Abstract:
A receiving part for receiving a rod for coupling the rod to a bone anchoring element includes a receiving part body including a first end and a second end, and having a substantially U-shaped recess at the first end forming a channel for receiving the rod, and an accommodation space for accommodating a head of the bone anchoring element, the accommodation space having an opening at the second end for introducing the head; and a pressure element arranged at least partially in the accommodation space, the pressure element including a first section having a second recess for receiving the rod, and a second section having a flexible portion to clamp the head, the first section and the second section being fixed relative to each other, wherein said pressure element is insertable from the opening.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/233,406, filed Aug. 12, 2009, the contents of which are hereby incorporated by reference in their entirety, and claims priority to European Patent Application EP 09 167 751.8, filed Aug. 12, 2009, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The invention relates to a receiving part for receiving a rod for coupling the rod to a bone anchoring element. The receiving part includes a receiving part body with a channel for receiving a rod and an accommodation space for accommodating a head of a bone anchoring element, and a pressure element for clamping the head. The accommodation space has an opening at a bottom side for inserting the pressure element and for inserting the head. 
         [0004]    2. Description of Related Art 
         [0005]    Various designs of polyaxial bone screws are known wherein a head is clamped from the side to lock a rotational position of the bone screw. 
         [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 a pressure onto the head from above and from the side. 
         [0007]    U.S. Pat. No. 5,669,911 describes a polyaxial orthopaedic device for use with a rod implant apparatus. The device includes a screw having a curvate head, a locking collar disposed therearound, and a receiving member having a linearly tapered socket in which the screw and the collar are nested. The collar is introduced from the top of the receiving member. The head of the screw can be inserted from the bottom. 
         [0008]    U.S. Pat. No. 6,063,090 relates to a device used to connect a longitudinal support to a pedicle screw by an accommodating head having a channel to accommodate the longitudinal support. The pedicle screw and the accommodating head are connected via a conical collate chuck in the accommodating head and by a spherical head on the pedicle screw. The device allows engagement of the pedicle screw in the accommodating head after the pedicle screw has been inserted into the bone. 
       SUMMARY 
       [0009]    Embodiments of the invention 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, where the embodiments have fewer parts, a low profile, and provide for improved handling during surgery. 
         [0010]    Embodiments of the invention provide a receiving part including a receiving part body including a first end and a second end, and having a substantially U-shaped recess at the first end forming a channel for receiving a rod, and an accommodation space for accommodating a head of a bone anchoring element, the accommodation space having an opening at the second end for introducing the head; and a pressure element arranged at least partially in the accommodation space, the pressure element including a first section having a second recess for receiving the rod, and a second section having a flexible portion to clamp the head, the first section and the second section being fixed relative to each other, wherein said pressure element is insertable from the opening, a bone anchoring device including such a receiving part, and a method of using such a receiving part. 
         [0011]    For the receiving part according to an embodiment of the invention, the pressure element has a slim design which allows it to be introduced from the bottom into the receiving part body. An internal end stop within the receiving part body may form an abutment for the pressure element to be positioned in an insertion position for the head. Therefore, there may be no additional components for holding the pressure element in the insertion position. 
         [0012]    In some embodiments, the receiving part has a low profile and improved or maximum stiffness, since a wall thickness of the receiving part body can be increased due to the slim design of the pressure element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Further features and advantages of the invention will become apparent from the description of embodiments using the accompanying drawings. In the drawings: 
           [0014]      FIG. 1  shows a perspective exploded view of a first embodiment of the bone anchoring device. 
           [0015]      FIG. 2  shows a perspective view of the bone anchoring device of  FIG. 1  in an assembled state. 
           [0016]      FIG. 3  shows a cross-sectional view of the bone anchoring device of  FIG. 1 , the section being taken perpendicular to a rod axis. 
           [0017]      FIG. 4  shows an enlarged perspective view of a pressure element according to an embodiment of the bone anchoring device. 
           [0018]      FIG. 5  shows a cross-sectional view of the pressure element according to  FIG. 4 . 
           [0019]      FIG. 6  shows a side view of the pressure element of  FIG. 4 . 
           [0020]      FIG. 7  shows a bottom view of the pressure element of  FIG. 4 . 
           [0021]      FIGS. 8   a  to  8   d  show schematic views of design modifications of a pressure element and a receiving part body in a locking position according to embodiments of the bone anchoring device. 
           [0022]      FIGS. 9   a  and  9   b  show schematic views of a further design modification of the pressure element and the receiving portion in a pre-locking position and a locking position, respectively, according to an embodiment of the bone anchoring device. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    As shown in  FIGS. 1 and 2 , a bone anchoring device according to a first embodiment includes a bone anchoring element  1  in the form of a bone screw having a threaded shaft  2  and a head  3 , which in this embodiment is a spherical segment-shaped head. The head  3  has a recess  4  for engagement with a screwing-in tool. The bone anchoring device further includes a receiving part body  5  for receiving a rod  6  to connect the rod to the bone anchoring element  1 . Further, a closure element  7 , in the form of an inner screw or set screw in some embodiments, is provided for securing the rod  6  in the receiving part body  5 . In addition, the bone anchoring device includes a pressure element  8  for locking the head  3  in the receiving part body  5 . 
