Patent Publication Number: US-2023149049-A1

Title: Polyaxial bone anchoring device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 17/721,116, filed Apr. 14, 2022, which claims priority to and the benefit of U.S. Provisional Patent Application No. 63/175,281, filed Apr. 15, 2021, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 21 168 627.4, filed Apr. 15, 2021, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     The application relates to a polyaxial bone anchoring device with an outer locking member for locking a head of an anchoring element in a receiving part. 
     Description of Related Art 
     A polyaxial bone anchoring device is used in orthopedic surgery, in particular in spinal surgery, for coupling a rod to a bone anchor. Typically, a head of the bone anchor is pivotably received in a receiver which also connects to the rod. The receiver can assume various angular positions with respect to the bone anchor to provide a suitable orientation with respect to the rod to be connected, and the receiver and head can be locked in this orientation. Various design concepts have been developed for such polyaxial bone anchoring devices. For example, one concept is to lock the head within the receiver by means of a pressure member placed in the receiver and pressing the head firmly into a seat in the receiver. Another concept is to accommodate the head in a flexible portion of the receiver and to compress the flexible portion by means of a locking ring embracing the flexible portion so as to lock the head therein. 
     The concept using an outer locking ring is described, for example, in U.S. Pat. No. 5,733,285, which describes a polyaxial colletted locking mechanism for use with an orthopedic apparatus that includes a screw or other orthopedic implant element having a curvate head and a coupling element. The coupling element has a tapered and colletted portion having an interior chamber in which the curvate head is initially polyaxially disposed. A locking collar is disposed around the tapered and colletted portion such that translation thereof in the direction of the expanding taper causes the interior volume to contract onto the curvate head and lock it therein. 
     U.S. Pat. No. 8,506,609 B2 describes a polyaxial bone anchoring device with a receiving part for coupling a rod to a bone anchor, wherein the receiving part has a head receiving portion and a rod receiving portion. The head receiving portion is flexible for inserting and clamping the head. A locking ring extends around the head receiving portion. In a pre-locking position the locking ring exerts a first force onto the head receiving portion and the head is prevented from removal. In a locking position the locking ring exerts a second force greater than the first force onto the head receiving portion to lock the head. 
     Another polyaxial bone anchoring device is known from U.S. Pat. No. 9,895,171 B2. It includes a threaded screw with a head portion and a tulip assembly that selectively “snaps” and locks onto the head portion of the threaded screw in a desired orientation. The threaded screw may be placed in bone before the tulip assembly is engaged. 
     SUMMARY 
     It is an object underlying the invention to provide an improved polyaxial bone anchoring device that is simple and safe to use and that provides a variety of features or applications. 
     According to an aspect of the invention, the polyaxial bone anchoring device includes an anchoring element with a shank for anchoring in bone and a head, a receiving part having a head receiving portion with a wall defining an accommodation space for accommodating the head of the anchoring element and a central axis extending through the accommodation space. The polyaxial bone anchoring device further includes a locking member mountable to the receiving part such that it at least partially embraces the wall of the head receiving portion and is movable between at least a first position and a second position, wherein the head receiving portion is configured to assume at least a first configuration in which the head can pivot in the accommodation space when the locking member is in the first position and a second configuration in which the head is locked in the accommodation space when the locking member is in the second position. The head receiving portion has at least one cavity in the wall that is configured such that a thickness of the wall in a radial direction with respect to the central axis between the accommodation space and the locking member is reduced by the cavity. 
     Since the head receiving portion has a partially reduced wall thickness, the spreading of the head receiving portion may be more easily facilitated. Thus, the receiving part may be mounted more easily onto the head of the bone anchor. On the other hand, another thicker wall portion forming a seat against which the head is pressed by action of the locking member has a sufficient strength and stability against loosening of the clamping. 
     The polyaxial bone anchoring device includes only few parts, which renders it more convenient to handle. 
     Moreover, the cavity in the head receiving portion enables other applications and/or uses. For example, in one embodiment, pharmaceutical substances may be deposited in the cavity. Once the polyaxial bone anchoring device has been implanted into a patient&#39;s body, the pharmaceutical substances may be released over time. In another embodiment, the cavity may be used for accommodating a spring portion of a pressure member that is resiliently connected to the receiving part. 
     According to another aspect of the invention, a polyaxial bone anchoring device includes an anchoring element with a shank for anchoring in bone and a head, a receiving part having a head receiving portion defining an accommodation space for accommodating the head of the anchoring element, and a pressure member which is configured to exert pressure onto the head in the accommodation space, wherein the pressure member is configured to assume at least a first position in which the head can pivot in the accommodation space and a second position in which the head is locked in the accommodation space, and wherein the pressure member is monolithic with the receiving part. In a particular embodiment, the pressure member is movable from the first position to the second position by separating the pressure member from the receiving part, preferably at a predefined breaking region. In another particular embodiment, the pressure member is resiliently connected with the receiving part. 
