element, the bone fixation assembly can be manipulated to cover a broader range of angles for capturing an orthopedic stabilizing rod. Once bone fixation element  12 , connector  14  and spinal rod  16  have all been positioned correctly, set screw  20  is translated downwardly through set screw bore  44  until the elements are secured relative to one another, as previously described. Achieving sufficient angulation between bone fixation elements while engaging the orthopedic rod is essential for assemblies mounted in spines having abnormal curvatures. Sufficient angulation is also important in the cervicothoracic junction of the spine. Thus, bone fixation assembly  10  can be locked at a wide range of angles relative to a vertebral body using a one-step process.  
         [0046]     As shown in  FIG. 9 , in an alternate embodiment, set screw bore  144  may be positioned between first bore  140  and channel  142  with regard to connector  114 . Set screw  120  is then translated downward within set screw bore  144 , so as to come into contact with ball ring  118  as well as a second ball ring  119 . Ball ring  118  works substantially the same as previously described. Specifically, set screw  120  applies a force against ball ring  118  constricting ball ring  118  so that it tightens around rod  116  disposed within the opening  180  of the ball ring. Additionally, as set screw  120  is translated downward into the set screw bore, the set screw simultaneously applies a pressure against second ball ring  119  located in first bore  140 . A bone fixation element  112  is disposed in first bore  140  and has housed within its aperture  181  bone fixation element  114 .  
         [0047]     According to a preferred embodiment, bone fixation assembly  10  is supplied as a kit including a plurality of connectors  14  with ball rings  18  and set screws  20  pre-seated in each connector  14  to reduce the number of loose parts and prevent any small loose parts from being lost, or from having to be handled and manipulated during surgery. As used herein, the terminology “pre-seated” means that the elements are pre-assembled in a manner to then prevent from being inadvertently removed from their respective bores, as opposed to being loose in a package and requiring assembly of the individual components. However, the kit could include the set screws and rings as loose parts. The kit may also include a plurality of rods  16  and fixation elements  12 . The size of the rods, as well as the size and type of fixation elements, may be varied within each kit and may also be different from kit to kit.  
         [0048]     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.