Abstract:
A bone anchor includes an anchor member including a first material, and having a tubular section extending along a longitudinal axis, a region configured to expand radially outwards from the longitudinal axis, and a bone engagement structure projecting from at least a portion of the expandable region of the tubular section, and a core member including a second material, and configured for insertion into and connection to the anchor member. When the core member is in the anchor member and is actuated relative to the anchor member, the tubular section is adjustable between a first position where the expandable region of the tubular member has a first length and a first maximum diameter, to a second position where the expandable region of the tubular member has a length shorter than the first length and a maximum diameter greater than the first maximum diameter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/661,234, filed Jun. 18, 2012, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 12 172 413.2, filed Jun. 18, 2012, 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 bone anchor including an anchor member with an expandable tubular section and a core member insertable into the anchor member and connectable thereto. The core member is configured to cooperate with the anchor member, such that by actuating the core member with respect to the anchor member, the tubular section is expandable. The anchor member is made from a material that is more flexible than a material of the core member. In particular, the bone anchor can be used in a bone fixation or stabilization arrangement, and is especially suitable for anchoring fixation or stabilization devices in osteoporotic bones. 
         [0004]    2. Description of Related Art 
         [0005]    US 2009/0131992 A1 describes an attachment device with a radially expandable section. The attachment device can have helical threads to facilitate screwing the attachment device into a bone. The attachment device can be positioned to radially expand the expandable section in cancellous bone substantially surrounded by cortical bone. In particular, the expandable attachment device can have an expandable thread on the expandable section and an unexpandable thread on the unexpandable section. It can be radially expanded by applying a distally-directed force to the distal end. 
         [0006]    US 2011/0319946 A1 describes a device for deploying a bone implantation and stabilization assembly into bone tissue, e.g. a spinal structure, vertebrae, cancellous bone, cortical bone etc., in order to stabilize bones and/or bone segments. The bone stabilization device includes an anchor region in the form of an expanding tube or the like including a plurality of anchoring elements or arms that are deployable in a direction away from the axis of the assembly. An actuator is provided for deployment of the anchoring elements. 
       SUMMARY 
       [0007]    As such, there is a need for an improved bone anchor that has a simple design and is easy to manufacture. 
         [0008]    It is an object of embodiments of the invention to provide an improved bone anchor that is particularly useful for application in weak bones. 
         [0009]    A bone anchor according to embodiments of the invention includes an expandable anchor member and a core member. The anchor member is made of a material that is more flexible than a material of the core member. Hence, the ability of the anchor member to expand in a radial direction results from the material of the anchor member that exhibits a greater flexibility than the material of the core member. This allows for a more simple construction of the anchor member. Preferably, in some embodiments, the material of the anchor member is a polymer material, and the material of the core member is a metallic material. 
         [0010]    The expansion of the anchor member can be effected after the anchor member has been inserted into a bone, for example, in situ during surgery. 
         [0011]    A separate head may be provided that can be made, preferably, in some embodiments from metallic material. With such a head, the bone anchor can be used with known polyaxial receiving parts and other stabilization devices. Because the head may be a separate part made from a material that is commonly used for stabilization devices, the head can in some embodiments be effectively connected and fixed to such stabilization devices. 
