Patent Publication Number: US-2022226025-A1

Title: Anchoring member for a polyaxial bone anchoring device and polyaxial bone anchoring device with such an anchoring member

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 16/686,714, filed Nov. 18, 2019, which claims priority to and the benefit of U.S. Provisional Patent Application No. 62/769,854, filed Nov. 20, 2018, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 18 207 354.4, filed Nov. 20, 2018, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     The application relates to an anchoring member for a polyaxial bone anchoring device and to a polyaxial bone anchoring device including the anchoring member. In particular, the application is related to devices for the treatment of spinal disorders or for use in trauma surgery. 
     Description of Related Art 
     Polyaxial bone anchoring devices for use in the treatment of spinal disorders are well known in the art. For example, U.S. Pat. No. 5,443,467 describes a polyaxial bone anchoring device with an anchoring element comprising a shaft for anchoring in bone and a spherical segment-shaped head, wherein the anchoring element is pivotably held in a receiving part that is configured to couple the anchoring element to a spinal rod. 
     US 2017/0245894 A1 describes a polyaxial bone anchoring device comprising an anchor body and a fastener that includes a head, a threaded shaft that extends out with respect to the head in a distal direction, and a neck between the head and the threaded shaft. The head includes an outer surface, at least a portion of which is convex and defines a portion of a sphere that defines a first diameter. The neck defines a second diameter, and the fastener defines a ratio of the first diameter to the second diameter in a range between about 2 to 1 and about 3 to 1. 
     SUMMARY 
     A portion of an anchoring element or anchoring member located between a head and a shaft may be a potential area where the anchoring member may fail under load. 
     It is an object of the invention to provide an anchoring member and a polyaxial bone anchoring device including the anchoring member that is improved with respect to a possible failure under load. 
     According to an aspect of the disclosure, an anchoring member for a polyaxial bone anchoring device includes a bone anchoring section having a first end, a second end, and a bone engagement structure on at least a portion thereof, a head having an outer surface portion that defines a portion of a sphere, a central axis extending through respective centers of the head and the first and second ends of the bone anchoring section, and a neck between the first end of the bone anchoring section and the head, the neck including a first portion that is closer to the head than to the bone anchoring section, wherein in at least one plane including the central axis, the first portion of the neck is concave with a first curvature, the neck further including a second portion between the first portion and the bone anchoring section, wherein in the at least one plane, the second portion is concave with a second curvature that is smaller or less curved than the first curvature. On at least one side of or in at least one radial direction relative to the central axis, the first portion joins the second portion in an axial direction. 
     By means of this design, a transition between the bone anchoring section or a cylindrical portion of the neck and the head is shifted toward a center of the head in the axial direction. Thereby the distribution of loads acting onto the head and/or the neck is improved. As a result, the strength, in particular the fatigue strength, of the anchoring member against breaking, in particular in the region of the neck, is increased. 
     According to another aspect of the disclosure, a polyaxial bone anchoring device includes the anchoring member and a receiving part including a seat configured to receive the head of the anchoring member, such that the bone anchoring section can assume a plurality of angular positions relative to the receiving part. 
     A maximum pivot angle that the anchoring member can form relative to the receiving part may also be increased compared to an anchoring member that has a substantially cylindrical neck. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages will become apparent from the description of 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 of the invention. 
         FIG. 2  shows a perspective view of the polyaxial bone anchoring device of  FIG. 1  in an assembled state. 
         FIG. 3  shows a side view of an anchoring member of the polyaxial bone anchoring device according to the first embodiment. 
         FIG. 4  shows an enlarged view of a portion of the anchoring member of  FIG. 3 . 
         FIG. 5  shows a cross-sectional view of the bone anchoring device according to  FIGS. 1 and 2 , with the cross-section taken in a plane extending through a center of a head of the anchoring member and perpendicular to a longitudinal axis of a rod received by the bone anchoring device. 
         FIG. 6  shows an enlarged view of a portion of  FIG. 5 . 
         FIG. 7  shows a further enlarged view of the bone anchoring device of  FIG. 5 . 
         FIG. 8  shows, in comparison to  FIG. 7 , an enlarged view of a portion of a bone anchoring device including an example of an anchoring member that has a substantially cylindrical neck. 
         FIG. 9  shows a side view of a portion of an anchoring member according to a second embodiment of the invention. 
