Patent Publication Number: US-2015066097-A1

Title: Bone anchor and bone anchor assembly comprising the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 61/874,174, filed on Sep. 5, 2013, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 13183246.1, filed on Sep. 5, 2013, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a bone anchor for use in clinical surgery, for example in the treatment of traumatic fractures caused by osteoporosis of bones, among others. The bone anchor has a main body with a head and a shank. One or more recesses extend through the head and along a portion of the shank. The one or more recesses may extend generally parallel with respect to a longitudinal axis of the main body, and may receive each a pin-shaped element to support and improve the anchoring stability of the bone anchor. 
     2. Description of the Related Art 
     Bone anchoring assemblies comprising a bone anchor and one or more associated pins may help in preventing loosening of the bone anchors, when the pins are mounted to the anchors and extend, for example, into the surrounding bone material in an inclined fashion with respect to a longitudinal axis of the bone anchor. However, even if the associated pins extend parallel to the implanted bone anchor, a rotational support may be provided, since an unscrewing movement of the bone anchor is inhibited. 
     One example is disclosed in U.S. Pat. No. 4,657,001. A lag screw is implanted into a femoral head, and a pin antirotation-locking assembly comprising four elongated pins connected with each other via a head is attached to the lag screw. The pins of the assembly may each slide into a respective groove provided along the entire length of the lag screw including its head portion. The pins have tips, which slope up and away from a longitudinal center line causing the pins to lift slightly out of the grooves when being driven into the bone. The pins serve to positively lock the lag screw. 
     A locking screw for an intramedullary nail having a head including a passage is disclosed in U.S. 2006/0064095 A1. A longitudinal wedging element can be inserted through the passage of the head thereby extending substantially parallel to a central longitudinal axis of the screw and along a flat ramp recessed from the screw shaft. In this case, the wedging element wedges the shaft of the locking screw in a transverse bore hole which formed in the intramedullary nail. 
     Document U.S. 2008/0262497 A1 discloses a medical device for treating fractures at the femoral head. The device has an outer tube provided with recesses and an inner tube that is connected with a distal end piece via two strips whose position upon insertion of the inner tube into the outer tube corresponds to the position of the recesses. Using a screw to advance the inner tube towards the distal end piece the two strips expand through the recesses of the outer tube and into the surrounding bone material. This bending prevents the device from loosening from the bone material in an axial direction. 
     Document U.S. 2009/0204216 A1 discloses an expandable implant for stabilizing the vertebrae or bones. The implant functions like a stent and has a flexible tubular section extending between first and second ends, and by decreasing the distance between both ends, a plurality of strips of the flexible tubular section expand radially outwards pushing aside cancellous bone material and thereby stabilizing an osteoporotic vertebrae body. 
     SUMMARY 
     It is an object to improve the anchoring stability and support a bone anchor when being implanted in particularly in osteoporotic, cancellous, or fractured bony material. 
     The object is solved by a bone anchor according to claim  1 . The object is further solved by a bone anchoring assembly according to claim  11  or claim  16 . Advantageous aspects and embodiments become apparent from the appended claims. 
     According to embodiments of the invention, a bone anchor is provided with a main body including a head and a shank. A recess extends through at least a portion of the head and along at least a portion of the shank, and the recess is configured to receive a pin-shaped element. 
     A first end of the recess provided at the head may for example be represented by an opening which allows inserting therethrough the pin-shaped element. A second end of the recess provided at the shank may be formed as a stop. The stop is configured to be abutted or engaged by a distal end portion of the pin-shaped element. A locking structure may be provided at the first end of the recess and exert a biasing force on a proximal end portion of the pin-shaped element in a direction of the longitudinal axis and towards the stop. The stop exerts a counterforce and as a consequence, the pin-shaped element is compressed in the longitudinal direction. The pin-shaped element, however, has few axial compressibility but a sufficient degree of bending flexibility, and thus its intermediate bends radially outwards away from the shank upon receiving compressing forces. 
