Abstract:
A bone anchoring device includes an anchoring element having a shaft and a head; a receiving part for receiving the head and a rod, the receiving part being formed in one piece and having a top end and a bottom end, a bore extending from the top end to the bottom end, an inner wall defined by the bore, a substantially U-shaped channel adjacent to the top end for receiving the rod and a seat adjacent to the bottom end for accommodating the head; a pressure element; and a spring element having a spring force. The anchoring element is inserted from the top end and is pivotable with respect to the receiving part. The pressure element is movable in the bore so as to be able to fix the anchoring element at an angle with respect to the receiving part by exerting pressure onto the head. The spring element acts between the pressure element and the receiving part and allows the pressure element to be introduced from the top end but withstands against removal of the pressure element through the top end.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/890,341, filed Sep. 24, 2010, now U.S. Pat. No. 8,764,810, which claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/245,940, filed Sep. 25, 2009, and claims priority from European Patent Application Number EP09171416.2, filed Sep. 25, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The invention relates to a bone anchoring device for anchoring a stabilization rod in a bone or in a vertebra. The bone anchoring device includes an anchoring element, a receiving part for receiving a head of the bone anchoring element and for receiving a stabilization rod to be connected to the anchoring element. The anchoring element is pivotably connected to the receiving part and can be fixed at an angle by exerting pressure onto the head via a pressure element that is arranged in the receiving part. The anchoring element and the pressure element are to be inserted from a top end of the receiving part and a spring element withstands against removal of the pressure element through the top end once the pressure element is inserted. 
     U.S. Pat. No. 5,716,356 describes a polyaxial bone screw including a screw element and a receiving part that is pivotably connected to the screw element and a pressure element to exert pressure onto the head of the screw element to fix the angle between the screw element and the receiving part. The receiving part as an U-shaped channel for receiving a stabilization rod. The pressure element comprises a cylindrical recess that is to be aligned with the U-shaped channel to receive the rod therein. In order to hold the pressure element in a position aligned with the U-shaped channel, the position of the pressure element is fixed by crimping through bores provided in the receiving part. 
     US 2005/0080420 A1 describes a multi-axial bone screw that includes a screw element and a receiver member, a base member for retaining the screw element in the receiver member and a crown element exerting pressure onto the head of the screw element. The crown element comprises a snap-ring assisting in holding the crown element within the receiving part by interfering with a stop surface of the receiving part. The multi-axial bone screw is a so-called bottom loading type screw wherein the screw element is introduced from the bottom into the receiving part. 
     WO 2006/116437 A2 describes a bone anchor for spinal fixation in form of a polyaxial bone screw including a screw element, a housing, a sleeve and a collet arranged in the housing for exerting pressure onto the head of the screw element. The head of the screw element is introduced through the bottom of the housing. The sleeve has retention tabs that snap into slots in opposite wall portions of the housing. Alternatively, the retention tabs may be formed on the housing and the slots may be formed on the sleeve. 
     Usually, the polyaxial bone screws of the above described type are delivered, for example by the manufacturer, in a pre-assembled condition. In this condition a specific screw element, e.g. a screw element having a specific length and shaft diameter of a specific thread form, is connected to receiving part and the pressure element is arranged therein so that it can not escape. For the surgery, the necessary number and types of such pre-assembled polyaxial bone screws are selected and provided in advance as a full set of implants. 
     When the known polyaxial bone screws are in the pre-assembled condition, the pressure element is in a position that enables the head to freely pivot within the receiving part. In certain situations during surgery such an unrestrained pivoting is not desirable. 
     SUMMARY 
     It is the object of the invention to provide a bone anchoring device wherein parts can be selected and assembled by the surgeon or by any other personnel in a simple manner at the operation site or at any other place after the parts have been manufactured. In addition, the bone anchoring device shall provide an improved handling during surgery. 
     The bone anchoring device has only few parts. The parts are of a simple design. This provides for a lower cost manufacturing and a convenient handling. It is possible to assemble the bone anchoring device in any condition of the delivery after the parts have been manufactured and before the screw element is inserted into the bone. Therefore, the assembling of the polyaxial bone screw can be carried out by anybody, in particular by the surgeon or any personnel assisting him or her before or during surgery. 
     With the bone anchoring device a modular system can be provided, which allows the combination of various anchoring elements with any suitable receiver on demand, depending on the actual clinical requirements. This reduces the costs of polyaxial screws, reduces the inventory and gives the surgeon a substantial choice of implants. In addition, existing receiving parts may be upgraded to form the bone anchoring device according to the invention. 