         FIG. 10  shows a cross-sectional view of the anchoring member of  FIG. 9 , the cross-section taken in a plane including a central axis of the anchoring member. 
         FIG. 11  shows a cross-sectional view of a portion of a bone anchoring device according to the second embodiment of the anchoring member shown in  FIGS. 9 and 10 , with the cross-section taken in a plane extending through the center of the head of the anchoring member and perpendicular to the longitudinal axis of a rod received by the bone anchoring device. 
         FIG. 12  shows a cross-sectional view of a still further embodiment of a polyaxial bone anchoring device, in the form of a bone plate. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , a polyaxial bone anchoring device according to a first embodiment includes an anchoring member  1  including a shank  2  for anchoring in a bone or a vertebra, and a head  3 . Further, a receiving part  4  for pivotably receiving the head  3  of the anchoring member  1  is provided that is configured to couple the anchoring member  1  to a stabilization rod  100 . Moreover, a pressure member  5  may be provided to exert pressure onto the head  3  in the receiving part  4 , to lock the head  3  in a specific angular position with respect to the receiving part  4 . Furthermore, a locking member  6 , for example in the form of a set screw, may be part of the bone anchoring device for securing and fixing rod  100  in the receiving part  4 . 
     As can be seen additionally in  FIG. 5 , the receiving part  4  has a first end or top end  4   a  and an opposite second end or bottom end  4   b,  an axis of symmetry M and a coaxial bore  8  extending from the first end  4   a  in the direction of the second end  4   b . Adjacent to the first end  4   a,  a substantially U-shaped recess  9  is provided that serves as a channel for receiving the rod  100 . By means of the recess  9 , two free legs are formed which are provided with an engagement structure, such as an internal thread  10 , for cooperating with the locking member  6 . 
     Adjacent to or close to the second end, the passage provided by the coaxial bore  8  narrows towards the second end  4   b  and forms an opening  11 , whose inner diameter is of a size such that the anchoring member  1  can be guided through with the shank  2  until the head  3  is seated in a seat  12  adjacent to the opening  11 . The seat  12  for the head  3  may be designed as a spherical segment-shaped portion that matches an outer surface portion of the head  3 . 
     Referring now further to  FIGS. 3 and 4 , the anchoring member  1  has a central axis C extending through a center of the head  3  and being coaxial to a longitudinal axis of the shank  2 . Preferably, the anchoring member  1  is a monolithic piece. The shank  2  has a first end  21  facing the head  3  and an opposite second end  22  that may be shaped as a tip. On at least a portion of the outer surface of the shank  2 , a bone engagement structure  20 , for example a bone thread, may be provided. Hence, the shank  2  forms a bone anchoring section of the anchoring member  1 . The head  3  has a spherically-shaped outer surface portion  31  that defines a sphere S (see  FIG. 4 ) with a diameter D 1 . In greater detail, in the embodiment, the head  3  has an axial length, such that the diameter D 1  of the sphere S forms the largest width of the head  3 . A section of the sphere S in a plane P that includes the central axis C forms part of a circle (see for example  FIG. 6 ). At the side opposite to the second end  22  of the shank, the head  3  has a free end surface  32  that may include a recess  3   a  for engagement with a driver to drive the anchoring member  1  into bone. The free end surface  32  may be flat. 
     Between the head  3  and the first end  21  of the shank  2 , a neck  23  is formed. The neck  23  includes a first portion  24  that is closer to the head  3  and a second portion  25  that is closer to the bone anchoring section. The first portion  24  has an outer surface that is concave with a first curvature. In the plane P, the surface of the first portion  24  of the neck  23  defines a circular segment with a first radius of curvature R 1 . Preferably, the neck  23  and the outer surface portion  31  of the head  3  are rotationally symmetrical around the central axis C. Hence, the first radius of curvature R 1  of the first portion  24  is the same in every radial direction, and therefore in every plane that includes the central axis C. At the position where the first portion  24  of the neck  23  meets the head  3 , an edge  33  is formed. By the concave shape of the first portion  24 , the width of the anchoring member decreases from the head  3  towards the shank  2 . 