     According to an embodiment, the recess may include a groove portion, which is open towards the outside, i.e., towards the surrounding bony material, when the bone anchor is implanted. The groove portion may be located at the shank, but may also be located at the head. The compressed pin-shaped element may thus bend with an intermediate portion between the two end portions thereof towards the outside through the open groove portion, while it is held at the end portions. The intermediate portion bending outside thereby expands into the bony material thereby improving the anchoring stability of the implanted bone anchor. In particular, loosening by rotational movement may be inhibited. 
     It is not necessary that the intermediate portion of the pin-shaped element bends and expands through the open groove portion. Alternatively the recess may comprise two or more non-contiguous parts, and the pin-shaped element inserted into the multiple portion recess bends and expands radially outward in a free section extending between the end portions of the recess. For example, a portion of the shank or head of the bone anchor, for example an extended neck portion, may be configured to be considerably thinned between a through hole formed at the head and a bore hole at the distal end of the recess. 
     It is also possible that the groove portion is closed by a thin or weak material in a state of assembly of the parts, and breaks up only when a compressive force is exerted on the pin-shaped element inserted therein. 
     Embodiments provided in the detailed explanation below provide for bone anchors having each two recesses and respective pin-shaped elements on opposite sides thereof. However, it is contemplated that bone anchors according to the invention may also comprise one, two, three, four, or even more recesses and pin-shaped elements respectively. Nevertheless, a bone anchor having two recesses and assembled with two pin-shaped elements is preferred due to the symmetry and the lesser number of parts. 
     The locking structure which provides for the biasing force for compressing the pin-shaped element may be embodied by different mechanisms and the examples provided below are purely illustrative and do not limit the scope of the invention. Examples provided herein refer to a bayonet catch, an undercut recess, and a locking cap, respectively. Other locking mechanisms, which maintain the biasing force being exerted, are possible as well. It is noted that the locking structure as defined herein provides for maintaining the biasing force. The initiation of the biasing force, however, will have to be effected by an external tool that is not part of the claimed bone anchor. 
     An alternative embodiment of a bone anchoring assembly comprises a bone anchor having a main body including a head and a shank. A recess extending extends through at least a portion of the head and along at least a portion of the shank, and the recess is configured to receive a pin-shaped element. The pin-shaped element is made of a material that has shape memory properties, such as a shape memory alloy, for example a nickel titanium alloy such as Nitinol. The pin-shaped element is configured to assume a first configuration at a first temperature in which it is insertable into the recess and a second configuration at a second temperature different from the first temperature in which an intermediate portion of the pin-shaped element is bent in a transverse direction away from the shank. The bone anchoring assembly is inserted into the bone in the first configuration and, by changing the temperature, the pin-shaped element transforms into the second configuration whereby its intermediate portion is bent outward in a transverse direction away from the shank. The bone anchoring assembly does not require the aid of a mechanical locking structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further aspects and advantages will become apparent from the following detailed description of embodiments taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a perspective view of a bone anchor assembly with bone anchor and pin-shaped element according to a first embodiment; 
         FIG. 2  shows a cross-sectional profile of the bone anchor and pin-shaped element of  FIG. 1  according to the first embodiment; 
         FIG. 3  shows an enlarged perspective view of the head portion of the bone anchor of  FIG. 1  according to the first embodiment; 
         FIG. 4  shows a top view of the head portion of the bone anchor of  FIG. 1  according to the first embodiment; 
         FIG. 5  shows a cross-sectional profile of the head portion taken along a line AA shown in  FIG. 4  according to the first embodiment; 
         FIG. 6  shows a cross-sectional profile of the head portion taken along a line BB shown in  FIG. 4  according to the first embodiment; 
