Abstract:
A stabilization device for bone parts or vertebrae includes two bone anchoring devices for anchoring in the bone parts or vertebrae. At least one of the bone anchoring devices includes an anchoring element with an anchoring section for anchoring in a bone part or a vertebra and a head, and a receiving part for receiving a stabilization rod. The receiving part has a seat for receiving the head so that the head can pivot with respect to the receiving part. The stabilization device includes a first pressure element which is movable in the receiving part so that it can be pressed onto the head to lock the angular position of the head. The stabilization device includes at least two stabilization rod sections, and at least two guiding channels within the receiving part which have a distance from each other for guiding through the at least two stabilization rod sections so that the rod sections do not touch each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 12/571,299, filed Sep. 30, 2009, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/103,858, filed Oct. 8, 2008, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application No. 08 017 644.9, filed Oct. 8, 2008, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a bone anchoring device, in particular to a polyaxial bone screw which is connected to two stabilization rods and to a stabilization device having such a bone anchoring device, in particular for the stabilization of the spinal column. 
         [0003]    A dynamic stabilization device for bones, in particular for vertebrae, is described in US 2004/0049190 A1. The stabilization device includes two bone anchoring elements, at least one of which is a polyaxial bone screw and a rigid rod with a longitudinal axis connecting them. An elastic element is inserted between the two bone anchoring elements. The elastic element acts on the bone anchoring elements to exert a force in a direction of the longitudinal axis. One of the bone anchoring elements is fixedly connected to the rod to prevent translational movement of the rod and the other bone anchoring element is slidably connected to the rod. 
         [0004]    EP 1 800 614 A 1 describes a dynamic stabilization device for bones or for vertebrae having at least two bone anchoring elements and at least one connection element in the form of an elastic loop connecting the bone anchoring elements. In one embodiment, the bone anchoring element is in the form of a polyaxial bone screw having a receiving part which accommodates to two elastic loops each of which can be connected to a second bone anchoring element. 
         [0005]    Based on the foregoing, there is a need to provide a bone anchoring device and a stabilization device comprising such a bone anchoring device which allows the dynamic stabilization of bone parts or vertebrae and which allows a variable design of elastic properties of the dynamic stabilization device. 
       SUMMARY 
       [0006]    The provision of a modular double-rod, i.e. two rods, allows to design the bone anchoring device more compact in terms of the height of the bone anchoring device, since each rod can be designed smaller than a single rod. The low profile cross-section of two rods compared to one single rod has also the advantage that the stiffness of the rods is enhanced. The stability in view of bending or torsional loads of the double-rod system is also enhanced. 
         [0007]    The dynamic properties of the stabilization device can be adjusted by selecting appropriate rods and/or adjusting the sliding motion of the rods by stops and/or dampening elements. The dynamic properties of the rods can vary. For example the rods can have the same or different elastic properties. They can be made of the same or different material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a perspective side view of the stabilization device. 
           [0009]      FIG. 2  shows a perspective exploded view of the stabilization device. 
           [0010]      FIG. 3  shows an exploded view of the bone anchoring device according to a first embodiment. 
           [0011]      FIG. 4  shows a perspective view of the bone anchoring device of  FIG. 3  in an assembled state. 
           [0012]      FIG. 5  shows a perspective view from the side of the first pressure element in a first embodiment. 
           [0013]      FIG. 6  shows a perspective view of the second pressure element in a first embodiment. 
           [0014]      FIG. 7  shows a partially sectional view of the bone anchoring device with the first and second pressure element according to the first embodiment. 
           [0015]      FIG. 8  shows a partially sectional view of the bone anchoring device with the first and second pressure element according to a second embodiment. 
           [0016]      FIG. 9  shows an exploded perspective view of the bone anchoring device with a first and second pressure element according to a third embodiment. 
           [0017]      FIG. 10  shows a perspective view of the bone anchoring device of  FIG. 9  in an assembled state. 
