Patent Publication Number: US-11395684-B2

Title: Head to head cross connector

Description:
FIELD OF THE INVENTION 
     The subject invention relates generally to the field of spinal stabilization systems and more particularly to a cross connector that connects to a bone anchor at the juncture of the connection with the longitudinally extending connecting rod. 
     BACKGROUND OF THE INVENTION 
     Cross connectors are known to provide transverse rigidity to a dual rod spinal stabilization in a patient. Cross connectors are typically fastened to two parallel connecting rods spanning a length of the spine on opposite contralateral sides of the median plane of the spine. Cross connectors in general can be clumsy to place on the rods in a rod/screw construct, a difficulty that is enhanced by the limited ability to manipulate or position the typical cross connector. For instance, certain cross connectors only permit relative movement of rod-engaging ends towards or apart from each other. In some cases the cross connector may permit relative rotation between the rod-engaging ends within a single plane parallel to the axis of the cross connector. Other cross connector designs allow rotation of a rod-engaging end about its own longitudinal axis. 
     One known version of a cross connector is used to connect the ends of the cross connector to a portion of the longitudinal connecting rods extending between bone anchors, such as hooks or pedicle screws. An example of this type of cross connector is shown and described in commonly assigned U.S. Pat. No. 8,372,120, entitled “Multi-axial Cross Connector”, issued on Feb. 12, 2013 to Anthony James (the &#39;120 patent), the entire contents of which are incorporated herein by reference. 
     Another type of known cross connector is used to attach to the longitudinal rods at the location where such rods are joined to the bone anchors. This type of cross connector is often considered when the space between bone anchors and hence the extent of the longitudinal connecting rod therebetween is minimal. This condition tends to occur in the cervical spine where vertebrae are smaller than the thoracic or lumber regions of the spine and space for spinal fixations systems is limited. Examples of this type of head to head cross connector are shown and described in U.S. Pat. No. 5,397,363, entitled “Spinal Stabilization Implant System”, issued on Mar. 14, 1995 to Gelbard, and U.S. Pat. No. 6,592,585, entitled “Spine Fixing System”, issued on Jul. 15, 2003 to Lee et al. Adjustability of these cross connectors is somewhat limited 
     As these systems have evolved, various degrees of freedom of relative orientation were integrated into the systems in order to accommodate misaligned spinal curvature as well as to more flexibility adjust to space limitations as well as anatomic conditions. Advances in head to head cross connectors with improved flexibility and adjustability are shown and described in U.S. Pat. No. 8,672,978, entitled “Transverse Connector”, issued on Mar. 18, 2014 to Dant et al., and U.S. Pat. No. 8,784,452, entitled “Transconnector” issued on Jul. 22, 2014 to Saidha et al. While showing improvement in flexibility and adjustability, a drawback of these cross connectors is the increase in height profile at the location of the connection to the bone anchor 
     Nevertheless, there is a need for a head to head cross connector that provides enhanced degrees of freedom to address the wide range of spinal treatment protocols that may be encountered, as well as to provide a relatively low profile at the juncture of the cross connector, bone anchor and connecting rod is a spinal stabilization system. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved spinal stabilization system that includes a cross connector that provides multiple degrees of freedom for connecting an elongate connecting rod to a bone screw in the spinal stabilization system. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a top perspective view of a spinal stabilization system utilizing a head to head cross connector in accordance with an embodiment of the present invention. 
         FIG. 2  is an exploded view of the spinal stabilization system of  FIG. 1   
         FIG. 3  is an exploded view of the polyaxial bone screw used in the spinal stabilization system of  FIG. 1 . 
         FIG. 4  is an end elevation view of the spinal stabilization system of  FIG. 1 . 
         FIG. 5  is a top plan view of the spinal stabilization system of  FIG. 1 . 
         FIG. 6  is a cross-sectional view of the spinal stabilization system as seen along viewing lines VI-VI  FIG. 5 . 
         FIG. 7  is an enlarged view of the portion of  FIG. 6  within circle A. 
         FIG. 8  is a top perspective view an alternative connecting element with a circular rod used in the cross connector of the spinal stabilization system. 
         FIG. 9  is an enlarged view of the connecting portion of one of the cross connector connecting elements. 
         FIG. 10  is an enlarged view of the portion of  FIG. 6  within circle B showing one end of the connecting elements of the cross connector in a substantially perpendicular orientation with respect to the bone screw. 
         FIG. 11  is an enlarged view of the portion of  FIG. 6  within circle B showing one end of the connecting elements of the cross connector in an angular orientation with respect to the bone screw. 
