Abstract:
A rod-shaped implant for stabilizing bone structures, in particular the spinal column includes a rod-shaped implant having a rod-shaped member with a first end and a second and a longitudinal axis. The rod-shaped member is at least partly made from a plastic material exhibiting flexibility and includes a longitudinal bore. A reinforcing rod is accommodated in the bore and is made from a material which is more rigid than the material of the rod-shaped member itself. The reinforcing rod is slidable in the bore.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/094,207, filed Sep. 4, 2008, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 08 015 662.3, filed Sep. 4, 2008, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The application relates to a rod-shaped implant in particular for stabilizing the spinal column and to a spinal stabilization device including such a rod-shaped implant. 
         [0003]    EP 0 669 109 B1 discloses a stabilizing apparatus for stabilizing neighbouring thoracic vertebrae. The apparatus includes two monaxial pedicle screws and a strap that is fixed in the receiver member of each pedicle screw by means of a clamping screw and a support element that is mounted on the strap and is designed as a pressure resistant body. The stabilization apparatus, however, fails to be torsionally stiff and does not allow for axial extension. In addition, the use of monoaxial pedicle screws limits the application of this stabilization apparatus. 
         [0004]    US 2007/0093820 A1 discloses a dynamic spinal stabilization comprising a flexible rod made of an elastomer material which is clamped in the receiving parts of monoaxial bone screws. EP 1 795 134 A1 and EP 1 900 334 A1 describe a spinal stabilization system with a flexible elastomer rod and polyaxial bone screws. 
         [0005]    The dynamic stabilization systems comprising a flexible elastomer rod are suitable for the control, in particular the damping, of axial compression and extension of motion segments of the spinal column. The elastomer material is advantageous with respect to obtaining the suitable length of the rod-shaped implant by cutting an elastomer rod and the implant is simple to manufacture. 
         [0006]    In clinical cases of early degeneration or partial damages or injuries of intervertebral discs the corresponding motion segments of the spinal column are subject to increased rotational movements and shearing forces. Such rotational movements and shearing forces can cause strong pain. 
         [0007]    US 2007/049937 A1 discloses a rod-shaped implant which includes a metallic hollow rod with a flexible section in form of a helix-shaped recess in wall of the rod. In the hollow rod a longitudinal core is provided which can be fixed with respect to one end of the rod and which can be moveable with respect to the other end of the rod. The problem of kinking of the rod-shaped implant caused by shearing movements of the spinal column is considerably reduced. 
         [0008]    Based on the foregoing, there is a need to provide an implant and a stabilization system for the spinal column which is particularly suited for cases in which rotational and shearing movements of the spinal column shall be suppressed. 
       SUMMARY 
       [0009]    The rod-shaped implant according to the disclosure includes a flexible rod made at least partly of an elastomer material and further includes a reinforcing rod for strengthening the implant in particular against rotational movements of the spinal column in an axial direction and against shearing forces. The spinal stabilization system includes such a rod-shaped implant and at least two bone anchoring elements to be connected to the rod-shaped implant. 
         [0010]    The rod-shaped implant according to the disclosure uses an elastomer rod but has enhanced stability compared to purely elastomeric rods and is a modular system allowing various combinations of elastomeric rods and reinforcing rods. 
         [0011]    The rod-shaped implant and the stabilization system allows a dynamic damping of the axial tension and compression movements by using the elastomer rod and considerably enhances the resistance against rotational and/or shearing and/or bending movements due to the reinforcing rod. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a schematic sectional view of the stabilization system, the section being taken along the rod axis. 
           [0013]      FIG. 2  shows a schematic side view of the stabilization system. 
           [0014]      FIG. 3  shows a schematic perspective view of a bone anchoring element which is adapted to the rod-shaped implant. 
           [0015]      FIG. 4  shows a schematic sectional view of the bone anchoring element of  FIG. 3  with an inserted rod-shaped implant, the section being taken perpendicular to the rod axis. 
           [0016]      FIG. 5  shows a schematic sectional view of the bone anchoring element of  FIG. 3  with a modified rod-shaped rod-implant. 
           [0017]      FIG. 6  shows a schematic sectional view of the bone anchoring element of  FIG. 3  with a still further modified rod-shaped implant. 
           [0018]      FIG. 7  shows a schematic perspective view of a bone anchoring element which is modified concerning the adaptation to the rod-shaped implant. 
