Patent Abstract:
A locking assembly for securing a rod in a receiving part of a bone anchoring device includes a first locking element having a first end and a second end and a longitudinal axis of rotation and an outer surface provided with an external thread, a coaxial bore passing entirely through said first locking element and an internal thread provided at said bore, a second locking element having a: longitudinal axis of rotation and an outer surface with an external thread cooperating with the internal thread of said first locking element. The first locking element has a recess between the first end and the second end, that defines a circumferentially closed wall portion. The interior of the wall portion has a longitudinally extending structure for engagement with a tool. Furthermore, a tool is provided which has sections which can be independently engaged with the first and second locking element, respectively.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 60/787,901, filed Mar. 31, 2006, and claims priority from European Patent Application EP06006832.7, filed Mar. 31, 2006, the entire disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The invention relates to a locking assembly for securing a rod member in a receiver part connected to a shank for use in spinal or trauma surgery. The invention further relates to a bone anchoring device using such a locking assembly and to a tool for cooperating with such a locking assembly. 
     U.S. Pat. No. 6,224,598 B1 discloses a threaded plug closure adapted for use in securing a rod member to a bone screw implant, said closure comprising a plug having a threaded cylindrically-shaped outer surface, said plug being received between a pair of arms of a medical implant during use, a central coaxial bore passing entirely through said plug, said central bore having an internal threaded surface which is shaped to receive a set screw. The plug closure and the set screw can be independently installed and the set screw tightened to cooperatively provide capture and locking of the rod in order to secure the rod against translational and rotational movement relative to the bone screw. 
     US 2003/0100896 A1 discloses a bone anchoring device with a shank and a receiving part connected to it for connecting to a rod. The receiving part has a recess having a U-shaped cross-section for receiving the rod with two open legs and an internal thread on the open legs. A locking assembly is provided comprising a nut member with an external thread which cooperates with the internal thread of the legs and a set screw. The nut member has on one end slits for engagement with a screw tool. The shank has a spherically shaped head which is pivotably held in the receiving part and a pressure element is provided which exerts pressure on the head when the nut member is tightened. By tightening the set screw the rod is fixed in the receiving part. Hence, the rod and the head can be locked independently from each other. The internal thread and the cooperating external thread of the nut member are designed as a flat thread. The implant has a compact design, since an outer ring or nut to prevent splaying of the legs is not necessary. 
     The outer diameter of the locking assembly is under various aspects determined by the required tightening torque and the thread form. In turn, the overall dimensions of the upper portion of the bone anchoring device are determined by the size of the locking assembly. 
     Therefore, there is a need for a locking assembly and a bone anchoring device with a locking assembly which has the same reliability as the known devices but which has smaller dimensions of the upper portion. Furthermore, there is a need for a tool for such a locking assembly. 
     SUMMARY 
     The locking assembly according to the invention can be designed with a smaller outer diameter compared to the known locking assemblies. Therefore, the size of the bone anchoring device can be reduced. The bone anchoring device with such a reduced size is particularly suitable for application to the cervical spine or other areas where a limited available space requires compact implants. 
     Furthermore, the locking assembly is structured so as to allow nesting of two or more locking elements. 
     With the tool according to the invention a simultaneous but independent fixation of the locking elements of the locking assembly is possible. 
     Further features and advantages of the invention will become apparent and will be best understood by reference to the following detailed description of embodiments taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of an embodiment of the bone anchoring device with the locking assembly. 
         FIG. 2  shows a perspective elevational view of the bone anchoring device of  FIG. 1 . 
         FIG. 3  shows a perspective view from the top of the locking assembly. 
         FIG. 4  shows a cross-sectional view of the locking assembly of  FIG. 3 . 
         FIG. 5  shows a side view of the locking assembly of  FIG. 3 . 
         FIG. 6  shows a perspective view of a tool. 
         FIG. 7  shows a cross-sectional view of the lower part of the tool cooperating with the locking assembly. 
         FIG. 8  shows a perspective view of the locking assembly and the lower part of the tool. 
         FIG. 9  shows a perspective view of the locking assembly with cooperating portions of the tool shown in section wherein the other parts of the tool are omitted. 