         [0024]    The receiving part body  5  is now explained with reference to  FIGS. 1 to 3 . The receiving part body  5  includes a first end  5   a  and a second end  5   b , and an axis of symmetry M passes through the first and the second end. A bore  9 , which is coaxial with the axis of symmetry M, extends from the first end  5   a  to the second end  5   b . The bore  9  may have a smallest diameter at, about, or adjacent to its middle section  91  in a center region of the receiving part body  5 . In a first region adjacent to the first end  5   a , the receiving part body  5  has a U-shaped recess  10 , which may be symmetric with respect to the symmetry axis M. The recess  10  has a bottom which may be directed towards the second end  5   b  and provides two free lateral legs  11   a ,  11   b  extending towards the first end  5   a . In the region of the legs  11   a ,  11   b  an internal thread  12  may be provided which may cooperate with the closure element  7 , which in this embodiment is a set screw. A channel formed by the U-shaped recess  10  is sized so as to receive the rod  6  therein, the rod for connecting a plurality of bone anchoring devices. 
         [0025]    At a second region near the second end  5   b , the receiving part body  5  has a narrowing portion  13 , which narrows in a direction towards the second end  5   b . The narrowing portion  13  provides a seat for the screw head and the pressure element. The narrowing portion  13  is located at a distance from the second end  5   b  to, for example, enable the screw shaft  2  to be pivoted in a certain pivot angle range. An opening  14  is provided at the second end  5   b , the diameter of which may be the same as or larger than the diameter of the narrowing portion  13  in some embodiments. Between the narrowing portion  13  and the middle portion  91  of the bore  9 , a portion with an inner diameter larger than the diameter of the narrowing portion  13  is provided, which forms a space  15  for allowing the pressure element to expand, as described later. At its side opposite to the narrowing portion  13 , the space  15  connects to the middle section  91  of the bore  9  having the smaller diameter, thereby providing a circular shoulder  16 . 
         [0026]    The middle section  91  of the bore  9  includes a circular projection  17  at its side opposite to the space  15 , which can also act as a stop, as described below in more detail. 
         [0027]    As can be seen in  FIGS. 1 and 3  to  7 , a pressure element  8  according to an embodiment of the invention includes a first section  81  which is substantially cylindrical and has an outer diameter which may be slightly smaller than the inner diameter of the middle section  91  of the bore  9 , so that the pressure element  8  is movable in the bore  9 . It further has a second section  82 , which has a hollow interior  83  which is substantially spherically-shaped and is sized to clamp the spherical head  3  therein. The outer diameter of the first section  81  may be, as can be seen in  FIG. 3 , smaller than an outer diameter of the head  3  and a largest outer diameter of the second section  82  is greater than the outer diameter of the cylindrical section  81 . A free end of the second section  82  provides an opening  84  for introduction of the head  3 . Further, the second section  82  may have a plurality of slits  85  extending from the edge of the opening  84  through the second section  82  to define or form slightly resilient legs  90 . The number and the dimension of the slits  85  may be such that the wall of the second portion is flexible enough to snap onto the head  3  when the head  3  is being inserted. The slits  85  can extend into the first cylindrical section  81  as shown in the figures to enhance flexibility. The outer wall surface of the second section  82  may include a first portion  86  which may be spherically-shaped, and a second portion  87  adjacent to the opening  84  which may be tapered or curved or otherwise narrowing towards the opening  84 . The portion  87  cooperates with the narrowing portion  13  of the receiving part body when the head is locked in the receiving part body. 
         [0028]    Further, the pressure element has a coaxial bore  88  for providing access to the screw head by a tool. The first section  81  may have at its free end a cylindrically-shaped or cylinder segment-shaped recess  89  for receiving the rod  6  therein. 
         [0029]    The dimensions of the pressure element  8  are such that the pressure element  8  can be inserted through the opening  14  at the second end  5   b  of the receiving part body  5 , with the cylindrical first section  81  inserted first. When the second section  82  passes through the opening  14 , it may be compressed due to the flexibility of the second section  82 , or the legs  90  thereof, which allows the pressure element  8  to be fully introduced into the receiving part body  5 . 
         [0030]    The pressure element  8  can be pushed into the receiving part body  5  until an upper edge of the first section  81  abuts against the stop  17  provided by the annular projection in the receiving part body  5 . When the pressure element is in this position, the flexible second section  82  is located in the portion between the middle section  91  of bore  9  and the narrowing portion  13  of the receiving part body  5 . In this position, a free space in the space  15  between the outer wall of the second section  82  of the pressure element and the inner wall of the receiving part body  5  provides room for the flexible second section  82  to expand when the head  3  is inserted. 
