Abstract:
A polyaxial bone anchoring device includes an anchoring element having a shaft and a head, a receiving part having a first end and a second end, a bore extending from the first end towards the second end, a seat for receiving the head, and at least one threaded through hole transverse to the bore, a pressure element to exert pressure onto the head, such that the head is pivotable and can be locked at an angle relative to the receiving part, the pressure element having at least one hole configured to at least partially align with the through hole, and at least one set screw threadable into the through hole to engage the pressure element at the hole such that a force is exerted by the pressure element onto the head that maintains the head at an adjustable angular position relative to the receiving part by friction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/525,093, filed Aug. 18, 2011, the contents of which are hereby incorporated by reference in their entirety, and claims priority to European Patent Application EP 11 178 034.2, filed Aug. 18, 2011 the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to a polyaxial bone anchoring device for anchoring a stabilization rod relative to a bone or a vertrebra. The bone anchoring device includes an anchoring element, a receiving part for receiving a head of the anchoring element and for receiving a stabilization rod to be connected to the anchoring element. The anchoring element is pivotably connected to the receiving part and can be locked at an angle by exerting pressure onto the head via a pressure element that is arranged in the receiving part. The pressure element and the receiving part are configured to cooperate in such a way that the pressure element can frictionally clamp the head to maintain a desired angular position before locking of the polyaxial bone anchoring device. 
     2. Description of Related Art 
     U.S. Pat. No. 5,716,356 describes a polyaxial bone screw including a screw element and a receiving part which is pivotably connected to the screw element and a pressure element to exert pressure onto the head of the screw element to lock the angle between the screw element and the receiving part. The receiving part has a U-shaped channel for receiving a stabilization rod. The pressure element comprises a cylindrical recess, which is to be aligned with the U-shaped channel to receive the rod therein. In order to hold the pressure element in a position aligned with the U-shaped channel, the position of the pressure element is fixed by crimping through bores provided in the receiving part. 
     However, when the head of the bone anchoring element remains freely pivotable with respect to the receiving part before locking the head in a final angular position, alignment of the receiving part and the insertion of the rod may be difficult in more complex clinical applications, for example, when a multitude of bone anchors have to be connected to the rod. 
     U.S. Pat. No. 7,604,656 (Shluzas) describes a fastener engageable with a bone portion to connect a longitudinal member to the bone portion. The housing that receives the fastener also receives a spacer, which is engageable with the fastener and the longitudinal member. In one embodiment, the spacer is urged by a pin member into frictional engagement with the fastener and with the housing. 
     SUMMARY 
     It is an object of the invention to provide a polyaxial bone anchoring device and a method for manufacturing the same which allows for improved handling during surgery and which can be manufactured in a simple manner. 
     With the polyaxial bone anchoring device according to embodiments of the invention, a temporary clamping of a head of an anchoring element in a desired angular position with respect to a receiving part, without locking the head, can be achieved. This allows maintaining of the receiving part in an adjustable angular position. In this condition, a pressure element exerts a preload onto the head, where the head is not locked, but is prevented from freely pivoting. When the head is temporarily clamped, alignment of the receiving part with respect to a rod and insertion of the rod is more easily facilitated, in particular, in situations in which a multitude of bone anchors are to be connected to the rod or rods. 
     Furthermore, when the rod is already inserted into the receiving part, adjustments of the rod are still possible without completely unlocking the head. 
     The polyaxial bone anchoring device according to embodiments of the invention comprises only few parts which are each of simple design, and where relatively large manufacturing tolerances are possible. A mechanism utilized to frictionally maintain the head before locking is free from any spring members or portions. This may facilitate easier manufacturing of the polyaxial bone anchoring device. Furthermore, existing receiving parts and pressure elements can be used without substantially modifying their forms. For example, it is possible to change existing crimp bores to threaded through bores. 
     An amount of preload exerted onto the head by the pressure element or member can be exactly predefined in a simple manner, for example, by selecting a position and shape of the bores and a shape of the set screw or screws, and furthermore by a force applied by the set screw or screws that is dependent on how deep the screws are advanced. By means of the set screws, a stepless adjustability of the pressure which is exerted on the head of the anchoring element is achievable. By screwing in the set screw tightly, for example, a polyaxial screw can be used as a monoaxial screw. Further, a friction force exerted by the set screws is reversible by loosening the set screws. 
     The polyaxial bone anchoring device according to embodiments of the invention can be provided to the surgeon in a pre-assembled manner, where the pressure element is axially and rotationally fixed by the set screw to such an extent that it can not fall out of the receiving part or be rotated out of its aligned position. This allows for safer insertion by a surgeon or other practitioner. It is also possible to provide receiving parts with pre-mounted set screws, which allows the surgeon to introduce the desired bone screw, to insert the pressure element into the receiving part, and to lock and/or clamp the polyaxial bone anchoring device by means of the set screw or the set screws. 
