Abstract:
A receiving part for coupling a rod to a bone anchoring element includes a receiving part body having a first end, a second end, a channel for receiving a rod, an accommodation space for accommodating a head of a bone anchoring element, and an opening at the second end for inserting the head, and a pressure element configured to be positioned at least partly in the accommodation space, the pressure element having a first end, a second end, a central axis, and a flexible portion to clamp an inserted head. The flexible portion has a first slit spaced apart from the second end of the pressure element that extends at least partially around the central axis, and a second slit that extends from the second end of the pressure element into the first slit. The first slit extends away from the second slit and is longer than the second slit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/652,646, filed May 29, 2012, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 12 169 894.8, filed May 29, 2012, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field 
     The invention relates to a receiving part for receiving a rod for coupling the rod to a bone anchoring element, where the receiving part includes a receiving part body with a channel for receiving the rod and an accommodation space for accommodating a head of the bone anchoring element, and a pressure element with a flexible portion to clamp the head. The flexible portion has a first slit provided in a circumferential direction with respect to a head of the bone anchoring element and a second slit extending from an open end of the pressure element into the first slit, so that a slit ring is integrally formed in the pressure element. Such a receiving part, together with a multitude of different bone anchoring elements, may constitute a modular polyaxial bone anchoring system. 
     2. Description of Related Art 
     Various designs of polyaxial bone screws, where a head of a bone anchoring element is clamped from the side to lock an angular position of the bone anchoring element relative to a receiving part, are known. 
     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. 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 a receiving part while the rod is still movable in order to allow the adjustment of its position. 
     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 collet 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. 
     US 2010/0234902 A1 describes a receiving part for receiving a rod for coupling the rod to a bone anchoring element wherein the receiving part includes a receiving part body and a pressure element arranged therein. The pressure element is movable along a longitudinal axis of the receiving part body from an inserting position insert the head, to a pre-locking position to clamp the head in the receiving part by a pre-stress exerted by the pressure element, to a locking position to lock the head in the receiving part. 
     SUMMARY 
     It is an object of embodiments 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 number of parts and facilitates improved or easier handling during surgery. 
     The receiving part according to embodiments of the invention includes a pressure element that has a slit ring at its bottom end. The slit ring is integrally connected to the pressure element, which means the slit ring and the remainder of the pressure element are a monolithic piece. The slit ring can expand in a radial direction to allow for insertion of the head of the bone anchoring element. The force necessary for introducing the head into such a flexible portion of the pressure element is reduced compared to known pressure elements that have only longitudinal or coaxial slits. 
     Furthermore, a distance the pressure element needs to move in an axial direction until expansion of the slit ring allows for insertion of the head of the bone anchoring element can be reduced. This allows and more easily facilitates an in situ mounting procedure of the receiving part to a bone anchoring element that has already been inserted into a bone. 
     By means of the elasticity of the slit ring, the head of the bone anchoring element can be clamped by friction between the head and the slit ring. A friction fit between the head of the bone anchoring element and the pressure element allows for a preliminary holding of the bone anchor relative to the receiving part in a desired angular position of the bone anchoring element. This position can be changed by overcoming the friction force, as long as the bone anchoring element is not finally locked with respect to the receiving part. 
     The receiving part and the bone anchoring element can be easily disassembled using a simple tool. 
     Furthermore, the polyaxial bone anchoring device according to embodiments of the invention has only a few parts, which reduces the costs for manufacturing and facilitates easier handling. Because the slit ring is integrally formed with the pressure element, it will not get lost. Further, the pressure element can be secured in the receiving part body against falling out. 