     In a still further embodiment a method of manufacturing a polyaxial bone anchoring device is provided, wherein the method includes manufacturing the receiving part with the cavity and/or with the pressure member by means of an additive manufacturing method, such as three dimensional printing, for example, laser sintering or laser or electron beam melting. With such a method, complex shapes can be easily manufactured on the basis of CAD data of the object to be manufactured. Hence, the cavity of the head receiving portion and/or the pressure member can be designed to have any shape regardless of conventional manufacturing restrictions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become apparent from the detailed description of embodiments by means of the accompanying drawings. In the drawings: 
         FIG.  1    shows a perspective exploded view of an embodiment of a polyaxial bone anchoring device. 
         FIG.  2    shows the polyaxial bone anchoring device of  FIG.  1    in an assembled state. 
         FIG.  3    shows a cross-sectional view of the polyaxial bone anchoring device of  FIGS.  1  and  2    in an assembled state, the cross-section being taken in a plane extending through centers of legs of the receiving part and perpendicular to an inserted rod. 
         FIG.  4    shows a perspective view from a top of a receiving part of the polyaxial bone anchoring device of  FIGS.  1  to  3   . 
         FIG.  5    shows a perspective view from a bottom of the receiving part of  FIG.  4   . 
         FIG.  6    shows a top view of the receiving part of  FIGS.  4  and  5   . 
         FIG.  7    shows a cross-sectional view of the receiving part of  FIGS.  4  to  6   , the cross-section being taken along line A-A in  FIG.  6   . 
         FIG.  8    shows a perspective view of a locking member in the form of a locking ring of the polyaxial bone anchoring device of  FIGS.  1  to  3   . 
         FIG.  9    shows a top view of the locking ring of  FIG.  8   . 
         FIG.  10    shows a cross-sectional view of the locking ring of  FIGS.  8  and  9   , the cross-section being taken along line B-B in  FIG.  9   . 
         FIGS.  11   a  to  11   e    show cross-sectional views of steps of assembling the polyaxial bone anchoring device of  FIGS.  1  to  3   , wherein  FIG.  11   c    shows an enlarged view of a detail of  FIG.  11     b.    
         FIGS.  12   a  to  12   f    show cross-sectional views of steps of locking the head of the bone anchor in the receiving part of the assembled polyaxial bone anchoring device of  FIGS.  1  to  3   , wherein  FIG.  12   b    shows an enlarged view of a detail of  FIG.  12   c    and  FIG.  12   d    shows a further enlarged view of a detail of  FIGS.  12   b    and  12   c.    
         FIG.  13    shows a perspective view from a top of a receiving part of a polyaxial bone anchoring device according to a modified embodiment. 
         FIG.  14    shows a perspective view from a bottom of the receiving part of  FIG.  13   . 
         FIG.  15    shows a top view of the receiving part of  FIGS.  13  and  14   . 
         FIG.  16    shows a cross-sectional view of the receiving part of  FIGS.  13  to  15   , the cross-section being taken along line D-D in  FIG.  15   . 
         FIG.  17    shows a perspective view from a top of a receiving part of a polyaxial bone anchoring device according to a further modified embodiment. 
         FIG.  18    shows a perspective view from a bottom of the receiving part of  FIG.  17   . 
         FIG.  19    shows a top view of the receiving part of  FIGS.  17  and  18   . 
         FIG.  20    shows a cross-sectional view of the receiving part of  FIGS.  17  to  19   , the cross-section being taken along line F-F in  FIG.  19   . 
         FIG.  21    shows a side view of another embodiment of a polyaxial bone anchoring device in an assembled state. 
         FIG.  22    shows a cross-sectional view of the polyaxial bone anchoring device of  FIG.  21   , the cross-section being taken in a plane extending through centers of legs of the receiving part and perpendicular to an inserted rod. 
         FIG.  23    shows a cross-sectional view of the polyaxial bone anchoring device of  FIGS.  21  and  22   , the cross-section being taken in a plane extending through a rod channel of the receiving part and along an inserted rod. 
         FIG.  24    shows a perspective view from a top of the receiving part of the polyaxial bone anchoring device of  FIGS.  21  to  23   . 
         FIG.  25    shows a perspective view from a bottom of the receiving part of  FIG.  24   . 
         FIG.  26    shows a cross-sectional view of the receiving part of  FIGS.  24  and  25   , the cross-section being taken perpendicular to the rod channel, similar to the cross-section in  FIG.  22   . 
         FIG.  27    shows a cross-sectional view of the receiving part of  FIGS.  24  to  26   , the cross-section being taken along the rod channel, similar to the cross-section in  FIG.  23   . 
         FIG.  28    shows a cross sectional view of a still further embodiment of a polyaxial bone anchoring device, the cross-section taken in a plane extending through legs of the receiving part and perpendicular to an inserted rod. 
         FIG.  29    shows a perspective view from a top of the receiving part of the polyaxial bone anchoring device of  FIG.  28   . 
         FIG.  30    shows a perspective view from a bottom of the receiving part of  FIG.  29   . 
         FIG.  31    shows a cross-sectional view of the receiving part of  FIGS.  29  and  30   , the cross-section being taken perpendicular to the rod channel, similar to the cross-section of  FIG.  28   . 
         FIG.  32    shows a perspective view of a yet still further embodiment of a polyaxial bone anchoring device in an assembled state. 
         FIGS.  33   a  to  33   d    show perspective views of a receiving part and a locking ring of the polyaxial bone anchoring device of  FIG.  32   , and steps of mounting and operating the locking ring of the polyaxial bone anchoring device of  FIG.  32   . 