     The receiving part and the pressure element can be manufactured in series at low costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantageous of the invention will become apparent from the description of the embodiments by means of the accompanying drawings. In the drawings: 
         FIG. 1  shows a perspective exploded view of a polyaxial bone anchoring device according to a first embodiment; 
         FIG. 2  shows the polyaxial bone anchoring device of  FIG. 1  in an assembled state; 
         FIG. 3  shows a cross-sectional view of the polyaxial bone anchoring device of  FIGS. 1 and 2  in an assembled state before final locking of the head, with two non-assembled set screws; 
         FIG. 4  shows a cross-sectional view of the polyaxial bone anchoring device of  FIGS. 1 and 2  in an assembled state; 
         FIG. 5  shows a perspective top view of a pressure element according to an embodiment; 
         FIG. 6  shows a perspective bottom view of the pressure element of  FIG. 5 ; 
         FIG. 7  shows a cross-sectional view of the pressure element of  FIGS. 5 and 6 ; 
         FIG. 8  shows a top view of the pressure element of  FIGS. 5 and 6 ; 
         FIG. 9  shows a perspective view of a set screw according to an embodiment; 
         FIG. 10  shows a side view of the set screw of  FIG. 9 ; 
         FIG. 11  shows a top view of the set screw of  FIGS. 9 and 10 ; 
         FIG. 12  shows a perspective view of a polyaxial bone anchoring device according to a second embodiment in an assembled state; 
         FIG. 13  shows a cross-sectional view of the polyaxial bone anchoring device of  FIG. 12 ; 
         FIG. 14  shows a cross-sectional view of a polyaxial bone anchoring device according to a third embodiment in an assembled state; 
         FIG. 15  shows a cross-sectional view of a polyaxial bone anchoring device according to a fourth embodiment in an assembled state; 
         FIG. 16  shows a perspective exploded view of a polyaxial bone anchoring device according to a fifth embodiment; 
         FIG. 17  shows the polyaxial bone anchoring device of  FIG. 16  in an assembled state; 
         FIG. 18  shows a cross-sectional view of the polyaxial bone anchoring device according to the fifth embodiment in an assembled state; 
         FIG. 19  shows a perspective view of a polyaxial bone anchoring device according to a sixth embodiment in an assembled state; and 
         FIG. 20  shows a cross-sectional view of the polyaxial bone anchoring device according to the sixth embodiment in an assembled state. 
     
    
    
     DETAILED DESCRIPTION 
     A polyaxial bone anchoring device  1  according to a first embodiment as shown in  FIGS. 1 to 4  includes a bone anchoring element  2  in the form of a screw member having a threaded shaft  3  and a head  4 . The head  4  is generally spherical and includes a recess  4   a  at its free end for engagement with a tool to insert the threaded shaft  3  into a bone. The bone anchoring device  1  further includes a receiving part  5  for connecting the anchoring element  2  to a rod  20 . A pressure element  6  is arranged in the receiving part  5  on top of the head  4  (as seen, for example, in  FIGS. 3 and 4 ). For securing the rod  20  in the receiving part  5 , and in some embodiments, for exerting pressure onto the head  4 , a locking device, for example an inner screw  7 , which cooperates with the receiving part  5 , is provided. 
     The receiving part is a substantially cylindrical one piece part and has a top end  51  and a bottom end  52 . A passageway extending from the top end  51  to the bottom end  52  is formed by a coaxial bore  53  followed by a seat portion  54  for receiving the head  4  of the anchoring element  2 . The seat portion  54  has an opening  55  at the bottom end  52  through which the shaft  3  of the anchoring element  2  extends. The seat portion  54  is shown to be spherically-shaped, but in other embodiments, the seat portion can be tapered or can have any other shape that allows the head  4  to be received so that the head  4  can pivot with respect to the receiving part  5 . At the top end  51 , a substantially U-shaped recess  56  is provided by means of which two free legs  57 ,  58  are formed that form sidewalls of a channel for receiving the rod  20 . An internal thread  59  is provided at the legs  57 ,  58  for cooperating with the inner screw  7 . 
     As can be seen from  FIGS. 5 to 8 , the pressure element  6  in this embodiment is formed in one piece. The pressure element  6  is of substantially cylindrical construction, and has an outer diameter which allows it to move in an axial direction within the bore  53  of the receiving part  5 . The pressure element  6  has a top end  61  and a bottom end  62 . When the pressure element  6  is inserted into the receiving part  5 , the bottom end  62  faces the head  4  of an inserted anchoring element  2 . At the bottom end  62 , a spherical recess  63  is provided, which is adapted to the size and shape of the head  4 . The spherical recess  63  is configured to come into frictional engagement with the spherical surface of the head  4 . At the top end  61 , a U-shaped recess  64  is provided by means of which two free legs  65 ,  66  are formed that form a channel to receive the rod  20  therein. Furthermore, the pressure element  6  includes a coaxial bore  67  for accessing the head  4  with a tool (not shown). The pressure element  6  may be a solid member, without any spring portions which could render it flexible. It is arranged in the receiving part  5  such that the U-shaped recess  56  of the receiving part  5  and the U-shaped recess  64  of the pressure element  6  are aligned. 