     The polyaxial bone anchoring device that includes the receiving part according to embodiments of the invention allows for a modular system, where various bone anchoring elements and/or different receiving parts may be available and can be assembled, on demand, prior to use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become more apparent from the description of various embodiments, using the accompanying drawings. In the drawings: 
         FIG. 1  shows a perspective exploded view of a first embodiment of a bone anchoring device; 
         FIG. 2  shows a perspective view of the bone anchoring device of  FIG. 1  in an assembled state; 
         FIG. 3  shows a cross-sectional view of the bone anchoring device according to the first embodiment of  FIGS. 1 and 2 , the section taken perpendicular to an axis of an inserted rod; 
         FIG. 4  shows a perspective view of a pressure element according to an embodiment; 
         FIG. 5  is a side view of the pressure element of  FIG. 4 ; 
         FIG. 6  shows a top view of the pressure element of  FIG. 4 ; 
         FIG. 7  shows a cross-sectional view of the pressure element of  FIGS. 4 to 6 , the section taken along line A-A of  FIG. 6 ; 
         FIG. 8  shows a bottom view of the pressure element of  FIG. 4 ; 
         FIGS. 9 to 14  show steps of assembling a bone anchoring element and a receiving part of the bone anchoring device according to the first embodiment; 
         FIGS. 15 and 16  show two positions of the pressure element in the receiving part with inserted bone anchoring element according to the first embodiment; 
         FIG. 17  shows a step of disassembling the receiving part and the bone anchoring element according to the first embodiment using a tool; 
         FIG. 18  shows a perspective exploded view of a bone anchoring device according to a second embodiment; 
         FIG. 19  shows a perspective view of a pressure element of the bone anchoring device according to the second embodiment; 
         FIG. 20  shows a side view of the pressure element according to the second embodiment; 
         FIG. 21  shows a cross-sectional view of the pressure element of  FIG. 19 ; 
         FIGS. 22 and 23  show steps of assembling and locking the bone anchoring device according to the second embodiment; 
         FIG. 24  shows a perspective view of a further embodiment of a pressure element of the bone anchoring device; 
         FIG. 25  shows the pressure element of  FIG. 24  from a bottom perspective view; 
         FIG. 26  shows a side view of the pressure element of  FIG. 24 ; 
         FIG. 27  shows a top view of the pressure element of  FIG. 24 ; 
         FIG. 28  shows a bottom view of the pressure element of  FIG. 24 ; and 
         FIG. 29  shows a cross-sectional view of the pressure element of  FIGS. 24 to 27 , the section taken along line D-D shown in  FIG. 27 . 
     
    
    
     DETAILED DESCRIPTION 
     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 . The head  3  has a spherical segment shaped outer surface portion, including a greatest outer diameter E of the sphere, and a flat free end with a recess  4  for engagement with a screwing-in tool. The bone anchoring device further includes a receiving part for receiving a stabilization rod  100  and for coupling the stabilization rod  100  to the bone anchoring element  1 . The receiving part includes a receiving part body  5  and a pressure element  6  configured to be arranged in the receiving part body  5 . The pressure element  6  serves for locking the head  3  in the receiving part body  5 . Pins  8   a ,  8   b  may also be provided for holding the pressure element  6  in the receiving part body  5 . 
     Further, a closure element  7  in the form of an inner screw is provided for securing the rod  100  in the receiving part body  5 . 
     The receiving part body  5  will now be explained with reference to  FIGS. 1 to 3 . The receiving part body  5  has a top end  10 , an opposite bottom end  11 , and an axis of symmetry NI passing through the top end  10  and the bottom end  11 . A bore  12  is provided that is coaxial to the axis of symmetry M. In a first region adjacent the top end  10 , the receiving part body  5  has a U-shaped recess  13  with a bottom directed towards the bottom end  11 , and two free lateral legs  14   a ,  14   b  extending towards the top end  10 . In the region of legs  14   a ,  14   b , an internal thread  15  is provided that cooperates with the inner screw  7 . The channel formed by the U-shaped recess  13  is sized so as to receive the rod  100  therein for connecting a plurality of bone anchoring devices. In the region of the legs  14   a ,  14   b , a first portion of the bore  12  has a first inner diameter. In a region below the legs  14   a ,  14   b  the bore  12  has a widened portion  12   a  with a diameter greater than the first inner diameter of the first portion. Between the bottom end  11  and the widened portion  12   a , the bore  12  has a narrowing portion  12   b  that tapers towards the bottom end  11  with a cone angle. An opening  16  is provided at the bottom end  11 , the diameter of the opening  16  being larger than the diameter of the head  3  to allow the introduction of the head  3  from the bottom end  11  of the receiving part body  5 . The widened portion  12   a  and the narrowing portion  12   b  define an accommodation space  17  for the head  3  of the bone anchoring element  1 . 