         FIG. 12  is an enlarged view of the circled portion of  FIG. 5  with the connecting rod removed for clarity showing one end of the connecting elements of the cross connector in a substantially perpendicular orientation with respect to the channel of the bone screw yoke. 
         FIG. 13  is an enlarged view of the circled portion of  FIG. 5  with the connecting rod removed for clarity showing one end of the connecting elements of the cross connector in an angular orientation with respect to the channel of the bone screw. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     As shown in  FIGS. 1 and 2 , a spinal stabilization system  10  spans between successive vertebrae of the spine. A connection member, such as elongate connecting rods  12 , each having a longitudinal axis  12   a , extends along the length of the spine and provides an anchor point for connecting each vertebra to the system  10 . Connecting rods  12 , typically formed of stainless steel, are contoured by bending to approximate the normal curvature of the spine for the particular instrumented spinal segments. Bone anchors  14  are provided for connecting each of the vertebral segments to the rods  12  on contralateral sides of the spine. These bone anchors  14  may include hooks, bolts or screws that have bone engaging portions for engaging a vertebra. In a particular arrangement of the present invention, the bone anchor  14  is a bone screw, more specifically, a polyaxial bone screw. Polyaxial bone screw  14  includes features that provide for polyaxial connection to rod  12  in a relatively high degree of angulation. A head to head cross connector  16  is adapted in spinal stabilization system  10  to attach to connecting rods  12  at the location where such rods  12  are joined to the bone screws  14 . In a particular example, spinal stabilization system  10  is configured and sized for connection to the cervico-thoracic spine although use in the thoracic and lumbar spine regions is also contemplated. 
     Referring now to  FIG. 3 , the elements of polyaxial bone screw  14  are shown in exploded view. Polyaxial bone screw  14  comprises a threaded fastener  18  defining a bone engaging portion, a yoke  20 , a crown  22 , a screw support  24 , a connecting element  26  for rotatably connecting screw support  24  and yoke  20 , and a fastening element  28 . Yoke  20  has a top surface  20   a  and a bottom surface  20   b  and a yoke axis  20   c  extending through top surface  20   a  and bottom surface  20   b . Yoke  20  has a connecting rod receiving channel  20   d  defined by a pair of opposed upstanding arms  20   e  and  20   f . Channel  20   d  is generally U-shaped defining a channel axis  20   g  (see  FIG. 2 ) and extending through yoke top surface  20   a . Bone engaging portion  18  projects downwardly from yoke bottom surface  20   b . Interior surfaces  20   h  of arms  20   e  and  20   f  are threaded to threadably receive the external threads  28   a  of fastening element  28 , which may be a set screw. Polyaxial bone screw  14  is more particularly described in commonly assigned U.S. patent application Ser. No. 15/378,521 entitled “Polyaxial Bone Screw”, filed on Dec. 14, 2016, the entire contents of which are incorporated herein by reference. It should be appreciated that other polyaxial bone screws that include a yoke supporting a threaded bone engaging portion for rotational and articulating movement relative to the yoke may also be used. In addition, monolithic bone anchors, such as pedicle screws or hooks may also be used in the context of the subject invention where less system flexibility and fewer degrees of freedom of component movement within the spinal stabilization system may be tolerated. 
     Turning now also to  FIGS. 4-7 , details of head to head cross connector  16  are described. Cross connector  16  includes an elongate connecting element  30  of extent to span the lateral distance between contralateral bone screws  14 . Connecting element  30  comprises one end  30   a  defining a connecting portion  32  for connection to bone screw  14  and an opposite other end  30   b  defining a connecting portion  34  for connection to a bone screw  14  on the contralateral side of the spine. Elongate element  30  comprises a first “adjustable” element  36  and a second “stationary” element  38 . For the purposes of the present description, the designation of one element as being “adjustable” and the other as “stationary” is arbitrary, with the understanding that the intent is to describe that the two elements  36  and  38  are movable relative to each other, as will be described. An elongate bar  36   a  extends axially from one end  30   a  of first adjustable element  36  and terminates in an opposite first coupling end  36   b . An elongated bar  38   a  extends axially from other end  30   b  of second stationary element  38  and terminates in an opposite second coupling end  38   b  defined by a flange  40 . Flange  40  is cup-like and defines an opening  42  extending therethrough as best seen in  FIG. 7 . An inner surface  44  of flange  40  is spherical to form an annular spherical interface capable of allowing articulation or pivoting in multiple degrees of freedom or about multiple separate axes, as will be described. While elongate bars  36   a  and  38   a  are shown as being generally linear, it should be appreciated that elongate bars  36   a  and  38   a  and he also be curved. 