     In  FIG. 3 , the head  4  of the anchoring element  2  is located in the seat  54  and the pressure element  6  is arranged on top of the screw head  4 . The heights of the free legs  65 ,  66  of the pressure element  6  are configured such that the free legs  65 ,  66  extend above the rod  20  when the rod  20  is inserted and rests on a bottom of the channel of the pressure element  6 . The two set screws  8  are in a non-assembled state in  FIG. 3 . 
     In  FIG. 4  the two set screws  8  are in an assembled state. As can be seen from  FIGS. 9 to 11 , the set screw (or grub screw)  8  is a headless screw having an outer thread  81 , a first end, and a second end. On the first end, a engagement structure  8   a  for engagement with a tool (not shown), for example, a screw driver, is provided. In  FIGS. 9 and 11 , it can be seen that the engagement structure  8   a  is an internal hexagon for engagement with a hexagon socket screw key (not shown). However, any other socket or engagement structure may instead be formed on the set screws. The second end of the set screw  8  in this embodiment is a truncated cone-shaped or taper-shaped portion  82  having a flattened tip. 
     The locking device in the form of the inner screw  7  has a projection  71  extending into the channel formed by the free legs  65 ,  66  of the pressure element  6 . A size of the projection  71  in an axial direction is such that when the inner screw  7  is tightened, the projection  71  presses onto the rod  20  while there is still a gap  21  between the top end  61  of the pressure element and a lower side of the inner screw  7 . Therefore, with a single inner screw  7 , pressure can be exerted onto the rod  20  only, which in turn can exert pressure onto the pressure element  6 . It should be noted that, instead of the single part locking device in the form of the inner screw  7 , in other embodiments, a two-part locking device can be used (not shown). Such a two-part locking device may include a first part to be screwed in between the legs  57 ,  58  of the receiving part, and which acts onto the top end  61  of the pressure element  6 . Further, a second part in the form of an inner screw may be provided in the first part, which is configured to press onto the rod  20 . By means of this, the head  4  and the rod  20  can be independently fixed relative to the receiving part  5 . 
     The receiving part  5  includes two threaded through bore holes  500   a ,  500   b  extending from a outer surface of the receiving part  5  into the coaxial bore  53 . The bore holes  500   a ,  500   b  may be arranged at 180° offset from each other on the receiving part  5  and at 90° with respect to the channel formed by the U-shaped recess  56 . The bore holes  500   a ,  500   b  may be aligned perpendicular with respect to a bore axis M of the coaxial bore  53 . The bore axes A and B of the bore holes  500   a ,  500   b  are provided at a distance H from the second end  52  of the receiving part  5 . 
     The pressure element  6  according to this embodiment correspondingly includes two through bore holes  600   a ,  600   b  which are 180° offset from each other and 90° offset from the channel formed by the U-shaped recess  64 . The bore holes  600   a ,  600   b  have center axes a, b, respectively, which are perpendicular to the bore axis M. In the embodiment shown, the bore holes  600   a ,  600   b  each have a conical shape. Downwardly extending flanks or surfaces  601   a ,  601   b  of the bore holes  600   a ,  600   b  may each be arranged at an angler β of approximately 60° with the central bore axis M. When the pressure element  6  is inserted and rests on the head  4  of the anchoring element  2 , the central axes a, b of the bore holes  600   a ,  600   b  each has a distance h from the second end  52  of the receiving part  5  that is greater than the distance H of the bore axes A, B of the bore holes  500   a ,  500   b . In other words, the bore holes  600   a ,  600   b  are arranged above the threaded bore holes  500   a ,  500   b . In some embodiments, the bore holes of the pressure element  6  can be blind holes having open sides facing an inner wall of the receiving part  5 . 