     On each leg  14   a ,  14   b , bores  19   a ,  19   b  are respectively provided, extending through the legs  14   a ,  14   b  in a direction substantially perpendicular to the central axis NI for receiving the pins  8   a ,  8   b . The bores  19   a ,  19   b  are located approximately at a center of each leg. The pins  8   a ,  8   b  preferably have such a length that once inserted into the bores  19   a ,  19   b , the pins  8   a ,  8   b  extend a short distance into the bore  12  to provide a stop for the pressure element  6  described below. The pins  8   a ,  8   b  may be flush with the outer surface of the receiving part body  5  when inserted. 
     As can be seen in particular in  FIGS. 1 and 4 to 8 , the pressure element  6  has a top end  61  and an opposite bottom end  62 . Adjacent the top end  61  there is a substantially cylindrical portion  63 , which has an outer diameter that is slightly smaller than the inner diameter of the bore  12 , so that the pressure element  6  is moveable in the bore  12 . There is a U-shaped recess  64  that is open to the top end  61  for receiving the rod  100  therein when the pressure element  6  is arranged in the receiving part body  5 , such that the U-shaped recess  64  can be aligned with the U-shaped recess  13  of the receiving part body  5 . Adjacent the cylindrical portion  63 , a recessed portion  65  is provided that continues into a head receiving portion  66 . 
     The head receiving portion  66  has a hollow interior chamber  67  that is substantially spherical segment shaped and configured to accommodate the head  3  therein. At the bottom end  62 , the head receiving portion  66  has an opening to allow the introduction of the head  3 . In a region adjacent the recessed portion  65 , the head receiving portion  66  has a spherical segment shaped outer surface portion that continues into a tapered outer surface portion in a direction towards the second end  62 . The taper corresponds substantially to a taper of the narrowing portion  12   b  of the receiving part body  5 . 
     At a distance from the bottom end  62 , a circumferentially extending slit  70  is provided. The slit  70  extends circumferentially around the central axis M of the receiving part body  5  along a plane substantially perpendicular to the central axis M when the pressure element  6  is arranged in the receiving part body  5 . The slit  70  extends around more than 180°, preferably more than 270°, and further preferably more than 340°, around the central axis M. Hence, by means of the slit  70 , a ring-shaped portion  71  at the bottom end  62  is provided that is integrally connected to the rest of the pressure element  6  by a wall portion forming a connecting strip  72 . 
     The connecting strip  72  has such a length in a circumferential direction that it provides a stable connection of the ring-shaped portion  1  to the rest of the pressure element  6 . At one end of the circumferentially extending slit  70 , there is a slit  73  that extends from the second end  62  fully through the ring-shaped portion  71  into the slit  70 . By means of this, the ring-shaped portion  71  is cut through or split in a circumferential direction, and forms a slit ring that can be xpanded and compressed in a radial direction. A width of the vertical slit  73  is preferably smaller than the width of the circumferential slit  70 . This can be seen in particular in  FIGS. 4 and 7 . The outer surface of the slit ring  71  is tapered towards the bottom end  62  of the pressure element  6 . The connecting strip  72  and the vertical slit  73  are arranged at an angle (i.e., are offset) relative to the U-shaped recess  64 . A position and size of the slit ring  71  is such that when the head  3  of the bone anchoring element  1  is inserted from the open bottom end  62  of the pressure element  6 , the slit ring  71  expands so that the width of the vertical slit  73  becomes larger, and when the head  3  has been fully inserted into the hollow interior chamber  67 , the slit ring  71  encompasses the head  3  at and/or below the greatest diameter E of the head  3  in a direction towards the shaft  2 . 