     With the bone anchoring device it is possible to exert a slight biasing force onto the head of the screw element in the pre-assembled condition, so that a force, which can be applied by hand, for example, is needed to pivot the head. In certain situations, this improves the handling of the bone anchoring device during surgery. Different pressure elements can be provided so that the degree of biasing the pressure element against the head can be selected by selecting an appropriate pressure element when assembling the bone anchoring device. 
     Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective side view of a first embodiment of the bone anchoring device. 
         FIG. 2  shows a perspective side view of the bone anchoring device of  FIG. 1  in an assembled state. 
         FIG. 3  shows a sectional view of the bone anchoring device of  FIG. 2 , the section being taken perpendicular to the rod axis. 
         FIG. 4  shows an enlarged view of a portion of  FIG. 3 . 
         FIG. 5  shows a perspective view of the snap-ring of the first embodiment. 
         FIG. 6  shows a top view of the snap-ring of the first embodiment. 
         FIG. 7  shows a sectional view of the snap-ring of  FIG. 6  along line A-A in  FIG. 6 . 
         FIG. 8  shows a perspective view of a modified embodiment of the snap-ring. 
         FIGS. 9-11  show steps of assembling the bone anchoring device according to the first embodiment in a sectional view, respectively. 
         FIG. 12  shows a perspective exploded view of a bone anchoring device according to a second embodiment. 
         FIG. 13  shows a perspective side view of the bone anchoring device of  FIG. 12  in an assembled state. 
         FIG. 14  shows a perspective view of the receiving part of the second embodiment according to  FIGS. 12 and 13 . 
         FIG. 15  shows a sectional view of the bone anchoring device according to the second embodiment of  FIG. 13  in a sectional view, the section being taken perpendicular to the rod axis. 
         FIG. 16  shows an enlarged view of a portion of the bone anchoring device of  FIG. 15 . 
         FIG. 17  shows the step of assembling the bone anchoring device according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The bone anchoring device  1  according to a first embodiment shown in  FIGS. 1 to 4  includes a bone anchoring element, in this case a screw member  2  having a threaded shaft  3  and a head  4 . The head  4  is, for example, shaped as a spherical segment. The head  4  has a recess  4 ′ at its free end for engagement with a tool. The bone anchoring device  1  further includes a receiving part  5  for connecting the screw member  2  to a rod  20 . A pressure element  6  is arranged in the receiving part  5  on top of the head  4 . For securing the rod  20  in the receiving part  5  and for exerting pressure onto the pressure element  6 , a locking device in form of an inner screw  7  is provided. It should be understood that the locking device shown is exemplary and can be realized in many other designs, in particular as a two-part locking device and with different thread form. In addition, a spring element is provided in the form of snap-ring  8 , which serves for holding the pressure element  6  within the receiving part  5 . 
     The receiving part  5 , as shown in particular in  FIGS. 1 to 3 , is formed in one piece. In the example shown, it is substantially cylindrical. It has a top end  51  and a bottom end  52  and a plane of symmetry M. A coaxial bore  53  extends from the top end  51  to the bottom end  52 . The diameter of the bore  53  becomes smaller towards the bottom end  52 , thereby providing a seat  54  for accommodating the screw head  4 . The shape of the seat  54  can be spherical, conical or otherwise tapering. It can have any other shape that allows to accommodate the head  4  so that it can pivot with respect to the receiving part  5 . The diameter of the bore  53  at the bottom end is sized such that the threaded shaft  3  can pass therethrough. The diameter of the bore  53  in the middle portion and until the top end  51  is sized such that the threaded shaft  3  and the head  4  of the screw element  2  can pass therethrough. At the top end  51 , the receiving part has a substantially U-shaped recess  55  by means of which a channel is formed for receiving the rod  20 . An internal thread  56  is provided near the top end  51  for cooperating with the inner screw  7  of the locking device. 
     The pressure element  6  is formed in one piece. It is of substantially cylindrical design construction and has an outer diameter that allows it to be inserted from the top end  51  and to be moved in axial direction within the coaxial bore  53  of the receiving part  5 . On its side facing the head  4  of the screw element  2 , a spherical recess  61  is provided that is adapted to the size of the head  4 . Any other shapes of the underside of the pressure element  6  are conceivable, such as a flat surface or any other structure, however, a surface that is adapted to the head  4  of the screw element  2  provides a homogenous load distribution onto the head  4 . On its side opposite to the head  4 , the pressure element  6  has a cylindrical recess  62  adapted to receive the rod  20  therein. By means of this recess  62 , a channel to receive the rod is formed. In the embodiment shown, the depth of the recess  62  is smaller than the diameter of the rod so that the inner screw  7  can come into contact with the rod  20 . A coaxial bore  63  extends through the pressure element to allow access to the recess  4 ′ with a tool for screwing-in the screw element  2 . 
