Abstract:
A bone anchoring device is provided that includes an anchoring element with a shank to be anchored in a bone or a vertebra, a connection element for connecting at least two anchoring elements, the connection element being made of an elastic material, a receiving part being connected to the shank for receiving the connection element and connecting the connection element to the shank, a seat for the connection element the seat being provided in the receiving part, and a locking element being engageable with the receiving part for fixation of the connection element in the seat. At least the portion of the connection element which is received in the recess has a substantially smooth-surface. The locking element has an engagement structure which contacts the surface of the connection element in such a way that a form-fit connection is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/825,841, filed Sep. 15, 2006, and claims priority from European Patent Application EP06019341.4, filed Sep. 15, 2006, the entire disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present application generally relates to a bone anchoring device for the dynamic stabilization of bones, in particular for the dynamic stabilization of the spine, and in particular, to a bone anchoring element which can be connected with a flexible rod which has a smooth surface and which is made of an elastic material. 
     Bone anchoring devices comprising a bone screw and a flexible rod which is made of an elastic material are known from EP 1 364 622 A2 and EP 1 527 742 A1. The rod has a corrugated surface which cooperates with a rib structure provided on the bottom of the receiving part of the bone screw to form a form-fit connection. The bone anchoring device according to EP 1 527 742 A1 additionally comprises a closure element which has a rib structure engaging the corrugated surface of the rod. The positioning of the corrugated surfaces of the rod relative to the rib structure requires a precise insertion of the rod into the receiving part to avoid tilting. Furthermore, a continuous positioning is not possible. This makes the adjustment of the position of the rod relative to the receiving part difficult and time consuming. 
     US 2004/0138660 A1 discloses a locking cap assembly for locking a rigid rod to a receiving body of a bone screw. The locking cap assembly includes an inner and an outer locking element. The outer locking element is a nut-like member and the inner locking element is rotatably connected to the outer locking element. The inner locking element has on its side facing the rod a ring-shaped deformable contacting element which comes into contact with the rod. Upon tightening of the outer locking element, the deformable element is deformed which provides feed-back to the surgeon to allow him to determine whether the locking cap assembly is tightened to the required extent. In one example, the deformable element is a deformable metallic ring which becomes cold welded to the rod. 
     Based on the above, there is a need to provide a bone anchoring device which can be used with a flexible rod made of an elastic material and which has a convenient handling while simultaneously providing a safe locking. 
     SUMMARY OF THE INVENTION 
     In a bone anchoring device according to the disclosure, during tightening of the locking element the deformation of the elastic material of the rod can lead to an indirect or dynamic form-fit connection between the elastic rod, the receiving part and the locking element without harming the integral structure of the rod. 
     Since the rod has a smooth surface, continuous positioning of the rod is possible. 
     In the bone anchoring device according to the disclosure a flow of the material of the rod in a direction along the longitudinal axis of the rod is minimized. 
     The fixation of the rod is achieved with a small number of parts. Therefore, the handling of the bone anchoring device during surgery is facilitated without loss of reliability of the fixation. 
     Further features and advantages of the invention will become 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 a perspective exploded view of a first embodiment of the bone anchoring device. 
         FIG. 2  shows the bone anchoring device of  FIG. 1  in an assembled state. 
         FIG. 3  shows a perspective view of the locking element of the bone anchoring device of  FIG. 1 . 
         FIG. 4  shows a sectional view of a portion of the bone anchoring device of  FIG. 1 and 2 . 
         FIG. 5   a  shows a schematic view of the fixation mechanism of the bone anchoring device according to the invention. 
         FIG. 5   b  shows a schematic view of another fixation mechanism. 
         FIG. 6  shows an perspective exploded view of a bone anchoring device according a second embodiment. 
         FIG. 7  shows a perspective view of the bone anchoring device shown in  FIG. 6  in an assembled state. 
         FIG. 8  shows a portion of the bone fixation device according to  FIG. 7  in an assembled state in a sectional view the section being taken along the rod axis. 
         FIG. 9  shows a sectional view of the bone anchoring device of  FIG. 8  along the line M-M. 
         FIG. 10  shows a sectional view of the locking element of the second embodiment. 
         FIG. 11  shows a perspective view of the pressure element of the bone anchoring device according to the second embodiment. 
