PATENT ABSTRACT
A bone anchoring device includes an anchoring element having a shank to be anchored in a bone or a vertebra, a rod for connecting at least two anchoring elements, the rod being made of an elastic material. The bone anchoring device further includes a receiving part being connected to the shank for receiving the rod, a seat for the rod being provided in the receiving part the seat having a rod contacting surface, and a locking device cooperating with the receiving part for fixation of the rod in the seat. The locking device includes a rod contacting surface, wherein the rod contacting surface of the seat and/or the rod contacting surface of the locking device includes an engagement structure for engaging the rod, the engagement structure having an asymmetric cross section.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application, Ser. No. 60/951,128, filed Jul. 20, 2007, the contents of which are hereby incorporated by reference in their entirety, and claims priority European Patent Application EP 07 014 318.5, filed Jul. 20, 2007, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present application relates to a bone anchoring device for the dynamic stabilization of bones, in particular for the dynamic stabilization of the spine. The bone anchoring device includes a bone anchoring element which can be connected to a flexible rod being made of an elastic material. For fixation of the rod, an engagement structure is provided which has such a shape that the pressure distribution on the rod in the fixed state is equalized. 
     EP 1 759 646 A1 discloses a spinal implant for the dynamic stabilization of the spine which uses a flexible rod made of an elastomer material. The rod is fixed in the receiving part by means of a locking device which clamps the rod by frictional forces with indirect form-fit contribution. 
     EP 1 795 134 A1 discloses a polyaxial screw for use with a flexible rod made of an elastomer material. To fix the rod or hold the rod in place, an engagement structure is provided in the receiving part receiving the rod and on the locking device locking the rod in the receiving part. The engagement structure comprises ribs or grooves which have a symmetric cross section in each sectional plane. The ribs press onto the elastomer rod causing a depression in the surface of the rod, while leaving the surface structure of the rod intact. 
     In some circumstances, in particular if high tensional loads act onto the rod, it is necessary to apply a high clamping force on the rod via the locking device to fix the rod. In such a case, there might be the risk that local pressure peaks could result in a structural damage with increased abrasion of the surface of the rod caused by the engagement structure. To avoid this, the height of the engagement structure could be reduced. 
     Based on the above, there is a need for a bone anchoring device which provides a safe fixation of the rod under high load conditions, and in particular, under high tensional load conditions acting onto the rod 
     SUMMARY 
     According to aspects of the disclosure, a bone anchoring device is provided which has a bone anchoring element and a rod, the rod being made of an elastic material, which can be used under high load conditions, in particular under high tensional load conditions acting onto the rod, and which nevertheless provides a safe fixation of the rod. 
     The bone anchoring device according to aspects of the disclosure optimizes the load distribution on the rod caused by the engagement structure in such a way that pressure peaks acting onto the surface of the rod in certain areas are avoided and the pressure distribution is made more uniform. Therefore, the risk of an abrasion or violation of the surface of the rod which can cause a loosening of the fixation is avoided. With the bone anchoring device according to the disclosure it is possible to transfer a high axial force from the rod to the bone anchoring element without rupture of the rod and to generate as little abrasion as possible when repeatedly fixed and loosened, for example during secondary adjustments. 
     Further features and advantages of the disclosure will be come apparent and will be best understood by reference to the following detailed description of embodiments taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exploded perspective view of a bone anchoring device according to a first embodiment of the disclosure. 
         FIG. 2  shows the bone anchoring device of  FIG. 1  in an assembled state. 
         FIG. 3  shows a perspective view of the locking device of the bone anchoring device according to  FIG. 1 . 
         FIG. 4  shows a sectional view of the bone anchoring device according to  FIG. 2  in a plane containing the rod axis. 
         FIG. 5  shows a side view of the bone anchoring element of  FIG. 1 . 
         FIG. 6  shows an enlarged portion of containing the engagement structure of the bone anchoring element of  FIG. 5 . 
         FIG. 7  shows partially sectional view of the bone anchoring device according to  FIG. 4  without the locking element. 
         FIG. 8  shows a schematic view of the forces acting onto the rod in the conventional bone anchoring device. 
         FIG. 9  shows a schematic sectional view of the forces acting onto the rod and the engagement structure in a bone anchoring device according to the first embodiment disclosure. 