         [0012]    The bone anchor according to embodiments of the invention provides for a safe, long term anchoring, in particular in osteoporotic or weak bones. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Further features and advantages will become apparent from the description of various embodiments by means of the accompanying drawings. In the drawings: 
           [0014]      FIG. 1  shows a perspective exploded view of a bone anchor according to a first embodiment; 
           [0015]      FIG. 2  shows a perspective exploded view of the bone anchor of  FIG. 1  with a core member mounted to a head; 
           [0016]      FIG. 3  shows the bone anchor of  FIGS. 1 and 2  in an assembled state in a first, non-expanded configuration; 
           [0017]      FIG. 4  shows the bone anchor of  FIGS. 1 and 2  in an assembled state in a second, expanded configuration; 
           [0018]      FIG. 5  shows a cross-sectional view in a plane containing a central longitudinal axis of an anchor member of the bone anchor according to the first embodiment; 
           [0019]      FIG. 6  shows a side view of the anchor member of  FIG. 5 ; 
           [0020]      FIG. 7  shows a top view onto a first end of the anchor member of  FIG. 5 ; 
           [0021]      FIG. 8  shows a top view onto a first end of the core member of the bone anchor according to the first embodiment; 
           [0022]      FIG. 9  shows a side view of the core member of  FIG. 8 ; 
           [0023]      FIG. 10  shows a perspective view from a top onto the head of the bone anchor according to the first embodiment; 
           [0024]      FIG. 11  shows a perspective view from a bottom of the head of the bone anchor of  FIG. 10 ; 
           [0025]      FIG. 12  shows a top view of the head of  FIG. 10 ; 
           [0026]      FIG. 13  shows a cross-sectional view of the head of  FIGS. 10 to 12 , along line A-A in  FIG. 12 ; 
           [0027]      FIG. 14  shows a cross-sectional view of the anchor member of the bone anchor according to the first embodiment in a first step of anchoring the bone anchor in a bone; 
           [0028]      FIG. 15  shows a cross-sectional view of mounting the core member with the head to the anchor member inserted into the bone; 
           [0029]      FIG. 16  shows a cross-sectional view of a further step of connecting the core member to the anchor member; 
           [0030]      FIG. 17  shows a cross-sectional view of rotating the core member to expand the anchor member; 
           [0031]      FIGS. 18 to 21  show steps of anchoring the bone anchor in a pedicle of a vertebra according to the first embodiment; 
           [0032]      FIG. 22  shows a cross-sectional view of a polyaxial bone anchoring device with the bone anchor according to the first embodiment; 
           [0033]      FIG. 23  shows a cross-sectional view of a bone plate with the bone anchor according to the first embodiment; 
           [0034]      FIG. 24  shows a perspective view of a bone anchor in an assembled state in a first non-expanded configuration according to a second embodiment; 
           [0035]      FIG. 25  shows a perspective view of the bone anchor in an expanded configuration according to the second embodiment; 
           [0036]      FIG. 26  shows a perspective view of the bone anchor of  FIG. 25  in the second, expanded configuration, where a traction member has been broken off; 
           [0037]      FIG. 27  shows a perspective view of a core member of the bone anchor according to the second embodiment; 
           [0038]      FIG. 28  shows a top view from the first end onto the core member of  FIG. 27 ; 
           [0039]      FIG. 29  shows a side view of the core member of  FIG. 27 ; 
           [0040]      FIG. 30  shows a cross-sectional view of the core member of  FIGS. 27 to 29 , along line B-B in  FIG. 29 ; 
           [0041]      FIG. 31  shows a perspective view from a bottom of a head of the bone anchor according to the second embodiment; 
           [0042]      FIG. 32  shows a top view of the head of  FIG. 31 ; 
           [0043]      FIG. 33  shows a cross-sectional view of the head of  FIGS. 31 and 32 , along line C-C in  FIG. 32 ; 
           [0044]      FIG. 34  shows a cross-sectional view of a first step of using the bone anchor according to the second embodiment; 
           [0045]      FIG. 35   a  shows a cross-sectional view of the bone anchor of  FIG. 34  in a second, expanded configuration; 
           [0046]      FIG. 35   b  shows an enlarged portion of a detail of  FIG. 35   a;    
           [0047]      FIG. 36  shows a cross-sectional view of the bone anchor according to the second embodiment, inserted into the bone and expanded, where the traction member has been broken off; and 
           [0048]      FIGS. 37 to 39  show enlarged cross-sectional partial views of further modifications of a connection between an anchor member and a core member, for example, as seen in the second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    As shown in  FIGS. 1 to 4 , a bone anchor according to a first embodiment includes an anchor member  1  in the form of a screw member, a core member  2 , and a head  3 . The core member  2  can be inserted into the head  3 , and an assembly including the core member  2  and the head  3  can be inserted into the anchor member  1  to form the bone anchor. By rotating the core member  2  relative to the anchor member  1 , the anchor member  1  can be expanded radially. 