     The second portion  25  of the neck  23  is also concave, with a second curvature that is smaller than the first curvature. In other words, the surface of the second portion  25  is less strongly curved than that of the first portion  24 . In the embodiment, the surface of the second portion  25  also defines a circular segment in the plane P, with a second radius of curvature R 2  that is greater than R 1 , as depicted in  FIG. 4 . Due to the rotational symmetry, the second radius of curvature R 2  of the second portion  25  of the neck  23  is the same in every radial direction, and therefore in every plane that includes the central axis C. As can be seen in the figures, the first portion  24  continuously merges into the second portion  25 . 
     A diameter D 2  of the neck  23  in a plane perpendicular to the central axis C is, at any axial position of the first portion  24  or the second portion  25 , smaller than the first diameter D 1 . More specifically, in the second portion  25 , the diameter D 2  is such that a ratio D 1 :D 2  is about 7:4. Preferably, the ratio D 1 :D 2  may be between about 1 and about 2. An axial length of the second portion  25  is greater than an axial length of the first portion  24 . More specifically, the axial length of the second portion  25  may be two times as long or more than the axial length of the first portion  24 . 
     Between the second portion  25  and the shank  2 , a third, cylindrical portion  26  may be provided. The second portion  25  continuously merges into the cylindrical portion  26 . An axial length of the third, cylindrical portion  26  may be smaller than an axial length of the second portion  25 . The outer diameter D 3  of the third cylindrical portion  26  may be the same as or slightly greater than a core diameter D c  of the shank  2 . At the transition between the second portion  25  and the cylindrical portion  26 , D 3  may be the same as D 2 . 
     The pressure member  5  will now be explained, with reference to  FIGS. 1 and 5 . The pressure member  5  may be formed as a monolithic piece, may be of substantially cylindrical construction, and has an outer diameter that allows it to move in the axial direction within the bore  8  of the receiving part  4 . Moreover, the pressure member  5  has a substantially spherical recess  51  at its side that faces the head  3  when the pressure member  5  and the head  3  are assembled within the receiving part  4 . The substantially spherical recess  51  is adapted to the size of the head  3 . At the opposite side facing away from the head  3 , a substantially cylindrically-shaped recess  52  is formed that is configured to receive the rod  100  therein. In this embodiment, the surface of the rod extends above the sidewalls of the recess  52  when the rod  100  is inserted into the recess  52 . A coaxial bore  53  formed in the pressure member  5  permits access with a tool such as a driver to the recess  3   a  in the head  3  when the head  3  of the anchoring member  1  is assembled in the receiving part  4 . 
     The locking member  6  may be formed as a set screw that can be screwed between the legs of the receiving part  4 . The locking member  6  is configured to contact the rod  100  when the rod  100  is inserted into the receiving part  4 . 
     The anchoring member  1 , the receiving part  4 , the pressure member  5 , and the locking member  6 , as well as the rod  100 , may each be made of one or more bio-compatible materials, for example of titanium or stainless steel, of a bio-compatible alloy, such as NiTi-alloys, for example Nitinol, of magnesium or magnesium alloys, or from a bio-compatible plastic material, such as, for example polyether ether ketone (PEEK) or poly-L-lactide acid (PLLA). In addition, the parts can be made of the same as or of different materials from another. 
     Turning now to  FIGS. 5 to 7 , the polyaxial bone anchoring device may be preassembled with the anchoring member  1  being inserted from the first end  4   a  into the receiving part  4  until its head  3  rests in the seat  12 . The pressure member  5  may be rotationally fixed within the receiving part  4 , for example, by crimping using crimp bores  54  at the outer surface of the pressure member  5 . 
     In use, at least two anchoring members  1  with receiving parts  4  are inserted into the bone or in adjacent vertebrae and connected through the rod  100 . For inserting the rod  100  into a receiving part  4 , the receiving part  4  may be pivoted relative to the anchoring member  1  to facilitate insertion of the rod  100 . As shown in  FIGS. 5 and 6 , a maximum pivot angle α is defined by the abutment of the neck  23  against an edge  11   a  of the opening  11  of the receiving part  4 . The sizes of the head  3  and the neck  23  relative to the seat  12  in the receiving part  4  are such that the edge  11   a  abuts against the second portion  25  of the neck that has the smaller or less curved curvature. In the position of the maximum pivot angle α, the first portion  24  of the neck  23  is preferably located within the seat  12  of the receiving part  4  at the side to which the anchoring member  1  pivots. 