         FIG. 10   a  schematically shows a second embodiment of the disclosure. 
         FIG. 10   b  shows a schematic view of the pressure distribution acting onto the rod according to the second embodiment. 
         FIG. 11  shows a perspective view of the bone anchoring device according to a third embodiment in an assembled state. 
         FIG. 12  shows an exploded view of the bone anchoring device according to  FIG. 11 . 
         FIG. 13  shows a perspective view of the pressure element of the bone anchoring device according to  FIG. 12 . 
         FIG. 14  shows a bottom view of the pressure element of  FIG. 13 . 
         FIG. 15  shows a top view of the pressure element of  FIG. 13 . 
         FIG. 16  shows a perspective view of a bone anchoring device according to a fourth embodiment in an assembled state. 
         FIG. 17  shows an exploded view of the bone anchoring device of  FIG. 16 . 
         FIG. 18  shows a perspective view from the lower side of the filling piece of the bone anchoring device according to  FIG. 17 . 
         FIG. 19  shows a sectional view of the filling piece of  FIG. 18 . 
         FIG. 20  shows a side view of the filling piece of  FIG. 18 . 
         FIG. 21  shows a bottom view of the filling piece according to  FIG. 18  and 
         FIG. 22  shows a top view of the filling piece according to  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIGS. 1 to 5 , a bone anchoring device according to a first embodiment includes a bone anchoring element  1  having a shank  2  with a bone thread and a tip at one end and a receiving part  3  at the opposite end. The receiving part  3  is substantially cylindrically shaped and includes a substantially U-shaped recess  4  forming two free legs  5 , 6 . An internal thread  7  is provided on the legs. The bottom of the U-shaped recess forms a seat  8  for receiving a rod  9 . The rod  9  is used to connect several bone anchoring elements. To secure the rod  9  in the recess  4 , a locking device in the form of an inner screw  10  is provided which can be screwed-in between the legs  5 , 6 . 
     The rod  9  is made of a biocompatible, elastic material, preferably of plastics. In particular, the material is a free-flowing material. For example, the rod  9  is made of an elastomer material including polycarbonate-polyurethane or polycarbonate urethane (PCU). 
     As can be seen in particular in FIGS.  1 , 4  and  5 , rib-like projections  11  are provided on the surface of the seat  8 . The rib-like projections  11  extend in a direction perpendicular to the longitudinal axis L of the recess  4 . Therefore, the rib-like projections  11  extend perpendicular to the longitudinal axis L R  of the rod  9 . The rib-like projections  11  have such a length that they form a curvature corresponding to the seat  8 . The rib-like projections  11  end at a distance from the internal thread  7 . The rib-like projections  11  may run out on one or on either side in groove-like recesses which provide depressions in the surface of the seat  8  (not shown). Alternatively, depressions in the surface of the seat adjoining the rib-like projections can be provided to allow a material to flow into these depressions. 
     In the embodiment shown, the two rib-likes projections  11  are provided at a distance from the center of the seat  8  in the direction of the rod axis L R . Preferably the distance from the outer ends of the seat  8  to the rib-like projection  11  is smaller than the distance between the rib-like projection  11  and the center of the seat  8 . Accordingly, the rib-like projections  11  are provided in the outer region of the seat  8 . To achieve secure clamping, two rib-like projections  11  on the seat are sufficient. 
     The inner screw  10  which is to be screwed in between the legs  5 , 6  includes at its lower side  10   a  which faces the rod  9  a ring-shaped projection  12  in the form of an annular rib with a central cavity. When, as shown in  FIG. 4 , the ring-shaped projection  12  comes into contact with the rod  9 , two contact areas  12   a ,  12   b  are provided where the ring-shaped projection  12  presses onto the rod  9 . The diameter of the ring-shaped projection  12  is such that the contact areas  12   a,   12   b  are located at the same position but at the opposite side of the surface of the rod  9  where the contact areas  11   a  and  11   b  of the rib-like projections  11  of the seat  8  are located. 
