Abstract:
A bone anchoring assembly for use in spinal surgery or trauma surgery includes an anchoring element having a shaft and a receiving part having a substantially U-shaped cross-section with a base connected to said shaft and two free legs forming a channel for receiving a rod having an at least partially structured surface. The bone anchoring assembly further includes a locking member cooperating with the receiving part for locking said rod in the channel. At least on the locking member or in the receiving part an engagement portion for engaging said structured surface of the rod is provided. The locking member, the rod and the receiving part cooperate in such a manner that as soon as the engagement portion comes into engagement with the structured surface the rod is held in the receiving part in a form-locking manner preventing movement of the rod.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/758,342, filed Jan. 11, 2006, and claims priority from EP 06000496.7, filed Jan. 11, 2006, the disclosures of which are incorporated by reference herein. 
     
    
     BACKGROUND  
       [0002]     The invention relates to a bone anchoring assembly for use in spinal surgery or trauma surgery. In particular, the invention relates to a bone anchoring assembly comprising at least two bone anchoring elements with a receiving part to receive a rod and such a rod for connecting the bone anchoring elements wherein the rod is fixed in the receiving part by means of a form-locking connection.  
         [0003]     U.S. Pat. No. 5,207,678 discloses a pedicle screw comprising a screw and a receiver member to connect the screw with a rod. The rod is a threaded rod which is locked in the receiver member by means of two nuts engaging the outside of the receiver member and being adjustable on the threaded rod. For re-adjusting the position of the rod relative to the receiver member the nuts are rotated on the threaded rod. This procedure is time consuming since always two nuts have to be adjusted.  
         [0004]     U.S. Pat. No. 5,443,467 discloses a bone anchoring assembly comprising a bone anchoring element with a receiving part for receiving a rod and an inner screw and an outer lock nut for securing and fixing the rod in the receiving part. The rod has a smooth surface. The fixation of the rod is achieved by means of frictional locking. Even in a case in which a temporary locking of the rod is desired for allowing a re-adjustment in a later step, a considerable torque has to be applied to achieve a sufficient clamping force.  
         [0005]     U.S. Pat. No. 5,716,356 discloses an anchoring member which is to be connected with a threaded rod. The anchoring member has a shaft to be anchored in the bone and a receiving part with a U-shaped cross-section to receive the rod. By means of the recess two open legs are formed which are provided with an external and an internal thread. A nut member is provided which is to be screwed on the external thread and a locking member is to be screwed between the legs. Either the nut member or the locking member or both have a projection which engages with the threaded rod. The anchoring member further comprises a pre-loaded spring member on the bottom of the U-shaped recess acting onto the rod in order to, on the one hand, obtain a rigid connection between the anchoring member and the rod and, on the other hand, to allow an easy manipulation for precisely adjusting the position between the anchoring member and the rod before finally locking the position. Due to the presence of the spring member the rod is still displaceable in its longitudinal direction if the locking member is not yet fully tightened. Also due to the presence of the spring member, the connection between the rod and the receiving part always has a frictional-locking contribution requiring a large torque to be applied for sufficient clamping.  
         [0006]     U.S. Pat. No. 5,961,517 discloses an adjustment tool for adjusting the position of the rod in such an anchoring member.  
         [0007]     Based on the foregoing, there is a need for a bone anchoring assembly which allows a quick and precise adjustment and re-adjustment of the position of the rod relative to the anchoring member without having to use large torque forces.  
       SUMMARY OF THE INVENTION  
       [0008]     A bone anchoring assembly according to the disclosure provides a large clamping force on the rod by the application of only a small torque via the locking element when compared to the fixation of a rod via frictional locking. This facilitates a temporary fixation of the rod via the application of small torques on the locking element during the process of adjusting and re-adjusting the position of the rod. Due to a form-locking connection, the temporary fixation has sufficient strength to withstand loosening when manipulations are carried out on the rod. Further, the temporary fixation can be loosened quickly by only slightly turning back the inner screw. Therefore, the bone anchoring device is suitable in the application of correcting spinal deformities such as scoliosis which during surgery require re-adjusting the position of the rod several times until it is finally locked.  