           [0018]      FIG. 11  shows a perspective view of a rod according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The invention is now described in detail with reference to the embodiment of the stabilization device shown in  FIGS. 1 to 8 . The stabilization device includes a first polyaxial pedicle screw  1 , a second pedicle screw  2  and two rods  3   a,    3   b  connecting them for stabilizing two adjacent vertebrae. The two rods  3   a,    3   b  may be separate rods as shown in  FIG. 2 . Alternatively, as shown in  FIG. 11 , the rods  3   a,    3   b  may be connected or formed in one-piece to define a single rod  3 . 
         [0020]    On each rod a spring element  4   a,    4   b  is provided and the rods  3   a,    3   b  are connected by rod connectors  5 ,  6 . The rods  3   a,    3   b  are fixedly clamped in the second pedicle screw  2  and can slide through the first pedicle screw  1  as shown by the arrows. The sliding motion is limited by means of the rod connector  6  which connects the free ends of the rods  3   a,    3   b  and acts as a stop. The springs  4   a,    4   b  and the rod connector  5  limit the sliding motion of the rods  3   a,    3   b  relative to the first pedicle screw  1  in the direction of the second pedicle screw  2 . The springs provide elastic dampening. The rod connectors  5 ,  6  are sleeve shaped with two channels  5   a,    5   b,    6   a,    6   b , respectively, for guiding through the rods  3   a,    3   b.  The distance of the channels corresponds to the distance of the rods in which they are guided through the pedicle screws. The rod connectors  5 ,  6  connect the rod  3   a,    3   b  by means of a press-fit connection i.e the diameter of the channels is selected such that the rods are firmly connected. The rod connectors  5 ,  6  can be made of an elastomer material or any other body compatible material. 
         [0021]    The springs  4   a,    4   b  in this embodiment are shown as helical springs encompassing the rods  3   a,    3   b  like sleeves. They can be made of any body compatible material, in particular of titanium, nickel titanium alloys, for example nitinol, or other materials. 
         [0022]    The rods  3   a,    3   b  exhibit a flexibility under forces having a component perpendicular to the rod axis, such as bending forces. For this purpose the rods are made of non-compressible materials, such as stainless steel, titanium, nickel titanium alloys, such as nitinol, PEEK or carbon reinforced PEEK or other body compatible materials. 
         [0023]    It should be noted that the rod connectors and the springs are only examples for the function of connecting the two rods, providing a stop and providing a dampening to the sliding motion. 
         [0024]    Next, the first pedicle screw  1  will be described in detail with reference to  FIGS. 3 to 7 . The pedicle screw  1  comprises a screw element  10  with a threaded shank  11  and a spherically segment-shaped head  12 . At the free end of the head  12  a recess  13  is provided for engagement with a tool. The pedicle screw  1  further comprises a receiving part  20  with a first end  21  and a second  22  and a coaxial bore  23  extending from the first end in the direction of the second end. At the second end  22  the bore  23  tapers to provide an opening and a seat  24  for the screw head  12  as shown in particular in  FIG. 7 . 
         [0025]    The receiving part  20  further comprises a recess  25  extending from the first end  21  in the direction of the second end  22  which provides a channel through the receiving part in a direction perpendicular to the bore axis of bore  23  for guiding through the rods  3   a,    3   b.  The recess provides two free legs  26   a,    26   b.  Near the first end  21  the free legs  26   a,    26   b  have an internal thread  27  for cooperation with a fixation screw  30 . The screw element  10  and the receiving part  20  as well as the fixation screw  30  are made of a rigid body compatible material, such as a body compatible metal like stainless steel or titanium or a titanium alloy, such as nitinol. 
         [0026]    For locking the head  12  and in consequence the angular position of the screw element  10  within the seat  24  of the receiving  20  a first pressure element  40  and a second pressure element  50  are provided. The first pressure element  40  and the second pressure element  50  also form guiding elements for guiding the rods  3   a,    3   b  through the receiving part  20 . The first pressure element  40  has a substantially cylindrical body part  41  which is sized such that the first pressure element  40  can be inserted in the receiving part and moved in an axial direction within the bore  23 . At its side facing the head  12  of the screw element the first pressure element  40  comprises a cylindrical recess  42  shown in  FIG. 7  in which a cylindrical insert  43  is provided. The insert  43  has on its side facing the head  12  of the screw element a spherical recess  44  the radius of which fits to the radius of spherical head  12  of the screw element. 