         FIGS.  34  and  35    show perspective views of steps of locking a head in the receiving part of the polyaxial bone anchoring device of  FIG.  32   . 
         FIGS.  36  and  37    show cross-sectional views of steps of locking the head in the receiving part of the polyaxial bone anchoring device of  FIG.  32   , the cross-sections corresponding to  FIGS.  34  and  35   , respectively. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS.  1  to  3   , an embodiment of the polyaxial bone anchoring device includes an anchoring element  1  having a shank  2  and a head  3 . The anchoring element  1  may be in the form of a bone screw with the shank  2  being at least partially threaded. The head  3  has a spherical outer surface portion. More specifically, the head  3  may be formed as a spherical segment, preferably including a region of a greatest outer diameter E of the sphere. In a free end surface of the head  3  opposite to the shank  2 , a recess  4  for engagement with a drive tool may be provided. The polyaxial bone anchoring device further includes a receiving part  5  which is configured to accommodate the head  3  of the anchoring element and for receiving a rod  100 . The rod  100  serves to connect the polyaxial bone anchoring device to at least one further bone anchor which may be part of an identical or similar polyaxial bone anchoring device. A locking member in the form of a locking ring  6  serves for locking the head  3  in the receiving part  5  with the shank  2  at a particular angular position. To fix the rod  100  in the receiving part  5 , a fixation member  7  may also be part of the polyaxial bone anchoring device. The fixation member  7  has an engagement structure  71  and a tool engagement recess  72 . The fixation member may be, for example, a set screw that can be screwed into the receiving part  5 . 
     Referring further to  FIGS.  4  to  7   , the receiving part  5  has a first or upper end  5   a  and a second or lower end  5   b , and a substantially cylindrical outer shape that extends around a central axis C. A coaxial passage  51  extends from the first end  5   a  to a distance therefrom. The passage  51  narrows in a narrowing portion  51   a  towards a reduced diameter intermediate portion  51   b . The narrowing portion  51   a  may be spherical but can also have another shape such as, for example, a conical shape. Adjacent to the intermediate portion  51   b , an accommodation space  52  is formed that is shaped and sized to accommodate the head  3  therein. The accommodation space  52  has an opening  52   a  at the second end  52 . A substantially U-shaped recess  53  extends from the first end  5   a  to a distance therefrom, by means of which two upstanding legs  54  are formed. The legs  54  define a channel for receiving the rod  100 . A bottom  53   a  of the channel may have a substantially V-shaped upper contour which provides a rod support surface. By means of this, rods with different diameters can be inserted into the channel and can be more securely supported on the rod support surface. Any rod with a diameter between a greatest diameter and a smallest diameter that are respectively defined by the size of the recess  53  and by the shape of the bottom  53   a  can more safely and securely rest on the bottom  53   a . An internal thread  55  is formed on the legs  54  which is configured to cooperate with the external thread  71  of the fixation member  7 . The internal thread  55  may have any suitable threadform, preferably however, a threadform that prevents splaying of the legs  54  when the fixation member is tightened. For example, the threadform may be a flat thread or a square thread. By means of the U-shaped recess  53 , the upper portion of the receiving part defines a rod receiving portion. 
     A lower portion of the receiving part  5 , approximately from the intermediate portion  51   b  of the passage  51  to the second end  5   b , which includes the accommodation space  52 , defines a head receiving portion  50  of the receiving part  5 . The accommodation space  52  has a substantially spherical inner contour that matches an outer contour of the head  3 . The axial extension of the accommodation space  52  is such that it is configured to cover the region of the head  3  with the greatest outer diameter E. More specifically, the size of the accommodation space  52  may be such that the head  3  can be held therein by friction. A small outwardly tapering portion  52   b  that widens towards the second end  5   b  may be provided for facilitation of the insertion of the head and/or for providing a slightly greater range of pivot angles for the shank  2 . As can be seen in particular in  FIGS.  3  and  7   , the accommodation space  52  is divided into an upper portion  52   c  configured to cover the head  3  from the top, and a lower portion including a seat  52   d  for the head  3  in which the head  3  can pivot. 
     A wall of the receiving part  5  around the accommodation space  52  for the head  3  is expandable and compressible in a radial direction with respect to the central axis C. This is achieved by a plurality of axially extending slits  56  that are open to the second end  5   b  and that may extend up to a distance from the second end, preferably up to the intermediate portion  51   b  of the passage  51 . The size and number of the slits  56  may be selected to obtain a desired flexibility. Moreover, an end portion  56   a  of the slits  56  may be enlarged, for example with a diamond-shaped contour, a circular contour, or a triangular contour, or any other suitable shape. 