         FIG. 7A  shows a perspective view of the head portion with inserted pin-shaped elements during a first step of assembly according to the first embodiment; 
         FIG. 7B  same as  FIG. 7A , but for the complete bone anchor; 
         FIG. 7C  same as  FIG. 7B , but in cross-sectional profile; 
         FIG. 8A  shows a perspective view of the head portion with inserted pin-shaped elements during a second step of assembly according to the first embodiment; 
         FIG. 8B  same as  FIG. 8A , but for the complete anchor; 
         FIG. 8C  same as  FIG. 8B , but in cross-sectional profile; 
         FIG. 9  shows a perspective view of a bone anchor assembly with bone anchor and pin-shaped element according to a second embodiment; 
         FIG. 10  shows a cross-sectional profile of the bone anchor and pin-shaped element of  FIG. 9  along with a top view of the head portion according to the second embodiment; 
         FIG. 11A  shows in an enlarged cross sectional view of the head portion of the bone anchor of  FIG. 9  a first step of assembling the bone anchor with pin-shaped elements according to the second embodiment; 
         FIG. 11B  shows in an enlarged cross sectional view of the head portion of the bone anchor of  FIG. 9  a second step of assembling the bone anchor with pin-shaped elements according to the second embodiment; 
         FIG. 12A  shows a perspective view of the bone anchoring assembly in a state with expanded pin-shaped elements according to the second embodiment; 
         FIG. 12A  same as  FIG. 12A , but as a cross sectional profile; 
         FIG. 13  shows a perspective view of a head portion with locking cap of a bone anchor according to a third embodiment; 
         FIG. 14  same as  FIG. 13 , but as a cross sectional view and in an assembled and expanded state of the pin-shaped elements; 
         FIG. 15A  shows a cross-sectional profile of the bone anchor and pin-shaped element according to a fourth embodiment in a first configuration. 
         FIG. 15B  shows an enlarged perspective view of the head portion of the bone anchor of  FIG. 15A . 
         FIG. 16A  shows a cross-sectional profile of the bone anchor and pin-shaped element according to  FIG. 15A  in a second configuration. 
         FIG. 16B  shows an enlarged perspective view of the head portion of the bone anchor of  FIG. 16A . 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment of a bone anchoring assembly with a bone anchor according to the invention will be explained with regard to  FIGS. 1 through 8C . The bone anchor assembly of this first embodiment comprises a bone anchor  1  and two pin-shaped elements  4 , one of which is shown each in  FIGS. 1 and 2 . 
     The bone anchor  1  comprises a main body including a head  6  and a shank  2 . The main body shown in the embodiments is a contiguous, monolithic body, but may generally also consist of multiple parts, wherein for example the head and the shank, or additionally the tip portion  23 , are separate parts connectable to each other. The head  6  has a spherically segment-shaped contour  60 , a neck portion  61  forming a transition to the shank  2  and a flat top face  62 . In the top face, an engagement portion  63  is formed as a recess, which in this example is torx-shaped, but any other shape such as hexagonal socket or recess shape etc. is possible as well. 
     The shank  2  extends from the neck portion  61  up to the tip  23  and is of substantially cylindrical shape with a conical shape or a tapering towards the tip  23 . A bone thread  22  extends along the entire length of the shank  2 , wherein the thread  22  is formed by a helical crest  24  and a corresponding thread root  25  formed between respective crest  24  portions of each turn. 
     As can best be seen in  FIG. 2 , two recesses  3  extend each from one opening  37  formed at the bottom of the engagement portion  63  through a portion of the head  6  and its neck portion  61  along the shank  2  up to a bore hole  33  with a stop  36 . The bore hole  33  and stop  36  are located near the tip  23  of the bone anchor  1 , substantially where the tapering or narrowing towards the tip  23  starts. The opening  37  corresponds to a first end, and the stop  36  corresponds to a second end of the recess  3 . Each of the two recesses  3  are configured to receive one of the pin-shaped elements  4  shown on the left sides each of  FIGS. 1 and 2 . 
     The recesses of this embodiment have an almost straight and linear shape except the bore hole  33  adjacent the second end, or stop  36 , of each recess  3 , which is slightly inclined towards a central longitudinal axis  26  of the main body of the bone anchor  1 . More specifically, the recesses  3  extend substantially parallel to the central longitudinal axis  26  of the main body, and are arranged symmetrically and mutually opposite each other. 