     A maximum outer diameter of the head receiving portion  66  is slightly smaller than the inner diameter of the first portion of the bore  12  and is also smaller than the diameter of the bore in the widened portion  12   a . Hence, the slit ring  71  can expand in the widened portion  12   a  of the receiving part body  5 . 
     At lateral sides of the U-shaped recess, the pressure element  6  has two elongate recesses  69   a ,  69   b  that are elongate in a direction from the top end  61  to the bottom end  62 . The recesses  69   a ,  69   b  are provided in the outer wall of the pressure element  6 . These elongate recesses  69   a ,  69   b  serve for engagement by the pins  8   a ,  8   b , respectively, when the pressure element  6  is inserted in the receiving part body  5 , and provide for a stop when the pins  8   a ,  8   b  abut against one of the ends of the recesses  69   a ,  69   b  in an axial direction. 
     Finally, the pressure element  6  has a coaxial through hole  75  allowing access to the recess  4  of the head  3  of the bone anchoring element  1  with a tool (not shown). The through hole  75  is preferably provided with an internal thread in at least a portion thereof, as shown in  FIGS. 3 and 7 . The internal thread serves for engagement with a tool to fix the pressure element  6  when head  3  of the bone anchoring element  1  is to be removed from the head receiving portion  66 , to be described further below. 
     The pressure element  6  may be arranged in the receiving part body  5  by introduction from the top end  10  of the receiving part body  5 , and by aligning the U-shaped recesses  64 ,  13  of the pressure element  6  and the receiving part body  5 , respectively. The pins  8   a ,  8   b  may be inserted thereafter to extend into the elongate recesses  69   a ,  69   b  of the pressure element  6 , to hold the pressure element  6  within the receiving part body  5 , thereby preventing escaping of the pressure element  6  through the first end  10 . By means of this, the pressure element  6  and the receiving part body  5  can be pre-assembled and can be connected to a bone anchoring element  1 . 
     All the parts described above may be made of a bio-compatible material, such as a bio-compatible metal like stainless steel or titanium, or of a bio-compatible metal alloy, such as Nitinol, or of a bio-compatible plastic material, for example, Polyetheretherketon (PEEK). The parts may all be made of the same material or of different materials. 
     Steps of assembling the bone anchoring device will now be explained with reference to  FIGS. 9 to 14 . As shown in  FIG. 9 , the pressure element  6  and the receiving part body  5  can be pre-assembled as described above. The slit ring  71  may abut against the upper part of the narrowing portion  12   b . The bone anchoring element  1  is introduced from the bottom end  11  of the receiving part body  5  (arrow A). Next, as shown in  FIG. 10 , the head  3  of the bone anchoring element is introduced from the bottom end  11  and touches the slit ring  71  of the pressure element  6 , thereby shifting the pressure element  6  in a direction of the arrow B, until a lower end of the elongate recesses  69   a ,  69   b  abut against the pins  8   a ,  8   b , respectively, as shown in  FIG. 11 . In this position, the slit ring  71  is situated in the accommodation space provided by the widened portion  12   a  of the receiving part body  5 . 
     Further introduction of the head  3  into the head receiving portion  66  expands the slit ring  71  within the widened portion  12   a  of the receiving part body  5 . The head  3  can then be inserted. Because the slit ring  71  does not expand at the connecting strip  72 , the introduction of the head  3  may not be precisely coaxial with to the axis of symmetry M, but instead may be slightly out of or misaligned with the axis of symmetry M. By the further introduction of the head  3 , the slit ring  71  may be expanded to a maximum extent, as shown in  FIG. 13 . Still further introduction of the head  3  allows the head  3  to enter the upper space or portion of the hollow interior chamber  67 , until the head  3  abuts against the inner wall of the chamber  67 . Here, the slit ring  71  can elastically contract around the head  3 , as shown in  FIG. 14 . 