     The pressure element  6  further comprises a circumferential groove  64  that is provided in a portion of the outer surface of the pressure element below the cylindrical recess  62 . The cross-section of the groove  64  is rectangular, however, it can also be square-shaped or can have rounded corners. It can also have another shape, for example, it can be asymmetric. The diameter of the groove  64  is such that it can accommodate a portion of the snap-ring  8 . By means of the slot  83 , the snap ring is resilient and can vary in its diameter. 
     The receiving part  5  comprises a corresponding circumferential groove  57 , which is located below the bottom of the channel  55  so that when the snap-ring  8  is inserted, it is located below the rod  20 . The groove  57  is sized so as to allow accommodation of a portion of the snap-ring  8 . Like the groove  64  in the pressure element  6 , the groove  57  in the receiving part  5  has a rectangular cross-section, a square-shaped cross-section or can have rounded corners. It can also have another shape, for example, it can be asymmetric. 
     The snap-ring  8  is described with reference to  FIGS. 5 to 7 . The snap-ring according to the first embodiment is circular. It has a top side  81 , a bottom side  82  and a slot  83  extending completely through the snap-ring from the top side to the bottom side. The inner diameter of the snap-ring is smaller at the bottom side  82  than at the top side  81  and the cross-section of the snap-ring is substantially trapezoidal and an inclined surface  84  is formed on the top side  81 . By means of this, the snap-ring is shaped asymmetrical in the direction of movement of the pressure element. The position of the groove  57  is such that when the snap-ring  8  is inserted in the groove  57  and the pressure element  6  is then inserted so that the snap-ring engages the groove  64  of the pressure element  6 , the pressure element  6  is in a holding position that still allows pivoting of the head. The inner diameter of the snap-ring  8  is such that if the snap-ring is located in the groove  57 , it can expand therein when the pressure element  6  is introduced until it snaps into the groove  64  of the pressure element. 
     The inner screw  7  of the locking device can be screwed into the internal thread  56  of the receiving part  5  until the inner screw  7  presses onto the upper side of the pressure element  6 , which moves the pressure element  6  downward until it presses onto the head  4  to lock the head  4  in the receiving part  5 . 
     The parts of the bone anchoring device are all made of a body-compatible material, such as titanium or stainless steel, a body-compatible metal alloy, for example a Ti—Ni alloy, or a body-compatible plastic material, such as PEEK. In particular, the snap-ring  8  can be made of a body-compatible plastic material that allows for an easy manufacture and a safe function of the spring element. 
       FIG. 8  shows a modified example of a snap-ring  8 ′. A snap-ring  8 ′ is not circular but has the shape of a star. It also has a slot  83 ′ and inclined surfaces  84 ′ that are preferably located in the valleys between the peaks of the star. The star-shaped snap-ring  8 ′ can be made more flexible than the circular snap-ring  8 . Other shapes are also conceivable, for example other wave-shaped rings with peaks and valleys. 
     In a further modification, the cross-section of the snap-ring can be symmetric in the direction of insertion and the cross-section of the grooves can be asymmetric. 
       FIGS. 9 to 11  show the steps of assembly of the bone anchoring device. In a first step, shown in  FIG. 9 , the snap-ring  8  is introduced into the receiving part  5  until it rests in the groove  57 . The orientation of the snap-ring  8  is such that the inclined surface  84  faces the top end  51  of the receiving part  5 . The orientation of the snap-ring  8  in a circumferential direction is such that the slot  83  can be positioned at around 90° with respect to the longitudinal axis of the channel  55 . 
     In a next step, as shown in  FIG. 10 , the screw element  2  and the pressure element  6  are inserted into the receiving part  5  from the top end  51 , so that the threaded shaft  3  passes through the bore  53  at the second end  52 . 
     Then, as shown in  FIG. 11 , the pressure element  6  and the head  4  are moved downward toward the second end  52 , thereby passing the snap-ring  8  that projects radially into the bore  53 . Since the inclined surface  84  faces the top end  51  of the receiving part, the insertion force for the screw head  4  and the pressure element  6  is reduced. The screw head  4  and a lower edge of the pressure element  6  slide along the inclined surface  84 , thereby expanding the snap-ring  8 . The pressure element  6  is pushed downwards until the snap-ring  8  snaps into the groove  64  of the pressure element  6 . Once the snap-ring is accommodated in the groove  64 , the force necessary for removing the pressure element  6 , i.e. removing the snap-ring from the groove  64 , is increased since the upper part of the grooves  64 ,  57  act as a stop. Therefore, the pressure element  6  cannot fall out and can not be removed through the first end  51  without using a specific tool. Hence, the assembly of the pressure element  6  and the screw element  2  is unidirectional. 