         [0050]    As further shown in  FIGS. 5 to 7 , the anchor member  1  has a first end  11 , an opposite second end  12 , and a longitudinal axis L extending through the first end  11  and the second end  12 . The second end  12  of the anchor member is formed as a tip. Adjacent to the first end  11 , there is a tubular section  13  that has a first inner diameter and extends up to a distance from the second end  12 . At an end of the tubular section  13  farthest away front the first end  11 , a threaded bore  14  is provided with a second inner diameter that is smaller than the first inner diameter of the tubular section  13 . In the tubular section  13 , at least two, and preferably three or more slits  15  are provided in a wall of the anchor member, where the slits  15  extend in a longitudinal direction substantially parallel to the longitudinal axis L. The slits  15  end at a distance from the first end  11  and at a distance from the second end  12 , respectively. By means of the slits  15 , the tubular section  13  is rendered flexible, as further described below. 
         [0051]    At least a portion of an outer surface of the anchor member  1  is provided with a bone thread  16 . Preferably, the bone thread  16  is provided in at least a portion of the tubular section  13  where the slits  15  are located. 
         [0052]    At the first end  11 , a plurality of recesses  17  extend from a front face of the first end  11  into the tubular section  13 , and form engagement portions for a form-fit engagement with the head  3 . 
         [0053]    The anchor member  1  may be made of a material that has a smaller modulus of elasticity compared to a modulus elasticity of a material of the core member  2 . Preferably, the anchor member  1  is made of a polymer material, in particular, a biocompatible polymer material. For example, the material may be selected from among the group of polyaryletherketones (PAEK), including polyetheretherketone (PEEK), polyetherketoneketone (PEKK) and polyetherketone (PEK). However, in various other embodiments, other polymeric materials may also be used. The extent to which the tubular section  13  is expandable can be adjusted during manufacturing by selecting an appropriate material, a wall thickness of the tubular section  13 , and a length and width of the slits  15 . 
         [0054]    It shall be noted that the second end  12  need not to be in the shape of a closed tip. In some embodiments, the second end  12  may be open, and can include teeth at an outer edge. Further, in some embodiments, the inner diameters of the tubular section  13  and of the threaded bore  14  may also be the same. 
         [0055]    Furthermore, instead of the bone thread  16 , various other bone engagement structures may be provided, such as, for example, barbs or a textured surface. 
         [0056]    As further shown in  FIGS. 8 and 9 , the core member  2  has a first end  21 , an opposite second end  22 , and a rod-shaped central portion  23 , the outer diameter of which is only slightly smaller than the inner diameter of the tubular section  13  of the anchor member  1 . Adjacent to the second end  22 , a portion  24  with an outer thread is provided that cooperates with the threaded bore  14  of the anchor member  1 . Adjacent to the first end  21 , a cylindrical section  25  with an engagement portion in the form of a recess  26  at the free end forms a drive portion. The engagement portion  26  can have any shape that allows for engagement with a drive tool. For example, the engagement portion  26  can have a hexagon-shape or torx-shape. An outer diameter of the cylindrical portion  25  is greater than the outer diameter of the rod-shaped central portion  23 . The length of the rod-shaped central portion  23  is such that, when the core member  2  is assembled with the head  3  and the assembly is inserted into the anchor member  1  as shown in  FIG. 16 , the threaded portion  24  can engage the thread of the threaded bore  14  of the anchor member  1  at an open end of the threaded bore  14 . It should be noted that, instead of a threaded connection between the core member  2  and the anchor member  1 , any of various other connections can be used that allows for advancement of the core member into the anchor member, and for holding the core member at a certain position, for example, for withstanding inadvertent sliding back of the core member relative to the anchor member. 
         [0057]    The core member  2  is made of a material that is preferably stiffer than the material of the anchor member, for example, a material that has a higher modulus of elasticity. For example, the core member  2  may be made of a metallic material, such as of titanium or stainless steel, or of a body compatible metal alloy, such as, for example, a Ni—Ti alloy, such as Nitinol. 