     When the head  3  abuts against the inner wall of the head receiving portion  66  in the upper region, the slit ring  71  encompasses a portion of the head  3  below the equator E as can be seen in  FIG. 15 . In this condition, the slit ring  71  frictionally clamps the head  3 , so that the bone anchoring element  1  can be pivoted to a desired angular position relative to the receiving part body  5 , and can be held there by the friction fit between the pressure element  6  and the head  3 . 
     Finally, moving down the pressure element  6  presses the slit ring  71  into the narrowing portion  12   b , so that the slit ring  71  locks the head  3 , as shown in  FIG. 16 . 
     In clinical use, first the receiving part may be assembled with the bone anchoring element  1  as described above, then the bone anchoring element  1  may be inserted into a bone. Usually at least two bone anchoring devices are inserted into the bone and the receiving parts are aligned. Then, the rod  100  is inserted into the respective receiving parts. Finally, the inner screws  7  are screwed between the legs  14   a ,  14   b  of the devices, and the rod  100  presses down on the respective pressure elements so that the respective heads  3  are locked therein. 
     Alternatively, the bone anchoring element  1  can first be inserted into a bone, and the receiving part body  5  with the pressure element  6  can be mounted thereafter onto the head  3  of the bone anchoring element  1 . Because of the slit ring  71 , the force necessary to insert the head  3  into the head receiving portion  66  of the pressure element  6  can be reduced, and a distance needed to move the pressure element  6  within the bore  12  of the receiving part body  5  can also be reduced. 
     In the case that the receiving part body  5  and the head  3  are to be disassembled, for example, when it may be desirable for the head  3  to be removed from the receiving part body  5 , a tool  200  may be used as shown in  FIG. 17 . The tool  200  may have a first portion to be gripped by a person (not shown) and a threaded end portion  201  that engages the threaded through hole  75  of the pressure element  6 . By means of this, the pressure element  6  can be pulled in the direction of the top end  10  of the receiving part body  5  until the pins  8   a ,  8   b  abut against the lower end of the elongate recesses  69   a ,  69   b . In this position, the slit ring  71  is located within the accommodation space in the widened portion  12   a  of the receiving part body  5 . Then, the head  3  can be removed through the bottom end  11  of the receiving part body  5 . 
     A second embodiment of the bone anchoring device is shown in  FIGS. 18 to 23 . With reference to  FIG. 18 , the bone anchoring device according to the second embodiment differs from the bone anchoring device according to the first embodiment in that the receiving part body  5 ′ has only one lateral bore  690  provided in the leg  14   b , at a distance below the bottom of the U-shaped recess  13 . Hence, as shown in  FIGS. 22 and 23 , a single pin  80  extends through the bore  690  into the widened portion  12   a  of the bore  12 . All other parts of the receiving part body  5 ′ are identical or similar to the first embodiment, and the descriptions thereof will not be repeated. 
     The pressure element  6 ′ has at an end of the cylindrical portion  63  a first substantially semi-circular notch  630   a  that is open towards the bottom end  62  and is configured to engage with the pin  80  as shown in  FIG. 23 . Hence, the notch  630   a  may be provided at a 90° offset to the U-shaped recess  64 . 
     The pressure element  6 ′ further has a second notch  670   a  that is provided in an axial direction below the first notch  630   a  in an upper part of the head receiving portion  66 . The width of the second notch  670   a  is slightly larger than the diameter of the pin  80 , so that the pin  80  can engage the second notch  670   a . As can be seen in particular in  FIG. 21 , the second notch  670   a  has such a depth that it is recessed with respect to an outer surface of the slit ring  71 . The notches  630   a ,  670   a  may be arranged on an opposite side of the connection strip  72 , as best seen in  FIGS. 20 and 21 . 