         FIG. 12  shows a top view of the pressure element shown in  FIG. 11 . 
         FIG. 13  shows a side view of the pressure element shown in  FIG. 11 . 
         FIG. 14  shows a sectional view of the pressure element shown in  FIG. 11  along the line S-S in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 1 to 4  the bone anchoring device according to a first embodiment comprises a bone anchoring element  1  in the form of a monoaxial bone screw 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 comprises 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 element in the form of an inner screw  10  is provided which can be screwed-in between the legs  5 ,  6 . 
     As can be seen in particular in  FIGS. 1 and 4  a plurality of 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  and hence extend perpendicular to the longitudinal axis LR of the rod  9 . In the embodiment shown the projections  11  have a substantially triangular cross-section with a rounded tip. The projections  11  have such a length that they form a U-shape corresponding to the seat  8 . They end at a distance from the internal thread  7 . Each or several of 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. Alternatively, one or several depressions in the surface of the seat which adjoins one or several of the projections can be provided. 
     The rod  9  is made of an elastic flexible biocompatible material, preferably of plastics. For example, the rod  9  is made of an elastomer material on the basis of polycarbonate polyurethane or polycarbonateurethane (PCU). Hence, the rod shows elastic deformation under applied external loads. 
     The inner screw  10  which is to be screwed between the legs  5 ,  6  comprises at its side  10   a  facing the rod  9  a ring shaped projection  12  in form of an annular rib with a central cavity. As can be seen in particular in  FIG. 4 , the ring-shaped projection  12  has a cross-section which is similar to the cross-section of the rib-like projections  11  of the seat. When the ring-shaped projection  12  comes into contact with the rod  9 , two contact areas  12   a  and  12   b  are provided where the ring-shaped projection presses onto the rod. The diameter of the ring-shaped projection  12  is such that the contact areas  12   a ,  12   b  and the contact areas  11   a  and  11   b  of the two outer rib-like projections are located on opposite sides of the surface of the rod  9  on a vertical line Va, Vb which is parallel to the central axis C of the receiving part  3 , 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  1  are screwed into adjacent vertebrae, for example into the pedicles of the vertebrae. Thereafter rod  9  is inserted into the receiving parts  3  until it is seated in the seat  8 . Thereafter the rod is locked in its position by screwing-in the inner screw  10 . If the inner screw  10  is not yet tightened, the position of the rod can still be adjusted in a stepless manner, since the rod has a smooth surface. After adjusting the position of the rod the inner screw  10  is tightened until the ring-shaped projection  12  comes into contact with the surface of the rod. As can be seen in  FIG. 4  the opposite portions  12   a  and  12   b  of the ring-shaped projection are pressed down on the surface of the rod. Similarly the rib-like projections  11  are pressing on the surface of the rod from below. The projections do not harm the integrity of the surface of the rod. The rod 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 in place. 
     The diameter of the ring-shaped projection  12  can be equal or larger than the distance between the outmost rib-like projections  11 . 
     As can be seen in  FIG. 5   a , one arrangement of the engagement structure formed by the projections of the seat and the engagement structure formed by the projection on the locking element at corresponding locations on opposite surface portions of the rod provides a form-fit connection which is resistant to a force F acting in the longitudinal direction of the rod  9 . 
     An alternating arrangement of the projections of the seat and that of the locking element with respect to the rod as shown in  FIG. 5   b , also leads to an indirect form-fit connection as described above, which, however, is less resistant to a force F acting in the axial direction of the rod. In case of a large force F, the connection shown in  FIG. 5   b  cannot prohibit a wavy-like movement of the rod and hence provides less fixation of the rod. 
     The ring-shaped projection of the inner screw makes it possible that the engagement structure is provided at the locking element itself instead of using a filling piece or pressure piece. 