         [0058]    As further shown in  FIGS. 10 to 13 , the head  3  has a first end  31  and a second end  32 . Adjacent to the first end  31 , a spherical segment shaped surface portion  33  is provided. A diameter of the head  3  decreases towards the second end  32 , and adjacent to the second end  32 , a cylindrical neck portion  34  is present. A cylinder axis L corresponds to the longitudinal axis L of the anchor member  1  when the head  3  is connected to the anchor member  1 . Adjacent to the first end  31 , a first coaxial bore  35  is provided, an inner diameter of which is slightly larger than the outer diameter of the cylindrical portion  25  of the core member  2 . Also, a length of the coaxial bore  35  is the same or greater than a length of the cylindrical portion  25  in an axial direction. Adjacent to the cylindrical bore  35 , a second cylindrical bore  36  is provided with a diameter smaller than the diameter of the first cylindrical bore  35 , but greater than the diameter of the rod-shaped central portion  23  of the core member  2 , such that the central portion  23  of the core member  2  may extend therethrough. At a transition between the first cylindrical bore  35  and the second cylindrical bore  36 , a shoulder is formed that acts as a support surface  37  for supporting the cylindrical portion  25  in the head  3 . Hence, when the core member  2  is inserted into the head  3 , the cylindrical portion  25  of the core member  2  rests on the support surface  37 . 
         [0059]    At the second end  32 , a plurality of projections  38  are coaxially arranged at an edge of the second end  32 . The projections  38  are shaped and sized so as to allow a form-fit engagement with the recesses  17  of the anchor member  1 . 
         [0060]    The head  3  is preferably formed of a material that is stiffer than the material of the anchor member  1 . In some embodiments, the head  3  is made of the same material as the core member  2 , for example, of a metallic material, such as titanium or stainless steel, or of a body compatible metal alloy, such as, for example, a Ni—Ti alloy, such as Nitinol. 
         [0061]    The head  3  may facilitate for a connection to other devices, such as receivers of polyaxial anchoring devices or plates, as further described below. 
         [0062]    A first embodiment of a method of using the bone anchor will be explained with reference to  FIGS. 14 to 17 . First, as shown in  FIG. 14 , the anchor member  1  is inserted into a bone, a bone part, or a vertebra (not shown). A core hole may be prepared in the bone in advance. A tool (not shown) may be used that engages the anchor member  1 . The engagement portion  17  that serves for a form-fit connection of the anchor member  1  with the head  3  may also be used as an engagement portion for a tool. In some embodiments, other engagement portions (not shown) may instead be provided. 
         [0063]    In a second step, as shown in  FIG. 15 , the preassembled core member  2  and head  3  are inserted into the anchor member  1 . Because the core member  2  is supported on the support surface  37  of the head  3 , pushing the core member  2  into the tubular section  13  will also move the head  3  towards the anchor member  1 . Then, as shown in  FIG. 16 , the projections  38  of the head  3  engage with the recesses  17  at the first end  11  of the anchor member  1 , so that the head  3  and the anchor member  1  are rotationally fixed with respect to each other. In the configuration shown in  FIG. 16 , the threaded portion  24  just engages the open end of the threaded bore  14 . By further rotation of the core member  2 , the core member  2  is advanced into the threaded bore  14 . Because the core member  2  abuts against the support surface  37  of the head  3 , a compressive force is exerted that serves to shorten a distance between the first end  11  and the second end  12  of the anchor member  1 . By means of this, the tubular section  13  is radially expanded due to the slits  15 , as shown in  FIG. 17 . 
         [0064]      FIGS. 18 to 21  show a method of use, as described above, with an application of the bone anchor to a pedicle of a vertebra  100 , corresponding to the steps shown in  FIGS. 14 to 17 .  FIG. 18  depicts the insertion of the anchor member  1  with a tool  101 . As shown in particular in  FIG. 18 , the expansion of the tubular section  13  is effected when the core member  2  has been inserted into the anchor member  1 . Especially in weak osteoporotic bone regions, the anchoring strength of the bone anchor can be increased when the anchor member  1  is expanded. 
         [0065]    In a second embodiment of a method of use the pre-assembled anchor member  1  with core member  2  and head  3  as depicted in  FIG. 16  are inserted as a whole assembly into an osteoporotic bone, and then the anchor member  1  is expanded as shown in  FIG. 17  by rotating the core member  2 . 