         FIG. 10  shows a perspective view of a modification of the first locking element of the locking assembly. 
         FIG. 11  shows a perspective view of a further modification of the first locking element of the locking assembly. 
         FIG. 12  shows a second embodiment of the locking assembly in a top view. 
         FIG. 13  shows a further embodiment of the locking assembly in a perspective view. 
         FIG. 14  shows the locking assembly of  FIG. 13  in a top view. 
         FIG. 15  shows the locking assembly of  FIG. 13  in a sectional view along line A-A of  FIG. 14 . 
         FIG. 16  shows a tool cooperating with the locking assembly of  FIG. 13  in a sectional view. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  show the locking assembly used in a polyaxial bone anchoring device  1 . The bone anchoring device comprises a bone screw  2  having a shank  3  with a bone thread and a spherically-shaped head  4 . The bone screw  2  is received in a receiving part  5  which has a first end  6  and a second end  7  and is of substantially cylindrical construction. The two ends are perpendicular to a longitudinal axis L. Coaxially with the longitudinal axis L a bore  8  is provided which extends from the first end  6  to a predetermined distance from the second end  7 . At the second end  7  an opening  9  is provided, the diameter of which is smaller than the diameter of the bore  8 . The coaxial bore  8  tapers towards the opening  9 . In the embodiment shown it tapers in form of a spherically shaped section  10 . However, the section  10  can have any other shape such as, for example, a conical shape. 
     The receiving part  5 , further, has a U-shaped recess  11  which starts at the first end  6  and extends in the direction of the second end  7  to a predetermined distance from said second end. By means of the U-shaped recess two free legs  12 ,  13  are formed ending towards the first end  6 . Adjacent to the first end  6 , the receiving part  5  comprises an internal thread  14  at the inner surface of the legs  12 ,  13 . In the embodiment shown, the internal thread  14  is a flat thread having horizontal upper and lower thread flanks. 
     Additionally, a pressure element  15  is provided which has a substantially cylindrical construction with an outer diameter sized so as to allow the pressure element  15  to be introduced into the bore  8  of the receiving part and to be moved in the axial direction. On its lower side facing towards the second end  7 , the pressure element  15  comprises a spherical recess  16  cooperating with a spherical section of the head  4 . On its opposite side the pressure element  15  has a U-shaped recess  17  extending transversely to the longitudinal axis L by means of which two free legs  18 , 19  are formed. The lateral diameter of this U-shaped recess is selected such that a rod  20  which is to be received in the receiving part  5  can be inserted into the recess  17  and guided laterally therein. The depth of this U-shaped recess  17  is larger than the diameter of the rod  20  so that the legs  18 ,  19  extend above the surface of the rod  20  when the rod is inserted. 
     The bone anchoring device comprises a locking assembly  30 . The locking assembly  30  includes, as shown in particular in  FIGS. 2 to 5  a first locking element  31  and a second locking element  32 . The first locking element has a first end  33  and a second end  34  and a substantially cylindrical shape between the first and the second end and with an outer surface having an external thread  35  which is, in the embodiment shown, a flat thread which matches with the internal thread  14  of the receiving part  5 . Further, the first locking element comprises a coaxial bore  36  extending from the second end  34  in the direction of the first end  33 . The coaxial bore  36  comprises an internal thread, which is in the embodiment shown a metric thread. The first locking element  31  further comprises a coaxial recess  37  starting from the first end  33  and extending to a predetermined distance from the second end  34 . The mean diameter of the recess  37  is larger than the diameter of the coaxial bore  36 . As can be seen in particular in  FIG. 3 , by means of the recess  37  a substantially ring-shaped wall is formed. A plurality of longitudinal grooves  38  are formed extending from the first end  33  along the wall to the bottom  39  of the recess  37 . The grooves  38  shown in this embodiment have an approximately semi-circular cross section. They are equidistantly distributed in a circumferential direction of the recess  37 . Preferably, at least two grooves are formed. The wall of the recess  37  can have a slanted surface  40  adjacent to the first end  33  in order to facilitate the introduction of a tool. The depth of the recess  37  is selected such that the length of the bore  36  is still sufficient to cooperate with the second locking element  32  for a good fixation. On the other hand the depth of the recess  37  is such that an area sufficient for engagement with a tool is provided. 