     A cavity  57  is formed in the wall circumferentially around the accommodation space  52 . The cavity  57  may extend in the axial direction from a position below a region accommodating the greatest diameter E of the head up to the end portions  56   a  of the slits  56 , as can be seen in particular in  FIG.  3   . Thus, the cavity  57  encircles an inserted head circumferentially and extends along a substantial region of the head  3  in the axial direction. Moreover, the cavity  57  is in communication with the accommodation space  52  through an opening  58  which has a smaller height in the axial direction than a radially outer portion of the cavity  57 . More specifically, the opening  58  has the shape of an annular slit as can be seen in particular in  FIGS.  5  and  7   . An outwardly directed wall portion  57   a  defining the cavity  57  may make the cavity  57  slightly convex and may be formed at a radial position away from the central axis C, so that a radially outermost wall portion  50   a  of the head receiving portion  50  is thinner than a lowermost wall portion  50   b  below the cavity  57  at or adjacent to the second end  5   b . Also, the end portions  56   a  of the slits  56  are in communication with the cavity  57 . An upper wall portion  57   b  of the wall defining the cavity may be inclined and tapered to narrow towards the second end  5   b  up to the opening  58 . A lower wall portion  57   c  of the wall defining the cavity  57  may have a portion that is substantially flat in a direction substantially perpendicular to the central axis and may have another portion that rises in a curved or inclined manner that narrows towards the opening  58 . It shall be noted that the detailed shape of the cavity is not limited to the embodiment shown but may vary. More generally, the axial height of the cavity  57  is greatest at a particular radial distance from the central axis C and decreases towards the central axis C. 
     By the opening  58  of the cavity  57 , the accommodation space  52  is divided into the upper portion  52   c  and the seat  52   d  which can be pressed against the head  3  by the locking ring  6 . 
     The wall portion  50   a  is thin compared to the wall portion  50   b  in the radial direction. Moreover, the wall portion  50   b  is divided in the circumferential direction by the slits  56  into a plurality of sections which behave as flexible wall sections. Due to the cavity  57 , the flexible wall sections of the wall portion  50   b  can be easily spread, although they are relatively thick at the second end  5   b  of the receiving part. When the head receiving portion  50  is compressed by the locking ring  6 , as described below, the lowermost wall portion  50   b  is configured to exert pressure onto the head  3  from an axial position at or close to the region with the greatest diameter E of the head and from below this region, as depicted in  FIG.  3   . As the wall sections of the wall portion  50   b  are thicker, a sufficiently strong clamping force can be achieved. Also, the strength against loosening may be enhanced by this design. In other words the wall sections provide a stable seat  52   d  for the head  3  in which the head can be safely clamped. 
     The outer surface of the wall of the head receiving portion  50  widens in a tapered portion  50   c  close to the second end  5   b  towards the second end  5   b . Thereby, the compression force increases when the locking ring  6  is moved along the head receiving portion  50  in a direction towards the second end  5   b . Except for the tapered portion  50   c , the outer diameter of the receiving part  5  may be substantially the same along the axial length of the receiving part  5 . Between the tapered portion  50   c  and the second end  5   b , the outer surface may be convexly rounded, and an edge  105  may be formed between the rounded surface and the tapered portion  50   c.    
     At an axial position above the bottom  53   a  of the U-shaped recess  53  and at a distance from the first end  5   a , a first holding structure for temporarily holding the locking ring  6  at an insertion position in which the head is insertable into the accommodation space  52  is provided. At the outside of each leg  54 , the first holding structure may be in the form of a circumferentially extending first rib  59   a . A thickness of the first rib  59   a  in the radial direction is such that when the locking ring  6  passes the first rib  59   a , the locking ring experiences a friction force such that the locking ring  6  is held by friction in the respective axial position. As also shown in  FIGS.  11   b  and  11   c   , the position of the first holding structure may be such that the locking ring  6  can be held completely above the head receiving portion  50 . 
     A second holding structure is provided at a distance from the first holding structure towards the second end  5   b . The second holding structure may also be in the form of a circumferential rib  59   b  at the outer surface of the receiving part  5 . The axial position of the second holding structure may be slightly below the bottom  53   a  of the substantially U-shaped recess  53  and above the end portion  56   a  of the slits. Also the second circumferential rib  59   b  has a thickness such that the locking ring  6  experiences a friction force when it passes along the outer surface of the receiving part. Thereby, the locking ring  6  can be held temporarily at a pre-locking position in which the head receiving portion  50  is compressed but the head  3  is still pivotable without being finally locked. As additionally shown in  FIG.  12   a   , in the pre-locking position, the locking ring  6  still has not fully engaged the tapered outer surface portion  50   c  of the head receiving portion  50 . In the pre-locking position, the head  3  is prevented from removal through the lower opening  52   a  of the accommodation space  52 . 
     At the center of each leg  54  in a circumferential direction, a through hole  500  or another engagement recess may be provided for engagement with an instrument. Lastly, inclined cutaway portions  501  may be formed at either end of the channel for the rod close to the bottom  53   a  of the substantially U-shaped recess. The cutaway portions  501  may extend until below the end portions  56   a  of the slits and may have a substantially triangular contour with the tip oriented towards the second end  5   b . Additional chamfered portions  502  may extend from the cutaway portions  501  to some extent towards the edges of each of the legs  54  on either side of the channel. The cutaway portions  501  and chamfered portions  502  may facilitate the mounting of the locking ring from the first end  5   a  of the receiving part  5  and/or generally reduce the size of the receiving part. 