     As shown in particular in  FIGS. 4 and 6 , the rib-like projection  11  and the ring-shaped projection  12  have an asymmetric cross section in a plane containing the longitudinal axis L R  of the rod  9  or in a plane parallel thereto. As shown in  FIG. 6 , the cross section is substantially triangle shaped resulting in a first flank  14   a  and an opposite second flank  14   b  which have a different inclination. In the embodiment shown, the first flank  14   a  includes an angle α with the normal N to the surface of the seat  8  which is approximately  100 , and the second flank  14   b  includes an angle β with the normal N to the surface of the seat  8  which is approximately 35°. Other angles are also possible, as long as one flank is steeper than the other flank, i.e., α&lt;β, for example, α&lt;45° and α&lt;β. The angle α is preferably between greater than 0° and smaller than approximately 15°. The angle β is preferably between approximately 30° and approximately 45°. As shown in  FIG. 6 , the edge  14   c  of the rib-like projection  11  is rounded. The second projection on the seat  8  which is shown in  FIG. 1  has a similar structure except that the orientations of the first and second flank are mirrored. Accordingly, the steep flanks  14   a  are oriented towards the center C of the seat  8 , as shown in  FIG. 4 . 
     As shown in  FIGS. 3 and 4  the ring-shaped projection  12  of the locking device also has an asymmetric cross section having a first flank  15   a  and a second flank  15   b  with different inclinations. The steep flank  15   a  is facing the center of the ring-shaped projection  12  and the flank  15   b  is directed outward. The angles α and β are preferably the same as those of the rib-like projections  11 . 
     As can be seen in  FIG. 4 , the orientation of the flanks of the rib-like projection  11  and of the ring-shaped projection  12  are such that the steep flanks are directed towards the center of the seat and of the inner screw, respectively. As a consequence thereof, the outer flanks having the smaller inclination are directed to the outer areas of the anchoring element or the locking device, respectively. 
     The bone anchoring element  1  and the inner screw  10  are made of a biocompatible rigid material, preferably of a metal, such as titanium or a titanium alloy. 
     In use, first at least two bone anchoring elements are screwed into adjacent vertebrae, for example into the pedicles of the vertebrae. Thereafter, the rod  9  is inserted into the receiving parts  3  until it is seated in the seat  8 . Then the rod  9  is locked in its position by screwing-in the inner screw  10 . When the inner screw  10  is not yet tightened, the position of the rod  9  can still be adjusted in a stepless manner, since the rod  9  has a smooth surface. After adjusting the position of the rod  9 , the inner screw  10  is tightened until the ring-shaped projection  12  comes into contact with the surface of the rod  9 . As can be seen in  FIG. 4 , the opposite contact areas  12   a  and  12   b  of the ring-shaped projection  12  are pressed down into the surface of the rod  9 . Similarly, the rib-like projections  11  are pressing on the surface of the rod  9  from below. The projections do not harm the integrity of the surface of the rod  9 . The rod  9  begins to flow under applied pressure. This material flow results in an indirect form-fit connection. The combination of direct frictional forces and indirect form-fit forces holds the rod  9  in place. 
       FIG. 7  shows a partial sectional view of a part of the receiving part  3  with seat  8  and rib-like projection  11  shown in section and the rod  9 . A depression  16  is shown which is caused by pressure of the rib-like projection  11  pressing onto the rod  9 . When the rod  9  is fixed and a force F acts in the longitudinal direction of the rod  9 , the steep flank  14   a  provides the counterforce against translational movement of the rod  9 . The counterforce provided by the steep flank  14   a  is larger than the counterforce which would be provided by a flank having the same inclination than the flank  14   b  at the same local pressure. 
     As can be seen in  FIGS. 8 and 9 , the normal force F normal  of the force F acting in longitudinal direction of the rod  9  is larger in the conventional design of the rib which has symmetric shape ( FIG. 8 ), compared to the normal force F normal  for the asymmetric engagement structure ( FIG. 9 ). In  FIGS. 8 and 9 , F x  represents the component of force F in the longitudinal direction of the rod  9 . Hence, to fix the rod with the symmetric rib-like projection  11  as shown in  FIG. 8 , a larger clamping force exerted by the locking device is necessary than in the case of the asymmetric rib-like projection  11  as shown in  FIG. 9 . Therefore, the local pressure applied to the rod via the engagement structure can be reduced with the asymmetric design of the engagement structure. 