     The recesses  3  according to the first embodiment comprise three portions: (a) a first portion formed as a bore or through hole  31  extending through the head  6  and neck portion  61 , (b) a second portion formed as a groove portion  32  extending along a surface of the shank  2 , and (c) a third portion corresponding to the above mentioned bore hole  33 . Through hole  31  and bore hole  33  fully enclose a pin-shaped element  4  received therein, as can be seen for example in  FIGS. 7C and 8C , while the groove portion  32  is open in radial outward direction, as can best be seen in  FIG. 1 . The groove portion  32  has a bottom  34  and sidewalls  35 . The bottom  34  of the groove portion  32  may have a flat or a rounded face (e.g. a hollow semi-cylindrical face). Groove portion  32  thus interrupts the thread  22 , and in particular each turn of crests  24  and roots  25 . As will be explained below, the groove portion  32  allows the pin-shaped element  4  inserted in the recess  3  to bend and expand radially outwards within a range between respective openings  39   a,    39   b  of through hole  31  and bore hole  33  facing each other. 
     The stop  36  is formed as a flat, rounded, conical, tapered or otherwise shaped face at the bottom of the bore hole  33  and is configured to receive and engage with the distal end portion  43  of the pin-shaped element, when it is inserted, and to exert a counterforce in axial direction (longitudinal axis  26 ), when the pin-shaped element  4  is compressed. 
     The pin-shaped element  4  has an elongated straight and linear shape wherein a proximal end portion  42  is kinked at a right angle with respect to a remainder portion  41  of the pin-shaped element. The recesses  3  of the bone anchor  1  have a width between the first and second ends, and a diameter of the pin-shaped element  4  is equal to or slightly smaller than said width such as to be received in the recess  3 . The length of the pin-shaped element  4  is larger than the length of the recesses  3  between the first and second ends, or between the opening  37  and the stop  36 , respectively, such that the pin-shaped element  4  protrudes from the opening  37  in an uncompressed, unbent or unbiased state. 
     Compression, bending and radial expansion of the pin-shaped element  4  is maintained by a locking structure  5 , which will be explained in the following: 
     The embodiments as described herein mainly differ from each other by the respective mechanism of locking. Details of the locking structure  5  according to the first embodiment are depicted in  FIGS. 3 through 6 . The locking structure  5  of the first embodiment refers to a bayonet catch. It comprises each one channel  50  and a corresponding catching recess  51  adjacent to channel  50 . Each one locking structure  5 , or bayonet catch is provided for each recess  3 . The channels  50  extend transverse to the longitudinal axis  26  of the main body, and—as can be seen in FIG.  4 —extend adjacent to the opening  37  of the recess  3  such as to receive the proximal end portion  42  which protrudes from the opening and which is kinked at right angle at the pin-shaped element  4 . 
     The openings  37  of respective recesses  3  are advantageously formed at the bottom of the engagement portion  63  within each one of the six mutually opposite lateral recesses of the torx-shape. As a consequence, the two channels  50  which open adjacent the openings  37  cut a wall formed between the engagement portion  63  and the outer spherically segment-shaped contour  60  of the head  6 . It is noted that the term “adjacent” as used in this document with regard to the locking structure does not necessarily mean that for example the opening and the locking structure contact each other or are contiguous. As small distance is possible. The distance should not extend a length of the engagement portion. 
     The catching recesses  51  extend laterally from a bottom portion of respective channels  50  such as to receive the proximal end portions  42  of the pin-shaped elements  4 , when these are rotated in an azimuthal direction around the longitudinal axis  26 . As shown in the top view of  FIG. 4 , the channels  50  have an axis BB and the catching recesses have an axis AA which is slightly rotated with respect to axis BB of the channels  50  around the central longitudinal axis  26  of the main body. 
     A length between the second end, or stop  36 , of the recess  3  and an upper wall of the catching recess  51  measured along the longitudinal axis direction  26  is less than a length between respective ends of the pin-shaped element  4  including distal and proximal end portions  42 ,  43 , respectively. As a consequence, when the proximal end portion  42  is received in the catching recess  51 , a pin-shaped element  4  is compressed and bent as can be seen in  FIGS. 8B and 8C  detailed below. The catching recess  51  preferably has an upper wall with a further recessed portion where the proximal end portion  42  may latch in. Such further recess portion may help to retain the proximal end portion  42  in the latched state. As common in the bayonet catch mechanism, further compression force and bending is then necessary to unlock the bent pin-shaped element  4 . 