     Referring additionally to  FIGS.  8  to  10   , the locking ring  6  will be described in greater detail. The locking ring  6  is formed as a closed ring with a first or upper end  6   a  and a second or lower end  6   b . The inner surface  61  is substantially cylindrical with an inner diameter sized such that the locking ring  6  fits around the receiving part  5  and is configured to slide along the outer surface of the receiving part  5 . Adjacent to the first end  6   a , a small beveled portion  62  may be formed that may serve for sliding the locking ring from the second end  5   b  of the receiving part along the rounded outer surface of the lower wall  50   b  and over the tapered portion  50   c  for mounting. Adjacent to the second end  6   b , the locking ring  6  may include a tapered inner surface portion  63  that tapers and narrows towards the first end  6   a  and is configured to cooperate with the tapered outer surface portion  50   c  of the receiving part  5 . The angles of the tapered portions of the locking ring  6  and the receiving part  5  preferably correspond to each other. Preferably the angle is selected such that a self-locking occurs when the surfaces engage each other. Thus, once the locking ring  6  is mounted and the tapered surfaces engage each other, the locking ring is restricted from being inadvertently moved upward and loosening the locking. However, it may be possible to disengage the cooperating surfaces, for example, by using an instrument. 
     The outer surface of the locking ring  6  has a groove  64  that may serve for engagement with an instrument. Adjacent to the first end  6   a , the outer surface may be cylindrical and adjacent to the second end  6   b  the outer surface may be tapered and narrow towards the second end  6   b . However, the shape is not limited to such a design. The axial length of the locking ring  6  may be at least the axial length of the cavity  57 , and preferably greater than the axial length of the cavity  57 . Thus, the locking ring is configured to exert a radial compression force onto a major part of the head receiving portion  50 . 
     The locking ring  6  can also be mounted to the receiving part  5  from the first end  5   a  thereof. In this case, the tapered portion  63  can slide along rounded edges of the legs  54  at the first end  5   a  of the receiving part  5 . 
     Parts and portions of the polyaxial bone anchoring device may be made of any material, preferably, however, of a bio-compatible material, such as titanium or stainless steel, or of any other bio-compatible metal or metal alloy, or of a plastic material. For bio-compatible alloys, a NiTi-alloy, for example Nitinol, may be used. Other materials that can be used are Magnesium or Magnesium alloys, and bio-compatible plastic materials that can be used may be for example, Polyether ether ketone (PEEK) or Poly-L-lactide acid (PLLA). The parts can be made of the same or of different materials from one another. 
     A preferred method of manufacturing the receiving part and optionally also the locking ring and/or the bone anchoring element is an additive manufacturing method, more preferably an additive layer manufacturing method, such as three-dimensional printing. More particularly, preferred methods are laser sintering or laser melting or electron beam melting. In such methods, subsequent layers of a powder material, such as a metal or plastic powder, are solidified with an energy beam, particularly a laser or an electron beam, at positions corresponding to the cross-section of each part at each respective layer. By such a method, complex shapes including undercuts, etc., can be more easily manufactured. 
     Referring to  FIGS.  11   a  to  11   e   , steps of mounting the locking ring  6  to the receiving part  5  and assembling the polyaxial bone anchoring device will be explained. According to a first method of assembling, the locking ring  6  is mounted to the receiving part  5  from the second end  5   b  of the receiving part  5 . As shown in  FIG.  11   a   , the locking ring  6  is oriented with its first end  6   a  towards the second end  5   b  of the receiving part and subsequently pushed onto the receiving part  5  as shown in  FIG.  11   b   . During this step, the beveled portion  62  slides along the rounded lower surface of the receiving part  5  and compresses the head receiving portion  50  slightly so that the locking ring can move over the edge  105  between the rounded portion and the tapered portion  50   c . Then, the locking ring  6  can be moved further towards the first end  6   a . The locking ring passes the second rib  59   b  which is slightly retracted due to the compression of the head receiving portion so that it does not present an obstacle during mounting. When the locking ring  6  reaches the first rib  59   a , the locking ring is held there by the friction that occurs between the first rib  59   a  and an upper region of the cylindrical section  61  of the locking ring that is close to the beveled portion  62 , as shown in  FIG.  11   c   . In this position, which is an insertion position of the locking ring for permitting the head  3  to be inserted into the accommodation space  52 , the second end  6   b  of the locking ring is above the head receiving portion  50 . Hence, the head receiving portion  50  is free to expand to permit the head  3  to be inserted, as shown in  FIG.  11   d   . Due to the flexibility of the wall of the head receiving portion  50 , the head  3  can be easily inserted and is held in the seat  52   d  by the friction force of the flexible wall sections, as shown in  FIG.  11     e.    
     Alternatively to the steps shown in  FIGS.  11   a  and  11   b   , in some embodiments, the locking ring can also be mounted from the first end  5   a  of the receiving part. 
     Referring to  FIGS.  12   a  to  12   d   , the further steps of locking the head  3  in the receiving part  5  will be explained. In  FIG.  12   a   , once the head  3  has been inserted into the accommodation space  52 , the locking ring  6  is moved towards the second end  5   b  of the receiving part, whereby the friction force between the first rib  59   a  and the locking ring  6  is overcome, until the locking ring  6  passes the second rib  59   b  where the locking ring is held temporarily by friction. This position of the locking ring  6  is a pre-locking position where the head receiving portion  50  is compressed to an extent such that the head  3  cannot be removed through the opening  52   a . Depending on the size of the head and the size and flexibility of the accommodation space, the head  3  may also be held with friction in the pre-locking position. This allows the head  3  to be held at a preliminary angular position relative to the receiving part. The preliminary angular position can be changed by overcoming the friction force, either manually or with an instrument. 