     The second locking element  32  is shaped as a set screw with an external thread  42  cooperating with the internal thread of the coaxial bore  36 . The axial length of the second locking element  32  is such that when the second locking element  32  is completely screwed into the first locking element  31  it projects slightly from the second end  34  of the first locking element. As can be seen in particular in  FIG. 4 , the second locking element  32  comprises a coaxial recess  43  with grooves  44  extending in longitudinal direction, similar to the recess  37  and the grooves  38  of the first locking element. The recess  43  and the grooves  44  serve for a form-fit cooperation with a tool to be described hereinafter. 
     A tool for cooperating with the locking assembly is shown in  FIGS. 6 to 9 . The tool  50  comprises a tube  51  and a bar  52  which is slidable in the tube  51 . The tube  51  has an end section  53  for cooperation with the locking assembly and a second end with a grip portion  58  which has, for example, a hexagonal outer shape. As can be seen in particular in  FIGS. 7 to 9 , the end section  53  has a reduced outer diameter, corresponding to the inner diameter of the recess  37  of the first locking element. The end section  53  comprises a plurality of projections  54  the number of which is less than or equal to the number of the grooves  38  of the first locking element. The projections  54  are structured and designed to engage with the grooves  38  of the first locking element to provide a form-fit connection between the end section  53  of the tool and the recess  37  of the first locking element. The axial length of the end section  53  is preferably equal to or larger than the depth of the recess  37 . 
     The bar  52  comprises an end section  55  which is structured and designed to cooperate with the recess  43  of the second locking element  32 . For this purpose, the end section  55  has a plurality of projections  56  the number of which is equal to or less than the number of grooves  44  of the second locking element and which are structured and designed to engage with the grooves  44 . On its opposite end, the bar  52  has a grip portion  57  which allows to grip the bar  52  and to rotate it independently from the tube  51 . The length of the bar  52  is selected such that when the end section  53  of the tube is engaged with the first locking element, the second locking element  32  can be independently engaged by the end section  55  of the bar and screwed into the first locking element. 
     In operation, first, at least two usually preassembled bone anchoring devices comprising the bone screw  2 , the receiving part  5  and the pressure element  15  are screwed into the bone. Thereafter, the rod  20  is inserted into the U-shaped recess  11  of the receiving part  5 . Then, the locking assembly  30 , comprising the first locking element  31  and the second locking element  32  which are preferably preassembled, is screwed-in between the legs  12 ,  13  of the receiving part  5 . The first locking element is tightened by applying the tool  50  such that the end section  53  of the tube engages with the recess  37  and the grooves  38  of the first locking element to form a form-fit connection. In this way, pressure is exerted by the lower side of the first locking element onto the free legs  18 ,  19  of the pressure element which presses onto the head  4  of the bone screw  2  to lock the head in its rotational position relative to the receiving part  5 . 
     Then, the second locking element  32  is tightened by application of the tool in that the end section  56  of the bar engages the recess  43  of the second locking element and torque is applied. In this way, the position of the rod  20  relative to the receiving part is fixed. 
     A fine tuning of the position of the receiving part  5  relative to the bone screw  2  and of the rod  20  relative to the receiving part can be performed by loosening either the first locking element  31  or the second locking element  32 . 
       FIG. 9  shows a perspective view of the locking assembly  30  with the end sections  53  and  56  of tool engaging the locking elements with the tool shown in section. For the purpose of illustration only, the remainder of the tool is not shown. As can be seen in  FIG. 9 , the end sections of the tool and the recesses of the first and second locking elements form a form-fit connection for the application of torque to screw-in the locking elements. The external thread  35  of the first locking element is of continuous form, without recesses or interruptions. Therefore, the dimension of the locking element can be reduced. This guarantees safe locking of the first locking element. The area required for engagement of the tool with the first locking element is located within the recess  37 . This allows to design the first locking element  31  with a reduced diameter. If the number of grooves  38  is increased, the depth of the grooves can be reduced. Hence, the size of the first locking element can be reduced corresponding to the increase of the number of grooves. 