         [0066]    A first application of the bone anchor together with a stabilization device is shown in  FIG. 22 . The bone anchor according to the first embodiment is received in a receiving part  4  of a polyaxial bone anchor. The receiving part  4  according to one embodiment is substantially cylindrical and has a top end  41 , a bottom end  42 , and a coaxial bore  43  extending from the top end  41  to a distance from the bottom end  42 . The bore  43  narrows towards the bottom end  42  and provides for an opening  44 . Near the bottom end  42 , a seat  45  is provided for pivotably receiving the head  3 . A U-shaped recess extends from the top end  41  to a distance from the bottom end  42  for receiving a rod  5 . By means of the U-shaped recess, two free legs  46 ,  47  are provided which have an internal thread  48  for inserting a locking member  6 . Further, a pressure member  7  is provided that exerts pressure onto the head  3 , such that the head  3  can be locked in a certain angular position by tightening the locking member  6 . In other embodiments, the bone anchor may be used with other parts of receiving parts and designs for polyaxial bone screws. Also, for example, the head  3  of the bone anchor may be designed such that it has a section for receiving a rod and/or for receiving a locking member to fix the rod, as known from, for example, monoaxial bone screws. 
         [0067]    A second example of an application is shown in  FIG. 23 , where bone anchors according to the first embodiment are used together with a bone plate  9  that has holes  10  and seat portions  11  for receiving the head  3 . Many different configurations of such bone plates are conceivable. Also here, the head  3  may be adapted in its shape to correspond to receiving portions provided in the bone plate. 
         [0068]    Referring to  FIGS. 24 to 26 , a second embodiment of a bone anchor is shown. The anchor member  1  is identical or similar to the anchor member  1  of the first embodiment. The core member  2 ′ acts as a traction element, and may include a traction portion  20 . When the core member  2 ′, that has been connected to the anchor member  1  at the second end  12 , is pulled using the traction portion  20 , a distance between the first end  11  and the second end  12  of the anchor member  1  can be shortened, and the tubular section  13  can be expanded, as shown in  FIG. 25 . The traction portion  20  may be a break-off portion, and may be broken off after expansion of the tubular section  13 , as shown in  FIG. 26 . 
         [0069]    The core member  2 ′ will be described in more detail with reference to  FIGS. 27 to 30 . The core member  2 ′ includes at its second end  22  an externally threaded portion  24  that cooperates with the threaded bore  14 . Further, a rod-shaped central portion  23  is provided. At an end of the central portion  23 , a plurality of flexible lips  28  are provided that are arranged around a circumference of an outer surface of the rod-shaped central portion  23  and that extend towards the first end  21 . The flexible lips  28  have teeth  29  adjacent to the first end  21  that extend in a circumferential direction and cooperate with corresponding grooves in a head  3 ′, as described further below. The traction portion  20  is a rod-shaped or bar-shaped portion that extends from an end of the central portion  23  between the lips  28  and that projects over or past the first end  21 . The traction portion  20  has a break-off portion that is formed as a thinned neck  20   a  at a position between the lips  28 , as shown in  FIG. 30 . The neck portion  20   a  has a diameter such that the traction portion  20  may be broken off, for example, by hand or with a tool. A length of the traction portion  20  is such that the traction portion  20  can be gripped and broken off. 
         [0070]    Referring further to  FIGS. 31 to 33 , the head  3 ′ differs from the head  3 , in that the head  3 ′ has a first coaxial bore provided with an engagement structure  35 ′ that is located adjacent to a first end  31 , and a coaxial bore  36 ′ at a second end  32 ′ that is provided with a groove structure  36   a  corresponding to the teeth  29  of the core member  2 ′. Projections  38 ′ at the second end  32  may also have a groove for engagement with the neck  29 . All other parts are the same as or similar to the first embodiment. 
         [0071]    The teeth  29  and the groove structure  36   a  of the second bore  36 ′ may have a substantially saw-tooth shape, such that a horizontal flank is oriented towards the second end  32  of the head  3 ′. The core member  2 ′ and the head  3 ° cooperate in such a manner that a movement of the core member  2 ′ towards the first end  31  of the head  3 ′ is allowed, whereas a movement in the opposite direction is prevented or restricted due to the engagement of the teeth  29  in the groove structure  36   a.  It shall be noted that any other engagement structure that fulfils the same or similar function may also be utilized. 
