Abstract:
A receiving part for receiving a rod for coupling the rod to a bone anchoring element, the receiving part including: a receiving part body including a rod receiving portion with a channel for receiving a rod, and a head receiving portion for accommodating a head of a bone anchoring element, the head receiving portion having an open end and being flexible for inserting and clamping of the head; a locking ring around the head receiving portion to exert a first force onto the head receiving portion to lock the head therein; and a pre-locking ring separate from the locking ring and around the head receiving portion to exert a second force less than the first force onto the head receiving portion to clamp the head in a temporary pre-locking position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/140,612, filed Dec. 23, 2008, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 08 022 389.4, filed Dec. 23, 2008, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a receiving part for receiving a rod for coupling the rod to a bone anchoring element and a bone anchoring device with such a receiving part. The head of the bone anchoring element is locked in the receiving part by compression of a head receiving portion of the receiving part laterally surrounding the head by means of a locking ring. The receiving part further comprises a pre-locking ring for clamping the head by the pre-stress exerted onto the head receiving portion for safe handling during surgery. The locking ring is movable to a final locking position for locking the head. The bone anchoring device can be realized, for example, in the form of a polyaxial bone screw. 
         [0004]    2. Description of Related Art 
         [0005]    Various designs of polyaxial bone screws wherein the head is clamped from the side to lock the rotational position of the bone screw are known. 
         [0006]    U.S. Pat. No. 5,672,176 describes a bone screw with a receiving part with a conically shaped seat and a conically shaped pressure element which exerts pressure onto the head from above and from the side. If the cone angle has a value laying within a specific range self-locking of the pressure element within the receiving part takes place which allows to preliminary lock the head within the receiving part while the rod is still movable in order to allow the adjustment of its position. 
         [0007]    U.S. Pat. No. 5,728,098 describes a bone screw for connection to a spinal rod comprising a screw element and a receiver member which has slits provided at the bottom of the rod receiving channel and wherein two ring-shaped compression members made of a shaped-memory alloy are provided at the lower side and the upper side of the receiver member, respectively. The compression members contract about the portions of the receiver member when the temperature is elevated so that the rod is clamped in the channel. 
         [0008]    WO 2007/038350 A2 discloses an apparatus for connecting a bone anchor to a support rod, the apparatus including a connector body and a cap. The connector body has a socket for insertion, angulation and removal of a bone anchor. A sleeve is provided which is configured to fit over the connector body in a temporary position in which the sleeve permits insertion of the bone anchor, to move to a provisional locking position in which the sleeve permits angulation but prevents removal of the bone anchor, and to move to a locking position in which the sleeve prevents both angulation and removal of the bone anchor. The sleeve extends over the whole length of the socket. 
       SUMMARY 
       [0009]    It is the object of the invention to provide an improved receiving part for receiving a rod for coupling the rod to a bone anchoring element and a bone anchoring device with such a receiving part, which has a small size while simultaneously providing for safer handling during surgery and a safer and more secure final locking. 
         [0010]    A pre-locking ring pre-locks a head of a bone anchoring element in a head receiving portion of a receiving part body which prevents removal of the head from the receiving part body and/or prevents movement of the head with respect to the receiving part body during surgery. This allows for safer handling of the bone anchoring device during surgery. 
         [0011]    The receiving part and the bone anchoring device according to an exemplary embodiment of the invention comprises few elements, which reduces the costs of manufacturing and which facilitates handling. It makes use of the principle of clamping the head of the bone anchoring element circumferentially from the lateral sides, which reduces the force necessary to safely clamp the head. The design of the receiving part allows to further reduce the dimension in terms of height as well as in terms of the bottom outer diameter which is particularly suitable for applications where small-sized anchoring devices are utilized, such as in the field of cervical spinal surgery or paediatric applications, trauma and minimally invasive applications. 
         [0012]    By providing various bone anchors with different receiving parts, a modular system is available prior to surgery. 
         [0013]    Since the height of the locking ring is smaller than the height of the head receiving portion the receiving part has a profile with a small diameter. The pressure exerted via the locking ring onto the head receiving portion is largest at a position of the largest diameter of the head of the bone anchoring element. Therefore, the locking ring does not have to extend up to the open end of the head receiving portion which allows for a locking ring with a reduced diameter at the bottom end. 
         [0014]    The locking ring is movable between a position in which the head is not clamped and a position in which the head is locked. The locking ring can be releasably held in either of the two end positions which makes handling convenient. 