     The operation of compression and radial expansion is shown with reference to  FIGS. 7A through 8C .  FIGS. 7A-7C  show a first step wherein pin-shaped elements  4  are inserted into respective recesses  3  through openings  37  with proximal end portions  42  received in channels  50  when distal end portions  43  of the pin-shaped elements  4  abut on respective stops  36  at the second ends of the recesses  3 . In this state, a distance  52  remains in the compression-free state between the proximal end portions  42  and a bottom wall of channels  50 . The amount of bending depends on the value of this distance  52 . In a next step (not shown) a tool (also not shown) is employed to apply a biasing force F onto proximal ends  42  of pin-shaped elements  4  in the longitudinal axis direction  26 . Consequently, pin-shaped element  4  is further pressed against stop  36  at the distal end portion  43 . As explained above, respective bore holes  33  include a central axis  38  indicated in  FIG. 7C  which is inclined with respect to the longitudinal axis  26  of the main body at an acute angle. Further insertion thus slightly bends the distal end portion  43  towards the central longitudinal axis  26  thereby predefining a radial bending direction E for an intermediate portion  44  of the pin-shaped element  4  as shown in  FIGS. 8B and 8C . Since the groove portion  32  is open, the intermediate portion  44  may leave the groove portion upon bending and expands radially outwards. 
     When the bone anchor  1  in this state has already been implanted in a bone, the intermediate portions  44  of both pin-shaped elements  4  advance into the surrounding bony material to improve and further support the anchoring stability of the bone anchor  1 . 
     In a next step shown in  FIG. 8A-8C , when the proximal end portion  42  has reached the bottom wall of the channel  50 , the tool (not shown) is used to rotate the proximal end portions  42  in an azimuthal direction D to latch the same into the catch recesses  51 . Once the proximal end portions  42  are latched into the catching recesses, the bending of the pin-shaped elements  4  is locked and maintained by the locking structure  5 . Unlocking of the pin-shaped elements may be performed with the same steps in the reverse order. 
     A second embodiment of a bone anchoring assembly is explained with reference to  FIGS. 9 through 12B . An overview of the second embodiment of a bone anchor  101  is given in  FIGS. 9 and 10 , wherein like parts with respect to the first embodiment are denoted with the same reference numerals and repeated explanation thereof shall be avoided herein. This particularly refers to features of the shank  2  of the bone anchor  101 . Differences arise with respect to the head  106 . While the head  106  also has a spherically segment-shaped contour  160  and a flat annular top face  162 , the locking structure  105  formed therein comprises features of an undercut  150  in order to lock the proximal end portions  142  of pin-shaped element  104 . 
     As shown in  FIG. 11A , which shows a first step of inserting the pin-shaped elements  104  into respective openings  137  of recesses  103 , an annular opening  151  is formed in the top face  162 , wherein the annular opening  151  has a diameter which is smaller than that of the undercut recess  150  adjacent below the opening  151 . Accordingly, in a state where the bone anchor  101  has been implanted in the bone and the pin-shaped elements  104  are inserted into openings  137  providing first ends of respective recesses  103 , a biasing force G has to be exerted onto pin-shaped elements  104  in order to bend their intermediate portions  44  in a radial direction E (shown in  FIG. 12B ). 
     The proximal end portions  142  of the pin-shaped elements  104  in this embodiment are not kinked at a right angle as in the first embodiment, but at a more obtuse angle, such that when these abut on an edge between the annular opening  151  and the planar top face  162 , these proximal end portions  142  slidingly bend inwards (direction H in  FIG. 11A ), and finally latch into the undercut recess  150  as shown in  FIG. 11B  (direction J). The proximal end portions  142  are thus locked against an expansion in the axial direction (longitudinal axis  26 ) by an upper wall  152 , or abutment face, of the undercut recess  150 . As can be seen in  FIG. 12B , the intermediate portion  44  of the pin-shaped element  4  is compressed and bent between the stop  36  adjacent the second end of the recess  103  and the upper wall  152  of the undercut recess  150  of locking structure  150  adjacent the opening  137 . 