     The size of the outer dimension of the first locking element  31  determines the size of the receiving part and the other elements of the bone anchoring device. Further, since the external thread of the first locking element remains intact over its whole length, the height of the locking element can be reduced. 
       FIG. 10  shows the first locking element  31  with a modified shape of the grooves. The grooves  38 ′ have a triangular cross section.  FIG. 11  shows the first locking element  31  with a further modification of the shape of the grooves. The grooves  38 ″ have a square cross section. However, the cross section of the grooves may have another shape as well. 
       FIG. 12  shows a second embodiment of the locking assembly. The locking assembly  300  comprises three locking elements. The first locking element  301  is shaped like the locking element  31  of the first embodiment. The second locking element  302  differs from the second locking element  30  of the first embodiment in that is has coaxial threaded bore, like the first locking element, to receive the third locking element  303 . It is also possible to design the locking assembly with more than three nested locking elements such that each of the locking elements has a threaded coaxial bore to receive a further locking element. In this manner, for certain applications, an improved fixation can be achieved, for example, if used in complex minimally invasive surgery procedures. 
     The locking assembly according to a further embodiment includes one single locking element  400  which has an external thread  401  and a coaxial bore  402  extending through the entire locking element from the first end  403  to the second end  404 . A coaxial ring-shaped recess  405  extends from the first end  403  in the direction of the second end. The wall of the recess comprises a plurality of longitudinal grooves  406  for engagement with a tool. By means of the recess  405  and the coaxial bore  402 , a coaxial hollow cylindrical section  407  is formed in the locking element. 
     A tool for engagement with the locking element is adapted to be engageable with the ring-shaped recess  405 .  FIG. 16  shows an exemplary tool  500  cooperating with the locking element  400 . The end section  501  comprises a ring-shaped projection  502  adapted to engage the recess  405  in a form-fitting manner. The end section also  501  comprises a central projection  503  with a retaining spring  504  engaging the coaxial bore  402  for facilitating alignment and handling of the locking element. 
     The locking element can be used in such applications where it is necessary to introduce an instrument or a wire through the bore  402 , for example in the case of minimally invasive surgery. 
     Further modifications are possible. The external and the internal thread can have any thread shape, such as, for example, a metric thread. Using a flat thread, a saw-tooth thread or a negative angle thread for the external thread of the first locking element and the cooperating internal thread of the receiving part, however, has the advantage that it prevents splaying of the legs of the receiving part. Therefore, an outer ring or a nut to prevent splaying is not needed. By using the locking assembly of the invention together with a flat thread as the external thread, the implant can be further downsized. 
     The number of the grooves and the shape of the grooves can vary. 
     It is conceivable to design the first locking element with a recess having a quadrangular or hexagonal or otherwise polygonal cross section with or without grooves. In this case, the end section of the tool has a matching shape. This also provides for a form-fit connection between the tool and the first locking element with the external thread of the first locking element remaining intact. 
     The second locking element  32  or the third locking element  303  in the case of the locking assembly  300  of the second embodiment or, in general, the inmost locking element in the case of a locking assembly having multiple locking elements may not need to have a recess with grooves as shown in the embodiment. It is sufficient, that the inmost locking element has a recess for engagement with screwing-in tool, such as a hexagon recess. The corresponding end section of the tool is then adapted to this shape. 
     The disclosure is not limited to the polyaxial bone anchoring device as shown in the first embodiment. It can be used in the case of a monoaxial bone anchoring device in which the receiving part is fixedly connected to the shank of the bone screw as well. Furthermore, the polyaxial bone anchoring device can have a different construction. It is possible to have a design of the receiving part which allows that the screw is inserted from the bottom instead from the top of the receiving part. 
     The locking assembly can also be used in such kind of bone anchoring devices in which the receiving part is designed and structured so that the rod is fixed laterally apart from the central axis of the bone screw.

Technology Classification (CPC): 5