         [0015]    When the locking ring has a curved interior surface portion, jamming between the locking ring and the head receiving portion does not occur. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Further features and advantages of the invention will become apparent from the description of embodiments using the accompanying drawings. In the drawings: 
           [0017]      FIG. 1  shows a perspective exploded view of an embodiment of the bone anchoring device. 
           [0018]      FIG. 2  shows a perspective view of the bone anchoring device of  FIG. 1  in an assembled state. 
           [0019]      FIG. 3  shows a perspective view of the receiving part body according to a first embodiment. 
           [0020]      FIG. 4  shows a side view of the receiving part body of  FIG. 3 . 
           [0021]      FIG. 5  shows a side view of the receiving part body of  FIG. 4  rotated by 90°. 
           [0022]      FIG. 6  shows a top view of the receiving part body of  FIG. 3 . 
           [0023]      FIG. 7  shows a perspective view of the locking ring. 
           [0024]      FIG. 8  shows a cross-sectional view of the locking ring of  FIG. 7 , the section being taken along line A-A. 
           [0025]      FIG. 9  shows a perspective view of the pre-locking ring. 
           [0026]      FIG. 10  shows a cross-sectional view of the pre-locking ring of  FIG. 9 , the section being taken along line B-B. 
           [0027]      FIG. 11  shows an enlarged portion of a cross-sectional view of the receiving part of  FIGS. 1 and 2  with a further enlarged section of a pre-locking ring in a first embodiment. 
           [0028]      FIG. 12  shows an enlarged portion of a cross-sectional view of the receiving part of  FIGS. 1 and 2  with a further enlarged section of a pre-locking ring in a second embodiment. 
           [0029]      FIG. 13  shows an enlarged portion of a cross-sectional view of an embodiment of a receiving part with a further enlarged section of a pre-locking ring in a third embodiment. 
           [0030]      FIG. 14  shows an enlarged portion of a cross-sectional view of another embodiment of a receiving part with a further enlarged section of a pre-locking ring in a fourth embodiment. 
           [0031]      FIG. 15  shows a cross-sectional view of an embodiment of a bone anchoring device in a first position. 
           [0032]      FIG. 16  shows a cross-sectional view of the embodiment of the bone anchoring device of  FIG. 15  in a second position. 
           [0033]      FIG. 17  shows a cross-sectional view of the embodiment of the bone anchoring device of  FIG. 15  in a third position. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    As shown in  FIGS. 1 and 2 , the bone anchoring device according to one embodiment comprises a bone anchoring element  1  in the form of a bone screw having a threaded shaft  2  and a head  3  with a curved surface portion, in this embodiment a spherical segment-shaped head. The head  3  has a recess  4  for engagement with a screwing-in tool. The bone anchoring device further comprises a receiving part body  5  for receiving a rod  6  to connect it to the bone anchoring element  1 . Further, a closure element  7  in the form of an inner screw is provided for securing the rod  6  in the receiving part body  5 . In addition, the bone anchoring device includes a locking ring  8  for locking the head  3  in the receiving part body  5 , and a pre-locking ring  80  for pre-locking the head  3  in the receiving part body  5 . 
         [0035]    As can be seen in particular in  FIGS. 3 to 6 , the receiving part body  5  comprises a rod receiving portion  9  which is substantially cylindrical and which has a first end  9   a  and a second end  9   b  opposite the first end  9   a . The rod receiving portion  9  has a coaxial first bore  10  provided at the second end  9   b . The diameter of the first bore  10  is smaller than the diameter of the head  3  of the bone anchoring element  1 . The rod receiving portion  9  also comprises a coaxial second bore  11  extending from the first end  9   a  to a distance from the second end  9   b . The diameter of the second bore  11  is larger than that of the first bore  10  and larger than the diameter of the rod  6 . A substantially U-shaped recess  12  is provided in the rod receiving portion  9  which extends from the first end  9   a  towards the second end  9   b , the diameter of the recess  12  being slightly larger than the diameter of the rod  6  in such a way that the rod  6  can be placed in the recess and can be guided therein. The recess  12  forms two free legs  12   a ,  12   b , on which an internal thread  13  is provided. The internal thread can be a metric thread, a flat thread, a negative angle thread, a saw-tooth thread, or any other thread type. Preferably, a thread form such as a flat thread or negative angle thread is used which prevents or reduces splaying of the legs  12   a ,  12   b  when the inner screw  7  is screwed-in. The depth of the recess  12  is such that the rod  6  and the inner screw  7  can be inserted between the legs. Between the bottom of the recess and the legs  12   a ,  12   b , a flat section  14  is provided, forming the end of the bore  11 . 