           [0019]      FIG. 8  shows a schematic sectional view of the bone anchoring element of  FIG. 7  with the rod-shaped implant inserted, the section being taken perpendicular to the rod axis. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    As shown in  FIGS. 1 and 2 , the stabilization device  1  includes a rod-shaped implant  2  and a plurality of bone anchoring elements  3 . The bone anchoring elements can be anchored in bony structures, in particular in adjacent vertebrae of the spinal column. The rod-shaped implant  2  can be connected to the bone anchoring elements so that it is anchored in the vertebrae. Hence, the total length of the rod-shaped implant  2  is such that it spans the distance of at least two vertebrae of the spinal column. 
         [0021]    In the embodiment shown, the rod-shaped implant is formed of a cylindrical rod  20  comprising a first end  2   a  and a second end  2   b.  A coaxial bore  21  extends from the first end  2   a  to the second  2   b  through the cylindrical rod  20 . In this embodiment the coaxial bore has a circular cross section. 
         [0022]    In the coaxial bore  21  a reinforcing rod  22  is accommodated which has a circular cross section and a diameter which is sized such that the reinforcing rod  22  can slide within the coaxial bore  21 . The reinforcing rod  22  extends from the first end  2   a  to at least the second end  2   b  and may project beyond second end  2   b  of the cylindrical rod  20 . At one end, for example at the first end  2   a,  the reinforcing rod  22  is limited with respect to its movement relative to the rod  20  with a stop  23 . The stop  23  can be formed, for example, as a disc which is mounted to the end of the reinforcing rod. Other constructions for the stop  23  are conceivable. For example, the stop can be adjustable along an end portion of the reinforcing rod  22 . This can be realized, for example, by providing a thread on the outer surface of the end portion of the reinforcing rod  22  and by providing a nut to be screwed on the threaded end portion which abuts against the end  2   a  of the cylindrical rod  22  (not shown). 
         [0023]    The reinforcing rod is movable at the second end  2   b.  However, a second stop (not shown) can be provided also at a distance from the second end  2   b.  The second stop may be adjustable. If a second stop is provided, the distance between the first stop and the second stop is greater than the distance between the first end  2   a  and the second end  2   b  so that the reinforcing rod  22  is freely movable. 
         [0024]    The material of the rod  20  is a plastic material which exhibits flexibility when the rod  20  experiences compression or tension forces acting in an axial direction. Particularly suitable are polymer materials exhibiting such flexibility, preferably elastomer materials such as polyurethanes, polycarbonate urethanes (PCU) or polysiloxanes. Any other material exhibiting such elastic features is, however, suitable. Since the material exhibits flexible properties, the cylindrical rod  20  also reacts on rotational forces around the rod axis and on shearing forces having a component perpendicular to the rod axis. Such forces arise from torsional and shearing motions of the motion segments of the spine. 
         [0025]    In order to control and dampen such rotational and shearing forces the reinforcing rod  22  is made from a material which is less flexible than the cylindrical rod  20  or which exhibits no flexibility under forces acting along the rod axis. Particularly suitable materials are body compatible metals, such as stainless steel or titanium or titanium alloys, such as Nitinol, or rigid plastic materials, for example PEEK or carbon fiber reinforced PEEK or others. 
         [0026]    The reinforcing rod  22  can be coated to facilitate sliding within the coaxial bore of the cylindrical rod  20 . Alternatively a sliding guidance or a sliding bearing can be provided to facilitate sliding of the reinforcing rod. 
         [0027]    The bone anchoring element  3  can be any monoaxial bone screw or bone hook, but is preferably a polyaxial bone screw as depicted in  FIGS. 1 and 2 . The polyaxial bone screw  3  includes a screw element  31  with a threaded shank and a spherically shaped head which is pivotably held in a receiving part  32  which receives the rod. To fix the screw element  31  in a desired angular position with respect to the receiving part  32  a pressure element  33  acting onto the head of the screw element  31  is provided. The rod-shaped implant  2  is received in the receiving part  32  and is fixed with a fixation element  34 . 
         [0028]      FIGS. 3 and 4  show a bone anchoring element  30  which is specifically adapted for use with the rod-shaped implant. The bone anchoring element  30  includes a screw element  31  with a threaded shank and a spherically-shaped head and a receiving part  32  for receiving the rod-shaped implant  2 . The receiving part is substantially cylindrical or cuboid-shaped and includes a coaxial bore  32   a  which extends from one end to the opposite end and which tapers with respect to one end in order to pivotably accommodate the screw head of the screw element  31 . The receiving part  32  further includes a U-shaped recess  32   b  for receiving the rod-shaped implant  2 . 