     An adjustment mechanism  46  couples first coupling end  36   b  and said second coupling end  38   b  of connecting element  30 . Adjustment mechanism  46 , the details of which are illustrated in  FIG. 7 , includes in one embodiment a pivot element  48  defined at its upper portion by a yoke element  50  that includes two opposing branches  52  projecting upward from a base  54  at the lower portion of the pivot element  48 . Branches  52  extend through opening  42  in the flange  40  of stationary element  38  and define a slot  56  (see  FIG. 3 ) therebetween that is sized to receive the elongate bar  36   a  of first adjustable element  36 . The bar end  36   c  of the adjustable element  36  is preferably sized larger than elongate bar  36   a  so that it cannot pass through the slot  56  when elongate bar  36   a  is received within the slot  56  to thus prevent disengagement of the two cross connector elements  36 ,  38  when the elements are moved apart. An upper outer surface  54   a  of the base  54  is a partially spherical bearing surface forming an articulating joint with the spherical inner bearing surface  44  of the flange  40 . It can be appreciated that the yoke element  50  can swivel or pivot in multiple directions or degrees of freedom, or about at least three independent axes, relative to the flange  40  so that the branches  52  of yoke element  50  can be oriented at a range of angles relative to the bar  38   a  of the stationary element  38 . 
     Adjustment mechanism  46  includes a spring  58  that contacts the elongate bar  36   a  of first element  36 . Spring  58  is calibrated to provide some resistance or friction to hold the cross connector  16  in a particular orientation while permitting continued articulation or pivoting of first element  36  relative to second element  38  until the first and second elements  36 ,  38  are finally rigidly locked. 
     Branches  52  of yoke element  50  extend generally parallel to each other and further define an inner threaded surface  60  for receiving a locking element, such as set screw  62 . As shown in  FIG. 7 , set screw  62  has outer threads  62   a  that are threaded into the branches  52  to bear against elongate bar  36   a  of first adjustable element  36 . A cap  62   b  may be provided on the top of set screw  62  to partially surround branches  52  so as to prevent splaying of branches  52  during tightening of set screw  62  into yoke element  50 . Cap  62   b  includes an opening  62   c  configured to permit access to a socket  62   d  in set screw  62  by a conventional driving tool to thread set screw  62  to branches  52 . Although an inner thread and set screw arrangement is described for locking first element  36  within yoke element  50 , other clamping or fixation mechanisms are contemplated. For instance, in lieu of the inner threads  60 , yoke element  50  may be provided with exterior threads on the branches  52  that are engaged by an internally threaded nut, rather than the externally threaded set screw  62 . 
     Set screw  62  is threaded to branches  52  of the yoke element  50  initially to only loosely retain elongate bar  36   a  within yoke element slot  56  in a non-locked position. In this position, the spherical interface of adjustment mechanism  46  provided by flange spherical inner surface  44  and yoke base spherical surface  54   a  permits articulation or pivoting of the two elements  36  and  38  in multiple degrees of freedom or along multiple separate axes. In an addition degree of freedom, first adjustable element  36  may translate axially relative to second stationary element  38  in such non-locked position. In a further degree of freedom as shown in  FIG. 8 , elongate bar  36   a  of first adjustable element  36  may formed as a rod  36   d  having a circular cross-section allowing first adjustable element  36  to also rotate within slot  56  of yoke element  50 . 
     Once the cross connector  16  is arranged in its desired orientation with the respect to bone screws  14 , set screw  62  can be fully tightened within the yoke element  50 . As set screw  62  is advanced into the inner threaded surface  60  of yoke element  50 , the screw  62  pushes the first element  36  downward and pulls the yoke element  50  upward, thereby compressing the first element  36 , spring  58  and flange  40  of second element  38  between the set screw  62  and the base  54  of the yoke element  50  to thereby rigidly lock first element  36  and second element  38  in a locked position. Further details of the structure and function of adjustment mechanism  46  are described in the &#39;120 patent, incorporated herein by reference. 