         [0036]    As can be seen in  FIGS. 1 ,  2 ,  3  and  6 , cuts  24  are provided in the rod receiving portion on either end of the channel formed by the recess  12 . 
         [0037]    The rod receiving portion  9  of the receiving part body  5  further comprises a plurality of coaxial slits  15  extending from the second end  9   b  to a distance from the first end, wherein the distance corresponds approximately to the length of the internal thread  13 . The slits  15  are open at the second end  9   b  and extend, as can be seen in particular in  FIGS. 1 ,  3  and  6  through the flat section  14  and the substantially U-shaped recess  12 . At least one slit  15 , and preferably more than one slit, is provided on either side of the recess  12 . The number of slits is provided according to the degree of flexibility which is to be provided by the slits. It may depend, for example, on the material and wall thickness and/or other factors. 
         [0038]    Adjacent to the second end  9   b  the receiving part body  5  comprises a head receiving portion  16  providing an accommodation space for the head  3  of the bone anchoring element  1 . The head receiving portion  16  has an outer surface which tapers towards the second end  9   b  and which has an open end  17  opposite to the second end  9   b . The exterior surface of the head receiving portion  16  can be partly or fully tapered. It is tapered at least in the region of the largest diameter of the head  3 . The open end  17  can have a rounded edge. 
         [0039]    As can be seen in particular in  FIGS. 3 to 5 , the outer diameter of the rod receiving portion  9  at the second end  9   b  is larger than the outer diameter of the head receiving portion  16  adjacent to the second end  9   b , and is also larger than the outer diameter of the head receiving portion at the open end  17 . Hence, the head receiving portion  16  is recessed with respect to the rod receiving portion  9 . 
         [0040]    As can be seen in particular in  FIG. 5 , the head receiving portion  16  has an internal hollow section  18  forming a seat for the head  3  of the bone anchoring element  1 . The hollow section  18  is adapted in its shape to correspond to the shape of the head  3 , in the embodiment shown it is a spherical section for accommodating the spherical head  3 . The hollow section  18  is dimensioned in such a way that it encompasses the head  3  of the bone anchoring element from the side, covering a region including the largest diameter of the head  3 . 
         [0041]    A plurality of slits  19  are provided in the head receiving portion  16  which are open to the open end  17  and extend from the open end  17  to the second end  9   b  of the rod receiving portion, and some of which may continue into the slits  15  of the rod receiving portion  9 , thereby forming continuous slits extending from the open end  17  of the head receiving portion  16  into the rod receiving portion  9 . The number of slits  19  may be equal to the number of slits  15 , however, the number of slits can be smaller or larger depending on the desired flexibility of the head receiving portion  16 . In addition, slits  20  are provided on the side of the head receiving portion  16  which is adjacent to the substantially U-shaped recess  12  of the rod receiving portion, as shown in  FIG. 3 . The slits  20  end at a distance from the second end  9   b . The flexibility of the head receiving portion  16  is such that the head  3  of the anchoring element can be inserted by expanding the head receiving portion  16  and can be clamped by compressing the head receiving portion  16 . The slits  15  in the rod receiving portion facilitate mounting of the receiving part body  5  onto the head  3  manually. 
         [0042]    As can be seen in particular in  FIGS. 3 to 5 , adjacent to the open end  17 , the head receiving portion  16  has a recessed area  17   a  for accommodation of a pre-locking ring  80 , to be described later. The recessed area  17   a  has, in the embodiment shown, the shape of a circular recess with a rectangular contour. 
         [0043]    The locking ring  8  will now be described with reference to  FIGS. 1 ,  2 ,  7  and  8 . The locking ring  8  has a substantially cylindrical outer surface with an outer diameter corresponding substantially to the outer diameter of the rod receiving portion  9  of the receiving part  5 . The height of the locking ring  8  in an axial direction is smaller than that of the head receiving portion  16  of the receiving part body  5 , so that, as shown in particular in  FIG. 2 , there is a distance between the locking ring  8  and the second end  9   b  of the rod receiving portion  9 , when the locking ring  8  is in the position in which the head  3  is locked. 
         [0044]    The locking ring  8  has on its inner side a curved interior surface portion  8   a . The curvature is directed to the center of the locking ring  8 . The curved interior surface portion  8   a  can have a spherical curvature. Other types of curvatures are also possible. The radius of the curvature is smaller than the radius of the head  3 . The dimensions of the locking ring  8  with respect to its inner portions are such that the locking ring  8  can slide along the outer surface of the head receiving portion  16 , thereby increasingly compressing the head receiving portion  16  when sliding downward. 