       FIG. 6 to 14  show a second embodiment of the bone anchoring device. The bone anchoring device comprises 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 comprises 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 predetermined distance from the second end. At the second end  27  an opening  30  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  comprises a spherical recess  36  the radius of which corresponds to the radius of the spherical thread  22  of the screw element. On the opposite side pressure the element  35  comprises 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  3  can be inserted into the recess  37  and guided laterally therein. The depth of the U-shaped recess  37  is selected such that in an assembled state when the rod is placed into the U-shaped recess  37 , legs  37   a ,  37   b  of the recess extend up to the upper surface of the rod. Preferably, the depth of the recess  37  is equal to the diameter of the rod  9 . The lateral diameter of the recess in the area of the ribs and/or the depth of the recess  37  can be slightly larger than the diameter of the rod to allow local plastic flow of the material of the rod. 
     The bottom  38  of the U-shaped recess of the pressure element  35  forms a seat for the rod  9 . Similar to the first embodiment a plurality of rib-like projections  39  are provided on the surface of the seat  38 . In the embodiment shown, there are two rib-like projections  39 , extending in a direction transversely to the longitudinal axis  1  of the U-shaped recess  37  and, therefore, transversely to the longitudinal axis LR of the rod  9 . Furthermore, the pressure element comprises a coaxial bore  40  to allow access to the recess  23  of the head  22  with a screwing-in tool. 
     The locking element 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  is such that, as shown in  FIG. 8 , the contact areas  12   a  and  12   b  of the ring-shaped projection  12  with the rod are located at positions corresponding to the contact areas  39   a ,  39   b  of the rib-like projections of the pressure element on opposite sides of the rod. 
     The dimensions of the pressure element  35  and the inner screw  10  are such that in the assembled state the lower side  10   a  of the inner screw rests on the upper end surface  41   a ,  41   b  of the legs of the pressure element. The dimensions of the projections  39  of the pressure element and the ring-shaped projection  12  of the inner screw are such that safe fixation by means of a press-fit with indirect form-fit is achieved without harming the integrity of the surface of the rod. 
     In use, the bone anchoring element  20  is 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 down until it clamps the rod. The function of the clamping is the same as in the first embodiment. When tightening the inner screw presses onto the upper end surface  41   a ,  41   b  of the pressure element and thus presses down the pressure element onto the head  22  to lock the angular position of the head in the receiving part. 
     Since the lower side  10   a  of the inner screw rests on the upper end surface  41   a ,  41   b  of the pressure element, a flow of the material of the elastic rod  9  when the projections  12 ,  39  press on the rod does not lead to a loosening of the fixation of the head  22  in the receiving part  25 . 
     As the locking of the rod is achieved by pressing the projections  39 ,  12  into the surface of the rod without harming the integral structure of the rod secondary adjustments are possible. 
     Modifications of the above described embodiments are possible. 
     The number of the rib-like projections may vary. Instead of having only rib-like projections provided at the surface of the locking element and the seat a combination of projections and depressions can be provided. Hence, this allows the material which is displaced when the projections press onto the surface of the rod to flow in the depressions to generate a form-fit connection. Additionally, the volumes of projections and depressions can be similar or have the same size such that after the flow of material is completed, the volume of the rod in a region of the connection with the receiving part has approximately the same size as before. 
     The projections and/or depressions need not to have a rib-like or groove-like structure but can have any shape. The projection which comes into contact with the surface of the rod can have another shape than a ring shape. Depending on the external load, at least one tooth should be provided which can be shaped so as to sink into the surface of the rod to create an indirect form-fit between the locking element and the rod or between the seat and the rod, respectively. A ring-shaped structure of the projection on the locking element is preferable, since it facilitates tightening of the locking element by means of a turning motion. 
     The rod needs not to have a circular cross section. It can have an oval rectangular or square cross section. 
     The second embodiment, to avoid a flowing out of material through the bore  40  provided in the pressure element, a cap can be provided for closing the bore after the bone anchoring element is screwed into the bone. 
     The locking element can also be modified. For example, the locking element can be a nut with a coaxial pin which is formed in one piece with the nut. The pin has the projection which comes into contact with the surface of the rod. In this case, the free legs of the receiving part have an external thread which cooperates with the thread of the nut. In the second embodiment described above the bone anchoring element is introduced from the top into the receiving part. However, the bone anchoring element can also be introduces from the bottom of the receiving part if the receiving part is constructed to allow this. 
     The head of the bone anchoring element and the shaft can be constructed as separate parts which can be assembled. 
     The invention is not limited to screws as bone anchoring elements but can be realized with bone hooks or any other bone anchoring element.