     A third embodiment will be described with reference to  FIGS. 13 and 14 . Like parts are denoted with the same reference numerals as in the previous embodiments and repeated explanation shall be avoided herein. 
     As can be seen in  FIG. 13 , the bone anchor  201  comprises a main body with a shank  2  and a head  206 , wherein the shank  2  is similar to that of the previous embodiments. The head  206  has an external thread  261  which is provided to receive a corresponding inner thread  255  of a locking cap  257 . The locking cap  257  and respective channels  251 , which receive proximal end portions  242  kinked at right angle at one end of respective pin-shaped elements  204  form the locking structure  205  of the third embodiment. The channels  251  are formed transverse to the central longitudinal axis  26  of the main body and cut through a wall 
     The locking cap  257  comprises a substantially cylindrical outer surface  250  and a spherical top face  252  with a flat centre face  253 , in which a hexagon-shaped engagement portion  254  for engagement with an external tool is formed. 
       FIG. 14  shows the head portion in a state where the locking cap is attached and locks the proximal end portions  242  of the pin-shaped elements. In this state, the intermediate portions  44  of the pin-shaped elements  204  are expanded. 
     As can be seen in  FIG. 13 , when the pin-shaped elements  204  are inserted into respective recesses  203  of the bone anchor  201 , end portions  242  protrude from openings  237  of the recesses  203  and even from the channels  251  above the flat top face  262 , when the distal end portions  43  abut on the stops  36 . 
     As can be seen in  FIG. 14 , a biasing force is exerted onto the proximal end portions  242  in an axial direction, i.e., along the longitudinal axis  26  of the bone anchor  201 . More specifically, upon screwing the locking cap  257  with its inner thread  255  onto the outer thread  261  of the head  206 , an engagement surface  256  provided in an inner space of the locking cap  257  engages an upper face of the proximal end portions  242  and presses the same down. As a consequence, like in the previous embodiments, the distal end portions  43  of the pin-shaped elements  204  abut on the stop  36  adjacent the second end of the recesses  203  which then exert a counter-force compressing the pin-shape element, such that the intermediate portions  44  bend radially outwards. Simultaneously, an extent of the pin-shaped element measured along the axis  26  shrinks and the proximal end portions  242  are pressed and move into the channels  251 . 
     It is not necessary that the end portions  242  are kinked at a right angle. The locking cap may, for example, further bend the proximal end portions  242 , when these are already only slightly kinked, further downwards upon locking. 
     The same or similar effect may be achieved as described with respect to the previous embodiments. Nevertheless, the third embodiment involves an additional part, i.e. the locking cap  257 , which avoids latching mechanisms as provided in the first and second embodiments. Installation and removal of the bone anchor  203  may thus be facilitated. Only a common screw driver is needed. 
     A fourth embodiment will be described with reference to  FIGS. 15A to 16B . Like parts are denoted with the same reference numerals as in the previous embodiments and repeated explanation shall be avoided herein. 
     As can be seen in  FIG. 15A , the bone anchor  301  comprises a main body with a shank  2  and a head  306 , wherein the shank  2  is similar to that of the previous embodiments. The head  306  has a spherically segment-shaped contour  360 , a neck portion  61  forming a transition to the shank  2  and a flat top face  62 . In the top face, an engagement portion  63  is formed as a recess as in the previous embodiments. The recesses  303  for receiving the pin-shaped element  304  are shaped as in the first embodiment. In the head  306  a groove  500  is formed that extends in a transverse direction to the longitudinal axis  26  of the main body and that has a depth that is greater than a thickness of the proximal end portion  342  of the pin-shaped element  304 . The groove  500  is configured to receive the end portion  342  of the pin-shaped element  304 . 