         [0045]    The locking ring  8  further has on its side facing the second end  9   b  two projections  21  located diametrically opposite to each other. The projections  21  have a height such that they project above the bottom of the substantially U-shaped recess  12  and extend into the cuts  24  when the locking ring  8  is in a position in which the head  3  is not yet clamped. The free end  22  of the projections  21  can be curved, particularly inwardly curved, with a curvature corresponding to that of the rod  6 . The locking ring is arranged in such a way around the head receiving portion  16  of the receiving part body  5  that the projections  21  are located at the positions of the recess  12 . By means of this, the projections  21  which project into the recess  12  prevent the locking ring  8  from rotating when the rod  6  is not inserted. 
         [0046]    The flexibility of the head receiving portion  16  and the size of the head receiving portion at the open end  17  allows mounting of the locking ring  8  by assembling it from the free end  17  onto the head receiving portion  16 . Since the outer diameter of the head receiving portion is smaller than that of the rod receiving portion  9 , the locking ring  8  does not project or only minimally project beyond the rod receiving portion in a radial direction. The locking ring  8  may have a tapered exterior surface  8   b  as shown in  FIG. 11  to further reduce its diameter in a direction of the open end  17 . 
         [0047]    The pre-locking ring is now explained with respect to  FIGS. 1 ,  2 ,  9  and  10 . The pre-locking ring  80  has a substantially rectangular cross section as shown in  FIGS. 9 and 10 , wherein the lower outer edge  80   a  may be rounded. The inner diameter of the pre-locking ring is slightly smaller than the outer diameter of the recess  17   a  provided at the open end  17  of the head receiving portion  16 . The dimensions of the recess  17   a  and the pre-locking ring  80  are such that when the pre-locking ring  80  is mounted onto the head receiving portion  16  from the open end  17  it slightly compresses the head receiving portion  16  so that the head  3  of the bone anchoring element is pre-locked by the pre-stress exerted by the pre-locking ring. Pre-locking means that under conditions arising during surgery the angular position of the bone anchoring element  1  with respect to the receiving part body  5  is maintained in a temporary position and can be loosened only by exerting an additional force onto the receiving part body or the bone anchoring element. 
         [0048]    in the embodiment shown and depicted in an enlarged view in  FIG. 11 , the inner contour  80   b  of the pre-locking ring  80  is rectangular to fit into the rectangular recess  17   a  of the head receiving portion  16 . Other geometries are also possible. For example, as shown in the embodiment of  FIG. 12 , the inner contour  80   b ′ of the locking ring  80 ′ can be rounded. As shown in the embodiment of  FIG. 13 , the recess  17   a ′ at the open end  17  of the head receiving portion can be tapered and the contour  80   b ″ of the pre-locking ring  80 ″ can be tapered to fit to the contour of the recess  17   a ′. As shown in the embodiment of  FIG. 14 , the recess  17   a ″ can be circular-shaped and the pre-locking ring  80 ″ has a circular cross-section which fits into the recess. Various other shapes are also conceivable. 
         [0049]    The inner screw  7  has a thread corresponding to the internal thread  13  provided on the legs. If a thread form which prevents the legs from splaying is used, a single closure element such as the inner screw  7  is sufficient. This reduces the size of the bone anchoring device in a radial direction. Other closure elements such as, for example, an outer nut, are also possible. 
         [0050]    The receiving part body  5 , the locking ring  8 , the inner screw  7  and the bone anchoring element  1  are made of a bio-compatible material, for example, of titanium or a stainless steel or bio-compatible alloy or bio-compatible plastic material with sufficient strength. The pre-locking ring is also made of one of these materials. However, the pre-locking ring  80  can also be made from a material which exhibits elastic properties, such as, for example, an elastomer material. For example, the pre-locking ring  80  can be made of an elastomer material which exhibits properties similar to that of an O-ring or other kinds of rubber rings. 
         [0051]    The bone anchoring device may be preassembled with the locking ring  8  mounted on the head receiving portion  16  of the receiving part body  5  and with the screw head  3  inserted and the pre-locking ring  80  mounted in the recess  17   a.    
         [0052]    The function of the bone anchoring device with respect to the locking of the head is now explained with reference to  FIGS. 15 to 17 .  FIG. 15  shows a first condition in which the locking ring  8  is in a first position P 1  near the second end  9   b  of the rod receiving portion  9  of the receiving part body  5 . The head  3  of the bone anchoring element is not yet inserted into the hollow space  18  and the pre-locking ring  80  is not yet mounted. Means can be provided (not shown) which releaseably hold the locking ring  8  in the position P 1 . Such means can be, for example, a catch engaging a groove. 