     Preferably, only two rib-like projections  11  are provided in the seat as shown in  FIG. 4 , the distance of which corresponds to the diameter of the ring-shaped projection  12  on the lower surface of the inner screw  10 . The orientation of the flanks is mirror-symmetrical, so that the effect described above can be obtained for tensional loads acting in one or the opposite longitudinal direction of the rod  9 . 
       FIG. 10   a  shows a schematic view of the receiving part  3  of the bone anchoring device according to a second embodiment seen in the direction of the rod axis. The rib-like projection  11 ′ in this embodiment is non-concentric about the rod axis L R . The distance from the rod axis L R  varies between a radius R 1  and R 2 , wherein R 2  is greater than R 1 . At the bottom of the seat  8  the height of the rib-like projection  11 ′ is the smallest while the height of the projection  11 ′ is increasing in a direction varying from the center of the seat and thereafter decreasing again when it runs out into the direction of the legs  5 , 6 . 
     As can be seen in  FIG. 10   b  this results in a more uniform pressure distribution acting onto the rod  9 . Uniform pressure distribution means that there are no local pressure peaks. In this embodiment, the cross section of rib-like projection  11 ′ can be symmetric or asymmetric in a plane containing the longitudinal axis of the rod L R . 
       FIGS. 11 to 15  show a third embodiment of the disclosure in the form of a polyaxial bone anchoring device. The bone anchoring device  1 ′ includes a bone anchoring element  20  in the form of a polyaxial bone screw having a screw element with a shank  21  with a bone thread, a tip at one end and a spherical head  22  at the opposite end. A recess  23  for engagement with the screwing-in tool is provided at the side of the head  22  which is opposite to the shank. 
     The bone anchoring element  20  further includes a receiving part  25  which has a first end  26  and a second end  27  opposite to the first end and a central axis C intersecting the plane of the first end and the second end. Coaxially with the central axis C a bore  29  is provided which extends from the first end to a distance from the second end. At the second end  27  an opening  30  (shown in dashed line) is provided the diameter of which is smaller than the diameter of the bore  29 . The head  22  is pivotably held in the receiving part  25  with the shank extending through the opening  30 . 
     The receiving part  25  further has a substantially U-shaped recess  31  which starts at the first end  26  and extends in the direction of the second end  27 . By means of the U-shaped recess two free legs  32 , 33  are formed, which have an internal thread  34 . 
     A pressure element  35  is provided which has a substantially cylindrical construction with an outer diameter which is only slightly smaller than the inner diameter of the bore  29  to allow the pressure element  35  to be introduced into the bore  29  of the receiving part and to be moved in the axial direction. On its lower side facing towards the second end  27 , the pressure element  35  includes a spherical recess  36  the radius of which corresponds to the radius of the spherical head  22  of the screw element. On the opposite side the pressure element  35  includes a U-shaped recess  37  extending transversely to the central axis C. The lateral diameter of this recess is selected such that the rod  9  which is to be received in the receiving part  25  can be inserted into the recess  37  and guided laterally therein. The depth of the U-shaped recess  37  is such that in an assembled state when the rod is placed into the U-shaped recess  37 , the pressure element  35  does not project over the upper surface of the rod  9 . 
     The bottom of the U-shaped recess  37  of the pressure element  35  forms a seat  38  for the rod  9 . Similar to the first embodiment two rib-like projections  39  are provided on the surface of the seat  38 . The rib-like projections  39  extend in a direction transversely to the longitudinal axis of the U-shaped recess  37  and, therefore, transversely to the longitudinal axis L R  of the rod  9 . As can be seen in particular in  FIG. 15 , the rib-like projections  39  have an asymmetric shape with a steep flank directed to the center of the pressure element  35 . The pressure element further includes a coaxial bore  40  to allow access to the recess  23  of the head  22  with a screwing-in tool. 
     The locking device is the inner screw  10  as in the first embodiment which has the ring-shaped projection  12  on its side  10   a  facing the rod  9 . The dimensions of the ring-shaped projection  12  are such that the projection contacts the surface of the rod  9  at the opposite side of the rib-like projections  39 , respectively. 