         [0027]    The first pressure element  40  further comprises a cuboid body part  45  which is shaped so as to fit in the recess  25  of the receiving part  20  as shown in particular in  FIGS. 3 and 4 . The width of the body part  45  corresponds to the width of the recess  25  and the length is selected such that the first pressure element is flush with the outer surface  28  of the receiving part  20  as shown in  FIG. 4 . On its side opposite to the recess  42  the cuboid body part includes two cylinder segment-shaped recesses  46   a,    46   b  the cylinder radius of which is slightly larger than the radius of the rods  3   a,    3   b.  The recesses  46   a,    46   b  extend perpendicular to the axis of the coaxial bore  23  of the receiving part  20 . The recesses  46   a , 46   b  form channels for receiving the rods  3   a,    3   b.  Since the recesses  46   a,    46   b  are spaced apart from each other a rib  47  is formed between them. The depth of the recesses  46   a,    46   b  is preferably slightly larger than the radius of the rods  3   a,    3   b.  The first pressure element  40  also has a coaxial bore  48  for providing access to the head  12  of the screw element with a tool. Similarly, the cylindrical insert  43  has a coaxial bore  49 . The cylindrical body part  41  and the cuboid body part  25  are shown to be made in one piece so that cylindrical segment-shaped flanges  41   a,    41   b  are provided on each side of the channel  46   a,    46   b.  The flanges facilitate the guidance of the first pressure element  40  within the receiving part  20 . The cuboid body part  45  prevents rotation of the first pressure element within the receiving part once the first pressure element is inserted into the recess  25 . 
         [0028]    The second pressure element  50  is substantially cuboid shaped with a width and length corresponding to that of the cuboid body part  45  of the first pressure element  40 . Therefore, it also fits into the recess  25  of the receiving part. On its long sides it comprises two cylindrical segment-shaped flanges  51   a ,  51   b  corresponding to the flanges  41   a,    41   b  of the first pressure element. On its side opposite to the first pressure element  40 , the second pressure element  50  comprises a cylindrical recess  52  and a coaxial cylindrical projection  53  in which a corresponding ring-shaped projection  31  and a cylindrical recess  32  of the fixation screw  30  engage, as shown in  FIG. 7 . Thereby, the fixation screw  30  can be rotatably connected to the pressure element  50 . 
         [0029]    On its side facing the first pressure element, the second pressure element  50  comprises two longitudinal cylinder segment-shaped recesses  56   a,    56   b  which are complementary in their size and distance to the channels  46   a,    46   b  of the first pressure element. The channels  56   a,    56   b  are spaced apart by a rib  57 . 
         [0030]    In the assembled state shown in  FIG. 7  the first pressure element presses via the insert  43  onto the head  12 . The second pressure element  50  presses onto the first pressure element  40  thereby providing closed channels for the rods  3   a,    3   b  which are accommodated therein with a gap  60  to the wall of the channel. Since the fixation screw  30  is rotatably connected to the second pressure element, the fixation screw  30  can be tightened when the second pressure element  50  is inserted. 
         [0031]    The first pressure element and the second pressure element can be made of a material which facilitates sliding of the rods  3   a,    3   b.  For example, the pressure elements can be made of titanium or a nickel titanium alloy which is coated or of PEEK or carbon reinforced PEEK. The insert  43  is preferably made of the same material as the head  12  of the screw, for example of a body compatible metal. Instead of providing the insert  43  the first pressure element itself can have a spherical recess to press onto the head. Instead of providing the first and second pressure element of a material which facilitates sliding or which is coated or treated to facilitate sliding, the rods  3   a ,  3   b  themselves can have a surface which facilitates sliding, for example a coated or otherwise treated surface. 