         [0031]    A depth of the cylindrical recess  89  of the pressure element is smaller than a radius of the rod  6 , so that the pressure element can be pressed down by means of screwing-in the inner screw  7 , which presses onto the rod  6 , which in turn presses onto the pressure element  8 . The pressure element  8  is oriented in the receiving part body  5  such that its cylindrical recess  89  is aligned with the U-shaped recess of the receiving part body  5 . 
         [0032]    The material of which the components of the bone anchoring device are made is preferably a body compatible metal, such as stainless steel or titanium, or a body compatible alloy such as a nickel titanium alloy, in particular Nitinol. However, body compatible plastic materials, such as medical-grade polyether ether ketone (PEEK), can also be used. 
         [0033]    In use, first, the pressure element  8  is introduced via the bottom opening  14  into the receiving part body  5  until it abuts against stop  17 . This can be done either by the surgeon, or before so that the receiving part body is preassembled with the pressure element  8 . Thereafter, the head  3  is introduced through the bottom opening and pushes the pressure element  8  upwards against stop  17 . This is the insertion position, which allows the screw head  3  to be introduced through the opening  84  into the pressure element  8 , thereby widening the hollow interior  83 , or the legs  90  respectively, until the pressure element  8  snaps onto the head  3 . This can be done either before screwing the threaded shaft  2  into the bone, or after having screwed in the threaded shaft  2  into the bone, to mount the receiving part body  5 , with the pressure element inside, onto the head  3 . When the head  3  is inserted, it can still pivot within the pressure element  8  if the flexible section  82  is not compressed. A plurality of bone anchoring devices may be implanted into the bone(s), for example, in the pedicles of adjacent vertebrae, and the rod  6  may be inserted into the bone anchoring devices, respectively. Angular positions of the receiving part bodies may then be adjusted. Finally, the set screw  7  for each bone anchoring device may be tightened, thereby pressing down the rod  6  onto the respective pressure elements, which are pressed down until the second portion  87  of the outer wall of the second section  82  of each pressure element engages with the narrowing portion  13  of the respective receiving part bodies. In this condition, the head of each bone anchoring device is clamped in an interior of the pressure element, so that it is locked in a desired angular position. 
         [0034]      FIGS. 8   a  to  8   d  show various designs of the narrowing portion  13  of the receiving part body  5  and the portion  87  of the pressure element  8  which engages the narrowing portion  13  according to embodiments of the invention.  FIG. 8   a  shows the two portions tapering lineally at substantially a same angle. This provides a substantially even pressure distribution between the second portion  87  of the pressure element  8  and the narrowing portion  13  of the receiving part body  5 .  FIG. 8   b  and  FIG. 8   c  show two different designs where the respective portions are tapered at different angles.  FIG. 8   b  shows a main contact area at a bottom of the narrowing portion  13 , and  FIG. 8   c  shows a main contact area at a top of the narrowing portion  13 .  FIG. 8   d  shows a tapering second portion  87  of the pressure element  8  and a rounded portion  13  of the receiving part body, wherein a curvature of the rounded portion is directed towards a center of the receiving part body  5 . With such a configuration, a contact area positioned at the rounded portion can be achieved. 
         [0035]    In  FIGS. 9   a  and  9   b  a narrowing portion  13 ′ of a modified example of a receiving part body  5  has a double spherical radius formed by two curved portions  13   a ,  13   b  on top of (i.e., adjacent to) each other, with a groove  13   c  therebetween. A curvature of the curved portions are directed towards the central axis M. Correspondingly, a modified pressure element  8 ′ has at its lower end two invertedly curved portions  8   a ,  8   b  which correspond to the curved portions  13   a ,  13   b , with a crest  8   c  therebetween, and an outer crest  8   d  at an outer yielding edge of the pressure element  8 ′. 
         [0036]    As shown in  FIG. 9   a , when the pressure element  8 ′ moves downward, its lower most edge  8   d  engages in the groove  13   c . In this position there is a frictional clamping of the head  3 , which still allows the head  3  to be pivoted upon exertion of a force, where this force is greater than a force needed to pivot the head  3  when the head  3  is introduced in an insertion position. This may be characterized as a pre-locking condition. As shown in  FIG. 9   b , by further pressing the pressure element  8 ′ downward, the curvatures of the pressure element  8 ′ engage with the corresponding curvatures of the narrowing portion  13 ′ to finally lock the head  3 . 
         [0037]    Further modifications of the bone anchoring device are possible. For example, in one embodiment the pressure element  8  can have a recess corresponding to the cylindrical recess  89  described above, which may be U-shaped and provides legs extending above the rod. A dual part closure element can then be used to separately clamp the head and the rod. A device for preventing the pressure element from rotating can be provided (not shown). Such a device can be realised, for example, by crimp bores or by a pin extending from the wall into a recess (not shown) of the pressure element. 
         [0038]    Further, in some embodiments, the abutment or stop  17 , on which a pressure element may abut after it is inserted through the bottom opening and pushed further inwards, can be provided at other locations in the receiving part, for example, at the circular shoulder  16 , which may then interact with a corresponding projecting part of the pressure element to provide an abutment for the pressure element. 
         [0039]    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.