     The pin-shaped element  304  comprises the kinked proximal end portion  342 , an intermediate portion  404  and a distal end portion  43 . In this embodiment, the pin-shaped element  304  has shape memory properties and can assume a first configuration at a first temperature in which the intermediate portion  404  is substantially straight and in which the pin-shaped element  304  is insertable into the longitudinal recess  303  and movable towards the stop  36  of the longitudinal recess  303 . In the first configuration, the length of the intermediate portion  404  is such that when the pin-shaped element  304  is inserted into the longitudinal recess  303 , and the distal end portion abuts against the stop  36 . The lower side of the proximal end portion  342  has a distance from the bottom of the groove  500  as can be seen in  FIG. 15B . The pin-shaped element  304  can further assume a second configuration at a second temperature, that is higher than the first temperature, wherein in the second configuration the intermediate portion  404  of the pin-shaped element is bent in a transverse direction away from the shank  2 . In the second configuration, when the distal end portion  43  abuts against the stop  36 , the total length of the pin-shaped element  304  in an axial direction is reduced, such that the lower side of the proximal end portion  342  rests on the bottom of the groove  500 , as can be seen in  FIGS. 16A and 16B . 
     The pin-shaped element  304  may be configured such that the transformation from the first configuration to the second configuration takes place when the pin-shaped element  304  is heated from the first temperature, that may be room temperature, to the second temperature, that may be body temperature. 
     In use, the bone anchoring assembly consisting of the bone anchor  301  and the pin-shaped element  304  is preassembled, wherein the pin-shaped element  304  is in the first configuration. Then the bone anchor is inserted into the bone. Through heating to the body temperature the pin-shaped element  304  can assume the second configuration in which the intermediate portion  404  is bent in a transverse direction away from the shank. The heating step can be performed using body heat or using an separate heating device. 
     The pin-shaped elements  4 ,  104 ,  204  of the first to third embodiments are preferably made from a flexible wire material, such as Kirschner wire, such as stainless steel, titanium alloys or other suitable, bio compatible materials. Sufficient bending flexibility is achieved by a diameter of 1 mm or less, preferably 0.75 mm or less, or more preferably 0.5 mm or less. The pin-shaped element  304  of the fourth embodiment is made preferably of Nitinol but other shape memory materials can also be used. These could be for example other metal alloys or plastic materials exhibiting a shape memory effect. 
     Materials for the main bodies of bone anchors employed for these or other embodiments can be taken from bio compatible materials including metals such as titanium, titanium alloys, nitinol, stainless steel, or plastic materials including PEEK, PCU, or similar materials. 
     The application field of the bone anchors described in these and other embodiments is not restricted to a treatment of fractures or osteoporosis, or to trauma surgery. For example, specific applications in the field of the vertebra column may also be envisaged. 
     In the above embodiments, bone anchors with shanks having a bone thread are shown. However, other types of shanks having thread less surfaces or being formed with barb elements may also be used. 
     In the above embodiments, spherically segment-shaped heads of bone anchors are described. However, any other shape of heads are possible. For example cylindrical, conical, etc. Further embodiments encompass bone anchors, in which no dedicated head is provided. For example, an end portion of the shank includes an engagement portion, which whereby defines a head portion. 
     In the above embodiments, the recess configured to receive the pin-shaped elements is described to have a substantially straight shape. However, it is also possible that the recesses extend helically around the shank portion. 
     In the above embodiments, the pin-shaped elements are described to have a round cross sectional profile. However, triangular, square or other profiles are possible as well, for example strip-like profiles. Additionally, the pin-shaped elements can be made from plastic material. In this case, however, fatigue breakage or damage has to be considered here. 
     In the above embodiments an inclined bore hole  33  having a stop  36  is provided at a second end of the recess receiving the pin-shaped element. However, the bore hole may not need to be inclined, and further, the bore hole needs not to have a constant diameter, but can have a conical or any other profile like being tapered towards the distal end. Still further, the stop  36  needs not can refer to a clamping means firmly holding the second distal end portion of the pin-shaped element. 
     In the above embodiments, proximal end portions are kinked at an angle with respect to a main portion of the pin-shaped elements. However, other embodiments include straight, non-kinked end portions and the locking structure presses on an end face or tip of the proximal end portion. 
     Modifications and variations of the above described embodiments are possible, and are contemplated to be covered by the scope of the appended claims.