         [0053]    As shown in  FIG. 16 , after inserting the head  3  of the bone anchoring element and mounting the pre-locking ring  80 , the head  3  is held in the head receiving portion  16  in a pre-locked condition due to the pre-stress exerted by the pre-locking ring  80  onto the head receiving portion  16 . In this condition, a pivoting of the head  3  within the head receiving portion is only possible by exerting an additional force to move the receiving part body  5  relative to the head  3 . When the pre-locking ring  80  is mounted, a removal of the head  3  from the head receiving portion  16  is no longer possible. 
         [0054]    As shown in  FIG. 17 , for final locking of the head  3  within the head receiving portion the locking ring  8  is moved downwards. This can be accomplished, for example, when the inner screw  7  presses onto the rod  6  and the rod presses onto the locking ring  8  via the projections  21 . In the course of tightening the inner screw  7  the locking ring  8  is shifted downward until the curved portion  8   a  engages the tapered exterior surface portion of the head receiving portion  16 . When the locking ring is at a position of the greatest diameter of the head  3  in a direction of the screw axis of the bone anchoring device, the pressure exerted by the curved portion  8   a  of the locking ring onto the head receiving portion  16  is largest, whereby the head  3  is clamped in such a way that it is locked. Simultaneously, a further downward movement of the locking ring  8  is prevented. 
         [0055]    The bone anchoring device can be preassembled either by the manufacturer or in the course of preparation of the surgery or at any other time. Advantageously, the surgeon selects prior to surgery the necessary receiving parts and bone anchoring elements according to the specific requirements of the clinical application. The design of the bone anchoring device allows selection of an appropriate bone anchoring element in terms of diameter, length and other features of the anchoring section. Hence, a modular system is provided which includes receiving parts and several bone anchoring elements, which then individually can be chosen and adapted. 
         [0056]    In the pre-locking condition the head  3  remains clamped due to the pre-locking ring  80  when the inner screw  7  is loosened. This allows further adjustments of the rod  6 . 
         [0057]    In use, the preassembled bone anchoring device is screwed into the bone. The recess  4  of the head can be accessed with a screw tool through the first bore  10 . The locking ring  8  is in its first position P 1  close to the second end  9   b  where it does not clamp the head  3 . The head  3  is, however, pre-locked by means of the pre-locking ring  80 . To correctly align the receiving part with respect to the rod to which it shall be connected, an additional force is exerted onto the receiving part either manually or by application of an instrument. Once the correct position of the rod with respect to other bone anchoring devices is achieved, the inner screw  7  is screwed between the legs until it presses onto the rod. The rod is pressed against the bottom of the substantially U-shaped recess  12  thereby engaging the free ends  22  of the projections  21 , respectively, and shifting down the locking ring  8 . When the locking ring  8  is moved towards the free end  17  of the head receiving portion it compresses the head receiving portion, thereby locking the head. Since the force which is exerted by the locking ring  8  acts with the interior curved surface  8   a  from the lateral side, the force utilized for safely immobilizing the head is smaller than when a force acts from above the head  3 . This also allows for reducing the size of the device by allowing a wall thickness of the receiving part to be reduced. Final tightening of the inner screw locks the rod and the head substantially simultaneously. 
         [0058]    Further modifications of the embodiment shown are possible. For example, the head of the bone anchoring element can have any other shape, such as, for example, a cylindrical shape, whereby a monoaxial bone screw is provided allowing rotation of the screw element with respect to the receiving part around a single axis. The head  3  can also be conically shaped or otherwise shaped, where the internal hollow section  18  of the head receiving portion  16  is adapted to any such shape. In a further modification, the receiving part body  5 , or at least the head receiving portion  16 , is made of bio-compatible plastic material which provides for elasticity to a certain degree. In this case, slits may be omitted. 
         [0059]    The projections of the locking ring which engage the rod can have various other shapes. For example, the surface of the free end can be flat or be otherwise shaped. In some embodiments, the projections can be omitted. 
         [0060]    The head receiving portion can have an inclined open end  17  to allow for a greater angulation of the head in one direction. 
         [0061]    In one embodiment, the pre-locking ring  80  is made of a material exhibiting elasticity, and it may be possible to first mount the pre-locking ring  80  before inserting the head  3  of the bone anchoring element. 
         [0062]    In one embodiment, the outer surface of the head receiving portion  16  can be spherical and the inner surface of the locking ring  8  can be tapered. 
         [0063]    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.