         [0009]     If the bone anchoring assembly comprises a ratchet-type rod, a lower torque for fixing is required to achieve the same holding force as compared to a threaded rod or a smooth rod.  
         [0010]     Since a pre-fixation of the rod is achieved with the application of a lower torque, a splaying of the legs of the receiving part is reduced. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Further features and advantages of the invention will become apparent and will be best understood by reference to the following detailed description taken into conjunction with the accompanying drawings.  
         [0012]      FIG. 1  is a perspective view of a bone anchoring device according to a first embodiment in the assembled state.  
         [0013]      FIG. 2  is a side view of the rod and the inner screw of the bone anchoring device of  FIG. 1 .  
         [0014]      FIG. 3  is a sectional view of the bone anchoring device of  FIG. 1  the section taken along the rod axis.  
         [0015]      FIG. 4  is enlarged view of detail of  FIG. 3 .  
         [0016]      FIG. 5  is a sectional view of the inner screw of  FIG. 2 .  
         [0017]      FIG. 6  is a perspective view of the receiving portion of the bone anchoring device of FIGS.  1  to  4  seen from above.  
         [0018]      FIG. 7  is a top view of the receiving portion shown in  FIG. 6 .  
         [0019]      FIG. 8  shows a perspective view of a further modification of the receiving portion shown in  FIG. 6 .  
         [0020]      FIG. 9  shows a top view of the receiving portion shown in  FIG. 8 .  
         [0021]      FIG. 10   a  shows a perspective view of a modification of the receiving portion.  
         [0022]      FIG. 10   b  shows a top view of the receiving portion shown in  FIG. 10   a.    
         [0023]      FIG. 11  shows a side view of the rod and the inner screw for a modification of the bone anchoring device according to the first embodiment.  
         [0024]      FIG. 12  shows a detail of a sectional view with the modified rod similar to the section shown in  FIG. 4 .  
         [0025]      FIG. 13  shows an exploded perspective view of a bone anchoring device according to a second embodiment.  
         [0026]      FIG. 14  shows a sectional view of the bone anchoring device of the second embodiment in a section along the longitudinal axis of the rod.  
         [0027]      FIG. 15  shows a detail of  FIG. 14 .  
         [0028]      FIG. 16  shows perspective view of the pressure element of the bone anchoring device of FIGS.  13  to  15 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     The first embodiment of the anchoring assembly shown in FIGS.  1  to  7  comprises a bone anchoring element  1  having a threaded shaft  2  and a receiving part  3 . The receiving part  3  is substantially cylindrical and has a U-shaped recess  4  with a bottom  5  at the side of the threaded shaft. By means of the U-shaped recess  4 , two free legs  6 ,  7  are formed. A bore having an internal thread  8  is provided within the channel formed by the U-shaped recess  4  and formed coaxially with the center axis of the threaded shaft  2 . The internal thread  8  can have any known thread shape. A flat thread or a negative angle thread however can be used so as to prevent splaying of the legs  6 ,  7 .  
         [0030]     The anchoring assembly further comprises a longitudinal rod  9  with a structured surface. In the embodiment shown, the rod has a circular cross section. The structure is a ratchet-type structure comprising a plurality of circumferential grooves  10  separated by circumferential crests  11 .  
         [0031]     In order to allow the anchoring of the rod  9  in the receiving part, a locking member  12  is provided. The locking member  12  in this embodiment is an inner screw  12  or set screw which is to be screwed in the internal thread  8  between the legs  6 ,  7 . The inner screw  12  comprises a ring shaped projection  13  on its side facing the rod as shown in FIGS.  2  to  5 . The dimensions of the ring shaped projection  13  are selected as a function of the dimensions of the grooves  10  of the rod  9  so that the ring-shaped projection  13  engages the grooves  10  in a position screwed onto the rod  9 , as shown in  FIG. 3  and  4 . Preferably, the shape of the grooves  10  and the corresponding projection  13  is round.  