     For locking the head  3  in the head receiving portion  50 , the locking ring  6  is further moved towards the second end  5   b  until the tapered outer surfaces  63 ,  50   c  of the locking ring and the receiving part, respectively, substantially engage each other, as depicted in  FIGS.  12   b  to  12   d   . This results in an increased compression of the head receiving portion  50  around the head  3 , so that the head  3  is finally locked in the head receiving portion  50 . The angle of the tapered surfaces preferably results in a self-locking, which means that the friction force between the two surfaces is high enough to prevent disengagement of the surfaces, and thereby, upward movement of the locking ring under normal conditions of use. As shown in the enlarged views of  FIGS.  12   b  and  12   d   , the tapered surfaces are in full engagement when the lower end  6   b  of the locking ring is substantially flush with the lower end of the tapered surface  50   c . Hence, the force that compresses the head receiving portion is transmitted mainly at the lower stable wall section  50   b  in a radial direction to lock the head  3  in the seat  52   d.    
     After locking the head  3 , the rod  100  can be inserted into the substantially U-shaped recess  53  and the fixation member  7  can be inserted between the legs  54 . For final locking, as shown in  FIG.  12   f   , the fixation member  7  is tightened to press the rod  100  into the bottom  53   a  of the substantially U-shaped recess  53 . As the rod  100  may also press onto the first end  6   a  of the locking ring  6 , depending on the size of the rod, the compression force exerted by the locking ring may increase. In this manner, the entire polyaxial bone anchoring device can be finally locked. 
     In clinical use, at least two polyaxial bone anchoring devices are inserted into bone parts or vertebrae and connected through the rod  100 . It shall be noted that the polyaxial bone anchoring device can be used in a pre-assembled condition where the head is already inserted into the head receiving portion and with the locking ring being in the pre-locking position, as shown in  FIG.  12   a   . The polyaxial bone anchoring device can be inserted in this pre-assembled condition into bone. In another way of use, the bone anchoring element  1  can be inserted first into the bone or vertebra, and the receiving part  5  with the locking ring in the insertion position as shown in  FIGS.  11   b  and  11   d    is mounted to the head  3 . The cavity may also be used for depositing a pharmaceutical substance therein that is released over time. 
     Referring to  FIGS.  13  to  16   , a modified receiving part of the polyaxial bone anchoring device will be explained. The receiving part  5 ′ differs from the receiving part of the previous embodiment mainly in the design of the cavity and the accommodation space. Also, in this modified embodiment, the preliminary holding structures like the first rib and the second rib are omitted. Parts and portions of the receiving part  5 ′ that are identical or similar to the parts and portions of the receiving part  5  of the previous embodiment are designated with the same reference numerals, and the descriptions thereof will not be repeated. 
     In the embodiment in  FIGS.  13  to  16   , the accommodation space  52 ′ forms a seat  52   d ′ that is shorter in the axial direction compared to the seat  52   d  of the previous embodiment. The widening portion  52   b ′ may have an axial length that is greater than in the receiving part of the previous embodiment. Generally, the widening portion  52   b ′ and the seat  52   d ′ may have approximately the same axial length. Adjacent to the seat  52   d ′ on a side opposite the widening portion  52   b ′, there may be a conical portion  52   e ′ that widens towards the first end  5   a  of the receiving part  5 ′. The cavity  57 ′ is elongate in the axial direction. More specifically, the height of the cavity  57 ′ in the axial direction is greater than the width of the cavity in the radial direction. Thus, the shape of the cavity  57 ′ is approximately that of a hollow cylindrical ring or an oblong torus, which is located substantially at an axial position around the widening portion  52   e ′ and the upper portion  52   c  of the accommodation space  52 ′. The opening  58 ′ that connects the cavity  57 ′ with the accommodation space  52 ′ in this embodiment extends from an upper portion of the cavity  57 ′ closer to the first end  5   a  of the receiving part into the widening portion  52   e ′. The lower wall portion  57   c ′ of the cavity is located at the middle or around the middle portion of the cavity  57 ′ in the axial direction. Thus, the cavity  57 ′ is separated to a greater extent from the accommodation space  52 ′, for example, compared to the previous embodiment. 
     The locking ring  6  can be temporarily held in the insertion position or in the pre-locking position only by friction with the outer surface of the receiving part  5 ′. 