     Turning now to  FIGS. 9 and 10 , details of connecting portions  32  and  34  of cross connector  16  are described. In a particular arrangement, connecting portions  32  and  34  are identical, and as such only the details of connecting portion  32  are set forth herein. As seen in  FIG. 9 , connecting portion  32  includes an enlarged plate  64  at the one end  30   a  of connecting element  30 . Plate  64  comprises a pair of spaced opposing side walls  66   a  and  66   b  interconnected by a connecting bar  68 . Side walls  66   a ,  66   b , connecting bar  68  and elongate bar  36   a  define a fully bounded opening  70 . Opening  70  is sized and configured to receive one of yoke arms  20   e ,  20   f  of bone screw  14  and is formed in a shape to generally mimic the cross-section of one of yoke arms  20   e ,  20   f . In a particular arrangement, opening  70  is generally crescent-shaped, as depicted in  FIG. 9 . Connecting bar  68  includes an upper surface  68   a  and a lower surface  68   b  as shown in  FIG. 10 , and a pair of spaced opposing side surfaces  68   c  and  68   d  extending between upper surface  68   a  and lower surface  68   b . Side surface  68   c  communicates with opening  70  and side surface  68   d  defines a terminal end of connecting element  30 . Upper surface  68   a  of connecting bar is formed to have a convexly curved surface while lower surface  68   b  is formed to have a substantially flat surface, as illustrated in  FIG. 10 . In a particular arrangement, side surface  68   d  may include a protruding portion  68   e  projecting outwardly therefrom and side surface  68   c  may have a protruding portion  68   f  protruding inwardly into opening  70 . Protruding portions  68   e  and  68   f  are formed to generally conform to the shape of yoke channel  20   d  and to provide enhanced construct strength. 
     In use in the stabilization system  10 , once the desired angulation of bone screw  14  relative to yoke  18  and the orientation of channel  20   d  of bone screw yoke  20  are properly achieved, connecting rod  12  may then be introduced into yoke channel  20   d  for securement to polyaxial bone screw  14 . Cross connector  16  in the non-locked position is manipulated by articulating first and connecting elements  36  and  38  relative to each other and/or translating or rotating adjustable first element  36  relative to stationary element  38  so as to place connecting portions  32  and  34  in proper position for connecting to contralateral bone screws  14 . Once so positioned, the connecting bar  68  of each connecting portion  32 ,  34  is placed into yoke channel  20   d  of respective bone screws  14 , as shown in  FIGS. 10-11 , in a non-secured position as yoke arm  20   e  is simultaneously received in and through opening  70  of connecting portion  32 . 
     In this non-secured position, lower surface  68   b  of connecting bar  68  of each connecting portion  32 ,  34  contacts connecting rod  12  and is capable of allowing movement of connecting elements  36 ,  38  respectively in at least two degrees of freedom relative to bone screw yoke  20 . Such movement is further facilitated while adjustment mechanism  46  of cross connector  16  is in the non-locked position. 
     In the first degree of freedom as shown in  FIGS. 10-11 , connecting bar  68  is situated in yoke channel  20   d  and is sized and configured to move within yoke channel  20   d  and on the surface of elongate connecting rod  12  about the axis  12   a  of elongate connecting rod  12 . Elongate bar  36   a  has a longitudinal axis  36   e  that, as shown in  FIG. 10  in one condition, lies substantially perpendicular to axis  20   c  of yoke  20 . Lower surface  68   b  of connecting bar  68  is in contact with elongate connecting rod  12  and also lies substantially perpendicular to axis  20   c . In the adjusted condition shown in  FIG. 11 , elongate bar  36   a , as a result of the movement of connecting bar  68  within yoke channel  20   d , is rotated downwardly relative to the perpendicular position of  FIG. 10  by an adjustable angle, α. Such movement is facilitated by the curvature of side surfaces  68   c  and  68   d  of connecting bar  68 , and the substantially flat surface of lower surface  68   b  of connecting bar  68 . As elongate bar  36   a  is rotated, flat lower surface  68   b  moves as a tangent on outer surface of connecting rod  12  further enabling rotation of elongate bar  36   a  relative to yoke  20 . It should be appreciated that elongate bar  36   a  may be similarly rotated upwardly relative to the perpendicular position of  FIG. 10 . In a particular configuration adjustable angle, α may be about 3° from perpendicular in both directions, thereby establishing a range of adjustment in this first degree of freedom of about 6°. It should also be understood that adjustable angle, α may be increased or decreased by varying the size of opening  70  and/or the width of connecting bar  68 , defined by the spacing between side surfaces  68   c  and  68   d.    