         [0072]    The length of the rod-shaped central portion  23  is such that, when the core member  2 ′ is fully inserted into the anchor member  1 , the lips  28  may not yet engage the groove structure  36   a  of the second bore  36 ′ of the head  3 ′, as shown in  FIG. 34 . 
         [0073]    In use, as shown in  FIG. 34 , in a first step, the bone anchor is pre-assembled. The core member  2 ′ is inserted into the anchor member  1  until its threaded portion  24  is fully screwed into the threaded bore  14 , such that the rod-shaped central portion  23  abuts against a shoulder formed between the threaded hole  14  and the tubular section  13 . The head  3 ′ is mounted to the anchor member  1  in a rotationally fixed manner by engagement of the recesses  17  of the anchor member  1  with the projections  38  of the head  3 ′. The traction portion  20  extends beyond the head  3 ′ out of the rest of the bone anchor, such that the traction portion  20  can be gripped. In this configuration, the bone anchor is screwed into a bone or bone part or vertebra. 
         [0074]    Thereafter, as shown in  FIGS. 35   a  and  35   b , the core member  2 ′ is pulled in a direction of the arrow, or in other words, away from the anchor member  1  until the lips  28  with the teeth  29  engage the groove structure  36   a  of bore  36 ′ of the head  3 ′. By means of this, because the head  3 ′ is fixed to the anchor member  1 , a distance between the first end  11  and the second end  12  of the anchor member  1  can be reduced and the flexible section  13  can be expanded. Such expansion is maintained because the cooperation of the teeth  29  with the groove structure  36   a  prevents relative movement of the core member  2 ′ in a direction that would enlarge the distance between the first end  11  and the second end  12  of the anchor member  1 . 
         [0075]    When a desired expansion is achieved, the traction portion  20  can be broken-off at the neck portion  20   a,  as shown in  FIG. 36 . 
         [0076]    The connection between the core member  2 ′ and the anchor member  1  at the second end  12  need not be a threaded connection. For example, as depicted in  FIG. 37 , the anchor member  1  may have a threadless bore hole  14 ′ and the core member  2 ′ may have a corresponding threadless portion  24 ′ that can be press-fit into the bore hole  14 ′. In a further modification, as seen in  FIG. 38 , a transverse pin  18  can be used to fix the core member  2 ′ and the anchor member  1  relative to each other at the second end  12 . In a still further modification, as seen in  FIG. 39 , a spherical or cylindrical projection  190  at an end of the core member  2 ′ can snap into a correspondingly shaped recess  19  provided inside the anchor member  1 . In various other embodiments, any of various other connections may also be possible for fixing the core member  2 ′ and the anchor member  1  relative to each other at the second end  12 , so that a tip of the anchor member can be pulled in a direction towards the head. 
         [0077]    Various other modifications of the second embodiment are also possible. For example, the core member can be made of two or more parts. For example, the traction portion with the break-off neck portion can be formed as a separate part that is screwed into a corresponding hole of the core member. Also, it may be conceivable that the engagement mechanism between the core member and the head is also formed as a separate part. For example, the teeth portion can instead be made as a separate ring element that is mounted, for example, screwed, onto the core member. 
         [0078]    The bone anchor according to the second embodiment may also be used with receiving parts of polyaxial bone anchoring devices, or may also have an adapted head with legs and a channel to receive a rod, for example, in the form of a monoaxial bone screw. The bone anchor according to the second embodiment may also be used with bone plates or various other stabilization devices. 
         [0079]    Also in the second embodiment, like in the first embodiment, the tip at the second end of the anchor member may be omitted. Any other bone engagement structure may be provided, such as for example, barbs. Also, the head may have various different shapes, depending on the intended use of the bone anchor. In some embodiments, the materials and combination of materials may be the same as or similar to the first embodiment. 
         [0080]    In still further modifications, the bone anchor may be cannulated. In such embodiments, the core member may be cannulated. This may allow for introduction of substances such as bone cement or drugs to surrounding bone regions. 
         [0081]    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.