     Referring to  FIGS.  17  to  20   , a further modified embodiment of the receiving part will be described. The receiving part  5 ″ of the polyaxial bone anchoring device differs from the receiving part  5 ′ of the previous embodiment in the design of the cavity, the accommodation space and the slits. The cavity  57 ″ is similar to the cavity  57 ′ of the previous embodiment. That means, the cavity  57 ″ is substantially hollow cylindrical ring-shaped or torus-shaped and extends around the widening portion  52   e ″ and the upper section  52   c  of the accommodation space. An inclined circumferential slit  58 ″ forms the opening that connects the upper portion of the cavity  57 ″ with the accommodation space  52 ″. The accommodation space  52 ″ may have an small widening section  52   b ″. Different from the previous embodiments, the number and length of the slits  56 ″ is reduced. In the embodiment shown, six equidistantly arranged slits  56 ″ are provided compared to more slits in the previous embodiments. However, the number of slits is not restricted thereto. A smaller number of slits may result in larger wall portions in the circumferential direction. Hence, to obtain enough flexibility, the area of the end portions  56   a ″ can be increased, for example, the end portions  56   a ″ are elongate in the axial direction. Therefore, the head receiving portion  50  has sufficient flexibility to be easily expanded and compressed around the head  3 . With the cavity  57 ″ designed in this manner, the entire axial length of the receiving part, and mainly that of the head receiving portion  50 , can be reduced, contributing to a smaller size of the receiving part  5 ″. 
     Referring to  FIGS.  21  to  27    a further embodiment of the polyaxial bone anchoring device will be described. The polyaxial bone anchoring device in  FIGS.  21  to  27    differs from the previous embodiment in the design of the receiving part. The receiving part  5 ′″ is similar to the receiving part  5  of the first embodiment with respect to the lower portion of the accommodation space  52  and with respect to the shape and position of the cavity  57 . 
     The passage  51 ′″ accommodates a pressure member  8  that is configured to exert pressure onto the head  3  from the top of the head  3 . The pressure member  8  is a substantially cylindrical part that is sized such that it fits into the passage  51 ′″ but is not tightly guided by the wall of the passage. The pressure member has a first or upper end  8   a  and an opposite second or lower end  8   b . At the second end  8   b , there is a substantially spherically-shaped recess  81  with a radius corresponding to the radius of the head  3  that allows engagement with the upper surface of the head  3  and exerts pressure thereto. At the first end  8   a , there is an elongate recess  82  with a longitudinal axis that is substantially perpendicular to the central axis C. The recess  82  has a substantially V-shaped cross-section to provide a support surface for rods of different diameters. Close to the first end  8   a , there are circumferentially extending projections  83 , in the embodiment two projections, that are axially spaced from each other and that are configured to engage corresponding circumferentially extending grooves  503  provided at the inner surface of the receiving part at an axial position such that, when the head is inserted, the inner surface of the spherical recess  81  may not yet contact the upper surface of the head  3 . The projections  83  fit into the grooves  503  with axial and radial play. The recess  82  is aligned with the substantially U-shaped recess  53  of the receiving part so that the rod  100  can be inserted into the receiving part and rests on the rod support surface formed by the recess  82 . Moreover, the pressure member  8  is at a position in the receiving part  5 ′″ such that the rod support surface formed by the recess  82  is slightly above the rod support surface  53   a  of the substantially U-shaped recess  53  of the receiving part  5 ′″, as shown in  FIG.  21   . 
     The pressure member  8  is monolithically connected to the receiving part  5 ′″, with one or more connection portions  84  that may be arranged at approximately 90° from centers of the legs  54  in a circumferential direction, on both sides of the rod channel, as can be seen in particular in  FIGS.  23  and  27   . The connection portion  84  is thin so that it can be easily broken when a force is exerted onto the pressure member  8 . Thus, the connection portion  84  forms a predefined breaking region that is configured to break under load, so that the pressure member  8  can be separated from the receiving part  5 ′″. The receiving part  5 ′″ and the pressure member  8  are manufactured preferably as a monolithic part, for example, by using an additive manufacturing method as described above. 
     It shall be noted that the pressure member  8  lacks a central hole that provides access to the head  3 . Hence, the polyaxial bone anchoring device in the embodiment of  FIGS.  21  to  27    is particularly suitable for in-situ placement of the bone anchoring element  1  into bone, and then mounting the receiving part with assembled locking ring thereafter. In a still further modified embodiment, the pressure member  8  may have a central hole providing access to the head  3  with a tool. 
     In use, as shown in  FIGS.  22  and  23   , the head  3  is first inserted into the receiving part  5 ′″ and clamped by means of the locking ring  6 , which is moved into the locking position where the tapered surfaces  50   c  and  63  of the receiving part and the locking ring, respectively, are engaged. Then, the rod  100  is inserted until it rests on the rod support surface of the recess  82  of the pressure member  8 . Finally, the fixation member  7  is inserted between the legs and tightened. The pressure exerted by the fixation member  7  onto the rod  100  is transferred onto the pressure member  8 . Thereby, the connection portion  84  breaks and the pressure member  8  can move to some extent in the axial direction, so that the pressure is transferred via the pressure member  8  to the head  3  to finally lock the head in the head receiving portion  50 . Due to the axial play, the pressure member can move slightly downward. Due to the radial and the axial play, the pressure member  8  can also be tilted slightly or wobble with respect to the head  3 . This permits adjusting of pressures more precisely. 