     In the second degree of freedom as initially illustrated in  FIG. 12 , connecting bar  68  is situated in yoke channel  20   d  such that longitudinal axis  36   e  of elongate bar  36   a  lies substantially perpendicular to axis  20   g  of yoke channel  20   d . The width W 1  of connecting bar  68  is defined by the spacing between side surfaces  68   c  and  68   d . The spacing between yoke arms  20   e  and  20   f  adjacent connecting bar side surfaces  68   c  and  68   d  defines a channel width, W 2 . Connecting bar width W 1  is formed to be less than channel bar width W 2  in an amount sufficient to allow connecting bar  68  to rotate within yoke channel  20   d  in a second degree of freedom about yoke axis  20   c . In the adjusted condition shown in  FIG. 13  elongate bar  36   a , as a result of the difference between connecting bar width W 1  and channel width W 2 , is rotated about yoke axis  20   c  relative to the perpendicular position of  FIG. 12  by an adjustable angle, β. It should be appreciated that elongate bar  36   a  may be similarly rotated angularly in an opposite direction relative to the perpendicular position of  FIG. 12 . In a particular configuration, connecting bar width W 1  may be formed to have a dimension of approximately 0.100 inches and channel width W 2  may be formed to have a dimension of approximately 0.160 inches with the outer diameter of yoke being approximately 0.354 inches. As such, the ratio of connecting bar width W 1  to channel width W 2  is approximately 62.5%. In such a configuration adjustable angle, β may be about 6° from perpendicular in both directions, thereby establishing a range of adjustment in this second degree of freedom of about 12°. It should also be understood that adjustable angle, β may be increased or decreased by varying the size of connecting bar width W 1  and channel width W 2 , and hence the ratio of connecting bar width W 1  to channel width W 2 , considering the potential effect on construct strength and desired angulation. 
     After achieving appropriate orientation of each connecting element  36  and  38  relative to each of the respective contralateral bone screws  14 , bone screw fastening element  28 , such as a set screw, may then be threaded into yoke  20 , by referring again to  FIGS. 10-11 . In a particular arrangement, the bottom surface  28   b  a fastening element  28  is formed to have a substantially flat surface. Each set screw  28  is inserted until it loosely contacts upper surface  68   a  of connecting bar  68  in a manner to allow final manipulation of connecting elements  36  and  38  relative to respective yokes  20 . In the event element  36 , for example, is in the perpendicular orientation shown in  FIG. 10 , flat bottom surface  28   b  of set screw  28  will contact the apex  68   g  of convexly curved surface  68  as a tangent. In the event element  36  is adjusted by an adjustable angle, α, as shown in  FIG. 11 , flat bottom surface  28   b  of set screw  28  will contact upper surface  68   a  of connecting bar  68  tangentially but at a location between apex  28   g  and either of side surfaces  68   c  or  68   d . Such tangential contact together with the tangential contact between lower flat surface  68   b  of connecting bar  68 , will allow set screw  28  upon further tightening to apply a uniform compressive force to rigidly secure connecting bar  68  to elongate connecting rod  12  and elongate connecting rod  12  to yoke  20  in a secured position. 
     Having secured connecting elements  36  and  38  to respective contralateral bone screws  14 , coupling ends  36   a  and  38   a  of respective connecting elements  36  and  38  may be rigidly locked by adjustment mechanism  46 . Set screw  62 , as described with reference to  FIG. 7 , having been loosely placed is then fully tightened within the yoke element  50 . As set screw  62  is advanced into the inner threaded surface  60  of yoke element  50 , the screw  62  pushes the first element  36  downward and pulls the yoke element  50  upward, thereby compressing the first element  36 , spring  58  and flange  40  of second element  38  between the set screw  62  and the base  54  of the yoke element  50  to thereby rigidly lock first element  36  and second element  38  in a locked position. Further details of the structure and function of adjustment mechanism  46  are described in the &#39;120 patent, incorporated herein by reference. Thus, as can be seen in  FIGS. 10 and 11  with upper surface  68   a  of connecting bar  68  being disposed within yoke channel  20   d  and below yoke top surface  20   a , cross connector  16  may be attached at the respective contralateral bone screws  14  with minimal, if any, height added to the profile of the bone screws  14 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. For example, there may be anatomic conditions wherein it would be desirable for the lower surfaces  68   b  of respective connecting portions  32  and  34  to lie on substantially the same plane when adjustment mechanism  46  of cross connector  16  is in either the non-locked or locked position. By reference to  FIG. 4 , it can be seen that first adjustable element  36  is elevated with respect to stationary second element  38  as a result, in part, of the interconnection of elements  36  and  38  at adjustment mechanism  16  where the elongate bars  36   a  and  38   a  are in different planes. To allow lower surfaces  68   b  of each connecting portion  32  and  34  to lie in substantially the same plane  72 , a lower surface  68   b  of connecting portion  34  is offset by a distance S 1  from bottom surface  38   c  of stationary second element  38  in a manner to compensate for the elevation difference at the connection of elements  36  and  38  at adjustment mechanism  46 . 
     It is therefore understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.