     The position of the snap-ring  8  can be selected such that the pressure element  6  exerts a slight biasing force onto the head  4  in the pre-assembled condition. In this way, different pressure elements, for example, a first pressure element with a groove that is located higher than that of a second pressure element can be provided. When assembling the bone anchoring device, the appropriate pressure element to have a desired amount of biasing force can be selected. 
     Receiving parts and pressure elements of existing polyaxial screws can be upgraded with the grooves and the snap-ring. 
     In use, after assembly of the bone anchoring device  1 , the screw element  2  is inserted into the bone. Thereafter, the receiving part  5  is pivoted until it has the correct orientation to take up the rod  20 . The rod  20 , which connects several bone anchoring devices, is inserted and the inner screw  7  is tightened to move the pressure element  6  downward to clamp the head  4  such that it is locked. Simultaneously the rod  20  is fixed by the inner screw  7 . Since the inclined surface  84  faces the first end  51 , the force needed for further pressing down the pressure element  6  is small because the upper edge of the groove  64  of the pressure element  6  slides along the inclined surface  84 . 
     A second embodiment is shown with respect to  FIGS. 12-17 . The second embodiment differs from the first embodiment in that the spring element is not provided as a snap-ring but is provided as a portion of a receiving part. The description of features of the second embodiment that are the same as features of the first embodiment is not repeated. A receiving part  5 ′ has on its two opposite sides longitudinal recesses  58 , in which resilient fingers  59  are arranged. The resilient fingers  59  extend from the bases of the recesses  58  that are facing the second end  52 , in the direction of the first end  51  and are flexible in a direction into and out of the recesses  58 , respectively. Hence, the recesses  58  provide a space for taking up the resilient fingers  59  when they are pressed outward by a pressure element  6 ′. At their end facing the first end, the fingers  59  have a stop surfaces  59   a , respectively, extending substantially perpendicular to the plane M. On the opposite side of each stop surface  59   a  there is an inclined surface  59   b  that faces the first end  51 . The surface  59   b  is inclined downward to provide a sliding face along which the pressure element  6 ′ can slide. The inner diameter between the resilient fingers  59  in the region of the stop surface  59   a  is smaller than the outer diameter of the pressure element  6 ′. 
     The pressure element  6 ′ is shaped like the pressure element  6  of the previous embodiment, however, without the groove  64 . An upper side  65  of the pressure element  6 ′ comes into contact with the stop surface  59   a  of the resilient fingers  59 . 
     The steps of assembly of the bone anchoring device according to the second embodiment include first providing the receiving part  5 ′ and then inserting the screw element  2  with pressure element  6 ′, as shown in  FIG. 17 , from the top end  51  of the receiving part  5 ′. The pressure element  6 ′ and the head  4  are moved downward until the pressure element  6 ′ contacts the inclined upper surface  59   b  of the resilient fingers  59 , thereby pressing the resilient fingers  59  outward into the recesses  58  respectively. Since the inclined surface  59   b  faces the first end  51 , the insertion force is small. When the pressure element  6 ′ has passed the resilient fingers  59 , the resilient fingers  59  snap back toward the bore  53  and removal of the pressure element  6 ′ in the direction to the first end  51  is prevented by the stop surfaces  59   a . Therefore, the assembly of the pressure element  6 ′, the screw element  2  and the receiving part  5 ′ is also unidirectional. 
     Further modifications of the embodiments are conceivable. For example, for the anchoring element, all kinds of anchoring elements can be used and combined with a receiving part. These anchoring elements are e.g. screws of different length, with different diameters, cannulated screws, screws with different thread forms, nails, etc. 
     All kinds of receiving parts can be used, in particular such with different locking elements, e.g. such as two-part locking devices that have an outer screw and an inner screw, which lock the rod and the head separately. In this case, the pressure element has a channel for the rod the depth of which is larger than the diameter of the rod. Other locking devices such as outer nuts, outer caps, bayonet locking devices or others are conceivable. The shape of the receiving part is not limited to the embodiments shown. For example, the receiving part can have an asymmetric end portion for allowing a greater pivot angle of the screw member to one side. 
     The shape of the pressure element is also not limited to the embodiments shown. For example, the pressure element can be designed so as to exert pressure onto the head from the side of the head, e.g. via a conically tapering portion and/or can have a slotted portion clamping the head or can have other shapes.