         [0032]     As shown in  FIGS. 3, 4 ,  6  and  7 , the receiving part  3  has on the bottom  5  of its U-shaped recess  4  a structure which can engage the structure of the surface of the rod  9 . In the embodiment shown, the structure comprises a plurality of projections  14  separated by incisions  15  which extend circumferentially in a direction transverse to the longitudinal axis L of the channel which is defined by the recess  4 . The projections  14  have dimensions which are selected so as to allow engagement with the grooves  10  of the rod  9 . The number of projections  14  depends on the dimensions of the grooves  10  and the available space on the bottom of the recess  4 .  
         [0033]     As shown in particular in  FIGS. 6 and 7 , the receiving part  3  further can have a central recess  16  on the bottom of the channel formed by the U-shaped recess  4 . By means of the recess  16  the projections  14  are divided into two groups located on the right side and the left side of the recess  16 , respectively. This arrangement allows the use of a bent rod as compared to the case in which the projections are provided throughout the bottom  5  of the channel.  
         [0034]     The incisions  15  between the projections  14  of the structure shown in  FIGS. 6 and 7  are fine or sharp. In a modification, the incisions  15  have a width so as to form valleys between the projections  14 . The valleys can be rounded or shaped otherwise. Similarly the structure of the ratchet-type rod  9  can have sharp crests  11  or rounded or otherwise shaped crests  11 . The structure used for the ratchet-type rod  9  and the corresponding ring-shaped projection  13  of the inner screw and for the bottom of the receiving part  3  is selected according the material used and/or according to the desired forces acting when the structures come into engagement.  
         [0035]     Similarly, the number of projections  14  on the bottom  5  of the receiving part  3  is selected according to the specific requirements. As shown in  FIGS. 8 and 9 , it is possible to provide only one single projection  14 ′ on each side of the recess  16 . Also, the inner screw  12  can have more than one single projection to engage with the rod  9 .  
         [0036]     The material from which the parts of the bone anchoring assembly are made is a biocompatible material such as, for example, titanium, stainless steel or another material which can be used for bone screws. The material can be different for different elements. In particular, the material is rigid enough to permit a solid form-locking connection between the rod  9  and the bone anchoring element  1 .  
         [0037]     In operation, first, at least two bone anchoring elements  1  are screwed into the bone, in particular into adjacent vertebrae. Then, the rod  9  is placed into the U-shaped recesses  4  of the receiving parts  3  of bone anchoring elements  1  such that its surface structure  10 ,  11  engages with the structure  14 ,  15  on the bottom  5  of the receiving part  3 . Thereafter, the inner screw  12  is screwed-in. As soon as the ring-shaped projection  13  engages the structure of the surface of the rod  9 , the rod  9  is held in the receiving part  3  in a form-locking manner, i.e. a positive connection, which prevents movement of the rod  9  in any direction. Due to the form-locking or positive locking connection, the application of a low torque is sufficient to obtain a sufficient pre-fixation.  
         [0038]     In a following phase, fine adjustment is carried out whereby the engagement position of each anchoring element  1  at the rod  9  relative to the longitudinal axis of the rod  9  is adjusted. For fine adjustment, the inner screw  12  is turned back slightly so as to loosen the form-locking engagement of the structure of the rod  9  with the structure of the inner screw  12 . Since the pre-fixation is obtained with low torque, it can be loosened by a quick turning back of the inner screw  12 . This facilitates the handling and accelerates the procedure. After the final positions of the anchoring elements  1  relative to the rod  9  are found, the inner screws  12  are tightened so as to firmly fix the rod  9  in the receiving parts  3  of the anchoring elements  1 .  
         [0039]     With the anchoring assembly described, a pre-fixation of the rod  9  can be obtained with low torque compared to a pre-fixation in the case in which a smooth rod is fixed via frictional locking only. This is also true compared to the case in which a structured rod is fixed using a spring member. Furthermore the anchoring assembly has a simpler construction compared to that having a spring element for clamping the rod. This facilitates the production.  
         [0040]     A modification of the first embodiment is shown in  FIGS. 10   a,    10   b,    11  and  12 . It differs from the first embodiment in that the rod is a threaded rod  9 ′. Hence, the grooves  10 ′ and the crests  11 ′ have a helical structure with a thread pitch. Correspondingly the structure  14 ″ in the bottom of the receiving part has a thread pitch. The threaded rod  9 ′ is easier to produce compared to the ratchet-type rod.  