         [0032]    The second pedicle screw  2  shown in  FIGS. 1 ,  2  and  8  differs from the first pedicle screw  1  in the design of the first and second pressure elements. All other parts are identical and have the same reference numerals. Therefore, the description thereof is not repeated. The shape of the first pressure element  40 ′ and of the second pressure element  50 ′ of the second pedicle screw  2  is the same as that of the first pressure element  40  and the second pressure element  50  of the first pedicle screw  1 . However, the size of the channels  46   a ′,  46   b ′,  56   a ′,  56   b ′ is smaller than that of the channels of the first and second pressure element of the first pedicle screw. The radius of the channels is adapted to the radius of the rods  3   a,    3   b  and depth of the channels is smaller than the radius of the rods  3   a,    3   b  such that, as shown in  FIG. 8 , in the assembled state the rods  3   a,    3   b  are clamped between the first pressure element  40 ′ and the second pressure element  50 ′. 
         [0033]    A second embodiment of the stabilization device is shown in  FIGS. 9 and 10  without the rods. The second embodiment differs from the first embodiment described with reference to  FIGS. 1 to 8  only in the shape of the first and second pressure elements  400 ,  500 . The length of the channels  460   a,    460   b  is smaller than the diameter of the cylindrical body part  410 . Therefore, the first pressure element  400  and the second pressure element  500  are arranged completely within the cylindrical bore  23  of the receiving part. 
         [0034]    Modifications of the above described embodiments are conceivable. For example, the pedicle screws and the design of the pressure elements can be such that more than two rods can be accommodated. It is possible to use rods with different elastic properties. It is sufficient, if one of the pressure elements has the channels for guiding the rods, however, it is advantageous if the rods are guided from below and from the top by the channels. The shape of the lower part of the first pressure element can be flat, however, a shape adapted to the shape of the head of the screw  12  is advantageous for distributing the pressure onto the head. 
         [0035]    The fixation element can be a two-part fixation screw wherein the first screw element of a bushing type presses onto the first pressure element for locking of the head  12  and a second screw element of a set screw type arranged within the first screw element presses onto the second pressure element for fixation of the rods in the embodiment shown in  FIG. 8 . 
         [0036]    The receiving part can be shaped as a top loader as shown in the figures or a bottom loader in which the screw element  10  is introduced from the bottom, i.e. the second end of the receiving part. 
         [0037]    The shank of the screw element does not have to have a thread. It can be in the form of a hook, a nail or can have barb elements for anchoring in the bone. 
         [0038]    The springs can be also provided adjacent the outer stop  6 . It is also conceivable that the rods themselves have an axial elastic spring portion. 
         [0039]    In use, first the screw elements of the pedicle screws  1 ,  2  which have been inserted into the receiving parts  20  are screwed into adjacent vertebrae. The first pressure elements can be preassembled so that after alignment of the receiving parts the rods  3   a,    3   b  can be inserted into the receiving parts and the channels of the first pressure element, respectively. The rods  3   a,    3   b  with the stops and the springs can be preassembled as well and can be inserted as a double-rod system. For specific clinical applications the first pedicle screw and the stop  6  points in the direction the patient&#39;s head. However, the arrangement of the pedicle screws depends on the specific clinical situation. 
         [0040]    Next, after the receiving parts and the rods are aligned the angular position of the screw elements relative to the receiving parts is fixed by inserting the fixation screw together with the second pressure element and tightening the fixation screw. In the case of the second pedicle screw  2  as shown in  FIGS. 1 ,  2  and  8  the rods  3   a,    3   b  are fixed simultaneously with the screw head  12 . In the case of the first pedicle screw only the head  12  of the screw element is fixed while the rods can still slide within the channels. 
         [0041]    As shown in  FIG. 1  the rods can slide through the receiving part of the first pedicle screw during flexion or extension of the spinal motion segment, whereby the sliding movement is limited by the rod connectors  6  and  5  acting as stops and dampened by the springs  4   a,    4   b . Simultaneously, the rods may experience bending forces and can bend to some extend provided by the flexibility of the material of the rods.