         [0029]    A first pressure element  33  is provided which is substantially cylindrical and movable in the bore. It presses onto the head of the screw element and has a U-shaped recess  33   b  to accommodate the rod-shaped implant  2 . The U-shaped recess  33   b  has a depth such that the pressure element extends above the surface of the rod-shaped implant  2  when the rod-shaped implant  2  is inserted. 
         [0030]    A fixation screw  34  is provided which can be screwed into the receiving part  32  from the free ends of the U-shaped recess  32   b.    
         [0031]    Furthermore, a second pressure element  35  is provided which can be pressed downwards with the fixation screw  34 . 
         [0032]    The first pressure element  33  and the second pressure element  35  are shaped in such a way that the rod-shaped implant  2  is enclosed therebetween and fixed in an axial direction without pressing onto the reinforcing rod  22 . Therefore, the reinforcing rod is still moveable. The fixation screw  34  also presses onto the first pressure element  33  in order to fix the head of the screw element  31  in the receiving part, independently from the fixation of the rod-shaped implant. 
         [0033]    The surfaces of the first pressure element  33  and the second pressure element  35  which contact the rod-shaped implant may have engagement structures engaging the surface of the rod without harming the surface structure of the rod-shaped implant. 
         [0034]      FIGS. 5 and 6  show variations of the cross-section of the reinforcing rod and the corresponding bore in the elastomer rod  20 . In  FIG. 5  a modified rod-shaped implant  2 ′ is shown which differs from the rod-shaped implant  2  of the previous embodiment in that the cross-section of the reinforcing rod  22 ′ is rectangular and the corresponding bore in the rod  20  has also a rectangular cross-section. In the embodiment shown in  FIG. 5  the rod-shaped implant  2 ′ is arranged such that the long side of the rectangle of the reinforcing rod  22 ′ is aligned perpendicular to the axis of the screw element  31 . However, any other orientation is possible. In the modification shown in  FIG. 6  the cross-section of the reinforcing rod  2 ′ is cross-shaped. 
         [0035]    Other cross-sections of the reinforcing rod are conceivable, for example a polygon-shaped cross-section. The resistance against rotational and/or shearing forces and/or bending forces can be enhanced by using a reinforcing rod with a non-circular cross-section. In addition, if necessary, the rod-shaped implant can be provided with an orientation dependent bending flexibility by using a reinforcing rod with a non-circular cross-section. 
         [0036]      FIGS. 7 and 8  show a modified bone anchoring element  300  which differs from the bone anchoring element  30  in that the pressure element  33 ′ has a U-shaped recess  33   b ′ which has a depth such that the pressure element projects only slightly above the surface of the rod-shaped implant  2  when the rod-shaped implant is inserted. On the bottom of the U-shaped recess one or several pin-shaped projections  36  are provided which engage the surface of the rod-shaped implant  2 . 
         [0037]    The fixation element  34 ′ is a two-part fixation element with an outer screw  37  and an inner screw  38 . The outer screw  37  is screwed into the receiving part  32  and presses onto the pressure element  33 ′ in order to lock the angular position of the screw element  31  in the receiving part. The inner screw  38  has a pin-shaped projection  39  on its side facing the rod-shaped implant  2 . The inner screw  38  together with the projection  36  clamp the rod-shaped implant  2  independently of the fixation of the head of the screw element  31  in the receiving part  32 . The dimension of the pin-shaped projections  36  and  39  and the dimension of the inner and outer screw of the fixation element  34 ′ is such that only the rod  20  is clamped whereas the reinforcing rod  22  is still freely movable. 
         [0038]    In use, first at least two bone anchoring elements are inserted into the respective vertebrae. Thereafter, the necessary length of the rod-shaped implant is determined and an appropriate rod-shaped implant is cut from the rod consisting of the elastomer rod with the reinforcing rod. If necessary, a stop is included at one or both ends. 
         [0039]    Thereafter, the rod-shaped implant is inserted into the receiving parts of the bone anchoring elements. Then, the vertebrae are adjusted in their position with respect to each other and the polyaxial position of the screw element  31  with respect to the receiving parts is locked. After adjusting the distances of the receiving parts the rod-shaped implant is fixed. 
         [0040]    While a particular form of the disclosure has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the disclosure be limited, except as by the appended claims.