         [0041]     A second embodiment is described with reference to FIGS.  13  to  15 . The second embodiment differs from the first embodiment in that the bone anchoring element  101  is formed as a so-called polyaxial screw whereby the receiving part receiving the rod is pivotably connected to the associated threaded shaft. The same reference numerals are used for those parts of this embodiment which correspond to the above described embodiments.  
         [0042]     The bone anchoring element  101  comprises a threaded shaft  102  and a head  102   a  having a spherical segment-shaped portion with a recess  102   b  for engagement with a screwing-in tool. The receiving part  103  is substantially cylindrical and comprises a coaxial bore with a section  103   a  for receiving the head  102   a  so that the head is pivotably held in the receiving part  103 . The receiving part  103  further has a U-shaped recess  104  for receiving the rod  9 . By means of the U-shaped recess  104 , two legs  106 ,  107  are formed which comprise an inner thread  108 .  
         [0043]     A pressure member  120  is provided which is substantially cylindrical and has an outer diameter allowing it to be displaced back and forth in the receiving part  103 . The pressure member  120  has on its side facing the head  102   a,  a preferably spherical, recess  121  for engagement with at least a part of the head  102   a.  On the opposite side, the pressure member  120  has a cylindrical recess  122  the cylinder axis extending transverse to the screw axis. The radius of the cylindrical recess  122  is adapted to receive the rod  9 .  
         [0044]     The surface of the cylindrical recess  122  comprises projections  114  separated by incisions  115  extending transverse to the rod axis in a circumferential direction for engagement with the grooves  10  of the rod  9  similar to the structure of the bottom  5  of the receiving part  3  of the first embodiment. The pressure element  120  further has a coaxial bore  123  for permitting access with a screwing-tool. The size of the pressure element  120  is such that in the assembled state the bottom of its cylindrical recess  123  projects above the bottom  105  of the U-shaped recess of the receiving part as shown in  FIG. 14  and  15 . The screw (the threaded shaft  102  and the head  102   a ), the receiving part  103  and the pressure element  120  can be pre-assembled.  
         [0045]     The rod  9  and the inner screw  12  are identical to that of the first embodiment.  
         [0046]     In operation, at least two bone anchoring elements are placed into two bone fragments or two adjacent vertebrae. Thereafter the rod  9  is inserted into the U-shaped recesses of the receiving parts. Since the receiving parts are pivotable with respect to the screws, the angular position of the receiving part relative to the threaded shaft can be adjusted. When the inner screw is screwed-in, it exerts pressure on the rod which exerts pressure on the pressure element  120  to press it against the section  103   a  of the receiving part. A temporary fixation or pre-fixation can be achieved if the inner screw is tightened with a low torque. The temporary fixation of the rod  9  is caused by the form-locking connection between the inner screw and the rod  9  on the one hand and the rod  9  and the pressure element  120  on the other hand. The fine adjustment is carried out in the same way as with the first embodiment.  
         [0047]     Further modifications of the invention are conceivable. The structure of the surface of the rod  9  may vary, for example, it can have wavy helical convolutions. For example sharp crests forming an obtuse angle are also possible. If such sharp crests are provided a finer graduation can be achieved since the formed teeth are smaller in the longitudinal direction of the rod. However, with a rounded shape the fine adjustment can be performed more reliable. The structure of the bottom of the receiving part and of the projection provided at the locking element can also vary.  
         [0048]     It is possible to provide the structure only on one part, either on the locking member or on the receiving part or the pressure element, respectively.  
         [0049]     In a further modification the U-shaped recess is not open to the top of the receiving part but to the lateral side.  
         [0050]     In a further modification of the second embodiment, the head and the rod can be fixed independently. In this case the pressure element has instead of the cylindrical recess a U-shaped recess by means of which legs are formed which extend above the rod. The locking member comprises a first screw pressing on the pressure element only and having a threaded coaxial bore in which a set screw is provided which comes into engagement with the rod.  
         [0051]     The invention is not limited to screws as bone anchoring elements but can be realized with bone hooks or any other bone anchoring element.