     In use, the pressure element  6 ′ and the receiving part body  5 ′ can be pre-assembled. The head  3  is introduced like in the first embodiment from the bottom end  11 . When the head  3  is introduced and abuts against the bottom end of the pressure element  6 ′, the slit ring  71  abuts against the pin  80  as shown in  FIG. 22 . Therefore, the pin  80  provides a stop for the slit ring  71  and prevents bending of the slit ring  71  in a direction towards the top end  10  of the receiving part body  5 . Simultaneously, the pin  80  acts as a securing device against escaping of the pressure element  6 ′ and as a positioning device for a correct alignment of the pressure element  6 ′ with respect to the receiving part body  5 ′. 
     A further embodiment of the pressure element  6 ″ will be described with reference to  FIGS. 24 to 29 . The pressure element  6 ″ has, as in the previous embodiments, a top end  61  and a bottom end  62 . Adjacent the top end  61 , a substantially cylindrical portion  63 ′ is provided that extends to a distance from the bottom end  62 , and has, adjacent the top end  61 , the U-shaped recess  64 . A central bore  75  with a threaded portion is present as in the previous embodiments. 
     At a distance from the bottom end  62 , two slits  700 ,  701  are provided that extend in a circumferential direction around a central axis M along a plane transverse to the central axis M. At the end of each of the horizontal slits  700 ,  701 , vertical slits  703 ,  704  are respectively provided, where the vertical slits  703 ,  704  are located around a circumference of the pressure element  6 ″ at opposite places. The vertical slits  703 ,  704  extend from the bottom end  62  frilly into the horizontal slits  700 ,  701 , respectively. Hence, two ring portions  710   a ,  710   b  that are opposite to each form the flexible section of the pressure element  6 ″. The ring portions  710   a ,  710   b  are connected with the cylindrical portion  63  via connecting strips  702   a ,  702   b  located opposite to each other. By means of the two ring portions  710   a ,  710   b , a slit ring  71 ′ is formed. The slit ring  71 ′ can expand when the vertical slits  703 ,  704  are enlarged, and can elastically return to a non-expanded condition. 
     Because the pressure element  6 ″ is almost not flexible in the region of the connecting strips  702   a ,  702   b , a recess  705  may be provided on one or both of the inner wall portions of the connecting strips  702   a ,  702   b , such that an inner diameter of the pressure element  6 ″ in the region of the connecting strips  702   a ,  702   b  is the same or is slightly larger than the largest outer diameter E of the head  3 . 
     Further, the pressure element  6 ″ has an interior chamber  67  with a preferably spherical segment shaped section to accommodate the head  3 . 
     The pressure element  63 ′ can be combined with a receiving part body  5  or  5 ′ according to the first or second embodiments. In addition, the various other features of the embodiments can be combined among each other. The pressure element can be further modified, for example, such that more than two horizontal and/or more than two vertical slits may be provided. 
     Further modifications of the embodiments shown are also possible. For example, the circumferential slits  70 ,  700 ,  701  need not be perpendicular or level relative to the central axis M, but can instead be inclined. The vertical slits  73 ,  703 ,  704  can also be inclined or can have a particular contour, as long as the slits extends fully through the respective rings. The head of the bone anchoring element can also have various other shapes, for example, a cylindrical shape where a monoaxial bone screw is provided, allowing for rotation of the bone anchoring element with respect to the receiving part around a single axis. The head can also be, for example, conically shaped. In such embodiments, the hollow interior chamber would then be adapted to correspond to the shape of the head. In a further modification, the receiving part body can have an inclined bottom end to allow for greater angulation of the bone anchoring element in one direction. 
     In another modification, the narrowing portion of the receiving part body and/or the corresponding outer surface portion of the slit ring may not be tapered, but may instead be spherically curved. 
     For the bone anchoring element, various different kinds of bone anchoring elements can be used, such as screws of different lengths, of different diameters, of different thread forms, nails with barbs or without barbs, cannulated screws or nails, or hooks among others. 
     In addition, for the closure element, various known kinds of closure elements can also be used, for example, single part closure elements or double part closure elements, among others. 
     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.