     Referring to  FIGS.  28  to  31   , a still further embodiment of the polyaxial bone anchoring device is described. The receiving part  5 ″″ is similar to the receiving part  5 ′″ of the previous embodiment, except with respect to the shape and connection to the pressure member  8 ′ and the shape of the cavity. The pressure member  8 ′ is permanently connected to the receiving part  5 ″″. In order to render the pressure member  8 ′ movable, the pressure member  8 ′ includes a spring portion  85  that connects the pressure member  8 ′ and the receiving part  5 ″″ monolithically in a resilient manner. The spring portion  85  is formed by two portions forming an angle. In greater detail, the spring portion includes a first portion  85   a  that extends from the outer edge of the second end  8   b  of the pressure member in an inclined manner upwards towards the first end  5   a  of the receiving part, and a second portion  85   b  that extends from the first portion  85   a  in an oppositely inclined manner to a downwardly facing upper wall portion  57   b ″″ of the cavity  57 ″″. The cavity  57 ″″ has a large opening that opens into the passage to accommodate the spring portion  85 . Due to its shape, the spring portion  85  provides resiliency to the pressure member  8 ′ in the axial direction and also to some extent in the radial direction. Preferably, the pressure member  8 ′ and the receiving part  5 ″″ are manufactured monolithically, for example, by using an additive manufacturing method. 
     In use, when the head has been inserted into the head receiving portion and the rod has been inserted into the rod channel and rests on the rod support surface of the recess  82 , tightening of the fixation member  7  transfers the pressure via the rod onto the pressure member  8 ′ and finally onto the head  3 , so that the head is locked in the accommodation space. Loosening the fixation member also loosens the clamping of the head. For example, when the locking ring is in the pre-locking position, several correction steps can be carried out in this manner, before final locking. 
     Referring to  FIGS.  32  to  37   , a still further embodiment of the polyaxial bone anchoring device is shown that is modified with respect to the cooperation of the receiving part with the locking ring. Parts and portions that are identical or similar to those of the embodiment according to  FIGS.  1  to  12     f  are designated with the same reference numerals, and the descriptions thereof will not be repeated. 
     The locking ring  6 ′ has, at a particular position in the circumferential direction, an extension portion  67  that extends from the first end  6   a  in a direction above the first end. The extension portion  67  may be inclined towards the central axis and may be resilient to some extent. In the illustrated embodiment, the extension member  67  has a substantially rectangular cross-section. 
     The receiving part  5000  is different from the receiving part  5  of  FIGS.  1  to  12     f  in the design of the holding structures at the outer surface of the receiving part. A first holding structure includes a rib  590  that is located above the bottom  53   a  of the substantially U-shaped recess  53  and at a distance from the first end  5   a . The rib  590  extends only over about half a leg  54  from one edge  54   a  of one leg  54  in a circumferential direction up to the through hole  500 . Adjacent to the rib  590 , at the side towards the first end  5   a , is a groove  591  that runs concentrically with the rib  590 . The groove  591  provides space for the free end of the extension portion  67  of the locking ring. Further, a guiding groove  592  is formed that extends from the through hole  500  in the axial direction downward to the second end  5   b  of the receiving part. At a distance from the through hole  500 , a downwardly inclined ramp or obstacle  593  that widens downwardly is provided in the groove  592 . The ramp  593  permits the extension portion  67  to snap over the ramp when moving downward, but prevents the extension portion  67  from moving upward again after passing downwardly past it. The ramp  593  is located at an axial position such that the locking ring is in the pre-locking position when the extension portion  67  has passed downwardly past the ramp  593 . 
     A mounting groove  595  is formed at the receiving part  5000  at a side of the rod channel. In greater detail, the mounting groove  595  extends from the second end  5   b  up to a small distance from the bottom  53   a  of the substantially U-shaped recess  53 . The mounting groove  595  permits the extension portion  67  to extend therein when the locking ring  6 ′ is mounted to the receiving part from the second end  5   b.    
     As shown in  FIGS.  33   a  to  33   d   , the locking ring  6 ′ can be mounted to the receiving part  5000  from the second end  5   b  of the receiving part in an orientation in which the extension portion  67  is aligned with the mounting groove  595 . The extension portion  67  engages the mounting groove  595  and the locking ring  6 ′ is moved upwards as depicted in  FIG.  33   b   . When the extension portion  67  has reached the circumferentially extending groove  591 , the locking ring  6 ′ is rotated until the extension portion  67  snaps into the through hole  500 . In this position, the extension portion  67  engages the bottom of the edge of the through hole  500 , as also shown in  FIGS.  34  and  36   , and is temporarily held there in the insertion position where the head can be inserted. 
     Once the head is inserted into the accommodation space  52  of the head receiving portion, the locking ring  6 ′ can be moved downward until the extension portion  67  snaps over the ramp  593  into the pre-locking position, as shown in  FIGS.  35  and  37   . In the pre-locking position the head cannot be removed from the receiving part. Thereafter, the locking ring can be moved further downward until the head  3  is locked. 
     Additional modifications of the embodiments are also conceivable. The parts are not limited to their detailed shape as depicted in the embodiments. The features of the various embodiments can be combined with each other to produce a number of further embodiments. In particular, the detailed shapes of the passage and the cavity in the receiving part may vary. Also, the receiving part and/or the head may be designed such that the head can only pivot in preferred directions and/or can pivot in one direction to a greater maximum angle than in another direction. For the bone anchoring element, all kinds of bone anchoring elements can be used, such as screws, nails, etc. 
     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.