     Referring to  FIG. 8 , a comparative example of an anchoring member  101  is shown in which the neck  123  is devoid of any portions that extend into the sphere S defined by the spherical segment-shaped head  103 . Comparing the anchoring member of the embodiment according to  FIG. 7  with the example of the anchoring member of  FIG. 8 , the maximum pivot angle α is increased when the neck  23  extends into the sphere S, as shown in  FIG. 7 . Moreover, the strength of the anchoring member  1  of the embodiment shown in  FIG. 7  against loads is increased. Ultimately, forces can be distributed in an improved manner between the head  3  and the neck  23 . 
     Referring to  FIGS. 9 to 11 , a second embodiment of the anchoring member will be described. The anchoring member  1 ′ differs from the anchoring member  1  of the first embodiment by the shape and size of the head. All other portions are identical or similar, and the descriptions thereof will not be repeated. Like portions are provided with the same reference numerals as in the first embodiment. The head  3 ′ of the anchoring member  1 ′ has a diameter D 1 ′ that, when compared relative to the diameter D 2  of the neck  23 , is relatively larger than the diameter D 1  of the anchoring member  1  of the first embodiment. More specifically, the ratio of D 1 ′:D 2  may be between about 1 and about 3. At the side opposite to the free end surface  32 , the head includes an undercut portion  34  that extends between the edge  33  and the first portion  24  of the neck  23 . The undercut portion  34  is recessed from a virtual plane that extends perpendicular to the central axis C and through the outermost point of the edge  33 , in a direction towards the free end surface  32 . Preferably, the undercut portion  34  is curved in the same manner as the first portion  24  of the neck  23 . In greater detail, in the embodiment, the surface of the undercut portion  34  defines part of the circular segment in the plane P that continues from the second portion  24  to the edge  33 . Hence, the radius of curvature R 1  of the undercut portion  34  and that of the first portion  24  of the neck  23  may be the same. However, the undercut portion  34  may also have another shape, for example a non-circular curvature. 
     By means of this design, the strength of the anchoring member can be further increased. As illustrated in  FIG. 11 , due to the enlarged diameter of the head  3 ′, the maximum pivot angle α can be further increased. 
     Referring to  FIG. 12 , a second embodiment of a polyaxial bone anchoring device is shown that includes a bone plate  200  as a receiving part for the anchoring member  1 . The bone plate  200  has a first or bone-contacting surface  201  and an opposite second surface  202 . At least one, preferably more than one through-hole  203  extends from the second surface  202  through the plate to the first surface  201 . In the wall that defines the through-hole  203 , a seat  204  is formed for the head  3  of the anchoring member  1 . The seat  204  has a shape corresponding to the outer spherical-surface portion of the head  3  so that, as long the bone plate and the anchoring member  1  are not implanted into the body, the anchoring member  1  can pivot within the seat  204  with a maximum pivot angle α to at least one side. The anchoring member is, for example, the anchoring member  1  of the first embodiment or may be the anchoring member  1 ′ of the second embodiment. In use, the anchoring member can be inserted into the bone at a specific angle that is delimited by the maximum pivot angle of the polyaxial bone anchoring device. The strength of the anchoring member under load is improved. If necessary, a locking cap (not shown) may further be provided in the hole  203  to prevent backing-out of the anchoring member. 
     The polyaxial bone anchoring device is not limited to the embodiments shown. The anchoring member can be used with any receiving part that provides a seat for the head. Such receiving parts can be of the top-loading type, where the anchoring member is inserted from the top of the receiving part, or of the bottom-loading type, where the anchoring member is inserted with the head through a lower opening. Also, any bone plate or other surgical implant that is combined with an anchoring member of the above described embodiments can form a polyaxial bone anchoring device according to embodiments of the invention. The seat may have another shape, and/or may be provided within an insert member that is within the receiving part. Many different polyaxial bone anchoring devices may be conceivable. 
     Modifications of the anchoring member are also conceivable. For example, the bone anchoring section may have any bone anchoring structure or shape that is suitable for anchoring in bone or in a vertebra. The head may also be non-rotationally symmetrical. For example, the head may have two opposite flat surface portions, so that the spherical surface portion allows the anchoring member to pivot in the receiving part only in a single plane. The symmetry of the neck may correspond to the symmetry of the head. 
     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.