     A distance between the axes of the bore holes  600   a ,  600   b  and the axes of the bore holes  500   a ,  500   b  in an axial direction is such that when the set screws  8  are screwed into the bore holes  500   a ,  500   b , the tapered surfaces of the set screws  8  press onto the lower surfaces  601   a ,  601   b  of the bore holes  600   a ,  600   b , respectively, to exert a downward force onto the pressure element  6 . The resulting force onto the pressure element  6  generates a preload onto the head  4 , which clamps the head  4  by means of friction. By controlling the thread turning and advancement of the set screws  8 , a desired friction force can be achieved. The screwing-in process can be force-actuated and/or path-controlled. By this friction force, the head  4  can be maintained in a desired angular position, and can be moved out of the position by applying a force greater than the friction force either on the anchoring element  2  or on the receiving part  5 . Simultaneously, via the set screws  8 , the pressure element  6  is secured against rotation and secured against escaping through the top end  51  of the receiving part  5 . The bore holes  600   a ,  600   b  provide space for relative to cones of the set screws  8  for the pressure element  6  to move downward to finally lock the head  4 . 
     A second embodiment is shown in  FIGS. 12 and 13 . The only difference relative to the first embodiment is the shape of the receiving part  5 ′. The other parts of the bone anchoring device  1 ′ are the same as or similar to those of the first embodiment, and the descriptions thereof shall not be repeated. 
     The receiving part  5 ′ has an asymmetric bottom end  52 ′ for allowing a greater pivot angle of the anchoring element  2 ′ to one side. Such an asymmetric bottom end  52 ′ can be achieved, for example, by cutting away a portion of the receiving part  5 ′ at an inclined angle. 
     Furthermore, the receiving part  5 ′ may be formed with a coaxial tube shaped extension  9  that has a slot  91  for inserting the rod  20 ′ and an internal thread  92 . Such an extension may be used for minimally invasive surgery, and can be broken away after tightening of the inner screw  7 ′. 
     It shall be noted that the shape of the set screws and/or the bore holes may also vary. In particular, the angle of the conical portion of the set screws may vary or the bottoms may have a rounded or other shape. The bore holes provided at the pressure element may also have various different shapes. 
     According to a third embodiment, shown in  FIG. 14 , bore holes  600   a ″,  600   b ″ can have, for example, a substantially rectangular cross-section. 
     According to a fourth embodiment. shown in  FIG. 15 , a cross-section of bore holes  600   a ′″,  600   b ′″ of the pressure element  6 ′″ can be, for example, trapezoidal, with an inclined lower surface  601   a ′″,  601   b ′″ for engagement with truncated tapered surfaces of set screws  8 ′″. 
     In a fifth embodiment, shown in  FIGS. 16 to 18 , a bone anchoring device  10  may only include a single set screw  8 , instead of two set screws  8  as described in the embodiments shown in  FIGS. 1 to 4 and 12 to 15 . 
     In a sixth embodiment, shown in  FIGS. 19 and 20 , a bone anchoring device  10 ′ also may only include one set screw  8  instead of two set screws  8  as described in the previous embodiments, and may include an inclined lower edge or asymmetrical bottom end as shown in  FIGS. 12 and 13 . All other parts may be the same or similar to the previously described embodiments. 
     The parts of the bone anchoring device according to embodiments of the invention may be made of a body-compatible material, such as a body-compatible metal, for example, titanium, of body-compatible metal alloys such as, for example, Nitinol, or from a body-compatible plastic material, such as, for example, polyether ether ketone (PEEK), or combinations thereof. 
     Usually, several bone anchoring devices are utilized for stabilizing bone parts or vertebrae with a rod. In use, the bone anchoring devices may be pre-assembled, as shown in  FIG. 4 . The anchoring elements may be screwed into or otherwise attached to the bone or vertebra. Then, the receiving parts are pivoted by applying a force greater than the friction force until each receiving part has a correct orientation for insertion of the rod. Due to the friction force, each receiving part is held in its adjusted angular position. Thereafter, the rod, which connects the bone anchoring devices, is inserted, and the set screws and/or the inner screw in some embodiments, are tightened to move the pressure element downwards to lock the head in the seat of the receiving part, so that the angular position of the anchoring element with respect to the receiving part is fixed. The rod is fixed by the inner screw. 
     The shapes of the through bores and/or bore holes of the embodiments described are not limited to the described forms. Also the angles of the tapers are not limited to the values described. Other shapes are also possible to achieve a downwardly directed force when the set screw or screws are screwed in. 
     For the anchoring element, various different kinds of anchoring elements can be used and combined with a receiving part. These anchoring elements may be, for example, screws with different lengths, with different diameters, cannulated screws, screws with different thread forms, nails, hooks, etc. In some embodiments, the head and the shaft of the anchoring elements can be separate parts which are connectable to each other. 
     The shape of the receiving part is also not limited to the embodiments shown. For example it is possible to have a recess allowing the rod to be introduced from a side of the receiving part instead of being introduced from the top, or a closed recess through which the rod is to be guided. In addition, various kinds of locking devices, including locking devices with two or more parts, outer nuts, outer caps, bayonet locking devices, or various other locking devices are also possible. 
     In a further modification, the receiving part may be configured to allow introduction of the anchoring element from the bottom end in some embodiments. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.