     In use, the bone anchoring device  1 ′ can be preassembled, i.e. the bone screw is pivotably held in the receiving part and the pressure element is inserted and slightly held in a position in which its U-shaped recess is aligned with the U-shaped recess of the receiving part. The bone anchoring element is screwed into the bone and the angular position of the receiving part relative to the bone screw is adjusted. The rod  9  is inserted and the inner screw  10  tightened until it clamps the rod. The function of the clamping is the same as in the first embodiment. When the inner screw is tightened, it presses onto the upper surface of the rod and hence presses down the pressure element onto the head  22  to lock the angular position of the head in the receiving part. 
     As the locking of the rod is achieved by pressing the projections  39 ,  12  into the surface of the rod  9  without harming the integral structure of the rod  9 , a reversal of the locking and secondary adjustments are possible. 
       FIGS. 16 to 22  show a fourth embodiment of the bone anchoring device. The bone anchoring device  1 ″ is of the polyaxial type as the third embodiment. Parts which are identical to the third embodiment are indicated with the same reference numerals and the description thereof is not repeated. The fourth embodiment differs from the third embodiment by the locking device. The locking device  100  is a two-part locking device and includes a filling piece  101  and an inner screw  102 . The filling piece  101  is shown in detail in  FIGS. 18 to 22 . The filling piece  101  has a substantially square shaped upper end  105  with a circular opening  106  and a cylindrical recess  107  the radius of which corresponds to the radius of the rod  9 . On the surface of the cylindrical recess  107 , two rib-like projections  108  are provided which extend in a direction transverse to the axis of the cylinderical recess  107 . The rib-like projections  108  each are shaped asymmetrically with a first steep flank facing the center of the filling piece and a second flank directed outwardly of the filling piece, respectively. The distance between the rib-like projections  108  corresponds to the distance of the rib-like projections  39  of the pressure element  35 . In an assembled state, the rib-like projections of the pressure element  35  and the filling piece  101  are located on opposite sides of the surface of the rod  9 , respectively. 
     The filling piece  101  further includes two projections  109  which fit into the space enclosed by the internal thread  34  to slide along the internal thread when the filling piece is inserted. 
     The dimension of the filling piece  101  and the pressure element  35  is such that the projections  109  come into contact with the upper end of the pressure element when the filling piece  101  is pressed onto the rod. 
     The inner screw  102  includes a cylindrical projection (not shown) fitting into the opening  106  of the filling piece in such a way that it can still rotate therein. 
     In use, the bone anchoring element and the receiving part and a pressure element can be preassembled. The bone anchoring element is screwed into the bone. Then, the rod  9  is inserted and the locking device comprising the filling piece  101  and the inner screw  102  is inserted. The inner screw is tightened thereby pressing the filling piece  101  onto the surface of the rod. Hence, the rod is clamped between the pressure element and the filling piece and the engagement structure in form of the ribs  39  and the ribs  108  engages the surface of the rod as in the previous embodiments. By pressing down the filling piece, the pressure element is also pressed down and locks the head  22  in its rotational position. 
     Modifications of the above described embodiments are possible. Features of one embodiment can be combined with that of another embodiment. 
     The number of the rib-like projections can vary. Also, a combination of projections and depressions can be provided. In this case, it is of advantage that also the depressions have an asymmetric cross section. This allows the material which is displaced when the projections press onto the surface of the rod to flow out of the depressions to generate an indirect form-fit connection. The depressions can have a symmetric or asymmetric cross section. If the depressions also have an asymmetric cross section the clamping force of the rod can be reduced further while maintaining the same axial reaction loads. 
     The rod needs not to have a circular cross section, it can have an oval, rectangular, square or triangular cross section as well. 
     The projections and/or depressions need not to have a rib or groove-like structure, but can have any shape, as long as at least the projections have an asymmetric cross section having at least two flanks, one of which is steeper than the other. 
     The receiving part and the pressure element can be modified in many known ways. For example, the receiving part can be designed such that the head  22  of the bone anchoring element can be introduced from the bottom. The pressure element can extend above the surface of the rod when the rod is inserted and the locking device  10  can be a two-part locking device including inner set screw and an outer screw in the known manner. In this case, the inner set screw has the engagement structure. The locking devices also can be an outer nut or can include an outer nut cooperating with the legs of the receiving part. 
     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.